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Identification and Characterization of

Typical Drinking Water Treatment

Residuals

Peer Involvement for the Drinking Water Treatment Effluent Guidelines

EPA Docket Number EPA-HQ-OW-2004-0035 (www.regulations.gov)

September 2007
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OVERVIEW OF THIS DOCUMENT

This document is the first of three EPA documents that summarize the major technical

and engineering issues related to the management of drinking water treatment residuals.

EPA developed these three documents to support the Drinking Water Treatment effluent

guidelines rulemaking.1 EPA invited a broad range of stakeholders to review these three

documents to allow for early stakeholder involvement into the rulemaking process and to

improve EPA’s record and analyses. The other two EPA documents included in this peer

involvement process are: (1) Identification of Technology Options; and (2) Evaluation of

Potential Subcategorization Options for the Category. This document and its peer

involvement will be placed in the Drinking Water Treatment effluent guidelines

rulemaking record (http://www.regulations.gov, Docket Number: EPA-HQ-OW-2004-

0035).

This document identifies and discusses the characteristics of typical waste products,

known as residuals, generated at drinking water treatment plants. Section 1 describes

the general classifications of water treatment plants. Section 2 describes the general

types of residuals generated at each type of water treatment plant. Section 3 lists the

questions EPA posed to reviewers. Section 4 lists the references used in developing this

document. EPA will use this document and other information in its record to develop

regulatory options for the proposed rulemaking.

Standard Disclaimer

Neither the United States government nor any of its employees, contractors,

subcontractors, or other employees makes any warranty, expressed or implied, or

assumes any legal liability or responsibility for any third party’s use of, or the results of

such use of, any information, apparatus, product, or process discussed in this report, or

represents that its use by such a third party would not infringe on privately owned rights.

References to proprietary technologies are not intended to be an endorsement by the

Agency.

1 EPA published the 2004 Effluent Guidelines Program Plan (2 September 2004; 69 FR 53705) and

identified the “drinking water treatment point source category” as a candidate for effluent guideline

rulemaking. See http://www.epa.gov/guide/dw.
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1.0 WATER TREATMENT PLANT CLASSIFICATION

Potable water systems use one or more water treatment processes to provide and

maintain reliable, high-quality water to their customers. The type of water treatment

process used at a given treatment plant is influenced by a number of factors, including

source water quality, the contaminant(s) to be removed, reliability of the treatment

process, flexibility to varying conditions, utility capabilities, costs, environmental

compatibility, and distribution system water quality.

Water treatment plant types can generally be classified into one of the following five

categories based on the primary treatment process used at the plant (ASCE, 1996):

 Coagulation and Filtration;

 Precipitative Softening;

 Membrane Separation;

 Ion Exchange; and

 Adsorption.

Drinking water processes may be used individually or together in a “treatment train”

design. The term “treatment train” refers to treatment units in series. The basic unit

processes of each primary treatment process are only described once. For example,

oxidation can apply to any of the above categories but is only described under the first

category, coagulation and filtration.

Drinking water treatment plants also contain other processes such as fluoridation or

disinfection (without filtration). These other processes are not discussed because they

typically do not generate residuals requiring management or disposal.

1.1 Coagulation and Filtration

Coagulation/filtration processes are traditionally referred to as conventional water

treatment. Conventional treatment is the most widely used technology for removing

turbidity and microbial contaminants from surface water supplies. These processes may

also be used to remove color and taste- and odor-causing compounds. The basic unit

processes may include screening, chemical pretreatment, presedimentation, aeration,

oxidation, coagulation/flocculation, sedimentation/flotation, filtration, and disinfection.

Other nonchemical variations include direct filtration and diatomaceous earth filtration.2

Depending on the raw water quality and the constituents to be removed, the unit

processes are combined to design a water treatment pant. The typical unit processes

that may be included in a coagulation/filtration plant are discussed below. Additionally,

drinking water treatment utilities may periodically use adsorption technologies such as

activated carbon to remove pesticides (e.g., atrazine) or other organic materials in the

surface waters. Activated carbon is an adsorbent material that possesses vast amounts

2 Slow sand filtration, which primarily relies on biological mechanisms to achieve the required

turbidity and microbial removal rates, is not being included in the analysis of water treatment plant

residuals. These types of filters are not backwashed, but rather are cleaned by periodic removal

of the surface media. The spent media is generally stored on site or disposed of in a landfill. As a

result, slow sand filtration does not typically generate solid or liquid waste streams that require

discharge to a surface water or wastewater treatment plant.
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of surface area on which contaminants can be extracted and reside. Intermolecular

attractive forces within the small pores of the carbon are the mechanism by which the

removal and retention of the contaminants are achieved. Activated carbon is available in

two common forms: powdered (PAC) and granular (GAC). The adsorptive properties of

PAC and GAC are fundamentally the same because they both depend on pore size and

surface area to effect separation and retention of the pollutants.

Although the two forms of activated carbon are similar in function, they differ in

application. PAC is usually applied early in the treatment process to reduce problems

associated with taste and odors. GAC is usually applied later in the treatment process to

remove dissolved organic chemicals and disinfection byproducts or their precursor

materials. When PAC is applied early in the treatment process, it is mixed with the

influent water before coagulation chemicals are added and travels through the treatment

process with removal in the sedimentation/filtration unit operations.

1.1.1 Screening and Presedimentation

Screening captures large debris such as leaves, logs, plastic bottles, sticks, and fish at

the intake where water is drawn into the treatment plant. Presedimentation allows

removal of settleable solids in the water by gravity prior to additional treatment. These

are physical separation processes that occur at or near the drinking water intake.

Drinking water utilities will typically use presedimentation on surface waters with high

suspended solids as a means to remove a large portion of the largest suspended solids

and to control organic material that can cause taste and odor problems. A typical

presedimentation train includes a rapid mixing chamber, a flocculation basin, and a

sedimentation basin equipped with sludge collection equipment. Treatment chemicals

used in presedimentation include powdered activated carbon, alum, potassium

permanganate, coagulant polymer, and chlorine.

1.1.2 Oxidation

Oxidants are used in water treatment to disinfect, control nuisance aquatic growths,

mitigate objectionable tastes and odors, remove color, remove iron and manganese,

and/or oxidate organic chemicals. In some cases, oxidants may also improve the

performance of, or reduce the required amount of, coagulants. (AWWA, 1998; AWWA,

1999)

Common oxidation processes include adding chlorine, potassium permanganate, ozone,

or chlorine dioxide. Ozone is sometimes used with hydrogen peroxide or ultraviolet

irradiation to produce radicals that have powerful oxidative properties. These oxidants

are often added at the head of the treatment plant, prior to or at the rapid mix basin, but

they can also be added after clarification and/or prior to filtration. (AWWA, 1999)

Chlorine is the most widely used oxidant in water treatment practice. Chlorine is typically

applied in one of three forms: chlorine gas, sodium hypochlorite, or calcium hypochlorite.

These compounds produce hypochlorite ions that inactive pathogens. Chlorine dioxide

exists as a gas and is highly soluble in water. Since it is explosive under pressure, it

must also be generated on site. Most commercial generators use sodium chlorite as the

common precursor feedstock to generate chlorine dioxide for drinking water application.
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While these disinfectants are effective in controlling many microorganisms, they react

with natural organic and inorganic matter in source water and distribution systems to

form DBPs. Results from toxicology studies have shown several DBPs (e.g.,

bromodichloromethane, bromoform, chloroform, dichloroacetic acid, and bromate) to be

carcinogenic in laboratory animals. Other DBPs (e.g., chlorite, bromodichloromethane,

and certain haloacetic acids) have also been shown to cause adverse reproductive or

developmental effects in laboratory animals. As a result of EPA’s Stage 1 Disinfection

Byproduct Rule requires water producers to further limit DBPs in their distribution

systems (see 16 December 1998; 63 FR 69389).

