performance study on dissolved air flotation (daf
TRANSCRIPT
PERFORMANCE STUDY ON DISSOLVED AIR FLOTATION (DAF) UNIT
AND PROCESS PERFORMANCE IMPROVEMENT STUDY IN THE PHYSICO-
CHEMICAL TREATMENT OF WASTEWATER
LEE SENG CHOW
A project report submitted in fulfillment of the
requirements for the award of the degree of
Master of Engineering
(Civil – Wastewater Engineering)
Faculty of Civil Engineering
Universiti Teknologi Malaysia
May 2007
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To my beloved parents and family
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ACKNOWLEDGEMENT
This study would not have been completed without the assistance and support
of those who guided me in the course of my masters project. In preparing this thesis,
I was in contact with many people, academicians and practitioners. They have
contributed towards my understanding and thoughts. In particular, I wish to express
my sincere appreciation to my honorable supervisor, Professor Ir. Dr. Mohd. Azraai
Kassim, for encouragement, support, guidance, critics and friendship. Without his
continued support and interest, this thesis would not have been the same as presented
here.
Secondly, I would like to extend my thankfulness to Universiti Teknologi
Malaysia (UTM) and librarians at UTM for their assistance in supplying the relevant
literatures.
My fellow postgraduate students should also be recognized for their support.
My sincere appreciation also extends to all my colleagues and others who have
provided assistance at various occasions. Their views and tips are useful indeed.
Unfortunately, it is not possible to list all of them in this limited space.
Lastly but not least, I am grateful to my family members for their love, care,
support and encouragement.
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ABSTRACT
From the time of the early 20th century, Dissolved Air Flotation (DAF) had
been used in the separation process. In the DAF system, air is dissolved in the
wastewater under a pressure of several atmospheres, followed by release of the
pressure to the atmospheric level. In this case, the recycle flow system has been
studied. For this kind of system, a portion of the DAF effluent is being recycled,
pressurized and semi-saturated with air. The main study object is the DAF System in
HACO Asia Pacific Sdn. Bhd. In the first stage, process performance improvement
study was carried out to determine the most effective operating parameters and
conditions of the DAF. Then, a performance monitoring study was continued by
operating the DAF in the most effective and optimum condition. Samples were being
taken before and after the DAF treatment to determine its pollutant removal
efficiency. Results from the performance monitoring study showed that the DAF is
able to meet its original design specification of Total Suspended Solids (TSS)
removal efficiency up 85%-95%. At a later stage, an emerging design system known
as Band-pass Filter (BF) was brought in for a comparative performance study with
the DAF system. The BF is intended for mechanical filtering of wastewater in
situations where rejections of waste products in connection with processing plants
are required. To obtain optimum cleansing, the BF is tuned for the imminent
cleansing process. Results from the comparative performance study showed that the
BF can achieve a slightly higher average TSS and Chemical Oxygen Demand (COD)
removal efficiency, with 93.9% and 49.6% respectively. On the other hand, DAF can
achieve a slightly higher average Oil and Grease (O&G) removal efficiency up to
91.9%.
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ABSTRAK
Daripada abad kedua-puluhan, “Dissolved Air Flotation (DAF)” telah
digunakan dalam proses pengasingan. Di dalam sistem DAF, udara akan dilarutkan
dalam air sisa pada tekanan yang tertentu, diikuti dengan pembebasan tekanan udara
mengikuti tahap tekanan udara biasa. Di sini, sistem pengedaran pengaliran semula
akan dikaji. Bagi sistem ini, sebahagian air daripada rawatan DAF akan diedarkan
semula ke dalam sistem, ditambahkan tekanan angin dan dilarutkan separa dengan
angin. Bahan kajian utama ialah sistem DAF di HACO Asia Pacific Sdn. Bhd.. Pada
peringkat pertama, proses peningkatan prestasi DAF telah dikaji untuk mendapatkan
parameter dan keadaan operasi yang paling berkesan. Seterusnya, kajian prestasi
DAF telah dilanjutkan pada keadaan operasi yang paling baik dan berkesan. Sampel
air sisa sebelum dan selepas rawatan DAF telah diambil dan dianalisis untuk
mengenalpastikan keberkesanan pengasingan bahan-bahan pencemar oleh DAF.
Keputusan daripada kajian prestasi DAF menunjukkan bahawa DAF berupaya untuk
mencapai spesifikasi reka bentuk asalnya yang dapat mengurang “Total Suspended
Solids (TSS)” sebanyak 85% - 95%. Pada peringkat akhir, satu reka bentuk terbaru
yang dikenali sebagai “Band-pass Filter (BF)” telah dibawa masuk untuk
membandingkan prestasinya dengan DAF sistem. BF ini adalah digunakan untuk
penapisan air sisa secara mekanikal dalam keadaan di mana penyinggiran bahan-
bahan sisa berhubungan dengan kilang pemprosesan adalah ddiperlukan. Untuk
mencapai pencucian optima, BF ini telah diubahsuai untuk proses pencucian yang
cepat. Keputusan daripada kajian pembandingan prestasi ini telah menunjukkan
bahawa BF dapat mencapai purata keberkesanan penyinggiran TSS dan “Chemical
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Oxygen Demand (COD)” yang lebih tinggi , iaitu 93.9% dan 49.6%. Manakala, DAF
dapat mencapai purata keberkesanan penyinggiran minyak dan gris yang lebih tinggi,
dengan bacaan sebanyak 91.9%.
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TABLE OF CONTENTS
CHAPTER TITLE PAGE
TITLE PAGE i
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENTS iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS viii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF ABBREVIATIONS xiv
LIST OF SYMBOLS xv
1 INTRODUCTION
1.1 General Overview in the Environmental
Control of Industrial Wastewater 1
1.2 Physico-Chemical Treatment of
Industrial Wastewater 4
1.2.1 History and Concept of Flotation 5
1.2.1.1 Type of Flotation Process 6
1.2.2 History and Concept of Filtration 9
1.3 Problem Statements 11
1.4 Objectives of the Study 12
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1.5 Scope of the Study 13
1.6 Significance of the Study 13
2 LITERATURE REVIEW
2.1 Coffee Production and Manufacturing
Factory Production Process Description 15
2.2 Wastewater Treatment Plant Design 19
2.2.1 Wastewater Treatment Plant
General Description 21
2.2.2 Wastewater Treatment Plant
Detailed Process Description 22
2.3 Chemical Treatment Process 28
2.3.1 pH Adjustment 30
2.3.2 Coagulation and Flocculation 31
2.4 Dissolved Air Flotation (DAF) System 34
2.5 Band-pass Filter (BF) System 35
3 RESEARCH METHODOLOGY
3.1 Wastewater Treatment Plant Site Survey,
Study and Planning 39
3.2 Dissolved Air Flotation (DAF) Unit 41
3.2.1 Site Operation Aspects of Installed DAF 45
3.2.2 Start-up of DAF 47
3.2.3 Shutting Down of DAF 49
3.3 Band-pass Filter (BF) Unit 50
3.3.1 Site Operation Aspects of BF Unit 53
3.3.2 Start-up of BF 54
3.3.3 Shutting Down of BF 55
3.4 Sampling at the Wastewater Treatment Plant 56
3.5 Analysis of Wastewater Samples 58
3.6 Jar Tests and Chemical Selection 59
3.7 Monitoring and Testing Design 59
3.7.1 Process Performance Improvement Study 60
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3.7.2 Performance Monitoring and Study of DAF 61
3.7.3 Comparative Performance Study between
DAF and BF 61
4 RESULTS AND ANALYSIS
4.1 Characterization of Soluble Coffee Production
Wastewater 63
4.2 Jar Test Results and Discussion 66
4.3 Process Performance Improvement Study on DAF 70
4.3.1 Design Calculation Review 70
4.3.1 Design Calculation Explanation 73
4.3.2 Analysis on the Mechanical Tuning
Adjustment Study 74
4.3.3 Analysis on the Chemical Tuning
Adjustment Study 76
4.4 Performance Monitoring of DAF 79
4.4.1 Analysis on the Wastewater Pollutant
Removal Efficiency 80
4.5 Comparative Performance Study between DAF and
BF 83
5 CONCLUSIONS AND RECOMMENDATIONS 86
REFERENCES 89
APPENDICES
Appendix A 92
Appendix B 95
Appendix C 97
Appendix D 100
Appendix E 102
Appendix F 104
Appendix G 106
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LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 WWTP design input and output 20
3.1 Recommended operating condition for DAF 47
3.2 Recommended operating condition for BF 54
4.1 Average values of incoming wastewater characteristics
at the equalization tank (results abstracted from
Appendix A) 64
4.2 Average values of incoming wastewater characteristics
at the equalization tank (results abstracted from
Appendix B) 64
4.3 Range of values for the incoming wastewater
characteristics at the equalization tank (results
abstracted from Appendix A & B) 66
4.4 Wastewater characteristics before and after chemical
treatment 67
4.5 Average characteristics and removal efficiency for pH,
COD, BOD5, TSS and O&G 80
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LIST OF FIGURES
CHAPTER TITLE PAGE
1.1 Dispersed-Air Flotation Unit. Air is induced and
dispersed into the liquid by pumping action of the
inductors 8
2.1 Soluble coffee production process flow chart 16
2.2 WWTP flow diagram 23
2.3 Small flocs formation during the coagulation process
at the coagulation reaction tank 33
2.4 Bigger flocs formation during the flocculation process
at the flocculation reaction tank 33
2.5 DAF system with recycle, in which only the
recycle flow pressurized 35
2.6 Process flow diagram for BF System 36
2.7 True unit of BF system 37
3.1 WWTP layout plan 40
3.2 The whole process equipments involve in
DAF treatment 42
3.3 C&F Reactor 42
3.4 Flotation tank with skimmer 43
3.5 Pressure vessel 43
3.6 Recirculation pump 44
3.7 Air injection valve 44
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3.8 P&ID of the studied DAF at the site 46
3.9 Overflow adjustment valve located at the discharge
collection basin 49
3.10 Roller filter with gear motor 51
3.11 Control panel 52
3.12 C&F reactor 52
3.13 Water jet system 53
3.14 Incoming sampling point at the equalization tank 57
3.15 Treated effluent after BF 58
4.1 TSS removal efficiency in relation to the differences
in recycle system pressure 75
4.2 TSS removal efficiency in relation to coagulant dosage 77
4.3 TSS removal efficiency in relation to polymer dosage 78
4.4 Wastewater pollutants removal efficiency (%) vs. date
for the 12 weeks of samples 82
4.5 Wastewater pollutant removal efficiency (%) after DAF
And BF treatment vs. date 84
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LIST OF ABBREVIATIONS
BF - Band-pass Filter
BOD5 - 5-days Biochemical Oxygen Demand
C&F - Coagulation and Flocculation
COD - Chemical Oxygen Demand
DOE - Department of Environment
DAF - Dissolved Air Flotation
IESWTR - Interim Enhanced Surface Water Treatment Rule
LT1ESWTR - Long Term 1 Enhanced Surface Water Treatment Rule
O&G - Oil and Grease
P&ID - Process and Instrumentation Diagram
SI - System International
SBR - Sequencing Batch Reactor
SWTR - Surface Water Treatment Rule
TSS - Total Suspended Solids
US - United States
USEPA - United States Environmental Protection Agency
WWTP - Wastewater Treatment Plant
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LIST OF SYMBOLS
% - Percent
g/d - Gram per day
g/m3 - Gram per cubic meter
gpd - Gallons per day
kg/d - Kilogram per day
lit. - Liter
lit./min - Liter per minute
lit./m2/min - Liter per meter square per minute
lit./hr - Liter per hour
lit./d - Liter per day
mm - Millimeter
m - Meter
m2 - Meter square
m3 - Cubic meter
m3/hr - Cubic meter per hour
m3/d - Cubic meter per day
mg/l - Milligram per liter
mL/mg - Milliliter per Milligram oC - Degree of Celsius
sec - Seconds
CHAPTER 1
INTRODUCTION
1.1 General Overview in the Environmental Control of Industrial
Wastewater
All industrial operations produce some wastewater which must be returned to
the environment. In Malaysia, these industries generally include the crude palm oil
mills, raw natural rubber factories, rubber-based industry, food and beverage
manufacturing industry, wood-based industry, textile industry and etc. Coffee
production and manufacturing factory, which can be categorized under the food and
beverage manufacturing industry, produce a lot of process wastewater during its
production cycle. Unlike the domestic wastewater, this kind of process wastewater
does not pose the potential for pathogenic microorganisms, but they do pose potential
damage to the environment through either direct or indirect chemical reactions.
