my thisis scale formation in reverse osmosis membranes eng
TRANSCRIPT
TABLE OF CONTENTS
Subject Page
ABSTRACT 7
ACNOWLEDGEMENTS 9
CHAPTER ONE :
1.1 INTRODUCTION
10
1. 2 Aim of the Present Work 17
CHAPTER TWO REVERSE OSMOSIS 18
2.1- Introduction 19
2.2 – Applications of Reverse Osmosis 21
2.3- Types of Membranes 24
2.4- Reverse Osmosis Theory 29
2.5 -Principles of Reverse Osmosis Processes 30
2.6- Basic Equation used in Reverse Osmosis 32
2.7 - Factors Affect Reverse Osmosis Performance 35
2.8 Pretreatment for Reverse Osmosis and Water Chemistry 42
2.8.1 - Introduction 42
2.8.2 Feed Water Analysis 43
2.9 Fouling Scale Problems and Control 44
2.9.1 Fouling in Reverse Osmosis Systems 44
2.9.2 Types of Fouling: 44
2.9.3 How Fouling Occurs in Reverse Osmosis 46
2.9.4 What Scale Is 46
2.9.5 Effects of Fouling Compounds on Reverse Osmosis Membranes 50
2.9.6 Effects of pH and Recovery on Fouling Compounds 51
1
Subject Page
2.10 -Methods Of Scale Control 53
2.10.1 -Acid Addition 54
2.10.2 -Scale Inhibitor Addition 54
2.10.3 - Lime Softening 56
2.10.4 -Preventive Cleaning 57
2.10.5 - Adjustment of Operating Variables 58
CHAPTER THREE : LITERATURE SURVEY 59
CHAPTER FOUR : RESULTS AND DISCUSSION 70
4.1 Introduction 71
4.2 Permacare Program 73
4.3 - Effect of CaSo4 Concentration 78
4.4 - Effect of CaCo3 Concentration 81
4.5 - Effect of Silica Content 83
4.6 Effect of PH on various Fouling Compounds 86
4.6.1 Effect of PH on CaCO3 Scale 86
4.6.2 Effect of PH on CaSO4 Scale 88
4.6.3 Effect of PH on IRON Scale 90
4.6.4 Effect of PH on SILICA Scale 92
4.7 - Effect of Process Recovery on the scalling potential of various Foulants 94
4.7.1 Effect of Recovery on the Fouling tendency of CaCO3 95
4.7.2 Effect of Recovery on the Fouling tendency of CaSO4 97
4.7.3 Effect of Recovery on the Fouling tendency of SILICA 99
4.7.4 Effect of Recovery on the Fouling tendency of IRON 101
2
Subject Page
4.8 - ACTUAL CASE STUDY ON FOULING PROBLEM FOR ONE OF RO
PLANTS
103
4.8.1 Introduction 103
CHAPTER FIVE CONCLUSIONS AND RECOMMENDATIONS 120
5.1 CONCLUSIONS 121
5.2 RECOMMENDATIONS 124
REFERENCES 125
APPENDEX 132
3
SCHEDULE OF TABLES
No. Title Page
Table 1 Drinking water Membrane Separation Technologies 13
Table 2 Factors Affect RO performance 41
Table 3 Operation Conditions of Considered RO plant 102
Table 4 CaSO4 Concentration Table 127
Table 5 CaCO3 Concentration Table 130
Table 6 SiO2 Feed Concentration Table 131
Table 7 PH on CaCO3 Scale Table 133
Table 8 PH on CaSO4 Scale Table 135
Table 9 PH on Iron Scale Table 136
Table 10 PH on SiO2 Scale Table 137
Table 11 Recovery on CaCO3 Scale Table 138
Table 12 Recovery on CaSO4 Scale Table 139
Table 13 Recovery on Iron Scale Table 140
Table 14 Recovery on SiO2 Scale Table 141
Table 15 Actual Case Study Phase 1 Table 143
Table 16 Actual Case Study Phase 2 Table 144
Table 17 Actual Case Study Phase 3 Table 145
4
SCHEDULE OF FIGURES
No. Title Page
Figure 1 Membrane and Conventional Process overview 23
Figure 2 Spiral Wound Membrane 25
Figure 3 Hollow Fine Fiber 26
Figure 4 Schematic Diagram of a Reverse Osmosis Membrane Element 27
Figure 5 Perma Care Program 31
Figure 6 Effect of CaSO4 Concentration 31
Figure 7 Effect of CaCO3 Concentration 32
Figure 8 Effect of Silica Content 42
Figure 9 Effect of PH on CaCO3 Scale 42
Figure 10 Effect of PH on CaSO4 Scale 43
Figure 11 Effect of PH on IRON Scale 43
Figure 12 Effect of PH on SILICA Scale 51
Figure 13 Effect of Recovery on the Fouling tendency of CaCO3 75
Figure 14 Effect of Recovery on the Fouling tendency of CaSO4 77
Figure 15 Effect of Recovery on the Fouling tendency of SILICA 79
Figure 16 Effect of Recovery on the Fouling tendency of IRON 81
Figure 17 Schematic Flow sheet of plant ( use in case study ) 83
Figure 18 Flux Decline (Product Flow Rate during the first phase of this case
study )
85
Figure 19 Recovery Decrease ( During this phase of the problem). 87
Figure 20 Pressure drop Increase ( during the first phase of the
problem)
90
Figure 21 Conductivity of the feed water to the Plant(during of the first 92
5
No. Title Page
phase of the problem).
Figure 22 PH of the Processed water of the Plant(during of the first phase of the problem).
94
Figure 23 Flux increase after washing (during the second phase of this problem).
96
Figure 24 Recovery Increase after Membranes Washing (during of the second phase of the problem).
99
Figure 25 Pressure Drop Decrease after Washing (during of the second phase of the problem ).
100
Figure 26 Recovery Decrease(during of the there'd phase of the problem) 101
Figure 27 Product decline (during of the third phase of the problem) 102
Figure 28 Pressure drop (during of the third phase of the problem) 104
6
ABSTRACT
Membrane applications in water technology have acquired significant importance
within the Kingdom as a result of its large use in sea water desalination and
brackish water treatment plants .
These membranes are facing serious problems that cause its deterioration or
failure. The most important of these problems are fouling on its surface This
fouling consists of deposition of undesired substances such as micro particles,
calcium salts, magnesium, metal silica as well as the organic substances on its
surface This fouling leads to significant reduction in the water production capacity
and increases the pressure drop inside the membranes .
In the present work, this fouling problem is investigated . The various fouling salts
like CaSO4 , CaCO3, , silica and iron compounds are studied concerning their
concentration effects on the membrane fouling . Moreover the process parameters
like recovery and pH of water are also investigated to clarify their effects on
membrane fouling .
An actual case study of a fouling problem occurring in one of water treatment
plants in Riyadh City was also investigated and discussed .
7
The obtained results in the present work identified the high fouling potential of
silica , iron compounds as well as Ca++ and Mg-- salts .
It is found that , the fouling tendency of these compounds is highly dependent on
its pH range which has to be considered during the pretreatment steps of RO
plants.
The data also revealed the significant effect of the process recovery on the fouling
potential of the fouling compounds . It is recommended that recovery is kept
between 70 and 80 % to avoid fouling troubles .
The case study considered in the present work showed the importance of precise
monitoring and observation of the RO process especially water analysis , ΔP
measurements and production rate ( or flux decline ) .
8
ACNOWLEDGEMENTS
I am greateful to my research advisor , Dr. Malik I . Al -Ahmad for his
encouragement and guidance through the course of research .
Special thanks are also due to Dr. Farag Abdul Aleem for his valuable help and
encouragement and Suggestions .
Last but not least .I would like to thank my parents for their sincere encouragement
and unfailing support over the years
9
CHAPTER ONE
INTRODUCTION
10
Chapter 1
1.1 Introduction
Kingdom of Saudi Arabia has a population of about 21 million, and an area of
2262000.Km2. Most of the country is within the arid or semiarid zones and has
long hot, dry summers and short cold winters, some areas of Saudi Arabia have a
little or not rain and the kingdom may be the world's largest country without any
perennial rivers or streams. Even though, the country doses not have dependable
supplies of surface water (lakes, rivers or streams).
Saudi Arabia is a fast growing country with limited drinking water resource, Most
of this water is required for domestic and industrial and daily needs. Ministry of
Water and Electricity- Riyadh Region is a governmental department to contact
water treatment up to suitable level of treatment satisfying standards and
requirements. It is depending on two sources of water desalinated seawater and
brackish water. Desalinated sea water which is pumped from Jubail to Riyadh area
across tow major pipes (with total full capacity of 850000 m3/day) is mixed with
well water (total full capacity 250000 m3/day) in big tanks, the mixed water has a
TDS concentration 250 ppm approximately.
Reverse osmosis membranes have recently acquired a potential interest in Saudi
Arabia due to its wide application in both desalination plants and waste water
11
treatment processes. Riyadh region has five large water treatment plants (Buwaib
Plant, Sulbukh, Manfouha, Shmessy and Malaz) They account for about 40% of
Riyadh water supply.
Scarcity of water resources is one of the stiffest challenges facing the kingdom
of Saudi Arabia as its population, and agricultural and industrial bases continue
to grow. To meet this challenges, Saudi Arabia invested heavily in the fields of
sea and brackish water desalination and water reuse from wastewater and
industrial effluents. As result, Saudi Arabia leads the world in desalination
capacity, producing about 30% of the world's total, and its experts have gained
considerable operating experiences with the various separation processes used
for desalination and, improving the performance of these separation processes is
a key factor in determining Saudi Arabia's ability to meet its increasing demand
in water in future. Reverse osmosis is one of the main separation processes as
shown in (Table 1) . It shows a growing potential usage for desalination water
reuse in Saudi Arabia. It is a simple process that uses hydrophilic
semipermeable membranes to separate water from dissolved solids as well as
organic and suspended solids without phase change as in the conventional
thermal separation processes. Reverse osmosis plants in Saudi Arabia produce
12
25% of the combined total production of desalination of sea and brackish water,
and reclaimed water from waste streams [1].
Reverse osmosis suffer from scaling problems. Commonly encountered scalants
in desalination processes fall into the following three basic categories:
Inorganic Scale
Suspended/colloidal matter
Biological material
Table ( 1 ) Drinking water Membrane Separation Technologies
Each will be looked at separately with a review of the causes, prevention, and
restoration of performance.
13
This thesis will focus on scaling only. Scaling is caused by the precipitation of
sparingly soluble salts dissolved in the feed water. During the desalination
process, the solubility of the sparingly soluble salts can be exceeded which will
lead to precipitation. Common scales encountered include calcium carbonate,
calcium sulfate, silica, metal silicates, oxides/hydroxides of aluminum, iron, and
manganese. Other less commonly encountered scales include calcium fluoride,
barium sulfate, strontium sulfate, and cupric sulfide.
Calcium carbonate is an important naturally occurring compound. It occurs in
colloidal and amorphous states as well as in at least three polymorphs: calcite,
vaterite, and aragonite. Many feed waters contain high concentrations of
calcium bicarbonate. The primary mechanism leading to the deposition of
calcium carbonate on membrane surface is the conversion of soluble calcium
bicarbonate salt into sparingly soluble calcium carbonate due to pressure drop
and /or increase in temperature. The scale may be easily removed by cleaning
with an acid.
Calcium sulfate is another mineral scale frequently deposited by brines. Calcium
sulfate can crystallize from solution in three forms: dihydrate (CaSO 4•2H2O,
14
gypsum), hemihydrate (CaSO4•1/2H2O, plaster of Paris), and anhydrite (CaSO4).
Most calcium sulfate deposits in the RO systems are gypsum (the predominant
form
at temperatures below 40 oC) whereas anhydrite and hemihydrate are the sulfate
deposits commonly found on heat exchangers in the distillation processes.
Though this scale is more soluble than calcium carbonate, once it has formed it
is more difficult to remove. The predominant method of removing this scale
involved the use of chelating agents.
Silica, a common constituent of most natural waters, can occur at extremely
high levels in well water supplies. At present, solubility is used to predict silica
scaling. The precipitation of silica from solution is strongly affected by
insoluble metal hydroxides/oxides present in the feed water. Silica, once
deposited on the membrane, is difficult to remove and should be treated with
caution. Two approaches which are often used to control silica in RO include
lime softening and running the RO system at low recovery to keep the soluble
silica concentration in the brine below the saturation point.
Iron in feed water may be present as colloidal or soluble species. The ferrous
(Fe++) form is quite soluble at pH ranges commonly encountered in RO. Ferrous
15
iron is not a problem as long as it remains in that form. Upon oxidation to the
ferric form, iron hydroxide can deposit on the membrane. It has been reported
that iron present in feed water, if not properly controlled, can co-precipitate with
soluble silica present in water which poses a completely different type of
fouling problem.
Aluminum based compounds have been used for years as coagulant aids to
clarify RO feed waters. Depending on the pH and mode of operation, high
concentrations of aluminum ions can be present in the feed water. Aluminum
hydroxide precipitation can occur when its solubility is exceeded or when an
acid is used to eliminate calcium carbonate scaling potential.
Manganese, although not as common as iron, is often found in iron-containing
water. Like iron, manganese can cause fouling problems when oxidized and
precipitated in RO systems. Iron and manganese bacteria are not common and
are usually not correctly identified, but where they occur, they may be factors in
deposition of iron and manganese-based foulants.
1.2 Aim of the Present Work :
16
Due to the significant importance of fouling and scale problems on the
performance of any RO process , the effects of certain salts like CaSO4 , CaCO3,
SiO2 and iron on the scalling potential of water are investigated in the present
work . More over the effect of pH of feed water as well as the designed recovery
of RO on its fouling problems is also studied and discussed .
