Water and Wastewater Engineering

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WATER PLANT RESIDUALS MANAGEMENT 15-7

Thus, on a theoretical basis, each mg/L of alu m yields 0.44 mg/L of sludge on a dry basis.

Suspended solids present in the water will produce an amount of sludge equal to the mg/L of

suspended solids. The amount of sludge produced per turbidity unit is not as obviou s ; however,

in many waters a correlation does exist. Carbon, polymers, and clay will produce about 1 kg of

sludge per kg of chemical addition. The sludge production for alum coagulation may then be ap-

proximated by (Davis and Cornwell, 2008):

MQ SSM

86 4 0 44..A()

(15-14)

where M

 mass of dry sludge produced, kg/d

Q  plant flow, m

/ s

A  alum dose, mg/L

SS  suspended solids in raw water, mg/L

M  miscellaneous chemical additions such as clay, polymer, and carbon, mg/L

In a similar fashion, the dry mass of sludge prod

uced from the addition of iron may be

estimated from the reaction chemistry described by Equation 6-10 in Chapter 6 which is repro-

duced here:

FeCl HCO H O Fe OH H O s CO

332322

33 3 3

 



() ()

CCl



(15-15)

The sludge production for ferric chloride coagulation may be approximated by:

MQ M

86 4 2 9..Fe()SS

(15-16)

where Fe is the iron dose in mg/L expressed as mg/L of Fe, and the other terms are as described

in the preceding equation. Because the units of expression are different, it appears that iron pro-

duces several times the amount of sludge that alum produces. Based on the molecular weights of

the produc

t, in coagulating equivalent, one mole of iron produces about 20 to 25 percent more

dry-weight sludge than one mole of aluminum. When iron is purchased as ferric chloride (FeCl

)

and the dose is as equivalent dry weight without waters of hydration, about 1.0 mg of solids is

produced for each milligram of FeCl

added.

The calculation of polyaluminum chloride (PACl) doses to solids production is not as

straight forward as alum and iron calculations because there is no uniform strength meas ure-

ment. A typical PACl liquid contains about 30 to 35 percent PACl and about 10 perc

ent Al

A very rough estimate of the dry solids production is about 0.8 mg for each mg of PACl added

expressed as PACl.

Solids concentrations from horizontal flow settling basins using continuous collection equip-

ment for alum and iron sludges resulting from coagulation of low- to m

oderate-turbidity raw water

will be in the range of 0.5 to 2 percent. It is often less than 1 percent. Coagulant sludges from

highly turbid water may be in the 2 to 4 percent range (Cornwell, 1999). Twenty to 40 percent of

the solids are organic constituents; the remainder are inorganic constituents or silts. The specific

gravity of alum coagulated soli

ds is typically in the range of 1.2 to 1.5. The range of specific

gravity for iron coagulated slud ge solids is 1.2 to 1.8 (MWH, 2005). The pH of alum sludge is

normally in the 5.5 to 7.5 range. Alum sludge from sedimentation basins may include large num-

bers of microorganisms

, but it generally does not exhibit an unpleasant odor. The sludge flow

rate is often in the range of 0.3 to 1 percent of the treatment plant flow.

15-8 WATER AND WASTEWATER ENGINEERING

Softening Sedimentation Basin

The residues from softening by precipitation with lime [Ca(OH)

] and soda ash (Na

) will

vary from a nearly pure chemical to a highly variable mixture. The softening process discussed in

Chapter 7 produces a sludge containing primarily CaCO

and Mg(OH)

Theoretically, each mg/L of calcium hardness removed produces 1 mg/L of CaCO

sludge;

each mg/L of magnesium hardness removed produces 0.6 mg/L of sludge; and each mg/L of lime

added produces 1 mg/L of sludge. The theoretical sludge production can be estimated as (Davis

and Cornwell, 2008):

864 2 26 16.. .CaCH MgCH CaNCH MgNCH CO(

)

