Lin S.D. Water and Wastewater Calculations Manual

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⫽ 0.05 day

⫺1

⫽ 0.8

⫽ 60 days

The terms YS

is neglected, since no primary sludge is applied.

⫽ 1383 m

⫽ 48,860 ft

Step 6. Check the diffused air requirement per m

of digester volume,

from Step 4

⫽ 0.025 m

/(min ⭈ m

)

⫽ 25 L/min ⭈ m

⫽ 25 ft

/min/1000 ft

This value is at the lowest range of regulatory requirement, 25 to 35 L/min ⭈ m

Example 4: Determine the size of an aerobic digester to treat the primary

sludge of 590 kg/d (1300 lb/d) plus activated sludge of 380 kg/d (840 kg/d). Assume

the mixed sludge has a solids content of 3.8% and a retention time of 18 days.

solution:

Step 1. Calculate the mass of mixed sludge

Mass ⫽ (590 kg/d ⫹ 380 kg/d)/0.038

⫽ 25,530 kg/d

> 25.5 m

Step 2. Calculate the volume (V ) of the aerobic digester

The volume of daily sludge production is approximately 25.5 m

/d, thus

V ⫽ 25.5 m

/d ⫻ 18 days

⫽ 459 m

Step 3. Check the solids loading rate assuming the solids are 80% volatile

VSS ⫽ (590 ⫹ 380) kg/d ⫻ 0.8 ⫽ 776 kg/d

Air 5

34.4 m

/min

1,383 m

58.8 m

/d 3 25,000 mg/L

18,750 mg/Ls0.05 day

3 0.8 1 1/60 daysd

V 5

1 YS

XsK

1 1/u

Wastewater Engineering 825

Loading rate ⫽ (776 kg VSS/d)/459 m

⫽ 1.69 kg VSS(m

⭈ d) [in the range of 1.1 to 3.2 kg VSS/(m

⭈ d)]

Lime stabilization. Chemical stabilization of wastewater sludge is an

alternative to biological stabilization. It involves chemical oxidation

(commonly using chlorine) and pH adjustment under basic conditions,

which is achieved by addition of lime or lime-containing matter. Chemical

oxidation is achieved by dosing the sludge with chlorine (or ozone, hydro-

gen peroxide). The sludge is deodorized and microbiological activities

slowed down. The sludge can then be dewatered and disposed of.

In the lime stabilization process, lime is added to sludge in sufficient

quantity to raise the pH to 12 or higher for a minimum of 2 h contact.

The highly alkaline environment will inactivate biological growth and

destroy pathogens. The sludge does not putrefy, create odors, or pose a

health hazard. However, if the pH drops below 11, renewed bacteria and

pathogen growth can reoccur. Since the addition of lime does not reduce

the volatile organics, the sludge must be further treated and disposed

of before the organic matter starts to putrefy again.

With 40 CFR Part 503 regulations (Federal Register, 1993) to reduce

pathogens, a supplemental heat system can be used to reduce the quan-

tity of lime required to obtain the proper temperature. Additional heat-

ing reduces the lime dosage and operating costs.

Composting. Composting is an aerobic biological decomposition of

organic material to a stable end product at an elevated temperature.

Some facilities have utilized anaerobic composting in a sanitary, nuisance-

free environment to create a stable humus-like material suitable for plant

growth. Approximately 20% to 30% of volatile solids are converted to

carbon dioxide and water at temperatures in the pasteurization range of

50 to 70⬚C (122 to 158⬚F). At these temperatures most enteric pathogens

are destroyed. Thermophilic bacteria are responsible for decomposing

organic matter. The finished compost has a moisture content of 40% to

50% and a volatile solids content of 40% or less (US EPA, 1991).

The optimum moisture content for composting is 50% to 60% water.

Dewatered wastewater sludges are generally too wet to meet optimum

composing conditions. If the moisture content is over 60% water, proper

structural integrity must be obtained. The dewatered sludge and bulk-

ing agent must be uniformly mixed. Sufficient air must be supplied to the

composting pile, either by forced aeration or windrow turning to main-

tain oxygen levels between 5% and 15%. Odor control may be required

on some sites.

