Lesson 6_ Filtration

8/21/2019 Lesson 6_ Filtration

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7/11/2014 Lesson 6: Filtration

http://water.me.vccs.edu/courses/ENV110/Lesson6_print.htm

Lesson 6:

Filtration

Objective

In this lesson we will answer the following questions:

How does filtration fit into the water treatment process?

How does filtration clean water?

What types of filters are used for water treatment?

How are filters cleaned?

What media are used in filters?

What factors affect filter efficiency?

Reading Assignment

Along with the online lesson, read Chapter 6: Filtration, in your textbook Operation of Water

Treatment Plants Volume I .

Lecture

Introduction to Filtration

Purpose

The purpose of filtration is to remove suspended particles from water by passing the water through

a medium such as sand. As the water passes through the filter, floc and impurities get stuck in the

sand and the clean water goes through. The filtered water collects in the clearwell, where it isdisinfected and then sent to the customers.

Filtration is usually the final step in the solids removal process which began with coagulation and

advanced through flocculation and sedimentation. In the filter, up to 99.5% of the suspended solids

in the water can be removed, including minerals, floc, and microorganisms.


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http://water.me.vccs.edu/courses/ENV110/Lesson6_print.htm 2

Requirements

Filtration is now required for most water treatment systems. Filters must reduce turbidity to less

than 0.5 NTU in 95% of each month's measurements and the finished water turbidity must never

exceed 5 NTU in any sample.

As you will recall, turbidity alone does not have health implications. So, why the strict regulations?

Although turbidity is not harmful on its own, turbid water is difficult to disinfect for a variety ofreasons. Microorganisms growing on the suspended particles may be hard to kill using disinfection

while the particles themselves may chemically react with chlorine, making it difficult to maintain a

chlorine residual in the distribution system. Turbidity can also cause deposits in the distribution

system that create tastes, odors, and bacterial growths.

However, turbid drinking water has other troublesome implications as well. Sand filtration removes

some cyst-forming microorganisms, such as Giardiawhich cannot be killed by traditional

chlorination. Cystsare resistant covers which protect the microorganism while it goes into an

inactive state.

Regulations require that at least 99.9% of Giardiacysts and 99.99% of viruses be removed fromdrinking water. Since it is difficult to test directly for these microorganisms, turbidity in water can

be used as an indicator for their presence. By requiring a low turbidity in the finished water,

treatment plants are ensuring that few or no Giardiaare present in finished drinking water.

In a few locations, surface waters are used for domestic purposes without filtration. In these

situations, the water is obtained from a watershed which includes only undeveloped areas. The

watershed is patrolled and carefully managed to prevent contamination.

Location in the Treatment Process

In the typical treatment process, filtration follows sedimentation (if present) and precedes

disinfection. Depending on the presence of flocculation and sedimentation, treatment processes are

divided into three groups - conventional filtration, direct filtration, and in-line filtration.

The most common method of filtration is conventional filtration, where filtration follows


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coagulation/flocculation and sedimentation. This type of filtration results in flexible and reliable

performance, especially when treating variable or very turbid source water.

Some treatment plants operate without some or all of the sediment removal processes which

precede filtration. If filtration follows coagulation and flocculation, without sedimentation, it is

known as direct filtration. This method can be used when raw water has low turbidity.

Another type of filtration, known as in-line filtration, involves operating the filters withoutflocculation or sedimentation. A coagulant chemical is added to the water just before filtration and

coagulation occurs in the filter. In-line filtration is often used with pressure filters, but is not as

efficient with variable turbidity and bacteria levels as conventional filtration is.

Polymer Aids

Although filtration does not require the addition of any chemicals, polymer aidsmay sometimes be

added to the influent water. These chemicals improve the quality of the effluent water by helping

the floc get caught in the filter.

Polymer aids come in two main types. Moderate molecular weight cationic polymers (DADMA)

are added ahead of flocculation to strengthen the floc while relatively high molecular weight nonionic

polymers (polyacrylamides) are added just before filtration to aid in floc removal.

Polymer aids can be troublesome in some respects. The powdered form of the polymer is very

slippery, so spills should be cleaned up quickly. In addition, extended use of polymer aids maygum up the filters. As a result, polymer aids are often used like coagulant aids - in extreme

situations to improve the water quality for a short time.

Mechanisms of Filtration

Introduction

How are particles removed from water using filtration? Four mechanisms have been found to be

part of the filtration process - straining, adsorption, biological action, and absorption. Each

mechanism will be explained below.


