sand filtration - jeremiah schreindl portfolio

UNIVERSITY OF IDAHO

Sand Filtration CE 404 Group Research Project

Group 4

Scott Fraser

Alex Grover

Anibal Hidalgo

Jeremiah Schreindl

12/1/2011

1

Table of Contents Introduction .................................................................................................................................................. 2

Purpose ..................................................................................................................................................... 2

Objective ................................................................................................................................................... 2

Sand Filtration Description ........................................................................................................................... 3

Rapid Sand Filtration ................................................................................................................................. 3

Filtration Process .................................................................................................................................. 4

Commonness ........................................................................................................................................ 6

Regulations............................................................................................................................................ 6

Slow Sand Filtration .................................................................................................................................. 9

Filtration Process .................................................................................................................................. 9

Commonness ...................................................................................................................................... 10

Regulations.......................................................................................................................................... 10

Sand Filtration Alternatives ........................................................................................................................ 11

Traveling Bridge Filter ............................................................................................................................. 11

Process Description ............................................................................................................................. 11

Equipment ........................................................................................................................................... 11

Features .............................................................................................................................................. 12

Advantages .......................................................................................................................................... 12

Continuous Backwash Filter .................................................................................................................... 13


Equipment ........................................................................................................................................... 13

Features .............................................................................................................................................. 14

Advantages .......................................................................................................................................... 14

Greensand Filtration ............................................................................................................................... 15


Applications ......................................................................................................................................... 17

Advantages .......................................................................................................................................... 17

Disadvantages ..................................................................................................................................... 17

2

Introduction Sand filtration is the oldest method of water treatment technologies known and has been in continuous

use since 1876 (1) (2). It is still the chosen method for water filtration in most municipalities around the

world because it is simple, inexpensive and reliable. This method of filtration is sometimes used solely

by itself or in conjunction with other technologies such as disinfection, coagulation and flocculation for

treating drinking and wastewater. Sand filtration reduces bacteria, cloudiness, and organic levels,

reducing the need for disinfection and, consequently, the presence of disinfection by-products in the

finished water. It makes better use of the local materials available in the developing country. Correctly

designed, constructed, operated and maintained sand filters produce a very high quality effluent.

Purpose

The purpose of this report is to describe the processes of rapid sand, slow sand, traveling bridge,

continuous backwash, and green sand filtration technologies. The frequency of use and regulation

governing operation of rapid and slow sand are explained. The features and advantages of specific

products available for traveling bridge, continuous backwash, and greensand filters are included.

Objective The following report will help to better understand the processes and various types of sand filtration in

water treatment.

3

Sand Filtration Description

Rapid Sand Filtration Rapid sand filtration is a technology for water treatment that was developed in 1896. Rapid sand filters

were developed to replace slow sand filters, which did not filter at sufficient rates for growing

populations. Slow sand filtration is a water treatment process that uses lower flow rates and larger

areas of land than rapid sand filtration and is discussed in detail in the report below. By the 1920’s, rapid

sand filtration had become the predominant method for water filtration (3). Wastewater treatment

plants and water treatment plants use rapid sand filters in sequence with multiple treatment processes.

Figure 1

In Figure 1 above, an overview of the sequence for surface water treatment is depicted. The water is

first collected from the source, which could be a lake, river, etc., and then passed through a settling tank

to settle out any particulate matter that passed through the inlet screen. After the water flows through

the settling tank, chemicals called coagulants are added that help precipitate almost all of the organic

matter in the water. This precipitate matter is grown in size, or flocculated, in the flocculation tank. As

the water passes through the sedimentation tank, the flocculated particles that have grown large and

heavy are settled out and removed. Any organic matter that is still left in the water is filtered out in the

rapid sand filter. The last step in the process is disinfection (usually adding chlorine) to kill any

pathogens in the water, leaving the water clean and safe for drinking.

Rapid sand filters are used in ground water filtration as well, though there are some differences in the

sequence of processes. Ground water filtration typically does not include a preliminary settling tank

because of its naturally filtered nature and sometimes does not need disinfection. Another difference in

the treatment of ground water and surface water is that ground water contains undesirably high levels

of natural minerals more often than surface water, requiring special treatment.

When used in waste water treatment plants, rapid sand filters are placed after the secondary treatment

processes (i.e. primary sedimentation, aeration basin, secondary sedimentation, etc.) as “tertiary” or

“advanced” treatment. The purpose of this tertiary treatment is to lower the levels of soluble BOD

(Biological Oxygen Demand) in the effluent from the secondary treatment processes from approximately

25 mg/L down to 10 mg/L (4). Rapid sand filtration used for filtering ground and surface water as well as

municipal wastewater is discussed in detail below.

