Water and Wastewater Engineering

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WATER PLANT RESIDUALS MANAGEMENT 15-37

Centrifuges are very sensitive to changes in the concentration or composition of the sludge,

as well as to the amount of polymer applied. Thus, the best way to evaluate centrifugation is by

using pilot tests. Cornwell (2006) provides a detailed proc edure for scaling up pilot results to

full scale. The best indi

cators of performance are the cake solids concentration and the centrate

suspended solids concentration.

For new plants without production sludge to test, similar facilities treating similar water are

the only option in evaluating this alternative for sludge dewatering.

Centrifuge Selection Considerations. Centrifuges can handle sludge solids

concentrations up

to about 4 percent. Centrifuges are often located on upper floors of the sludge building so that the

cake may be discharged into trucks or hoppers below. Because of the mass and vibration of the

centrifuge, special attention should be given to the structural requirements to handle the load and

vibration in evaluating the

cost of this option.

The best performance data for centrifuges have been obtained at 75 to 80 percent of the

manufacturer’s hydraulic or solids capacity. A typical selection of centrifuges is shown in

Table 15-8 .

Example 15-7. Select a centrifuge to dewater a coagulation sludge that has been thickened to

5%. The initial sludge flow rate is estimated to be 128.4 m

/ d, and the design sludge flow rate

is 171.2 m

/ d. The plant managers wish to operate on a two-shift basis, that is, 16 h/d to minimize

labor costs. Using Table 15-8 , select an appropriate centrifuge system.

Solution:

a . Calculate the hourly sludge flow rate for a 16-hour operation at the initial and des ign

sludge flow rates.

TABLE 15-8

Typical selection of centrifuges

Model

Flow rate,

Bowl

diameter, cm Length, m Width, m Height, m Weight, kg

25 4.5 25 2.1 1.1 0.8 1,300

309 30 2.4 1.1 0.9 1,500

35 18 35 2.8 1.4 1.0 2,500

4530453.2 1.5 1.0 3,500

55 60 55 3.7 1.6 1.1 4,500

65 70 65 4.4 2.0 1.2 7,500

75 110 755.9 2.7 1.3 13,000

Maximum solids concentration  4%

Note: These centrifuges are representative but do not represent actual choices. Actual manufacturers’ data

must be used for real world design.

15-38 WATER AND WASTEWATER ENGINEERING

sludge initial

m /d

h/d

or

128 4

8025 8

m /h

m /d

h/d

sludge design

171 2

10 7Q 

.oor m /h11

b. From Table 15-8 , the Model 30 centrifuge rated at 9 m

/h appears to be satisfactory for

the initial sludge flow rate, but not the the design flow rate. Using the 75 to 80% rule of

thumb as a safety factor,

()( )075 9675

..m /h m /h

would provide the best performance for this centrifuge. This is not satisfactory.

c. An alternate design that would also provide redundancy would be to provide two centri-

fuges rated at 9 m

/h. This scheme would allow for an operational time of

128 4

2675

9 511

m /d

centrifugesm/h()()

 oor h/d9 5.

The capacity of the two centrifuges at the design sludge flow rate would allow for operation

171 2

2675

12 68

m /d

centrifugesm/h()()

 oor h/d13

d. Another scheme would be to provide one centrifuge rated at 18 m

/h. While this scheme

would provide the same operating hours as the two centrifuge option, it would not provide

any redundancy.

Comments:

1 . Among the many other alternatives to be considered are redesign of the thickener to

yield a higher solids loading at a lower flow rate, or other manufacturers’ products with

a different sele

ction of sizes.

2. For the two centrifuge option, the initial operating schedule should alternate the centri-

fuges’ use to both extend their life and make use of the manufacturer’s warranty.

3. As may be noted, at the design life, there is no redundant centrifuge. Either sludge stor-

age or the provision of space for the addition of another

centrifuge when the capacity

of the two centrifuges does not allow for redundant operation are options that might be

considered.