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Straining

passing the water through a filter in which the pores are smaller than the particles to be removed.

This is the most intuitive mechanism of filtration, and one which you probably use in your daily life.

Straining occurs when you remove spaghetti from water by pouring the water and spaghetti into a

strainer.

The picture below shows an example of straining in a filter. As you can see, the floc cannot fitthrough the gaps between the sand particles, so the floc are captured. The water is able to flow

through the sand, leaving the floc particles behind.

In the past, straining has been assumed to be very important in the filtration process. However, in

many cases, the pores between sand particles in the filter are much larger than the particles

captured by the filter. It has been suggested that small particles become wedged between sand

grains as filtration occurs, making the pore spaces smaller and allowing the filter to strain out yet

smaller particles. However, a clean filter will produce clean water before any of this pore size-

reduction has occurred. Therefore, it is now believed that straining is not an important part of most

filtration processes.

Adsorption

The second, and in many cases the most important mechanism of filtration, is adsorption.

Adsorptionis the gathering of gas, liquid, or dissolved solids onto the surface of another material,

as shown below:


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Coagulation takes advantage of the mechanism of adsorption when small floc particles are pulledtogether by van der Waal's forces. In filtration, adsorption involves particles becoming attracted to

and "sticking" to the sand particles. Adsorption can remove even very small particles from water.

Biological Action

The third mechanism of filtration is biological action, which involves any sort of breakdown of theparticles in water by biological processes. This may involve decomposition of organic particles by

algae, plankton, diatoms, and bacteria or it may involve microorganisms eating each other.

Although biological action is an important part of filtration in slow sand filters, in most other filters

the water passes through the filter too quickly for much biological action to occur.

Absorption

The final mechanism of filtration is absorption, the soaking up of one substance into the body of

another substance. Absorption should be a very familiar concept - sponges absorb water, as do

towels.

In a filter, absorption involves liquids being soaked up into the sand grains, as shown below:

After the initial wetting of the sand, absorption is not very important in the filtration process.


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Types of Filters

Introduction

Filters can be categorized in a variety of ways. The table below shows the characteristics of four

types of filters which can be used in water treatment.

S low S an d Fi lte r Rapi d S an d Fi lte r Pre ssu re Fi lte r Di atom ace ou s e arth fi lte r

(Diatomite filter)

Filtration

rate

(GPM/ft2)

0.015-0.15 2-3 2-3 1-2

Pros Reliable. Minimum

operation and maintenance

requirements. Usually does

not require chemical

pret reatment .

Relatively small and

compact.

Lower installat ion

and operation costs

in small filtration

plant s.

Small size. Efficiency. Ease of

operat ion. Relatively low cost.

Produces high clarity water. Usually

does not require chemical

pret reatment.

Cons Large land area required.Need to manually clean

filters.

Requires chemicalpret reatment .

Doesn't remove

pat hogens as well as

slow sand filters.

Less reliable thangravity filters.

Filter bed cannot be

observed during

operation.

Sludge disposal problems. High headloss. Pot ential decreased reliability.

High maint enance and repair costs.

Filter Media Sand. Sand. Or sand and

anthracite coal. Or

sand and anthracite

coal and garnet .

Sand. Or sand and

anthracite coal. Or

sand and anthracite

coal and garnet .

Diatomaceous earth.

Gravity or

Pressure?

Gravity. Gravity. Pressure. Pressure, gravity, or vacuum.

Filtration

Mechanism

Biological action , straining,

and adsorption .

Primarily

adsorption. Alsosome straining.

Primarily

adsorption. Alsosome straining.

Primarily straining.

Cleaning

Method

Manually removing the top

2 inches of sand.

Backwashing. Backwashing. Backwashing.

Common

Applications

Small groundwater systems. Most commonly used

type of filter for

surface water

treatment.

Iron and manganese

removal in small

groundwater

systems.

Beverage and food industries and

swimming poo ls. Smaller systems.

We will discuss two types of filters below - the slow sand filter and the rapid sand filter. The

pressure sand filter is essentially a rapid sand filter placed inside a pressurized chamber while thediatomaceous earth filter is not commonly used in treatment of drinking water.

History


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The history of water treatment dates back to approximately the thirteenth century B.C. in Egypt.

However, modern filtration began much later. John Gibb's slow sand filter, built in 1804 in

Scotland, was the first filter used for treating potable water in large quantities. Slow sand filters

spread rapidly, with the first one in the United States built in Richmond, VA, in 1832. A set of slow

sand filters adapted from English designs was built in 1870 in Poughkeepsie, NY, and is still in

operation.