4

Filtration Process

Before the water is passed through the rapid sand filter for water and waste water treatment, salts of

aluminum or iron are added to stabilize the natural repulsion between particles in the water. The usual

dose of aluminum used is about 10-30 mg/L of water (5). This allows the pathogens and particles to stick

together and flocculate by means of van der Waal's forces (5) in order to be settled out in the

sedimentation basin and removed as sludge. For waste water, the coagulants are also sometimes added

to remove phosphorous and heavy metals. The water is then passed through the rapid sand filter as the

“polishing step” to strain out particulate matter that is larger than the pore sizes between the sand

grains (6). Matter that is smaller than these pores is adsorbed onto the sand grains (or other media

being used) (6). A diagram of this filtration process is shown in Figure 2 below.

Figure 2: Sand Filtration Mechanics (6)

Rapid sand filters are grouped into the two categories of gravity filters and pressure filters with the

difference being that gravity filters rely on gravity while pressure filters use a pump to force water

through the filter (7). The filters are constructed of a rectangular basin that typically uses multiple levels

of media. In figure 3 below, the different parts of a rapid sand filter are shown. The water flows over the

wash troughs and onto the sand media which is 0.5 to 0.75 m deep at a rate of 120 m/d to 235 m/d (4).

Directly below the sand is a layer of graded gravel that helps prevent the sand from escaping through

the underdrains.

http://en.wikipedia.org/wiki/Aluminium

http://en.wikipedia.org/wiki/Iron

5

Figure 3: Rapid Sand Filter Cutaway (8)

The particulate matter being filtered out eventually clogs the filter, increasing head losses. The rapid

sand filter box must be designed to be at least as deep as the highest design head loss or the water will

not flow through. The theory for head loss through the filter is based off hydraulics and fluid mechanics

and is calculated using the given formula. (4)

Head Loss through Clean Sand Filter

(

)( )

( )( )( )∑

( )( )

(4)

= head loss, m

= approach velocity, m/s

= depth of filter sand, m

= drag coefficient

= mass fraction of sand particles w/diameter “d”

= diameter of sand grains, m

= shape factor

= gravity, m/s^2

ε = porosity

Once the maximum head loss is reached, the filter is cleaned by backwashing which is performed by

reversing the flow of water at high rates to fluidize the sand bed (1). The fluidized sand particles knock

together and “scrub” the particulate matter off of each other.

6

The process of coagulation and rapid sand filtration removes 98-99% bacteria in the water, which

necessitates that chlorine or ozone be injected into the water to kill the remaining bacteria (9).

Commonness

Because of the small land areas needed for rapid sand filters and the high flow rates that they can

process, rapid sand filters are far more popular than slow sand filters and have become the most

common method of filtration for water and for certain “tertiary” treatments of waste water worldwide

(8) (10).

Regulations

Regulations for drinking and wastewater vary between different states and the federal government.

Some states such as Washington have their own state EPA to provide regulations for water treatment,

while other states like Idaho depend on the federal EPA to provide those regulations. The federal EPA

sets maximum contaminant levels (MCL) for indicators and pathogens in drinking water; when it is too

difficult to measure contaminants at very low concentrations, treatment technique are prescribed

instead of an MCL. Both treatment techniques and MCL can be enforced. The quality of the effluent, in

regards to EPA regulations, from rapid sand filters determines the need for disinfection. Federal EPA

regulations and treatment techniques for drinking water are shown below.

Indicators

Turbidity

“Surface Water Treatment Rule requirements:

EPA’s surface water treatment rules require systems using surface water or GWUDI to (1) disinfect their water, and (2) filter their water or meet criteria for avoiding filtration so that:

Surface water systems and GWUDI systems that use conventional and direct filtration: At no time can turbidity (cloudiness of water) be higher than one nephelometric turbidity unit (NTU); samples for turbidity must be less than 0.3 NTU in at least 95 percent of samples in any month.” (11)

Total coliforms

“Total Coliform Rule requirements:

Systems are required to take samples for total coliforms based on the population served, source type

and vulnerability to contamination. No more than 5.0 percent of samples for total coliforms can be

positive in one month. (For systems that collect fewer than 40 routine samples per month, no more than

one sample can be total coliform-positive per month). If a sample tests positive for total coliforms, the

system must collect a set of repeat samples within 24 hours, and also analyze for fecal coliform or E.

coli.” (11)

7

Fecal Coliforms and E. coli

“Total Coliform Rule requirements:

A routine sample that tests positive for fecal coliform or E. coli triggers repeat samples. If any repeat sample tests positive for total coliform, the system has an acute MCL violation.

