# Turbidity Through the Treatment Processes

## Introduction

In the arena of public water supply, water treatment is provided to
remove constituents from raw water which may pose a risk to public
health or are undesirable in finished water. Turbidity is a
characteristic related to the concentration of suspended solid particles
in water and has been adopted as an easy and reasonably accurate measure
of overall water quality. Turbidity can be used to measure the
performance of individual treatment processes as well as the performance
of an overall water treatment system. Common water treatment processes
intended to remove suspended solids and reduce turbidity, or aid in this
removal and reduction process, include:

-   Raw water screening;

-   Pre-sedimentation;

-   Coagulation;

-   Flocculation;

-   Sedimentation; and

-   Filtration.

This chapter provides a general description of each of these processes
and information on the level of turbidity reduction that is commonly
achieved through each. A typical water treatment system is shown
schematically in Figure 10-1.

![](media/image1.wmf){width="5.986111111111111in"
height="3.214583333333333in"}

[]{#__RefHeading___Toc448538655 .anchor}Figure 10-1. A Typical
Conventional Water Treatment System

## Intake Facilities/Raw Water Screening

Systems which obtain water from surface supplies such as lakes and
rivers employ intake facilities to allow water to be withdrawn from the
source. Most surface water intake facilities are equipped with some type
of screening device to prevent large rocks, sticks, and other debris
from entering the treatment system. Large bar racks with openings of 1
and 3 inches apart are commonly used for this purpose. They are designed
specifically to prevent large materials that could damage the intake
structure or downstream equipment from entering the treatment system.
Bar racks are usually designed for manual cleaning. When the raw water
source is a river and a bar rack is used, the rack is usually oriented
to take advantage of the hydraulics of the river to keep the rack
cleaned. Although trash racks have little effect on turbidity, they do
serve an important function in keeping large solids out of the treatment
system.

In other cases, intake screens are employed to perform the same
function. Intake screens are generally similar to bar racks except they
have smaller openings (typically 3/16 to 3/8 inch openings) and are
usually equipped with mechanical or hydraulic cleaning mechanisms.
Because intake screens remove particles much smaller than those
generally removed with bar racks, screens may provide some turbidity
reduction by removing larger solids that may be the source of smaller
particles further in the treatment train.

### Intake Location 

Intake facilities are typically the very first process in the water
treatment system. When the water source is a lake or reservoir,
substantial "pre-sedimentation" may occur within the reservoir itself.
This can serve to reduce the turbidity of water entering the treatment
system as well as dampen the impact of fluctuations in source water
turbidity resulting from storm events and other environmental
phenomenon. If the intake facility is located away from the reservoir's
water source (i.e., the river feeding the reservoir) this
pre-sedimentation may substantially reduce the turbidity of the water
entering the treatment system. On the other hand, if the intake facility
is located near the point where the supply stream enters the reservoir,
the benefit of the pre-sedimentation occurring within the reservoir can
be significantly reduced or lost.

When the water source is a river, the quality of water withdrawn may be
impacted by the location of the intake in relationship to sources of
pollution entering the river. For example, an intake structure located
upstream from a municipal or industrial wastewater discharge may supply
substantially higher quality water than if it were located downstream
from the discharge.

### Intake Depth

For many lakes and reservoirs the water surface elevation varies
seasonally due to environmental factors or reservoir management
practices. In such cases, it is essential to be able to withdraw water
from the source under a variety of different water surface level
conditions. Even in cases where the source water surface elevation does
not vary significantly, the intake structure may be designed to allow
withdrawal from the surface, or from different levels beneath the
surface. This capability can have a significant impact on the quality of
water entering the treatment system. Many times stratification within a
lake or reservoir exists continually or occurs on a seasonal basis. In
these cases, the quality of water may vary significantly from near the
surface to tens or hundreds of feet below the surface. For example, if
algae growth is a problem, water withdrawn from several feet beneath the
surface may have substantially lower turbidity than water withdrawn from
the surface, as algae needs sunlight to survive and is typically found
near the water surface.

### Effect on Turbidity

Intake screens are not intended to reduce the turbidity of the water
entering the treatment system. The solids removed by intake screens are
large enough that they typically do not directly impact turbidity,
though subsequent deterioration and break-up of these solids could
contribute to increased levels of turbidity later in the treatment
process. The physical location of the intake structure and the
flexibility it provides for varying the depth from which source water is
withdrawn can significantly influence the turbidity of the water
entering the treatment system.

## Pre-sedimentation

Pre-sedimentation is commonly used for water supplies where raw water
turbidity is continually high, is high on a seasonal basis, or is high
sporadically due to storms or other environmental events within the
watershed. Pre-sedimentation may also be used in situations where
substantial amounts of sand and gravel may be present in the source
water. Depending on the purpose of the pre-sedimentation process, the
pre-sedimentation basins may be relatively large settling ponds or small
concrete basins. When ponds are utilized, they are generally designed to
remove large quantities of silt from the raw water and typically provide
hydraulic detention times ranging from a few hours to a few days.
Smaller concrete basins that provide less than 20 minutes detention time
are sometimes used to provide grit removal. The larger settling ponds
are generally not equipped with mechanical sludge removal facilities and
must be periodically cleaned by dredging or other means. The concrete
pre-sedimentation basins may be equipped with mechanical equipment to
remove solids from the basin bottom, or they may be designed to promote
manual cleaning using a fire hose or other equipment.