In order to comply with the “process to significantly reduce pathogens”

(PSRP) requirements of 40 CFR Part 257 (Federal Register, 1993), the

compost pile must be maintained at a minimum operating temperature

826 Chapter 6

of 40⬚C (104⬚F) for at least 5 days. During this time, the temperature must

be allowed to increase above 55⬚C (131⬚F) for at least 4 h to ensure

pathogen destruction. In order to comply with the “process to further

reduce pathogens” (PFRP) requirements of 40 CFR Part 257 (Federal

maintained at a minimum operating temperature of 55⬚C for at least

3 days. The windrow pile must be maintained at a minimum operating

temperature of 55⬚C for 15 days. In addition, there must be at least three

turnings of the compost pile during this period.

Sludge conditioning. Mechanically concentrated (thickened) sludges

and biologically and chemically stabilized sludges still require some

conditioning steps. Sludge conditioning can involve chemical and/or

physical treatment to enhance water removal. Sludge conditioning is

undertaken before sludge dewatering.

Details of process chemistry, design considerations, mechanical com-

ponents, system layout, operation, and costs of conditional methods are

presented in Sludge Conditioning Manual (WAPCF, 1988a). Some addi-

tional sludge conditioning processes disinfect sludge, control odors, alter

the nature of solids, provide limited solids destruction, and increase

solids recovery.

Chemical conditioning. Chemical conditioning can reduce the 90% to 99%

incoming sludge moisture content to 65% to 80%, depending on the nature

of the sludge to be treated (WPCF, 1988a). Chemical conditioning results

in coagulation of the solids and release of the absorbed water. Chemicals

used for sludge conditioning include inorganic compounds such as lime,

pebble quicklime, ferric chloride, alum, and organic polymers (polyelec-

trolytes). Addition of conditioning chemicals may increase the dry solids

of the sludge. Inorganic chemicals can increase the dry solids by 20% to

30%, but polymers do not increase the dry solids significantly.

Physical conditioning. The physical process includes using hot and cold

temperatures to change sludge characteristics. The commonly used phys-

ical conditioning methods are thermal conditioning and elutriation. Less

commonly used methods include freeze–thaw, solvent extraction, irra-

diation, and ultrasonic vibration.

The thermal conditioning (heat treatment) process involves heating

the sludge to a temperature of 177 to 240⬚C (350 to 464⬚F) in a reaction

vessel under pressure of 1720 to 2760 kN/m

(250 to 400 lb/in

[psig])

for a period of 15 to 40 min (US EPA, 1991). One modification of the

process involves the addition of a small amount of air. Heat coagulates

solids, breaks down the structure of microbial cells in waste activated

sludge, and releases the water bound in the cell. The heat-treated sludge

is sterilized and practically deodorized. It has excellent dewatering

Wastewater Engineering 827

characteristics and does not normally require chemical conditioning to

dewater well on mechanical equipment, Yielding cake solids concen-

trations of 40% to 50%.

Heat treatment is suitable for many types of sludge that cannot be sta-

bilized biologically due to the presence of toxic materials and is relatively

insensitive to change in sludge composition. However, the process pro-

duces liquid sidestreams with high concentrations of organics, ammonia

nitrogen, and color. Significant odorous off-gases are also generated

which must be collected and treated before release. The process has

high capital cost because of its mechanical complexity and the use of

corrosion-resistant material. It also requires close supervision, skilled

operators, and O and M programs. More detailed description of thermal

processing of sludge can be found elsewhere (WPCF, 1988a; WEF and

ASCE, 1991b).

A physical conditioning method used in the past is elutriation. In elu-

triation, a washing process, sludge is mixed with a liquid for the pur-

pose of transferring certain soluble organic or inorganic components to

the liquid. Wastewater effluent is usually used for elutriation. The

volume of wastewater is two to six times the volume of sludge. The elu-

triation of the sludge produces large volumes of liquid that contain a high

concentration of suspended solids. This liquid, when returned to the

treatment plant, increases the solids and organic loadings. Elutriation

tanks are designed to act as gravity thickeners with solids loading rates

of 39 to 49 kg/(m

⭈ d) (8 to 10 lb/(ft

⭈ d)) (Qasim, 1985).

