Lin S.D. Water and Wastewater Calculations Manual

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pressure-tight chamber; the sludge is pumped into the press at pressure

up to 225 lb/in

(15 atm, 1550 kN/m

) and maintained for less than 2 h

(CPC Eng., 1980). The water passes through the filter cloth, while the

solids are retained and form a cake on the cloth. During the initial

phase of operation the fabric cloth retains the solids which then become

the filter media. This action results in high solids capture and a filtrate

of maximum clarity. At the end of a filtration cycle, the feed pump

system is stopped and the chamber plates are opened, allowing the cake

to fall into appropriate hoppers or conveyors. Filter cloth usually

requires a precoat (ash or diatomaceous earth) to aid retention of solids

on the cloth and release of cake. Drainage ports are provided at both the

top and the bottom of each chamber (Figure 6.61b). This figure shows

a fixed volume, recessed plate filter press. Another type, the variable

volume recessed plate filter press, is also used for sludge dewatering.

The advantages of the plate filter press include production of higher

solids cake, 35% to 50% versus 18% for both belt press (US EPA, 1991) and

vacuum filters, higher clarity filtrate, lower energy consumption, lower o

& M costs, longer equipment service life, and relatively simple operation.

However, sufficient washing and air drying time between cycles is required.

Example: A plate and frame filter press has a plate surface of 400 ft

and

is used to dewater conditioned waste activated sludge with a solids concen-

tration of 2.5%. The filtration time is 110 min at 225 lb/in

(gage) and the time

to discharge the sludge and restart the feed is 10 min. The total volume of

sludge dewatered is 3450 gal. (1) Determine the solids loading rate and the

net filter yield in lb/(h ⭈ ft

). (2) What will be the time of filtration required

to produce the same net yield if the feed solids concentration is reduced to 2%?

solution:

Step 1. Compute solids loading rate

Step 2. Compute the net filter yield

5 0.90 lb/sh

5 0.98 lb/sh

d 3

110 min

s110 1 10d min

Net yield 5 solids loading 3

filtration time

time per cycle

5 0.98 lb/sh

3450 gal 3 8.34 lb/gal 3 0.025

110 min/s60 min/hd 3 400 ft

Solids loading 5

mass of dry solids applied

filtration time 3 plate area

Wastewater Engineering 835

Step 3. Determine the filtration time required at 2% solids feed

Time ⫽ 110 min (2.5%/2%)

⫽ 137.5 min

Centrifugation. The centrifugation process is also used for dewatering

raw, digested, and waste activated sludge. The process is discussed in

more detail in the section on sludge thickening. Typically, a centrifuge

produces a cake with 15% to 30% dry solids. The solids capture ranges

from 50% to 80% and 80% to 95%, without and with proper chemical

sludge conditioning, respectively (US EPA, 1987b).

Sludge drying beds. Sludge drying beds remove moisture by natural

evaporation, gravity, and/or induced drainage, and are the most widely

used method of dewatering municipal sludge in the United States. They

are usually used for dewatering well-digested sludge. Drying beds

include conventional sand beds, paved beds, unpaved beds, wedge-wire

beds, and vacuum-assisted beds.

Drying beds are less complex, easier to operate, require less operat-

ing energy, and produce higher solids cake than mechanical dewatering

systems. However, more land is needed for them. Drying beds are usu-

ally used for small- and mid-size wastewater treatment plants with

design flow of less than 7500 m

/d (2 Mgal/d), due to land restrictions

(WEF, 1996b).

Sand drying beds. Sand drying beds generally consist of 10 to 23 cm

(4 to 9 in) of sand placed over a 20 to 50 cm (8 to 20 in) layer of gravel.

The diameters of sand and gravel range from 0.3 to 1.22 mm and from

0.3 to 2.5 cm, respectively. The water drains to an underdrain system

that consists of perforated pipe at least 10 cm (4 in) in diameter and

spaced 2.4 to 6 m (8 to 20 ft) apart. Drying beds usually consist of a 0.3

to 1 m (1 to 3 ft) high retaining wall enclosing process drainage media

(US EPA, 1989, 1991). The drying area is partitioned into individual

beds, 6 m wide by 6 to 30 m long (20 ft wide by 20 to 100 ft long), or of

a convenient size so that one or two beds will be filled in a normal cycle

(Metcalf and Eddy, Inc. 1991).