When pre-sedimentation is intended to remove silt and other fine
suspended solids, chemical addition is often used to enhance process
performance. Organic polymers are the chemicals most commonly added
prior to pre-sedimentation to enhance solids removal, but alum and
ferric chloride are also sometimes used. The chemicals are added to the
raw water as it enters the pre-sedimentation basin to promote solid
separation.

### Effect on Turbidity

Pre-sedimentation facility performance depends largely on facility
design. Factors such as the ability to provide low velocity plug flow
through the pre-sedimentation facility and the capability to add
chemicals are critical to achieving optimal system performance. The
characteristics of the suspended solids in the raw water also play a key
role in facility performance. In cases where well-designed
pre-sedimentation facilities are available and adequate hydraulic
detention times are provided (generally greater than 12 hours),
significant turbidity reduction can be achieved through the
pre-sedimentation process.

In cases where pre-sedimentation is used primarily to remove grit from
the raw water before it enters subsequent treatment processes, detention
times are generally limited to 10 to 20 minutes and very little
turbidity reduction is achieved.

## Coagulation

Coagulation is the process of conditioning suspended solids particles to
promote their agglomeration and produce larger particles that can be
more readily removed in subsequent treatment processes. In many cases,
dissolved organic substances are adsorbed on the surface of suspended
solids particles and effective coagulation can be an effective step in
their removal as well (AWWA and ASCE, 1990). The particles suspended in
raw water typically vary widely in size. Typical sizes for different
types of particles commonly found in water supplies can be seen in
Figure 8-1.

Colloidal size particles typically carry an electrical charge (AWWA and
ASCE, 1990). When the suspended particles are similarly charged, the
resulting repulsive forces between particles tend to "stabilize" the
suspension and prevent particle agglomeration. The process of
coagulation is complex and may involve several different mechanisms to
achieve "destabilization", which allows particle agglomeration and
enhances subsequent removal.

Coagulation is typically accomplished through chemical addition and
mixing. Following coagulation, the processes of flocculation,
sedimentation, and filtration are used to remove the "destabilized"
particles from suspension.

### Chemicals

Chemicals commonly used in the coagulation process include aluminum or
iron salts and organic polymers. The most common aluminum salt used for
coagulation is aluminum sulfate, or alum. Alum may react in different
ways to achieve coagulation. When used at relatively low doses (\<5
mg/l), charge neutralization (destabilization) is believed to be the
primary mechanism involved. At higher dosages, the primary coagulation
mechanism is entrapment. In this case, aluminum hydroxide (Al(OH)~2~)
precipitates forming a "sweep-floc" which tends to capture suspended
solids as it settles out of suspension.

Solution pH plays an important role when alum is used for coagulation
since the solubility of the aluminum species in water is pH dependent.
If the pH value of a mixed solution is between 4 and 5, alum is
generally present in the form of positive ions (i.e., Al(OH)^2+^,
Al~8~(OH)^4+^, and Al^3+^). However, optimum sweep and sweep coagulation
occur when negatively-charged forms of alum predominate, which occurs
when the pH is between 6 and 8. Figure 10-2 depicts the solubility of
some of these aluminum species present during a typical coagulation
process. Figure 10-2 depicts some of the aluminum species involved in
alum coagulation and the conditions of aluminum concentration and pH
under which they occur (AWWA and ASCE, 1990).

When alum is used and charge neutralization is the primary coagulation
mechanism, effective flash mixing is critical to the success of the
process. When the primary mechanism is entrapment, effective flash
mixing is less critical, and flocculation is the more important process.

Source: AWWA and ASCE, 1990.

[]{#__RefHeading___Toc448538656 .anchor}Figure 10-2. The Alum
Coagulation Diagram and Its Relationship to\
Zeta Potential

Ferric Chloride (FeCl~3~) is the most common iron salt used to achieve
coagulation. Its reactions in the coagulation process are similar to
those of alum, but the relative solubility and pH ranges differ
significantly from those of alum.

Both alum and ferric chloride can be used to generate inorganic
polymeric coagulants that have been used for coagulation. These
coagulants are typically generated by partially neutralizing
concentrated solutions of alum or ferric chloride with a base such as
sodium hydroxide prior to their use in the coagulation process (AWWA and
ASCE, 1990). The resulting inorganic polymers may have some advantages
over alum or ferric chloride in cold waters or in low alkalinity waters.

Organic polymers tend to be large molecules composed of chains of
smaller "monomer" groups (AWWA and ASCE, 1990). Because of their large
size and charge characteristics, polymers can promote destabilization
through bridging, charge neutralization, or both. Polymers are often
used in conjunction with other coagulants such as alum or ferric
chloride to optimize solids removal.

### Rapid Mixing

Mixing is utilized as part of the coagulation process to distribute the
coagulant chemicals throughout the water stream. When alum or ferric
chloride are used to achieve destabilization through charge
neutralization, it is extremely important that the coagulant chemical be
distributed quickly and efficiently because it is the intermediate
products of the coagulant reaction that are the destabilizing agents.
The life of these intermediate species is short and they must contact
the solids particles in the water if destabilization is to be achieved.
When other mechanisms are predominant in the coagulation process, or
when organic polymers are being used as the coagulant chemical,
immediate distribution of the coagulant chemical is not as critical and
less intense mixing may be acceptable, or even desirable. In some cases,
excessive mixing may serve to break-up coagulant molecules or floc
particles, thereby reducing the effectiveness of subsequent solids
removal processes.