17
CHAPTER TWO
THE REVERSE OSMOSIS PROCESS
18
Chapter 2
The Reverse Osmosis Process
2.1 Introduction
Reverse osmosis (RO) is a process used to separate solvents, most of the time
water, from solutes (dissolved solids). It also removes several ions and metals and
other contaminants—organic, inorganic and even bacteria. In addition, reverse
osmosis can also help in removing microorganisms, but this is seldom
recommended due to the strain it puts on the membrane [ 2]
Reverse osmosis is the finest level of filtration available. The RO membrane
acts as a barrier to all dissolved salts and inorganic molecules, as well as
organic molecules with a molecular weight greater than approximately 100.
Water molecules, on the other hand, pass freely through the membrane creating
a purified product stream. Rejection of dissolved salts is typically 95 to greater
than 99%.
Membrane separation systems are widely used to purify water of different
qualities including brackish water, seawater and wastewater. Dissolved and
suspended particulate matter in the feed water can deposit on the membrane
surface. This phenomenon is identified as fouling. Several types of fouling can
19
occur in the membrane systems, e.g. inorganic fouling or scaling, particulate and
colloidal fouling, organic fouling and finally biological fouling or biofouling.
During the process, the membrane concentrate absorbs salts. Inorganic salts, such
as calcium carbonate and barium sulphate, which are water-insoluble, can become
over-saturated. This causes them to precipitate. The precipitation of water-
insoluble salts on the membrane is more likely to occur when conversion is high.
Sodium hexametaphospahte (polyphosphate) is dosing to prevent calcium
sulphate deposition in the membrane
For the removal of small particles and dissolved salts, membrane separation
systems are utilized which use a different method than conventional grade
filtration. Termed crossflow membrane filtration ,this method uses a
pressurized feed stream which flows parallel to the membrane surface. A
portion of this stream passes through the membrane, leaving behind the
rejected particles in the concentrated remainder of the stream. Since there is a
continuous flow across the membrane surface, the rejected particles do not
accumulate but instead are swept away by the concentrate stream. Thus, one
feed stream is separated into two exit streams: the solution passing through the
membrane surface (permeate) and the remaining concentrate stream.
20
2.2 Applications of Reverse Osmosis
There are several industries that benefit from the use of reverse osmosis. The
various industries in which reverse osmosis is being used. RO can be used in
the preparation of various cosmetic products. It is also used in the desalination
of salt water in order to get potable water. RO also removes minerals in water
for the purposes of purifying drinking water. Moreover, RO helps reduce
impurities for manufacturing purposes in the electronics industry. It helps in
generating low sodium food preparation and in producing fruit juices. In
laboratories, it is also used in rinsing glassware. In the area of pharmaceuticals,
RO is used in the production of pure water. [3]
Reverse osmosis can be used in treating wastewater. Water with organic
pollutants as well as wastewater from the processes of electroplating, metal
finishing, paper processes, mining, textile and food processing, radioactivity,
and even contaminated wastewater may be treated using reverse osmosis. RO
is best used for this because designing and operating RO systems are simple,
low maintenance and modular. In addition, organic and inorganic pollutants
and contaminants can be removed easily by the membrane process used by the
RO process as shown in (Figure 1). In the process of recycling of waste
process, RO has no effect on the recovered material. The use of membrane in
21
RO process uses less energy and is set up with considerably less capital and
operating costs as compared with other treatment processes. Lastly, RO
reduces waste streams’ volume, thereby enabling a more efficient and cost-
effective way of treating them.[4]
The process of desalination becomes even more important for areas and
locations where potable water is scarce, such as in the Middle East. RO is the
most widely used desalination process. But 85% of desalinated water is still
produced in multi-stage plants. The Middle East, especially in Saudi Arabia,
due to the scarcity of surface water widely resorts to RO and multi-stage flash
desalination plants
During the process of reverse osmosis desalination, there are two
streams of products—one is pure water and the other is liquid with very high
concentration of saline. The challenge, therefore, is how to dispose of this
highly saline liquid [5]. In desalination plants near the sea, this poses no
problem as it can be put back into the ocean. However in inland sites, this can
be a problem. The common way of solving this is in dumping the concentrate
on available bodies of water, or injecting it to deep wells, or disposed in
publicly declared treatment areas. In addition, they may be disposed to saline
wetlands.
22
Figure ( 1 ) Membrane and Conventional Process overview
23
2.3 Types of Membranes
The type of membranes used in RO largely influences the outcome of the
separation ,The membrane should be chemical and microbe resistant,
mechanically and structurally stable for a long period and it should possess the
desired characteristics for separation. Furthermore, he identified the two types
of membranes that are being widely used as shown in (Figure 2) and (Figure
3 ): asymmetric membranes, which contains only one polymer and the other
one is thin-film, composite membrane, which consists of two or more layers of
polymer. Asymmetric membranes have thin skin layer supported by another
porous layer of the same material. The porous layer provides mechanical
support to the skin layer which selects the fluxes and selectivities of these
types of membranes. These asymmetric membranes are usually formed through
a phase inversion or the process of the precipitation of polymer.
On the other hand, thin film, composite membranes are constructed from
interfacial polymerization [6]. This process is done by coating a highly porous
membrane with a monomer or polymer and then combining it with a cross-
linking agent. The selectivities and water fluxes of these thin, highly cross-
linked polymer membranes can be higher compared with the asymmetric type.
This is because their layers are thinner. An example of this would be
24
composite membranes that consist of cross-linked aromatic polyamide on a
polysulfone support layer.
Among the most important materials used in RO membranes are the
cellulosic polymers (cellulose acetate and cellulose triacetate), linear and
cross-linked aromatic polyamide, and aryl-alkyl polyetherurea. Asymmetric
cellulose acetate membranes are still widely used despite to its susceptibility to
hydrolysis, biological attack, compaction, and lower upper temperature limits.
Other types of composite membranes include polyamide and polyurea which
possess higher fluxes and salt and organic segregations. They can also resist
higher temperature better and their pH variations are higher. Moreover, they do
not easily succumb to biological attack and compaction. These membranes,
however, are less chlorine-resistant and prone to oxidation [6]
Prior to the RO process, it is important for water to be tested first to determine
what types of contaminants and pollutants are present. Moreover, pre-treatment
is important because of the fragility of the membranes and the way they are
wound spirally. Moreover, most membranes are designed such that the flow of
water is one way only—water cannot be pulsed back. Moreover, accumulated
material cannot be recovered from the membranes because they may lose their
production capacity. There are four components of pre-treatment [7]. The first
25
component is that the solid particles in the water should be removed first and
water should be treated. This will ensure that the membranes will not be fouled
by very fine particles. The second significant factor is that microorganisms be
dealt with in order to avoid the fouling of membranes. Third, the acidity of the
water should be adjusted as well since this may also have negative effect on
the membrane and on the overall RO process. And lastly, the cartridge should
also be filtered.
26
Figure (2 ) Spiral Wound Membrane
27
Figure (3 ) Hollow Fine Fiber
28
2.4 Reverse Osmosis Theory
Reverse osmosis, as its name implies, is a process whereby the natural
phenomenon of osmosis is reversed by the application of pressure to a
concentrated solution in contact with a semipermeable membrane. If the
applied pressure is in excess of the solution's natural osmotic pressure, the
solvent will flow throw the membrane to from a dilute solution on the opposite
side and a more concentrated solution on the side to which pressure is applied.
If the applied pressure is equal to the solution's natural osmotic pressure, no
flow will occur, there will be flow from the dilute solution to the concentrated
solution.
The rate of water transport across the membrane depends on the membrane
properties, the solution temperature, and the difference in applied pressure,
across the membrane, less the difference in osmotic pressure between the
concentrated and dilute solutions. Osmotic pressure is proportional to the
solution concentrated and temperature and depends on the type of ionic species
present. For solutions of predominantly sodium chloride at ambient
temperatures. A role of thumb is that osmotic pressure is 10 psi (0.7 atm) per
1000 mg/1 concentration). [ 8 ]
29
2.5 Principles of Reverse Osmosis Processes :
The phenomenon of osmosis which a semipermeable membrane is placed
between two compartments. "Semipermeable" means that the membrane is
permeable to some species, and not permeable to others. Assume that this
membrane is permeable to water, but not to salt. Then, place a salt solution in
one compartment and pure water in the other compartment. The membrane will
allow water to permeate through it to either side. But salt cannot pass through
the membrane. As a fundamental rule of nature, this system will try to reach
equilibrium. That is, it will try to reach the same concentration on both sides of
the membrane. The only possible way to reach equilibrium is for water to pass
from the pure water compartment to the salt-containing compartment, to dilute
the salt solution. Osmosis can cause a rise in the height of the salt solution.
This height will increase until the pressure of the column of water (salt
solution) is so high that the force of this water column stops the water flow.
The equilibrium point of this water column height in terms of water pressure
against the membrane is called osmotic pressure. If a force is applied to this
column of water, the direction of water flow through the membrane can be
reversed. This is the basis of the term reverse osmosis. Note that this reversed
flow produces a pure water from the salt solution, since the membrane is not
30
permeable to salt. water diffuses through a semi-permeable membrane toward
region of higher concentration to equalize solution strength, while the Ultimate height
difference between columns is called "osmotic" pressure , and the applied pressure in
excess of osmotic pressure reverses water flow direction. Hence the term "reverse
osmosis."
31
2.6 Basic Equation Used in Reverse Osmosis :
Figure ( 4) Schematic Diagram of a Reverse Osmosis Membrane Element
The water transport (at constant temperature) through a semipermeable membrane is
described by the following equation The recovery of RO Process is given by :
QW = KW
Q W = water flow rate through the membraneK W = membrane permeability coefficient for water
= hydraulic pressure differential across membrane
= osmotic pressure differential across membrane A = membrane surface area
= membrane thickness
32
Osmotic pressure depends on solute concentration, solution temperature, and
the type of ions present, for dilute solutions, osmotic pressure is approximated
using the van't Hoff equation.
Where
= osmotic pressure
= molar concentration of the solute
= number of ions formed if the solute dissociates (e.g., for NaCl , =2) R = gas constant T = absolute temperature
The salt transport across a membrane is proportional to the concentration
difference across the membrane and is described by the following equation :
= salt flow through membrane
= membrane permeability coefficient for salt
= salt concentration differential across membrane
33
= membrane surface area
= membrane thickness
In this equation salt transport across the membrane is dependent only on the
concentration differential and is independent of applied pressure.
The transport of salt across a membrane is commonly expressed as salt passage
or salt rejection. Salt passage is the percentage of the salt in the feed that
passes through the membrane into the permeate, and is calculated as follows :
Where
= % salt passage
= salt concentration in product stream
= salt concentration in feed stream
The percentage of salt rejection is 100 percent minus the percentage of salt
passage . Salt Rejection (%) = 100 - Salt Passage which the recovery is given
by the ratio of permeate and feed flows . it can be given as :
Recovery (%)=( (Permeate Flow) / (Feed Flow) ) × 100
34
2.7 Factors Affecting Membrane Performance :
Over the past two decades reverse osmosis (RO) has grown to one of the most
widely chosen point-of-use (POU) and commercial water treatment categories
domestically and internationally. During this period, RO membrane and
system technologies have evolved to increase product reliability, improve
total dissolved solids (TDS) and contaminant rejection, and optimize
operation efficiency. Despite these important advancements, the fundamental
rules governing the proper selection and applications of RO systems must still
be observed. Whether working on a residential or commercial RO system,
always review the following checklist of water-quality parameters to ensure
satisfactory system performance .Permeate Flux' and salt rejection are the key
performance parameters of a reverse osmosis process. They are mainly
influenced by variable parameters which are as follows: Pressure ,
Temperature ,Feed water quality , Recovery and pH as show in Table (1)
Not to be neglected are several main factors which cannot be seen directly in
membrane performance. These are maintenance and operation of the plant as
35
well as proper pretreatment design. Consideration of these three 'parameters',
which have very strong impact on the performance of a reverse osmosis system
A ) Recovery
Recovery or conversion is commonly used to define the percentage of the feed
water is converted to permeate it is calculated as follows :
= % recovery or conversion
= Product stream flow
= Feed stream flow rate
In the case of increasing recovery, the permeate flux will decrease and stop if
the salt concentration reaches a value where the osmotic pressure of the
concentrate is as high as the applied feed pressure. The salt rejection will drop
with increasing recovery. Fouling and Scaling problems will also increase by
increasing the recovery.
36
The brine will contain most of dissolved solids in the feed and will be
approximately four times more concentrated than the feed .To conserve energy
, it is desirable to operate at as high a recovery rate as possible to minimize
upstream capital costs. Excessively high recovery can create high brine
concentrations, which reduce permeate flow and increase salt passage. This
may lead to membrane fouling or scaling resulting from the precipitation of
sparingly soluble salts from the concentrated brine
B ) Pressure of the feed water
For a given set of feed conditions; increasing pressure results in increased
water flow per unit of membrane area. Although the transport of salt (Q)
across a membrane is not affected by pressure, increased water flow with
pressure dilutes the salt passing through the membrane, which results in a
lower permeate salt concentration (a reduction in salt passage).
With increasing effective feed pressure, the permeate TDS will decrease while
the permeate flux will increase .
The effect of pressure on the RO process refers to the net pressure or
differential pressure across the membrane :
Net pressure =feed pressure -back pressure -osmotic pressure Where :
37
Feed pressure is the water pressure measured with a pressure gauge at the
installation site under static conditions and what acts on the tapwater side of
the membrane. Back pressure acts on the product side of the membrane,
opposing the feed pressure. For RO systems, it results from the pressurized
water in the storage tank and increases as the tank fills to capacity. Osmotic
pressure is due to the presence of TDS in the water which opposes the feed
pressure. The more TDS in the water, the greater the pressure requirements to
achieve the same productivity. As a general rule, for every 100 ppm of TDS,
about 1 psi of osmotic pressure must be overcome before water starts
permeating the membrane. Clearly, a higher feed water pressure to overcome
it. For example, if the TDS is 2,000 ppm, the osmotic pressure is
2,000/100=20psi. This has the effect of subtracting 20 psi from the feed
pressure. RO production rate is directly proportional to net pressure : Doubling
the net pressure doubles the production rate; halving the pressure halves the
production rate .