(15-17)

where M

 mass of dry sludge production, kg/d

Q  plant flow, m

/ s

CaCH  calcium carbonate hardness removed as CaCO

, mg/L

MgCH  magnesium carbonate hardness removed as CaCO

, mg/L

CaNCH  noncarbonate calcium hardness removed as CaCO

, mg/L

MgNCH  noncarbonate magnesium hardness removed as CaCO

, mg/L

 carbon dioxide removed by lime addition, as CaCO

, mg/L

When surface waters are softened, or when the softening process is followed by coagulation

and flocculation to remove the fine precipitate, this equation does not account for all of the solids

production. There will be additional sludge from coagulation of suspended materials and precipi-

tation of metal coagulants. Equation

s 15-14 and/or 15-16 may be used to estimate the additional

mass of solids that will be produced.

The specific gravity of lime softening sludge solids is about 1.9 to 2.5. The sludge pH will be

in the range 10.5 to 11.5. The solids content of lime softening sludge in the sedimentation basin

ranges between 2 an

d 15 percent. A nominal value of 10 percent solids is often used.

Spent Filter Backwash Water

All water treatment plants that practice filtration produce a large volume of was h water con-

taining a low suspended solids concentration. The volume of backwash water is usually 2 to 3

percent of the treatment plant flow. Spent filter backwash water (SFBW) will typically contain

10 to 20 percent of the total solids production. It will have a suspended solids concentration in

the range of 30 to 400

mg/L depending on the applied turbidity and the ratio of backwash water

to production water volume (Cornwell, 1999; Peck and Russell, 2005). From limited data, solids

production can range from less than 3 kg/1,000 m

to more than 16 kg/m

of production water

(Cornwell, 2006). The solids in backwash water resemble those found in sedimentation units. Be-

cause filters can support biological growth, the spent filter backwash water may contain a larger

fraction of organic solids than do the solids from the sedimentation basins. SFBW will contain

substantial concentrations of

microorganisms. It has been identified as a source of microorgan-

isms that increases the concentration of Cryptosporidium and Giardia in the water applied to

filters when it is recycled. This may result in an undesirable breakthrough of these organisms into

the water supply (Le Gouellec et al., 2004).

Iron and Manganese Precipitates

The oxidation products that are formed in the removal of iron and manganese are principally ferric

hydroxide, ferric carbonate, and/or manganese dioxide. For each mg/L of iron or m anganese in

WATER PLANT RESIDUALS MANAGEMENT 15-9

solution, 1.5 to 2 mg/L of sludge is produced (Peck and Russel, 2005). Because the iron and man-

ganese concentrations found in natural water is typically low, the sludge volumes are much less

than coagulant and softening sludge volumes. The iron and manganese oxides are captured on the

filters, and the solids are found in the spent backwash water.

Membrane Process Residuals

The constituents that do not pass through the membrane are termed reject, concentrate, or brine.

The volume of reject can be estimated as

QQ R

()1

(15-18)

where Q

 reject or concentrate flow rate, m

/ d

 feed water flow rate, m

/ d

R  recovery rate

The recovery rate is dependent on the source water quality, fouling, feed rate, operating pressure,

and type of membrane. Typical recovery rates and backwash or concentrate flow rates as a per-

cent of the feed water flow rate are shown in Table 15-2 .

If an ion or particle is completely rejected by a N

F/RO membrane, the concentration factor,

that is, the concentration in the residual waste stream compared to that in the feed stream, may be

estimated as (Peck and Russel, 2005):





(15-19)

In addition to the concentrate, clean in place (CIP) residuals must be disposed. While the reject

volume may range from 10 to 60 percent of the feed flow, CIP chemicals are typically less than

0.1 percent of the treated flow (AWWA, 2004).

For low pressure membranes (MF and UF), backwash water represents about 95 to 99 per-

cent of the residual waste. The remainder is c hemically enhanced backwash (CEB) and CIP

chemicals. The volume of backwash residuals is on the order of 2 to 15 percent of the plant feed

TABLE 15-2

Typical recovery rates for membrane processes

Process

Feed water

recovery rate, %

Backwash or concentrate

flow,

% of feed rate

Microfiltration 85 to 98 2 to 15

Ultrafiltration 85 to 98 2 to 15

Nanofiltration 75 to 90 10 to 25

Brackish water RO 60 to 85 15 to 40

Seawater RO 20 to 50 50 to 80

Backwash does not include chemically enhanced backwash (CEB) or clean in place

(CIP) chemicals.

