Chapter 5. Overview of Water Treatment Processes

5.2



CHAPTER Five



Introduction

Selection of appropriate unit processes and integration of the processes into an overall water

treatment plant when assessing site-specific treatment needs is a complex task. Factors

that are typically considered during treatment process evaluation and selection include

(1) source water quality, (2) regulatory compliance and contaminant removal requirements,

(3) process reliability and flexibility, (4) initial construction and annual operating and maintenance costs, (5) environmental impacts, (6) utility preferences and capabilities, (7) available

site space, and (8) residuals handling requirements and site constraints.

The purpose of this chapter is to provide a general description of the various unit processes included in the design of water treatment facilities and their effectiveness and applicability for achieving broad treatment objectives. It is intended to serve as a guide and to

direct the reader to more detailed information in the chapters that follow.



Source Water Quality Considerations

(Chap 3)

.

Foremost among the considerations for selecting sources of supply for a water system is

provision of adequate quantity to meet system demands over a wide range of conditions.

Other factors that must be considered are the amount of effort and cost to withdraw and

convey the water to the point of use and the water quality characteristics that affect treatment needs and the ultimate acceptability of the potential source. The complexity of these

issues varies with location. In some locations, adequate supplies of high-quality water are

available in reasonable proximity to the area to be served, whereas others involve vast conveyance systems and multiple jurisdictions.

In recent years, communities with limited available freshwater supplies have begun to

look for new types of sources, including brackish and saline waters that can be treated by

technologies such as desalination. The potential for reuse of wastewater for either nonpotable or potable applications is another consideration. In the case of potable applications, a distinction is made between indirect potable reuse, where the effluent is returned

to a source of supply and subsequently conveyed to a water treatment process, and direct

potable reuse, where highly treated effluent is conveyed to a water distribution system for

direct consumption (see Chap. 16).

Water sources traditionally have been categorized as either surface or groundwater supplies, both of which have several general characteristics, as discussed below. Details on the

compositions of these types of supplies are presented in Chap. 3.

Surface Water

Surface water is open to the atmosphere, which brings it into contact with oxygen, a key

regulator of oxidizing conditions in natural waters. Much of the chemistry of surface water

is dominated by the presence of oxidized constituents. An exception occurs in the case of

impoundments that undergo stratification, with the accompanying oxygen deficit in the lower

levels, where the presence of biodegradable substances leads to microbial decay of oxygen

residuals. In such impoundments, the level of withdrawal and the time of year dictate the

extent to which reducing conditions develop. Strategies such as mixing to limit stratification

and various methods of in-lake aeration can be considered to alleviate such conditions.

Potential for exposure to microbial pathogens in surface supplies leads to the use of

disinfection as a treatment step in these supplies. In addition, particle-removal processes







OVERVIEW OF WATER TREATMENT PROCESSES



5.3



such as filtration typically are applied as a component of a multiple-barrier approach to

provide high levels of safety.

Runoff that enters surface supplies contains natural organic matter (NOM) that reacts

with disinfectants to form by-products, some of which are currently regulated. Production

of NOM also can result from the activity of algae and other biota within water bodies. The

concentration of NOM and the associated potential formation of disinfection by-products

(DBPs) depend on the nature of the watershed and can vary seasonally as well. The extent

of this variation should be considered when developing treatment strategies.

Point-source waste discharges and non-point-source surface runoff can directly influence the presence of contaminants in surface supplies. Although the volume of flow available in most surface supplies often reduces contaminant concentrations to relatively low

levels, concerns have been raised even at extremely low concentrations for new categories

of contaminants such as endocrine-disrupting compounds (EDCs) and pharmaceutical and

personal care products (PPCPs). While the detrimental impacts of some of these contaminants on aquatic life and wildlife have been documented, the importance of low-level

human exposure to EDCs and PPCPs has not yet been determined (see Chap. 2).

Trophic status of impounded supplies can be an important consideration because algal

activity can lead to treatment problems that challenge a number of processes. Problems

with tastes and odors and with soluble iron and manganese that increase in oxygen-depleted

lower strata of impoundments also accompany algal activity. Problems that stem from

excess discharges of nutrients often can be resolved by watershed management measures

that are also applicable for limiting exposure to a broader range of contaminants.

Storm events can be detrimental to the quality of some surface supplies by increasing turbidity and NOM concentrations, as well as by increasing nutrient loadings and the concentration of microbial contaminants. Facility design and operating concepts should incorporate

provisions for dealing with these and other causes of extreme water quality conditions.