A few decades after the first slow sand filters were built in the U.S., the first rapid sand filters wereinstalled. The advent of rapid sand filtration is linked to the discovery of coagulation. By adding

certain chemicals (coagulants) to turbid water, the material in the water could be made to clump

together and quickly settle out. Using coagulation, clear water for filtration could be produced from

turbid, polluted streams.

By the end of the nineteenth century, there were ten times as many rapid sand filters in service as

the slow sand type. Currently, slow sand filtration is only considered economical in unusual cases.

The diatomaceous earth filter was developed by the U.S. Army during WWII. They needed a filter

that was easily transportable, lightweight, and able to produce pure drinking water. The

diatomaceous earth filter is used in smaller systems, but is not commonly part of water treatment

plants.

Slow Sand Filter

The slow sand filter is the oldest type of large-scale filter. In the slow sand filter, water passes first

through about 36 inches of sand, then through a layer of gravel, before entering the underdrain. The

sand removes particles from the water through adsorption and straining.

Unlike other filters, slow sand filters also remove a great deal of turbidity from water using

biological action. A layer of dirt, debris, and microorganisms builds up on the top of the sand. This

layer is known as schmutzdecke, which is German for "dirty skin." The schmutzdecke breaks

down organic particles in the water biologically, and is also very effective in straining out even very

small inorganic particles from water.


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Maintenance of a slow sand filter consists of raking the sand periodically and cleaning the filter by

removing the top two inches of sand from the filter surface. After a few cleanings, new sand must

be added to replace the removed sand.

Cleaning the filter removes the schmutzdecke layer, without which the filter does not produce

potable water. After a cleaning the filter must be operated for two weeks, with the filtered water

sent to waste, to allow the schmutzdecke layer to rebuild. As a result, a treatment plant must havetwo slow sand filters for continuous operation.

Slow sand filters are very reliable filters which do not usually require coagulation/flocculation before

filtration. However, water passes through the slow sand filter very slowly, and the rate is slowed

yet further by the schmutzdecke layer. As a result, large land areas must be devoted to filters when

slow sand filters are part of a treatment plant. Only a few slow sand filters are operating in the

United States although this type of filter is more widely used in Europe.

Number of slow sand filters operating in each s tate as o f 1991. (Sims)

Rapid Sand Filter

The rapid sand filter differs from the slow sand filter in a variety of ways, the most important of

which are the much greater filtration rate and the ability to clean automatically using backwashing.The mechanism of particle removal also differs in the two types of filters - rapid sand filters do not

use biological filtration and depend primarily on adsorption and some straining.

Since rapid sand filters are the primary filtration type used in water treatment in the United States,

we will discuss this filter in more detail.


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A diagram of a typical rapid sand filter is shown above. The filter is contained within a filter box,

usually made of concrete. Inside the filter box are layers of filter media(sand, anthracite, etc.)

and gravel. Below the gravel, a network of pipes makes up the underdrainwhich collects the

filtered water and evenly distributes the backwash water. Backwash troughshelp distribute the

influent water and are also used in backwashing (which will be discussed in a later section.)

In addition to the parts mentioned above, most rapid sand filters contain a controller, or filter

control system, which regulates flow rates of water through the filter. Other parts, such as valves,

a loss of head gauge, surface washers, and a backwash pump, are used while cleaning the filter.

Operation of a rapid sand filter during filtration is similar to operation of a slow sand filter. The

influent flows down through the sand and support gravel and is captured by the underdrain.

However, the influent water in a rapid sand filter is already relatively clear due to

coagulation/flocculation and sedimentation, so rapid sand filters operate much more quickly than

slow sand filters.


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The rest of this lesson will be concerned primarily with rapid sand filters, though many of the factors

discussed can carry over to other filter types.

Filter Cleaning

When to Backwash

Rapid sand filters, pressure filters, and diatomaceous earth filters can all be backwashed. During

backwashing, the flow of water through the filter is reversed, cleaning out trapped particles.

Three factors can be used to assess when a filter needs backwashing. Some plants use the length

of the filter run, arbitrarily scheduling backwashing after 72 hours or some other length of filter

operation. Other plants monitor turbidity of the effluent water and head loss within the filter to

determine when the filter is clogged enough to need cleaning.