Coffee production and manufacturing factories have been long operated in
Europe and United States before their production lines started to be shifted to
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Malaysia. In the United States, the discharge of process wastewater from this kind of
food and beverage industry has fully emphasized on the removal of constituents that
may cause long-term health effects and environmental impacts since 1980. In here,
the important US federal regulation that govern the control of wastewater issues, and
have brought about the changes in the planning and design of wastewater treatment
facilities in the US are as follows:
i. Clean Water Act (CWA)(Federal Water Pollution Control Act
Amendments of 1972)
� Establishes the National Pollution Discharge Elimination System
(NPEDS), a permitting program based on uniform technological
minimum standard for each discharger.
ii. Water Quality Act of 1987 (WQA) (Amendment of the CWA)
� Strengthen federal water quality regulations by providing changes
in permitting and adds substantial penalties for permit violations.
(U.S.EPA, 1997)
As one of the fast developing countries, Malaysia has also planned and
implemented its environmental protection management policy and activities in the
control of the industrial wastewater discharge by referring to the control strategies as
presently adopted in the US. In Malaysia, the achievements and progress in the
works of pollution abatement and control is mainly via the enforcement of pollution
control and regulations under the Environmental Quality Act, 1974 carried out by the
Department of Environment (DOE). The enforcement of the existing environmental
laws and legislation is essential and has been stepped up so as to ensure the
capability of the industrial sector, in particular to control the production of
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environmental pollutants and to practice effective storage and disposal systems.
Some technological progress has been noted in treating and controlling pollution
resulting from the agro-based industries. However, on the whole, performance
records show that the status of compliance of these industries is still far from
satisfactory. The problems are attributed to improper management of treatment
systems, the use of under-sized system as well as increased milling capacity (Abu
Bakar Jaafar, KMN, 1992).
More concerted efforts are needed to curb down pollution problems resulting
from all these manufacturing industries. With stricter enforcement of the Environ-
mental Quality (Sewage and Industrial Effluents) Regulations 1979, it is envisaged
that the river pollution problems can be minimized. Strict revision on the issuance of
contravention licenses made under the Section 22(1) and Section 25(1) of the
Environmental Quality Act, 1974 will help to facilitate further the compliance to
these regulations. In addition, factors that aggravate or increase non-compliance,
such as the incompetence of some waste management consultants in designing
effective treatment systems, have been recognized, and efforts initiated to address
and resolve these problems. The implementation and enforcement of the mandatory
environmental impact assessment (EIA) procedure and requirements under the
Environmental Quality (Prescribed Activities)(Environmental Impact Assessment)
Order 1987 have been stepped up in tandem with the country's increasing rate of
development and inclination to industrialize (Laws of Malaysia, 2003)
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1.2 Physico-chemical Treatment of Industrial Waste Water
Premises other than those prescribed (crude palm oil and raw natural rubber
mills) i.e. largely manufacturing industries, are subjected to the Environmental
Quality (Sewage and Industrial Effluents) Regulations 1979, the Environmental
Quality (Clean Air) Regulations 1978 and the Environmental Quality (Scheduled
Wastes) Regulations 1989 (Laws of Malaysia, 2003). As a coffee production and
manufacturing factory that have a great production capacity, HACO Asia Pacific Sdn.
Bhd. should be categorized under a general term known as “Food & Beverages”
manufacturing industries.
This kind of industry does contribute significantly to water pollution in the
country. Their non-compliance is largely due to the absence of a proper wastewater
treatment system, under capacity of the existing treatment system to cater for the
increased production capacity of the industry and lack of maintenance of the
wastewater treatment system. For the year 1991, about 32 industries have applied for
the contravention license under Section 25(1) of the Environmental Quality Act. This
constitutes about 39.5 per cent of the total contravention licenses issued under
Section 25(1) of the Environmental Quality Act for that year (Abu Bakar Jaafar,
KMN, 1992).
In order to implement a proper wastewater treatment system and to avoid any
ineffective treatment system design that was mentioned previously, physico-chemical
treatment system has always be a wise choice to be adopted in the primary treatment
stage. Common physico-chemical treatment systems normally adopted in the
wastewater treatment plant design are Dissolved Air Flotation Systems and
Sedimentation Tank Systems. Due to advances in the material developing technology,
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some kind of filtration system known as the Band-pass Filter can even breakthrough
the cost constraints and is beginning to be adopted in the primary treatment stage for
small WWTP. The main focus of this study is on the DAF which is operated based
on the concept of flotation. At the latter part of this thesis, a preliminary study on the
BF which functions based on the concept of filtration is also included.
1.2.1 History and Concept of Flotation
Over 2000 years ago, the ancient Greeks used a flotation process to separate
the desired minerals from the gangue, the waste material (Gaudin, 1957). Crushed
ore was dusted onto a water surface, and mineral particles were retained at the
surface by surface tension while the gangue settled. In 1860, Haynes patented a
process in which oil was used for the separation of the mineral from the gangue
(Kitchener, 1984). The mineral floated with the oil when the mixture was stirred in
water.
In 1905, Salman, Picard, and Ballot developed the froth flotation process by
agitating finely divided ore in water with entrained air. A small amount of oil was
added, sufficient enough to bestow good floatability to the sulfide grains (Kitchener,
1984). The air bubbles, together with the desired mineral, collected as foam at the
surface while the gangue settled. The first froth flotation equipment was developed
by T. Hoover in 1910 (Kitchener, 1984), and except for size, it was not much
different than the equipment used today.
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Elmore suggested in 1904 the use of electrolysis to produce gas bubbles for
flotation. This process, although not commercially used at that time, has been
developed into electrolytic flotation (Bratby, 1976). Elmore also invented the
dissolved-air (vacuum) flotation (DAF) process, whereby air bubbles are produced
by applying a vacuum to the liquid, which releases the air in the form of minute
bubbles (Kitchener, 1984). The original patent for the dissolved-air pressure flotation
process was issued in 1924 to Peterson and Sveen for the recovery of fibers and
white water in the paper industry (Lundgren, 1976).
1.2.1.1 Type of Flotation Process
Flotation is a unit operation used to separate solid or liquid particles from a
liquid phase. Separation is brought about by introducing fine gas (usually air)
bubbles into the liquid phase. The bubbles attach to the particulate matter, and the
buoyant force of the combined particle and gas bubbles is great enough to cause the
particle to rise to the surface. Particles that have a higher density than the liquid can
thus be made to rise. The rising of particles with lower density than the liquid can
also be facilitated (e.g., oil suspension in water). Different methods of producing gas
bubbles give rise to different types of flotation processes. These are electrolytic
flotation, dispersed-air flotation, and dissolved-air flotation (Lundgren, 1976).
i. Electrolytic Flotation:
The basis of electrolytic flotation, or electrolytic flotation, is the
generation of bubbles of hydrogen and oxygen in a dilute aqueous
solution by passing a DC current between two electrodes (Barrett, 1975).
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The bubble size generated in electrolytic flotation is very small, and the
surface loading is therefore restricted to less than 4 m/h (13.3 ft/h). The
application of electrolytic flotation has been restricted mainly to sludge
thickening and small wastewater treatment plants in the range 10 to 20
m3/h (50,000 to 100,000 gpd).
ii. Dispersed-air Flotation
Two different dispersed-air flotation systems are foam flotation and
froth flotation (Sherfold, 1984). Generally, dispersed-air flotation is
seldom used in municipal wastewater treatment, but it is used in
industrial applications for the removal of emulsified oil and suspended
solids from high volume of waste or process waters. In dispersed-air
flotation systems, air bubbles are formed by introducing the gas phase
directly into the liquid phase through a revolving impeller. The spinning
impeller acts as a pump, forcing fluid through disperser openings and
creating a vacuum in the standpipe (see Fig. 1.1). The vacuum pulls air
(or gas) into the standpipe and thoroughly mixes it with the liquid. As
the gas/liquid mixture travels through the disperser, a mixing force is
created that causes the gas to form very fine bubbles. The liquid moves
through a series of cells before leaving the unit. Oil particles and
suspended solids attach to the bubbles as they rise to the surface. The oil
and suspended solids gather in dense froth at the surface and are
removed by skimming paddles (Tchobanoglous et al, 2003). The
advantages of a dispersed-air flotation system are: (1) compact size, (2)
lower capital cost, and (3) capacity to remove relatively free oil and
suspended solids. The disadvantages of induced-air flotation include
higher connected power requirements than the pressurized system;
performance is dependent on strict hydraulic control, and less
flocculation flexibility. The quantities of float skimming are
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significantly higher than the pressurized unit: 3 to 7 percent of the
incoming as compared to less than 1 percent of dissolved-air systems
(Eckenfelder, 2000).
Figure 1.1: Dispersed-air flotation unit. Air is induced and dispersed into the liquid
by pumping action of the inductors. (Courtesy Eimco) (Tchobanoglous et al, 2003).
iii. Dissolved Air Flotation (DAF)
There are three main type of DAF, and it is known as vacuum flotation
(Zabel and Melbourne, 1980), microflotation (Hemming, Cottrell, and
Oldfelt, 1977), and pressure flotation (Barrett, 1975). Of these three,
pressure flotation is currently the most widely used. In pressure flotation,
air is dissolved in water under pressure. Three basic pressure DAF
processes can be used: full-flow, split-flow, and recycle-flow pressure
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flotation (Zabel and Melt, 1980). For water treatment applications
requiring the removal of fragile floe, recycle flow pressure flotation is
the most appropriate system. Further details on the DAF description will
be explained later in Section 2.4.
1.2.2 History and Concept of Filtration
Filtration is a process that is widely used for removing particulate matter
from water. Besides raw water treatment, the filtration concept is beginning to be
popularly utilized in wastewater treatment process. In raw water treatment, nearly all
surface water treatment facilities and some groundwater treatment facilities employ
some form of filtration. Most surface waters contain algae, sediment, clay, and other
organic or inorganic particulate matter, and filtration improves the clarity of water by
removing these particles. More importantly, all surface waters contain
microorganisms that can cause waterborne illnesses, and filtration is nearly always
required in conjunction with chemical disinfection to assure that water is free of
pathogens. Groundwater is often low in microorganisms and particles but may
require filtration when other treatment processes (such as oxidation or softening)
generate particles that must be removed. In wastewater treatment, filtration is
commonly being used to achieve supplemental removals of suspended solids
(including particulate BOD) from wastewater effluents of biological and chemical
treatment processes to reduce the mass discharge of solids and perhaps more
importantly, as a conditioning step that will allow for the effective disinfection of the
filtered effluent.