In order to control the production of DBPs operators are increasingly switching from

chlorination to chloramination as chloramination exhibits less tendency to convert

organic materials to DPBs. Additionally, unlike chlorine, chloramine has a longer half life

in the distribution system and still maintains effective protection against pathogens. The

reason chloramines persist in the distribution is due to the relatively lower redox potential

in comparison to free chlorine. Chloramine is formed by the addition of ammonia into

drinking water to form mono-, di-, and trichloramines.

Ozone exists as a gas at room temperature. Because ozone is an unstable molecule, it

is generated on site at the point of application by passing a stream of air or oxygen

through a high voltage electrical discharge.

Potassium permanganate is used primarily to control objectionable taste and odors,

remove color, control biological growth in treatment plants, and remove iron and

manganese. Although potassium permanganate has many potential uses as an oxidant,

it is a poor disinfectant. In water treatment, potassium permanganate is usually added as

a solution prepared from the granular solids. The solids are added to a hopper and

metered into the makeup water with a screw feeder. The resulting solution is then

injected into the raw water by a metering pump. (USEPA, 1999)

1.1.3 Coagulation and Flocculation

Coagulation in water treatment refers to the process of destabilizing a particle

suspension using a chemical coagulant and gentle agitation to promote the formation of

settleable and/or filterable floc particles. Coagulation is performed in three separate and

sequential steps: coagulant formation, particle destabilization, and inter-particle

collisions. The coagulation and flocculation process has traditionally been used to

remove solids from drinking water supplies. However, the process is not restricted to the

removal of particles. Coagulants render some dissolved species (e.g., natural organic

matter, inorganics, and hydrophobic synthetic organic compounds) insoluble, and the

metal hydroxide particles produced when metal salt coagulants (typically aluminum

sulfate, aluminum chloride, ferric chloride, or ferric sulfate, and polymers) are added can

adsorb other dissolved species. Major components of a basic coagulation and

flocculation facility include chemical feed systems, mixing equipment, basins for rapid

mix, and flocculation basins.

In conventional treatment, the coagulant is dispersed evenly throughout the water. This

treatment step can occur in flash mix basins equipped with mechanical mixers, in an

influent pipe with static mixers, or in an open channel with a hydraulic jump. After the

rapid mixing, a longer period of slow mixing is needed to reduce surface charges,
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promote particle collisions, and enhance the growth of the floc. Flocculation basins are

typically equipped with slow moving mechanical mixers.

1.1.4 Sedimentation/Flotation

Sedimentation and flotation are solid-liquid separation processes used in water

treatment mostly to lower the solids concentration, or load, on filters and membranes. As

a result, filters and membranes can be operated more easily and cost effectively to

produce acceptable filtered water quality.

In most conventional treatment plants, once the floc is formed, the water enters a

quiescent basin where it is allowed to settle out. The accumulated sludge resting on the

floor of the sedimentation basin is then removed for eventual disposal. Sludge can be

removed continuously or intermittently through floor drains and scrapers or using suction

equipment.

In conventional water treatment, flotation has become accepted as an alternative to

sedimentation. Facilities use flotation mainly to treat nutrient-rich reservoir waters that

may contain heavy algal blooms and low-turbidity, low-alkalinity, colored waters. These

types of water are difficult to treat by sedimentation, because floc produced by chemical

treatment has a low settling velocity. Flotation is a separation process in which gas

bubbles attach to solid particles to cause the apparent density of the bubble-solid

agglomerates to be less than that of the water, thereby allowing the agglomerate to float

to the surface. Flotation is used to remove low-density solids from source water.

Dissolved air flotation (DAF) is the most commonly used flotation method for drinking

water. In DAF, bubbles are produced by the reduction in pressure of a water stream

saturated with air. In this process, the whole influent flows either through the flocculation

tank or directly to the flotation tank if separate flocculation is not required. Part of the

clarified effluent is recycled, pressurized, and saturated with air. The pressurized

recycled water is introduced to the flotation tank through a pressure-release device. As

the pressure is reduced to atmospheric pressure, air is released in the form of fine

bubbles. The air bubbles attach themselves to the flocs, and the aggregates float to the

surface. The floating material is removed from the surface, and the clarified water is

taken from the bottom of the tanks. Float or sludge can be removed either continuously

or intermittently by flooding or mechanical scraping.

1.1.5 Filtration

Any particles remaining after sedimentation or flotation are removed from the water

through the filtration process. Filters for suspended particle removal can be made of

graded sand, granular synthetic material, diatomaceous earth, mixed media (layers of

various sizes of gravel, high-density garnet, sand, and anthracite coal), screens of

various materials, and fabrics. The most widely used filters are gravity filters consisting

of two or three different granular media in rectangular tanks. When the water passes

through the filter, solid particles are removed by adhering to the media.

At a point in the operation, the pores in the filter media become clogged with particles

causing increased headloss and deteriorated effluent quality. To restore filter

performance, the media has to be backwashed. Backwashing entails reversing the flow

through the filter at a rate sufficiently high to fluidize the bed until the backwash effluent
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has become reasonably clear (turbidity < 10 NTU) (AWWA, 1999). The filter backwash

residual will contain any solids collected by the filters. Additionally, finished water is used

for the filter backwash process so this residual will contain chlorinated compounds.

Pressure filters operate in the same manner as gravity filters. However, instead of an

open tank, the media is contained in a cylindrical steel vessel that can be pressurized.

Pressure filters are widely used in small conventional plants or in membrane

applications.

1.1.6 Filter-to-Waste

This residual is also generated at the filters and is generated immediately after the filter

backwash process. Operators will typically ‘ripen’ their filters immediately after

backwashing and will waste the filtered water for 15 minutes to one hour (ASCE, 1996).

Typically this is residual is clear but will often contain chlorine compounds as chlorinate

will almost always precede filtration. This residual is most often equalized and recycled

to the head of the plant (ASCE, 1996).

1.2 Precipitative Softening

The precipitative softening treatment process uses some of the same processes as

coagulation and filtration process such as screening and presedimentation, activated

carbon, oxidation, sedimentation, filtration, and filter-to-waste. The key difference

between the two treatment processes is that in addition to providing safe drinking water

the precipitative softening treatment reduces the natural hardness in the surface waters.

This section describes the precipitative softening process.

Precipitative softening uses chemical precipitation with slaked lime (calcium hydroxide)

and other chemicals to reduce the water’s hardness and, in some cases, to enhance

clarification before filtration. Lime is the most commonly used chemical in precipitative

softening and is sometimes used with soda ash. To a lesser extent, sodium hydroxide is

used in place of lime. (AWWA, 1999)

Hardness is usually caused by calcium and magnesium compounds in water. Lime

softening removes this hardness by creating a shift in the carbonate equilibrium. Adding

lime to water raises the pH. As the pH increases, bicarbonate is converted to carbonate

and calcium is precipitated as calcium carbonate. Soda ash (sodium carbonate) is added

if insufficient bicarbonate is present in the water. Softening for calcium removal typically

takes place at a pH range of 9 to 9.5. To remove magnesium, excess lime is added

beyond the point of calcium carbonate precipitation. Magnesium hydroxide precipitates

at pH levels greater than 10.5. The recarbonation with carbon dioxide is generally used

to neutralize the pH if it exceeds 9.5.

The lime softening process generally consists of pretreatment (where required); singlestage, two-stage, or split-treatment softening; and filtration, supplemented by

disinfection. Softening can be combined with iron and manganese removal, coagulation

for turbidity removal, or both. The conventional softening basins are similar to

conventional basins used for coagulation and clarification, in which rapid mixing,

flocculation, and sedimentation occur. Lime, soda ash, coagulant, and a polymer

coagulant aid (if required) are added at the rapid mix basin and mixed using mechanical

mixers, with a detention time of approximately 30 seconds. Flocculation and
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sedimentation follow as unit processes that are similar to the conventional

coagulation/filtration process. Alternatively, upflow solids blanket clarifiers may be used

in place of conventional sedimentation basins.

Depending on the source water type and concentration of contaminants, facilities use

both precipitation and coprecipitation with calcium carbonates and magnesium

hydroxides to remove both hardness and nonhardness contaminants. The four types of

coprecipitation are:

 Inclusion. A portion of the solution surrounding the growing particle is

mechanically trapped.