Mixing intensity is typically quantified with a number known as the
"velocity gradient" or "G value". The G value is a function of the power
input into the mixing process and the volume of the reaction basin.
Typical G values for coagulation mixing range from 300 to 8000 sec^--1^
(Hudson, 1981).

The time required to achieve efficient coagulation varies, depending on
the coagulation mechanism involved. When charge neutralization is the
mechanism involved, the detention time required may be one second or
less. When sweep floc or entrapment is the mechanism involved, longer
detention times on the order of 1 to 30 seconds may be appropriate
(Kawumara, 1991; AWWA and ASCE, 1998; Hudson, 1981).

### Effect on Turbidity

Coagulation by itself does not achieve turbidity reduction, in fact
turbidity may increase during the coagulation process due to the
additional insoluble compounds generated through chemical addition. The
subsequent processes of flocculation, sedimentation, and filtration are
used in conjunction with coagulation to achieve suspended solids and
turbidity reduction.

## Flocculation

Flocculation is the physical process of agglomerating small particles
into larger ones that can be more easily removed from suspension.
Flocculation is almost always used in conjunction with, and preceded by
coagulation. During the coagulation process the repulsive forces between
solids particles are reduced or eliminated. Flocculation is the process
of bringing the destabilized particles into contact with one another to
form larger "floc" particles. These larger particles are more readily
removed from the water in subsequent processes.

Flocculation is generally accomplished by mixing the destabilized
suspension to provide the opportunity for the particles to come into
contact with one another and stick together.

### Slow Mixing 

Mixing is a key aspect of the flocculation process. Often the intensity
of mixing is reduced as the water proceeds through the flocculation
process to achieve optimum performance.

At the beginning of the process, the mixing is fairly intense to
maximize the particle contact opportunities. Mixing intensity values (G
values) in this area are typically in the range of 60 to 70 sec^-1^
(Kawamura, 1996).

Toward the end of the flocculation process, mixing intensity is
generally reduced to minimize the potential for breaking up the floc
particles that have begun to form. In this portion of the process, G
values are commonly in the 10 to 30 sec^-1^ range (Kawamura, 1996). Many
times mixing intensity is tapered through several different stages of
the flocculation process to optimize process effectiveness.

A wide variety of flocculation mixing mechanisms have been used in water
treatment. These include vertical shaft mechanical mixers, horizontal
shaft mechanical mixers, and hydraulic mixing systems.

### Detention Time

The amount of time the water spends in the flocculation process is a key
performance parameter. Adequate time must be provided to allow
generation of particles sufficiently large to allow their efficient
removal in subsequent treatment processes. The optimum particle size may
vary significantly depending on the downstream treatment processes
utilized. For example, when sedimentation is used, large floc particles
are typically desirable because they tend to settle out of suspension
readily. If filtration directly follows the flocculation process,
smaller floc particles may be the most desirable since they tend to be
stronger and less susceptible to break-up by the shear forces
encountered within the filters. Overall detention time in the
flocculation process typically ranges from 10 to 30 minutes and is
generally provided in several different basins or basin segments. This
allows the mixing intensity to be varied through the process.

### Effect on Turbidity

As with coagulation, the purpose of the flocculation process is not to
directly reduce turbidity or suspended solids levels, but rather to
prepare the solids for subsequent removal. The reduction in number of
suspended solids particles in suspension is typically achieved in the
flocculation process as the smaller particles are combined to form
larger ones. This process may, or may not result in a reduction in
turbidity.

## Sedimentation/Clarification

Sedimentation is the process by which solids are removed from the water
by means of gravity separation. In the sedimentation process, the water
passes through a basin in which relatively quiescent conditions prevail.
Under these conditions, the floc particles formed during flocculation
settle to the bottom of the basin while the "clear" water passes out of
the basin over an effluent baffle or weir. As shown in Figure 10-3, the
solids collect on the basin bottom and are removed, typically by a
mechanical "sludge collection" device. The sludge collection device
scrapes the solids (sludge) to a collection point within the basin from
which it is pumped directly to disposal or to a sludge treatment
process.

Conventional sedimentation typically involves one or more basins. These
"clarifiers" are relatively large open tanks, either circular or
rectangular in shape. In properly designed clarifiers, velocity currents
are reduced to the point where gravity is the predominant force acting
on the water/solids suspension. Under this condition, the difference in
specific gravity between the water and the solids particles causes the
solids particles to settle to the bottom of the basin.

High rate sedimentation is similar to conventional sedimentation except
that the sedimentation basin has been modified through the addition of
some mechanical or other device to aid in the settling process. These
mechanical devices typically consist of plates or tubes intended to
reduce the distance the solids particles must settle through the water
before they reach the bottom of the basin and can be removed. Figure
10-4 illustrates a plate settler used for high rate sedimentation.

Another high rate clarification process employs an "adsorption
clarifier" and is designed to provide flocculation and clarifications
within a single process. These clarifiers consist of a basin filled with
adsorption media, generally small particles of either plastic or rock,
about the size of pea gravel. As the water passes through the media,
hydraulic mixing promotes flocculation and the flocculated particles
adhere to the surface of the media particles. The media is cleaned
periodically using an air or air and water backwash process to remove
the solids.