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Filters have been used to clarify water for thousands of years. Medical lore
written in India, dating to perhaps 2000 BC, mentions filtration through sand and
gravel as a method of purifying water. Venice, Italy, used rainwater stored in cisterns
as a freshwater supply but drew the water from wells in sand that surrounded the
cisterns (Baker, 1948). In 1852, the first regulation mandating filtration was passed,
required all river water supplied by the Metropolitan District of London to be filtered.
The regulation was prompted by rampant pollution in the Thames River and
suspicions that cholera was transmitted by water (Fuller, 1933), a suspicion
confirmed by Dr. John Snow in his famous investigation of a cholera outbreak in
London just 2 years later.
In 1880, rapid filtration had its origin in the United States. Elements of
modern design, such as mechanical or hydraulic systems to assist with cleaning the
media during backwashing, appeared during that decade. The first municipal plant
employing coagulation and other critical elements of rapid filtration was in
Somerville, New Jersey, in 1885 (Fuller, 1933). Both slow sand and rapid filters
were common in early filter installations (Fuller, 1933), but by the middle of the
twentieth century, rapid filters were commonplace and slow sand filters were rarely
used.
By the latter part of the twentieth century, most surface water was filtered
before municipal distribution. Nevertheless, the Surface Water Treatment Rule
(SWTR), passed in 1989, was the first regulation in the United States requiring
widespread (but not universal) mandatory filtration of municipal water (U.S. EPA,
1989), with the recognition that chemical disinfection alone was ineffective for
protozoa such as Giardia lamblia and Cryptosporidium parvum. Rapid filters were
used in almost all cases (99 percent), but the SWTR caused a resurgence of interest
in slow sand filter, particularly among small utilities that had unfiltered water
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Since passage of the SWTR and more recently the Interim Enhanced SWTR
(IESWTR) (U.S. EPA, 1998) and Long Term 1 Enhanced SWTR (LT1ESWTR)
(U.S. EPA, 2002), finished water turbidity requirements have become more stringent
and the remaining utilities with unfiltered surface water supplies have been under
increasing pressure to install filtration. In short, filtration is and will continue to be a
central feature in surface water treatment plants.
1.3 Problem Statements
DAF, being one of the important physico-chemical separation devices, is
being used currently in the primary treatment stage of industrial WWTP at the
coffee production factory, HACO Asia Pacific Sdn. Bhd. Since the actual pollutant
concentration and loading discharged from the factory is higher than the designed
loading, this physico-chemical separation device is facing a great challenge to
remove the pollutant concentration up to the expected requirement. Furthermore,
due to the upset of the bacteria in the secondary biological treatment system after
the DAF treatment at the early stage of the operation, it is very important to ensure a
sufficient and consistent pollutant removal before the wastewater is being
transferred into the biological treatment system. As such, a process performance
improvement study and performance monitoring needs to be carried out on the DAF.
Firstly, it is important to carry out a process performance improvement study to
maximize the performance of the DAF and to determine the most effective
operating parameters; then, to analyze the pollutant concentrations before and after
the DAF treatment. The results from this study were very important for the plant
designer, i.e. CST Engineering Sdn. Bhd. to propose solutions to the clients.
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At the later stage of the study, a newly invented equipment, the Band-pass
Filter (BF) test unit will be tested out for its performance. This test unit performs the
same functions as DAF and had been brought in by CST from Denmark. A
comparative performance study shall be carried out between the DAF and the BF
due to the availability of the facility at the site.
1.4 Objectives of the Study
This study is aimed at improving and monitoring the performance of the
installed DAF and the existing physico-chemical treatment system and process. The
objectives of the study are as follows:-
i. To determine the most effective operating parameters of the existing DAF.
ii. To determine methods to improve the performance of DAF process.
iii. To determine the effectiveness of DAF in the primary treatment stage of
the WWTP in the coffee production factory.
iv. To compare the performance effectiveness between the DAF and BF in
the primary treatment stage.
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1.5 Scope of the Study
The first target of the study was to perform a process performance
improvement study on the DAF and to determine the most effective operating
parameters. This is to find out all the possible design improvement modification if any.
Then, it was followed by a performance monitoring study on DAF. The study consists
of a through on-site monitoring works at Haco Asia Pacific’s factory at Kota
Kemuning, Shah Alam using the installed DAF unit in the wastewater treatment plant.
The study shall focus on the determination of the Chemical Oxygen Demand (COD),
Biochemical Oxygen Demand (BOD5), Total Suspended Solids (TSS) and Oil and
Grease (O&G) reduction efficiencies of the DAF.
At the later stage of the study, a BF test unit shall be brought to HACO Asia
Pacific S/B, Shah Alam. A comparative performance study was carried out between
the DAF and the BF to compare the removal efficiencies of COD, O&G and TSS.
1.6 Significance of the Study
In the portion of the process improvement study, the results from the study can
be used to operate the DAF more effectively and always in the peak performance.
These effective operating parameters shall be determined and adjusted during the
performance monitoring works.
14
As previously stated, this performance monitoring study was very important
especially in determining the COD, BOD5, TSS and O&G. before and after the DAF
treatment. By measuring and knowing these values, the removal efficiencies can be
determined and a judgment can be made on the performance of the DAF unit.
At the later stage of the study, the BF offers a very good opportunity to study
an emerging design from overseas. It will give an alternative choice that can be
adopted in the WWTP design. More importantly, the results from the comparative
performance study between the DAF and BF are very helpful for the project design
and management team of CST Engineering Sdn. Bhd. in the WWTP design decision
making.
CHAPTER 2
LITERATURE REVIEW
2.1 Coffee Production and Manufacturing Factory Production Process
Description
Since this study is focusing on the coffee production and manufacturing
factory, it is very important to understand the coffee and wastewater production
process before going into the details of the WWTP process. Figure 2.1 captures a
general idea of the soluble coffee production process. In this flow chart, coffee
production mainly consists of 8 main production processes namely:
i. Process (1): Green Coffee Cleaning:
In this first process, green beans are received in 60 kg sacks, big bags or
loose in containers. In here, there is a cleaning unit used to separate all
foreign material such as metal, wood, stones and dust from the green
beans. It is a clean physical separation process with metal detection,
16
Figure 2.1: Soluble coffee production process flow chart
17
sieve and vacuum system (de-stoner). The cleaned coffee beans are
stored in compartment silos or big bags.
i. Process (2): Roasting:
In this process, the different origins are now blended together according
to the confidential recipes. In this step, the end product quality is
influenced depending on the type of green beans used. The beans are
now filled into the drum-roasting chamber and with induction heated to
more than 200°C. This phase lasts for about 15 minutes and in this time
period, the cell structure is breaking open, and the typical coffee flavor
and aesthetic oils are created. The volume of the beans increases by
more than 30%. The roasting process must be interrupted fast and in two
stages, the beans are cooled down to temperatures below 80°C in order
to stop all chemical reactions. After this cooling process and de-stoner,
the beans are pneumatically transported to the roast Coffee Silos.
ii. Process (3): High Pressure Extraction:
The roasted beans are now divided in batch sizes and pass through a
coffee granulator. Before the extraction process, the granulated coffee
must be pre-moistured. The extraction batteries (percolators) are under
16 bar working pressure and the extraction temperature can exceed more
than 180°C. The aroma-extract stream is stripped in a distillation process
to save the premium flavor. This aroma concentrate is given back to the
process directly before the drying process to optimize the flavor profile
of the end product. Before entering the tank farm, the stripped aroma
and hydrolyze extract must be cleaned and concentrated in a vacuum-
fall-stream process.
18
iii. Process (4): Tank farm:
The purpose of the tank farm is to store concentrated extract ready to
spray dry. The storage temperature is kept below 15°C to prohibit
microbiological growth. The Rework station is used to blend non
conforming product back to the main product stream.
iv. Process (5): Liquid Coffee Concentrated (S):
Concentrated Coffee can also be sold as finished goods mainly for
industrial purpose. The coffee is filled and packed in cans or containers.
v. Process (6): Spray Drying (A):
The spray drier is changing the liquid concentrated extract into dry
soluble coffee. The feed tank system adjusts the pH. After the pre-
pressure pump, the liquid extract must be foamed in order to control the
density and color of the finished product. The high-pressure system is
important to stabilize the product dispersing through the nozzles. Inside
the drying chamber, the extract is spread into a hot air stream. The
moisture content of the end product is less than 3.5%. The spray-dried
product is stored in hermetic closed containers and forwarded to the
product on hold area. All sensorial, analytical and bacteriological
analysis must be completed before the product is released to the
packaging area.
vi. Process (7): Bulk Packaging & Process (8):Packaging in Pouches:
As the word imply, packaging of the finished product is being carried
out in here and the product to be sold out.
(CST, 2007)
19
In here, process (2) to process (6) is the major processes that waste materials
to the environment. Gas emissions discharge to the atmosphere is normally
contributed by process 2, 3, 4 and 6. In terms of the solids waste generation, it is
purely generated by process 3. However, in terms of the waste water generation, it is
mainly contributed by process 3, 4, 5 and 6.
2.2 Wastewater Treatment Plant Design
Although the main scope of study in this thesis is focused on the DAF, it is
better to understand the whole process design of the WWTP before going into details
of DAF. The proposed treatment plant is designed to produce secondary effluent of a
quality that will meet the requirements of the Malaysian Government for discharge
according to DOE Standard B provided that the influent is having the wastewater
characteristics shown in Table 2.1.
20
Table 2.1: WWTP design input and output
Parameter
Raw influent
Treated effluent (DOE
Std. B)
Flow Rate (m3/day) 600 -
Peak Flow (m3/hour) 30 (max. 2 hours) -
BOD5 ( mg/L)
1000 - 2200 < 50
COD (mg/L) 4000 - 6500 < 100
TSS (mg/L) 700 < 100
O&G ( mg/L) 50 < 10
Temperature (oC) < 60 < 40
pH 4 – 7 6 – 9
Plant Operating Hours 20 -
WWTP Operating Hours 24 -
(Laws of Malaysia, 2003)
21
2.2.1 Wastewater Treatment Plant General Description
First of all, the industrial wastewater from the factory is discharged into the
pump sump and consequently, pumped to the WWTP. In the WWTP, the proposed
treatment process comprises of the following main steps, namely physical, chemical
and biological treatment:
i. Physical Treatment:
The physical pretreatment involves a solid screening system. The
wastewater stream passes through a static fine screen that will remove
any solid particles in excess of 1.0 mm.
ii. Chemical Treatment:
After the static fine screen and equalization tank, the wastewater is
being treated by chemical means through coagulation and flocculation
in a Dissolved Air Flotation (DAF) unit. Efficient removal of
emulsified oil and grease, fine solids and certain dissolved proteins
will be achieved in this step.
iii. Biological Treatment:
Organic pollutants are removed aerobically in a Sequencing Batch
Reactor (SBR) to a level required by the DOE. The mechanism
involved microbial metabolism that will transform BOD and COD into
environmentally inoffensive compounds. Adequacy of COD removal,
however, will depend on overall biodegradability of the wastewater.