C ) Temperature of the feed water
Temperature changes affect both osmotic pressure and water flux (K w )The
effect on osmotic pressure . (K w ) is also directly proportional to temperature
and is usually proportional to the variation in the water viscosity from one
38
temperature to another. A rule of thumb is that membrane capacity increases
about 3 percent per degree Celsius increase in water temperature.
If the temperature increases and all other parameters are kept constant, the
permeate flux and the salt passage will increase .
Feed temperature also determines RO production rate because of the effect it
has on the viscosity of water. The industry standard for determining production
rate is 77 ْ F ( 25 ْ C ). For predicting the production rate at higher or lower
temperatures, refer to a temperature conversion supplied with the particular
membrane being used .
The water temperature must be known to ensure that the maximum operating
temperature is not exceeded, or membrane degradation will occur over time.
D ) Compaction
The water transport, or flux, through a clean membrane can decrease with
time as a result of membrane compaction. Compaction is caused by creep
deformation of polymeric membranes over time and is dependent on the
membrane material, applied pressure, and temperature. As pressure and
temperature increase, the tendency to creep is greater. This tightens the
membrane's rejecting layer and reduces its water transport as a log function of
time. versus time at a given temperature and pressure produces a straight
39
line. This effect is more pronounced in asymmetric homogeneous membranes.
These. data, available from membrane manufacturers, are used to predict
performance at a future time to provide the design basis for RO system
capacity.Initially, an RO system has excess membrane capacity, which is
offset by lower operating pressures that are gradually increased to the design
pressure over time .
E ) Feed water quality
The level of TDS in feed water is a major factor that will determine the RO
product water quality, because RO membranes reject a percentage of the feed
TDS. There is a direct relation between TDS levels and the minimum feed
pressure required. TDS causes what can be best described as an osmotic "back-
pressure", which reduces the effective feed water pressure and must be
overcome before the RO process can take place . TDS in the feed water supply
is measured with a conductivity meter or from a water analysis.
F ) pH of the feed water
Also determines the type of membranes that can be used. Cellulose
membranes can slowly deteriorate by hydrolysis at high pH levels (typically
40
over 8.0 ) and gradually lose their rejection of TDS. By contrast, Thin Film
membranes can be safely subjected to higher pH levels up to 11.0.Membrane
rejection of certain dissolved solids such as arsenic is also affected by feed
water pH levels, as well as determining how some dissolved solids such as
iron, manganese and hardness minerals will remain in solution and whether
they pose a potential fouling /scaling problem.
Table (1) Factors Affect RO performance
:
Increasing of : Effect on Permeate flow Effect on Salt passage
Effective pressure Direct Proportional(↑) Inverse Proportional(↓)
Temperature Direct Proportional(↑) Direct Proportional(↑)
Recovery Inverse Proportional(↓) Direct Proportional(↑)
TDS Inverse Proportional(↓) Direct Proportional(↑)
41
2.8 Pretreatment for Reverse Osmosis and Water Chemistry
2.8.1 Introduction:
Pretreatment is the key to successful long-term RO performance, and its
importance in system design should not be underestimated. The purpose of
pretreatment is to guard against feed water upsets, remove suspended and
colloidal material, prevent membrane scaling resulting from precipitation of
sparingly soluble salts, and to prevent biological growth. Much has been
written about pretreatment throughout the years, and the techniques have
become state-of-the-art for RO practitioners. Membrane manufacturers,
systems designers and manufacturers and/or consulting engineering
organizations should be consulted about pretreatment requirements for
specific feed waters and membrane types and configurations . [ 9 ]
42
To increase the efficiency and life of a reverse osmosis system, effective
pretreatment of the feed water is required. Selection of the proper pretreatment
strategy will maximize efficiency and membrane life by minimizing: Fouling ,
Scaling and membrane degradation . The net result of the above will be the
optimization of :Product flow ,Salt rejection, Product recovery and Operating
costs. In this study, "fouling" will refer to the entrapment of particulates such
as iron floc or bio compounds, whereas "scaling" will refer to the precipitation
and deposition within the system of sparingly soluble salts such as calcium
sulfate (CaSO4) or barium sulfate (BaSO4).
2.8.2 Feed Water Analysis:
The major water types being treated by RO are: low salinity brackish waters
with (TDS) of up to 5,000 ppm. , High salinity brackish waters with TDS in
the range of 5,000 -15,000 ppm. And Sea water with TDS in the range of
35,000 ppm. . Sea water with TDS of 35,000 mg/l is considered as "standard
sea water" as this constitutes by far the largest amount of water
worldwide ,except the Arabian gulf and the red sea (45000 mg/l). The
composition is nearly the same all over the world. The actual TDS content
may, however, vary within wide limits from e.g. the Baltic Sea with 7,000
mg/L to the Red Sea and Arabian Gulf with up to 45,000 mg/l. The actual
43
compositions can be proportionally estimated from the "standard sea water"
composition. Be aware that seashore wells depending on the soil, influx from
inland, etc. can often have salinity and composition quite different from that of
a sample taken from the sea itself. In sea water treatment the limiting factor is
of a physical nature, i.e. the osmotic pressure caused by the high TDS. In
brackish water treatment the limiting factor is mostly of a chemical nature, i.e.
precipitation and scale formation by e.g. calcium carbonate or sulfate.
2.9 Fouling and Scale Problems and Control :
2.9.1 Fouling in Reverse Osmosis Systems
As previously discussed, fouling is one of the major problems occurred
during the process of reverse osmosis. Fouling is defined as the accumulation
of debris and particles on the surface of the membrane used in reverse osmosis
such as insoluble corrosion products, scale deposits, mud, slit, clay, airborne
and process contaminants, and biological matters (e.g., insects, pollen). The
frequency of fouling depends on various reasons such as the system recovery
rate, reverse osmosis feedwater characteristics, pretreatment, and system
operation [10] .
2.9.2 Types of Fouling :
44
Membrane foulants are classified into three major types ( as show in Figure
( 5) ) depending on their physical type and location on the membrane, namely,
dissolved solids, suspended solids, biological organisms, and nonbiological
organics [10]
1. Dissolved solids – These materials form scales which are soluble in
feedwater like calcium and barium. Dissolved solids can be cations
(positively charged ions) or anions (negatively charged ions). These
dissolved solids settle and precipitate in the brine stream when their
concentrations increased during the process of reverse osmosis.
Calcium carbonate, calcium sulfate, barium sulfate, and strontium
sulfate are examples of precipitated cations or anions.
2. Suspended solids – These precipitates such as colloidal forms of
metal oxides (e.g., iron, aluminum, silica) retain their suspension on
the membrane through a process called repulsion by a double layer of
charge.
3. Biological foulants – These foulants include bacteria, fungus, algae,
and the metabolic wastes they produced. These foulants are anaerobic
and aerobic in nature. Biological foulants are produced in large
45
numbers, which tend to be the cause of blockage of flow through the
membrane. These are also present in low concentrations.
Nonbiological organic foulants – These are nonliving organisms which have
carbon-based chemical structures such as oil, plant materials, cationic
surfactants, and hydrocarbons
2.9. 3 How Fouling Occurs in Reverse Osmosis :
Fouling occurs when dissolved solids increase their concentrations and
settle on the reverse osmosis membrane. It happens when a nucleus is formed
because the water elements become concentrated surpassing the point of
supersaturation. This formed nucleus will settle on the membrane or attached
itself to a then-fouled area of the membrane, which is called as the site of
nucleation. Ions like calcium and bicarbonate and barium and sulfate also form
a nucleus that precipitates on the membrane. It happens when the recovery
rates during reverse osmosis increased; thus, salts concentration in the brine
also increases. Other ions that precipitate on the membrane are magnesium,
aluminum, iron, and silica. Silica can block the flow of fluids through the
membrane and blind the membrane if a gel is formed and thickened due to
phosphates or phosphonates.
46
2.9.4 What Is Scale ?
Scale, as previously discussed, is also one of the major problems occurred in
reverse osmosis. It is the precipitates of water-soluble inorganic salts, which
are localized on the membrane surface. There are several factors that lead to
the formation of scales on the membrane: calcium and magnesium salts
concentration, pH and alkalinity, and total dissolved solids concentration.
Scale formation usually results in inability of the membrane to perform well
during the process of reverse osmosis.
Scaling of a reverse osmosis membrane may occur when sparingly
soluble salts are concentrated within the element beyond their solubility limit.
For example, if a reverse osmosis plant is operated at 50% recovery, the
concentration in the concentrate stream will be double the concentration in the
feed stream. As the recovery of a plant is increased, so is the risk of scaling.
Therefore, care must be taken not to exceed the solubility limits of slightly
soluble salts, or precipitation and scaling may occur. Scaling problems in
membrane filtration systems will most likely become more important in the
future due to emphasis on the use of lower quality water resources in order to
save and recycle water.
47
The most frequent scaling problems come from calcium carbonate (CaCO3)
because it precipitates fast, once concentrated beyond its solubility limit and
also most natural waters are almost saturated with respect to CaCO3.
CaCO3 Scaling can be prevented by acid addition, a scale inhibitor, softening
of the feed water, preventive cleaning and low system recovery. CaSO4 scaling
is preventable by the same methods as CaCO3 Scaling except the acid addition.
In fact, using sulfuric acid to lower pH for the prevention of CaCO3 scaling
would increase the probability of the sulfate scaling. The successful operation of
any RO plant depends on how free the feed water is of particulate matter and
suspended solids. Most natural waters contain some suspended matter, and the
variation in size and type of such particles can be substantial. The particles can
be classified into two groups: colloidal and noncolloidal. A major fouling
problem with reverse osmosis systems is scale deposits on the membrane. Scale
buildup causes system performance to deteriorate. Drop in flow rate and an
increase in salt passage. It is usually indicated by an increase in pressure drop
across the filters.
The correct pH, inhibitor dosage and recovery rate depend on the feed water
chemistry and can vary considerably. This is why it's crucial to control these three
parameters as close to design standards as possible.
48
In a reverse osmosis system the most common sparingly soluble salts
encountered are CaSO4 , CaCO3 and silica.
49
Figure ( 5 ) Scaling and Fouling
50
2.9.5 Effects of Fouling Compounds on Reverse Osmosis Membranes
Concentrations of fouling compounds like calcium sulfate, calcium
carbonate, silica, and iron greatly affect the integrity of the membrane. According
to recent studies, presence of high levels of silica in water results in stern
membrane fouling. High levels of silica often hinder to attain the level of reverse
osmosis recovery [11]. furthermore, when calcium carbonate and silica increased
concentration over their saturation values, they are deposited on the membrane
resulting in the loss of purified water production or loss of permeate flux through
the membrane [12] . Calcium sulfate also contribute to scale formation where
permeates flux decline and shortens the life of the membrane [13 ]. Presence of
iron on the membrane should also be measured because it also causes fouling
problem. It can cause oxidation to insoluble ferric salts even though it is soluble
[ 14]. Moreover, calcium carbonate impedes the fluid flow through the
membrane when it forms in the brine spacers just above the surface of the
membrane. Calcium sulfate, on the other hand, precipitated on the membrane
surface in small groups. Silica forms a gel that wets out and spreads in a thin
layer over the surface of the membrane, which causes the blinding off the
membrane.
51
2.9.6 Effects of pH and Recovery on Fouling Compounds
Reduced operating recovery and feedwater pH reduction are often used to
control membrane fouling caused by fouling compounds such as calcium
carbonate, calcium sulfate, silica, and iron. Reducing the pH of the feedwater
with acid (either sulfuric or hydrochloric acid) can decrease the concentration of
calcium carbonate. This method shifts equilibrium to the left that keeps the
calcium ions in solution [15] . However, it increases the growth of other foulants
like silica and is effective for carbonate scale only. Moreover, the use of sulfuric
acid can cause problem with sulfate scale because of the additional sulfate ions
the acid contributes. The solubility of silica decreases when lowering the pH to
less than 7; thus, it precipitates on the surface of the membrane. Reduction of
feedwater pH has also an effect on permeate water quality. When thw pH of feed
water below 8.2, carbon dioxide will be produced affecting the quality of
permeate water [15] . In this case, low pH and alkalinity should be used to attain
good permeate water quality and system performance.
Reducing system recovery is another way of preventing scale formation. It is
advisable to operate the process at lower recovery. The concentration of salts in
the brine steam decreases when the system recovery is also decreased; thus,
52
preventing precipitation. Operating at lower recovery produces less permeate and
more brine water [15] . Lower recovery reduces the concentrations of all fouling
compounds; thus, preventing or minimizing membrane fouling.
The integrity of reverse osmosis membranes are influenced by pH and alkalinity,
temperatures, recovery rate, salt passage, and pressures. It is necessary that the
membrane could withstand these factors to prevent membrane fouling and scale
formation. The performance of the membrane during the process of reverse
osmosis can be affected by the concentrations of dissolved and suspended solids.
If the concentration of the debris is high, the osmotic pressure is also high.
Therefore, there is a tendency that the suspended solids set and precipitate on the
membrane resulting in scale formation. In certain circumstances, temperatures
above 35°C can create hydrolysis and membrane degradation. Salt passage is also
affected if temperatures and pressures do not support the type of membrane being
used in reverse osmosis.