Groundwater

Groundwater supplies are confined to substrata, thereby limiting the availability of oxygen,

which can lead to the need for different treatment methods than those used with surface

supplies. These conditions often lead to treatment to deal with reduced substances such as

iron, manganese, hydrogen sulfide, ammonia, and reduced arsenic.

The substrata also can protect the water against contamination by microbial pathogens,

resulting in treatment approaches for this category of contaminant that differ from those

applied for surface-water supplies. Multiple-barrier approaches that combine disinfection

with particle-removal processes are not needed for some groundwater supplies. However,

not all groundwater supplies are the same, and some are subject to direct surface-water

influence, requiring the use of treatment barriers for pathogens in such cases.

Quality of groundwater varies widely depending on the nature of the geology through

which it flows (see Chap. 3). Variations can be regional or across the aquifer layers within

a region, resulting in wide differences in required treatment sequences that depend on general characteristics such as hardness and total dissolved solids and the presence of specific

natural contaminants such as radon and radionuclides. While total organic carbon (TOC)

concentrations in most groundwater supplies are generally low (<2 mg/L), some groundwater supplies contain elevated concentrations of TOC that require treatment strategies to

limit the formation of regulated disinfection by-products.

The confinement of groundwater can restrict flow, which can affect the rate of withdrawal

from an aquifer and the corresponding yield for a given well. Often a system is supplied from

multiple wells, each of which may have its own water quality characteristics and susceptibility

to contamination that affects blended quality, thereby requiring management and operating

decisions to make best use of these resources.



5.4



CHAPTER Five



Groundwater aquifers have long residence times and thus are renewed with less frequency

than surface water systems. Therefore, should aquifers become contaminated, the contaminants

can occur at much higher concentrations than surface supplies, which places consumers at greater

risk of extended exposure to contaminants unless effective treatment measures are provided.

Water Quality

Several general water quality variables must be considered when selecting and configuring

the treatment sequence for a given water source regardless of whether it is a surface-water

or groundwater supply, including the following:

1. pH. This variable has major effects on the chemistry of constituents in water and on treatment-process performance. It is therefore imperative that the pH of the source water is monitored routinely and that the pH profile through the treatment process is controlled. pH also

affects corrosion and water quality in the distribution system and thus must be controlled to

minimize water quality impacts.

2. Alkalinity. Alkalinity is the measure of the acid-neutralizing capacity of a solution. It is

defined by the amount of acid required to reduce the pH to a defined end point. For the

pH conditions of most supplies, alkalinity is due mainly to the bicarbonate concentration.

Alkalinity is an important factor in coagulation and in selecting the corrosion-control methodology to be implemented. Acidity, a related parameter, is a measure of the base-neutralizing capacity. Principles pertaining to alkalinity and acidity are discussed in Chap. 3.

3. Hardness. The level of hardness can dictate treatment concepts that should be applied.

A high level of hardness tends to favor softening over clarification only and can affect a

range of treatment processes. Like alkalinity, it also can affect the choice of corrosioncontrol methods.

4. Turbidity. Turbidity is a measure of the particulate matter in the water. It affects the

choice of clarification methods and can dictate whether or not there is a need for pretreatment upstream of some other processes.

5. Natural organic matter. NOM presents a concern for disinfection by-product formation, increases coagulant and oxidant demand, and can affect a number of treatment

processes. It can be characterized by several surrogate measurements, including TOC

concentration and ultraviolet (UV) absorbance (see Chap. 3).

6. Total dissolved solids (TDS). This measure of salt and mineral content can affect treatment needs as well as the acceptability of a source of supply.

7. Dissolved oxygen. Dissolved oxygen is an important regulator of oxidation-reduction conditions that determine the chemical speciation of a number of constituents in water (see Chap. 3).

Oxidizing conditions are produced even when oxygen is present at low concentrations.

In addition to these general parameters, water sources can have unique quality concerns

that must be addressed.



Characteristics and General Capabilities

of Unit Processes

This section provides an introduction to the treatment processes discussed in the chapters

that follow. The objectives are to present a broad overview of the various process components and their typical applications and capabilities and to serve as a “road map” to the







OVERVIEW OF WATER TREATMENT PROCESSES



5.5



contents of the remainder of the book. A more detailed discussion of each unit process is

presented in the referenced chapters.