Vacuum Filtration. A vacuum filter consists of a cylindrical drum covered with a filtering

material or fabric, which rotates partially submerged in a vat of conditioned sludge. A vacuum is

applied

inside the drum to extract water, leaving the solids, or filter cake, on the filter medium. In

practice, a differential pressure of about 70 kPa is applied. As the drum completes its rotational

cycles, a blade scrapes the filter cake from the filter and the cycle begins again. Two basic types

WATER PLANT RESIDUALS MANAGEMENT 15-39

of rotary drum vacuum filters are used in water treatment: the traveling medium and the precoat

medium filters. The traveling medium filter is made of fabric or stainless steel coils. This filter is

continuously removed from the drum, allowing it to be washed from both sides without dilu

ting

the sludge in the sludge vat. The precoat medium filter is coated with 50 to 75 mm of inert mate-

rial, which is shaved off in 0.1 mm increments as the drum moves.

Vacuum filters are not recommended for coagulation sludges.

Continuous Belt Filter Press (CBFP). The belt filter press operates on the principle that ben

ing a sludge cake contained between two filter belts around a roll introduces shear and compres-

sive forces in the cake, allowing water to work its way to the surface and out of the cake, thereby

reducing the cake moisture content. The device employs double moving belts to continuous ly

ewater sludges through one or more stages of dewatering ( Figure 15-9 ). Typically the CBFP

includes the following stages of treatment:

1 . A reactor/conditioner to remove free-draining water.

2. A low pressure zone of belts with the top belt being s olid and the bottom belt being a

sieve; here further water removal occurs, and a sludge mat having significant dim

en-

sional stability is formed.

3. A high pressure zone of belts with a serpentine or sinusoidal configuration to add shear

to the pressure dewatering mechanisms.

Gravity drainageCompression dewatering

Low-pressure section

Gravity

drainage

stage

Compression

dewatering

stage

Chemical

conditional

stage

Polyelectrolyte

solution

Sludge

Conditioned

sludge

Wash spray

Washwater

Filtrate

Sludge

cake

High-pressure section

Mixer

FIGURE 15-9

Continuous belt filter press.

( Source: U.S. EPA, 1979.)

15-40 WATER AND WASTEWATER ENGINEERING

The design and selection of a belt filter press is often based on the “throughput” of the

press. Throughput i s the rate at which residuals can be dewatered. The throughput can be either

hydraulically or solids limited. A belt press having a particular type and width of belt has a max-

imum loading capacity for a given type of res

idual. The solids loading is considered the m ost

critical factor, and throughput is expressed in terms of solids loading: kg/meter of belt width per

hour. For coagulant sludges the typical loading rate is about 150 kg/m · h, but sludges thickened

to 4 perc

ent may be loaded at a rate of 400 to 570 kg/m · h (Cornwell, 2006). Lime sludges up to

30 percent solids have been dewatered to 60 percent solids (MWH, 2005). Typical belt widths

are 1.0, 1.5, 2.0, and 3.0 m.

Discharge from the press may be directly to a truck. Other options include conveyors and

hoppers or roll-off boxe

The belt press has a relatively low energy requirement compared to other mechanical de-

watering devices . To achieve acceptable solids conc entrations, the sludge fed to the CBFP

must be conditioned. Polymers are often used for conditioning. In addition, belt wash water

must be provided. This wash water represent

s another disposal issue as it will be high in sus-

pended solids.

Plate and Frame Filter Press. The basic component of a filter press is a series of recessed

vertical plates. Each plate is covered with cloth to support and contain the slu dge cake. The

plates are mounted in a frame consisting of two head support

s connected by two horizontal paral-

lel bars. Conditioned sludge is pumped into the pressure filter and passes through feed holes in

the filter plates along the length of the filter and into the recessed chambers. As the sludge cake

forms and builds up in the chamber, the pressure gradually increases to a point where further

slud ge injection would be counterproductive. At this time the injection ceases. A variation of

the standard plate and frame filter press is the diaphragm filter press. The construction is similar

to the standard recessed chamber, but the drainage surfaces on the face of the plates are flexible

membrane

s or diaphragms. After the filtration cycle is completed and the recessed chambers

are filled with solids, and before the press is opened, air or water pressure is applied behind the

diaphragms causing them to flex outward to exert additional pressure on the filter cake. This

squeezes the cake and further reduces any re

maining moisture.