53
2.10 Methods Of Scale Control:
There are a number of ways to pretreated raw water to prevent or reduce
undesirable scaling. These include, for example, lime-soda softening and ion
exchange. Calcium carbonate is often termed an alkaline scale because high
pH favors the carbonate species in the equilibrium that exists between
bicarbonate and carbonate ions in water .The addition of hydrogen ions,
usually in the form of sulfuric or hydrochloric, increases the bicarbonate
concentration. The most common water pretreatment method for reducing
carbonate scale is acid dosing. Acid dosing invariably is used where the RO
membrane is cellulose acetate, which must operate in an acidic environment.
Acid dosing, however, is not effective against the sulfate scales. (In fact
sulfuric acid would contribute toward the problem were it used). Neither is
acid dosing effective for fluorides. Where acid dosing is used, often a second
chemical, SHMP (sodium hexametaphosphate) must be added to control
calcium sulfate deposition. A preferred approach - particularly where the RO
membranes are not of cellulose acetate and therefore have higher operational
pH limits - is to use a modern polymeric multifunctional additive.
54
2.10.1 -Acid Addition:
Most natural surface and ground waters are almost saturated with respect to
CaCO3. The solubility of CaCO3 depends on the pH, as can be seen from the
following equation:
Ca++ + HCO3-↔ H+ + CaCO3
Accordingly, by adding H+ as acid, the equilibrium can be shifted to the left
side in order to keep calcium carbonate dissolved. The acid used should be of
food grade quality. Sulfuric acid is easier to handle and in many countries
more readily available than hydrochloric acid, but on the other hand, additional
sulfate is added to the feed stream. This might be critical with respect to
sulfate scaling. In order to avoid calcium carbonate scaling, CaCO 3 should tend
to dissolve in the concentrate stream rather than to precipitate.
2.10.2 -Scale Inhibitor Addition:
Scale inhibitors (antiscalants) can be used to control carbonate scaling, sulfate
scaling and calcium fluoride scaling.
Scale inhibitors have a "threshold effect", which means that minor amounts
adsorb specifically to the surface of microcrystals thereby preventing further
55
growth and precipitation of the crystals. The most widely used scale inhibitor
is sodiumhexametaphosphate (SHMP). Food grade quality should be used.
Care has to be taken in order to avoid hydrolysis of SHMP in the dosing feed
tank. Hydrolysis would not only decrease the scale inhibition efficiency, but
also create a calcium phosphate scaling risk.
SHMP should be dosed to give a concentration in the concentrate stream of 20
mg/L. For example, the dosage into the feed stream of a system with 75%
recovery will be 5 mg/L.
Polymeric organic scale inhibitors are more effective than SHMP. However,
precipitation reactions may occur with cationic polyelectrolytes or multivalent
cations, e.g. aluminium or iron. The resulting gumlike products are very
difficult to remove from the membrane elements.
Overdosing should be avoided. In RO plants operating on sea water with TDS
in the range of 35,000 to 45000 mg/L, scaling is not such a problem as in
brackish water plants, because the recovery of sea water plants is limited by
the osmotic pressure of the concentrate stream to 30-45%. However, for safety
reasons using a scale inhibitor when operating above a recovery of 35% is
recommended .
56
2.10.3 - Lime Softening:
Lime softening can be used to remove carbonate hardness by adding hydrated
lime:
Ca(HCO3)2 + Ca(OH)2 →2CaCO3↓ + 2 H2O
Mg(HCO3)2 + 2 Ca (OH)2 → Mg(OH)2↓ + 2 CaCO3 + 2H2O
The noncarbonate calcium hardness can be further reduced by adding sodium
carbonate (soda ash):
CaCl2 + Na2CO3 →2NaCI + CaCO3 ↓
The lime-soda ash process can also be used to reduce the silica concentration.
When sodium aluminate and ferric chloride are added, the precipitate will
include calcium carbonate and a complex with silicic acid, aluminium oxide
and iron. With the hot lime silicic acid removal process at 60-70°C, silica can
be reduced to 1 mg/l by adding a mixture of lime and porous magnesium
oxide. With lime softening, barium, strontium and organic substances are also
reduced significantly. The process requires a reactor with a high concentrdtion
of precipitated particles serving as crystallization nuclei. This is usually
achieved by up flow solids-contact clarifiers. The effluent from this process
needs media filtration and pH adjustment prior to the RO elements. Iron
57
coagulants with or without polymeric flocculants (anionic and non-ionic) may
be used to improve the solid liquid separation.
Lime softening should be considered for brackish water plants of capacity
larger than 200 m3/h (880 gpm).
2.10.4 - Preventive Cleaning:
In some applications, scaling is controlled by preventive membrane cleaning.
This allows the system to run without softening or dosage of chemicals.
Typically, those systems operate at low recovery in the range of 25%, and the
membrane elements are replaced after 1-2 years. Accordingly, those systems
are mainly small single-element plants for potable water from tap water or sea
water. The simplest way of cleaning is a forward flush at low pressure by
opening the concentrate valve. Short cleaning intervals are more effective than
long cleaning times, e.g. 30 seconds every 30 minutes. Cleaning can also be
carried out with cleaning chemicals. In batch processes like in waste water
treatment it is common practice to clean the membranes after every batch. The
cleaning procedure, cleaning chemicals and the frequency of cleaning, has to
be determined and optimized case by case. Special care has to be taken not to
allow scaling layer to develop in the course of time.
58
2.10.5 - Adjustment of Operating Variables:
When other scale control methods do not work, the operating variables of the
plant have to be adjusted in such away that scaling will not occur. The
precipitation of dissolved salts can be avoided by keeping their concentration
below the solubility limit, that means by reducing the system recovery until the
concentrate concentration is low enough.
Solubility depends also on temperature and pH. In the case of silica, increasing
temperature and pH increases its solubility .Silica is usually the only reason for
considering adjustment of operating variables as a scale control method,
because these adjustments have economic drawbacks (energy consumption) or
other scaling risks (CaCO3 at high pH).
For small systems, a low recovery combined with a preventive cleaning
program might be a convenient way to control scaling.
59
CHAPTER THREE
LITERATURE SURVEY
60
Chapter 3
Literature Survey
Scaling or precipitation fouling occurs when a dissolved material comes out of
solution and deposits on the membrane surface. The precipitants are inverse
soluble salts and, most commonly, calcium carbonate. Precipitation fouling
usually follows a linear fouling curve. In general, for precipitation to occur, the
solubility limit at given conditions, particularly the temperature must be
exceeded, or in other words for the solution to be supersaturated. If the region of
supersaturation is at the membrane surface, then precipitation is likely. If the
supersaturation is away from the membrane surface, crystals will form in the
bulk fluid and deposit in the same manner as a particulate [16]. The degree of
supersaturation was found to affect what type of crystals form. Calcium
carbonate will crystallize in two forms, calcite and aragonite. Aragonite forms at
higher supersaturation ratios and has the greater effect on the fouling resistance,
while calcite forms at lower supersaturation ratios and has less of an effect on
the fouling resistance [17].
Exact knowledge of the solubilities of the most common minerals such as
calcium carbonate, calcium sulfates and silica, in brackish waters and seawater,
61
is vital since the solubility determines whether the brine is saturated or not. It is
necessary to calculate the solubility product constant for the sparingly soluble
salts in the concentrate stream to determine if they present a potential scaling
problem. The main factors affecting scale formation is salt concentration [18],
operating temperature [18-19], fluid velocity [19], pH of water environment [20]
and operating time [21].
Ritter conducted scaling tests for two salts, lithium sulfate and calcium sulfate
[22]. He found that the scaling rates of the different salts could be correlated
using the supersaturation and mass transfer coefficient among other parameters.
He also found that supersaturation was the most important parameter in
determining the fouling rate. The other parameters varied in importance
depending on which salt was tested. For example, the mass transfer coefficients
was the second most important parameter in correlating the fouling in calcium
sulfate and surface temperature in lithium sulfate.
Hasson lists three process conditions which lead to supersaturation [23]:
1. A solution is evaporated beyond the solubility limits of a dissolved salt.
62
2. A solution containing a dissolved salt of normal solubility is cooled below its
solubility temperature or a solution containing a dissolved salt of inverse
solubility is heated above its solubility temperature.
3. Mixing of different streams may also lead to the creation of supersaturated
conditions, as is the case of CaSO4 scale formation in phosphoric acid
evaporators.
He also lists the significant parameters which determine the superaturation
potential that causes precipitation fouling:
1. The composition of the fresh water feed or of the process feed solution
2. The concentration effects occurring in the process through the separation of
near pure water by evaporation or by a single-phase diffusion process
3. The temperature level of the process or operation which affects the solubility
limits of the scaling salts, many of which have inverse solubility characteristics
4. The temperature gradient characterizing the heat-transfer operation to which
the solution is exposed and the heat transfer mechanism
Most of the research done in the field of precipitation fouling is on calcium
carbonate. Watkinson et al. investigated precipitation of calcium carbonate in
63
internally-finned tubes and spirally-indented tubes [24]. The experiments were
conducted on a shell-and-tube heat exchanger apparatus, which had water
flowing from a mixing tank through two tubes in parallel in the shell. One of the
tubes was plain, and the other was enhanced. Steam was condensed on the shell
side of the heat exchanger. The effects of salt concentration were briefly
studied, and the fouling resistance was found to increase as the concentration
increased. For the internally-finned tubes, the fouling resistance was found to be
between 15 and 35 % greater then in plain tubes for the same velocity. On the
other hand, the spirally-indented tubes showed a fouling resistance 25 to 50%
smaller than that of the plain tubes at water velocities greater then 3 ft/s.
Watkinson and Martinez [25] conducted tests on a competing brand of spirally
indented tubes from the ones used by Watkinson et al. [24]. The same set up and
experimental procedures were used. The water had 300 ppm suspended particles
and 3000 ppm dissolved solids. The tests were run at velocities between 0.5 m/s
and 3.0 m/s. They found that the spirally-indented tubes had roughly the same
asymptotic fouling resistances as the plain tubes under the same condition,
whereas in the previous study the fouling resistances were found to be less than
the plain tube. They also observed that there appeared to be no clear effect of
the pitch of the spirals on the fouling resistance.
64
Knudsen and a colleague conducted a series of experiments to determine the
factors affecting the formation of scale in cooling tower water [26]. The tests
were run by flowing the cooling tower water through a tube in which a heated
rod was inserted. The water quality was monitored by the Ryznor stability
index.
Most of the work published in literature concentrates on scale control and scale
prediction methods. Kelle, et.al investigated novel antiscalent monitoring
systems to optimize the antiscalent dose required and reported that effective
monitoring and control provided by the TRASAR technology which is based on
the use of a fluorescent molecule which provides a clear and identifiable
fluorescence that can be separated from the natural background [27].
Al-Ahmad et al investigated the main factors affecting biofouling in membrane
processes and recommended a mechanism for this type of fouling in RO
membranes used in either water treatment or waste water systems [28]. The
design of spiral wound membrane element makes them an ideal environment for
the growth of microorganisms that form a biofilm on the membrane surface and
on the spacing material in the narrow feed channels. This biofilm acts as a trap
for other particulate matter which quickly build up as a biomass. Michael et al
reported that: the most frequent scaling problems come from calcium carbonate
65
(CaCO3) because it precipitates fast, once concentrated beyond its solubility
limit and also most natural waters are almost saturated with respect to CaCO 3
[29]. CaCO3 scaling including SrCO3 and BaCO3 can be prevented by acid
addition, a scale inhibitor, softening of the feed water, preventive cleaning and
low system recovery. CaSO4 scaling including BaSO4, SrSO4 and CaF2 are
preventable by the same methods as CaCO3 scaling except the acid addition. In
fact, using sulfuric acid to lower pH for the prevention of CaCO 3 scaling would
increase the probability of the sulfate scaling.
David et al found that when there are only a few calcium and sulfate ions in a
solution, they won't form a permanent scale since it causes the salt to diffuse
away from the membrane surface and back into the brine [30]. However, as a
solution is concentrated, scaling occurs whenever certain parameters are met.
The point at which scaling occurs depends primarily on the concentration of the
particular cation making up the scaling compound, the concentration of the
particular anion making up the scaling compound, the temperature of the
solution and the ionic strength of the solution.
The Langelier Saturation index (LSI) is an equilibrium model derived from the
theoretical concept of saturation and provides an indicator of the degree of
saturation of water with respect to calcium carbonate.
66
Since scale formation is a problem often encountered in reverse osmosis, several
studies have been conducted to resolve this phenomenon. There are chemicals,
better known as scale inhibitors, which control the growth of scales on the
membrane. The chemicals used in controlling scale growth depend on the type
of scale predicted and system operating conditions. There are several methods
used in controlling scale growth. To name a few are the following: bleed-off or
blow-down to limit the cycles of concentration; converts calcium and
magnesium hardness to more soluble salts; and ion-exchange softening which
removes calcium and magnesium hardness salts.
Rahardianto et al found out that membrane scale formation increases as the level
of surface gypsum (calcium sulfate dihydrate) supersaturation increases [31].
They used the antiscalant treatment to study the membrane scaling
susceptibility.
Howe conducted a study aiming to substitute brackish water for the limited
supply of fresh water by desalination through the process of reverse osmosis
[32]. The salinity of brackish water is between 1 000 and 10 000 mg/L. The
feasibility of using a water source for a portable supply depends on the water
salinity, the lower the total dissolved solids, the more feasible the supply [32].
However, there are factors that limit the recovery in reverse osmosis such as
67
solubility and osmotic pressure. In terms of solubility, there are many ions
present in brackish water like calcium, barium, strontium, carbonate, fluoride,
and sulfate. Therefore, it will limit the recovery because ions tend to concentrate
more as recovery increases. Salts also precipitate on the membrane surface when
solubility limit is reached. In terms of osmotic pressure, the externally applied
pressure should overcome the osmotic pressure to produce water. Furthermore,
when recovery increases, osmotic pressure increases. Scale formation is another
factor that needs to be addressed in this study [32].