A routine sample that tests positive for total coliform but tests negative for fecal coliform or E. coli triggers repeat samples. If any repeat sample then tests positive for fecal coliform or E. coli, the system has an acute MCL violation.” (11)

Fecal indicators (Enterococci or coliphage), and E. coli

“Ground Water Rule:

Public water systems that use ground water* must take corrective action if a sufficient deficiency is identified, or if the initial source sample (if required by the state) or one of the five additional ground water source samples tests positive for fecal contamination (E. coli, Enterococci, or coliphage). The systems must implement at least one of the following corrective actions:

Correct all significant deficiencies Provide an alternate source of water Eliminate the source of contamination Provide treatment that reliably achieves at least 4-log treatment of viruses (using inactivation,

removal, or a state-approved combination of 4-log virus inactivation and removal) before or at the first customer for the ground water source.” (11)

Pathogens

Cryptosporidium


Systems using surface water or ground water under the direct influence of surface water (GWUDI) must

disinfect and filter their water so that 99 percent of Cryptosporidium oocysts are removed or inactivated

(killed). Unfiltered systems (systems that meet criteria for avoiding filtration) are required to include

Cryptosporidium in their existing watershed control provisions.” (11)

Giardia lamblia


Systems using surface water or GWUDI must disinfect and filter their water so that 99.9 percent of

Giardia lamblia is removed or inactivated. Unfiltered systems (systems that meet criteria for avoiding

filtration) are also required to include Giardia lamblia in their existing watershed control provisions.”

(11)

8

Viruses


Systems using surface water or GWUDI must disinfect and filter their water so that 99.99 percent of

viruses are removed or inactivated.” (11)

Legionella


Systems using surface water or GWUDI must (1) disinfect their water, and (2) filter their water or meet

criteria for avoiding filtration.

There is no limit specific to Legionella, but EPA believes that if Giardia lamblia and viruses are

removed/inactivated according to the treatment techniques in the surface water treatment rules,

Legionella will be controlled.” (11)

9

Slow Sand Filtration Slow sand filtration was developed in the 1800s and is one of the oldest water treatment technologies.

This technology filters drinking water through fine-grained sands to remove pathogens and reduce

turbidity. Slow sand filters are technologically simple and require relatively little operational

supervision, making them effective for treating drinking water in rural or developing regions. The down

side to using slow sand filters is that they require manual labor for cleaning and large surface areas of

sand to improve loading rates (4).

Slow sand filters are usually the last step in the water treatment facility. The pre-treatment process is

similar to rapid sand pre-treatment but neglects coagulation and flocculation, as they can cause the

filters to clog prematurely (1).Multiple filter beds are installed in water treatment plants allow the plant

to operate when a filter bed is offline or being cleaned (16).

Filtration Process

While the design of slow sand filters can vary, most consist of fine-grained sand in a large filter tank or

basin. The filter sands are generally about a meter deep and have relatively larger horizontal

dimensions. This creates large surface areas to achieve higher loading rates. As water flows through the

sand, colloidal particles and pathogens in the water become trapped in pore spaces or stuck to the sand

particles.

Figure 4 (17)

Figure 1 above illustrates a basic slow sand filter and its operation. Water enters the filter tank and

begins filtering through the fine sands. Bacteria and other organisms build up in the top few inches of

sand. These bacteria consume harmful chemicals and pathogens as the water percolates through the

sand. This biological sand layer is called the “Schmutzdecke” layer and is responsible for biological

treatment the filter provides. Over time, the Schmutzdecke layer becomes saturated with sediment and

pathogens and must be scraped off. New sand is laid in its place for the next Schmutzdecke layer to

develop. This cleaning process is much simpler than that in rapid sand filters but requires manual labor

to remove the sand (4). A gravel layer prevents the filtration sands from washing out of the tank. The

treated water flows out of the tank through underdrains (17).

Slow sand loading rates vary from about 2.9 to 7.6 m/day. This is much lower than rapid sands, which

flow at 120 to 235 m/day (4). This difference in loading rates demonstrates the need for large surface

areas of sand to increase flow rates in slow sand filters.

10

Commonness

Slow sand filters were more common in the past than they are today, but they are still very useful in

rural areas and occasionally in developed areas. In 1991, 18 slow sand filters were used in Idaho water

treatment plants, significantly more than any other state (18). They were used in Western Europe until

recently and are still used in London (19). A deterrent from using slow sand filters is the large surface

areas of sand required to achieve faster flow rates. With land prices rising in populated regions, slow

sand filters are becoming less cost-effective. Rapid sand filters coupled with an alternative pathogen

treatment technology are often used in place of slow sand filters (4). Another deterrent from using slow

sand filters is their susceptibility to becoming clogged with fine particles when high turbidity water is

flowing into them (20).