Sources: AWWA, 1996; AWWA, 2005; Peck and Russel, 2005.

15-10 WATER AND WASTEWATER ENGINEERING

flow rate (AWWA, 2003). CEB and CIP chemicals range from 0.2 to 0.4 percent of the feed

water flow rate (AWWA, 2005). The total solids in the wash water is in the range of 100 to 1,000

mg/L. The specific gravity of the wash water is in the range 1.00 to 1.025 (MWH, 2005).

Ion Exchange Residuals

The residual waste streams from ion exchange are liquid. The production rate varies from 1.5 to 10

percent of the water treated. The typical constituents and their ranges are shown in Table 15-3 .

The ion exchange resin itself will leach 30 to 300 mg/L of BOD and 30 to 5,000 mg/L of COD.

In addition, it is solid waste that will have to be replaced approximately every

five to ten years.

Mass Balance Analysis

Clarifier sludge production can be estimated by a mass balance analysis of the sedimentation ba-

sin. Because there is no reaction taking place, the mass balance equation reduces to the form:

Accumulation Rate InputRate OutputRate

(15-20)

The input rate of solids may be estimated using Equations 15-14, 15-16, or 15-17 for the

appropriate chemical addition. An estimate of the concentration of solids and the flow rate is

required to estimate the mass flow (outpu t rate) of solids leaving the clarifier through the weir.

With these estimates, the mass flow out through the weir is

then

Weir solids mass outputrate C

effluent e

 ()(Q

fffluent

)

(15-21)

where C

effluent

 concentration of solids in effluent, g/m

effluent

 flow rate through the weir, m

/ d

E xample 15-2 illustrates the calculations to estimate the sludge production.

Example 15-2. A coagulation treatment plant with a flow of 0.5 m

/ s is dosing alum at

23.0 mg/L. No other chemicals are being added. The raw water suspended solids concentration is

37.0 mg/L. The effluent suspended solids concentration is measured at 12.0 mg/L. The sludge

solids content is 1.00% and the s pecific gravit

y of the sludge solids is 1.2. What volume of

sludge must be disposed of each day?

Consituent Range of concentration, mg/L

Calcium3,000 to 6,000

Magnesium 1,000 to 2,000

Sodium 2,000 to 5,000

Chloride 9,000 to 20,000

Total dissolved solids (TDS) 15,000 to 35,000

Total hardness (as CaCO

) 11,000 to 24,000

Sources: Mickey, 1993; MWH, 2005.

TABLE 15-3

Typical constituents of ion exchange brine

WATER PLANT RESIDUALS MANAGEMENT 15-11

Solution. The mass balance diagram for the sedimentation basin is

Mass of

solids in

influent

Mass of

solids in

effluent

Accumulation withdrawn as slud

a . Compute the accumulation of sludge in the clarifier. The mass of solids (sludge) flowing

into the clarifier is estimated from Equation 15-14 :

86 4 0 5004423 0 370

.. . . .( )[( ( )m /smg/L mg//L

kg/d





2035 58

]

Recognizing that g/m

 mg/L, the mass of solids leaving the weir is

Weir outpu trate g/mm ()()(12 0 0 508640

.. ,0010

518 4

s/d kg/g

kg/d

)( )



 .

The accumulation of sludge in the clarifier is then

Accumulation or2035 58 518 4 1 517 18 1 5,. . ,. ,117 kg/d

b. Using the specific gravity of the sludge solids, compute the specific gravity of the

sludge.







001 12 099

1 002

...

()( )

c. Estimate the volume of sludge produced that must be disposed of each day.