In recent years, low-pressure reverse-osmosis membrane desalination has been
explored as a viable technology for desalination of brackish water for potable
usage [33-38]. However, the problem of mineral salt scaling remains a serious
impediment to achieving high recovery in brackish water desalination. Mineral
salt scaling can occur when scale precursor ions (e.g. Ba2+, Ca2+, SO42-, CO32-,
etc.) in the membrane brain stream are concentrated above the solubility limit of
various sparingly water soluble mineral scalants such as calcium sulfate
dihydrate (gypsum), calcium carbonate (e.g. calcite), and barium sulfate (barite).
Mineral salt scaling leads to permeate flux decline and eventually the shortening
of membrane useful life. The use of antiscalants can suppress mineral salt
precipitation to some extent, thereby partially alleviating mineral scaling.
68
However, even with the use of antiscalants, which can significantly increase the
cost of RO desalting, mineral salt scaling remains an impediment to achieving
high product water recovery, partially due to the increased potential of fouling
(including biofouling) when excessive dose of antiscalants is applied [39].
Accelerated precipitation (AP) by inducing calcium carbonate crystallization
through chemical dosing (e.g., lime, caustic, and soda ash) has been shown to be
a promising approach for desupersaturation of primary RO concentrate [40].
This approach is analogous to the lime-soda or caustic softening processes (i.e.
precipitation softening) [41, 42]. The basis of the process is the strong pH
dependency of CaCO3 solubility that enables its precipitation through pH
control. Conventional precipitation softening, however, is characterized by the
production of fine suspension of mineral salt crystals that requires a long
sedimentation time (about 1.5 to 3 hours) and results in sludge that is of low
solids content (~2-30%) [43].
Precipitation softening technologies have been developed to improve both
precipitation kinetics and the efficiency of solid-liquid separation [44-46]. These
technologies are based on the concept of seeded precipitation softening (SPS).
Various studies have indicated that the use of crystal seeds in precipitation
softening can provide a preferential surface area for heterogeneous nucleation
69
and growth of mineral salts, thus accelerating the kinetics of mineral
precipitation [47]. In addition, control of the initial seed size, loading and type
provides a means of controlling the size of the final precipitates so as to
facilitate efficient liquid-solid separation. Variations of SPS reported in the
literature include fluidized-bed type reactors [48] and systems with slurry
recirculation through specially designed microfiltration units [49].
70
CHAPTER FOUR
RESULTS AND DISCUSSION
71
Chapter 4
Results And Discussion
4.1 Introduction
The first phase of the present work concentrates on the investigation of
the effect of the operating parameters like concentration of the main
fouling compounds as CaSO4 , CaCO3, SiO2 , iron salts and pH as well
as the process recovery on the fouling tendency of the processed water in
RO plants.
In order to clarify the significant effects of these operating parameters on
the fouling potential of the process, the PermaCare Simulation Program
was extensively applied ( due to its significant ability to analyze the
fouling phenomena in RO systems ) using single parameter each time and
finding its response on the fouling tendency as obtained by the program
( more explanations are given in the appendix for each case )
Scaling Tendency
The Scale Tendency ST is defined as the ratio of the activity product Q
for an equilibrium to the solubility product Ksp for the same equilibrium.
In other words, ST= Q / Ksp.
72
When the ratio of Q/ Ksp is greater than 1.0, the salt is said to have a
thermodynamic driving force to form. When the ratio is less than 1.0, the
solid does not have the driving force to form. When the ratio equals 1.0,
the solid is considered to be at saturation.
The solubility product, usually represented as Ksp, is the thermodynamic
equilibrium constant, a function of temperature and pressure. The
equilibrium constant, Ksp, can be estimated as a function of temperature
and pressure, for all salts in the solution.
73
4.2 Perma Care Program
This program is regarded to be the most important program used in evaluating
the feed water of RO membranes, and determining the scaling level, which is
expected to happen in the membranes. It is suitable for producing antiscalant
proposals for brackish water, sea water/high salinity waters, process water,
waste water and recycle systems .
It helps in calculating scaling easily. Moreover, through this program, the
occurrence of scale problems in the RO membranes system can be monitored
For the purpose of dealing with this program, adding a chemical analysis of the
water for these elements is required The program will accept data for the
following ionic species:
Anions: Ca, Mg, Na, K, Ba, Sr, Fe, Al and Mn
Cations: SO4, Cl, F, HCO3, CO3, NO3, SiO2, PO4
As the species are entered the anion/cation balance is shown as milli equivalents
(meq/litre) are given and the total dissolved solids (TDS) .
In addition to that, the program requires determining the type of the used
membrane and its model, To select a membrane, The desired manufacturer’s
74
button should be click and the list will open up with the first membrane of the
selected manufacturer highlighted. Scroll down and The desired membrane and
the list will close with the chosen membrane selected. Once selected the
‘assumed % salt passage’ (ASP) for the membrane will be displayed at the
bottom of the screen. and this done through the program choices. Then, the
program requires the insertion of plant operation data, which is Pressure ,
Temperature, Recovery , Product Flow , pH , Hours per Day
Operating Data Limitations
Operating parameters must be within the limits set as follows:
Pressure : The pressure must be between 5 - 100 bar or between 73.5 - 1470 psi.
Recovery : The recovery must be between 1- 95%.
Product Flow : The product flow rate must be <500,000 m3 / day or the
equivalent in the chosen units.
pH : The raw water and feed water pH must be between 4 and 10.
By the previous inputs, the program give us a table that illustrates the scaling
Tendency of the following salts( CaCO3 , CaSO4 , iron , silica …. )
75
Through this primary evaluation, we can detect the types of scaling that occur,
and the level at which they happen.
Units available:
Flow: I/min, m3/hr, m3/d, gal/d, USgal/d, USgal/min.
Concentration: mg/l as ion, ppm as CaCO3, meq/l as ion, mmole/l, degress
French and degrees German.
Pressure: psi, bar.
Sample Scaling Calculations
Scaling calculations have to be carried out in order to determine whether a
sparingly soluble salt presents a potential scaling problem in a reverse osmosis
system
To determine the scaling potential, one has to compare the ion product IPc of
the considered salt in the concentrate stream, with the solubility product Ksp of
that salt under conditions in the concentrate stream.
The solubility product Ksp is generally also expressed in molal concentrations
and is dependent on ionic strength and temperature .
The temperature in the concentrate stream is about the same as in the feed
stream.
76
The ionic strength of the feed water is:
Ic = 1/2 Σ(mi x Zi2)
where:
mi = molal concentration of ion i (mol/kg)
zi = ionic charge of ion i
With the ionic strength of the concentrate stream, the solubility product K sp of
scaling salt can be obtained
CaSO4 → Ca++ + SO4 --
0.01 Mole of CaSO4
The ionic strength of the feed water is:
IC = 1/2 Σ(mi x Zi2)
= (1 / 2) ([Ca++] (2)2 + [SO4--] (2)2 ) =(1 / 2) ([0.01] (2)2 + [0.01] (-2)2 ) = 0.04 Mole
Debye – Huckel Equation :
γ = Activity Coefficient
α = ion size from tables α Ca++ = 0.60 nm α SO4-- = 0.35 nm
for [Ca++ ] Zi = +2 α Ca++ = 0.60 nm
- log γ Ca++ = 0.511(+2) 2 ( 0.04) 1/2 / ( 1+3.3*0.6*(0.04) 1/2 = 0.293
γ Ca++ = 0.51
77
» activity of [Ca++ ] = γ Ca++ * [Ca++ ] = 0.51 * 0.01 = 0.005 M
Find Activity [SO4--] = γ SO4-- * [SO4--]
for [SO4--] Zi = - 2 α SO4-- = 0.35 nm
- log γ SO4-- = 0.511(-2) 2 ( 0.04) 1/2 / ( 1+3.3*0.35 * (0.04) 1/2 = 0.332
γ SO4-- = 0.487
» activity of [SO4--] = γ SO4-- * [SO4--] = 0.487 * 0.01 = 0.00487 M
Compare
If γ Ca++ * [Ca++ ] . γ SO4-- * [SO4--] > Ksp CaSO4 will
precipitate
If γ Ca++ * [Ca++ ] . γ SO4-- * [SO4--] < Ksp no scale in
expected
If γ Ca++ * [Ca++ ] . γ SO4-- * [SO4--] = Ksp Saturation
In this case
γ Ca++ * [Ca++ ] . γ SO4-- * [SO4--] = Ksp = 4.93 × 10 -5 for CaSO4
0.51 * 0.01 * 0.487 * 0.01 = 0.005 * 0.00487 = 2.435× 10 -5 < 4.93 × 10 -5
for CaSO4 no scale expected
78
4-3 : Effect of calcium sulfate Concentration :-
It is aimed by this experiment to study the effect of CaSO4 salt on the
performance of membrane and through knowledge of its concentration at start
of this type of deposition, Perm care program is used to study this salt, where it
is assumed that there is only CaSO4 salt in feed water .
Hence , CaSO4 Concentration in the feed water was increased from 100 ppm to
1000 ppm ( Since this range depend on the actual practical conditions of water
quality in Riyadh ) then the corresponding fouling tendency " or Scalling
tendency " was obtained by the PermaCare Program .
Then a graph was plotted for the effect of CaSO4 concentration on the scalling
tendency of the processed feed water . Fig (6) indicates this plot .
As shown in this figure , increasing CaSO4 concentration in feed water affects
strongly the scalling tendency of water due to the expected increase in the over
saturation of CaSO4. ( for more details see Appendix A )
It is worth mentioning that , some investigators [4,6,8,13] reported that the
dependence of scalling tendency on CaSO4 concentration is not liner , but it is of
the second order type as it depends on both concentration of [Ca++] and [So4
--] i.e their product according to the following thermodynamic equilibrium
79
CaSO4 → [ Ca++] [ SO4 --]
Ks = [ Ca++] [ SO4 --]
Ks is the reaction rate constant
As recently reported in literature, The degree of super saturation for CaSO 4 is given by the following scaling index model solution
The solubility product Ksp is generally expressed in molal concentrations and is
dependent on ionic strength and temperature .
80
Fig.(6) Effect of CaSO4 Concentration on the Scalling Tendency of the Feed Water
81
4-4 : Effect of CaCO3 Concentration :-
In this experiment (experiments ) the aim is to study the effects of calcium
carbonate CaCO3 salt on the performance of membranes using Perm care
program where several different values for the concentration of salt start from
10 to 200 ppm since this range depend on the actual practical .
The same previous procedure was applied here to find the effect of CaCO3
concentration in feed water on its scaling tendency , but the concentration range
selected for CaCO3 was smaller than CaSO4 runs due to the expected lower
solubility of this salt . The extracted scaling tendency from the program are also
plotted below in Fig .(7) .
As shown in this figure , CaCO3 concentration has strong effect on the scaling
tendency of water even at small concentrations ( 10 ppm to 200 ppm ) , which
reflects the numerous research activities conducted by other workers in fouling
studies ( 6). . ( for more details see Appendix B )
Moreover the dependence of fouling tendency on CaCO 3 concentration is due to
the ions concentration present in the solution like :-
Ca [HCO3] 2 → [ Ca ++] + 2 HCO3 –
82
Ks = [ Ca++] [ HCO3-] 2
Most probable , a third order dependence can be observed from the
thermodynamic point of view for this case [13,14,15,20]
Fig (7).Effect of CaCO3 Concentration on the Scalling Tendency of the Feed Water
83
4-5 Effect of Silica Content :-
Dissolved silica (SiO2) is naturally present in most feed waters in the range of
1-100 mg/l. Supersaturated, SiO2 can be polymerized to form insoluble colloidal
silica or silica gel, which can cause membrane scaling. The maximum allowable
SiO2 concentration in the concentrate stream is based on the solubility of SiO 2.
The scaling potential for the concentrate stream will be quite different from that
of the feed solution, because of the increase in the concentration of SiO 2, and the
change in pH. It can be calculated from the feed water analysis and the reverse
osmosis operating parameters [28]
Due to the potential importance of Silica content on fouling problems as
suffered by most workers in RO treatment plants , these runs were carried out to
elucidate this significant effect . As previously done with CaSO 4 and CaCO3 ,
the silica content of feed water was changed from 1 to about 30 ppm , and the
corresponding scalling tendency of these solutions was extracted by the
PermaCare Program .
Then the obtained results are plotted as usual in Fig(8) which indicates a linear
dependences of scalling tendency on the concentration of SiO 2 in the processed
water . ( for more details see Appendix c )
84
It is interesting to notice that ; the dependence here is linear ( different from the
case of CaSO4 and CaCO3) due to the thermodynamic deriving force to this salt
which is its physical solubility and not concentration of ionic species , as silica
does not ionize in water and only its saturation solubility affects its fouling
tendency .
85
Fig(8): Effect of silica Concentration on the Scalling Tendency of the Feed Water
86
4-6 Effect of pH on various Fouling Compounds :-
Due to the potential importance of this parameter on RO fouling problems ,
its effect was investigated on each foulant ( e.g. CaCO3 ,CaSO4 , Silica ……
etc ) as follows :-
4-6-1 : Effect of pH on CaCO3 Scale :
In these runs , the PH of CaCO3 water solution was changed from 6 to 9 and
the scalling tendency of that feed water solution was obtained by the
Program as previously mentioned (CaCO3 concentration in feed water was
kept constant in all these runs , at ( 250 ppm ) .
The obtained results are plotted in Fig(9) as usual.
It is clear from this figure that; the PH increase has strong effect on
increasing the scalling tendency of CaCO3.
This can be ascribed to the alkaline behaviour of CaCO3 scale .
Hence , in practice , acid dosing is normally applied to reduce this type of
alkaline scale . ( for more details see Appendix D )
87
Fig(9): Effect of pH on calcium carbonate ( CaCO3 ) scale.
