Lin S.D. Water and Wastewater Calculations Manual

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(d) Compute total solids out

Solids out ⫽ (Step 2b) ⫹ (Step 2c) ⫽ (12,240 ⫹ 290) kg/d

⫽ 12,530 kg/d

(e) Compare Steps 2a and 2d

Solids in 19,200 kg/d ⬎ solids out, 12,530 kg/d

Answer: The sludge blanket will increase in depth.

Note: the withdrawal rate also can be expressed as per minute

Dissolved air ﬂotation thickening. Flotation thickeners include dissolved

air flotation (DAF), vacuum flotation, and depressed-air flotation. Only

DAF is used for wastewater sludge thickening in the US. It offers sig-

nificant advantages in thickening light sludges such as activated sludge.

The DAF thickener (Fig. 6.56) separates solids from the liquid phase in

Wastewater Engineering 805

Figure 6.56 Dissolved air flotation thickener (source: US EPA, 1991).

an upward direction by attaching fine air bubbles (60 to 100 mm) to

particles of suspended solids which then float. The influent stream at

the tank bottom is saturated with air, pressurized (280 to 550 kPa),

then released to the inlet distributor. The retention tank is maintained

at a pressure of 3.2 to 4.9 kg/cm

(45 to 70 lb/in

) (US EPA, 1987b).

The ratio of the quantity of air supplied and dissolved into the recy-

cle or waste stream to that of solids (the air-to-solids, A/S ratio) is prob-

ably the most important factor affecting the performance of the flotation

thickener. Normal loading rates for waste activated sludge range from

10 to 20 kg solids/(m

d) (2 to 4 lb/(ft

d)). DAF thickening produces

about 4% solids with a solids recovery of 85% (Hammer, 1986). The

sludge volume index, SVI, is also an important factor for DAF operation.

Example 1: Determine hydraulic and solids loading rates for a DAF thick-

ener. The thickener, of 9 m (30 ft) diameter, treats 303 L/min (80 gal/min) of

waste activated sludge with a TSS concentration of 7800 mg/L.

solution:

Step 1. Calculate liquid surface area

Area ⫽ p (9 m/2)

⫽ 63.6 m

Hydraulic loading ⫽ (303 L/min)/63.6 m

⫽ 4.76 L/(min

)

Step 2. Calculate solids loading rate

TSS in WAS ⫽ 7800 mg/L ⫽ 0.78%

Solids loaded ⫽ 303 L/min ⫻ 60 min/h ⫻ 1 kg/L ⫻ (0.78%/100%)

⫽ 141.8 kg/h

Solids loading rate ⫽ (141.8 kg/h)/(63.6 m

)

⫽ 2.23 kg/(m

⫽ 53.3 kg/(m

⫽ 10.9 lb/(ft

Example 2: A DAF thickener treats 303 L/min (80 ft/min) of waste acti-

vated sludge at 8600 mg/L. Air is added at a rate of 170 L/min (6.0 ft

/min).

Determine the air-to-solids ratio. Use 1.2 g of air per liter of air (0.075 lb/ft

)

under the plant conditions.

solution:

Step 1. Compute mass of air added

Air ⫽170 L/min ⫻ 1.2 g/L

⫽ 204 g/min

806 Chapter 6

Step 2. Compute mass of solids treated

Solids ⫽ 8.6 g/L ⫻ 303 L/min

⫽ 2606 g/min

Step 3. Compute A/S ratio

A/S ⫽ (204 g/min)/(2606 g/min)

⫽ 0.078 g air/g solids

or ⫽ 0.078 lb air/lb solids

Example 3: Determine the concentration factor and the solids removal effi-

ciency of a DAF thickener with conditions the same as in Example 2. The

thickened sludge or float has 3.7% solids and the effluent TSS concentration

is 166 mg/L.

solution:

Step 1. Compute the concentration factor CF

From Example 2,

The influent sludge solids content ⫽ 8600 mg/L

⫽ 0.86%

⫽ 4.3

Step 2. Compute the solids removal efficiency

Centrifuge thickening. A centrifuge acts both to thicken and to dewater

sludge. The centrifuge process separates liquid and solids by the influence

of centrifugal force which is typically 50 to 300 times that of gravity (WEF

and ASCE, 1996b). Centrifuges may be used for thickening waste acti-

vated sludge or as dewatering devices for digested or conditioned sludges.

Three basic types (solid bowl, imperforate basket, and disc-nozzle) are

commonly installed to thicken or dewater wastewater sludge.

The solid bowl scroll centrifuge (Fig. 6.57) is the most widely used type.