22
2.2.2 Wastewater Treatment Plant Detailed Process Description
In this section, the detailed process design of the WWTP will be explained.
By referring to the WWTP flow diagram as shown in Figure 2.2, a better conceptual
understanding can be obtained when reading through the process description as
follows:
i. Pump Sump:
This sump receives different wastewater sources from the processing
plant. The wastewater will then be pumped by submersible pumps into
subsequent high load damping system and equalization tank
respectively after passing a screening system for solid and particle
removal, depending on the effluent quality from the processing plant.
This unit has two chambers with a volume of 20 m3 for normal load
and 5 m3 for high load wastewater.
ii. Screening System:
It is proposed that the wastewater to be directed into a static fine
screen with aperture size approximately 1.0 mm. Particles with size
greater than 1.0 mm, which may cause operational problems
downstream, will be retained in the screen. The screened wastewater
will flow by gravity into the high load tank and equalisation tank
downstream. The solids on the screen will be removed and stored in a
bin prepared by client. The designed screening system is capable to
handle wastewater of 600 m3/day.
23
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INTERMEDIATE
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24
i. High Load Dumping System:
During the factories processing, maintenance and cleaning schedule or
change of process system in the plant, high organic loading (COD &
BOD) wastewater will be dampened by this specially designed system
to avoid organic shock to the wastewater treatment plant. High loading
wastewater will be held temporary in a High Load Damping Tank
(HLDT) to avoid abrupt increment in organic loading, which will pose
detrimental effect to aerobic system. During the time of normal
loading, wastewater in HLDT will be fed gradually into the
equalisation tank. A floating surface aerator is installed in the tank to
ensure proper mixing, to equalise the high loading wastewater from
different sources and to enhance the temperature level.
ii. Equalization Tank:
During the normal operation of the processing plant, the equalisation
tank is used to overcome the operational problems caused by flow rate
variations and organic loading fluctuation, thus improve the
performance of downstream processes. Therefore the subsequent
processes will be steady hydraulically. A floating surface aerator is
used to keep the wastewater properly mixed and enhance the
temperature reduction prior to be transferred into the following
chemical treatment system. This unit has a total volume of 600 m3.
iii. DAF System:
Our design calculations are based on chemical assistance be provided
to alter the physical state of dissolved proteins, dispersed fine solids
and/or emulsified oil droplets to facilitate their removal in the DAF
unit. With proper chemical dosing, the DAF unit and scraper system
25
will be able to achieve the stated COD removal. A flocculator and
coagulator reactor system shall be installed as part of the DAF unit to
facilitate the injection of chemicals for treatment of the wastewater.
iv. Intermediate Holding / Hydrolysis Tank:
The chemical pre-treated wastewater will be held temporarily in an
intermediate holding tank cum hydrolysis reactor prior to being
transferred to the biological treatment system at the reaction phase of
the sequencing batch reactor. This tank will have a capacity of 1000 m3.
v. Sequencing Batch Reactor (SBR):
The biological treatment involves a Sequencing Batch Reactor, which
has a total volume of 1700 m3. This design is done to meet the
optimum removal of the COD and BOD in the influent wastewater to
meet local government required standard.
In the SBR process, the tank serves as both aeration and
sedimentation basins for the system. This is achieved by operating the
SBR in a sequential mode instead of the standard continuous mode
commonly adopted for conventional activated sludge processes. This
mode of operation is ideal for the type of processing plant in question
due to its batch operation, and the undulating flow pattern
characterized by periods of high flow followed by sustained periods of
low or no flow.
The SBR cycle will be structured so that filling of the reactor will be
done mostly during high flow periods. Aeration (REACT phase) will
26
be carried out for most of the FILL phase and for a short period after
filling has stopped.
Upon completion of REACT, the biomass will settle for a nominal
period of an hour (SETTLE phase). At the end of SETTLE, the reactor
will be decanted (DECANT phase), leaving most of the sludge mass
behind for the next cycle. The end of DECANT will incorporate an
IDLE phase when excess sludge is drawn off by a submersible
centrifugal pump to the sludge holding tank to await dewatering and
final disposal.
The SBR serves to reduce the concentration of soluble organic
compounds in the wastewater that survived the upstream treatment
steps. Removal of these compounds is done mainly through oxidation
by aerobic micro-organisms. To ensure proper performance, it is
essential that sufficient oxygen be supplied to the activated sludge
biomass. Oxygen will be supplied in this proposed scheme by floating
surface aerator. The micro-organisms in the tank will convert the
organic contaminants (substrate) by biochemical reaction to CO2, H2O
and mineral compounds and, as a result, reduce the concentration of
BOD5 and COD. At the same time, the micro-organisms multiply
themselves, producing a growing amount of activated sludge which
has to be withdrawn from the system as excess sludge.
Other than the aeration, the SBR can also provide an anoxic period for
the Denitrification system during the operating cycle. Hence, the NO3-
generation from the Nitrification system during aeration can be
reduced to required standard. For nitrification, SBR aerobic react times
may range from 1.0 to 3.0 h (WEF, 1998). A submersible mixer will be
27
used to promote the good mixing of wastewater during the
Denitrification stage.
vi. Sludge Holding Tank:
The sludge holding tank has a volume of 160 m3. Settled sludge from
the clarifier will be stored temporarily in this tank for 1-2 days before
sludge is transferred to sludge conditioning tank downstream. The tank
will be provide by a submersible mixer/aerator used to keep the sludge
properly mixed and prevent potential odour problem.
vii. Sludge Conditioning & Dewatering System:
This system comprises a sludge-conditioning unit and a semi-
automatic chamber filter press. In the sludge-conditioning unit a
polymer conditioner (to be selected during commissioning of the plant)
will be added into a sludge conditioning tank to improve water
expulsion characteristics of the sludge. Polymer can bind with sludge
to become bigger in size and heavier particles for considerable gravity
sedimentation. An air diaphragm feed pump will be used to feed the
sludge into the chamber filter press.
Filter press is a type of filtration devices to separate solid and liquid. A
chamber of vertical filter plate, in series arrangement, is playing the
role of filtration by retaining solid or sludge on its surface. Filter press
is operating at 2-3 cycles per day at the operating pressure of 7 bars
and capable to handle sludge dry solid content of 53.4 kg SS/d. Sludge
in the holding tank is assumed to contain 1- 2% dry solids. Dry solids
content of the dewatered sludge cake will be about 30-40%, depending
28
on the sludge characteristics. Dewatered sludge cake will be stored in a
bin prepared by client prior to final disposal.
Filtrate and from the filter press and excess water in sludge bin will be
drained back to the equalization tank for further treatment.
viii. Filtration System:
This filtration system comprises of a supernatant holding tank, a disc
filter and pump system and a treated water tank. Treated water from
the SBR tank will be discharged into the supernatant holding tank with
a volume of 600 m3, where the water will be held temporary in this
tank before going through the filtration system for water reuse at
selected areas of the factory or drained off for final discharge.
2.6 Chemical Treatment Process
Before going into details about the DAF and the BF, it is important to first
understand the chemical treatment process. It is vital and important because the
wastewater will be treated chemically first before it goes through the 2 process unit
for separation purpose. Chemical processes, in conjunction with various physical
operations, have been developed for the complete secondary treatment of untreated
(raw) wastewater, including the removal of either nitrogen or phosphorus or both.
Chemical processes have also been developed to remove phosphorus by chemical
precipitation, and are designed to be used in conjunction with biological treatment.
Other chemical processes have been developed for the removal of heavy metals and
29
for specific organic compounds and for the advanced treatment of wastewater
(Tchobanoglous et al, 2003), in case, chemical pretreatment prior to filtration is more
critical to success than the physical facilities at one plant. This was fully agreed and
reported by Cleasby et al (1989) who recommended that the plant staff used a well-
defined coagulant chemical control strategy that considers variable raw-water quality
(Cleasby et al, 1989).
The concern here is in the pH adjustment, coagulation and flocculation
process. Before going into details of each, it is important to differentiate the 2
confusing terms- “Coagulation” and “Flocculation”. The terminology of coagulation
has not been standardized. However, in most of the water treatment literature,
coagulation refers to all the reactions and mechanisms that result in particle
aggregation in the water being treated, including in situ coagulant formation (where
applicable), particle destabilization, and physical inter-particle contacts. The physical
process of producing interparticle contacts is termed “Flocculation” (Camp, 1955).
However, in the colloidal sciences literature, LaMer (1964) considered only chemical
mechanisms in particle destabilization and used the term coagulation and
flocculation to distinguish between two of them. LaMer defined destabilization by
simple salts such as NaCl (a so-called indifferent electrolyte) as “coagulation”.
Destabilization of particles by adsorption of large organic polymers and the
subsequent formation of particle-polymer-particle bridges was termed “flocculation”
(LaMer, 1964).
The water treatment literature sometimes makes a distinction between the
terms “coagulant” and “flocculant”. When this distinction is made, a coagulant is a
chemical used to initially destabilize the suspension and is typically added in the
rapid-mix process. In most cases, a flocculant is used after the addition of a
coagulant; its purpose is to enhance floc formation and to increase the strength of the
30
floc structure. It is sometimes called a “coagulant aid”. Flocculants are often used to
increase filter performance (they may be called “filter aids” in this context) and to
increase the efficiency of a sludge dewatering process. In any case, depending on
how and where it is used and at what dosage, a coagulant is sometimes a flocculant
and vice versa.
2.3.1 pH Adjustment
The removal of excess acidity or alkalinity by treatment with a chemical of
the opposite composition is termed neutralization. In general, all treated wastewater
with excessively low or high pH require neutralization before they can be dispersed
to the environment. In a variety of waste water treatment operation and processes,
there is often a need for pH adjustment. Because a number of chemicals are available,
the choice will depend on the suitability of a given particular application and
prevailing economics.
Neutralization of acidic wastewater in small plants or for treatment where
small quantities are adequate commonly employ sodium hydroxide (NaOH, also
known as caustic soda) and sodium carbonate, although somewhat expensive, are
convenient and used widely by these small plants. Lime, which is cheaper but
somewhat less convenient and slower in reaction rate, is the most require widely
used chemicals.
31
Alkaline wastes are less of problem than acid but nevertheless often require
treatment. If acidic waste streams are not available or are not adequate to neutralize
alkaline waste, sulfuric acid is commonly employed. In some treatment plants,
carbon dioxide in the form of flue gas had been used to neutralize alkaline
wastewater.
Most importantly, chemical neutralization or pH adjustment also functions to
be used in the chemical precipitation reaction. Adjustment of pH will cause the
adjustment of solubility of different constituents in the wastewater and can be used to
precipitate out the necessary pollutants especially heavy metals. Besides that, pH
adjustment of the wastewater is required in getting the best floc during the
coagulation/ flocculation process (Tchobanoglous et al, 2003). In this case, caustic
soda (NaOH) is used to achieve a pH range of 4.3 to 4.8, in which a best floc can be
gotten.