88
4-6 -2 : Effect of pH on CaSO4 Scale :
In this case , a feed water containing fixed concentration of CaSO 4 is used
(340 ppm ) while its pH is varied from 5 to 8 and the corresponding salling
tendency of them are obtained as the usual .Then a plot was carried out
between pH and scalling tendency as shown in Fig. (10) . In contrast to
CaCO3 scale , PH has small ( weak ) effect on the reduction of CaSO4 scale.
This scale can be considered as slightly acidic , hence alkaline medium can
reduce it . ( for more details see Appendix E )
89
5 6 7 8PH
80
82
84
86
88C
aso
4 S
calli
ng
Ten
den
cy "
Per
cen
tag
e O
ver
Sat
ura
tio
n"
Fig(10): Effect of pH on calcium sulphate ( CaSO4 ) scale.
90
4-6 -3 Effect of pH on Iron Scale :
This type of scale is usually encountered in RO plants especially those
dealing with Brackish water , it has also bad effect on the membrane
performance , hence it is investigated in the present work at various pH
values ( ranging from 5 to 8 ) by the same previous procedure and collected
results from program are plotted here below in Fig , (11) , As shown from
this figure , Iron scale increases by increasing pH due to its alkaline nature ;
this scale is usually deposited from the solution in the From of Fe(OH) 2 or
Fe(OH)3 .
More over, at higher pH like 8 , this scale reaches an asymptotic value " or
quasi equilibrium value " depending on the Solubility of iron scale at these
conditions. ( for more details see Appendix F )
More over , acid dosing is also helpful to reduce this type of scale.
91
Fig (11) : Effect of pH on Iron scale.
4-6 - 4 Effect of pH on silica Scale :
92
As previously mentioned , SiO2 scale is very critical scale type for RO
plants , hence the effect of pH on it is conducted in this work using the same
procedure as before , and the obtained results from the program are plotted in
Fig (12) which indicates an interesting effect for pH on silica scale . By
increasing pH from 5 to about 6.5 the scale is increased , then it gives
maximum value at about ( 6.5) and starts to decrease more deeply in the
alkaline medium .This behavior reflects the amphoteric nature of SiO 2 .
SiO2 dissolves ( or reacts ) in both alkaline and acid mediums but with better
solubility in the alkaline medium .Hence , both lime-soda process and acid
dosing can reduce the fouling potential of silica .
. ( for more details see Appendix G )
93
Fig.(12): Effect of pH on silica scale.
4-7 Effect of Process Recovery on the scalling potential of various Foulants :-
94
Recovery is an important parameter of RO treatment plants, high recoveries are
desirable and attractive for most experts in this field , but fouling and scale
problems are the bottleneck for achieving high recovery target. Hence the actual
effect of increasing recovery on the fouling tendency of the processed water
needs to be carefully understood .
For this reason , the present runs are conducted to clarify the effect of increasing
the recovery on each fouling salt of the present work as follows:-
4-7 -1 Effect of Recovery on the Fouling tendency of CaCO3 :-
95
In this case the recovery was changed from 30 to 85 % while the CaCO3
concentration in feed water was kept constant " at 250 ppm." and the scalling
tendency was obtained by the program for each recovery value investigated .
Then the collected results are plotted in Fig .(13) .
As shown in this figure , the recovery has strong effect on scalling tendency of
CaCO3 .
This effect is directly ascribed to the increase in salt concentration of water at
high pressure side ( brine) by extracting more product water from it .some
investigators reported the following equation for the effect of recovery on
increasing the concentration ratio CF(Cb/Cf) . [14,15,17,18]
CF = ø (Rw) non linear
CF = Cb\Cf Cb = brine concentration , Cf = feed concentration
Rw = water Recovery ( mass fraction )
Rs = Salt Rejection ( mass fraction )
Hence the effect has the same shape like the concentration effect .
96
Fig(13): Effect of Process Recovery on CaCO3 scale.
97
4- 7 -2 Effect of Recovery on the Fouling tendency of CaSO4 :-
Using the same pervious approach , the process recovery was varied from 30 to
85 % while the concentration of the feed water was kept constant " at 340 ppm
and corresponding scalling tendency was obtained by the program .
Then the collected results were plotted in Fig(14) which indicates the strong
dependence of fouling tendency on the process recovery .
The effect is similar to CaCO3 as previously mentioned .It is also clear from
this figure that sharp increase in scalling tendency is observed for recovery
values larger than 80 % .Hence in practice , most of RO plants are operated
around 80 % recovery " or between 70 and 80 % "
. ( for more details see Appendix I )
98
Fig.(14) Effect of Process Recovery on CaSO4 scale.
99
4 – 7 -3 Effect of Recovery on the Fouling tendency of Silica :-
In this case , the process recovery was varied from 30 to 85 % and silica
content of the feed water was kept constant at ( 15 ppm ) . as previously
done with other foulants . Then the corresponding scalling tendency was
obtained by the program . These results are plotted as usual in Fig (15) which
indicates the sensitivity of this scale to recovery. It is clear from this figure
that ; increasing the recovery higher than 80 % , lot of silica scale problems
are expected . . ( for more details see Appendix L )
100
Fig(15): Effect of Process Recovery on Scalling Tendency of Silica.
101
4 – 7 -4 Effect of Recovery on the Fouling tendency of Iron :-
The variation of recovery on iron scale is shown in Fig. (16). The saturation
varied from 3 % to 35 % depending on the recovery. At higher recovery, the
saturation of iron was also high and decrease in recovery yielded a decrease
in saturation. One important conclusion can be drawn from the simulation is
that the saturation increases steeply when the recovery was increased from 70
to 90%.. ( for more details see Appendix M )
102
Fig(16): Effect of Process Recovery on Scalling Tendency of Iron .
103
4 – 8 Actual Case Study On Fouling Problem For One Of RO Plants : -
4-8-1 Introduction :
This case Study is an actual fouling problem occurred in one of RO plants in
Riyadh which was established in 1986 with a designed capacity of 800 m3 / hr .
Then , RO plant has been recently up-dated by exchanging the old complete system
with new membrane modules .and applying a new control system in all its
operating steps .(as show in figure 17)
This plant , in general, consists of pretreatment part , RO membrane Section
consists of four units (Blocks) with total production capacity of 800 m3/hr, ( 200
m3/hr for each block) , The plant has been designed for Recovery of 85%, salts
passing rate not exceeding 5%, and its membranes consist of a number of layers for
each block, the first stage consists of 102 membranes, the second stage consists of
66 membranes, and the third stage consists of 36 membranes, feed Temperature
is 35 C o, , feed TDS is about 2500 ppm , feed pressure is 15 bar .
The membranes of this plant are of the spiral wound type ( BW30-400 FILMTEC
Membranes ) made of polyamide.,
The system is equipped by a washing unit, and it can conduct flushing for blocks
automatically.
The plant has been designed on the following operating conditions .
104
Analysis
RO. FEED
Minimum Maximum
Temperature CO 30 40
PH 5.5 7
Hardness Calcium mg/l as CaCO3 150 240
Hardness Megnesium mg/l as CaCO3 130 250
Alkalinity Total mg/l as CaCO3 12 50
Salinity (TDS ) mg/l 1500 2500
Chloride mg/l 250 350
Sulphate SO4 mg/l 620 800
Nitrate as NO2 mg/l 0.5 2.0
Nitrate as NO3 mg/l 17 40
Iron mg/l 0.05 0.1
Aluminum mg/l 0.02 0.05
Silicate as SiO2 mg/l 15 30
Turbidity (NTU ) 0 1
SDI ‹ 3The new system has been operated in the plant starting from 2005.
Table (2 ) Operating Conditions of the Considered RO Plant
105
Fig 17 : Schematic Flowsheet of the Plant " used in case study "
106
Coolingtower
Clarifier
Ca(OH)2
Na2CO3
Polyelectrolyte
Sandfilter
Cartridgefilter
High pressurepump
Raw water
Blending Line
H2SO4
Sodium aluminateCl2
SBSAnti-scalant
H2SO4
Buffertank
mixingchamper
Sandfilter
Boosterpump
RO
Na2Co3
During the first operation of this plant , the product started to decrease suddenly
and strongly from 200 m3/ hr to about 50 m3 / hr ( about 75 % reduction as shown
in Fig (18) .
This was strange behavior since the system is new and modern.
Fig(.18): Flux Decline (Product Flow Rate during the first phase of this case study )
107
The Recovery also decreased during this period from 85 % to about 50% as
shown in Fig (19)
Fig(.19): Recovery Decrease ( During this phase of the problem).
108
More over the Pressure drop increases during this period from 3 bar to about 5
bar , as shown in Fig.(20)
Fig(20): Pressure drop Increase ( during the first phase of the problem).
109
Then , it was started to think about the main reasons for this problem to find out
the suitable solutions for it .
It is quite clear that the type of the present problem is a fouling case due to the
sharp flux decline and the rise in the pressure drop .
Hence , it was decided to consider what changes occurred to the feed water
during this period .
Fig. ( 21) illustrates the variation of feed conductivity with number of days of
operation. It is very clear from this figure that the conductivity of the feed
varied from 1970 to 1990 ; i.e. no significant changes occurred to the feed
water
110
0 10 20 30Time ( Days )
1900
1920
1940
1960
1980
2000
Fee
d C
on
du
ctiv
ity
Fig.(21) Conductivity of the feed water to the Plant(during of the first phase of the problem ).
111
Later on , PH values of the process water was considered to account for
any reason of the present problem .But , as shown in Fig( 22) , no significant
changes can be detected .
Fig.(22): PH of the Processed water of the Plant(during of the first phase of the problem ).
112
Then , it was decided to wash the membranes .( as it is normal practice in such
cases ) , After washing , the product starts to rise directly to it is design values
shown in Fig.( 23)
Fig(.23) : Flux increase after washing (during the second phase of this problem).
113
More over , the recovery started to increase directly after washing (as shown in
Fig (24)
Fig(24):Recovery Increase after Membranes Washing (during of the second phase of the problem).
114
The pressure drop also decreased after washing (as shown in Fig.(25)
Fig.(25): Pressure Drop Decrease after Washing (during of the second phase of the problem ).
115
But , After about one month and during the third phase of this case , the plant
recovery and the product started to drop again ( the problem reappeared ) as
shown in Fig. (26) and (27).
Fig(26). Recovery Decrease(during of the there'd phase of the problem)
116
0 10 20 30Time (Days)
40
80
120
160
200
240P
RO
DU
CT
FL
OW
RA
TE
(m
3/h
r)
Fig(27). Product decline (during of the third phase of the problem)
117
But pressure drop did not detect any significant change during this third phase
of the problem as shown in Fig.( 28)
Fig(28). Pressure drop (during of the third phase of the problem)
118
As previously mentioned , during the initial period “ 1 st Phase “ of this problem;
the drastic decline in the production rate of the plant and the significant decrease in
the process recovery as well as the pronounced increase in the pressure drop ; were
considered as a strong evidence that the plant was suffering from a harmful fouling
problem.
In order to find out the possible , reasons leading to this problem ; the feed quality
was reconsidered as expressed by its electrical conductivity ; but no significant
changes were observed in this feed water ( as shown in Fig .(21) then the SDI
index of the input water to the membrane section was also rechecked several times
during this period and again no detectable increases were found in that index .
More over , the PH of the process water of the plant was monitored during this
phase and no significant changes were observed ( as shown in Fig (22) ) .
Therefore , it was decided to do the routine washing of the membrane
section extensively , and consequently after that ; the productivity of the RO plant
starts directly to increase until it reached the designed capacity again ( as shown in
Fig .23) . both recovery and pressure drop started also to be as designed . ( as
shown in Fig .(24) (25) ). Hence , it was started to think about biofouling problem
(as it is the only type of the fouling which is not detectable by SDI or feed
conductivity , or pH).
119
Therefore, we have conducted bacterial examination for input water to the
membrane , and it showed that there is an over count in feed water, then we
controlled the chloration dosing to get rid of this problem
But after about one month of plant operation , the problem started again as
observed before ( Figs.(26) and (27) ) for flux decline and recovery decrease
In conclusion of this case study it has been clear to us that the problem is a
biofouling one and not scale, and we get rid of it by precise controlling of the
chlorine dosing to remove the bacterial growth in the feeding lines as well as
through washing of the membranes .
As previously mentioned the problem began to appear again according to the
following interpretation :
The operations of chlorine injection were not conducted in the proper required
way, where there are some areas in the system which are not taking its enough
dose, consequently, the bacteria has grown and appeared once again.
After dealing with this case study and also using ROSA and PermaCare programs,
it was clear that these programs are not giving bio fouling indications, but they are
only giving the indications of scaling problems, hence, it is now clear that ; there is
need from practical point of view to have other programs that deal with this type
of harmful fouling problem .
120
CHAPTER FIVE
CONCLUSIONS AND RECOMMENDATIONS
121
Chapter 5
Conclusions And Recommendations
5.1 Conclusions:
The Present work led to the following conclusion:-
1. CaCO3 salt has strong effect on the scalling tendency of water even at low
concentrations ( about 100 to 200 ppm ) ,which reflects the numerous
research activities on this salt in the fouling field .
2. CaSO4 salt has also similer effect like CaCO3 but at higher concentration
levels ( about 1000 ppm ) due to the increased solubility of it in water
medium . Both effects ( of CaCO3 and CaSO4 ) are non liner due to the
effects of their ionic concentration on the scalling tendency .
3. Silica effect on the scalling tendency was even deeper than other salts, and
its scale started to appear at lower concentrations ( about 20-30 ppm ). The
effect on the scalling tendency is of the liner type due to dependence on the
thermodynamic as a result of over-saturation and .It is not ionized in water .
4. The pH investigations for the scalling tendencies of various fouling
compounds revealed that :
122
A. Direct increase in CaCO3 scale with increasing pH as it is an alkaline
scale .