 Adsorption. An impurity in attached onto the surface of a particle of precipitate.

 Occlusion. A contaminant is trapped in the interior of a particle of precipitate.

 Solid-solution formation. A particle of precipitate becomes contaminated with a

different type of particle that precipitates under similar conditions.

1.3 Membrane Separation

Membrane processes can be classified according to driving force, membrane type,

configuration, and removal capabilities. Membrane processes in the drinking water

industry are classified as desalting membranes or low-pressure membranes. Desalting

membranes are used to remove dissolved contaminants, both inorganic and organic.

Low-pressure membranes are designed for particle removal. Desalting includes the

high-pressure-driven processes of reverse osmosis (RO) and nanofiltration (NF) and the

electrically-driven processes of electrodialysis (ED) and electrodialysis reversal (EDR).

The low-pressure membranes include ultrafiltration (UF) and microfiltration (MF). Highpressure and low-pressure membranes use semipermeable membranes to remove

contaminants from feed water. Pressure is the driving force for these membranes.

Increased pressure on the feed side of the membrane transports some of the water from

the feed side, through the membrane, to the permeate side. The membrane may reject

other constituents depending on the properties of the particular membrane used and the

operating conditions. Water and other constituents not permeating the membrane flow

out of the membrane system as the concentrate stream. ED and EDR use an electrical

potential to force dissolved ions through ion-exchange membranes.

Membrane fouling and scaling are the biggest problems in the membrane industry and

cause an increased permeate rejection rate and increased operating costs. To minimize

fouling and scaling, low-pressure membrane systems are designed to include an online

backwash system and isolated chemical cleaning. Desalting membrane processes

require the feed water to be pretreated to minimize scaling and fouling as feed water

flows across the membrane surface. The desalting membranes produce concentrate on

a continuous basis and chemicals are added to inhibit scale formation. These

membranes are typically chemically cleaned every 3 to 12 months.

1.3.1 Low-Pressure Membranes
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Microfiltration (MF) and ultrafiltration (UF) are low-pressure membrane filtration

processes that have gained considerable acceptance in the drinking water industry over

the past 10 years. MF and UF are primarily used for particle removal as a stand-alone

treatment, or as a retrofit of existing conventional treatment plants (as a replacement for

conventional particle removal processes). MF and UF have been demonstrated to

remove protozoa cysts to below detection levels and meet the turbidity requirements of

current and future surface water treatment regulations.

For low-pressure membranes, residuals include backwash, concentrate (for cross-flow

and immersed systems), and chemical cleaning wastes (clean in place (CIP) and

chemically-enhanced backwash (CEBW)).

Membrane backwashing uses air, water, or a combination of both. A typical range of

backwash frequencies for MF/UF systems is between 15 and 60 minutes, and the

backwash duration ranges from 30 seconds to 3 minutes. Most systems experience a

gradual increase in post-backwash transmembrane pressure (TMP) that must be

addressed by chemical cleaning.

Chemical cleaning is used to restore the post-backwash TMP to its initial level (i.e., the

TMP of a new, clean membrane). In the chemical cleaning process, acid, caustic,

chlorine and/or surfactants are circulated through the membrane system to dissolve or

dislodge contaminants that have not been removed by backwashing. Most systems are

chemically cleaned once every 1 to 6 months, depending on system design and

operation. Irreversible fouling is known to occur in all membrane systems, requiring the

membranes to be replaced.

1.3.2 Desalting Membranes

Desalting membrane processes include two classes, pressure driven and electrical

driven. The former includes NF and RO, which utilize semi-permeable membranes

designed to separate dissolved substances from water using a pressure differential. The

electrically-driven processes include ED and its variant, EDR. ED/EDR membranes are

ion exchange membranes using an electrical potential to separate impurities from water.

For desalting membranes, residuals include concentrate (produced on a continuous

basis during normal operation) and spent cleaning solutions (produced periodically from

off-line chemical cleaning of the membrane train through a CIP process). (AWWA, 2004)

1.3.2.1 Nanofiltration and Reverse Osmosis

NF and RO are desalting membranes used in applications that require removing

dissolved contaminants, as in the case of softening or desalination. NF/RO membranes

are designed to remove dissolved solids through reverse osmosis.

RO membranes are typically used for seawater or brackish water applications. Most ions

are completely rejected (rejections greater than 99 percent). The NF process softens

water by removing calcium and magnesium ions. This process has removed monovalent

ions by 50 to 70 percent and multivalent ions (hardness) by 90 percent. It is also

effective in removing other dissolved organics, biological contaminants, and suspended

solids that are not removed in pretreatment steps; however, pretreatment is used to

remove biological material and particulates.
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A high quality of feed water with reduced turbidity and total suspended solids (TSS) is

required to prevent plugging of the flow spacers in the membrane elements. Generally,

turbidity levels higher than 0.3 NTU or silt density index levels higher than 3 units can

cause physical damage to the membrane elements by plugging the flow path. In

addition, membrane fouling and scaling will increase the required pressure to maintain a

constant water production. Chemical scaling occurs when the concentration of scaleforming compounds reaches the solubility limit and begins to precipitate.

1.3.2.2 Electrically-Driven Processes

The ED/EDR process is designed for desalting brackish water supplies and is not

applicable to the removal of non-charged solutes, such as silica, or particles and

pathogens. It is inefficient at removing dissolved organics, which are weakly charged.

(AWWA 2004)

ED and EDR use an electrical potential to force dissolved ions through ion-exchange

membranes that are virtually impermeable to water. The water is purified by the

movement of contaminants, not water, across the membrane. The ED process places

alternating pairs of cation (+) and anion (-) membranes between positively and

negatively charged electrodes. When a voltage is applied, a current causes cations to

move in the direction of the negatively charged electrode (cathode). The cations move

through the cation membrane but are restrained by the anion membrane. The anions

move in the direction of the positively charged electrode (anode) through the anion

membrane, but are restrained by the cation membrane. Water flows in all channels; flow

from channels where contaminants are removed becomes the desalted product stream.

Flow from channels to which salts are transferred becomes the concentrate. Feed

pressures for ED/EDR are typically between 50 and 70 psi to overcome head losses in

the ED/EDR membrane modules or “stacks.” (AWWA 2004)

With ED, the direction of electrical charge is always the same. With EDR, the direction of

charge is reversed several times hourly by reversing the electrical polarity, which

changes the direction of ion movement through the membranes and "electrically flushes"

fouling ions from the membrane surfaces to control the buildup of scale, improve

recovery, and reduce the need for scale-inhibiting chemicals. (AWWA 2004)

1.3.2.3 Forward Osmosis

Forward osmosis (FO) is a method of desalination in development for drinking water

applications. Osmosis is the natural tendency for water to flow from areas of lower

concentration to higher concentration. FO uses this tendency to desalinate seawater. A

FO unit uses a “draw solution” with dissolved molecules at higher concentration than sea

water. The draw solution is separated from the feed water by a semi-permeable

membrane. The greater concentration of the draw solution to the feed water will cause

water to cross the membrane into the draw solution. The dissolved molecules in the

draw solution are removed, typically though evaporation.

Forward osmosis is hypothesized to be less energy intensive than reverse osmosis. RO

requires high pressure on the feed side of the membrane, which can be energy

intensive. FO does not require pressure to force water through the membrane, but does

require heat to evaporate the dissolved molecules from the draw solution. This treatment
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may also be applicable to further concentrating the brine generated from other

membrane technologies and the ion exchange technology.

1.4 Ion Exchange

Ion exchange (IX) is a physical-chemical process by which objectionable ionic

contaminants are removed from the water. In this process, an ion on the solid phase is

exchanged for an ion in the feed water. This solid phase is typically a synthetic resin that

has been chosen to preferentially adsorb the particular contaminant of concern. To

accomplish this exchange of ions, feed water is continuously passed through a bed of

ion exchange resin beads in a down-flow or up-flow mode until the resin is exhausted

(i.e., when all sites on the resin beads have been filled by contaminant ions). At this

point, the bed is regenerated by rinsing the IX column with regenerant brine- a

concentrated solution of ions initially exchanged from the resin. Most useful IX reactions

are reversible. The number of bed volumes that can be treated before exhaustion varies

with resin type and influent water quality. Typically 300 to 60,000 bed volumes can be

treated before regeneration is required. (USEPA, 2000)

IX can remove a number of contaminant cations (e.g., hardness, barium, strontium, and

radium) and anions (e.g., fluoride, nitrate, DOC, fulvates, humates, arsenate, selenate,

chromate, perchlorate, and anionic complexes of uranium) from water. Residuals include

backwash and regeneration waste.

