Lin S.D. Water and Wastewater Calculations Manual

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Wastewater Engineering 735

The use of aerobic ponds is limited to warm and sunny climates, espe-

cially where a high degree of BOD removal is required but the land

area is not limited. However, very little coliform die-off occurs.

25.3 Anaerobic ponds

Anaerobic ponds are usually deep and are subjected to heavy organic

loading. There is no aerobic zone in an anaerobic pond. The depths of

anaerobic ponds usually range from 2.5 to 5 m (8 to 16 ft). The deten-

tion times are 20 to 50 days (US EPA, 1983b).

Anaerobic bacteria decompose organic matter to carbon dioxide and

methane. The principal biological reactions are acid formation and

methane fermentation. These processes are similar to those of anaero-

bic digestion of sludge. Odorous compounds, such as organic acids and

hydrogen sulfide, are also produced.

Anaerobic ponds are usually used to treat strong industrial and agri-

cultural wastes. They have been used as pretreatment to facultative or

aerobic ponds for strong industrial wastewaters and for rural commu-

nities with high organic load, such as food processing. They are not in

wide application for municipal wastewater treatment.

The advantages of anaerobic ponds compared with an aerobic treat-

ment process are low production of waste biological sludge and no need

for aeration equipment. An important disadvantage of the anaerobic

pond is the generation of odorous compounds. Its incomplete stabiliza-

tion of waste requires a second-stage aerobic process. It requires a rel-

atively high temperature for anaerobic decomposition of wastes.

Normal operation, to achieve a BOD removal efficiency of at least

75%, entails a loading rate of 0.32 kg B0D/(m

⭈ d) (20 lb/(1000 ft

⭈ d)),

a minimum detention time of 4 days, and a minimum operating tem-

perature of 24⬚C (75⬚F) (Hummer, 1986).

26 Secondary Clariﬁer

The secondary settling tank is an integral part of both suspended-

and attached-growth biological treatment processes. It is vital to the

operation and performance of the activated-sludge process. The clar-

ifier separates MLSS from the treated wastewater prior to discharge.

It thickens the MLSS before their turn in the aeration tank or their

wasting.

Secondary clarifiers are generally classified as Type 3 settling. Types

1 and 2 may also occur. The sloughed-off solids are commonly well-

oxidized particles that settle readily. On the other hand, greater viability

of activated sludge results in lighter, more buoyant flocs, with reduced

settling velocities. In addition, this is the result of microbial production

of gas bubbles that buoy up the tiny biological clusters.

736 Chapter 6

TABLE 6.19 Typical Design Parameters for Secondary Sedimentation Tanks

Hydraulic loading Solids loading,*

gal/(d ⭈ ft

)† lb solids/(d ⭈ ft

)†

Type of treatment Average Peak Average Peak Depth, ft

Settling following 400–600 1000–2000 _ _ 10–12

tracking filtration

Settling following 400–800 1000–1200 20–30 50 12–15

air activated

sludge (excluding

extended aeration)

Settling following 200–400 800 20–30 50 12–15

extended aeration

Settling following 400–800 1000–1200 25–35 50 12–15

oxygen activated

sludge with primary

settling

* Allowable solids loading are generally governed by sludge thickening characteristics

associated with cold weather operations.

† gal/(d ⭈ ft

) ⫻ 0.0407 ⫽ m

/(m

⭈ d)

lb/(d ⭈ ft

) ⫻ 4.8827 ⫽ kg/(d ⭈ m

)

SOURCE: US EPA (1975a)

26.1 Basin sizing for attached-growth

biological treatment efﬂuent

The Ten States Standards (GLUMRB, 1996) recommends that surface

overflow rate for the settling tank following trickling filter should not

exceed 1200 gal/(d ⭈ ft

) (49 m

/(m

⭈ d)) based on design peak hourly flow.

In practice, typical overflow rates are 600 gal/(d ⭈ ft

) (24.4 m

/(m

⭈ d)) for

plants smaller than 1 Mgal/d (3785 m

/d) and 800 gal/(d ⭈ ft

) (33 m

/(m

⭈ d))

for larger plants. The minimum side water depth is 10 ft (3 m) with

greater depths for larger diameter basins. The retention time in the sec-

ondary settling tank is in the range of 2 to 3 h. The maximum recom-

mended weir loading rate is 20,000 gal/(d ⭈ ft) (250 m

/(d ⭈ m)) for plants

equal or less than 1 Mgal/d and 30,000 gal/(d ⭈ ft) (373 m

/(d ⭈ m)) for plants

greater than 1 Mgal/d (3785 m

/d).