In a typical sand drying bed digested and/or conditioned sludge is

discharged (at least 0.75 m/s velocity) on the bed in a 30 to 45 cm (12 to

18 in) layer and allowed to dewater by drainage through the sludge

mass and supporting sand and by evaporation from the surface exposed

to air. Dissolved gases are released and rise to the surface. The water

drains through the sand and is collected in the underdrain system and

generally returned to the plant for further treatment.

Design area requirements for open sludge drying beds are based on

sludge loading rate, which is calculated on a per capita basis or on a

unit load of mass of dry solids per unit area per year. On a per capita

836 Chapter 6

basis, surface areas range from 1.0 to 2.5 ft

/person (0.09 to 0.23 m

person), depending on sludge type. Sludge loading rates are between

12 and 23 lb dry solids/(ft

⭈ yr) (59 and 113 kg/(m

⭈ yr) (Metcalf and

Eddy, Inc. 1991).

The design, use, and performance of the drying bed are affected by the

type of sludge, sludge conditioning, sludge application rates and depth,

dewatered sludge removal techniques, and climatic conditions.

Example: A sand drying bed, 6 m by 30 m (20 by 100 ft), receives conditioned

sludge to a depth of 30 cm (12 in). The sludge feed contains 3% solids. The sand

bed requires 29 days to dry and one day to remove the sludge from the bed for

another application. Determine the amount of sludge applied per application

and the annual solids loading rate. The sp. gr. of sludge feed is 1.02.

solution:

Step 1. Compute the volume of sludge applied per application, V

V ⫽ 6 m ⫻ 30 m ⫻ 0.3 m/app.

⫽ 54 m

/app.

⫽ 1907 ft

/app.

⫽ 14,300 gal/app.

Step 2. Compute the solids applied per application

Solids ⫽ 54 m

/app ⫻ 1000 kg/m

⫻ 1.02 ⫻ 0.03

⫽ 1650 kg/app.

⫽ 3640 lb/app.

Step 3. Compute annual solids loading rate

range)

Paved beds. Paved beds consist of a concrete or asphalt pavement

above a porous gravel subbase with a slope of at least 1.5%. There are

two types of drainage, i.e. a drainage type and a decant type. Unpaved

areas, constructed as sand drains, are placed around the perimeter or

5 22.8 lb/sft

yrd

5 111 kg/sm

yrd sOK within 592113 kg/sm

yrd

1650 kg/app 3 365 d/yr

180 m

3 30 d/app.

Loading rate 5

solids applied 3 365 d/yr

surface area 3 cycle

Surface area of the bed 5 6 m 3 30 m 5 180 m

Wastewater Engineering 837

along the center of the bed to collect and convey drainage water. The

advantage of the paved bed is that sludge can be removed by a front-

end loader. The bed areas are larger and typically rectangular in shape,

being 6 to 15 m (20 to 50 ft) wide by 21 to 46 m (70 to 150 ft) long with

vertical side walls as sand beds. Cake solids contents as much as 50%

can be achieved in an arid climate.

Well designed paved beds may remove about 20% to 30% of the water

with good settling solids. Solids concentration may range from 40% to

50% for a 30 to 40 days drying time in an arid climate for a 30 cm (1 ft)

sludge layer (Metcalf and Eddy, Inc. 1991).

Unpaved beds. Unpaved beds may be used in warm and dry climate

areas where groundwater is not a concern. The beds are similar to paved

beds with decanting. Sufficient storage area is required during a wet

weather period, because access to the beds is restricted during wet

weather.

Vacuum-assisted drying beds. The vacuum-assisted drying bed consists

of a reinforced concrete ground slab, a layer of supporting aggregate,

and a rigid porous media plate on top. The space between the con-

crete slab and the multimedia plate is the vacuum chamber, con-

nected to a vacuum pump. This is to accelerate sludge dewatering and

drying.

Polymer-preconditioned sludge is applied to the surface of the media

plates until it is entirely covered. The vacuum is then applied to remove

the free water from the sludge. The sludge is air dried for 24 to 48 h.

Essentially all of the solids remain on top of the media plates form a cake

of fairly uniform thickness. The cake has solids concentration from 9%

to 35%, depending on sludge feed (WEF, 1996b).

The dewatered sludge is removed by a front-end loader. The porous

multimedia plates are washed with a high-pressure hose to remove the

remaining sludge residue before another application.

The vacuum-assisted drying bed has a short cycle time and needs

less footage. However, it is labor intensive and is expensive to operate.

Drying beds are sometimes enclosed in a greenhouse-type glass struc-

ture to increase drying efficiency in wet or colder climates. The enclo-

sure also serves for odor and insect control, and improves the overall

appearance of the treatment plant. Good ventilation is required to allow

moisture to escape.