Aeration and Air Stripping (Gas-Liquid Processes; Chap. 6)

Air stripping and aeration are gas-transfer processes that have played important roles in

water treatment for many years, including transfer of oxygen for oxidation of constituents

such as iron and manganese, removal of certain tastes and odors, and removal of dissolved

gases such as hydrogen sulfide and carbon dioxide. Air stripping typically is used to remove

volatile organic compounds (VOCs) from contaminated groundwaters that became a concern in the 1980s and newer contaminants such as methyl-tert-butyl-ether (MTBE) and

radon. Air stripping to remove trihalomethanes (specifically, chloroform) has been used

in some facilities, but this approach has proven somewhat limited in scope because most

disinfection by-products are not amenable to removal by air stripping.

Aeration is not appropriate for treatment of groundwater containing reduced arsenic and

iron because iron is oxidized more quickly than arsenic by oxygen. For effective removal

of arsenic, both arsenic and iron need to be oxidized at the same time by a strong oxidant

such as free chlorine.

Gas transfer processes are most applicable when there is an imbalance in the activity of

the constituent in the gaseous and liquid phases, where activity in the gas phase is generally characterized by the partial pressure, and activity in the liquid phase is characterized

by concentration. This imbalance causes constituents to flow from the condition of higher

activity in the direction of lower activity to balance the conditions in the two phases. The

rate of transfer depends on the extent of the imbalance, the surface area available for transfer, the degree of mixing and turbulence, temperature, and intrinsic diffusion rates of the

target constituent. Constituents such as VOCs are readily removed by air stripping. For

constituents that can react in solution, the rate and extent of the transfer are also affected

by factors such as pH and other reactive constituents in solution, such as chlorine, carbon

dioxide, hydrogen sulfide, ammonia, and oxygen. Constituents that have little or no tendency for reaction in solution, such as radon and VOCs, recently have become targets for

treatment by air stripping.

The susceptibility of a target constituent to gas transfer depends on its solubility in water

relative to its corresponding partial pressure or concentration in the gas phase. Gases that

are poorly soluble in water relative to their concentrations in the gas phase can be readily

removed. The most common measure of these relationships is Henry’s constant, which is the

ratio of the activity of a constituent in gas to the activity in solution at equilibrium. The higher

the Henry’s constant of the constituent, the more readily removable it is by gas transfer.

Equipment for bringing gas and water together to produce gas transfer generally is configured to produce a large surface area at the boundary between the two phases and, in some

cases, a degree of turbulence, both of which are necessary for accelerating the rate of transfer. This equipment includes spray nozzles, diffusers that control the size of the bubbles for

the gaseous phase, and tray aerators and packed-bed towers that spread the surface of the

interface across specially designed media. In some cases, treatment of aerator/stripper offgas may be required to remove contaminants in order meet air quality criteria.



Chemical Oxidation (Chap. 7)

The basic purpose of chemical oxidation is to change the oxidation state of the constituents

involved. In the context of water treatment, the intent is to produce changes that improve

water quality, but there is also the potential for the formation of undesired reaction products



5.6



CHAPTER Five



(such as halogenated disinfection by-products when chlorine is used as the oxidant), an

outcome that can require wise decision making.

Two different types of chemical oxidation are used commonly in water treatment. The

more direct method consists of applying the chemical oxidant to produce the desired treatment outcome in a single step, such as in the oxidation of taste- and odor-causing compounds by ozone. With the second method, chemical oxidation is a component of a multistep

sequence whereby the oxidant is used to alter the oxidation state of the target constituent to

a form that is more readily removable by subsequent treatment steps. Oxidation of reduced

forms of iron and manganese to higher oxidation states that will readily precipitate is one

example of this method. Removal of sulfides and arsenic in multistep sequences is another

example. Since all chemical disinfectants are oxidants, oxidation will occur to some degree

even when the objective is inactivation of microbial contaminants.

Chemical oxidants used most commonly in water treatment include

1. Chlorine. Chlorine is used widely in water treatment as a disinfectant. It can oxidize

iron and manganese, color, and some taste- and odor-causing compounds. It is especially effective in manganese removal when applied ahead of filtration to maintain an

oxidized coating on the granular media. This method of manganese removal, while

often inadvertent, is an important consideration if points of chlorine addition are modified to meet other treatment objectives. Chlorine is available commercially in several

forms: as a compressed gas, in aqueous liquid form (sodium hypochlorite), and in solid

form (calcium hypochlorite). Another alternative that has received increased attention

in recent years is on-site electrolytic generation of sodium hypochlorite from salt (see

Chap. 17).