Solids Contact clarifiers represent an entirely different approach to
high rate clarification. They consist of a basin similar to that used
for a conventional clarifier but with a sludge recycle system to promote
development of a dense sludge blanket as depicted in Figure 10-5.

![](media/image4.wmf){width="4.847916666666666in"
height="7.704166666666667in"}

Source: AWWA and ASCE, 1990.

[]{#__RefHeading___Toc448538657 .anchor}Figure 10-3. Circular
Radial-flow Clarifier

![](media/image5.wmf){width="6.0in" height="4.516666666666667in"}

Source: AWWA and ASCE, 1998.

[]{#__RefHeading___Toc448538658 .anchor}Figure 10-4. Plate Settlers Used
for High Rate Sedimentation

![](media/image6.wmf){width="5.9944444444444445in"
height="2.3604166666666666in"}

Source: AWWA and ASCE, 1998.

[]{#__RefHeading___Toc448538659 .anchor}Figure 10-5. Accelerator Solids
Contact Unit

As the water enters the bottom of the basin and passes upward through
the sludge blanket, the flocculated solids in the blanket tend to
contact and capture or adsorb the solids from the water.

### Effect on Turbidity

Suspended solids removal and turbidity reduction rates achieved through
sedimentation may range from about 50 to 90 percent, depending on the
nature of the solids, the level of pretreatment provided, and the design
of the clarifiers. Common values are in the 60 to 80 percent range
(Hudson, 1981). A primary function of the sedimentation or clarification
process is to reduce the load of solids going to the filters.
Optimization of the clarification process will minimize the solids
loading on the filters and contribute to enhanced filter performance and
better overall treated water quality.

## Filtration

Like clarification, filtration is a process in which solids are removed
from water and substantial turbidity removal is achieved. Optimization
used prior to the filtration process will control loading rates while
allowing the system to achieve maximum filtration rates. In fact,
filtration is the final step to achieve turbidity reduction in most
water treatment operations. The water leaving the filtration process
should be well within turbidity limits.

In the filtration process, the water passes through a bed of granular
filter media or other filtering material and solids are physically
retained on the media. After passing through the filter media, the
"filtered" water is collected and removed from the filter. The solids
retained on the media are also periodically collected and removed. As
with the sedimentation process, the performance of most filters depends
largely on the preparatory treatment processes of coagulation and
flocculation. Without effective use of these processes, only marginal
filter performance can be expected.

Filters are classified according to the type of media used and the
operational conditions employed. The primary types of filters used in
domestic water treatment include:

-   Rapid Sand Filters;

-   Pressure Filters;

-   Slow Sand Filters; and

-   Precoat Filters.

### Conventional Rapid Sand Filters

Rapid sand filters are the most commonly used type of filters in water
treatment systems today. They get their name from the type of media
employed (sand) and from the rate at which they are hydraulically
loaded. A sectional drawing of a typical rapid sand filter is shown in
Figure 10-6.

![](media/image7.wmf){width="5.521527777777778in"
height="3.707638888888889in"}

Source: AWWA and ASCE, 1998.

[]{#__RefHeading___Toc448538660 .anchor}Figure 10-6. Typical Rapid Sand
Filter

Water enters the filter unit above the media and flows by gravity
downward through the filter media to the underdrain or collection
system, where it is removed from the filter. When the filter media
becomes clogged with solids it is cleaned through a "backwash" process.
In the backwash process water, and in some instances, air is introduced
to the filter at a relatively high rate through the underdrain system.
The water and air flow upward through the media, expanding the media bed
and creating a scrubbing or scouring action which removes solids
accumulated on the media surface and in inter-particle sites within the
media bed. After passing through the media bed, the backwash water and
the solids it contains are removed from the filter with a series of
collection troughs.

#### Media

A variety of different types of media are used in rapid sand filters. As
the name implies, the primary media is sand. In some cases all of the
sand is the same size, but more commonly the media consists of particles
of varying composition, size, and density. Filters with more than one
type and size of media are referred to as dual media, mixed media, or
multi-media filters, depending on the media provided. As the backwashing
process in these filters concludes and the media particles settle back
into position in the filter bed, the particles become stratified due to
their differing sizes and densities. The largest and least dense media
particles accumulate near the top of the media bed and the smallest and
most dense particles migrate to the bottom. With this media
stratification, when the filter is placed back into service and the
water passes down through the bed of media, it first encounters large
particles and then finer and finer sand until it reaches the underdrain
system. This stratification tends to minimize the "blinding" effect that
occurs when solid particles accumulate at the very top of the media bed.
It also provides a much greater volume for solids storage within the
filter bed, which allows longer filter runs between backwash operations.

The top, or coarsest layer of media is often composed of anthracite coal
rather than sand. The relatively light coal remains at the top of the
filter bed after filter cleaning (backwashing), even though its particle
sizes are relatively large. Conversely, high density garnet sand is used
for the smallest layer of filter media. The high density of this
material causes it to settle quickly to the bottom of the filter bed
following backwash operations, even though its particles are relatively
small.