Wedge-wire drying beds. The wedge-wire (wedgewater) drying bed was

developed in England and there exist a few installations in the United

States. The material for the drying bed consists of stainless steel wedge

wire or high-density polyurethane. It is a physical process similar to the

vacuum-assisted drying bed.

The bed consists of a shallow rectangular watertight basin fitted with

a false floor of artificial media, wedge-wire panels (Fig. 6.62). The panels

838 Chapter 6

(wedge-wire septum) have wedge-shaped slotted openings of 0.25 mm

(0.01 in). The false floor is made watertight with caulking where the

panels abut the walls. An outlet valve to control the drainage rate is

located underneath the false floor. Water or plant effluent is introduced

to the bed from beneath the wedge-wire septum up to a depth of 2.5 cm

(1 in) over the septum. This water serves as a cushion and prevents com-

pression or other disturbance of the colloidal particles. The sludge is

introduced slowly onto a horizontal water level to float without causing

upward or downward pressure across the wedgewire surface. After the

bed is filled with sludge, the initially separate water layer and drainage

water are allowed to percolate away at a controlled flow rate through

the outlet valve. After the free water has been drained, the sludge fur-

ther concentrates by drainage and evaporation until there is a require-

ment for sludge removal (US EPA, 1987b).

The advantages of this process include no clogging in the wedgewire,

treatability of aerobically digested sludge, constant and rapid drainage,

higher throughput than the sand bed, and ease of operation and main-

tenance. However, the capital cost is higher than that of other drying beds.

Sludge drying lagoons. A sludge drying lagoon is similar to a sand bed

in that the digested sludge is introduced to a lagoon and removed after

a period of drying. Unlike the sand drying bed, the lagoon does not have

an underdrain system to drain water. Sludge drying lagoons are not

suitable for dewatering untreated sludges, lime treated sludges, or

sludge with a high strength of supernatant, due to odor and nuisance

potential.

Sludge drying lagoons are operated by periodically decanting the

supernatant back to the plant and by evaporation. They are periodically

dredged to remove sludge. Unconditioned digested sludge is introduced

Wastewater Engineering 839

Figure 6.62 Crosssection of a wedgewire drying bed (source: US EPA, 1991).

to the lagoon to a depth of 0.75 to 1.25 m (2.5 to 4 ft). It will dry mainly

by evaporation in 3 to 5 months, depending on climate. Dried sludge is

removed mechanically at a solids content of 20% to 30% percent. If

sludge is to be used for soil conditioning, it needs to be stored for fur-

ther drying. A 3-year cycle may be applied: lagooning for 1 year, drying

for 18 months, and cleaning and resting for 6 months.

Solids loading rates are 36 to 39 kg/(m

⭈ y) (2.2 to 2.4 lb/(ft

⭈ y)) of

lagoon capacity (US EPA, 1987b). The lagoons are operated in parallel;

at least two units are essential. Very little process control is needed.

Sludge lagoons are the most basic treatment units.

Other sludge volume reduction processes. Processes such as heat treatment

and thermal combustion can reduce the moisture content and volume

of the sludge. Detailed discussions are presented in the section on solids

treatment.

Example: A POTW generates 1800 kg/d (3970 lb/d) of dewatered sludge at

33% solids concentration. The moisture content of composted material and

compost mixture are 35% and 55%, respectively. What is the mass of com-

posted material that must be blended daily with the dewatered sludge?

solution:

Step 1. Compute moisture of dewatered sludge

Sludge moisture ⫽ 100% ⫺ solids content ⫽ 100% ⫺ 33%

⫽ 67%

Step 2. Compute composted material required daily

Let X and Y be the mass of dewatered sludge and composted material, respec-

tively. Then X ⫽ 1800 kg/d and

(X ⫹ Y ) (mixture moisture) ⫽ X (s1. mois.) ⫹ Y (comp. mois.)

(X ⫹ Y ) 55% ⫽ X (67%) ⫹ Y (35%)

Y (55 ⫺ 35) ⫽ X (67 ⫺ 55)

Y ⫽ (1800 kg/d) (12/20)

⫽ 1080 kg/d

⫽ 2380 lb/d

29.3 Sewage sludge biosolids

Sewage sludge processing (i.e. thickening, stabilization, conditioning,

and dewatering) produces a volume reduction, and this reduction in

volume decreases the capital and operating costs. Digestion or com-

posting of the sludge reduces the level of pathogens and odors. The

840 Chapter 6

degree of the sludge treatment process is very important to eliminate

pathogens when considering land application of the sludge, when dis-

tributing and marketing it, and when placing it in monofills or on a sur-

face disposal site.