1 517

1 000 1 002 0 01

,..

kg/d

kg/m()()()

114 150

.or m /d

15-4 MINIMIZATION OF RESIDUALS GENERATION

Coagulation

The sludge production rate may be reduced by 30 to 80 percent from the amount produced by

conventional complete treatment using alum or iron if either of two methods is employed: simul-

taneous use of polymer and coagulant or the adoption of a direct filtration alternative (Kawamura,

2000). While the use of polym

er has been adopted widely, the direct filtration techniques (direct

filtration and in-line filtration) are limited by the raw water turbidity.

Frequent assessment of coagulant dosage by jar testing has been shown to reduce the genera-

tion of sludge because it minimizes the potential for overdosing with coagulant.

15-12 WATER AND WASTEWATER ENGINEERING

Softening

Blending softened water and raw water to achieve a final hardness greater than the practical solu-

bility limits will significantly reduce the amount of sludge produced. Customers that have been

consuming water with a hardness over 300 mg/L as CaCO

will be pleased to have a water with

130 to 150 mg/L as CaCO

hardness. Not only will there be a savings in sludge production, there

will be associated cost savings in sludge disposal and chemical purchases that can be passed on

to the customer. Each 10 mg/L of hardness as CaCO

left in the water reduces the sludge quantity

by about 1,200 kg/y per m

/ s of flow.

Air stripping of carbon d ioxide rather than neutralization with lime is another method for

reducing sludge production. Because there is a cost associated with build ing and operating the

stripping column, a careful economic analysis is required. Suggestions for a starting point in

investigation of stripping bas

ed on the CO

concentration are given in Chapter 7.

In softening plants where a significant fraction of hardness is attributable to magnesium, split

flow lime softening can reduce the total sludge production compared with excess lime softening

(Peck and Russell, 2005).

Spent Backwash Water

Filter design criteria that are relevant to determining waste wash water frequency in granular

filters are the unit filter run volume (UFRV) and the unit backwash volume (UBWV). The UFRV

is the volume of water that passes through a unit area of the filter during a run. The UBWV is the

volume per unit area required to backwash the filter.

The efficiency of water production is called the recovery. It is d

efined as the ratio of the net

to total water filtered:

Recovery

UFRV UBWV UFWV

UFRV





(15-22)

where UFWV  unit filter to waste volume, m

/ m

of filter area.

Filters designed to achieve a recovery of 95 percent or more will generate less spent backwas h

water. To achieve 95 percent recovery, UFRV will have to be at least 200 m

/ m

of filter area,

and a filter run will have to be at least 1,000 minutes between backwash cycles. (MWH, 2005).

Recycling

Recycling both saves product water and minimizes the volume of residuals. Possible streams that

may be recycled include (Cornwell, 1999):

1 . Filter to waste.

2. Spent filter backwash water.

a . With the solids from filtration.

b. Without the solids from filtration (after settling).

3. Clarifier or settling basin sludge from softening.

4. Slud

ge thickener supernatant.

WATER PLANT RESIDUALS MANAGEMENT 15-13

5. Sludge lagoon overflow.

6. Dewatering operation liquid waste.

a . From filter press.

b. From centifuge.

c. Leachate from sand drying beds.

Recycling must be evaluated carefully. These wastes may upset the treatment processes and

affect the quality of the finished water. The prin

cipal constituents that may be of concern include

(Cornwell, 1999):

• Microbiological contaminants.

• Total organic carbon.

• Disinfection byproducts.

• Turbidity and suspended solids.

• Metals.

• Taste and odor causing compounds.

High

concentrations of many of these constituents can be removed by coagulation, sedimen-

tation, and other treatment processes. As the composition of the waste stream is unique for each

plant, careful on-site analysis, including laboratory and pilot experiments, is rec

ommended.

15-5 RECOVERY OF TREATMENT CHEMICALS

Although not widely practiced, the technologies for recovery of alum and iron coagulant and lime

have been available in the United States for over 40 years. The low cost of virgin chemicals, as

well as the availability of several sludge disposal options, has made the economics of chemical

recovery

unfavorable except in some special localized cases. The specialized cases will become

more numerous as local circumstances reduce the options for sludge disposal.

Alum Recovery

Four methods are available for recovery of alum: acidification, liquid ion exchange, alkaline

recovery, and Donnan dialysis.