Secondary clarifiers following trickling filters are usually sized on the

basis of the hydraulic loading rate. Typical design parameters for sec-

ondary clarifiers are listed in Table 6.19 (US EPA, 1975a). Referring to the

hydraulic loading rates from Table 6.20, sizing should be calculated for both

peak and design average flow conditions and the largest value calculated

should be used. At the selected hydraulic loading rates, settled effluent

quality is limited primarily by the performance of the biological reactor,

not of the settling basins. Solids loading limits are not involved in the size

of clarifiers following trickling filters. Where further treatment follows the

clarifier, cost optimization may be considered in sizing the settling basins.

Wastewater Engineering 737

Example 1: A trickling filter plant treats a flow of 1.0 Mgal/d (3785 m

/d) with

a raw wastewater BOD

of 200 mg/L and total suspended solids of 240 mg/L.

Estimate the daily solids production of the plant assuming that the primary

clarifiers remove 32% of the BOD

and 65% of TSS.

solution:

Step 1. Calculate the BOD

loading to the secondary clarifiers

BOD

⫽ (1 – 0.32) ⫻ (200 mg/L) ⫻ 1.0 Mgal/d ⫻ 8.34 lb/(Mgal ⭈ mg/L)

⫽ 1134 lb/d (⫽515 kg/d)

Step 2. Estimate solids (S

) production of the secondary clarifiers

Note: The biological solids generated in secondary treatment range from 0.4

to 0.5 lb per lb (kg/kg) BOD

applied. Use 0.45 lb/lb in this example.

⫽ 0.45 lb/lb ⫻ 1134 lb/d

⫽ 510 lb/d (⫽ 32 kg/d)

Step 3. Estimate solids (S

) generated from the primary clarifiers

⫽ 0.65 ⫻ (240 mg/L) ⫻ 1.0 Mgal/d ⫻ 8.34 lb/(Mgal ⭈ mg/L)

⫽ 1300 lb/d (⫽ 590 kg/d)

TABLE 6.20 Recommended Design Overﬂow Rate and Peak Solids Loading Rate for

Secondary Settling Tanks Following Activated-Sludge Processes

Surface loading at design

peak hourly flow,* Peak solids loading rate,‡

Treatment process gal/(d ⭈ ft

) (m

⭈ m

⭈ d) lb/(d ⭈ ft

) (kg/(d ⭈ m

)

Conventional 1200 (49) 50 (244)

Step aeration or

Complete mix 1000 (41)†

Contact stabilization

Carbonaceous state of

separate-stage

nitrification

Extended aeration 1000 (41) 35 (171)

Single-stage nitrification

Two-stage nitrification 800 (33) 35 (171)

Activated sludge with 900 (37)§ as above

chemical addition to

mixed liquor for

phosphorus removal

*Based on influent flow only.

†Computed on the basis of design maximum daily flow rate plus design maximum return

sludge rate requirement, and the design MLSS under aeration.

‡

For plant effluent ⱕ 20 mg/L.

§When effluent P concentration of 1.0 mg/L or less is required.

SOURCE: GLUMRB (1996)

738 Chapter 6

Step 4. Calculate the total daily solids production (S)

S ⫽ S

⫹ S

⫽ 1300 lb/d ⫹ 510 lb/d

⫽ 1810 lb/d (⫽ 822 kg/d)

Or solve this problem using SI units

Step 1. Calculate the BOD

loading to the secondary clarifiers

BOD

⫽ (1 – 0.32) ⫻ (200 mg/L) ⫻ (3785 m

/d) ⫻ 0.001 [(kg/m

)/(mg/L)]

⫽ 515 kg/d (⫽ 1134 lb/d)

Step 2. Estimate solids (S

) production of the secondary clarifiers

Note: The biological solids generated in secondary treatment range from 0.4

to 0.5 lb per lb (kg/kg) BOD

applied. Use 0.45 kg/kg in this example.