The diaphragm filter press yields a higher cake solids and has a shorter cycle time than the

standard plate and frame press. Although the filter press may be highly automated, the operation

will require significant operator attention. Current models are provided with “cake breakers” to

dislodge the cake from the press. These m

ay need operator attention to ensure that the cake is com-

pletely dislodged. The operation and schematic cross sections are illustrated in Figure 15-10.

A typical pressure filtration cycle begins with the closing of the press to the position shown on

Figure 15-10a. Sludge is fed for a 20- to 30-minute period

until the press is effectively full of cake.

The pressure at this point is generally the designed maximum (700 to 1,700 kPa) and is maintained

for one to four hours, during which more filtrate is removed and the desired cake solids content

is achieved. The filter is then mechanically opened

, and the dewatered cake is dropped from the

chambers onto a conveyor belt or hopper for removal. Cake breakers are usually required to break

up the rigid cake into conveyable form. Because plate pressure filters operate at high pressures and

because many units use lime for conditioning, the cloths require routine wa

shing with high-pressure

water, as well as periodic washing with acid.

While filter presses work well for lime sludges, they require large quantities of conditioning

agents, including lime and fly ash, to produce a dry cake from coagulation sludges. In either case,

thickening before filtration is typical.

WATER PLANT RESIDUALS MANAGEMENT 15-41

FIGURE 15-10

Plate and frame filter press. ( a ) Filling the press, ( b ) cake breaking, ( c ) plate details for recessed plate, ( d ) diaphragm plate details,

( e ) plate and frame solids off-loading system.

Polypropylene

plates

Filter cloth

Clear

filtrate

Head

Inlet feed

of slurry

(a)

Head plate

Tail plate

Back-up

plate

Follower

Intermediate plates

Filter cake

ilter

ake

Filter cloth

Plate separates

Filter plate

Cake falls

(b)

Cake falls

Plate

support

handle

Feed eye

Wash water or

air blowdown por

Stay boss

Discharge eye

(c)

Gasket

Chambers

(e)

Roll-off box

Center Web

Piped drainage

surface

Flexible

diaphragm

Cloth sealing

area

Diaphragm

supports

Two halves of

molded diaphragm

plate laminated to

center web

(d)

15-42 WATER AND WASTEWATER ENGINEERING

Alum sludges are conditioned using lime and/or fly ash. Lime dosage is typically in the range

of 10 to 15 percent of the sludge solids. Fly ash is applied at dosage of about 100 percent of the

sludge solids. Polymers are dosed in the range of 1 to 2 g/kg. While cake solids may be between

45 and 50 percent dry solids, as much as 30 percent of the dry solids may be conditioning chemi-

cals and/or fly ash. To reduce the volume of sludge, only polymer may be used for conditioning.

This is an economic i

ssue as well as an operational issue. The cost of polymer, which is very

expensive, must be weighed against the cost of disposing of a larger volume of solids.

E xample 15-8 illustrates the method for selecting an appropriate size filter press from manu-

facturer’s data like that shown in Table 15-9 .

TABLE 15-9

Typical filter press manufacturer’s data

Press size Volume, L No. of chambers Length, m

H  3.5 m

W  2.7 m

3,500 64 7.7

4,300 77 8.7

5,000 90 9.5

5,700 103 10.4

6,400 115 11.2

7,100 128 12.1

7,800 141 13.0

H  4.2 m

W  2.7 m

5,700 74 8.4

6,500 84 9.1

7,000 91 9.6

7,700 100 10.2

8,500 110 10.9

9,300 120 11.6

10,000 130 12.2

H  3.9 m

W  3.9 m

8,500 89 9.5

9,600 100 10.2

10,600 110 10.9

11,500 120 11.6

12,500 130 12.2

13,400 140 12.9

14,400 150133.6

Note: These presses are representative but do not represent actual choices.

Actual manufacturer’s data must be used for real world design.