Sludge dewatering. Following stabilization and/or conditioning, waste-

water sludge can be ultimately disposed of, or can be dewatered prior to

further treatment and/or ultimate disposal. It is generally more eco-

nomical to dewater before disposal. The primary objective of dewatering

is to reduce sludge moisture. Subsequently, it reduces the costs of pump-

ing and hauling to the disposal site. Dewatered sludge is easier to handle

than thickened or liquid sludge. Dewatering is also required before com-

posting, prior to sludge incineration, and prior to landfill. An advantage

of dewatering is that it makes the sludge odorless and nonputrescible.

Sludge can be dewatered by slow natural evaporation and percolation

(drying beds, drying lagoons) or by mechanical devices, mechanically

assisted physical means, such as vacuum filtration, pressure filtration,

centrifugation, or recessed plate filtration.

Vacuum ﬁltration. Vacuum filtration has been used for wastewater

sludge dewatering for almost seven decades. Its use has declined owing

to competition from the belt press. A rotary vacuum filter consists of a

cylindrical drum covered with cloth of natural or synthetic fabric, coil

springs, or woven stainless steel mesh. The drum is partly submerged

(20% to 40%) in a vat containing the sludge to be dewatered.

828 Chapter 6

The filter drum is divided into compartments. In sequence, each com-

partment is subject to vacuums ranging from 38 to 75 cm (18 to 30 in)

of mercury. As it slowly rotates, vacuum is applied immediately under

the mat formation or sludge pick-up zone. Suction continues to dewa-

ter the solids adhering to the filter medium as it rotates out of the

liquid. This is called the drying zone of the cycle. The cake drying zone

represents from 40% to 60% of the drum surface. The vacuum is then

stopped, and the solids cake is removed to a sludge hopper or a conveyor.

The filter medium is washed by water sprays before reentering the vat.

For small plants, 35 h operation per week is often designed. This allows

daily 7 h operation with 1 h for start-up and wash-down. For larger

plants, vacuum filters are often operated for a period of 16 to 20 h/d

(Hammer, 1986).

Example: A 3 m (10 ft) diameter by 4.5 m (15 ft) long vacuum filter dewa-

ters 5400 kg/d primary plus secondary sludge solids. The filter is operated 7 h/d

with a drum cycle time of 4 min. The dewatered sludge has 25% solids con-

tent. The filter yield is 17.3 kg/(m

⭈ h). Determine the filter loading rate and

percent solids recovery. What is the daily operating hours required if the

dewatered sludge is 30% solids with the same percentage of solids recovery

and a filter yield of 9.8 kg/(m

⭈ h)?

solution:

Step 1. Compute the surface area of the filter

Area ⫽ p(3 m)(4.5 m) ⫽ 42.4 m

Step 2. Compute the filter loading rate

Step 3. Compute the percent solids recovery

Step 4. Compute the daily filter operation required with 30% solids

Since filter yield is determined by

Filter yield 5

solids loading 3 factor of recovery

h/d operation 3 filter area

95%

% 5

s17.3 kg/sm

hdd 3 100%

18.2 kg/sm

5 3.7 lb/sft

5 18.2 kg/sm

Loading 5

solids loading

area 3 operating time

5400 kg/d

42.4 m

3 7 h/d

Wastewater Engineering 829

rearranging the above:

Pressure ﬁltration. There are two types of pressure filtration process as

used for sludge dewatering. Normally, they are the belt filter press and

the plate and frame filter press. The operating mechanics of these two

filter press types are completely different. The belt filter press is popu-

lar due to availability of smaller sizes and uses polymer for chemical floc-

culation of the sludge. It consumes much less energy than the vacuum

filter. The plate and frame filter press is used primarily to dewater

chemical sludges. It is a large machine and uses lime and ferric chloride

for conditioning of organic sludge prior to dewatering. The filtered cake

is very compact and dry.

Belt filter press. A belt filter press consists of two endless, tensioned,

porous belts. The belts travel continuously over a series of rollers of

varied diameter that squeeze out the water from the sludge and produce

a dried cake that can be easily removed from the belts. Variations in belt

filter press design are available from different manufacturers.