#### Hydraulics

Filter performance is affected significantly by its hydraulic
characteristics. Typical rapid sand filter loading rates range from 2 to
8 gallons per minute per square foot of filter bed surface area. As the
filter bed becomes dirty and clogged with solids, the resistance to flow
increases. Ultimately flow will cease when the resistance to flow is
greater that the gravitational force compelling it. As the "head"
required to push water through the filter increases, the rate of flow
tends to decrease and solids particles are pushed further and further
into the bed of media. Ultimately, if sufficient head is available,
solids will be driven completely through the bed and appear in the
filtered water, a condition known as "breakthrough". The filter run
should be terminated and backwash initiated before breakthrough occurs.

#### Controls 

Typical filter control parameters include:

-   Filter loading rate;

-   Filter run length;

-   Headloss;

-   Filtered water turbidity;

-   Backwash rate; and

-   Backwash duration.

Control of these parameters gives the operator a great deal of influence
in determining the performance of a filter. Often, however, there are
conflicting objectives associated with filter operation. For example,
the goal of maximizing water production may conflict with the objective
of minimizing treated water turbidity. The operator must use good
judgement in establishing operational goals and exercising process
control to achieve optimal finished water quality and production.

### Slow Sand Filters

Slow sand filters have been used for nearly two centuries and have been
proven to an effective "low-tech" method of treating some waters (AWWA
and ASCE, 1990). A slow sand filter consists of a bed of uniform,
relatively fine grain sand underlain by an underdrain system as depicted
in Figure 10-7.

![](media/image8.wmf){width="6.0in" height="2.158333333333333in"}

Source: AWWA and ASCE, 1998.

[]{#__RefHeading___Toc448538661 .anchor}Figure 10-7. Typical Covered
Slow Sand Filter Installation

Water is introduced at the top of the bed and under the influence of
gravity it passes downward through the bed to the underdrain system.
Slow sand filters are loaded at much lower rates than rapid sand
filters, with typical hydraulic loading rates ranging from 0.04 to 0.1
gallons per minute per square foot of filter bed area (AWWA and ASCE,
1998). Since the sand used in slow sand filters is relatively uniform in
size and fine grained, most of the solids removal and turbidity
reduction occurs at the very top of the sand bed. As operation of the
filter continues, a layer of dirt and micro-organisms builds up at the
surface of the bed. This layer is known as the "schmutzdecke" and
contributes to the effectiveness of the filter in removing suspended
solids and reducing turbidity. After a period of operation the headloss
through the filter becomes excessive and the filter must be cleaned.
Cleaning is accomplished by letting the water level drop below the top
of the filter media and then physically removing the schmutzdecke along
with the top 0.8 to 1.2 inches of sand. Typical slow sand filter runs
between cleaning are one to six months (Kawamura, 1996).

Generally slow sand filters are not proceeded by coagulation or
flocculation processes, making them one of the simplest water filtration
processes available. However, relatively large land areas are required
and, with their simplicity comes little operational flexibility. The
only basic operational controls available to the slow sand filter
operator are hydraulic loading and frequency of cleaning.

Slow sand filters can be very effective in removing suspended solids and
reducing turbidity, depending on the nature of the solids particles
involved, however, they have been found to be limited in their
capability to remove clay particles and color (AWWA and ASCE, 1990).

### Pressure Filters

Pressure filters are essentially a variation of the conventional rapid
sand filter. They employ the same types of media and function in much
the same way. The primary difference is that pressure filters are
contained within a pressurized vessel, usually made of steel, and
pressure is used to push the water through the filter bed rather than
gravity as depicted in Figure 10-8.

Since pressure filters function much like conventional rapid sand
filters, their capability to remove suspended solids and reduce
turbidity is similar.

![](media/image9.wmf)The primary advantage of pressure filters is that
they do not require the vertical space for several feet of water above
the filter bed and the water leaves the filter under pressure, thus
eliminating the potential for air binding associated with conventional
rapid sand filters. Disadvantages include the lack of access for visual
observation of the filter bed and the possibly greater potential to
experience turbidity "breakthrough" due to the higher pressure of force
driving the filtration process.

Source: AWWA and ASCE, 1990.

[]{#__RefHeading___Toc448538662 .anchor}Figure 10-8. Cross Section of a
Typical Pressure Filter

### Precoat/Diatomaceous Earth Filters 

Precoat filters represent an entirely different kind of filtration
mechanism. Instead of using sand or other granular material as the
filtration medium, precoat filters use a thin layer of "diatomaceous
earth" or similar material to form a thin layer of filter media over a
supporting fabric or "septum", which in turn is supported by a rigid
filter support structure. The filter coat material is applied in a
slurry before the filtration cycle begins. When water passes through the
filter under pressure, usually supplied by a pump but sometimes by
gravity, the solids in the water are captured on the surface of the
filter media. As the filtration process proceeds, additional filter
media is added to the water going to the filter. This supplemental media
or "body feed", like the suspended solids in the water, accumulates on
the surface of the filter coat, increasing the depth of the media and
preventing the surface blinding effect that would otherwise occur. When
the pressure loss through the filter becomes excessive, filtration is
discontinued and the filter media coat is washed off through a backwash
process, a new pre-coat is applied, and the filtration process begins
again.

Precoat filters have the capability to remove particles down to about
one micron in size. Hydraulic loading rates are typically in the range
of 0.5 to 2 gallons per minute per square foot of coated filter surface
(ASCE, 1990). Advantages include relatively low capital cost and no need
for the preliminary processes of coagulation and flocculation.
Disadvantages include the inability to handle high turbidity water, the
potential for particle pass-through if the precoat process is not
effective or cracking occurs during filter operation, and the relatively
poor capability to remove color and taste and odor causing compounds.