Acid Recovery. This process consists of three steps:

• Thickening of the sludge.

• Acidification of the sludge to a pH between 1.0 and 3.0.

• Decanting the dissolved aluminum

for reuse.

Aluminum recoveries range from 60 to 80 percent. The reaction of about 1 kg of sulfuric acid

with 0.5 kg of aluminum hydroxide yields approximately 1 kg of alum. Other metals such as

15-14 WATER AND WASTEWATER ENGINEERING

chromium and copper can be converted to a soluble form during acidification. This has raised

concerns for the potential buildup of toxic metals.

Liquid Ion Exchange (LIX). The process steps for liquid ion exchange are the same as those

used for acid recovery e

xcept that the aluminum is extracted into the LIX medium. The LIX is

immiscible in water and is separated by flotation. The aluminum is recovered from the LIX by

adjusting the pH of the solution. The LIX can then be reused. Unlike the acid recovery process,

the LIX process is selective for aluminu

m (Westerhoff and Cornwell, 1978). In laboratory studies

this process achieved 95 percent recovery of alum.

An example of a specialized case of the application of this technique is one where a

nearby wastewater treatment plant can us e the recovered alum to remove phosphorus from the

wastewater. This approach increase

s the potential for favorable economics while reducing the

potential for recycling and concentrating toxic metals.

Alkaline Recovery. If sodium aluminate can be used as coagulant, then the aluminum can

be dissolved by raising the pH from about 12 to 12.5 with sodium hydroxide. This converts the

uminum hydroxide to sodium aluminate. Aluminum recoveries of 90 to 95 percent are reported

(Peck and Russell, 2005).

Donnan Membrane Process. Also known as Donnan dialysis, this process is driven by an elec-

trochemical potential gradient across a semipermeable ion exchange membrane. In the Donnan

membrane cell, the feed side of the membrane contains coagulant acidified to a pH in the range

of 3 to 3.5. The recovery side contains a 10 percent sulfuric acid solution. In laboratory experi-

ments, a 70 percent alum recovery was obtained, and the recovered alum was essentially

free of

particulate matter and NOM. Concentrations of metals that were examined were low but not zero:

As  0.5 mg/L, Cu  1 mg/L, Zn  7 mg/L. While these data were obtained at laboratory scale,

they show promise for future development (Prakash and Sengupta, 2003).

Iron Coagulant Recovery

In a process similar to the acid recovery process for alum, this process requires a pH of 1.5 to 2.0

to recover 60 to 70 percent of the iron. Because of the expense and poor dewatering characteris-

tics of the sludge, there has been little interest in this process.

The laboratory scale Donnan process has also been used to recover iron c

oagulant with simi-

lar success to alum recovery (Prakash and Sengupta, 2003).

Lime and Magnesium Recovery

Recalcining. Lime sludge that is predominantly CaCO

can be subjected to high heat in a pro-

cess similar to that used to form quicklime (CaO) from CaCO

that has been mined. This heating

process is called recalcining. This process is energy intensive and requires cheap energy and

substantial quantities of sludge to achieve a scale that can be economical.

Magnesium Carbonate. Black and Thompson (1975) developed a method that softens

the

water while coagulating turbidity. Magnesium carbonate is used as the coagulant. Lime is added

to precipitate the magnesium. The resulting sludge is composed of CaCO

, Mg(OH)

, and the

coagulated turbidity. The sludge is carbonated by injecting CO

gas, which selectively dissolves

WATER PLANT RESIDUALS MANAGEMENT 15-15

the Mg(OH)

. The carbonated sludge is filtered and soluble magnesium bicarbonate is recovered

as filtrate. The lime is recovered by recalcining the CaCO

15-6 RESIDUALS CONVEYANCE

With appropriate selection of materials to deal with the corrosive nature of the sludge, waste back-

wash water, reject from membrane processes, brine from ion exchange processes, and sludges with

solids concentrations less than 1 percent may be conveyed by standard pump and pipe sy

stems used

for water. As shown in Example 27-2, the Hazen-Williams equation may be used for pipe design.