⫽ 0.45 kg/kg ⫻ 515 kg/d

⫽ 232 kg/d (⫽ 510 lb/d)

Step 3. Estimate solids (S

) generated from the primary clarifiers

⫽ 0.65 ⫻ (240 mg/L) ⫻ 3785 m

/d ⫻ 0.001 [(kg/m

)/(mg/L)]

⫽ 590 kg/d ⫽ (1300 lb/d)

Step 4. Calculate the total daily solids production (S)

S ⫽ S

⫹ S

⫽ 590 kg/d ⫹ 232 kg/d

⫽ 822 kg/d (⫽ 1810 lb/d)

Example 2: Design a two-stage trickling filter system with a design average

flow of 5680 m

/d (1.5 Mgal/d) and intermediate and secondary settling tanks

under conditions given as follows. The primary effluent (system influent)

BOD ⫽ 190 mg/L. The design BOD loading rate is 1.5 kg/(m

⭈ d) (93.8 lb/(1000

⭈ d)).

The recycle ratio of both filters is 0.8. Both clarifiers have 20% recycled to the

influent.

solution:

Step 1. Determine volume required for the filters (plastic media)

Volume of each filter ⫽ 360 m

5 719 m

Volume 5

5680 m

/d 3 0.19 kg/m

1.5 kg/sm

Wastewater Engineering 739

Step 2. Determine surface area of the filter

Use side water depth of 4 m (13 ft) since minimum depth is 3 m.

Area ⫽ 360 m

/4 m ⫽ 90 m

Diameter ⫽ (4 ⫻ 90 m

/3.14)

0.5

⫽ 10.7 m

⫽ 35 ft

Step 3. Check hydraulic loading rate

HRT ⫽ (5680 m

/d)(l ⫹ 0.8)/90 m

⫽ 113.6 m

/(m

⭈ d) (OK, ⬍375m

/(m

⭈ d))

Use 4 m (13 ft) deep filters with 90 m

(970 ft

) area.

Step 4. Sizing for the intermediate clarifier

Use HLR ⫽ 41 m

/(m

⭈ d) (1000 gal/(d ⭈ ft

)) and minimum depth of 3 m (10 ft)

Diameter ⫽ (4 ⫻ 166 m

/3.14)

1/2

⫽ 14.6 m

⫽ 48 ft

Step 5. Sizing for the secondary clarifier

Using HLR ⫽ 31 m

/(m

⭈ d) (760 gal/(d ⭈ ft

)) and minimum depth of 3 m (10 ft)

Diameter ⫽ (4 ⫻ 220 m

/3.14)

1/2

⫽ 16.7 m

⫽ 55 ft

26.2 Basin sizing for suspended-growth

biological treatment

In order to produce the proper concentration of return sludge, activated-

sludge settling tanks must be designed to meet thickening as well as sep-

aration requirements. Since the rate of recirculation of RAS from the

secondary settling tanks to the aeration or reaeration basins is quite

5 220 m

Area required 5

5680 m

/d 3 1.2

31 m

/sm

5 166 m

Area required 5

5680 m

/d 3 1.2

41 m

/sm

740 Chapter 6

high in activated-sludge processes, their surface overflow rate and weir

overflow rate should be adjusted for the various modified processes to

minimize the problems with sludge loadings, density currents, inlet

hydraulic turbulence, and occasional poor sludge settle-ability. The size

of the secondary settling tank must be based on the large surface area

determined for surface overflow rate and solids loading rate. Table 6.20

presents the design criteria for secondary clarifiers following activated-

sludge processes (GLUMRB, 1996). The values given in Tables 6.19 and

6.20 are comparable.

Solids loading rate is of primary importance to insure adequate func-

tion in the secondary settling tanks following aeration basins. In practice,

most domestic wastewater plants have values of solids volume index in

the range of 100 to 250 mg/L (WEF and ASCE, 1991a). Detail discussions

of the secondary clarifier can be found in this manual. Most design engi-

neers prefer to keep the maximum solids loading rates in the range of

4 to 6 kg/(m

⭈ h) (20 to 30 lb/(d ⭈ ft

)). Solids loadings rates of 10 kg/(m

⭈ h)

(50 lb/(d ⭈ ft

) or more have been found in some well-operating plants.

The maximum allowable hydraulic loading rate HLR as a function of

the initial settling velocity ISV at the design MLSS concentration was

proposed by Wilson and Lee (1982). The equation is expressed as follows:

(6.151)

where HRT ⫽ hydraulic retention time, h

Q ⫽ limiting hydraulic capacity, m

A ⫽ area of the clarifier, m

24 ⫽ unit conversion factor, 24 h/d

ISV ⫽ initial settling velocity at the design MLSS

concentration, m/h

CSF ⫽ clarifier safety factor, 1.5 to 3, typically 2

The values of ISV change with MLSS concentrations and other condi-

tions. Batch-settling analyses should be performed. The maximum antic-

ipated operational MLSS or the corresponding minimum ISV should be

used in Eq. (6.151).