1.5 Adsorption Technologies

There are numerous solid adsorptive media that are increasingly used for radionuclide

removal. Some media are capable of being regenerated, which would make a

regenerate waste stream in the same fashion as ion exchange. However, these media

are often regenerated offsite, or disposed as solid waste.

Carbon has been used as an adsorbent for centuries. Activated carbon removes

contaminants from water by adsorption. Adsorption is a natural process by which

molecules of a dissolved compound adhere to the surface of an adsorbent solid.

Adsorption occurs when the attractive forces at the carbon surface overcome the

attractive forces of the liquid. There are two different forms of activated carbon used in

the water industry: powdered activated carbon (PAC) and granular activated carbon

(GAC).

PAC is widely used in conventional treatment of surface waters to adsorb taste- and

odor-causing substances. On a limited basis, PAC is also used to remove seasonally

occurring pesticides such as atrazine. In most designs, PAC is fed from a silo into a

screw feeder to prepare a slurry that is metered into the raw water. Therefore, use of

PAC would not typically be considered as a stand-alone process.

GAC is a particularly good adsorbent medium due to its high surface-area-to-volume

ratio. One gram of a typical commercial activated carbon has a surface area equivalent

to 1,000 square meters. This high surface area permits a large number of contaminant

molecules to accumulate. It is generally used in beds or tanks through which the water

passes for treatment. As a contaminated water stream passes through the activated

carbon bed and exits, the effluent begins to slow contamination. This condition is

classified as “breakthrough” and the amount of material adsorbed is considered the
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breakthrough capacity. If the GAC is used in a polishing application to remove

undesirable organic chemicals, the carbon would not have a waste discharge stream,

but would require periodic replacement or rejuvenation as breakthrough of the carbon

occurred. If the GAC application is used in conjunction with particulate removal, a

backwash stream will result and have similar TSS properties to the conventional

treatment multimedia filters discussed earlier.

1.6 Miscellaneous Drinking Water Plant Wastewasters

In a previous discussion with the National Association of Clean Water Agencies

(NACWA), it was noted that other miscellaneous wastes that could be discharged are

discharged from some drinking water treatment facilities. These discharges include:

 Copper Sulfate, Cu(SO4), residual discharges. This chemical is used by DWT

facilities for control of algae in drinking water reservoirs and presedimentation lagoons at the DWT facility.

 Laboratory wastewaters.

 Re-generation wastewaters for boiler make-up waters

 Boiler blow-down wastewaters.

 Chlorine analyzer and flow-through sampler wastewaters

 Equipment washdown wastewaters which can be as much as 10,000 gallons

per day.

 Contact and non-contact cooling wastewaters.

EPA is gathering additional information about the characterization, management,

treatment, and disposal options for these residuals.

2.0 WATER TREATMENT RESIDUAL CHARACTERISTICS

As the water passes through different treatment processes, it is subjected to mixing,

chemical interaction, aggregation, and phase transformation to remove the contaminants

of concern. The contaminants removed from the water are called residual wastes and

vary in type, size, and characteristics. They range from large suspended particulates

such as leaves, debris, and other material that can be removed by screens in the intake

to dissolved gases such as volatile organic compounds and sulfides that can be

removed by aeration processes. Regardless of the raw water source and quality and the

processes used, water treatment plants can produce three types of residuals:

 Solids, including spent media, spent membranes, and dewatered residuals;

 Liquids, including brines, concentrates, backwash water, rinse water, and filterto-waste water; and

 Sludges, including coagulant/polymetric, chemical softening, and oxidized

iron/manganese sludges.

As indicated in Section 1, the quality and characteristics of the waste streams generated

by a water treatment plant are related to the treatment processes in use.
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Screening and presedimentation, coagulation, filter backwashing, filter-to-waste, lime

softening, iron and manganese removal, spent materials (spent IX resins, GAC, and filter

media), and slow sand and diatomaceous earth filtration all generate residuals. This

paper focuses on liquid and sludge residuals that may be discharged to surface waters

or POTWs.

Some limited studies have been conducted to analyze water treatment residuals, and

the results, though helpful in identifying possible pollutants of concern, confirm that the

concentrations of the pollutants in the residuals vary significantly from plant to plant. For

example, EPA’s study of eight water treatment plants published in 1992, Inorganic

Chemical Characterization of Water Treatment Plant Residuals, detected many

inorganic contaminants (e.g., aluminum, arsenic, barium, chromium, copper, iron,

magnesium, nickel) at concentrations that varied up to an order of magnitude from one

plant to another. (US EPA, 1992) [XXX - ERG to reference section the more recent

AWWA report.]

The remainder of this section discusses the typical characteristics and generation rates

of sludges and liquid residuals generated by the five treatment processes described in

Section 1 with only limited information on pollutant concentrations.

2.1 Solid and Sludge Residuals

The three most common types of sludges are coagulant/polymeric, chemical softening,

and oxidized ion/manganese

S  8.34Q2.0CaCH  2.6MgCH  CaNCH 1.6MgNCH  CO2  0.44Al  2.9Fe  SS  A

Where:

S = sludge production (lb/day)

Ca = calcium hardness removed as CaCO3 (mg/L)

Mg = magnesium hardness removed as CaCO3 (mg/L)

Fe = iron dose as Fe (mg/L)

Al = dry alum dose (mg/L) (as 17.1 percent Al2O3)

Q = plant flow (mgd)

SS = raw water suspended solids (mg/L)

A = other additives (mg/L)

CaNCH = calcium hardness removal as CaCO3 (mg/L)

MgNCH = noncarbonated magnesium hardness removed as CaCO2 (mg/L)

CO2 = carbon dioxide removed by lime addition, as CaCO3

Notes:

 PACl (polyaluminum chloride dose (mg/L)) can be used in the equation as 0.27

PACl, varying by manufacturer.

 EPA may need to correlate turbidity measurements to suspended solids.

The above equation presents the mass of residuals produced. The percentage (mass) of

these residuals that are due to the addition of treatment chemicals are estimated as XXX

(insert table or range). Additionally, the following table provides and estimate of the trace
14

impurities in the treatment chemicals (use AWWA report to provide table or range – this

should be mostly be done from the PR BPJ report).

The volume and solids concentration of the sludge leaving a presedimentation,

sedimentation basin, or clarifier is also a function of the sludge removal method. Sludge

can be removed using either a periodic cleaning method or a continuous removal

method. Periodical sludge removal systems accumulate a compact and thickened

stratification of solids. The volume of sludge produced depends on the amount of water

used to remove the solids. In a continuous sludge collection system, the solid

concentrations are generally as shown below:

 Circular or chain and flight - 1.5 to 2.5 percent

 Suction type – 1.5 to 2.0 percent

 Blanket type clarifiers – 0.5 to 1.0 percent

 Sand Ballasted type – 0.1 to 0.5 percent

 DAF (mechanical) – 3 to 4 percent

 DAF (hydraulic) – 0.1 to 0.5 percent

Specific characteristics of sludges and solids generated by coagulation/filtration,

precipitative softening, membrane separation, and ion exchange unit processes are

presented below.

2.1.1 Coagulation and Filtration Solid and Sludge Residuals

The types and amounts of coagulant used in a water treatment facility have a significant

effect on the type and amount of residual waste produced by the plant. An example is

enhanced coagulation where the use of higher coagulant dosages may produce a

residual that is much more difficult to dewater (Cornwell D.A., 1999). The amount of

residual (weight and volume) impacts the cost of treatment and the overall

environmental significance of the plant. In most conventional water treatment facilities,

coagulation generates the bulk of residual waste material.