### Effect on Turbidity

Conventional and direct filtration processes have the capability of
producing water with turbidity below the proposed SDWA turbidity of 0.3
NTU and even below the 0.1 NTU Partnership for Safe Water finished water
optimization goal if properly operated and maintained.

Filter performance depends largely on the characteristics of the solids
particles entering the filter and on the characteristics of the filter
itself. Generally treated water turbidity will be relatively high for a
short period immediately following the backwash cycle of rapid sand
filter operation, commonly known as post backwash turbidity spiking, but
will then improve rapidly to a level near the highest quality level the
filter can produce. The filter will then operate at or near this level
for an extended period. Figure 10-9 shows typical filtered water
turbidity as a function of filter run time.

![](media/image10.wmf){width="4.7659722222222225in"
height="2.622916666666667in"}

Source: Hudson, 1981.

[]{#__RefHeading___Toc448538663 .anchor}Figure 10-9. Typical Filter Run
Showing Progress of Floc Penetration and Effluent Turbidity

A variety of strategies may be used to minimize the impact of post
backwashing turbidity spikes on finished water quality. These include
routing the post backwash, high turbidity, water to waste or adding
organic polymers to the backwash water during the final stages of the
backwash process.

The performance of other types of filters is similar to that of rapid
sand filters, except that some may not experience the higher turbidity
levels following filter cleaning operations.

A filter run is typically terminated after a pre-set period of time,
when a certain pre-established headloss is reached, or treated water
turbidity deteriorates to a set level.

## Membrane Processes

Membrane processes are processes in which the water is passed through a
semi-permeable membrane to accomplish solids removal. They are much like
other filtration processes except the membrane is used in place of the
filter media. An additional benefit that may be realized with membrane
processes is the removal of dissolved organic or inorganic constituents.
Membrane technology has been in use for many years but recent
technological advances coupled with increased concern for particulate
removal have brought its use to the forefront of potable water treatment
technology.

Membranes are made of a variety of different materials and membrane
processes are typically classified according to the driving force
involved and the size of particles that pass through the membrane (AWWA
and ASCE, 1998). For most systems commonly used in larger water
treatment operations, the water is forced through the membrane by
applying pressure to one side creating a pressure differential across
the membrane. The four general classifications of pressure membrane
process are:

-   Microfiltration;

-   Ultrafiltration;

-   Nanofiltration; and

-   Reverse Osmosis.

Figure 10-10 shows the size of particles generally removed by these
different membrane processes.

![](media/image11.wmf){width="5.139583333333333in"
height="2.7680555555555557in"}

Source: AWWA and ASCE, 1998.

[]{#__RefHeading___Toc448538664 .anchor}Figure 10-10. Pressure-Driven
Membrane Process Application Guide

Table 10-1 provides information on the typical pressure operating ranges
for the different types of pressure-driven membrane processes. Most
pressure driven membrane processes utilize either cellulose acetate or
synthetic organic polymer membranes (AWWA and ASCE, 1998). Standard
pressure membrane configurations include spiral wound membrane units and
hollow fiber membrane units. Figures 10-11, 10-12, and 10-13 show the
standard membrane configurations commonly used for potable water
treatment systems.

####### Table 10-1. Typical Feed Pressures for Pressure Driven Membrane Processes {#table-10-1.-typical-feed-pressures-for-pressure-driven-membrane-processes .TabCaption}

  -------------------------------------- --------------------------------
  **Membrane Process**                   **Typical Feed Pressure (psi)**

  Reverse Osmosis - Brackish Water       
  Application                            

  Low Pressure                           125 to 300

  Standard Pressure                      350 to 600

  Reverse Osmosis - Seawater Application 800 to 1,200

  Nanofiltration                         50 to 150

  Ultrafiltration                        20 to 75

  Microfiltration                        15 to 30
  -------------------------------------- --------------------------------

Source: AWWA and ASCE, 1998.

![](media/image12.wmf) Source: AWWA and ASCE,1998.

[]{#__RefHeading___Toc448538665 .anchor}![](media/image13.wmf)Figure
10-11. Typical Spiral-Wound Reverse Osmosis Membrane\
Module Driven

Source: AWWA and ASCE, 1998.

[]{#__RefHeading___Toc448538666 .anchor}Figure 10-12. Typical Hollow
Fine-Fiber Reverse Osmosis Membrane Module

![](media/image14.wmf)Source: AWWA and ASCE, 1998.

[]{#__RefHeading___Toc448538667 .anchor}Figure 10-13. Representation of
Hollow-fiber UF Module

### Effect on Turbidity

Membrane systems provide a positive barrier to particles of a size
larger than will pass through the membrane. Consequently, the turbidity
of water produced by membrane treatment systems is usually well below
0.3 NTU. The size of particles that will pass through the membrane
depends on the structure of the membrane itself. Figure 10-10 contains
information on the sizes of particles removed by different types of
membrane systems. As shown in Figure 10-10 all conventional membrane
processes will effectively remove bacteria and other large organisms
such as *Giardia* and *Cryptosporidium*. Only the more restrictive
membranes are effective for removing viruses, small colloids, and
dissolved constituents. Many times membrane system performance is
determined not by treated water turbidity but by the level of other
constituents such as total dissolved solids that may be of concern in a
particular situation. Please note that this assessment is based on
absolute pore sizes outlined in Figure 10-10. It does not reflect
microbe pass-through resulting from nominal pore size membranes or
membrane failures (e.g., rupture, seal leakage).