2. Permanganate. Permanganate can be used for oxidation of iron and manganese and

for removal of some taste and odors. Traditionally, it has been available as potassium

permanganate, which is delivered as a solid that requires handling and dissolution prior

to application. Recently, liquid sodium permanganate has become available and can be

applied directly as delivered.

3. Ozone. Ozone is a strong oxidant that is effective for reduction of color, control of a

broad range of taste- and odor-causing compounds, and conversion of NOM to forms

more readily removed by biological filtration. It sometimes can improve coagulation

and filtration and reduce the formation of some disinfection by-products, notably trihalomethanes and haloacetic acids. While also effective for oxidation of iron and manganese, use of ozone for control of manganese has proved complex and problematic in

some cases because of the formation of colloidal manganese oxide particles. Overdosing

of ozone also can result in the formation of permanganate, which can pass through filters and later precipitate manganese oxides in the distribution system. Because ozone is

highly reactive and not sufficiently stable for storage, it is produced on site using generation equipment that applies a controlled electrical discharge to interact with oxygen

to form ozone. Sources of oxygen for this process include ambient air that has been

treated to remove moisture and impurities, commercially manufactured liquid oxygen,

and application of equipment to produce a concentrated oxygen stream on site. While

use of ambient air was the predominant approach in early ozone-generation systems,

newer designs typically use more concentrated oxygen streams that yield improved

generation efficiencies and higher ozone concentrations in the feed gas.

4. Chlorine dioxide. Chlorine dioxide can oxidize iron and manganese and some taste- and

odor-causing compounds, reduce color, and in some cases reduce trihalomethane and

haloacetic acid formation potential. It is an alternative to chlorine that reduces production of chlorinated organic by-products, but care must be exercised to manage application rates such that excessive amounts of chlorine dioxide and its inorganic degradation







OVERVIEW OF WATER TREATMENT PROCESSES



5.7



products (i.e., chlorite and chlorate) are not present in the finished water. Being highly

reactive, chlorine dioxide cannot be manufactured off site for delivery to the point of

use. It is therefore generated on site by reactions between (1) gaseous chlorine and

sodium chlorite, (2) hydrochloric acid, sodium hypochlorite, and sodium chlorite, or (3)

sulfuric acid and proprietary blended sodium chlorate–hydrogen peroxide products.

5. Advanced oxidation. Advanced oxidation processes produce highly reactive free radicals, most notably the hydroxyl radical. Much of the interest in these processes has been

directed toward removal of specific organic contaminants, such as taste- and odor-causing compounds that are resistant to treatment using other oxidants. Interest in removal of

a range of other organic contaminants also has emerged. Technologies used to generate

these radicals include (a) ozone and hydrogen peroxide, (b) UV radiation and hydrogen

peroxide, (c) UV radiation and ozone, and (d) various approaches involving catalysts.

6. Mixed oxidants. Mixed oxidants are generated on site from salt solutions and carefully

controlled operational settings to produce a hypochlorite-based solution with disinfection and oxidation properties different from hypochlorite alone. Mixed oxidants offer

an alternative to traditional forms of chlorination and chloramination for secondary

disinfection and residual maintenance in the distribution system.

Treatment results from use of these oxidants are influenced by pH, temperature, oxidant

dose, reaction time, and the presence of interfering substances. Characteristics of an oxidant in solution can be affected particularly by pH, whereas increased temperature almost

universally increases the rate of reaction. Reaction time and dose are fundamental considerations for providing conditions that affect the extent to which a target constituent will

be removed, whereas pretreatment processes and process sequencing may be important

considerations for limiting the adverse effects of interfering substances. The susceptibility

of various chemical bonds to oxidative attack are also critical considerations and therefore

influence selection of chemical oxidants. Each oxidant can lead to different treatment outcomes and effectiveness for a given application.

While chemical oxidants are most commonly used for oxidation of specific constituents,

they also can take part in other types of reactions where oxidation is not necessarily the only

outcome, such as substitution reactions between chlorine and ammonia to form chloramines.

This is a desirable outcome because chlorine is substituting into the structure of ammonia

without a change in oxidation state, thereby retaining the capability for disinfection. Other

substitution reactions include the formation of halogenated disinfection by-products, an undesired outcome of reactions between oxidants and precursor components of NOM.