WATER PLANT RESIDUALS MANAGEMENT 15-43

Example 15-8. A lime softening water treatment plant with a design capacity of 30,000 m

/ d

is being designed. A filter press has been selected as the method of dewatering the sludge for

landfilling. The calculated removals of hardness and the design characteristics of the sludge are

given below. Based on experience, a cycle time of four hours is anticipated. This incl

udes filling,

pressing, and discharging the sludge. From Table 15-9 select an appropriate press.

CaCH  198 mg/L as CaCO

MgCH  15 mg/L as CaCO

CaNCH  0.0 mg/L as CaCO

MgNCH  55 mg/L as CaCO

 18 mg/L as CaCO

Specific gravity of sludge  1.1

Specific gravity of dewatered sludge  1.176

Percent solids delivered to the filter press  10%

Dewatered solids  25%

Solution:

a . Begin by estim ating the daily dry solids sludge production using Equation 15-17. The

flow rate in appropriate units is

Q 

30 000

86 400

0 3472

m /d

s/d

m /s

Using Equation 15-17, the estimated mass of dry solids is

86 4 0 3472 2 198 2 6 15

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

mg/L mg/L kg/d

)

() ]  00 1655 18 16 229.. ,

b. Estimate the volume of sludge using Equation 15-9.



16 229

1000 11 010

147

,..

kg/d

kg/m()()()

.. ,5 147 500

m /d or L/d

c. The volume of the filter press in liters is estimated using Equation 15-11.

dewatered

L/d g/L



()()()(147 500 0 10 1 000 1,.,..

., .

1 176 1 000 0 25

55 187 55 2

)

()( )()g/L

or 000 L/d

d. At four hours per cycle, the volume of the press must be

()()55 200 4

9 200

L/d h/cycle

h/d

L/cycle

e. From Table 15-9 two models will work:

H  4.2 m and W  2.7 m with 120 plates has a capacity of 9,300 L

H  3.9 m and W  3.9 m with 100 plates has a capacity of 9,600 L

15-44 WATER AND WASTEWATER ENGINEERING

Comments:

1 . Of the two possible choices, the larger press ( H  3.9 m and W  3.9 m) has the advan-

tage of using fewer plates. Fewer plates means a smaller number of things that can go

wrong. The shorter length will also result in a better distribution of solids. Because the

frame for the larger plates can be expanded to 150 plates, this press

also offers the pos-

sibility of expansion by adding plates.

2. Not addressed here is redundancy and staff management, which are issues that also must

be considered.

15-8 MANAGEMENT OF LIQUID RESIDUALS

Conventional methods of disposal of liquid residuals from ion exchange and membrane pro-

cesses include discharge to surface water bodies, dilution and spray irrigation, deep well injec-

tion, drain fields, and discharge to municipal sewers. Table 15-10 summarizes

the regulatory and

other environmental requirements with these disposal methods. Decision trees for selection of a

management alternative for MF/UF and NF/RO residuals are shown in Figures 15-11 and 15-12

on pages 15-45 and 15-46.

TABLE 15-10

Concerns and requirements associated with conventional disposal methods

Disposal method Regulatory concerns Other requirements

Disposal to surface water Receiving stream limitations Mixing zone

Radionuclides Possible pretreatment

Odors (hydrogen sulfide) Multiple-port diffusers

Low dissolved oxygen levels Modeling of receiving stream

lfide toxicity

Low pH

Deep well injection Confining layer Well liner

Upconing to drinking water wells Monitoring well

Injection well integrity Periodic integrity test

Corrosivity Water quality of concentrate must

be compatible with the water

quality in the injection zone

Spray irrigation Groundwater protection Monitoring wells

Possible pretreatment

Backup disposal method

Need for irrigation water

Availability of blend waters

Drainfield or borehole Groundwater protection Monitoring wells

Proper soil conditions and/or rock

permeability

Sanitary sewer collection

systems

Effect on local wastewater

treatment plant performance

(toxicity to biomass or inhibited

settleability in clarifiers)

None

Source: AWWA, 1996.