Chemical conditioning of sludge is vital to the efficiency of a belt filter

press. Polymers are used as a conditioning agent, especially cationic

polymers. The operation of the belt filter press comprises three zones,

i.e. gravity drainage, low-pressure and high-pressure zones. Conditioned

sludge is first introduced uniformly to the gravity drainage zone and onto

the moving belt where it is allowed to thicken. The majority of free

water is removed from the sludge by gravity. Free water readily sepa-

rates from the slurry and is recycled back through the treatment system.

The efficiency of gravity drainage depends on the type of sludge, the

quality of the sludge, sludge conditioning, the belt screen mesh, and

design of the drainage section. Typically, gravity drainage occurs on a

flat or slightly inclined belt for a period of 1 to 2 min. A 5% to 10%

increase in solids content is achieved in this zone; that is, 1% to 5%

sludge feed produces 6% to 15% solids prior to compression (US EPA,

1991).

Following the gravity drainage zone, pressure is applied in the low-

pressure (wedge) zone. The pressure can come from the compression of

the sludge between two belts or from the application of a vacuum on the

lower belt. In the wedge zone, the thickened sludge is subjected to low

pressure to further remove water from the sludge matrix. This is to

5 12.3 h/d

5400 kg/d 3 0.95

9.8 kg/sm

hd 3 42.4 m

Operation, h/d 5

solids loading 3 factor of recovery

filter yield 3 filter area

830 Chapter 6

prepare an even firmer sludge cake that can withstand the shear forces

to which it is subjected by rollers.

On some units, the low-pressure zone is followed by a high-pressure

zone, where the sludge is sandwiched between porous belts and sub-

jected to shearing forces as the belts pass through a series of rollers of

decreasing diameter. The roller arrangement progressively increases the

pressure. The squeezing and shearing forces induce the further release

of additional amounts of water from the sludge cake.

The final dewatered filter cake is removed from the belts by scraper

blades into a hopper or conveyor belt for transfer to the sludge man-

agement area. After the cake is removed, a spray of water is applied to

wash and rinse the belts. The spray rinse water, together with the fil-

trate, is recycled back to primary or secondary treatment.

Belt filter presses are commercially available in metric sizes from 0.5

to 3.5 m in belt width, with the most common size between 1.0 and 2.5 m.

Hydraulic loading based on belt width varies from 25 to 100 gal/

(min ⭈ m) (1.6 to 6.3 L/(m ⭈ s)). Sludge loading rates range from 200 to

1500 lb/(m ⭈ h) (90 to 680 kg/(m ⭈ h)) (Metcalf and Eddy, Inc. 1991).

Typical operating parameters for belt filter press dewatering of polymer-

conditioned wastewater sludges are presented in Table 6.26.

Odor is often a problem with the belt filter press. Adequate ventilation

to remove hydrogen sulfide or other gases is also a safety consideration.

Wastewater Engineering 831

TABLE 6.26 Typical Operating Parameters for Belt Filter Press Dewatering of

Polymer Conditioned Sludge

Feed Hydraulic Solids Cake Polymer

solids, loading, loading, solids, dosage,

Type of sludge percent gal/(min ⭈ m

∗

) lb/(m

∗

⭈ h) percent lb/ton

Anaerobically digested

Primary 4–7 40–50 1000–1600 25–44 3–6

Primary plus WAS 2–6 40–50 500–1000 15–35 6–12

Aerobically digested 1–3 30–45 200–500 12–20 8–14

without primary

Raw primary and 3–6 40–50 800–1200 20–35 4–10

waste activated

Thickened waste 3–5 40–50 800–1000 14–20 6–8

activated

Extended aeration 1–3 30–45 200–500 11–22 8–14

waste activated

Heat-treated primary 4–8 35–50 1000–1800 25–50 1–2

plus waste activated

Notes: 1.0 gal/(min ⭈ m) ⫽ 0.225 m

/(m ⭈ h)

1.0 lb/(m ⭈ h) ⫽ 0.454 kg/(m ⭈ h)

1.1 lb/ton ⫽ 0.500 kg/tonne

∗

Loading per meter belt width

SOURCES: WPCF, 1983; Viessman and Hammer, 1993; WEF, 1996b

Hydrogen peroxide or potassium permanganate can be used to oxidize the

odor-causing chemical (H

S). Potassium permanganate can improve the

dewatering efficiency of the sludge and can reduce the quantity of poly-

mer required.