Head lossis a loss of pressure (also known as head) by water flowing through the filter. When

water flows through a clogged filter, friction causes the water to lose energy, so that the water

leaving the filter is under less pressure than the water entering the filter. Head loss is displayed on a

head loss gauge. Once the head loss within the filter has reached between six and ten hours, a filter

should be backwashed.

The Process of Backwashing

In order to backwash a filter, the influent valve is closed and a waste line is opened. A backwash

pump or tower forces treated water from the system back up through the filter bed. The dirty

backwash water is collected by the wash troughs and can be recycled to the beginning of the plant

or can be allowed to settle in a tank, pond, or basin.


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Backwashing should begin slowly. If begun too quickly, backwash water can damage the

underdrain system, gravel bed, and media due to the speed of the water. Beginning backwashing

too quickly will also force air bound in the filter out, further damaging the filter.

After a slow start, the backwash rate should be accelerated to reach around 10 to 25 gpm/ft.2 The

backwash water must have enough velocity and volume to agitate the sand and carry away the

foreign matter which has collected there.

Backwashing normally takes about 10 minutes, though the time varies depending on the length of

the filter run and the quantity of material to be removed. Filters should be backwashed until the

backwash water is clean.

Surface Washing

At the same time as backwashing is occurring, the surface of the filter should be additionally

scoured using surface washers. Surface washersspray water over the sand at the top of the filter

breaking down mudballs.

Filter Media

Introduction

The filter media is the part of the filter which actually removes the particles from the water being

treated. Filter media is most commonly sand, though other types of media can be used, usually in

combination with sand. The gravel at the bottom of the filter is not part of the filter media, merely

providing a support between the underdrains and the media and allowing an even flow of water


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during filtering and backwashing.

The sand used in rapid sand filters is coarser (larger) than the sand used in slow sand filters. This

larger sand has larger pores which do not fill as quickly with particles out of the water. Coarse

sand also costs less and is more readily available than the finer sand used in slow sand filtration.

Dual and Multi-Media Filters

In many cases, multiple types of media are layered within the filter. Typically, the layers (starting at

the bottom of the filter and advancing upward) are sand and anthracite coal, or garnet, sand, and

anthracite coal. The picture below shows a cross-section through a dual media filter.

Photo Cre dit: Christie Shinault

The media in a dual or multi-media filter are arranged so that the water moves through media with

progressively smaller pores. The largest particles are strained out by the anthracite. Then the sand

and garnet trap the rest of the particulate matter though a combination of adhesion and straining.

Since the particles in the water are filtered out at various depths in a dual or multi-media filter, the

filter does not clog as quickly as if all of the particles were all caught by the top layer.


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The largest particles are removed by the coal , the medium particles by the sand,

and the smallest particles by the garnet.

The media in a dual or multi-media filter must have varying density as well as varying pore size so

that they will sort back into the correct layering arrangement after backwashing. Anthracite coal is

a very light (low density) coal which will settle slowly, ending up as the top layer of the filter.

Garnet is a very dense sand which will settle quickly to the bottom of the filter.

Filter Efficiency

Monitoring

The filter efficiency can be measured in a variety of ways. Effluent turbidity, which should bemonitored continuously, gives an indication of the efficacy of the filtration process. Particle

counterscan be used to count the number of particles in the effluent which are within the size range

of Giardia and Cryptosporidiumto determine how efficiently the filter has removed these

microorganisms.

The length of the run time between backwashing can also be used as a measure of filter efficiency.

Filter run time depends largely on the clarity of the water passing through the filter since clearer

water will contain less material to be filtered out and clog the filter. This clarity, in turn, usually

reflects the operator's skill and knowledge at maximizing the efficiency of coagulation/flocculation


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and sedimentation. Physical features of the plant can also have considerable influence on the run

time.

The operator should test the influent and effluent turbidity, the effluent color, and head loss. These

factors, as well as the filter run time, should be recorded.

Factors Influencing Efficiency

The efficiency of a filter is influenced by a variety of factors. To a large extent, the efficiency is

determined by the characteristics of the water being treated and by the efficiency of previous stages

in the treatment process.

The chemical characteristics of the water being treated can influence both the preceding

coagulation/flocculation and the filtration process. In addition, the characteristics of the particles in

the water are especially important to the filtration process. Size, shape, and chemical

characteristics of the particles will all influence filtration. For example, floc which is too large will

clog the filter rapidly, requiring frequent backwashing, or can break up and pass through the filter,

decreasing water quality.

The types and degree of previous treatment processes greatly influence filtration as well.