2.3.2 Coagulation and Flocculation
Colloidal particles found in wastewater typically have a net negative surface
charge. The size of colloids (about 0.01 to 1 µm) is such that the attractive body
forces between particles are considerably less than the repelling forces of the
electrical charge. Under these stable conditions, Brownian motion keeps the particles
in suspension. Brownian motion (i.e. random movement) is brought about by the
constant thermal bombardment of the colloidal particles by the relatively small water
molecules that surround them. The term “chemical coagulation” used in here
includes all of the reactions and mechanisms involved in the chemical destabilization
32
of colloidal particles so that particles growth can occur as a result of particle
collisions, which means that in the formation of larger particles through perikinetic
flocculation (aggregation of particles in the size range from 0.01 to 1 µm).
In general, a coagulant is the chemical that is added to destabilize the
colloidal particles in wastewater so that the floc formation can result. A flocculant is
a chemical, typically organic, added to enhance the flocculation process. Typical
coagulants and flocculants include natural and synthetic organic polymer, metal salts
such as alum or ferric sulfate, and pre-hydrolized metal salts such as polyaluminium
chloride (PACl) and polyiron chloride (PICI). Flocculents, especially organic
polymers, are also used to enhance the performance of granular medium filters and in
the dewatering of digested biosolids. In these applications, the flocculant chemicals
are often identified as filter aids.
In the aspect of flocculation, there are two types of flocculation: (1)
microflocculation (also known as perikinetic flocculation), in which particle
aggregation is brought about by the random thermal motion of fluid molecules
known as Brownian motion or movement and (2) macroflocculation (also known as
orthokinetic flocculation), in which particle aggregation is brought about by inducing
velocity gradients and mixing in the fluid containing the particles to be flocculated.
Another form of macroflocculation is brought about by differential settling in which
large particles overtake small particles to form larger particles. The purpose of
flocculation is to produce particles, by means of aggregation, that can be removed by
inexpensive particle-separation procedure such as gravity sedimentation and
filtration. To further emphasize on this, macroflocculation is ineffectual until the
colloidal particles reach a size of 1 to 10 µm through contacts produced by Brownian
motion and gentle mixing (Tchobanoglous at al, 2003)(To have a general idea on
33
how the flocs formation during coagulation and flocculation, refer to Figure 2.3 and
Figure 2.4).
Figure 2.3: Small flocs formation during the coagulation process at the
coagulation reaction tank.
34
Figure 2.4: Bigger flocs formation during the floculation process at the
flocculation reaction tank..
2.4 Dissolved Air Flotation (DAF) System
DAF is a unit operation for the separation of solids and semisolid (floc)
particles from a liquid phase that has been used for the clarification of potable water
for over 40 years (Edzwald, 1995). Besides potable water, this concept is also widely
applicable in wastewater treatment. In here, air bubbles are introduced near the
bottom of the basin containing the water to be treated. As the bubbles move upward
through the water, they become attached to the particulate matter and floc particles,
and the buoyant force of the combined particles and air bubbles will causes the
particles to rise to the surface.
To achieve efficient clarification by DAF, particles and natural color present
in the water must be coagulated and flocculated effectively prior to the introduction
of micro-bubbles to form bubble-floc aggregates. Floatable bubble-floc agglomerates
might form by any of three distinct mechanisms: entrapment of bubbles within
condensing network of floc particles, growth of bubbles from nuclei within the floc,
and attachment of bubbles of floc during collision. All three mechanisms can occur
but that the principal mechanism in DAF for the potable water treatment is the
attachment mechanism (Kitchener and Gochin, 1981).
On the other hand, for the treatment of industrial waste and concentration of
solids, recycle-flow pressurized system is being considered and utilized in here and it
is shown in Figure 2.5. In this kind of system, a portion of the DAF effluent is
recycled, pressurized and semi-saturated with air. The recycled flow is mixed with
the un-pressurized main stream just before admission to the flotation tank, with the
35
result that the air comes out of solution in contact with particulate matter at the
entrance to the tank. A pressure release device is being utilized in here to control the
entrance of the pressurized recycle water. Here, the pressure is reduced to
atmospheric pressure, releasing the air in the form of fine bubbles (10 to 100µm in
diameter). The air bubbles attach themselves to the flocs, and the aggregates float to
the surface. The floated material (the float) is removed from the surface, and the
clarified water is taken from the bottom of the flotation tank.
Figure 2.5: DAF system with recycle, in which only the recycle flow is pressurized.
(Tchobanoglous et al, 2003).
.
2.5 Band-pass Filter (BF) System
BF is purely a chemical-mechanical filtration system with a proper electrical
device used in the controlling process. Before going through the unit in detail,
36
understanding of the BF process flow diagram is important. The process flow
diagram of a BF is shown in Figure 2.6 and the picture for the true unit is shown in
Figure 2.7.
Figure 2.6: Process flow diagram for BF System (CST, 2007)
37
Figure 2.7: True unit of BF System (CST, 2007)
Like DAF, the BF consists of a chemical treatment system at the front side.
The chemical treatment system in here basically consists of pH adjustment,
coagulation and flocculation reactor aiming in the formation of good floc. In other
words, the chemical treatment system acts as a separate stage from filter and is to
obtain a floc “designed” to be retained by the filter.
This kind of treatment system is seen to be a combined concept of
“Microstraining” and “Direct Filtration”. “Microstraining”, sometimes referred to
also as microfiltration, microscreening, or microsieving, is a water-screening process
that use very fine mesh fabrics to arrest all particles smaller than the mesh aperture
(Vigneswaran, 1989). On the other hand, for “Direct Filtration”, it is generally meant
that in this process there is no other treatment aiming at the removal of suspended
solids such as sedimentation, flocculation, hydrocyclones, etc. prior to filtration. In
practice, the concept of “Direct Filtration” has developed along with 2 different
guidelines, i.e. (1) Direct filtration with preflocculation- coagulation-flocculation as a
preliminary step well-separated from filters; and (2) contact filtration- coagulation-
38
flocculation as an integral part of those mechanisms governing filtration (Janssens et
al, 1985). In here, the target of coagulation-flocculation as a separate stage from filter
is to obtain a floc “designed” to penetrate the bed in depth and to resist the shear
forces, as a results, the different sizes of filter medium are necessary for “sieving”
flocs of different size. (Vigneswaran, 1989)
.
The concept of filtration is strictly connected to an idea of clearness, which in
turn is connected with a subjective sensory perception. The instruments commonly
employed to measure the turbidity of water are ineffective in providing objective
values of immediate application. This is the reason for the necessity to know the
concentration of suspended solids and the distribution of particle sizes. For instance,
the comparison between the turbidity value in formazine turbidity units (FTU) and
the concentration of suspended matter can provide useful information which is
known as “fitness coefficient”, i.e., the colloidal degree of suspension. Rather than
turbidity itself, this factor makes it possible to anticipate the coagulant quantity to be
employed for the treatment, thus rendering “Direct Filtration” feasible (Brathy, 1980).
What differentiates BF from common direct filtration and microstraining
method is its filtering materials and method. The BF has a constantly moving
polyethylene endless filter which means that the purchase, fitting and disposal of
expensive rolls of paper cloth can be avoided. When the water is passed through the
filter, the particles are carefully retained on the belt without breaking up into fine
particles. They are then lifted out of the water and deposited into a skid. The filter
belt is being flushed either intermittently or constantly with clean or recirculated
water in order to avoid the perforations from being blocked (CST Engineering, 2007).
As compared to other filtration system, BF utilize a very suitable filter media since
polyethylene is some kind of plastic media that are economical and have good
chemical resistance and weathering characteristics (Cheremisinoff, 2003).
CHAPTER 3
STUDY METHODOLOGY
3.1 Wastewater Treatment Plant Site Survey, Study and Planning
Planning at the site has been carried out to determine the most suitable study
methods and set-up procedure for the equipment. Figure 3.1 shows the WWTP
Layout Plan. In here, the most important study object would be the DAF and it is
represented by S1 in Figure 3.1. It has already been constructed at the site and it was
used back during the study. For the BF study, the test unit was brought to the site and
being located at C09- Filtration Feed Tank, near the V4- Azud Filter. It is shown in
red color in Figure 3.1.
During the performance monitoring and process performance improvement
study on the DAF, sampling will be carried out at C04 (Equalization Tank) and S1
(DAF). In the comparative performance study, sampling will be carried out together
at three different locations at the same time.
40
41
3.2 Dissolved Air Flotation (DAF) Unit
The DAF is the most important physico-chemical separation equipment in the
primary treatment design stage of the WWTP to knock down the residual TSS and
O&G, in order to enable the success of the downstream biological treatment process
functional ability. The DAF in this case study is of the rectangular type and the
whole process equipment of DAF is shown in Figure 3.2. The constructed DAF at
the WWTP consists of the following compartments:
i. Coagulation and Flocculation (C & F) Reactor (Figure 3.3)
ii. Flotation tank with skimmer (Figure 3.4)
iii. Pressure vessel (Figure 3.5)
iv. Recirculation pump (Figure 3.6)
v. Air injection valve (Figure 3.7)
vi. Chemical Dosing System
42
Figure 3.2: The whole process equipment involved in DAF treatment
Figure 3.3: Coagulantion & Flocculation Reactor
43
Figure 3.4: Flotation tank with skimmer
Figure 3.5: Pressure Vessel
44
Figure 3.6: Recirculation pump
Figure 3.7: Air injection valve
45
3.2.1 Site Operation Aspects of Installed DAF
Before going into details of the operation of DAF, it is better to have an
overall idea of the process flow of the DAF by looking at the detailed Process and
Instrumentation Diagram (P&ID) shown in Figure 3.8. By referring to this figure; the
equalized wastewater is pumped to the C & F reactor where Coagulant and flocculant
are added into the reactor. The contaminating solid is separated from the wastewater
in the C & F reactor. From the reactor, wastewater is overflowed into the flotation
tank. On top of the flotation tank a scraper chain conveying system is installed,
which intermittently scrapes the sludge into the sludge compartment tank, by a motor.
The purified effluent is discharged in the bottom of the flotation tank and
passed on to an outlet collection basin attached to the DAF. By means of overflow
control, the water level can be adjusted in the flotation tank. Both flotation tank and
outlet basin are made of stainless steel. Wastewater in the flotation tank can be
emptied by opening a bottom drain valve.
The dispersion system consists of a dispersion tank (Pressure Vessel),
compressed air, recirculation pump and pressure reducing valve. Dispersed water is
made continually and automatically. Dispersed water is made from recirculated water
and compressed air is mixed to supersaturation. The amount of water fed into the
flotation tank is adjustable by a manual valve.
The dispersed fluid is injected into the flotation tank forming microscopic air
bubbles, which would carries flocs to the surface. Sludge layer is formed on the
46
47
surface and is scrapped intermittently to the sludge compartment. During the study
periods, the DAF was operated with parameters shown in Table 3.1.
Table 3.1: Recommended operating condition for DAF:
Operating Parameter Unit Operating/
Recommended Range
1. Feed pump flow rate m3/hr 300 - 450
2. Recirculation pump flow rate m3/hr 5 - 10
3. Recirculation pump
pressure
Bar 2.6 - 3.0
4. Air pressure Bar 2.0 - 4.0
5. Caustic dosing pump
rate
lit./hr 3.0 – 5.0
6. Coagulant dosing pump
rate
lit./hr 24 – 36
7. Flocculant dosing pump
rate
lit./hr 90 -180
3.2.2 Start-up of DAF
During the time of performance study, start-up of the DAF system needed to
be done and the following procedure had been carried out as followed:
i. Recirculation pump, DAF skimmer and dosing pumps selector
switch were set to Auto Mode.