Numerous state regulations limit the maximum allowable weir load-

ing rates to 124 m

/(d ⭈ m) (10,000 gal/(d ⭈ ft)) for small plants (less than

3785 m

/d or 1 Mgal/d) and to 186 m

/(d ⭈ m) (15,000 gal/(d ⭈ ft)) for larger

treatment plants (WEF and ASCE, 1991a). It is a general consensus that

substantially high weir loading rates will not impair performance.

The depth of secondary clarifiers is commonly designed as 4 to 5 m

(13 to 16 ft). The deeper tanks increase TSS removal and RAS concen-

tration as well as costs. Typically, the larger the tank diameter, the

deeper the sidewall depth. The shapes of settling tanks include rectan-

gular, circular, and square. A design example of a secondary clarifier

HRT 5 Q/A 5 24 3 ISV/CSF

Wastewater Engineering 741

following the activated-sludge process is given previously in section

21.5, Example 1B.

Example: Determine the size of three identical secondary settling tanks for

an activated-sludge process with a recycle rate of 25%, an MLSS concentra-

tion of 3600 mg/L, an average design flow of 22,710 m

/d (6 Mgal/d), and an

anticipated peak hourly flow of 53,000 m

/d (14 Mgal/d). Use solids loading

rates of 4.0 and 10.0 kg/(m

⭈ h) for average and peak flow respectively.

solution:

Step 1. Compute the peak solids loading

3600 mg/L ⫽ 3600 g/m

⫽ 3.6 kg/m

Loading ⫽ 53,000 m

/d ⫻ 3.6 kg/m

⫻ 1.25

⫽ 238,500 kg/d

Step 2. Compute the design average solids loading

Loading ⫽ 22,710 m

/d ⫻ 3.6 kg/m

⫻ 1.25

⫽ 102,200 kg/d

Step 3. Compute the surface area required (each of three)

(a) At design flow

(b) At peak flow

A ⫽ 238,500/(10.0 ⫻ 24 ⫻ 3)

⫽ 331 m

⫽ 3563 ft

The design flow is the controlling factor; and the required surface area A ⫽

355 m

Step 3. Determine the diameter of circular clarifiers

⫽ 4A/p ⫽ 4 ⫻ 355 m

/3.14

d ⫽ 21.3 m

⫽ 70 ft

Use sidewall depth of 4 m (13 ft) for the tank diameter of 21 to 30 m (70 to

100

ft).

5 3820 ft

5 355 m

A 5

102,200 kg/d

4.0 kg/ sm

hd 3 24 h/d 3 3 sunitsd

742 Chapter 6

Step 4. Check hydraulic loading rate at the design flow

HLR ⫽ (22,710 m

/d)/(3 ⫻ 355 m

)

⫽ 21.3 m

/(m

⭈ d)

Step 5. Check HLR at the peak flow

HLR ⫽ (53,000 m

/d)/(3 ⫻ 355 m

)

⫽ 49.8 m

/(m

⭈ d)

(It is slightly over the limit of 49 m

/(m

⭈ d); see Table 6.20.)

Step 6. Compute the weir loading rate

Perimeter ⫽ pd ⫽ 3.14 ⫻ 21.3 m

⫽ 66.9 m

At average design flow

Weir loading ⫽ (22,710 m

/d)/(66.9 m ⫻ 3)

⫽ 113 m

/(m ⭈ d) (OK, ⬍ 125 m

/m ⭈ d limit)

At the peak flow

Weir loading ⫽ (53,000 m

/d)/(66.9 m ⫻ 3)

⫽ 264 m

/(m ⭈ d)

27 Efﬂuent Disinfection

Effluent disinfection is the last treatment step of a secondary or terti-

ary treatment process. Disinfection is a chemical treatment method

carried out by adding the selected disinfectant to an effluent to destroy

or inactivate the disease-causing organisms. The purposes of effluent

disinfection are to protect public health by killing or inactivating path-

ogenic organisms such as enteric bacteria, viruses, and protozoans, and

to meet the effluent discharge standards.

The disinfection agents (chemicals) include chlorine, ozone, ultraviolet

(UV) radiation, chlorine dioxide, and bromine. Design of UV irradiation

can be referred to the manufacturer or elsewhere (WEF and ASCE, 1996a).