Conventional, direct, and in-line filtration will all have different levels of efficiency.

Finally, the type of filter used and the operation of the filter will influence filter efficiency. The next

section will discuss problems caused by improper operation of the filter.

Filter Problems


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Photo Credit: Know Your Filters

Mudballsare approximately round conglomerations of filter material, ranging in size from pea-

sized to two inches or more in diameter. The picture above shows a very large mudball. Mudballs

form on the surface of filters when adhesive materials cause particles out of the water and media

grains to stick together. If the filter is not properly backwashed and surface washed, mudballs will

continue accumulating material and will grow larger, eventually sinking down into the filter media.Mudballs in the media result in shortened filter runs and in loss of filter capacity, since water will not

pass through the mudballs and must flow around them.

Another problem associated with filters is breakthroughs, cracking of the filter media and/or

separation of the media from the filter wall. Breakthroughs are caused by running the filter at an

excessive filtration rate or by extending filter runs too long between backwashing. Breakthroughs

can result in untreated water flowing through the filter, which in turn results in a sudden high turbidityin the effluent water. The untreated water may contain microorganisms such as Giardiaand is thus

not safe to drink.

Air bindingis the release of dissolved gases from the water into the filter or underdrain. Air

binding may result from low pressure in the filter (negative head) or from filtering very cold,

supersaturated water. The air in the filter and underdrain prevents water from passing through the

filter, which in turn results in abnormally high head loss even when the filter has recently been

backwashed. During backwash, the air in the filter can damage the filter media.
http://www.wioa.org.au/conf_papers/2001/paper9.htm


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Filtration Math

Introduction

In this lesson, we will design a rapid sand filter and a clear well chamber. Once again, these

calculations are similar to those used for flash mix, flocculation, and sedimentation basins.

For the rapid sand filter, the most important dimension is the surface area. Filters must be designed

so that the water flowing through is spread out over enough surface area that the filtration rate is

within the recommended range.

The clear wellis a reservoir for storage of filter effluent water. In this lesson, we will design a clear

well with sufficient volume to backwash the rapid sand filter we design. However, clear wells have

other purposes, most important of which is to allow sufficient contact time for chlorination. We willdiscuss chlorination in the next lesson.

Specifications

A water treatment plant will typically have several filters. Each filter in our calculations will be

assumed to have the following specifications.

Square tank

Basin depth: 10 ft

Media depth: 2-3 ft

Surface area:


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The backwash frequencyis the same as filter run time. Either term can be used to signify the

number of hours between backwashing.

The backwash periodis the length of time which backwashing lasts.

The backwash wateris the water used to backwash the filter. For the filters we're considering,

backwash water should be 1-5% of the water filtered during the filter run.

The backwash rateis the rate at which water is forced backwards through the filter during

backwashing. This rate is homologous to the filtration rate, only with water moving in the other

direction through the filter. The backwash rate is typically much greater than the filtration rate.

The filter rise rateis the speed at which water rises up through the filter during backwashing. This

is another way of measuring the backwash rate.

During backwashing, the water pushes the media up until it is suspended in the water. The height to

which the media rises during backwashing is known as the bed expansion. For example, if the

filter media is 2 feet deep, it may rise up to 3 feet deep during backwashing. This is a 50% bed

expansion:

Bed expansion = 50%

Most of these backwash specifications merely describe the type of filter we will be considering and

are not used in calculations. However, two factors - the filter rise rate and the backwash period -

will be used when calculating the volume of the clear well chamber.

Overview of Calculations

1. Calculate the approximate number of filters required.

2. Calculate the flow through one filter.

3. Calculate the surface area of one filter.

4. Calculate the length of the tank.

5. Calculate the clearwell volume.


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1. Number of Filters

The treatment plant's flow should be divided into at least three filters. You can estimate the number

of filters required using the following formula:

Where:

Q = Flow, MGD

So, for a plant with a flow of 1.5 MGD, then the approximate number of filters would be:

2. Flow

Next, the flow through one filter is calculated just as it was for one tank of the sedimentation basin:

Qc= Q / n

Qc= (1.5 MGD) / 3

Qc= 0.5 MGD

So the flow through each of our three filters will be 0.5 MGD.

3. Surface Area


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The required filter surface area is calculated using the formula below:

A = Qc/ F.R.