Example: A belt filter press (BFP) is designed to dewater anaerobically digested

primary plus waste activated sludge for 7 h/d and 5 d/wk. The effective belt width

is 2.0 m (commonly used). Estimate hydraulic and solids loading rates, poly-

mer dosage, and solids capture (recovery) with the following given data.

solution:

Step 1. Calculate the average weekly sludge production

Wet sludge ⫽ 20 gal/min ⫻ 1440 min/d ⫻ 7 d/wk ⫻ 8.34 lb/gal ⫻ 1.03

⫽ 1,731,780 lb/wk

Dry solids ⫽ 1,731,780 lb/wk ⫻ 0.035

⫽ 60,610 lb/wk

Step 2. Calculate daily and hourly dry solids dewatering requirement based

on the designed operation schedule: 7 h/d and 5 d/wk

Daily rate ⫽ (60,610 lb/wk)/(5 d/wk)

⫽ 12,120 lb/d

Hourly rate ⫽ (12,120 lb/d)/(7 h/d)

⫽ 1730 lb/h

Step 3. Select belt filter press size and calculate solids loading rate

Use one 2.0 m belt (most common size) and one more identical size for standby.

Calculate solids loading rate

Solids loading ⫽ (1730 lb/h)/2 m

⫽ 865 lb/(h ⭈ m) (OK, within 500 to 1000 lb/(h ⭈ m) range)

Step 4. Calculate hydraulic loading rate

Flow to the press ⫽ 20 gal/min ⫻ (7 days/5 days)(24 h/7 h)

⫽ 96 gal/min

Sludge production 20 gal/min

Total solids in sludge feed 3.5%

Total solids in cake 25%

Sp. gr. of sludge feed 1.03

Sp. gr. of dewatered cake 1.08

Sp. gr. of filtrate 1.01

Polymer dose (0.2% by weight) 7.2 gal/min

TSS in wastewater of BFP 1900 mg/L

Wash water 40 gal/min

832 Chapter 6

Wastewater Engineering 833

HL ⫽ 96 (gal/min)/2 m

⫽ 48 gal/(min ⭈ m)

(OK, within 40 to 50 gal/(min ⭈ m) range)

Step 5. Calculate dosage of polymer (0.2% by weight) with a 7.2 gal/min rate

⫽ 8.33 lb/ton

⫽ 4.16 kg/tonne

Step 6. Calculate solids recovery rate

(a) Estimate the volumetric flow of cake, q (from Step 4)

⫽ 12.4 gal/min

(b) Calculate flow rate of filtrate

Flow of filtrate ⫽ sludge feed flow ⫺ cake flow

⫽ 96 gal/min ⫺ 12.4 gal/min

⫽ 83.6 gal/min

(d) Calculate solids capture (recovery)

Plate and frame filter press. There are several types of filter press avail-

able. The most common type, the plate and frame filter press, consists

of vertical plates that are held rigidly in a frame and pressed together

between fixed and moving ends (Fig. 6.61a). As shown in Fig. 6.61b, each

chamber is formed by paired recessed plates. A series of individual

5 95.4%

s865 2 40d lb/sh

md 3 100%

865 lb/sh

Solids caputre 5

solids in feed 2 solids in filtrate

solids in feed

3 100%

5 40lb/sh

Solids 5

83.6 gal/min 3 60 min/h 3 1900 mg/L 3 8.34 lb/gal

2.0 m 3 1,000,000 mg/L

q 5

96 gal/min 3 0.035

s25%/100%d 3 1.08

Dosage 5

s7.2 gal/min 3 60 min/h 3 0.002d 3 8.34 lb/gal

2.0 m 3 865 lb/sh

md 3 s1 ton/2000 lbd

chambers have a common feed port and two or more common filtrate

channels. Fabric filter media are mounted on the face of each indi-

vidual plate, thus providing initial solids capture.

The filter press operates in batch manner to dewater sludge. Despite

its name, the filter press does not press or squeeze sludge. Instead,

when the filter is closed, the recessed faces of adjacent plates form a

834 Chapter 6

Figure 6.61 Plate and frame filter press: (a) side view of a filter press; (b) schematic cross

section of chamber area during fill cycle.