## Recycle Streams

Recycle streams are waste streams generated during the water treatment
process that are returned to the treatment train with or without prior
treatment. Though they are not related to one particular treatment
process, recycle streams may have a deleterious impact on treated water
quality, including turbidity. Consequently, proper management of recycle
streams is an important part of optimizing turbidity reduction in water
treatment. Any discharges of recycle streams must comply with Federal
and State regulations, including the National Pollution Discharge
Elimination System (NPDES) program (40 CFR 122) and the Pretreatment
program (40 CFR 403).

Waste streams may be handled in several different ways. Historically it
was a common practice to discharge waste streams directly to surface
waters. In some cases this may still be acceptable but state or federal
regulations have largely curtailed the practice of discharging water
treatment waste streams directly to surface water. If a waste stream is
to be discharged to a surface water, an NPDES permit must be obtained
from the appropriate permitting authority prior to any discharge
occurring. Due to these new regulatory requirements, or to conserve
water and reduce wastes, waste streams are now recycled by many systems,
with or without treatment, or discharged to a sanitary sewer system.

### Sources of Recycle Streams

The most common recycle streams found in potable water treatment systems
include:

-   Filter backwash water;

-   Sludge thickener supernatant;

-   Filter to waste flow; and

-   Sedimentation basin underflow.

These recycle streams can represent concentrated waste flows and contain
high levels of contaminants. The continued recycle of these contaminants
may affect treated water quality. Impacts may include higher turbidity
as well as higher concentrations of pathogens and other contaminants in
the plant influent. Because of its potential impact to finished water
quality, the handling of recycle streams should be carefully considered
during design and upgrade of all water treatment systems.

### Recycle Stream Quantity and Quality

The quantity and quality of recycle streams varies considerably
depending on the quality of the raw water, the treatment processes
employed and their efficiencies, and the type and amount of chemicals
used during treatment. Generally, the composite of all waste streams
generated in a conventional complete treatment system employing
coagulation, flocculation, sedimentation, and filtration will be in the
neighborhood of 2 to 10 percent of the total volume of water treated.
Contaminants present in recycle streams may include:

-   Suspended solids;

-   Organics;

-   Inorganics; and

-   Microorganisms.

Reported quality parameters associated with sludges generated from alum
coagulation include (Culp and Culp, 1974):

-   BOD -- 40 to 150 mg/l

-   COD -- 340 to 5000 mg/l

-   TSS -- 1100 to 14,000 mg/l

-   VSS -- 600 to 4000 mg/l

-   pH -- near neutral

Levels of *Cryptosporidium* oocysts ranging from 2,900 to 47,000 counts
per mL have been reported in recycle streams from sedimentation and
filtration processes after settling (Cornwell and Lee, 1994). Reported
values of spent filter backwash water turbidity typically range from 30
to 400 NTU (Kawamura, 1991; Cornwell and Lee, 1993).

Waste streams generated from membrane processes such as reverse osmosis
may also contain significant levels of other contaminants such as
dissolved solids and salts.

### Point of Recycle Stream Return

The point at which a recycle stream is introduced to the treatment train
is also important. Recycle streams should be introduced at the plant
headworks or as close to the beginning of the treatment system as
possible to provide the maximum level of recycle stream treatment. The
point of introduction should also be one where effective mixing is
provided to thoroughly disperse the recycled flow in the raw water
stream before it enters subsequent treatment processes. Studies have
shown that the timing or regularity of the recycle stream introduction
is also very important in determining it's impact on the performance of
the treatment process (Goldgrabe-Brewen, 1995). The continuous and
steady introduction of an equalized recycle stream will have much less
negative impact on the water treatment process than sporadic
introduction of larger volume recycle flows that vary in quantity and
quality.

### Effect on Turbidity

When waste streams are recycled they may have an impact on the amount of
solids loaded to the treatment system. A model, developed by Cornwell
and Lee (1993) to predict the impact of recycled spent filter backwash
water on the concentration of *Giardia* cysts and *Cryptosporidium*
cysts on water entering a treatment system, demonstrates the significant
impact recycle streams can have on water quality, even when a relatively
high level of treatment is provided for the recycle flow before it is
returned to the main flow stream. The model predicts a greater than
three-fold increase in cyst concentration in the water entering the
treatment process as a result of the recycle practice, even when 70
percent of the cysts initially present in the recycle stream are removed
through treatment. Similar impacts on turbidity can be expected.

## References

1.  AWWA. 1990. *Water Quality and Treatment.* Fourth Edition.
    McGraw-Hill, Inc.

2.  AWWA and ASCE. 1998. *Water Treatment Plant Design.* Third Edition.
    AWWA and ASCE, McGraw-Hill, Inc, New York, NY.

3.  AWWA and ASCE. 1990. *Water Treatment Plant Design*. Second
    Edition*.* ASCE and AWWA, McGraw-Hill, Inc, New York, NY.

4.  Cornwell, D.A. and R.G. Lee. 1994. "Waste Stream Recycling: Its
    Effect on Water Quality." *J. AWWA*, 86(11): 50-63.