Where:

A = filter surface area, ft2

Qc= flow into one filter, gpm

F.R. = filtration rate, gal/min-ft2

We will use a filtration rate of 4 gal/min-ft.2 We will also have to convert from gpm to MGD. The

calculations for our example are shown below:

A = 500,000 gal/day (1 day / 1440 minutes) / 4 gal/min-ft2

A = 87 ft2

4. Tank Length

Since the filter tank is a square, the length of the tank can be calculated with the following simple

formula:

Where:

L = Length, ft

A = Surface area, ft2

In the case of our example, the length of one tank is calculated as follows:


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This is the final calculation required for the design of the filter.

5. Clearwell Volume

The volume of the clearwell must be sufficient to provide backwash water for each filter. First we

calculate the total filter area:

Total filter area = A (Number of filters)

For our example, the total filter area is:

Total filter area = 87 ft2 3

Total filter area = 261 ft2

Then we calculate the volume of the clearwell as follows:

V = (Backwash period) (Total filter area) (Filter rise rate)

We will assume a 5 minute backwash period and filter rise rate of 30 in/min. So, for our example,

the volume of the clearwell would be calculated as follows:

V = (5 min) (261 ft2) (30 in/min) (1 ft / 12 in)

V = 3,263 ft3

You will notice that we translated from inches to feet.

Conclusions

For our plant, we need three filters, each with a surface area of 87 ft2and a length of 9.3 ft. In

order to accommodate backwashing all three filters at once, the clearwell volume should be 3,263


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ft.3

Review

Filtration removes suspended particles from water by passing the water through a medium.

Particles are removed through straining, adsorption, biological processes, and absorption.

Four types of filters are used in water treatment - slow sand, rapid sand, pressure, and

diatomaceous. Rapid sand filters are the most widely used in treating surface water. Rapid sand

filters are cleaned by backwashing and surface washing. Filter media may be sand, or layers

involving anthracite coal, sand, and garnet.

Filter efficiency is typically monitored using effluent turbidity, particle counters, and filter run time.

Problems associated with filters include mudballs, breakthroughs, and air binding.

References

Alabama Department of Environmental Management. 1989. Water Works Operator Manual.

American Water Works Association. Brief History of Filtration.

Kerri, K.D. 2002. Water Treatment Plant Operation. California State University: Sacramento.

Rust, Mary, and Katie MacArthur. 1996. Slow Sand Filtration. Virginia Tech, Blacksburg.

Schmitt, Dottie, and Christie Shinault. 1996. Rapid Sand Filtration. Virginia Tech, Blacksburg.

Sims, Ronald C., and Lloyd A. Slezak. 1991. "Slow Sand Filtration: Present Practice in the

United States." Slow Sand Filtration. American Society of Civil Engineers: New York.

Assignments

Part 1 of your Assignment: Answer the following questions. Show all of your work and circle the

answer for each math problem below. If there is insufficient information to find the answer, write

"Insufficient information". When you are done, either email, mail or fax the assignment to your

instructor. (Each question is worth 10 points)

1. During filtration, the filter bed is 30 inches deep. During backwashing, the filter bed is 50
http://www.cee.vt.edu/program_areas/environmental/teach/wtprimer/rapid/rapid.htmlhttp://www.cee.vt.edu/program_areas/environmental/teach/wtprimer/slowsand/slowsand.htmlhttp://www.awwa.org/advocacy/learn/info/HistoryofDrinkingWater.cfm


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inches deep. What is the percent of bed expansion?

2. Given a flow of 5 MGD, approximate the number of filters which should be used.

3. The flow into one filter is 0.75 MGD. The filtration rate is 4 gal/min-ft2. Calculate the filter's

surface area.

4. The flow is 4 MGD, divided into the recommended number of filters. The filtration rate is 4

gal/min-ft2. What is the length of one filter tank?

5. A plant has 4 filters, each with an area of 75 ft2. Assume a 5 minute backwash period and

filter rise rate of 30 in/min. What should the volume of the clearwell be to allow backwashing

of all four filters at once?

Part 2 of your Assignment: Work the following crossword puzzle that comes from definitions in

your textbook. You may either print the puzzle out, complete it and mail or fax back to the

instructor or you may send an email with the correct answers numbered accordingly. (Crossword

worth 50 points)

Quiz

Answer the questions in the Lesson 6 quiz . When you have gotten all the answers correct, print

the page and either mail or fax it to the instructor. You may also take the quiz online and submit

your grade directly into the database for grading purposes.
http://water.me.vccs.edu/courses/ENV110/quiz6.htmhttp://water.me.vccs.edu/courses/ENV110/crosswords/lesson6.pdf

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Lesson 6_ Filtration