B. Direct effect also in iron scale as it is also an alkaline scale ( as Fe(OH)2
C. Slight effect on CaSO4 Scale as it can be considered a neutral or slightly
acidic scale .
D. An interesting effect on silica scale in both acidic and alkaline range of
pH with an optimum value ( around pH~ 6 – 7 ) due to the amphoteric
nature of SiO2 it self . Moreover alkaline medium ( PH > 8 ) reduces
strongly silica scale .
5. The Process recovery investigations revealed strong effects on the
scalling tendency of fouling compounds due to the expected effect on the
over saturations of these salts by increasing the process recovery
especially after 80 % Recovery .
6. The case study on fouling revealed that the flux decline and pressure drop
are the significant operating factors indicating the presence of a fouling
problem .
But SDI and feed conductivity has nothing to detect the biofouling problem in
practice .
123
7. Precise monitoring of bacterial analysis of water entering the membrane
section as well as the proper chlorine dosing are only the significant
factors to overcome the biofouling problem in practice .
8. Biofouling problem , if start to happen , can be expected to reappear
again until the whole process flowsheet is free of any bacterial infection .
9. The present software programs ( or packages ) for RO plants , either ;
PermaCare or ROSA have nothing to do with biofouling as they
concentrate on the inorganic scale salts .
10. Precise membrane washing and proper pretreatment as well as efficient
monitoring of the RO production line are significant factors for
overcoming scale and fouling problems in these plants .
124
5.2 Recommendations
1. More attention has to be given to ; silica , iron and biological
fouling in RO plants either from the practical point of view or from
the research side .
2. An Actual need exists for a software program covering the
biofouling problems in practice similar to PermaCare and ROSA
programs .
3. The process recovery and pH of the processed water in RO plants
have to be given more consideration for their strong effects on
scale formation problems in these processes .
4. Proper monitoring and control of the process parameters in RO
plants are of vital importance to reveal early and overcome
properly the expected fouling and scale problems in this field .
125
REFERENCES
126
References:
1- M. Al-Ahmad, F.A. Abdul Aleem, A. Mutiri, A. Ubaisy, "Biofuoling in
RO Membrane Systems: Part 1: Fundamentals and Control", Conference
on Membranes in Drinking Water Production, Paris, France, 3-6 October
2000, pp. 173-179.
2- Edstrom, C S 2001, Reverse Osmosis: A Key Process for Controlling
Drinking Water Quality for Laboratory Animals. Edstrom Industries, Inc.
Retrieved 20 April 2007 from http://www.edstrom.com/DocLib/RO.pdf
3- Fisher, A, Reisig, J, Powell, P & Walker, M (no date), Reverse Osmosis
(R/O): How It Works. University of Nevada Cooperative Extension.
Retrieved 20 April 20, 2007 from
http://ag.arizona.edu/region9wq/pdf/nv_ROhow.pdf
4- Gold Coast City Council 2006, Understanding the Desalination Process.
Retrieved 20 April 20, 2007 from
http://www.goldcoast.qld.gov.au/attachment/goldcoastwater/EBWS_FS4.pdf
5- Henthorne, L 2003, Desalination Today. Southwest Hydrology,
May/June, pp. 12-13.
6- Kelter, P, Mosher, M & Scott, A 2007, Chemistry: The Practical Science.
New York: Houghton Mifflin.
7- Williams, M E 2003, A Review of Wastewater Treatment by Reverse
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132
APPENDICES
133
Appendix A
A.1 Effect of CaSO4 Concentration :
1- Conditions Feed Pressure = 15 bar Raw Water pH = 8Feed Water pH = 5.5Product Flow = 100 m3 / hr Recovery = 85 %Temp = 30 C o
Membrane Type ( DOW FilmTec BW30-400 )
2- CaSO4 Solution :
CaSO4 → Ca++ + SO4 --
The Con. ( ppm ) of the Ca and SO4 was calculated from the following equation :
3- Sample Calculation :-
Con. (ppm ) CaSO4 = 1000 ppm M. wt. of CaSO4 = 40+32+(16*4 ) = 136 M. wt. of Ca = 40M. wt. of SO4 = 32+(16*4 ) = 96
» Con. (ppm ) ca ++ = ( 1000/136 )* 40 = 294 ppm » Con. (ppm ) SO4 -- = ( 1000/136 )* 96 = 706 ppm
134
4- The Effect of the increase of calcium concentration ca++ and sulfate concentration SO4
-- in supplement water alone without any other factors was examined as these ions represent calcium sulfate in supplement water and the results showed the followings:
Table (4) CaSO4 Concentration
CaSO4 Feed ppm
SO4—( ppm )By Equation
Ca++( ppm )By Equation
CaCO3 scalling Tendency
By PermaCare Program
CaSO4 scalling
TendencyBy
PermaCare Program
100 71 29 0.1 5.2
150 106 44 5.9 11.2
200 141 59 10 18.9
250 176 74 13.1 28.1
300 212 88 15.4 38.4
350 247 103 17.6 50
400 282 118 19.4 62.6
450 318 132 20.9 75.9
500 353 147 22.3 90.3
550 388 162 23.6 105.4
600 424 176 24.7 120.8
650 459 191 25.7 137.2
700 494 206 26.7 154.1
750 529 221 27.6 171.6
800 565 235 28.4 189.2
850 600 250 29.2 207.6
900 635 265 29.9 226.5
950 671 279 30.6 245.4
1000 706 294 31.2 265
135
PermaCare Program Results
CaSO4 Concentration
136
Appendix B
B.1 . Effect of CaCO3 Concentration :
1- Conditions Feed Pressure = 15 bar Raw Water PH = 8Feed Water PH = 5.5Product Flow = 100 m3 / hr Recovery = 85 %Temp = 30 C o
Membrane Type ( DOW FilmTec BW30-400 )
2- CaCO3 Solution :
CaCO3 → Ca++ + CO3 --
The Con. ( ppm ) of the Ca and CO3 was calculated from the following equation :
137
3- Sample Calculation :-
Con. (ppm ) CaCO3 = 200 ppm M. wt. of CaCO3 = 40+12+ (16*3) = 100 M. wt. of Ca = 40M. wt. of CO3 = 12+(16*3 ) = 60
» Con. (ppm ) ca ++ = ( 200/100)* 40 = 80 ppm » Con. (ppm ) SO4 -- = ( 200/100)* 60 = 120 ppm
4- The Effect of the increase of calcium concentration Ca++ and carbonate concentration CO3 -- in supplement water alone without any other factors was examined as these ions represent calcium carbonate in supplement water and the results showed the followings
:
138
Table ( 5) CaCO3 Concentration
CaCO3 Feed ppm Ca++ CO3--
CaCO3 scalling
Tendency
CaSO4 scalling
Tendency
10 4 6 20.3
20 8 12 35.7
30 12 18 45.4
40 16 24 52.5
50 20 30 58.2 2.9
60 24 36 62.9 4
70 28 42 66.9 5.2
80 32 48 70.4 6.5
90 36 54 73.6 7.9
100 40 60 76.4 9.4
110 44 66 78.9 11.1
120 48 72 81.2 12.8
130 52 78 83.4 14.6
140 56 84 85.3 16.5
150 60 90 87.2 18.5
160 64 96 88.9 20.6
170 68 102 90.5 22.8
180 72 108 92.1 25
190 76 114 93.5 27.3
200 80 120 94.9 29.6
139
PermaCare Program Results
CaCO3 Concentration
140
Appendix C
C.1 Effect of Silica Concentration :
The Effect of the increase of Silica SiO2 in supplement water alone without any other factors and the results showed the followings:
Table (6) SiO2 Feed Concentration
SiO2 Feed ppm
SiO2 scalling Tendency
5 23.1
8 37
10 46.2
12 55.5
15 69.3
18 83.2
20 92.4
22 101.6
25 115.5
28 129.3
30 138.5
141
PermaCare Program Results
SiO2 Concentration
142
Appendix D
D1 Effect of PH on CaCO3 Scale :1- Conditions Feed Pressure = 15 bar Raw Water PH = 8Feed Water PH = range ( 5 to 8 )Recovery = 85 %Temp = 30 C o
Membrane Type ( DOW FilmTec BW30-400 )CaCO3 Concentration in feed water = 250 ppm
2- The Effect of PH from 5 to 8 was examined in order to know how the precipitates calcium carbonate salt is affected and the results showed the following:
143
Table (7) PH on CaCO3 Scale
PHCaCO3
scalling Tendency
5 33.2
5.2 46
5.4 58.6
5.6 70.8
5.8 82.5
6 93.7
6.2 104.1
6.4 113.7
6.6 122.6
6.8 130.9
7 138.7 7.2 146.1 7.4 153.3 7.6 160.3 7.8 167.2 8 174.1
144
Appendix E
E.1. Effect of PH on CaSO4 Scale :1- Conditions Feed Pressure = 15 bar Raw Water PH = 8Feed Water PH = from 5 to 8Product Flow = 100 m3 / hr Recovery = 85 %Temp = 30 C o
Calcium Sulfate Concentration in feed water = 340 ppm
2- The Effect of PH from 5 to 8 was examined in order to know how the precipitates calcium sulfate salt is affected and the results showed the following:
145
Table ( 8) PH on CaSO4 Scale
PHCaSO4
scalling Tendency
5 87.2
5.2 87.1
5.4 87
5.6 86.9
5.8 86.7
6 86.5
6.2 86.2
6.4 86
6.6 85.8
6.8 85.6
7 85.4 7.2 85.3 7.4 85.3 7.6 85.2 7.8 85.2 8 85.2
146
Appendix F
F .1 . Effect of PH on IRON Scale :1- Conditions Feed Pressure = 15 bar Raw Water PH = 8Feed Water PH = 5.5Recovery = 85 %Temp = 30 C o
IRON Concentration in feed water = .001 ppm2- The Effect of PH from 5 to 8 was examined in order to know how the precipitates iron is affected and the results showed the following:
Table (9 ) PH on IRON Scale
PHIRON
scalling Tendency
5 14.2
5.2 20.8
5.4 27.3
5.6 33.8
5.8 40.4
6 46.9
6.2 53.5
6.4 60
6.6 66.5
6.8 73.1
7 79.6 7.2 86.2 7.4 92.7 7.6 98.2 7.8 98.2 8 98.2
147
Appendix G
G 1 Effect of PH on SiO2 Scale :1-Conditions Feed Pressure = 15 bar Raw Water PH = 8Feed Water PH = 5.5Recovery = 85 %Temp = 30 C o
Concentration in feed water = 15 ppm2- The Effect of PH from 5 to 8 was examined in order to know how the precipitates silica is affected and the results showed the following:
Table (10) PH on SiO2 Scale
PHSiO2
scalling Tendency
5 62.3
5.2 64.3
5.4 66.5
5.6 68.6
5.8 70.7
6 72.4
6.2 73.7
6.4 74.2
6.6 73.9
6.8 72.5
7 70 7.2 66.5 7.4 62 7.6 57 7.8 51.7 8 46.4
148
PermaCare Program Results
Effect of PH
149
Appendix H
H1 . Effect of Recovery on CaCO3 Scale :1-Conditions Feed Pressure = 15 bar Raw Water PH = 8Feed Water PH = 5.5Product Flow = 100 m3 / hr Temp = 30 C o
2- The Effect of recovery rate from 30 to 85% was examined in order to know how the precipitates calcium carbonate is affected
and the results showed the following
Table (11) Recovery on CaCO3 Scale
RecoveryCaCO3
scalling Tendency
30 3.2
35 6.3
40 9.6
45 13.1
50 17
55 21.3
60 26.1
65 31.5
70 37.6
75 44.9
80 53.6 85 64.8
150
Appendix I
I. 1 Effect of Recovery on CaSO4 Scale :
1-Conditions Feed Pressure = 15 bar Raw Water PH = 8Feed Water PH = 5.5Temp = 30 C o