The sludge produced by the coagulation processes predominately contain the coagulant

metal hydroxides along with raw water natural organic matter, suspended solids,

microorganisms, radionuclides, and other raw water organic and inorganic constituents.

The metals found in coagulant sludges include aluminum, arsenic, and occasionally

cadmium, chromium, copper, iron, lead, manganese, nickel, and zinc (Cornwell D.A.,

1999).

If the source water has a high concentration of total suspended solids (TSS), the

alum/iron coagulant sludges will contain a high percentage of gelatinous, hydroxide

precipitates (e.g., Al(OH)3, Fe(OH)3) and will likely exhibit the characteristics in Table 2-

1. (ASCE, 1996)

Table 2-1. Typical Alum/Iron Coagulant Sludge Characteristics

Sludge Characteristics Percent Solids Content

Liquid 0 – 5

Spongy, semi-solid 8 – 12

Soft clay 18 – 25
15

Stiff clay 40 – 50

It has been observed that alum and iron coagulants tend to produce 0.1 to 1.0 percent

solids from conventional sedimentation basins when treating waters of low to moderate

raw water turbidity. Sludge blanket clarifiers can produce over 2 percent solids from the

same water. Flotation can produce 2 to 5 percent solids at the top of a unit.

Generally, alum sludge generation quantities range from 0.001 to 0.37 percent of the

treated water flow rate with an average of 0.11 percent. Table 2-2 presents typical

chemical coagulant sludge volumes by water treatment plant flow. Backwash uses

approximately 0.5 to 5 percent of the plant intake. (US EPA, 1993) Additionally, filter-towaste typically uses an equal amount of the plant intake.
16

Table 2-2. Typical Chemical Coagulation Sludge Volumes

Water Treatment Plant Flow

(mgd)

Typical Sludge Volume

Range (gal/day)

Average Sludge Volume

(gal/day)

0.7 7 – 2,600 770

1.8 18 – 6,700 2,000

4.8 48 – 17,800 5,300

11 110 – 40,900 12,100

18 180 – 66,800 19,800

26 260 – 96,600 28,600

51 510 – 189,400 56,200

210 2,100 – 779,900 231,300

430 4,300 – 1,596,900 473,500

2.1.2 Precipitative Softening Wastes

The sludge produced by softening processes contains predominately calcium carbonate

and magnesium hydroxide, other divalent ions, raw water natural organic matter,

inorganics, suspended solids, microorganisms, and radionuclides. Metals found in the

precipitate sludges include calcium, magnesium, arsenic, barium, cadmium, chromium,

lead, mercury, selenium, and silver. Calcium carbonate sludges are generally dense,

stable, and inert materials that dewater easily to a solid content up to 50 to 60 percent.

The sludge pH is normally greater than 10.5. However, if the hardness is due to

magnesium, the hydroxide sludge is more difficult to handle and dewater (Cornwell D.A.,

1999). If the water softening is performed by pellet reactor, a sludge pelletization occurs.

This precipitated hardness is withdrawn as a sand-like granular material. (AWWA, 1998).

In precipitative softening the sludge concentration is a function of the calcium carbonate

(CaCO3) to magnesium hydroxide (Mg(OH)2) ratio and the type of clarifier. Conventional

sedimentation basins may only produce solid concentrations of 2 to 4 percent, whereas,

sludge blanket clarifiers can produce solids concentrations up to 30 percent. Table 2-3

lists the solids content of typical chemical softening sludge. (ASCE, 1996)

Table 2-3. Typical Chemical Softening Sludge Characteristics

Sludge Characteristics Percent Solids Content

Liquid 0 – 10

Viscous Liquid 25 – 35

Semi-Solid 40 – 50

Crumbly Cake 60 – 70

Generally, the volume of sludge generated from lime softening plants ranges from 2,800

to 6,600,000 gallons per day depending on the water treatment plant capacity. The

typical lime sludge volumes by treated water design flow are summarized in Table 2-4
17

Table 2-4. Typical Lime Softening Sludge Volumes

Water Treatment Plant Flow

(mgd)

Typical Sludge Volume

Range (gal/day)

Average Sludge Volume

(gal/day)

0.7 2,800 – 10,700 8,500

1.8 7,200 – 27,400 21,900

4.8 19,300 – 73,100 58,300

11 44,200 – 167,500 133,600

18 72,300 – 274,100 218,600

26 104,400 – 395,900 315,800

51 204,800 – 776,600 619,400

210 843,400 – 3,198,000 2,550,600

430 1,726,900 – 6,548,200 5,222,700

2.1.3 Low-Pressure Membrane Separation Wastes

The solids generated by low-pressure (MF/UF) membranes is from the concentrate (for

cross-flow and immerse systems), backwashing, and spent chemical cleaning. Normally,

residuals in the concentrate and backwash systems contain raw water suspended solids,

metal hydroxides, and PAC if coagulation/precipitation and PAC are used as part of the

pretreatment process or if coagulant and PAC are used with the membrane process.

The solids from low-pressure membranes are similar to solids in spent backwash water

from conventional filters

2.1.4 Desalting Membrane Separation Wastes

The concentration of solids from high-pressure membranes (NF/RO) is typically low, less

than 10 mg/L of TSS in the concentrate. For ED/EDR, ionized contaminants in the feed

water are removed and exit the system through the concentrate stream, similar to

NF/RO.

2.1.5 Ion Exchange and Adsorption Wastes

The residuals formed in IX and adsorption systems include backwash water and spent

regenerant chemical and media. The challenge with IX and adsorption treatments is the

handling and disposal of spent regenerants and media. The spent media residuals are

mainly solid waste media and the contaminants absorbed to it.

GAC treatment is mainly for the removal of taste- and odor-causing organic compounds

along with organic compounds like disinfection by-product precursor material. As a

polishing step, activated carbon can be used to remove disinfection by-products

generated in the production of drinking water. As a result, the spent carbon is mainly

exhausted with organic compounds; however, some inorganics are strongly adsorbed on

the carbon surface like antimony, arsenic, bismuth, chromium, tin, silver, mercury,

cobalt, zirconium, chlorine bromine iodine, and fluoride.

2.2 Liquid Phase Waste

Liquid waste is generated from filter backwashing, filter-to-waste, ion exchange

regeneration, membrane reject water, chemical cleaning, and GAC transport water. The

waste characteristics are highly dependent on the nature of the source water and the
18

treatment processes incorporated. If the liquid waste contains a high total dissolved solid

(TDS) content, it is often referred to as brine waste. This would apply to ion exchange

processes and membrane processes used to remove salt from seawater or brackish

groundwater. Low-TDS wastes are referred to as concentrate.

2.2.1 Filter Wastes

Waste generated from filters includes filter backwash water and filter-to-waste from rapid

filters and pre-coat filters, and dirty sand from scraping of slow sand filters. The filter

backwash and filter-to-waste are liquid residual waste streams that contain particulates

including clay and silt particles, microorganisms (bacteria, viruses, and protozoan cysts),

colloidal and precipitated humic substances, and other natural organic particulates from

the decay of vegetation; and precipitates of aluminum or iron used in coagulation

(Cornwell D.A., 1999). Additionally, these liquid filter wastes will contain chlorinated

compounds as they are generated from chlorinated water, which typically contains a

chlorine concentration of XXX mg/L in the water treatment plant. When precoat filters

and slow sand filtration are used the spent filter media becomes a solid waste.

The filter backwash and filter-to-waste operations generate large volumes of water with

relatively low concentrations of solids. Filter backwash and filter-to-waste operations

each use approximately 1 to 5 percent of the drinking water intake. Normally, filter

backwash contains 50 to 400 mg/L of suspended solids. In some cases 15 to 20 percent

of the backwash water solids are spent filter media. In general, large treatment plants

generate a lower percentage of filter backwash than small treatment plants because they

tend to operate more efficiently (US EPA, 1993).