48
ii. Water was then filled into the flotation tank.
iii. Coagulant and flocculant were being prepared
iv. Recirculation pump was started and the pressure vessel was filled
v. Air pressure for the pressure vessel was being set to the ranges from
2.0 bars to 4.0 bars.
vi. Flotation manual feed valve and recirculation pump suction valve
were being adjusted until the desired flow rate approximately 10 -50%
of the main flow rate.
vii. Pressure reducing valve and the manual gate valve were adjusted until
the desired fine bubble.
viii. Dosage of coagulant and flocculant were adjusted as per Jar Test
results.
ix. When the flotation tank was filled, the water level was adjusted by
adjusting the overflow of the discharge collection basin (refer Figure
3.9).
x. Pressurized air or fluid in the pressure vessel should be released if the
air pressure in the vessel build up beyond setting requirement and
design pressure during start up. It was carried out by releasing the air
from the pressure vessel using the manual air release valve. The
incoming air pressure must be regulated as well to reduce the air
supply and if necessary use the manual valve to shut off the air supply.
The liquid in the pressure vessel can be drained off by opening the
49
bottom drain off valve should the water overflow to the air line
system.
xi. The recirculation pump to be shut off if abnormality of over pressure
occurred during start up as indicated on the pressure vessel pressure
gauge pressure build up and over 5 bars
Figure 3.9: Overflow adjustment valve located at the discharge collection basin.
3.2.3 Shut-down of DAF
Shutting down of the system needed to be carried out by our own self if the
operator is not in at that moment. These procedures were followed during the shut-
off time:
50
i. The overflow was adjusted to raise the sludge level.
ii. Then, the sludge layer was being scraped off.
iii. Inlet compressed air was shut off.
iv. The skimmer and dosing pumps were shut off..
v. The recirculation pump for the pump to be serviced was shut off.
vi. Pressurized air or fluid in the pressure vessel was released if the air
pressure in the vessel build up beyond design pressureduring shut
down.time
vii. Inlet valve to the DAF was shut off.
viii. Recirculation feed valve was shut off after the recirculation pump was
switch off.
ix. Drain valve in the DAF was opened
x. In servicing the Pressure vessel, the air pressure in the vessel was
release until it reaches zero, and the water in the vessel was being
drain out using bottom drain off line.
3.3 Band-pass Filter (BF) Unit
The BF fulfill the requirements of the laws of the Member States on the
safety of the machines 98/37/EEC. In operational condition, the requirements of the
51
laws of the Member States on electrical equipment 73/23/EEC and the laws of the
Member States on Electromagnetic compatibility 89/336/EEC are fulfilled.
This BF system consists of the main components as follows:
i. Roller filter with electrical gear motor (Figure 3.10).
ii. Control panel system (Figure 3.11).
iii. Coagulation and Flocculation (C&F) Reactor (Figure 3.12).
iv. Chemical dosing system.
v. Water jet system (Shown in Figure 3.13).
Figure 3.10: Roller filter with gear motor
52
Figure 3.11: Control panel
Figure 3.12: C&F reactor
53
Figure 3.13: Water jet system
3.3.1 Site Operation Aspect of BF Unit
During the actual site operation, Table 3.2 shows the recommended operating
condition.
54
Table 3.2: Recommended operating condition for BF.
Operating Parameter Unit Operating/
Recommended Range
1. Feed pump flow rate m3/hr 1 – 2
2. Band-pass filter
rotational speed
m/min 5 – 7
3. Flocculation mixer
speed
rpm 60 – 90
4. Caustic dosing pump
Rate*
lit./hr 3.0 – 6.0
5. Coagulant dosing
pump rate
lit./hr 2.0 – 4.0
6. Flocculant dosing
pump rate
lit./hr 6.0 -14.0
* 10 times dilution of caustic (50%) chemical is required because the existing
chemical pump is more accurate at greater dosing rates.
3.3.2 Start-up of BF
During the performance study period, start-up of the BF system had to be
done and the following procedures must be followed:
i. First, the Power supply is connected.
ii. Then, the feed pump flow rate, BF rotational speed, flocculation
mixer speed and chemical dosing rate was set manually according to
the recommended operating condition.
55
iii. Water is filled into the system and water jet systems that utilize
the water to flush the BF roller filter all the while is turned on.
iv. Coagulant and flocculant were then prepared.
v. Feed pump (submersible pump) is switched on and the system should
be filled up at that moment.
vi. Influent valve is adjusted until the recommended influent flow rate
is achieved.
vii. Dosing rate of coagulant and flocculant is adjusted as per Jar Test
results.
viii. The BF rotational speed is adjusted until the observed flocs removal
capacity and clear water is achieved.
3.3.3 Shut-down of BF
Shutting down of the system in here is very straight forward. We just needed
to turn off all the equipment at that moment except the BF filter roller and let it to
run for a while to clear the balance flocs inside the tank. Then, the discharge valve
can be opened at the bottom of the C & F reactor tank. Finally, the water jetting
systems that clean the BF roller filter can be turned off when the appearance of the
band-pass filter roller was clean at that moment.
56
3.4 Sampling at the Wastewater Treatment Plant
There are 3 major sampling points in our study. They are:
i. Incoming sampling point at the equalization tank (Figure 3.14).
ii. Treated effluent after DAF (Figure 3.9).
iii. Treated effluent after BF (Figure 3.15).
For all these 3 sampling points, we were collecting the sample based on composite
sampling methods, in which 4 samples would be collected during an operational
testing of 4 hrs and then mixed together before sending to the laboratories.
57
Figure 3.14: Incoming sampling point at the equalization tank
58
Figure 3.15: Treated effluent after BF
3.5 Analysis of Wastewater Samples
For the temperature reading, it can be directly recorded from the monitoring screen
of the Eutech pH controller that is equipped with a built-in temperature sensor installed at
the coagulation tank. Meanwhile, the pH at the Equalization tank and after the DAF
treatment can be measured directly with a portable HANNA pH Tester- model no: HI
96107 (pHep®). For the other parameters, it was tested when the entire wastewater sample
was sent to the third party laboratory and all the analysis on the wastewater was done in
accordance with Standard Methods (APHA, 1998).
59
3.6 Jar Tests and Chemical Selection
The purpose of the jar test is to simulate, to the extent possible, the expected or
desired conditions in the coagulation-flocculation facilities. Refer to the American Society
for Testing and Materials procedure (ASTM, 1976) for a review of the experimental
protocols. Generally, the test consists of a rapid-mix phase (high mixing intensity) with
simple batch addition of the coagulant or coagulants followed by a slow-mix period to
stimulate flocculation. Normally, flocs were allowed to settle and samples would be taken
from the supernatant. But in our case, since the operating equipment is a DAF, jar test was
to be simulated like DAF, in which air bubbles were being injected into the water at the
moment to ensure that all the flocs formed are floated to the surface of the beaker (Zakariya,
1998). The jar test was carried out by the water treatment chemical supplier.
3.7 Monitoring and Testing Design
Design of the testing and study would be firstly focused on the process performance
improvement study. By getting the optimum operating parameters, it was then possible to
carry out the performance monitoring study of the DAF by operating the DAF under
optimum conditions. Lastly and finally, the comparative performance study between the
DAF and the BF was designed to determine the superior removal functions of each
equipment.
60
3.7.1 Process Performance Improvement Study
Before any testing was carried out, the original design based on the original design
input was first reviewed back. Then, based on the actual condition, a design calculation
simulation was carried out. From this, a general idea on what to focus in the process
performance improvement study can be obtained.
In the aspect of the mechanical tuning of the DAF, variation of the recycle system
pressure on the DAF performance was studied. In here, the valve after the recycle pump
was adjusted to obtain the required pressure during the study. Performance of the DAF at
every different recycle pressure can be observed from the appearance of the effluent from
the DAF. To confirm about the observation results, 15 wastewater samples before and after
the DAF treatment were collected for analysis during the 3 days testing and commissioning
study.
In the aspect of the chemical tuning adjustment, it was mainly focused on the
chemical dosing pump capacity adjustment to determine the most effective and efficient
operating condition. The optimum pH condition would be based on the jar test results.
During the process performance improvement study, it was strictly required to vary
the studied operating parameters and maintain the other operating parameters as usual
(constant) (Ng, et al, 1988)
61
3.7.2 Performance Monitoring and Study of DAF
This monitoring and study work was carried out at the WWTP of HACO Asia
Pacific Sdn. Bhd. and the WWTP layout plan is shown in Figure 3.1. The wastewater will
be pumped from the Equalization Tank into the DAF as what was happening during the
normal operation periods. Then, the operating parameter of the DAF being set according to
the optimum operating parameter obtained from the process performance improvement
study. Sample of wastewater was collected from the Equalization Tank and after the DAF
treatment. This sampling activity had to be carried out once every week during the
operation of the DAF and the sample taken will be sent out to a third party laboratory for
analysis. The sampling method was based on composite sampling during the operation of
the DAF for a period of 4 hrs. In this way, performance of the DAF can be monitored every
week.
3.7.3 Comparative Performance Study between DAF and BF
The BF was transported to the site and located near the Filtration Feed Tank
(C09)(Its location is shown in Figure 3.1). Then, a submersible pump was temporary
installed at the Equalization Tank and connected with a rubber hose to the BF. Test runs of
the BF ware carried out before the actual comparative study until the best treated water
condition was obtained. After that, the actual comparative study can be carried out. In this
study, the BF was operated to match the design flow of 1m3/hr, with the flocculation mixer
set to 80rpm to get the best flocs. Meanwhile, rotational speed of the band-pass filter roller
was set to 6m/min. (maximum speed of the system is 7m/min). For the DAF, it was
62
operated with 15m3/hr, and the other operating parameters were set to match the optimum
operating parameter determined during the process performance improvement study.
In the chemical dosing part, the 2 process units were operated with the same
chemical concentration based on their own operational flow rate.
CHAPTER 4
RESULTS AND ANALYSIS
1.3 Characterization of Soluble Coffee Production Wastewater
Incoming wastewater at the equalization tank was analyzed at 2 different time
frames, one was analyzed during the performance monitoring of DAF, and the other
was analyzed during the comparative performance study between the DAF and the
BF. Both of the results are tabulated separately in Appendix A and Appendix B.
These results are summarized separately in Tables 4.1 and 4.2.
.
64
Table 4.1: Average values of incoming wastewater characteristics at the
equalization tank (results abstracted from Appendix A):
Waste water testing
parameters
Unit Reading
(Average Value)
1. Color - Dark brown
2. Temperature oC 31
3. pH - 4.1
4. COD mg/l 13,281
5. BOD5 at 20oC mg/l 3,759
6. TSS mg/l 1,896
7. O&G mg/l 429
Table 4.2: Average values of incoming wastewater characteristics at the
equalization tank (results abstracted from Appendix B):
Waste water testing
parameters
Unit Reading
(Average Value)
1. Color - Dark brown
2. Temperature oC 30
3. pH - 4.0
4. COD mg/l 11,223
5. TSS mg/l 1,168
6. O&G mg/l 206
When we compare the 2 results from Table 4.1 and Table 4.2, color,
temperature and pH characteristics are almost the same. However, results in
Appendix B show a lower COD, TSS and O&G readings. After our detailed studies,
we found that the differences are due to the different sampling timing.