2- The Effect of recovery rate from 30 to 85% was examined in order to know how the precipitates calcium sulfate is affected and the results showed the following
Table (12) Recovery on CaSO4 Scale
RecoveryCaSO4
scalling Tendency
30 7.7
35 8.8
40 10
45 11.6
50 13.6
55 16.2
60 19.6
65 24.3
70 30.9
75 40.9
80 57.1 85 86.9
151
Appendix M
M 1 . Effect of Recovery on IRON Scale :
1-Conditions Feed Pressure = 15 bar Raw Water PH = 8Feed Water PH = 5.5Product Flow = 100 m3 / hr Temp = 30 C o
2- The Effect of recovery rate from 30 to 85% was examined in order to know how the precipitates iron is affected and the results showed the following
Table (13) Recovery on IRON Scale
RecoveryIRON
scalling Tendency
30 3
35 3.4
40 3.9
45 4.6
50 5.3
55 6.3
60 7.6
65 9.4
70 11.8
75 15.3
80 20.9 85 30.6
152
Appendix L
Effect of Recovery on SiO2 Scale :1-Conditions Feed Pressure = 15 bar Raw Water PH = 8Feed Water PH = 5.5Product Flow = 100 m3 / hr Temp = 30 C o
Membrane Type ( DOW FilmTec BW30-400 )2- The Effect of recovery rate from 30 to 85% was examined in order to know how the precipitates silica is affected and the results showed the following
Table (14) Recovery on SiO2 Scale
RecoverySiO2
scalling Tendency
30 13.3
35 14.4
40 15.7
45 17.2
50 19
55 21.3
60 24.1
65 27.7
70 32.6
75 39.5
80 50 85 67.5
153
PermaCare Program Results
Effect of Recovery
154
PermaCare Program Pictures
155
Appendix N
ACTUAL CASE STUDY ON FOULING PROBLEM FOR ONE
OF RO PLANTS : -
156
ACTUAL CASE STUDY ON FOULING PROBLEM FOR ONE OF RO PLANTS : TABLE ( 15 ) PHASE 1
DAYFEED
PHFEEDCOND
FEEDTEMP
PRODUCTCOND
INLETFLOW
_REJECTFLOW
PRODUCTFLOW
_INLETPR
_REJECTPR dp
CON_RATE
TDS_STG1
TDS_STG2
TDS_STG3
1 6.93 1986.89 32.89 42.01 29.94 29.12 208.00 14.00 11.09 2.91 86.00 20.42 35.78 77.28
2 7.04 1986.86 33.01 39.95 28.51 28.80 192.00 14.10 11.00 3.10 85.00 20.49 35.09 69.68
3 6.85 1985.12 33.29 63.06 29.49 29.60 185.00 14.30 11.00 3.30 84.00 38.49 37.05 96.13
4 6.98 1987.83 33.54 46.13 29.23 32.58 181.00 14.50 10.86 3.64 82.00 21.85 35.29 68.66
5 7.36 1989.16 34.05 93.52 29.99 33.25 175.00 14.70 11.00 3.70 81.00 22.20 36.32 73.51
7 6.61 1986.58 33.78 92.25 30.08 34.20 171.00 14.80 11.00 3.80 80.00 22.70 36.61 70.98
8 6.60 1983.32 33.59 93.41 30.34 33.20 166.00 14.85 11.00 3.85 80.00 21.44 34.22 64.11
9 6.58 1980.11 33.55 95.16 29.54 33.81 161.00 14.90 11.00 3.90 79.00 22.55 34.78 62.83
10 6.58 1981.58 33.81 87.72 29.12 33.00 150.00 15.00 11.15 3.85 78.00 23.29 35.49 61.88
11 6.80 1980.81 33.94 41.28 27.52 34.04 148.00 15.10 11.19 3.91 77.00 21.38 33.95 59.84
12 6.69 1979.28 34.36 43.54 28.24 36.00 144.00 15.40 11.01 4.39 75.00 22.94 34.76 60.67
13 6.78 1983.29 34.40 45.43 28.80 39.20 140.00 15.45 10.86 4.59 72.00 23.80 34.88 60.94
14 6.90 1987.83 34.57 46.71 29.52 40.80 136.00 15.50 11.10 4.40 70.00 23.48 35.94 67.54
15 6.70 1984.74 34.66 48.51 29.84 40.50 135.00 15.55 11.00 4.55 70.00 23.75 36.52 69.96
17 6.97 1983.87 35.30 52.30 34.85 40.61 131.00 15.55 11.18 4.37 69.00 26.45 44.55 91.92
18 6.72 1988.49 35.71 54.22 33.67 39.68 128.00 15.55 11.15 4.40 69.00 25.41 43.19 88.45
19 6.91 1988.90 34.93 56.22 31.93 40.92 124.00 15.60 11.00 4.60 67.00 23.41 38.17 73.50
20 6.80 1987.27 35.28 59.41 31.96 41.48 122.00 15.65 11.09 4.56 66.00 24.21 40.94 81.60
21 6.80 1990.14 35.41 62.73 32.65 42.12 117.00 15.68 11.11 4.57 64.00 24.86 41.65 84.00
22 6.85 1987.49 36.16 65.85 32.53 41.80 110.00 15.70 11.01 4.69 62.00 24.99 41.38 83.89
23 6.90 1989.09 36.27 67.75 32.75 40.56 104.00 15.71 11.03 4.68 61.00 25.41 41.60 84.25
24 6.88 1988.09 36.02 70.57 31.24 40.40 101.00 15.75 11.06 4.69 60.00 23.99 40.17 81.94
25 7.16 1989.45 35.45 72.89 31.81 41.00 100.00 15.78 11.29 4.49 59.00 24.07 41.65 86.75
26 6.98 1985.64 35.87 74.20 30.61 40.42 94.00 15.80 11.15 4.65 57.00 23.35 39.88 82.70
27 7.11 1987.41 35.65 76.18 30.24 39.16 89.00 15.85 11.12 4.73 56.00 23.61 39.66 80.72
28 7.11 1988.18 34.84 78.30 30.11 36.90 82.00 15.90 11.18 4.72 55.00 23.74 39.55 80.59
29 7.31 1988.27 35.24 80.34 30.02 32.90 70.00 15.95 11.37 4.58 53.00 22.95 40.36 86.56
30 7.34 1990.27 35.44 81.73 32.10 30.38 62.00 16.00 11.30 4.70 51.00 24.38 44.69 106.74
157
ACTUAL CASE STUDY ON FOULING PROBLEM FOR ONE OF RO PLANTS : TABLE ( 16 ) PHASE 2
DAYFEED_
PHFEED
_CONDFEED
_TEMPPRODUCT
CONDINLETFLOW
REJECT_FLOW PRODUCT_FLOW
INLETPR REJECT_PR dP CON_RATE
TDS__STG1
TDSSTG2
TDS_STG3
1 7.14 2624.53 38.84 1061.97 32.43 24.09 60.00 15.06 12.10 2.96 59.85 19.53 186.55 417.37
2 7.03 2600.30 39.00 218.65 32.43 25.94 65.00 15.05 12.27 2.78 60.10 19.53 186.55 420.16
3 7.14 2630.11 38.80 47.46 31.38 26.58 66.00 15.04 12.29 2.75 59.73 19.87 188.99 417.72
4 6.95 2573.13 38.51 47.35 31.03 27.09 67.00 15.05 12.31 2.74 59.57 20.22 190.73 423.30
5 7.43 2620.24 38.68 49.49 31.73 27.85 70.00 15.04 12.39 2.65 60.21 20.22 191.43 426.09
6 7.44 2641.18 39.16 50.69 32.43 28.09 75.00 15.04 12.42 2.62 62.55 19.87 191.78 428.53
7 7.29 2500.49 39.98 1058.32 33.12 29.26 77.00 15.06 12.41 2.65 62.00 19.87 192.47 428.53
8 7.15 2490.72 39.15 1059.30 33.82 27.65 79.00 15.05 12.44 2.61 65.00 19.87 192.82 425.74
9 7.30 2744.67 39.37 1058.81 32.43 26.40 80.00 15.08 12.48 2.60 67.00 19.87 192.12 423.30
10 7.02 2661.65 39.46 1060.28 32.78 28.90 85.00 15.04 12.45 2.59 66.00 19.87 188.99 415.28
11 7.27 2705.60 38.22 1059.30 31.73 30.80 88.00 15.05 12.46 2.59 65.00 19.87 187.24 404.82
12 7.18 2480.95 39.58 1055.88 32.78 29.44 92.00 15.04 12.47 2.57 68.00 19.53 185.50 405.52
13 7.15 2832.58 39.95 1053.93 30.68 28.50 95.00 15.07 12.49 2.58 70.00 19.18 185.15 402.73
14 6.99 2759.33 37.97 1057.84 32.43 30.90 103.00 15.06 12.50 2.56 70.00 19.18 184.45 400.99
15 7.02 2544.44 38.97 1059.79 32.08 30.80 110.00 15.07 12.52 2.55 72.00 19.18 184.10 401.33
16 7.23 2778.86 38.29 1065.16 31.73 33.35 115.00 15.07 12.53 2.54 71.00 19.53 184.45 396.80
17 7.08 2754.44 39.15 1066.13 31.38 32.94 122.00 15.03 12.53 2.50 73.00 19.18 184.45 395.76
18 7.10 2749.56 37.88 1065.16 31.73 33.75 125.00 15.07 12.57 2.50 73.00 18.83 183.76 396.10
19 7.26 2759.33 39.54 1064.18 31.38 33.54 129.00 15.06 12.55 2.51 74.00 18.83 183.76 398.20
20 7.26 2710.49 39.85 1064.67 31.73 30.96 129.00 15.04 12.53 2.51 76.00 18.83 182.71 394.36
21 7.37 2471.18 37.88 1064.18 30.68 32.00 128.00 15.06 12.56 2.50 75.00 18.83 182.01 390.53
22 7.18 2759.33 38.17 1063.69 29.64 37.24 133.00 15.11 12.58 2.53 72.00 18.83 182.01 387.04
23 7.33 2476.07 39.71 1063.69 30.68 35.62 137.00 15.06 12.55 2.51 74.00 19.18 182.36 383.20
24 7.34 2681.19 38.39 1062.72 29.99 33.60 140.00 15.06 12.56 2.50 76.00 19.18 182.36 384.95
25 7.27 2632.35 38.88 1062.23 31.03 31.90 145.00 15.06 12.60 2.46 78.00 19.18 183.76 386.34
26 7.19 2593.28 38.24 1062.72 31.38 30.00 150.00 15.09 12.70 2.39 80.00 19.18 184.45 389.48
27 7.06 2451.65 37.70 1062.23 31.03 28.99 165.00 15.04 12.75 2.29 82.43 19.18 185.85 388.43
28 7.06 2495.60 37.51 1060.28 30.34 28.07 170.00 15.07 12.75 2.32 83.49 19.53 187.24 387.04
29 7.26 2661.65 38.75 1062.72 31.03 30.09 177.00 15.07 12.76 2.31 83.00 20.22 187.24 387.04
158
30 6.99 2490.72 39.56 1061.74 31.03 28.84 190.00 15.07 12.77 2.30 84.82 19.18 187.24 380.06
159
ACTUAL CASE STUDY ON FOULING PROBLEM FOR ONE OF RO PLANTS : TABLE ( 17 ) PHASE 3
DAY FEED_PHFEED
_CONDFEED
_TEMPPRODUCT
_COND_INLET_FLOW
_REJECTFLOW
PRODUCT_FLOW
INLET_PR
REJECT_PR dp
CON_RATE
TDS__STG1
TDSSTG2
TDSSTG3
1 6.93 1986.80 32.89 42.01 29.94 32.11 210.00 14.19 11.09 3.10 84.71 20.42 35.78 77.28
2 7.04 1975.08 33.01 39.95 28.51 29.91 200.00 14.18 11.00 3.18 84.00 20.49 35.09 69.68
3 6.85 1978.01 33.29 63.06 29.49 30.23 180.00 13.65 10.35 3.30 83.21 38.49 37.05 96.13
4 6.98 1979.96 33.54 46.13 29.23 43.95 168.00 14.18 10.86 3.32 83.00 21.85 35.29 68.66
5 7.36 1979.47 34.05 93.52 29.99 27.72 166.00 14.14 10.73 3.41 83.30 22.20 36.32 73.51
7 6.61 1992.17 33.78 92.25 30.08 25.50 170.00 14.17 11.11 3.05 82.00 22.70 36.61 70.98
8 6.60 1985.33 33.59 93.41 30.34 28.88 185.00 13.09 10.31 2.77 82.00 21.44 34.22 64.11
9 6.58 1999.00 33.55 95.16 29.54 30.15 190.00 13.78 10.73 3.04 80.00 22.55 34.78 62.83
10 6.58 1979.96 33.81 87.72 29.12 27.66 175.00 14.34 11.15 3.19 80.00 23.29 35.49 61.88
11 6.80 1978.50 33.94 41.28 27.52 28.59 180.00 14.41 11.19 3.22 80.00 21.38 33.95 59.84
12 6.69 1976.06 34.36 43.54 28.24 27.70 175.00 14.38 11.01 3.37 79.00 22.94 34.76 60.67
13 6.78 1991.68 34.40 45.43 28.80 27.21 165.00 14.18 10.86 3.32 77.00 23.80 34.88 60.94
14 6.90 1975.57 34.57 46.71 29.52 24.59 160.00 14.18 11.10 3.08 74.00 23.48 35.94 67.54
15 6.70 1992.17 34.66 48.51 29.84 22.06 140.00 13.74 10.61 3.12 73.00 23.75 36.52 69.96
17 6.97 1977.03 35.30 52.30 34.85 20.60 131.00 13.93 11.18 2.75 73.00 26.45 44.55 91.92
18 6.72 1987.77 35.71 54.22 33.67 20.72 130.00 14.07 11.15 2.93 70.00 25.41 43.19 88.45
19 6.91 1976.55 34.93 56.22 31.93 21.59 125.00 13.28 10.32 2.95 70.00 23.41 38.17 73.50
20 6.80 1985.82 35.28 59.41 31.96 22.38 135.00 14.17 11.09 3.08 70.00 24.21 40.94 81.60
21 6.80 1980.45 35.41 62.73 32.65 19.63 120.00 14.17 11.11 3.05 69.00 24.86 41.65 84.00
22 6.85 1960.00 36.16 65.85 32.53 20.68 125.00 14.18 11.01 3.16 68.00 24.99 41.38 83.89
23 6.90 1970.00 36.27 67.75 32.75 19.71 120.00 14.18 11.03 3.15 68.00 25.41 41.60 84.25
24 6.88 1980.00 36.02 70.57 31.24 18.18 110.00 14.17 11.06 3.11 65.00 23.99 40.17 81.94
25 7.16 1993.14 35.45 72.89 31.81 18.69 115.00 14.18 11.29 2.89 63.00 24.07 41.65 86.75
26 6.98 1996.56 35.87 74.20 30.61 21.85 130.00 14.17 11.15 3.03 61.00 23.35 39.88 82.70
27 7.11 1978.50 35.65 76.18 30.24 23.60 135.00 14.18 11.12 3.06 61.00 23.61 39.66 80.72
28 7.11 1986.31 34.84 78.30 30.11 22.15 125.00 14.17 11.18 2.99 60.00 23.74 39.55 80.59
29 7.31 1996.56 35.24 80.34 30.02 17.57 100.00 14.17 11.37 2.81 60.00 22.95 40.36 86.56
30 7.34 1977.52 35.44 81.73 32.10 15.38 90.00 14.18 11.64 2.54 60.00 24.38 44.69 106.74
160
161