Coagulation and Flocculation (Chap. 8)

Coagulation is a fundamental process used at water treatment plants worldwide as a pretreatment step not only for traditional rapid-rate filtration technologies that comprise much

of the water treatment infrastructure that still exists but also for many of the newer technologies. In rapid-rate filtration plants it serves as the mechanism for building particles to a size

that can be readily removed in the clarification step, as well as for conditioning particles to

adhere to the filter media to ensure maximum process efficiency. Its importance to filtration

was underscored by the results of a survey by Cleasby et al. (1989) that found coagulation

to be more important than physical filter characteristics and loading rates for meeting filtration goals at many of the sites surveyed.

The traditional view of coagulation is that it facilitates agglomeration of small colloidal

particles into larger particles of a size that can be physically removed. The mechanisms

for achieving this are discussed in detail in Chap. 8. The role of coagulation in removing



5.8



CHAPTER Five



TOC has emerged in the last 20 years, and for many surface-water supplies, TOC controls

coagulant dosing rather than raw-water turbidity.

Ferric and aluminum salts are the most common primary coagulants. Aluminum salts

include alum and various commercial products such as polyaluminum chlorides. Ferric salts

include ferric chloride and ferric sulfate and less commonly used formulations.

High-charge-density cationic polymers are sometimes used alone in direct filtration

applications or as a dual coagulant with alum or ferric coagulants. High-molecular-weight,

long-chained nonionic polymers or anionic polymers of very low charge are used as flocculant aids and filter aids. As flocculant aids, they are capable of bridging particles together to

increase floc size and strength, thereby increasing floc settling velocities. As filter aids,

they improve attachment of particles to filter grains and previously retained floc particles.

Typical points of application of flocculent aids are downstream from the rapid mix or near

the midpoint or the end of a conventional flocculation basin. Filter-aid polymers are typically

added immediately upstream of the filters.

Discussions of coagulation chemistry emphasize the importance of pH as a critical

process condition for coagulation with the common metal salts, which can be especially

sensitive to changes in this parameter (see Chap. 8). This is because pH affects the chemical

speciation of the dissolved coagulant in rapid mixing and the precipitation of aluminum or

ferric hydroxide.

Polyaluminum chlorides, aluminum chlorohydrates, and some other commercial formulations differ from the common metal coagulant salts in that their composition is adjusted

during the manufacturing process to maximize the presence of the most effective dissolved

aluminum species in rapid mixing. These prehydrolyzed coagulants, which are effective at

neutralizing the negative charge of particles, are less acidic than alum and ferric coagulant, so

their use in low- to medium-alkalinity waters without the need for addition of supplemental

base to control pH may be feasible. In addition to pH, coagulation is strongly affected by a

number of other factors, including particle characteristics, concentration and nature of NOM,

presence of anions that interact with the metal coagulants, and temperature. There is no single

“preferred” coagulant—the selection varies with the treatment conditions and goals.

In addition to these and other direct chemical influences, decisions regarding coagulant

application are affected by a number of other secondary effects that can result, such as the

production of residual solids, the introduction of contaminants such as manganese that

occur as impurities in the coagulant, and changes in pH that affect other aspects of treatment. A more recent concern has emerged regarding the effects that coagulant chemicals

have on the finished water chloride-to-sulfate ratio, which may lead to increased lead corrosion in some water systems (see Chap. 20). Corrosion of metal components within a water

treatment facility prior to final adjustment of finished water pH can also be affected by the

pH during coagulation.

Rapid mixing and flocculation provide the physical mechanisms for dispersion of chemicals and particle contacting following coagulation. Although the physical characteristics of

the equipment used in the coagulant dispersion and flocculation processes are similar, the

process goals are very different. The objective of mixing is rapid dispersion of the coagulant

chemical to achieve relatively uniform distribution. Long detention times are not needed to

achieve this goal and actually can be detrimental because dispersion tends to be most effective when it occurs rapidly within a confined space. Older rapid-mix basin configurations

based on detention-time criteria are no longer regarded as the best approach. Dispersionoriented configurations include basins in which mixing is confined to the smallest possible

volume as flow passes the mixer blade and a variety of in-pipe configurations that include

mechanical mixers, static mixers, and hydraulic-jet dispersion concepts. Other approaches

can involve distributed dispersion across weirs and hydraulic jumps. Even simple in-pipe

addition can provide better results than some of the older conventional rapid-mix basins

(Clark et al., 1994).