2.2.2 Low-Pressure Membrane Liquid Wastes

The primary residuals of MF/UF systems result from the wastes generated during

backwashing and chemical cleaning (either CIP or CEBW). Cross-flow filtration systems

additionally generate concentrate wastes, which can be considered similar in nature to

the backwash wastes. All MF/UF systems produce residuals on an intermittent basis,

with two exceptions: one type of submerged system that operates in a feed-and-bleed

mode, where backwash water is discharged into the membrane tank (containing feed

water) and waste (concentrate) is continuously drawn from the tank; and cross-flow

systems that use a continuous discharge of concentrate. However, it should be

recognized that when multiple MF/UF trains are assembled to produce larger permeate

flowrates (>10 mgd or 38 ML/day), backwash flowrates from the plant essentially

become continuous. (AWWA, 2003)

Backwash and Concentrate Wastes

MF and UF essentially remove a large size range of particles from water, including

turbidity, inorganic and organic colloids, and pathogens (protozoan cysts and bacteria).

UF, because of its smaller pore size, also removes viruses and some organic fractions.

Solids accumulate on the surface of the membrane or in the cross-flow concentrate

circulating through the membrane units. The solids are removed by frequent

backwashing or flushing, with or without air scour, or by bleeding-off a portion of the

concentrate flow to maintain an acceptable solids level on the feed side of the

membrane (as in cross-flow and certain immersed membrane systems). (AWWA, 2003)
19

Residuals produced from backwashing of low-pressure membranes will consist of filtrate

together with naturally occurring feedwater solids retained by the membrane. As such,

backwash residuals in general are not expected to have a significant environmental

impact. Where chemicals, such as a coagulant, oxidant, or adsorbent, are dosed to the

source water prior to membrane treatment for the purposes of removing dissolved

contaminants, membrane backwash water will be enriched in both the pretreatment

chemical and dissolved matter, both of which can make disposal of the backwash more

challenging. Likewise, when chemicals are dosed to filtrate during backwashing (known

as chemically enhanced backwash) to improve foulant removal, increased care must be

given to backwash disposal. (AWWA, 2003)

Specific characteristics of low-pressure backwash and concentrate residuals depend

upon the quality of the water being treated, and the recovery of the membrane system.

Net recoveries typically range from 85 to 98 percent, which results in concentrating the

feedwater solids by a factor ranging from 7 to 50 times the feedwater concentration.

Logically, as contaminant concentrations and recovery increase, the contaminant

concentrations in the residuals increase. With high recoveries and high feedwater

contaminant levels, MF/UF residuals can have very high concentrations of suspended

solids and turbidity. If chemicals are used during backwash (CEBW), backwash wastes

will resemble CIP wastes. Table 2-5 summarizes the characteristics of some typical

backwash waste solutions. (AWWA, 2003)

Table 2-5. Typical Characteristics of Low-Pressure

Membrane Backwash Residuals* (AWWA, 2003)

Frequency of application Every 10-60 minutes

Volume of waste produced 2-15% of plant feed flowrate for recoveries of 85-98%

(Daily CEBW wastes might be 0.2 to 0.4% of plant feed flowrate)

Algae, precipitated solids, possible chemical residues if using CEBW

TOC of 1 to 2 times the feedwater concentration (if no coagulant or

absorbent is used)

If coagulant is used, the TOC could be 5 times the feed concentration

For recoveries of 85 to 98%, wastes will have a concentration factor of 7

to 50 times the feedwater: TSS of 7 to 1,250 mg/L for feedwater of 1 to 25

mg/L, and Giardia and Cryptosporidium of 7 to 1,250 count per 100 liters

for feedwater of 1 to 25 count per 100 liters

Characteristics of

backwash residuals

If using CEBW (with chlorine, acid or base): pH may be <6 or >9, and

Chlorine residual may be up to 1,000 mg/L as Cl2

*Backwash characteristics will vary with feed flowrate (flux) and recovery / backwash frequency.

The volume of backwash flowrates is typically 2 to 15 percent of the plant flowrate for

systems operating at 85 to 98 percent recovery. Typically, these flows will represent 95

to 99 percent of the total volume of residual wastes from MF/UF treatment processes.

The remaining 1 to 5 percent of membrane residual wastes comes from chemical

cleaning procedures.

If coagulants, PAC, or other pretreatment chemicals (e.g., potassium permanganate) are

applied, the characteristics of the MF/UF residuals become more similar to the residuals

from a conventional water treatment plant (WTP). (AWWA, 2003)
20

Chemical Cleaning Wastes

The second type of membrane residual is generated from the chemical cleaning

processes used to control fouling (CEBW and CIP). Cleaning solution residuals reflect

the chemicals used in the cleaning process. Only a portion of the active chemical

ingredient is consumed during the cleaning process, so the resulting chemical cleaning

waste includes some remaining active chemical ingredient, as well as salts from

chemical reactions between the chemicals and foulants, “dissolved” organic materials,

and suspended solids. While some plants refresh the active ingredient in spent cleaning

solutions and reuse it to minimize waste quantities, this practice can result in more

concentrated CIP waste solutions. Table 2-6 summarizes the characteristics of some

typical chemical cleaning waste solutions. (AWWA, 2003)

Table 2-6. Typical Characteristics of Low-Pressure Membrane

Chemical Cleaning Solutions and Wastes (AWWA, 2003)

Frequency of application Daily to once every 3 or 4 months

Volume of waste produced Monthly CIP wastes normally < 0.05% of plant feed flowrate

(Daily CEBW wastes might be 0.2-0.4% of plant feed flowrate)

Sodium hypochlorite – 500 to 1,000 mg/L as Cl2

Citric or hydrochloric acid – pH 1 to 2

Caustic soda – pH 12 to 13

Chemicals commonly used

Surfactant – 0.1% by weight

pH from 2 to 14

Chlorine residual up to 1,000 mg/L as Cl2

Low concentrations of surfactants

TSS up to 500 mg/L (neutralization may precipitate additional solids)

TOC 10 to 30 times the feedwater concentration

Characteristics of spent

cleaning solution

BOD5 up to 5,000 or 10,000 mg/L (if citric acid is used)

Chlorine residuals in CEBW and CIP wastes may range from 1 to 1,000 mg/L as Cl2, and

pH may be acidic (pH<6) or basic (pH>9) depending on the chemicals used. Surfactants

may also be used at low concentrations, and may cause foaming when the spent

cleaning solution is discharged, depending upon the type of surfactant. While mixing the

acid and caustic waste solutions will minimize the need for additional neutralization

chemicals, it may also precipitate additional solids, such as calcium carbonate and iron

compounds, which increases the suspended solids concentration of the waste solution.

If citric acid is used in the CIP, the biochemical oxygen demand (BOD5) of the spent

cleaning solution is likely to be very high as well. (AWWA, 2003)

Chemical cleaning residuals have a different composition and typically a smaller volume

compared to the concentrate and backwash residuals generated during water

production. Since chemical cleaning is performed less frequently than backwashing

(from once a day to once every 3 or 4 months), the yearly residual volume is much

smaller than the volume generated as concentrate or backwash. The volume of cleaning

solution used and the amount of waste solution generated as residuals are functions of

the following factors: (AWWA, 2003)

 Source water characteristics (type and level of foulants present);

 Membrane process and module/system characteristics;

 Size/capacity of membrane train;
21

 Cleaning system design (piping diameter and lengths); and

 Frequency of cleaning (CIP).