The chlorination–dechlorination process is currently widely practiced

in the United States. Chlorine is added to a secondary effluent for a cer-

tain contact time (20 to 45 min for average flow and 15 min at peak flow),

then the effluent is dechlorinated before discharge only during warm

weather when people use water as primary contact. In the United States

most states adopt a coliform limitation of 200 fecal coliform/100 ml.

Wastewater Engineering 743

Chlorination of effluents is usually accomplished with liquid chlo-

rine. Alternative methods use calcium or sodium hypochlorite or chlo-

rine dioxide. Disinfection kinetics and chemistry of chlorination are

discussed in Chapter 5, numerous literature, and textbooks. An excel-

lent review of effluent disinfection can be found in Design Manual

(US EPA, 1986).

27.1 Chlorine dosage

If a small quantity of chlorine is added to wastewater or effluent, it will

react rapidly with reducing substances such as hydrogen sulfide and fer-

rous iron, and be destroyed. Under these conditions, there are no disin-

fection effects. If enough chlorine is added to react with all reducing

compounds, then a little more added chlorine will react with organic

materials present in wastewater and form chlororganic compounds, which

have slight disinfection activities. Again, if enough chlorine is introduced

to react with all reducing compounds and all organic materials, then a

little more chlorine added will react with ammonia or other nitrogenous

compounds to produce chloramines or other combined forms of chlorine,

which do have disinfection capabilities. Therefore chlorine dosage and

residual chlorine are very important factors of disinfection operation. In

addition to its disinfection purpose, chlorination is also applied for pre-

vention of wastewater decomposition, prechlorination of primary influent,

control of activated sludge bulking, and reduction of BOD.

Chlorinators are designed to have a capacity adequate to produce

an effluent to meet coliform density limits specified by the regulatory

agency. Usually, multiple units are installed for adequate capacity and

to prevent excessive chlorine residuals in the effluent. Table 6.21

shows the recommended chlorine dosing capacity for treating normal

TABLE 6.21 Recommended Chlorine Dosing Capacity for Various Types of Treatment

Based on Design Average Flow

Illinois EPA dosage, GLUMRB dosage,

Type of treatment mg/L mg/L

Primary settled effluent 20

Lagoon effluent (unfiltered) 20

Lagoon effluent (filtered) 10

Trickling filter plant effluent 10 10

Activated sludge plant effluent 6 8

Activated sludge plant with 4

chemical addition

Nitrified effluent 6

Filtered effluent following 4 6

mechanical biological treatment

SOURCE: Illinois EPA (1997), GLUMRBS (1996)

744 Chapter 6

domestic wastewater, based on design average flow (Illinois EPA,

1997; GLUMRB, 1996).

For small applications, 150-lb (68-kg) chlorine cylinders are typically

used where chlorine gas consumption is less than 150 lb/d. Chlorine

cylinders are stored in an upright position with adequate support brack-

ets and chains at 2/3 of cylinder height for each cylinder. For larger

applications where the average daily chlorine gas consumption is greater

than 150 lb, 1-ton (907-kg) containers are employed. Tank cars, usually

accompanied by evaporators, are used for large installations (⬎10

Mgal/d, 0.44 m

/s). In this case, area-wide public safety should be eval-

uated as part of the design consideration.

Example 1: Estimate a monthly supply of liquid chlorine for trickling filter

plant effluent disinfection. The design average flow of the plant is 3.0 Mgal/d

(11,360 m

/d).

solution:

Step 1. Find the recommended dosage

From Table 6.21, the recommended dosage for trickling filter plant effluent

is 10 mg/L.

Step 2. Compute the daily consumption

Chlorine ⫽ 3.0 Mgal/d ⫻ 10 mg/L ⫻ 8.34 lb/(Mgal ⭈ (mg/L)

⫽ 250 lb/d

The daily consumption is over 150 lb/d (68 kg/d); thus choose 1-ton (2000-lb,

907-kg) containers.

Step 3. Compute the number of 1-ton containers required for 1 month’s (M)

supply

Monthly need ⫽ 250 lb/d ⫻ 30 d/M

⫽ 7500 lb/M

The plant needs four 1-ton containers, which is enough for 1 month’s

consumption.

Example 2: Determine the feeding rate in gal/min of sodium hypochlorite

(NaOCl) solution containing 10% available chlorine. The daily chlorine dosage

for the plant is 480 kg/d (1060 lb/d).

solution:

Step 1. Calculate chlorine concentration of the solution

10% ⫽ 100,000 mg/L ⫽ 100 g/L