OVERVIEW OF WATER TREATMENT PROCESSES



5.9



Mixing of the coagulant is followed by flocculation to aggregate the chemically conditioned particles into floc that can be removed by downstream processes. The objective

of this process step is controlled introduction of mixing energy to induce contact between

particles that leads to a progressive increase in floc size. Ideally, this process will result

in a uniform floc size with particles in the optimal size range for removal by clarification

or filtration. Unlike coagulant dispersion/mixing processes, detention time is important in

the flocculation process for developing floc of the desired size. The most common types

of equipment for this process are vertical turbines and paddle-type flocculators. Hydraulic

flocculators, which rely on the turbulence created as flow is redirected around baffles

installed in the floc basin, are another alternative. In some cases, gates that can be adjusted

to vary the energy imparted to the process stream are provided to optimize the hydraulic

flocculation process. Turbulence within a pipeline or at the entrances to a series of unmixed

chambers is also applied in some cases. However, these hydraulic flocculation systems

generally offer less operational flexibility and process-control capability over a range of

flow rates than conventional mechanically mixed systems.

The design of the flocculation step is generally based on the floc characteristics desired

for downstream clarification and filtration. As an example, dissolved air flotation targets

smaller floc sizes than conventional sedimentation because the floc particles attach to air

bubbles to be floated; excessively large floc size would be detrimental to this process. Other

downstream processes are also optimized by using different floc size distributions. For

direct filtration, filterable floc size is the target; floc that is too large can cause filter “blinding.” Even for conventional settling, there is usually a maximum desirable floc size owing

to the lower floc densities and susceptibility to shear that is often encountered with large

floc. Therefore, it is essential that particle size be considered in configuring and operating

the flocculation process.

The performance of most unit processes within a water treatment plant is highly dependent on effective coagulation, mixing, and flocculation. The pretreatment benefits of coagulation can significantly influence the performance of other unit processes discussed in

this book, including chemical oxidation, ion exchange, adsorption of organic compounds,

chemical disinfection, and UV irradiation.

Sedimentation and Flotation (Chap. 9)

Sedimentation and flotation processes achieve the objective of separating particles from the

process stream. While these processes can be used alone in pretreatment or other applications, they usually follow coagulation or precipitation. In a larger context, these processes

fall into the category of clarification technologies. Both processes are gravity processes; in

the case of settling, since the density of the particles exceeds the density of water, settling

occurs, whereas in flotation, because the density of the bubble-floc aggregate is less than

the density of water, the aggregate rises. Principles of settling and rise velocity are similar,

and both processes are sized based on hydraulic loading rates—often referred to as the

overflow rate.

Central to an understanding of sedimentation processes is the concept of surface loading rate or overflow rate (OR), which was developed by Hazen (1904). Its importance

was further developed with regard to process effectiveness by Camp (1936). Calculated

by dividing the process flow rate by the unit process steeling area, OR corresponds to a

vertical particle settling velocity and therefore is commonly expressed in units of m/h

(gpm/ft2 or gpd/ft2).

Under the OR concept, settling basins with greater settling or surface areas perform

better. Recognition of this relationship led to the development of tray, tube, and inclinedplate settling concepts to increase sedimentation rates by incorporating additional settling



5.10



CHAPTER Five



surfaces within a given volume. The OR concept is commonly applied to all the sedimentation processes and is used as a fundamental basis for characterizing and evaluating these

processes.

A number of sedimentation processes are available, including a variety of proprietary

configurations. In general, they are divided into the following categories:

1. Conventional horizontal-flow sedimentation. This is a traditional approach that consists

of simple gravity settling in a basin or tank with no enhancements to accelerate settling.

It is simple to operate but requires a larger plan area than other clarification processes.

2. High-rate gravity settling. This process involves the use of additional configurations

or devices that increase the available effective surface area in the basin. Commercially

available inclined-plate and tube settler equipment is the most widely used equipment

of this type. Tray sedimentation basins that employ multiple “floors” within a given

sedimentation basin area to maximize flow path lengths are another example—these

were used in the past but are not used now in new plants.