The end product of wastewater sludge treatment processes is referred

to as “biosolids.” Webster’s Collegiate Dictionary (1997) defines biosolids

as solid organic matter recovered from a sewage treatment process and

used especially as fertilizer for plants. The McGraw-Hill Dictionary of

Scientific and Technical Terms (5th ed., 1994) defines biosolid as a recy-

clable, primarily organic solid material produced by wastewater treat-

ment processes.

The term “biosolids” has gained recently in popularity as a synonym

for sewage sludge because it perhaps has more reuse potential than the

term “treated wastewater sludge.” The name was chosen by the Water

Environmental Federation (WEF) in early 1990s.

29.4 Use and disposal of sewage

sludge biosolids

Biosolids are commonly used and disposed of in many ways. The bene-

ficial uses of biosolids include land application to agricultural lands, land

application to nonagricultural lands, and the sale or give-away of

biosolids for use on home gardens. Nonagricultural areas may include

compost, forests, public contact (parks, highways, recreational areas, golf

courses, etc.). Case histories of beneficial use programs for biosolids use

and management can be found elsewhere (WEF, 1994).

Disposal methods of biosolids include disposal in municipal landfills,

disposal on delicate sites, surface disposal, and incineration. Surface dis-

posal includes piles of biosolids left on the land surface and land appli-

cation to dedicated nonagricultural land, and disposal in sludge-only

landfills (monofills).

29.5 Regulatory requirements

In 1993, the US EPApromulgated “the standard for the use and disposal

of sewage sludge, final rule, title 40 of the Code of Federation

Regulations (CFR), Parts 257, 403, and 503” (Federal Register, 1993).

All the above three plus “Phase-in submission of sewage sludge permit

application: final rule, revisions to 40 CFR Parts 122, 123, and 501” are

reprinted by the Water Environmental Federation, stock #P0100

(1993b). “The Part 503 rule” or “Part 503” was developed to protect

public health and the environment from any reasonably anticipated

adverse effects of using or disposing of sewage sludge biosolids. Two

easy-to-read versions of the Part 503 rule were published by US EPA

(1994, 1995).

Wastewater Engineering 841

The CFR 40 Part 503 includes five subparts. They are general provi-

sions, requirement for land application, surface disposal, pathogen and

vector attraction (flies, mosquitoes, and other potential disease-carrying

organisms) reduction, and incineration. For each of the regulated use

or disposal methods, the rule covers general requirements, pollutant

limits, operational standards (pathogen and vector attraction reduction

for land application and surface disposal; total hydrocarbons or carbon

dioxide for incineration), management practices, and requirement for the

frequency of monitoring, record keeping, and reporting. The require-

ments of the Part 503 are self-implementing and must be followed even

without the insurance of a permit. State regulatory agencies may have

their own rules governing the use or disposal of sewage sludge biosolids

or domestic septage.

Part 503 applies to any person who applies biosolids to the land, or

burns the biosolids in an incinerator, and to the operator/owner of a sur-

face disposal site, or to any person preparing to use, dispose of, or incin-

erate biosolids. A person is defined as an individual, association,

partnership, corporation, municipality, state, or federal agency, or an

agent or employee thereof.

A person must apply for a permit covering biosolids use or disposal

standards if they own or operate a treatment works treating domestic

sewage (TWTDS). In most cases, Part 503 requirements will be incor-

porated over a time period into National Pollutant Discharge Elimination

System (NPDES) permits issued to Public Owned Treatment Works

(POTWs) and TWTDSs. Application for a federal biosolids permit must

be submitted to the appropriate US EPA Regional Office, not the state.

Until the biosolids management programs of individual states are

approved by the US EPA, the US EPA will remain the permitting

authority.

Most sewage sludge biosolids currently generated by POTWs in the

United States meet the minimum pollutant limits and pathogen and

vector attraction reduction requirements set forth in the Part 503. Some

biosolids already meet the most stringent Part 503 pollutant limit stan-

dards and requirements of pathogen and vector attraction reduction.