Sludges with concentrations above 1 percent behave as non-Newtonian fluids. There is

no simple relationship that can be used to predict headloss. There is no readily available

method for calculating headloss in chemi

cally generated sludges from water treatment pro-

cesses. The graphs that are available for calculating headloss in pumping sludge are based on

experiments with biological sludges found in wastewater treatment systems. Pec k and Russell

(2005) suggest that these may be used with appropriate

safety factors. These are discussed in

Chapter 27.

In addition to the difficulties in predicting headloss, conventional centrifugal pumps are inef-

fective in moving sludge with solids concentrations greater than about 3 or 4 percent. Progressive

cavity pumps, peristaltic pumps, and diaphragm pum

ps have been used for pumping residuals

from clarifiers and thic keners . As an alternative to s c rew or c onvey or systems, progressive cav-

ity pumps and high-pressure piston pumps have been used to move dewatered residuals (Peck

and Russell, 2005). The selec

tion and application of various alternatives for moving sludges and

dewatered residuals are discussed in Chapter 27.

15-7 MANAGEMENT OF SLUDGES

The treatment of solid/liquid wastes produced in water treatment processes involves the separa-

tion of the water from the solid constituents to the degree necessary for the selected disposal

method. Therefore, the required degree of treatment is a direct function of the ultimate disposal

metho

d. In turn, the ultimate disposal method is a function of regulatory constraints and the eco-

nomics of the disposal method.

There are several sludge treatment methodologies that have been practiced in the water

industry. Figure 15-1 shows the most comm

on s ludge handling options available, listed by the

categories of thickening, dewatering, and disposal. In choosing a combination of the possible treat-

ment process trains, it is best to first identify the available disposal options and their requirements

for a final cake solids concentration. Most landfill applications will require a “handleable” sl

udge,

and this may limit the type of dewatering devices that are acceptable. Methods and costs of trans-

portation may affect the decision how dry is dry enough? The criteria should not be to simply

reach a given solids concentration, but rather to reach a solids concentration that has

the properties

for handling, transport, and disposal. The required properties are a function of the management

options that are available.

Table 15-4 shows a generalized range of results that have been obtained for final solids con-

centrations from different dewatering devices for coagulant and lime sludges.

To give you an appreciation of these solids c

oncentrations, a sludge cake with 35 percent

solids would have the consistency of butter, while a 15 percent sludge would have a consistency

much like rubber cement.

15-16 WATER AND WASTEWATER ENGINEERING

The conventional sludge handling options shown in Figure 15-1 are discussed in the following

paragraphs.

Thickening

After removal of the sludge from the clarifier or sedimentation basin, the first treatm ent step is

usually thickening. Thickening assists the performance of any subsequent treatment, gets rid of a

lot of water quickly, and helps to equalize flows to the subsequent treatment device.

Softening

unit

Thickening Conditioning

Lime

Dewatering/treatment DisposalWaste source

Dewatering

lagoon

Recalcination

Direct

discharge

Landfill/

monofill

Land

application

Useable or

salable product

Wastewater

plant

Centrifuge

Pressure

filter

Va cuum

filter

Belt filter

press

Permanent

lagoon

Sand bed/

freeze-thaw

Conditioning

Alum

recovery

Coagulant

clarifer

Thickening

Alum

Gravity

thickening

Storage/

equalization

Spent filter

backwash

FIGURE 15-1

Sludge handling options. ( Source: Davis and Cornwell, 2008.)

Lime sludge, % Coagulation sludge, %

Gravity thickening 15–30 2–4

Dissolved air flotation 3–53–5

Basket centrifuge

N/A

10–15

Solid bowl, scroll centrifuge 55–65 20–25

Belt filter press 25–60 15–30

Vacuum filter 45–65

N/A

Pressure filter 55–70 30–40

Sand drying bed50 20–25

Storage lagoon 50–60 7–15

TABLE 15-4

Range of obtainable cake solid concentrations

N/A  not advised.

( Sources: Davis and Cornwell, 2008; MWH, 2005.)