The volume of chemical cleaning waste generated from CEBW is typically very low,

especially if CEBW is performed intermittently and at low chemical concentrations that

do not require segregation of CEBW wastes from regular backwash wastes. In other

instances where stronger chemical concentrations are used, a daily CEBW typically will

be less than 0.2 to 0.4 percent of the plant flowrate, and may be reduced further if the

solutions are reused. Recently, there has been a trend among low-pressure membrane

suppliers to move away from using low concentrations of chlorine in the regular

backwash (<5 ppm) to control biofilm formation. Instead, many manufacturers are

switching to less frequent use of higher concentrations of chlorine (100 to 500 ppm) and

acid (pH of 1 to 2) for more effective organic and precipitated metals removal. With this

trend comes the generation of wastes more similar to CIP wastes in chemical strength,

making their reuse more difficult and the need for neutralization prior to discharge more

likely. Specific CEBW solution volumes are relatively small for pressurized systems but

can be significant for immersed systems because of the need to fill up the entire tank in

which the membranes are housed. (AWWA, 2003)

Monthly CIP wastes normally will be less than 0.05 percent of plant flow. While specific

cleaning solution volumes will vary depending upon the system geometry and the type of

solution, typically they will be two to three times the volume normally present in the

membranes and piping for each cleaning solution (one volume for the actual cleaning

solution and one or two additional volumes to rinse the solution from the system) or for

immersed systems equal to the working volume of the membrane process tank. Rinse

waters have low levels of contamination and may be handled differently than the

cleaning solution, such as by recycling to the feedwater, combining with the backwash

wastes for recycling or disposal, or using the rinse water for making up the next batch of

cleaning solution. Annual volumes of spent CIP solutions can be calculated by

multiplying the volume for each cleaning by the number of annual cleanings. This small

volume of wastewater reduces the subsequent handling and disposal impacts, as it

represents a volume that is easier to collect, store, neutralize, and transport (if

necessary). (AWWA, 2003)

2.2.3 Desalting Membrane Liquid Wastes

Desalting membrane processes generate a residual waste stream from the concentrate

and chemical cleaning. Desalting membrane processes generate a concentrate that

contains mostly dissolved contaminants removed from the feedwater. In general, the

percentage of wastewater produced decreases as the size of the water systems

increase due to increased efficiencies (US EPA, 1993). Concentrate is produced

continuously while spent cleaning solutions are generated intermittently (typically every 3

to 12 months). Also, the volume of concentrate is much greater than the volume of spent

chemical solution.

Concentrate

The concentrate from NF, RO, and ED/EDR systems are considered similar. ED/EDR

systems also produce a specialized waste stream of limited flow called “electrode

waste,” which contains significant levels of hydrogen and chlorine gases that are
22

typically stripped from the electrode waste stream using a degasifier (which is part of the

EDR system). (AWWA, 2004)

Since desalting membrane processes produce a high-purity stream by rejecting high

percentages of contaminants in the feed stream, the concentrate stream is primarily a

more concentrated version of the feed water. As such, the characteristics of concentrate

are directly related to the feed water quality. Typical contaminants removed during

desalting are presented (by source water) in Table 2-7. For NF and RO, these

contaminants and other constituents in the feed water will be rejected by the membrane

and will become part of the concentrate stream. Contaminant concentrations are

typically 4 to 10 times the feed water concentration and depend upon the rejection

characteristics of the membrane and the water recovery. If pre-treatment is used, the

feed water to the desalting membranes will have lower levels of certain constituents and

particles. However, the pre-treatment may increase inorganic ions, such as sulfate, iron,

and aluminum, if coagulants are used, and possibly residual organics from polymer or

sulfuric acid use. (AWWA, 2004)

Table 2-7. Typical Target Contaminants by Source Water (AWWA, 2004)

Source Water Typical Target Contaminants

Total Organic Carbon (TOC)

DBP Precursors

Pathogens

Pesticides / Synthetic Organic Compounds (SOCs)

Surface water

Taste & Odor Compounds

Hardness

Color

TOC

Groundwater

Inorganic and organic compounds / chemicals

TDS

Hardness

Brackish surface water and groundwater

Chloride and sodium

TDS

Chloride and Sodium

Bromide

Seawater

Boron

Concentrate quantity is a function of the amount of feed water purified (or converted to

permeate), as defined by the product water recovery. Table 2-8 presents typical

recoveries for the various types of water sources treated by desalting membrane

processes and associated concentrate quantity as percent of feed. NF and brackish

water RO systems operate at recoveries of 80–90 percent and 65-85 percent,

respectively; seawater RO system recoveries are lower, typically 40–60 percent. NF and

brackish RO residuals differ somewhat because NF salt rejection is lower. Therefore, for

a given feed and recovery, NF concentrate is less saline than RO concentrate. Further,

NF provides low rejection of monovalent ions (e.g., sodium and chloride) compared to

multivalent ions (e.g., calcium and sulfate). Consequently, NF concentrate has a higher

ratio of multivalent to monovalent ions than feed water. Lastly, since RO typically treats

higher salinity waters, the TDS levels in the RO concentrate are much higher, especially

for seawater. (AWWA, 2004)
23

Table 2-8. Typical Desalting Membrane System Design Parameters (AWWA, 2004)

Parameter Surface

Water

Fresh

Groundwater

Brackish

Groundwater Seawater

Feed TDS (mg/L) 200-400 400-500 500-10,000 30,000-40,000

Water recovery

(% of feed)

80-90 80-90 65-85 40-60

Concentrate quantity

(% of feed)

10-20 10-20 15-35 40-60

Concentrate TDS (mg/L)

(at example recovery)

1,330-2,660

(85%)

2,660-3,330

(85%)

2,000-40,000

(75%)

60,000-80,000

(50%)

Concentration factor* 5-10 5-10 2.9-6.7 1.7-2.5

*Ratio of total dissolved solids in concentrate to total dissolved solids in feed, assuming 100 percent salt

rejection

As previously mentioned, the concentration of contaminants is proportional to water

recovery and membrane rejection, with concentration increasing as water recovery

increases. Typical source water quality, water recovery, and residuals concentrations

are included in Table 2-8, where the values represent the maximum concentrations for

residuals streams based on a simplifying assumption that all of the salts are rejected

(100% salt rejection). The amount of particles or TSS in desalting membrane

concentrates is low, typically <10 mg/L, because of the extensive pretreatment required

to control fouling. The acid and scale inhibitor added to the membrane feed water to

control mineral precipitation will be concentrated in the desalting membrane concentrate.

Scale inhibitor levels in the concentrate are typically <20–30 mg/L and consist of

phosphates or organic polymers (such as polyacrylates or dendrimers). (AWWA, 2004)

For simplicity, the residuals stream concentration can be estimated based on the feed

water quality and a concentration factor related to the water recovery, as shown in the

equation below:

r

w

r Q

C

C 

Where:

Cr = Concentration in the rejected water

Cw = Concentration in the raw water

Qr = Fraction of water rejected

Chemical Cleaning (CIP) Residuals

Cleaning solutions were developed to address specific contaminants that cause fouling

and loss of performance, and CIP residuals reflect the chemical characteristics of both

the spent cleaning solution and material removed from the membrane system during

CIP. Reactions with foulants will tend to raise the pH of acid solutions and lower that of

basic ones. (AWWA, 2004)

Typically, the "cleaning solution volumes" generated during a CIP of NF and RO are

about 3 gal/100 sq. ft. (1.2 liter/m2). This volume does not include rinse water volumes.

Typical cleaning solution volume is estimated by adding the total empty vessel volume

and pipe volume. Spent cleaning solutions, which may be diluted with rinse water (feed
24

or permeate), can contain detergents, surfactants, acid, caustic, or other chemicals used

to remove foulants from the membrane system. Typically, the spent cleaning solution

volume is an extremely small percentage of the treated flow (<0.1%). (AWWA, 2004)

2.2.4 Ion Exchange Brine Wastes

Ion exchange is used extensively in water treatment for the removal of hardness or

cations, such as magnesium and calcium and anions such as chloride. Anion exchange

resins have been employed to treat drinking water sources with high nitrate levels as

well as high concentrations of disinfection by-products precursor material. IX has been

observed to effectively remove barium, radium, arsenate, selenate, fluoride, lead, and

chromate (Cornwell D. A., 1999).

Brines are produced from rinsing, regenerating, and backwashing the ion exchange unit.

Of these three streams, the regeneration stream contains the highest levels of TDS,

ranging from 10 to 300 times the levels found in the feed water. This high TDS content is

due to the salts removed from the exchange resin during regeneration and excess

regeneration salts. The regeneration stream is often mixed with the rinse and backwash

streams, diluting the regenerant by as much as an order of magnitude. (US EPA, 1993).