3. Solids contact clarifiers. These basins generally contain mixing and flocculation equipment within a common reaction zone, with settling occurring in another zone of the

same basin. Solids are recycled to maintain a high solids inventory in the reaction zone

and to provide increased opportunities for contact between particles, thereby increasing

the size and settling rates of flocs or precipitates and allowing the use of higher surface

loading rates than in conventional gravity settling. One modification of this approach

is the IDI Densadeg process, which separates the mixing and settling compartments

between different basins; the solids inventory within the reaction zone is maintained

through collection and return of settled solids from the settling tank to the mixing/

reaction stage. Withdrawal of solids (termed blowdown) from the sludge blanket within

the solids contact clarifier is necessary to avoid loss of solids from the clarifier into the

process stream. Appropriate blowdown rates and duration are critical to maintaining a

properly balanced system; excessive rates will remove too many solids from the blanket,

whereas an inadequate rate will allow solids to overflow to the process stream.

4. Floc blanket clarifiers. Flow into these clarifiers is introduced at the bottom and withdrawn from the surface. The process relies on a solids blanket that is developed at the

bottom of the basin to entrain incoming particles. These clarifiers are available in several configurations, but all are equipped with a submerged weir over which the blanket

will overflow to be removed by a blowdown process. The appropriate blowdown rate is

also critical for this process because excess blowdown will result in larger than necessary waste streams, and inadequate blowdown will result in overflow of solids into the

process stream. One application of this approach is the SuperPulsator clarifier, in which

a pulsing action is applied to the floc blanket to keep it homogeneous.

5. Ballasted flocculation. This process accelerates settling by using polymer to attach floc

particles to a concentrated mass of fine sand (microsand). The density of the resulting

floc is increased by the sand, thereby allowing use of high overflow rates. Settled solids

are collected from the bottom of the settling compartment and pumped to hydrocyclones, where the sand is separated from the lighter floc and returned to the process

stream, and the waste solids are directed to the disposal process. The accelerated particle

settling features of this process allow use of high loading rates in the settling zone, with

corresponding reductions in the system footprint.

Dissolved air flotation (DAF) is an alternative to sedimentation. Available in several

configurations, DAF employs a pressurized recycle stream that is saturated with air. The

recycle stream discharges at the inlet to the flotation basin, and large quantities of fine

bubbles are released. Floc particles from the upstream coagulation/flocculation process







OVERVIEW OF WATER TREATMENT PROCESSES



5.11



attach to the bubbles and are floated to the surface. Conventional DAF systems operate at

hydraulic loading rates similar to filter rates. High-rate systems have been introduced in

the last 10 years (see Chap. 9).

The sedimentation and DAF processes have site-specific advantages that require recognition of respective benefits and limitations for proper process selection. For example, dissolved air flotation is most effective in treating reservoir supplies, which are low in mineral

turbidity (turbidity attributable to silts/clays). In addition, it can have application to supplies

with moderate to high color or TOC.

Polymer is used with some of the sedimentation processes, and possible effects of polymer residual on downstream processes are a potential concern depending on the nature and

quantity of polymer applied. Ballasted flocculation processes require the application of

polymer to attach floc to microsand; polymer usage cannot be avoided for this process, and

excessive carryover of polymer has been shown to be detrimental to downstream granular

media filter performance if dosing is not regulated properly. Other processes can also benefit from the application of polymer for improved settling. This can be an important consideration for both solids blanket and floc blanket clarifiers, where settling properties within

the floc blankets can be critical to effective performance. Gravity settling (both conventional and high rate) can benefit from polymer addition as well, but application generally

is less critical for these processes. Dissolved air flotation generally functions well without

polymer, although there can be isolated situations where polymer can be beneficial.

Granular Media Filtration (Chap. 10)

Conventional media filtration systems include granular bed, precoat, and slow sand filters.

Granular bed filtration, the most commonly used of these processes, consists of a filter

box that contains a bed of granular media placed on a support layer with an underdrain

below it. Sand, anthracite, granular activated carbon (GAC), and high-density garnet and

ilmenite represent common media options, and other commercially produced media have

been developed. The dual-media arrangement (typically anthracite over fine sand) is the

most common configuration. Efficient removal of particles requires an upstream coagulation step because attachment of flocs to filter grains and previously retained flocs are the

mechanisms of floc particle removal within the pores of the filter bed.

Water flows through the filters in the downward direction; filtered water is collected by

the underdrain system at the bottom of the filter and conveyed through filter effluent piping.

Flow and filter level are usually controlled by valves in the filter effluent piping; however,

some types of granular bed filters allow flow to diminish as head loss accumulates to produce a declining-rate mode of operation.