Regulations

EPA regulations that apply to slow sand filtration are explained below and apply to all public water

systems that process surface water or groundwater that is influenced by surface water (21). These

regulations apply in the United States unless a state has regulations that take precedence over the EPA

regulations. Washington is an example of such a case, in which the Office of Drinking Water has primacy

over the EPA (22).

Turbidity

EPA regulations state that turbidity measurements must be taken every four hours and must be less

than or equal to 1 NTU in at least 95 percent of measurements over a monthly period. Turbidity must

never be greater than 5 NTU for individual measurements (21). Water flowing into the filter must not

exceed 10 NTU or the filter sands may become prematurely saturated with fine particles (20).

Pathogens

EPA regulations require 99.99% removal/inactivation of viruses, 99.9% removal/inactivation of Giardia

lambia, and 99% removal/inactivation of Cryptosporidium (21).

11

Sand Filtration Alternatives

Traveling Bridge Filter Traveling bridge filters are a type of rapid sand filter that has multiple filter cells containing sand or

media. The benefits of traveling bridge filters are that they clean individual cells, allowing water to be

continually filtered compared to one compartment rapid sand filters that are out of service during the

backwash cycle (24).

Process Description

Traveling bridge filters are gravity filters that have a hood that travels along a track system above each

cell, backwashing one at a time. Backwashing can be triggered by the filter reaching a specific head loss

which is measured by water level probes or set on a timer basis. Some traveling bridge filters have

enhancements to help with the backwashing such as air scour or hydraulic spray jet. A scarifier blade is

sometimes used to plow or loosen up the solids mat on top of the media that results from particulate

matter clogging the sand (24). These systems are typically designed to operate at a normal loading rate

of 117 m/d or a peak loading of 293 m/d. They are designed to manage a solids loading rate of about 30

mg/L with a peak loading rate of 50 mg/L. One of the negatives of using this system is the need for

maintenance and repair due to the highly complicated machinery needed to operate the backwashing

mechanism. (25)

Equipment

AQUA-AEROBICS SYSTEMS, INC. markets a model of traveling bridge filter called the AquaABF®

Automatic Backwash Filter. A diagram of this model is shown below.

Figure 5 (27)

12

Features

Simple design and high quality corrosion resistant materials insure reliability and durability

Low maintenance. All drives, pumps, motors and bearings are located for easy access

Adaptable design utilizes cast-in-place concrete or steel package tanks.

Single media sand, dual media sand and coal, or granular activated carbon

Fully automatic. Minimal operation attention required

Standard widths of 6, 9, 12.5 and 16 ft. for site adaptability and cost efficiency

High efficiency filtration. Typical 79-90% SS reduction in domestic sewage

Advantages

General

o Continuous filtration, even during backwash mode

o No backwash or washwater holding tanks required

o As little as 6" of headloss through filter

o Low terminal head losses will not force solids into media

o Continuous static head above media prevents air entrapment under the porous plate

o Surface mat filtration – no mudball formation

o Only 1-2% of daily flow is normally required for backwash

o No pipe galleries or air bowers mean less maintenance

o Ideal for both municipal and industrial water filtration systems

Municipal Water

o Turbidity reduction

o Color reduction

o Organics removal

o Iron and mineral removal

o Taste and odor control

Municipal Wastewater

o Suspended solids reduction

o BOD, COD, TOC reduction

o Reuse

o Wastewater odor control

Industrial Water or Wastewater

o Solids removal

o Turbidity reduction of process makeup water

o Chemical precipitate removal

o Toxic chemical reduction

o Suspended solids, BOD, COD, TOC reduction (6) (9)

13

Continuous Backwash Filter Continuous backwash filters are up-flow, moving bed filters. They filter drinking water through sand

while simultaneously cleaning the filter sands and returning them to the filter bed. They have

advantages over rapid sand filters because they do not have to shut down for regular backwash

cleaning. They can be used for many applications including tertiary water treatment filtration,

phosphorus removal, algae removal, oil removal, metal finishing, and steel mills. Loading rates range

from about 175 to 300 m/d depending on application and can be as high as 580 m/d in steel mills (28).