The predominant ion in the spent regenerate streams is often the coion in the regenerant

(e.g., chloride, sulfate, carbonate, or sodium). (ASCE, 1996) The other ions present are

those removed by the resin and reclaimed by the regenerant. With chlorides present,

brine is corrosive to materials it contacts. The pH of the spent regenerant could be low or

high depending on the type of resin used. For example, a cationic exchange resin may

be regenerated using concentrated sulfuric or hydrochloric acid, while an anionic

exchange resin may be regenerated using a strong base. (AWWA, 1998) In addition to

the spent regenerant waste, IX generates backwash water used before and after

regeneration.

Table 2-9 presents typical ranges of ion concentrations in the ion exchange wastewater.

(ASCE, 1996)

Table 2-9. Typical Chemical Constituents of Ion Exchange Wastewater

Constituents Range of Averages (mg/L)

TDS 15,000 – 35,000

Ca++ 3,000 – 6,000

Mg++ 1,000 – 2,000

Hardness (as CaCO3) 11,600 – 23,000

Na+2,000 – 5,000

Cl9,000 – 22,000

The volume of brine generated from ion exchange units range from 1.5 to 10 percent of

the treated water depending on the hardness of the feed water and the type of ion

exchange unit used. In general, for every kilogram of hardness removed, approximately

3 to 15 gallons of brine are produced. Table 2-10 presents typical brine generation rates

by water treatment plant flow. (US EPA, 1993)
25

Table 2-10. Typical Ion Exchange Brine Volumes

Water Treatment Plant Flow

(mgd)

Typical Flow Range

(gal/day) Typical Volume (gal/day)

0.7 12,300 – 63,200 12,300

1.8 31,500 – 162,500 31,500

4.8 84,000 – 433,300 84,000

11 192,500 – 993,000 192,500

18 315,000 – 1,624,900 315,000

26 455,000 – 2,347,100 455,000

51 892,600 – 4,604,000 892,600

210 3,675,200 – 18,957,700 3,675,200

430 7,525,400 – 38,818,100 7,525,400

2.2.5 Adsorption Wastes

Adsorption processes often generate a backwash effluent (backwashing disengages

solids that have been entrapped in the bed). Backwashing of GAC filter-adsorbers is

essential to remove solids and to maintain the desired hydraulic properties of the bed.

The residual of the backwash wastewater is generally suspended solids, biological films,

and some filter media.
26

2.3 Summary of Residuals Generated by Drinking Water Treatment Process

Table 2-11 summarizes the typical residuals generated from drinking water treatment

processes and potential pollutants in each waste stream. (ASCE, 1996; AWWARF 1987)

Table 2-11. Drinking Water Treatment Processes and Waste Streams

Major Water Treatment Plant

Type

Typical Residual Waste

Streams

Typical Waste Stream

Constituents

Presedimentation,

sedimentation, and coagulation

basin residuals

Aluminum hydroxide

Ferric hydroxide

Suspended solids

Metals

Organics

Radionuclides

Biological

Silica

Polymers

Coagulation and Filtration

Filter backwash and filter-towasteSimilar to sludge,

but more dilute

Presedimentation,

sedimentation, and precipiitive

softening settling basin residuals

Calcium carbonate

Magnesium hydroxide

Unreacted lime

Suspended solids

Metals

Organics

Radionuclides

Biological

Polymers

Aluminum hydroxide

Ferric hydroxide

Precipitative Softening

Filter backwash and filter-towasteSimilar to sludge,

but more dilute

Concentrate stream

Suspended solids

Organics

Viruses

Metals

Radionuclides

Backwash See Table 2-5

Low-Pressure Membrane

Separation

Chemical cleaning See Table 2-6
27

Table 2-11. Drinking Water Treatment Processes and Waste Streams (Cont.)

Major Water Treatment Plant

Type

Typical Residual Waste

Streams

Typical Waste Stream

Constituents

Concentrate stream See Table 2-7 for a breakdown

by source water

Desalting Membrane Separation

Chemical cleaning

Reflects the spent cleaning

solution and material removed

from the membrane system

during CIP

Brine stream

(regenerate waste)

TDS

Metals

Hardness

Calcium

Magnesium

Sodium

Sulfate

Chloride

Nitrates

Radionuclides

Ion Exchange

Rinse water and/or backwash Dilutes regenerate waste

Adsorption Technologies Backwash

Suspended solids

Metals

Organics

Radionuclides

3.0 STAKEHOLDER INPUT

EPA solicited input from the EPA Drinking Water Treatment Costing Reviewers on the

following aspects of the identification and characterization of drinking water treatment

residuals.

1. Is it appropriate to focus on the five types of water treatment processes

described in Section 1?

2. EPA is gathering additional information about the characterization, management,

treatment, and disposal options for the miscellaneous wastes listed in Section

1.6. Are there any other miscellaneous wastes discharged from drinking water

treatment plants?

3. Is it appropriate to focus on the seven typical residual waste streams presented

in Table 2-11?

4. Is the listing of possible pollutants by waste stream presented in Table 2-11

reasonable and inclusive of all major pollutants types that are likely to be present

in the residual?

5. Should the volume of generated sludge be calculated using the equation

presented in Section 2.1? Or, should EPA develop a curve of typical sludge

generation volumes using data obtained from industry questionnaires?
28

6. Is it appropriate to use percent solid ranges presented in Tables 2-1 and 2-3 for

coagulation/filtration and precipitative softening sludges, respectively? Or, should

EPA develop ranges using data obtained from industry questionnaires?

7. Is it appropriate to use the membrane concentrate generation rates presented in

Table 2-8 to estimate the volume of concentrate/brine produced by membrane

separation processes? Or, should EPA develop generation rates using data

obtained from industry questionnaires?

8. EPA is planning on collecting discharge monitoring reports (DMR) for most if not

all of the large drinking water treatment plants (those serving greater than 50,000

people). Is it reasonable to use the DMR data to estimate pollutant

concentrations in the discharged residuals?

EPA documented its responses to these comments in a document titled, “Summary of

Peer Input: Identification and Characterization of Typical Drinking Water Treatment

Residuals,” which is located in the public record for the Drinking Water Treatment

effluent guidelines rulemaking (EPA-HQ-OW-2004-0035).
29

4.0 REFERENCES

American Society of Civil Engineers, American Water Work Association, U.S.

Environmental Protection Agency. 1996. Management of Water Treatment Plant

Residuals: Technical Transfer Handbook. ASCE & AWWA.

American Water Work Association, American Society of Civil Engineers. 1998. Water

Treatment Plant Design. 3rd ed. McGraw-Hill, New York.

American Water Works Association. 1999. Water Quality and Treatment: A Handbook of

Community Water Supplies. 5th ed. McGraw-Hill, New York.

American Water Works Association Research Foundation. 1987. Water Treatment Plant

Waste Management. AWWA & AWWARF.

American Water Works Association Research Foundation. 2005. Water Treatment

Residuals Engineering.

AWWA Residual Management Research Committee Subcommittee on Membrane

Residual Management, 2003 Committee Report: Residuals Management for LowPressure Membranes. Jour. AWWA, 95:6:68.

AWWA Residual Management Research Committee Subcommittee on Membrane

Residual Management, 2004 Committee Report: Residuals Management for Desalting

Membranes. Jour. AWWA, 96:12:73.

Cornwell, D.A. 1999. Water Treatment Plant Residual Management . In Water Quality

and Treatment: A Handbook of Community Water Supplies. 5th ed. McGraw-Hill, New

York.

U.S. Bureau of Reclamation. 1993. The Desalting and Water Treatment Membrane Manual: A

Guide to Membranes for Municipal Water Treatment. R-96-15

U.S. Environmental Protection Agency. 1992. Project Summary: Inorganic Chemical

Characterization of Water Treatment Plant Residuals. EPA 600/SR-92/117.

U.S. Environmental Protection Agency. 1993. Large Water System Byproducts

Treatment and Disposal Cost Document. EPA 811-D-93-002.

U.S. Environmental Protection Agency. 1999. Alternative Disinfectants and Oxidants

Guidance Manual. EPA 815-R-99-014.

U.S. Environmental Protection Agency. 2000. Technologies and Cost for Removal of

Arsenic from Drinking Water. EPA 815-R-00-028.