For the results in Table 4.1, the sample was taken every Wednesday for a
period of 12 weeks. Meanwhile, samples in Appendix B were only taken for testing
65
for a period of two weeks time and it was taken on Thursday, Friday and Saturday.
From the statistical point of view, results in Appendix A are more accurate due to a
longer sampling duration.
One possible reason contributing to such a difference is the different
sampling day. According to the production and maintenance personnel of the plant,
the production is having a peak production load from Monday to Thursday, and the
production of the plant start to slow down from Friday to Sunday. As a result, fewer
pollutants loading were discharged into the WWTP from Thursday to Saturday
leading to a lower pollutant concentration for the results in Table 4.2.
In order to have a general overview on the incoming wastewater
characteristics, the highest and lowest values from Appendix A and B are
summarized in Table 4.3.
66
Table 4.3: Range of values for the incoming wastewater characteristics at the
equalization tank (Results abstracted from Appendix A & B):
Wastewater testing
parameters
Unit Reading
(Average Value)
1. Color - Dark brown
2. Temperature oC 28 - 31
3. pH - 3.8 - 4.3
4. COD mg/l 9,873 - 15,048
5. BOD5 at 20oC mg/l 2,180 - 6,100
6. TSS mg/l 650 - 3,105
7. O&G mg/l 94 - 663
4.2 Jar Test Results and Discussion
Jar tests ware being carried out by the chemical supplier and the
recommended chemicals are:
i. pH Adjustment Chemical - Caustic (50%)
ii. Coagulant - Polyaluminum hydroxide
Chloride (15%)
iii. Flocculant - Polyacrylamine (100%)
67
Jar test results for the wastewater characteristics before chemical treatment
and after chemical treatment are shown in Table 4.4 as follows:
Table 4.4: Wastewater characteristics before and after chemical treatment:
Testing parameters Unit Influent
Characteristics
before treatment
Effluent
Characteristics
after treatment
1. pH - 3.9 4.5
2. COD mg/l 12,069 5,210
3. TSS mg/l 1,689 70
* The treated effluent characteristics are based on the optimum chemical
dosage as follows: - Caustic (50%) - 250 mg/l
- Polyaluminum hydroxide Chloride (15%) - 2000 mg/l
- Polyacrylamine (100%) - 20 mg/l
For the pH adjustment chemical and coagulant, it can be dosed directly by the
chemical dosing pump. But for flocculant, it appears in solids form and preparation
of flocculant solution is required. Polyacrylamine solution of concentration 0.3%
needs to be prepared from 100% polyacrylamine in solids form.
The chemical dosage calculation of the three types of chemicals is shown as
follows:
68
i. Caustic solution (50%) dosage calculation:
Caustic dosage = C x Q
1000
Where,
C = concentration of caustic to be maintained in the waste water
solution= 250 mg/l = 250 g/m3
Q = WWTP system flow rate = 360 m3/day = 15m3/hr
Caustic dosage = 250 g/m3 x 15m3/hr
1000
= 3.75 lit./hr
ii. Coagulant(Polyaluminum hydroxide Chloride solution (15%)) dosage
calculation:
Coagulant dosage = C x Q
1000
Where,
C = concentration of coagulant to be maintained in the waste water
solution = 2000 mg/l = 2000 g/m3
Q = WWTP system flow rate = 360 m3/day = 15m3/hr
Coagulant dosage = 2000 g/m3 x 15m3/hr
1000
= 30 lit./hr
69
iii. Flocculants (Polyacrylamine solution (0.3%)) dosage calculation:
Flocculant dosage = C x Q
1000
where,
- In here, preparation of 0.3% polyacrylamine solution needs to be
carried out. To maintain an amount of 20 mg of 100%
polyacrylamine in 1 liter of waste water solution, the
concentration of 0.3% polyacrylamine solution (C) to be
maintained in the waste water is as follows:
C = concentration of 0.3% polyacrylamine solution to be
maintained in the waste water solution = 20mg/l = 6667 mg/l
0.003
Q = WWTP system flow rate = 360 m3/day = 15m3/hr
Flocculant dosage = 6667 g/m3 x 15m3/hr
1000
= 100 lit./hr
From the calculations above, the amount of chemical dosage determined from
the Jar test is: - 3.75 lit./hr caustic solution (50%)
- 30 lit./hr Coagulant(Polyaluminum hydroxide Chloride solution
(15%))
- 100 lit./hr of Flocculants (Polyacrylamine solution (0.3%))
To prepare 0.3% polyacrylamine solution, the amount of 100%
polyacrylamine (in solids form) that needed to be dissolved and mixed with water is
calculated as follows:
70
Amounts added for 1 day = 20 g/m3 x 360 m3/d
1000
= 7.2 g/d of 100% polyacrylamine
(in solids form) into 1000 lit.
water
4.3 Process Performance Improvement Study on DAF
The main purpose of the process performance improvement study is to
determine the most optimum and effective DAF operating parameters and the study
was separated into 3 stages as follows:
i. Design calculation review to determine the area of optimization to be
focused in.
ii. Mechanical tuning adjustment study.
iii. Chemical tuning adjustment study.
4.3.1 Design Calculation Review
The original design calculation is shown in Appendix F. The design input of
the DAF system at the initial stage is based on 600m3/day of incoming wastewater
71
flow rate and 200mg/l TSS. And the design output from Appendix F can be
summarized as follows:
Recirculation rate = 3.92 m3/hr
Recirculation pump pressure = 3 bar
Required surface area = 5.36 m2
Saturation tube volume = 0.13 m3
Air volume required = 15,072 lit./day
Based on the above design input, the constructed DAF is having
characteristics as follows:
Recirculation pump specification = 10m3/hr @ 4 bar (Dry
mounted/ vertical multistage
pump)
Constructed DAF tank surface area = 6.0 m2
Constructed saturation tube volume = 0.15 m3
Air compressor capacity = 751 lit./min (2,703,600 l/day-
Single stage type)
Then, a simulation design calculation on the DAF based on the current
WWTP operating condition of 350m3/day incoming wastewater flow rate and 2,000
mg/l of TSS was carried out. The result of calculation is shown in Appendix G and it
is summarized as follows:
72
Recirculation rate = 22.86 m3/hr
Recirculation pump pressure = 3 bar
Required surface area = 6.93 m2
Saturation tube volume = 0.76 m3
Air volume required = 87,920 lit./day
When actual constructed DAF specification is compared with the
specification from the design simulation output based on currently assumed WWTP
operating condition, it shows that a lot of compartments of the DAF needed to be
modified or replaced. Based advice from the Chief Designer (Mr. Jeff Ong), the DAF
was operated first without doing any modifications because a lot of design
assumptions at the initial design stage is far higher and has a very high safety factor.
This is mainly due to lacking of wastewater during at the early design stage. As such,
a proper experiment cannot be carried out to determine the design assumptions and
the designer had to set a higher design assumption based on his own experiences.
To determine a proper design parameters, an experiment using a laboratory
flotation cell is normally needed to be carried out to be used to determine the A/S
ratio- “Volume of air to the mass of solids ratio” (Higbie, 1935), . In the design stage,
the designer assume the optimum A/S ratio to be 0.008 mL/mg, surface loading rate
of 90 L/m2/min and air saturation time of 120s. These data are gotten from the Joint-
Venture partner associate in Denmark who was previously involved in the design and
construction of DAF in coffee production factory’s WWTP and all these design
assumptions have a high safety factor.
73
4.3.2 Design Calculation Explanation
The design calculation shown in Appendix F and G is actually based on the
equation as follows:
Equation showing the relationship between A/S ratio and the solubility of air,
the operating pressure, and the concentration of solids for a system in which
there is a pressurized recycle flow(Tchobanoglous et al, 2003):
A / S = [1.3 sa ( f P - 1 )R ] / ( Q Sa ) (Equation 4.1)
Where,
A/S = air to solids ratio, mL (air)/ mg (solids)
sa = air solubility, mL/L
f = fraction of air dissolved at pressure P, usually 0.5
P = pressure, atm.
= (p + 101.35)/ 101.35 (SI Unit)
= (p + 14.7)/ 14.7 (US customary unit)
p = gage pressure, kPa (ib/in2 gage)
Sa = influent suspended solids, g/m3 (mg/l)
R = pressurized recycle, m3/d
Q = mixed-liquor flow, m3/d
For a better understanding of the formula, kindly please refer to Metcalf &
Eddy- Chapter 5. Besides that, additional information about the use of flotation for
treating oily wastewater can be found in Eckenfelder (2000).
74
4.3.3 Analysis on the Mechanical Tuning Adjustment Study
Before carrying out the mechanical and chemical tuning adjustment, advice
was sought from the Chief Designer who recommended us to maximize the
performance of the system by tuning adjustment of the recycle system pressure and
chemical dosage.
In the mechanical tuning adjustment, variation in the recycle system
operating pressure on the total suspended solids removal efficiency was studied and
the results were tabulated in Appendix C. A graph of TSS removal efficiency (%) vs.
recycle system operating pressure had been drawn and it is shown in figure 4.1 as
follows:
75
TSS Removal Efficiency (%) in relation to the Differences in Recycle System Operating Pressure(bar)
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
0 0.5 1 1.5 2 2.5 3 3.5
Recycle System Operating Pressure (bar)
TS
S R
em
ova
l Effi
cie
ncy
(%
)
.
Figure 4.1: TSS removal efficiency in relation to the difference in recycle system
pressure.
The graph trend show a remarkable increase in the TSS removal efficiency
when there is a substantial increment in the recycle system operating pressure from
1.8 bar to 2.6 bar. Nevertheless, the increments in TSS removal efficiency tend to be
stabilized from 3.0 bars to 3.2 bars.
When there is an increment in the recycle system operating pressure, it will
cause an increment in the quantity of air dissolved and the higher pressure would
also result in the release of finer air bubbles during the instantaneous release of
pressure. These two phenomenons are very important in producing a good separation
76
of solids from the water, and leads to a higher total suspended solids removal rate
(Ng at al, 1988).
Although it is known that the best TSS removal efficiency at the recycle
system can be obtained at an operating pressure of 3 bars, the recycle pump was still
operated at 2.6 bars only due to the fast clogging effects of the recycle pump. This
condition can actually be improved by installing a Y-strainer at the suction sides of
the pump or the system can be improved by installing filtration units just before the
recycled water flow into the pump.
Based on on-site observation, it was found that the air compressor capacity
was unable to cater for the DAF air requirement when there is a sudden air
requirement being acquired by the filter press during the “Air Blowing” cycle. As
such, DAF always faced “Carry Over” problem when the filters press “Air Blowing”
cycle is taking over. As such, it is strongly recommended that an additional air
compressor to be installed.
4.3.4 Analysis on the Chemical Tuning Adjustment Study
Jar tests show that the best floc formation is in the pH range of 4.3 to 4.8.
When this result was adopted in the actual plant operation, a very good floc can be
gotten. By using this information of pH, optimum coagulant and flocculant dosage
can be determined.
77
In the coagulant dosage study, the results ware tabulated in Appendix D and a
graph drawn by referring to these data. For the graph in Figure 4.2, it showed that an
optimum dosage of 2000mg/l to 2500mg/l of coagulant can help to achieve TSS
removal efficiency up to 93%.