Process Description

A diagram of a continuous backwash filter is shown below in Figure 6. The influent water is piped from

the top of the filter down to the base, where it is distributed evenly in the sand by multiple arms. The

water is under pressure and therefore filters upward through the sand. Once the filtered water reaches

the top of the tank, it is dispelled over the reject weir. While the water is filtering, the sand column is

slowly moving downward. When it reaches the bottom of the tank, an air pump blows the

contaminated sand up to a tray near the top of the tank, cleaning the sand in the process. The organic

contaminants are expelled into the air and the sand falls back into the filter tank (29).

Equipment

PARKSON markets a model of continuous backwash called the DynaSand®Filter. The model diagram is

shown in Figure 6 below.

A-Feed

B-Feed assembly

C-Distribution

D-Sand bed

E-Filtrate

F-Airlift pump

G-Airlift discharge

H-Reject compartment

I-Washer section

J-Filtrate weir

K-Reject weir

L-Reject line

Figure 6 (29)

14

Features

Sand bed is continuously cleaned

No underdrains or screens required

Sand is washed with filtrate Internal sand air lift

Low power requirements

Advantages

Filter does not shut down for backwashing

Up to 70% less compressed air than competitors

Reduced wear/maintenance

Minimized pressure drop

Reduced risk of plugging (28)

15

Greensand Filtration Greensand filtration devices are a newer technology that has only been around since the 1950’s. This

method of filtration is used to reduce and remove mineral impurities (mainly iron and manganese) from

drinking water. Trained workers are required to operate this type of filter due to its complexity and its

use of chemicals that must be mixed correctly based on the water entering the plant.

Process Description

This filtration method can remove pathogens and other substances found in the water to be treated (1),

but it is most efficient at removing the iron and manganese (31). There are two different ways in which

greensand filtration can operate, which are continuous regeneration and intermittent regeneration.

Both of these processes have the same end effect of removing amounts of sulfur, iron and manganese

from drinking water (31). The only real difference between Continuous Regeneration and Intermittent

Regeneration is the method of adding the chemicals to the water.

In Continuous Regeneration, the chemicals are continuously added to the water flowing into the filter,

allowing the chemicals to have their full effect on the wastewater impurities before entrance to the

filter (31). This is used to target more iron than manganese or sulfur.

Intermittent Regeneration is utilized when the objective is to remove more amounts of manganese. This

process uses oxidation of manganese, which then precipitates and catches to the sand and filter media

(31). The sand must be regenerated after a certain amount of water has been treated. The regeneration

process consists of backwashing the greensand in a diluted solution of potassium permanganate for a

few hours.

These two processes have the same loading rates, which depends on the concentration of iron and

manganese that are in the effluent as it enters the filter. When there are higher concentrations in the

effluent, loading rates are around 88 m/d. When concentrations are average or lower loading rates are

allowed to be around 175 m/d. The loading rate for backwash/ regeneration of the filter is 705 m/d

(.489 m/min)(31).

Both of these processes use a sand filter system that is similar a rapid sand filter. The only major

differences are that they use greensand, have extra piping to add the chemicals to the water and to the

sand and are pressurized throughout the process, as seen in Figure 7. With both processes, the

chemicals react with and oxidize the impurities and then they precipitate and stick to the greensand

(31). Continuous and Intermittent Regeneration systems are similar in design to the system in Figure 7.

16

Figure 7

17

Applications

Individual House Hold Filtration Water Treatment Facility Filtration

Figure 8: Household Greensand Filter

The application of these for household greensand filters (as seen in Figure 8) usage is practical and

useful. They are not excessively expensive, in the range from $500 to $2400, and they are user friendly.

Most are already set up to produce a certain amount of cleaned water per a given time. Other more

expensive greensand filters need to be professionally installed and can be adjusted to suit water

consumption needs. In both cases there is a maximum water production limit due to the amount of area

in which the water is passing through.

Advantages

Very effective at removing Iron and Manganese

Easy to clean, since cleaning is done through backwash system

Creates clean water that is storable in tanks

Disadvantages

For Water Treatment Plants greensand filters can take up a large amount of land depending on the amount of water needed to be filtered

Continual upkeep cost due to the uses of chemicals

Owner filter will need knowledge about greensand systems because of chemicals

Should have labs test samples of water every so often

18

Works Cited

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19

17. Slow Sand Filter. The Water Treatments. [Online] 2011. [Cited: November 27, 2011.]

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treatment-filtration.

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nd.pdf.

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[Online] December 17, 2003. [Cited: November 3, 2011.]

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Continuous-backwash-filter-for-sidestream-filtration.pdf.

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30. Bhardwaj, Vipin and Mirliss, Mel J. Diatomaceous Earth Filtration for Drinking Water. National

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http://www.nesc.wvu.edu/pdf/dw/publications/ontap/2009_tb/diatomaceous_dwfsom39.pdf.

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