Granular bed filters require periodic backwashing to displace particles that accumulate

on the media over the course of a filter run. Older backwashing methods relied on hydraulic

scour to clean the filter media during backwash, but newer air-scouring approaches offer

significant advantages, including improved media cleaning and reductions in the amount

of water required for backwashing.

Precoat filtration consists of applying a fine filter medium to a filter leaf assembly,

where it is restrained by a fine-mesh septum. The media are applied initially to build up

a cake on the surface of the septum in a precoat step. Naturally occurring diatomaceous

earth and commercially produced perlite are the typical sources of this media. Feed water

is applied following completion of the precoat step, and it is common to provide a supplemental body feed of the filter material as the run progresses. Precoat filtration differs from

granular bed filtration in two respects: (1) particles are removed by straining, and therefore,

the process does not require the upstream addition of coagulant for particle conditioning,

and (2) the media are disposed of at the end of each filter run. Since coagulant is not added,



5.12



CHAPTER Five



removal of TOC is not achieved, which limits the process to treating high-quality waters

with low TOC.

Slow sand filtration is similar to granular media filtration in that the water passes

through a bed of sand, but the process differs mechanistically. Hydraulic loading rates are

much lower [typically 0.1–0.3 m/h (0.04–0.12 gpm/ft2)], and much finer media are used.

As with precoat filtration, coagulation is not required because much of the removal takes

place through a gelatinous layer that develops on the top of the filter; this layer is commonly

referred to as the schmutzdecke. Biological activity that plays a role in the filtration process

can develop in this layer and in the upper portion of the sand bed. The filter is cleaned periodically by scraping the schmutzdecke and the top layer of sand when head loss becomes

excessive. Required cleaning frequencies can range from several weeks to up to one year,

depending on media design and applied water quality. GAC also has been incorporated into

slow sand filters to improve removal of organic contaminants, typically with an overlying

layer of sand media.

While filtration is important for producing high-clarity water for customers, its primary

function is to remove particles that may include microbial pathogens. This is a role that has

evolved in the last 20 years following promulgation of the U.S. Environmental Protection

Agency’s (USEPA’s) Surface Water Treatment Rule in 1989. Prior to that time, the regulated limit for filtered water turbidity was 1 nephelometric turbidity unit (ntu). Heightened

awareness of the importance of filtration for controlling waterborne pathogens in a multiple-barrier approach in conjunction with disinfection processes emerged in the 1980s with

the discovery of pathogens that were resistant to chemical disinfectants. Initially, the focus

was on Giardia, and the turbidity limit was lowered to 0.5 ntu for granular bed filters to

address this concern. Recognition of Cryptosporidium as an even more chlorine-resistant

waterborne pathogen led to further reduction in the turbidity limit for plants employing

coagulation and granular media filtration to 0.3 ntu level or less, and voluntary guidance

under the criteria developed for the Partnership for Safe Water set a turbidity goal of less

than 0.1 ntu.

These more restrictive requirements have profoundly altered the approach to granular bed filtration in recent years. The need to view it as a component of a larger process

sequence has increased, particularly with recognition of the importance of effective coagulation for producing conditioned particles that can be removed efficiently by attachment to

filter media. Older, shallow-depth, single-media sand filters that were once common have

been displaced by more efficient dual-media filters. Deep-bed monomedia filters are being

used in higher-rate filters. Whereas older single-media filters had a smaller sand size and

operated at lower hydraulic loading rates, these configurations ignored the importance of

attachment rather than straining mechanisms for achieving efficient filter performance.

These older configurations were more prone to surface blinding and accompanying turbidity breakthrough that results from shearing of floc even at lower filtration rates.

Hydraulic loading rates, or filter rates, for granular media filters have increased substantially in recent years with the application of newer and more efficient media configurations.

While rates as low as 5 m/h (2 gpm/ft2) were once common, filter rates of 10–15 m/h (4–6

gpm/ft2) are now routine, and rates of 20 m/h (8 gpm/ft2) and higher have proved acceptable in some circumstances. Media size and configuration affect the acceptable filter rate

because some configurations are more tolerant of higher rates than others. In most cases,

larger media applied in deeper beds are more accommodating to higher filter rates than

smaller, shallower media.

While granular bed filtration until recently has been the most common approach to

media-based filtration, precoat filters and slow sand filters may be preferable in some situations, and membrane filters (discussed in Chap. 11) offer a new option that is increasingly

being used in new treatment facilities. These processes differ from granular bed filters in

terms of the mechanism of particle removal; precoat and membrane filters remove particles