5.  Cornwell, D.A. and R.G. Lee. 1993. *Recycle Stream Effects on Water
    Treatment*. AWWARF and AWWA.

6.  Culp, G.L. and R.L. Culp. 1974. *New Concepts in Water
    Purification.* Van Nostrand Reinhold Environmental Engineering
    Series, Litton Educational Publishing, Inc.

7.  Goldgrabe-Brewen, J.C. 1995. "Impacts of Recycle Streams on Water
    Quality." Conference proceedings, AWWA Water Quality Conference,
    Anaheim, CA.

8.  Hudson, H.E. Jr. 1981. *Water Clarification Processes, Practical
    Design and Evaluation*. Van Nostrand Reinhold Environmental
    Engineering Series, Litton Educational Publishing, Inc.

9.  Kawumara, S. 1991. *Integrated Design of Water Treatment
    Facilities*. John Wiley & Sons, Inc.

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TABLE OF CONTENTS

10\. Turbidity Through the Treatment Processes
[10-1](#turbidity-through-the-treatment-processes)

10.1 Introduction [10-1](#introduction)

10.2 Intake Facilities/Raw Water Screening
[10-2](#intake-facilitiesraw-water-screening)

10.2.1 Intake Location [10-2](#intake-location)

10.2.2 Intake Depth [10-2](#intake-depth)

10.2.3 Effect on Turbidity [10-3](#effect-on-turbidity)

10.3 Pre-sedimentation [10-3](#pre-sedimentation)

10.3.1 Effect on Turbidity [10-3](#effect-on-turbidity-1)

10.4 Coagulation [10-4](#coagulation)

10.4.1 Chemicals [10-4](#chemicals)

10.4.2 Rapid Mixing [10-6](#rapid-mixing)

10.4.3 Effect on Turbidity [10-6](#effect-on-turbidity-2)

10.5 Flocculation [10-7](#flocculation)

10.5.1 Slow Mixing [10-7](#slow-mixing)

10.5.2 Detention Time [10-7](#detention-time)

10.5.3 Effect on Turbidity [10-8](#effect-on-turbidity-3)

10.6 Sedimentation/Clarification [10-8](#sedimentationclarification)

10.6.1 Effect on Turbidity [10-11](#effect-on-turbidity-4)

10.7 Filtration [10-11](#filtration)

10.7.1 Conventional Rapid Sand Filters
[10-11](#conventional-rapid-sand-filters)

10.7.2 Slow Sand Filters [10-14](#slow-sand-filters)

10.7.3 Pressure Filters [10-15](#pressure-filters)

10.7.4 Precoat/Diatomaceous Earth Filters
[10-16](#precoatdiatomaceous-earth-filters)

10.7.5 Effect on Turbidity [10-16](#effect-on-turbidity-5)

10.8 Membrane Processes [10-17](#membrane-processes)

10.8.1 Effect on Turbidity [10-20](#effect-on-turbidity-6)

10.9 Recycle Streams [10-20](#recycle-streams)

10.9.1 Sources of Recycle Streams [10-21](#sources-of-recycle-streams)

10.9.2 Recycle Stream Quantity and Quality
[10-21](#recycle-stream-quantity-and-quality)

10.9.3 Point of Recycle Stream Return
[10-22](#point-of-recycle-stream-return)

10.9.4 Effect on Turbidity [10-22](#effect-on-turbidity-7)

10.10 References [10-22](#references)

LIST OF TABLES

Table 10-1. Typical Feed Pressures for Pressure Driven Membrane
Processes
[10-18](#table-10-1.-typical-feed-pressures-for-pressure-driven-membrane-processes)

LIST OF FIGURES

Figure 10-1. A Typical Conventional Water Treatment System
[10-1](#__RefHeading___Toc448538655)

Figure 10-2. The Alum Coagulation Diagram and Its Relationship to Zeta
Potential [10-5](#__RefHeading___Toc448538656)

Figure 10-3. Circular Radial-flow Clarifier
[10-9](#__RefHeading___Toc448538657)

Figure 10-4. Plate Settlers Used for High Rate Sedimentation
[10-10](#__RefHeading___Toc448538658)

Figure 10-5. Accelerator Solids Contact Unit
[10-10](#__RefHeading___Toc448538659)

Figure 10-6. Typical Rapid Sand Filter
[10-12](#__RefHeading___Toc448538660)

Figure 10-7. Typical Covered Slow Sand Filter Installation
[10-14](#__RefHeading___Toc448538661)

Figure 10-8. Cross Section of a Typical Pressure Filter
[10-15](#__RefHeading___Toc448538662)

Figure 10-9. Typical Filter Run Showing Progress of Floc Penetration and
Effluent Turbidity [10-17](#__RefHeading___Toc448538663)

Figure 10-10. Pressure-Driven Membrane Process Application Guide
[10-18](#__RefHeading___Toc448538664)

Figure 10-11. Typical Spiral-Wound Reverse Osmosis Membrane Module
Driven [10-19](#__RefHeading___Toc448538665)

Figure 10-12. Typical Hollow Fine-Fiber Reverse Osmosis Membrane Module
[10-19](#__RefHeading___Toc448538666)

Figure 10-13. Representation of Hollow-fiber UF Module
[10-20](#__RefHeading___Toc448538667)