WATER PLANT RESIDUALS MANAGEMENT 15-45

15-9 DISPOSAL OF SPECIFIC RESIDUAL CONSTITUENTS

Arsenic Residuals

Unless the arsenic concentration is less than 5.0 mg/L, regenerant from ion exchange, activated alu-

mina and modified iron removal (MIR) plants, as well as reject from reverse osmosis will exceed

the toxicity characteristic and must be disposed of as a hazardous waste at an approve

d hazardous

waste treatment facility. Iron-based sorbents and sludges from iron and aluminum coagulation as

well as enhanced lime softening must be tested using the toxicity characteristic leaching procedure

(TCLP) or, in the case of California, the waste extra

ction test (WET) to determine if they are clas-

sified as a hazardous waste. Testing of sludges has resulted in the conclusion that sludges do not

qualify as a hazardous waste because the TCLP levels are under 5.0 mg/L (Cornwell et al., 2003).

However, these sludges m

ay not pass the WET, so individual evaluation is warranted.

The potential regulatory requ irements are shown in Figure 15-13. A decision tree for han-

dling and disposal of arsenic residuals is shown in Figure 15-14 on page 15-47.

Spent CIP?

Adjust pH to 6.0 to 9.0

Are coagulation

chemicals present?

Neutralize oxidants

Nautralize oxidants

Are levels of TSS,

organic compounds,

and TDS acceptable?

– Sewer discharge with

pretreatment permit

– Haul offsite for

appropriate disposal

– Recycle

– Surface discharge

(as permitted)

– Spray

irrigation

– Sewer discharge

Surface discharge

(as permitted)

Liquid–solid separation

– Recycle

– Surface discharge

– Sewer discharge

– Dewater for land

disposal

– Land application

Contaminant removal

required?

Solids removal required?

Treat as spent filter

backwash from

conventional WTP

djust pH to 6.0 to 9.0

Spend backwash and CEBW

Yes

Yes No

Yes

Treatment for

contaminant

removal

Yes

Would blending with

spent backwash make

it acceptable?

Solids

Treated

water

Yes

FIGURE 15-11

Decision tree for MF/UF residuals. (CIP—c lean in place chemicals, CEBW—chemically enhanced backwash, TSS—total suspended solids, TDS—total

dissolved solids.)

15-46 WATER AND WASTEWATER ENGINEERING

NF/RO Membrance Residuals

Adjust pH to 6.0 to 9.0

Spent CIP Concentrate

Adjust pH to 6.0 to 9.0

Neutralize oxidants/Cl

as needed

Are levels of TSS, organic

compounds, and TDS acceptable?

Contaminant removal requred?

– Sewer discharge with

pretreatment permit

– Haul offsite for appropriate

disposal

Solids and

concentrates

Treatment for

contaminant removal

Treated

water

Surface discharge

(as permitted)

Surface disc

harge?

– Increase dissolved oxygen level

(as required)

– Remove H

S (as required)

– Sewer discharge

– Deep well injection (as permitted)

– Evaporation pond

– Zero liquid discharge

Neutralize chlorine

(ED/EDR only) as

needed

Yes

FIGURE 15-12

Decision tree for NF/RO membrane residuals management. (CIP—clean in place chemicals, TSS—total suspended solids, TDS—total dissolved solids.)

Underground injection

Deep well

Hazardous landfill

Land disposal

Sanitary industrial

Reuse

Land application

Wetland/ocean disposal

Incineration

Discharge

Direct/indirect

surface water,

wetland, ocean,

sanitary sewer

Solid/sludge residuals

Liquid residuals

Interim treatment

Clean Water Act

NPDES program

40 CFR Parts 122–133

Pretreatment program

40 CFR Part 403

Resource Conservation

and Recovery Act

Subtitles C and D programs

40 CFR

Parts 257–270

Clean Water Act

Dredge and fill

programs

40 CFR

Parts 230–233

lean Water Act/

Resource Conservation

and Recovery Act

40 CFR

Parts 26, 50, 60–63

Safe Drinking Water Act

Underground injection

control program

40 CFR

Parts 141–149

FIGURE 15-13

A r senic residuals disposal—federal regulations. (Adapted from U.S. EPA, 2000.)