Pathogen reduction requirements. The Part 503 pathogen reduction

requirements for land application of biosolids are divided into two cat-

egories: Class A and Class B biosolids. In addition to meeting the require-

ment in one of the six treatment alternatives, the Class A requirement

is to reduce the pathogens in biosolids (fecal coliform, or Salmonella sp.

bacteria, enteric viruses, parasites, and viable helminth ova) in biosolids

to below the detectable levels. When this goal is reached, Class A biosolids

can be applied without any pathogen-related restriction on the site. The

six treatment alternatives for biosolids include: alternative 1, thermally

842 Chapter 6

treated; alternative 2, high pH–high temperature treatment; alternative

3, other processes treatment; alternative 4, unknown processes; alter-

native 5, use of the processes to further reduce pathogens (PFRP); and

alternative 6, use of a process equivalent to PFRP.

Class A biosolids. The pathogen reduction requirements must be met for

all six alternatives (503.32a) and vector attraction reduction (503.33) for

Class A with respect to pathogens. Regardless of the alternative chosen,

either the FC density in the biosolids must be less than 1000, the most

probable number (MPN) per gram dry total solids; or the density of

Salmonella sp. in the biosolids must be less than 3 MPN per 4 g of dry

total solids. Either of these requirements must be met at one of the fol-

lowing times: (1) when the biosolids are used or disposed of; (2) when the

biosolids are prepared for sale or give away in a bag or other container

for land application; or (3) when the biosolids or derived materials are pre-

pared to meet the requirements for excellent quality biosolids.

Table 6.27 lists the four time–temperature regimes for Class A

pathogen reduction under alternative 1 (time and temperature).

Alternative 2 (pH and time) raises and maintains sludge pH above 12

for 72 h and keeps it at 52⬚C for 12 h. Alternative 3 is an analysis of the

Wastewater Engineering 843

TABLE 6.27 The Four Time-Temperature Regimes for Class A Pathogen Reduction

Under Alternative 1

Time-temperature

Regime Applies to Requirement relationships

∗

A Biosolids with 7% Temperature of

solids or greater biosolids must be 50⬚C

(except those or higher for 20 min or

covered by longer

Regime B) (Eq. 3 of section 503.32)

B Biosolids with 7% Temperature of

solids or greater in biosolids must be 50⬚C

the form of small or higher for 15 s or

particles and heated longer

by contact with either

warmed gases or an

immiscible liquid

C Biosolids with less Heated for at least 15 s

than 7% solids but less than 30 min

D Biosolids with less Temperature of sludge

than 7% solids is 50⬚C or higher with

at least 30 min or

longer contact time

(Eq. 4 of section 503.32)

∗

D ⫽ time in days; T ⫽ temperature in degrees Celsius.

SOURCE: US EPA (1994, 1995)

D 5

50,070,000

0.14T

D 5

131,700,000

0.14T

D 5

131,700,000

0.14T

D 5

131,700,000

0.14T

treatment process effectiveness of the Enteric viruses and helminth ova

are to be determined prior to and after treatment. The final density is

to be less than one plaque-forming unit per 4 g of dry solids. Alternative 4

(analysis with unidentified treatment process) analyzes for fecal col-

iform, Salmonella, Enteric viruses, and helminth ova at the time of use

or disposal. Alternative 5 (PFRP process) treats sludge with one of com-

posting and Class B alternative 2 processes (aerobic digestion, air drying,

anaerobic digestion, heat drying, heat treatment, thermophilic aerobic

digestion, beta ray irradiation, gamma ray irradiation, and pasteur-

ization). Alternative 6 (PFRP equivalent process) allows the permitting

authority to approve a process not currently identified as a PFRP process

through a review by the Pathogen Equivalency Committee (Federal

Example 1: What is the required minimum time to achieve Class A pathogen

requirement when a biosolid with 15% solids content is heated at 60⬚C

(140⬚F)?

solution:

Step 1. Select the regime under the given condition

Referring to Table 6.27, Regime A is chosen.

Step 2. Compute minimum time required

⫽ 0.5243 days

⫽ 12.58 h

Example 2: Determine the required minimum temperature to treat a

biosolid containing 15% solids with a heating time of 30 min.

solution:

D ⫽ 30 min ⫽ 0.0208 day

D(10

0.14T

) ⫽ 131,700,000

0.14T

⫽ 131,700,000/0.0208 ⫽ 6,331,700,000

0.14T ⫽ 9.80

T ⫽ 70.0 (°C)

Class B biosolids. Class B requirements ensure that pathogens have

been reduced to levels that are unlikely to pose a threat to public

health and the environment under specific conditions of use. Class B

biosolids also must meet one of the three alternatives: (1) monitoring

D 5

131,700,000

0.14T

131,700,000

0.14s60d

844 Chapter 6