TSS Removal Efficiency in Relation to Coagulant Dosage
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
0 500 1000 1500 2000 2500 3000 3500
Coagulant Dosage (mg/l)
To
tal S
usp
en
ded
So
lids
Re
mo
val E
ffic
ien
cy (
%)
.
Figure 4.2: TSS removal efficiency in relation to coagulant dosage
78
TSS Removal Efficiency in Relation to Flocculant Dosage
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
0 5 10 15 20 25 30 35
Polymer Dosage (mg/l)
To
tal S
usp
en
de
d S
olid
s R
em
ova
l Eff
icie
ncy
(%
)
.
Figure 4.3: TSS removal efficiency in relation to polymer dosage
Based on the data points plotted on the graph in Figure 4.2, it is very obvious
that stability in the TSS removal efficiency is achieved when the coagulant dosage
concentration reached 2,000 mg/l. The optimum dosage of the coagulant matches
with the Jar Test results. Nevertheless, the plotted data shows that there is a slight
decrease in the TSS removal rate when the coagulant concentration is more than
2,500 mg/l. This is because of the sudden increment of the dosage causing the charge
on the surface of the precipitate to become increasingly positive, and this change
tends to destabilize the component particles and make them disperse.
In the flocculant dosage study, the detailed results of the wastewater
characteristics and analysis results in relation to the polymer dosage is shown in
Appendix E. A graph based on the results in Appendix E had been drawn and is
79
shown in Figure 4.3. The same general trend that was observed in Figure 4.2 can also
be seen in Figure 4.3. It also shows that stability in TSS removal efficiency is
achieved when the concentration of flocculant reaches 24ppm. When this is
compared to the jar test results, it can be seen that 20 mg/l of polymer dosage is
needed to achieve the desired floc and TSS removal rate. It means that the actual
plant operation require a higher dosage of flocculant.
In the other aspect, it is strongly believed that the polymer dosage can be
reduced if the condition of the flocculation tank’s mixer rotational speed is improved.
This is because the mixer is having a very high rotational speed as it is operated with
a speed of 295rpm. This high rotational speed will break the floc and caused a higher
flocculant requirement. As such, the mixer is recommended to be modified to operate
with a rotational speed of 50-100rpm.
4.4 Performance Monitoring of DAF
Performance monitoring on DAF was carried out for a period of 14 weeks
and the results are recorded in Appendix A. The periods for the performance
monitoring study started from 13 December 2006 until 14 March 2007 and 12 sets of
results were obtained due to the shut-off of the plant during week no.1 and week no.8
of year 2007.
80
4.4.1 Analysis on the Wastewater Pollutant Removal Efficiency
For a better comparison of all the data, the average characteristics and
removal efficiency for pH, COD, BOD5, TSS and O&G will once again be
summarized in Table 4.5 as follows:
Table 4.5: Average characteristics and removal efficiency for pH, COD, BOD5,
TSS and O&G.
Testing
parameters
Unit Influent
Characteristics
before DAF
treatment
Effluent
Characteristics
after DAF
treatment
Average
Pollutant
Removal
Efficiency
(%)
1. pH - 4.1 4.5 -
2. COD mg/l 13,281 7,147 45.98
3. BOD5 mg/l 3,759 1,981 45.11
4. TSS mg/l 1,896 188 90.01
3. O&G mg/l 429 27 91.28
The results above showed that the DAF is performing very well for TSS and
O&G removal. However, it showed a very much lower COD and BOD5 removal
efficiency. This is mainly because the total colloidal particles and emulsified organic
pollutants which contribute to the BOD5 and COD are not high. As a result, removal
of high level of TSS and O&G did not contribute to a high percentage of COD and
BOD5 reduction. But what is interesting here is that the DAF is able to meet its
design specification of 85% - 95% TSS removal although the incoming wastewater is
having a higher pollutant concentration, as well as higher pollutant loading.
81
A simple explanation on the calculation of the of pollutant removal efficiency
is shown below:
Average TSS Removal Efficiency = Influent TSS – Effluent TSS x 100%
Influent TSS
= (1896-188) mg/l x 100%
1896mg/l
= 90.01%
In order to study the trend of the wastewater pollutant removal efficiency
more clearly, a plot of wastewater pollutant removal efficiency (%) vs. time is shown
in Figure 4.4.
82
Wastewater Pollutants Removal Efficiency (%) Vs. Time
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
13 Dec.2006
20 Dec.2006
26 Dec.2006
10 Jan.2007
17 Jan.2007
24 Jan.2007
31 Jan.2007
7 Feb.2007
14 Feb.2007
28 Feb.2007
7March.2007
14March.2007
1 2 3 4 5 6 7 8 9 10 11 12
Date
Po
lluta
nt R
em
ova
l Effi
cie
ncy
(%
)
.
COD Removal % BOD5 Removal % TSS Removal % O&G Removal %
Figure 4.4: Wastewater pollutant removal efficiency (%) vs. time for the 12 weeks
samples.
This graph shows clearly the trend of the pollutant removal efficiency (which
consists of COD removal efficiency, BOD5, TSS removal efficiency and O&G
removal efficiency) from 13 December 2007 until 14 March 2007. It shows that the
DAF has a high and constant TSS and O&G removal rates. However, it was rather
odd that low TSS and O&G removal efficiencies were detected separately on 24 Jan.
2007 and 28 Feb. 2007 respectively.
83
It is possible that this reduction in the removal efficiency is due an increment
in temperature, because it will reduce the air solubility. However, when the
maintenance logbook was reviewed, it was found that the major contributing factor is
the clogging of the recycle pump which gives rise to the carry over effects. But it is
rather odd that, there is only one parameter that is lower in reading. Normally, the 2
parameters of TSS and O&G should upset. Besides that, the COD and BOD5
readings on these 2 particular days were seen to be acceptable and did not show any
sign of DAF upset. This may be due to the uneven distribution of pollutants during
the early stage of pump clogging. A second reason is that it may due to the
differences in sampling points or sampling methods that gives rise to this scenario.
On the other hand, the line diagram of Figure 4.4 shows that the COD
removal efficiency is quite constant. But for BOD5, the removal efficiency is very
inconsistent. This might be due to the inconsistency in the biodegradability of the
different batch of wastewater during the analysis.
4.5 Comparative Performance Study between DAF and BF
Before the comparative performance study was carried out, a trial run on the
BF showed that a lower concentration of flocculant (20ppm) based on 1m3/hr of
wastewater flow rate is able to achieve an equal or better floc condition when
compared to DAF. Nevertheless, this comparative study was still based on the
coagulant dosage of 2000ppm and flocculant dosage of 24ppm (that is the chemical
dosage that was applied during the operation of DAF).
84
The results of the comparative performance study were tabulated in Appendix
E. For a better comparative purpose, the results were presented in a graph format and
it is shown in Figure 4.5.
COD, TSS and O&G Removl Efficiency After DAF and BF Treatment Vs. Date
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
14 Dec. 2006 15 Dec. 2006 16 Dec. 2006 1 Mar. 2007 2 Mar. 2007 3 Mar. 2007
1 2 3 4 5 6
Date
Rem
oval
Eff
icie
ncy
(%)
.
COD- After DAF Treatment - Removal Efficiency (%) COD- After BF Treatment - Removal Efficiency (%)
TSS- After DAF Treatment - Removal Efficiency (%) TSS- After BF Treatment - Removal Efficiency (%)
O&G- After DAF Treatment - Removal Efficiency (%) O&G- After BF Treatment - Removal Efficiency (%)
Figure 4.5: Wastewater pollutant removal efficiency (%) of DAF and BF vs. date
From Figure 4.5, the BF showed a higher COD and TSS removal efficiency
than the DAF when we compared the results on a daily basis. Overall, the BF had an
average COD and TSS removal efficiency of 49.6% and 93.9% respectively, and the
DAF had an average COD and TSS removal efficiency of 47.1% and 89.8%
respectively. In contrast, the DAF had a higher oil & grease removal rate than the BF.
85
On the whole, the DAF showed an average O&G removal efficiency of 91.9% and
the BF showed a value of 90.9%.
The high suspended solids removal rate of the BF is mainly due to the good
retaining and filtering effects of the 130-µm polyethylene filter utilized in the BF
than the air utilized in the DAF. It is believed that the higher COD removal
efficiency of BF is because of the contributory effects from the suspended solids to
COD is greater than the contributory effects from oil & grease to COD. As such, the
BF had a higher COD removal rate. On the other hand, the DAF had a better oil &
grease removal efficiency. This is mainly because of the original floating
characteristics of the oil & grease matching the designed concept of DAF that
pressurized dissolved air is utilized in DAF to lift up floc and oil and grease to the
surface of the flotation tank. As a result, it makes the removal by DAF even easier.
From the operation point of view, both the BF and DAF had their advantages
and disadvantages. For DAF, it normally requires extra doses of polyelectrolyte to
re-flocculate and strengthen the floc. Furthermore, a high degree of operator skill is
required to operate DAF. Theses two points above are not merely by observation; bur
had been confirmed by site personnel (Beaumont, 1994). As for the BF, additional
costs are required to replace the BF’s filter roller.
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
From the performance improvement study that was carried out on the DAF,
the DAF was finally able to be operated in the most optimum condition and success
to achieve the designed removal efficiency. A comparative performance between the
DAF and BF was also carried out. All the outcomes drawn out as conclusions with
respect to the objectives of the study are as follows:
i. In the process performance improvement study, the DAF design
calculation was reviewed. Further to this, mechanical and chemical
tuning study had been carried. The results form this study indicated that
the DAF is best to be operated with a recycling system operating
pressure range of 2.6 to 3.0 bars, and the most optimum operating
condition is 2.6 bars. In the chemical dosing parts, optimum pH is 4.3 to
4.8 and the optimum coagulant concentration should be maintained
accurately at 2000ppm. Overdosing of coagulant should be avoided to
prevent upset of the system. Meanwhile, optimum flocculant dosage is
24ppm.
87
ii. From the study, it was found that the current air compressor and
flocculation mixer is disrupting the performance of the DAF. Both
systems should be upgraded.
iii. During the performance monitoring study of the DAF, average COD,
BOD5, TSS and O&G removal efficiencies of 45.98%, 45.11%, 90.01%
and 91.28% can be achieved respectively. All these results showed that
the DAF is meeting its design specification of 85% - 95% TSS removal
rate.
iv. In this study, the BF showed a higher average COD and TSS removal
efficiencies up to 49.6% and 93.9% respectively. On the other hand, the
DAF was showed a better average O&G removal efficiency of 91.9%.
Some recommendations that can be suggested for future study is as follows:
i. The comparative performance study between the DAF and the BF
should be continued for a longer period, so that more results can be
gotten to confirm the findings. Besides that, other comparative studies in
the aspects of cost, operation and maintenance benefits and design
differences should be considered to be carried out concurrently.
ii. Secondly, the possible application of the BF system in treating the
coffee wastewater which is darker in color after the biological treatment
process of the WWTP should be explored further.
88
iii. Referring back to the treated water quality after the DAF, the current
plant design which utilize a secondary biological sequencing batch
reactor is still facing difficulty in knocking down the COD level. As
such, a study on the different process application in reducing pollutant
loads up to DOE Std. B should be carried out.
89
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