It rotates along a horizontal axis and operates in a continuous-feed

manner. It consists of a rotating bowl having a cylindrical-conical shape

5 98.1%

s8600 2 166d mg/L 3 100%

8600 mg/L

Efficiency 5

influent TSS 2 effluent TSS

inf luent TSS

CF 5

float solids content

influent solids content

3.7%

0.86%

Wastewater Engineering 807

and a screw conveyer. Sludge is introduced into the rotating bowl

through a stationary feed pipe continuously and the solids concentrate

on the periphery. The gravitational force causes the solids to settle out

on the inner surface of the rotating bowl. A helical scroll, spinning at a

slightly different speed, moves the settled solids toward the tapered

end (outlet ports) and then discharges them. The light liquid pools above

the sludge layer flows toward the concentrate outlet ports. The unit

has a low cost/capacity ratio.

The basket centrifuge, also called the imperforate bowl (Fig. 6.58),

is a knife-discharge type and operates on a batch basis. Liquid sludge

is transported by a pipe through the top and is fed to the bottom of a

vertically mounted spinning bowl. Solids accumulate against the wall

of the bowl by centrifugal force and the concentrate is decanted. The

duration of the feed time is controlled by a preset timer or a concentrate

monitor (usually 60% to 85% of maximum depth). When the feed is

stopped, the bowl begins to decelerate. As a certain point a nozzle skim-

mer (plow or knife) enters the bowl to remove the retained solids. The

solids fall through the bottom of the bowl into a hopper. The plow

retracts and the bowl accelerates, starting a new cycle. The units needs

a skilled operator.

The disc-nozzle centrifuge (Fig. 6.59) rotates along a vertical axis and

operates in a continuous manner. The liquid sludge is fed normally

through the top of the unit (bottom feed is also possible) and flows

through a feedwell in the center of the rotor and to a set of some 50 con-

ical discs. An impeller within the rotor accelerates and distributes the

feed slurry, filling the rotor interior. The centrifugal force is applied to

808 Chapter 6

Figure 6.57 Solid bowl scroll centrifuge (source: US EPA, 1991).

the relatively thin film of liquid and solid between the discs. The force

throws the denser solid materials to the wall of the rotor bowl, where it

is subjected to additional centrifugal force and concentrated before it is

discharged through nozzles located on the periphery. The clarified liquid

passes on through the disc stack into the weir at the top of the bowl, and

is then discharged.

The performance of a centrifuge is usually determined by the per-

centage of capture. It can be calculated as follows:

(6.213)

where C

⫽ concentration of solids in rejected wastewater (concentrate),

mg/L, %

⫽ concentration of solids in sludge cake, mg/L, %

⫽ concentration of solids in sludge feed, mg/L, %

Percent capture 5 c1 2

2 C

d 3 100

Wastewater Engineering 809

Figure 6.58 Schematic of imperforate basket centrifuge (source:

US EPA, 1991.)

Example: A basket centrifuge is applied with 378 L/min (100 gal/min) of

waste activated sludge at a solids content of 0.77%. The basket run time for

80% depth is 18 min with a skimming operating time of 2 min. The average

solids content in the thickened sludge is 7.2%. The solids concentration in the

effluent is 950 mg/L. Determine hourly hydraulic loading and solids loading,

dry solids produced per cycle and per day, and the efficiency of solids capture.

solution:

Step 1. Calculate hydraulic loading (HL) rate

HL ⫽ 378 L/min ⫻ 60 min/h

⫽ 22,680 L/h (⫽22.7 m

/h)

⫽ 5990 gal/h

Step 2. Calculate solids loading SL

SL ⫽ 378 L/min ⫻ 60 min/h ⫻ (0.77%/100%) ⫻ 1 kg/L

⫽ 175 kg/h

⫽ 385 lb/h

810 Chapter 6

Figure 6.59 Schematic of a disc-nozzle centrifuge (source: US EPA, 1991).

Step 3. Calculate the solids production per cycle and per day

Step 4. Calculate the efficiency of solids capture

Given: C

⫽ 950 mg/L ⫽ 0.095%

⫽ 7.2%

⫽ 0.77%

Using Eq. (6.213)

Gravity belt thickening. The gravity belt thickener is a relatively new

sludge thickening technique, stemming from the application of belt

presses for sludge dewatering. It is effective for raw sludge and

digested sludge with less than 2% solids content. A gravity belt moves

over rollers driven by a variable-speed drive unit. The sludge is con-

ditioned with polymer and applied into a feed/distribution box at one

end. The sludge is distributed evenly across the width of the moving

belt as the liquid drains through, and the solids are carried toward

the discharge end of the thickener and removed. The belt travels

through a wash cycle. Additional material on gravity belt thickeners

and rotary drum thickeners may be found in WEF and ASCE (1991b,

1996b).

Example 1: An operator adds 2 lb (0.91 kg) of a dry polymer to 50 gal (189 L)

of water. What is the polymer strength?

5 88.8%

5 c1 2

0.095s6.2 2 0.77d

0.77s6.2 2 0.095d

d 3 100%

Efficiency 5 c1 2

2 C

d 3 100%

5 8330 lb/d

5 3780 kg/d

Solids produced per day 5

52.5 kg/cycle 3 1440 min/d

20 min/cycle

Total time for each cycle 5 18 min 1 2 min 5 20 min/cycle

5 52.5 kg/cycle

Solids produced per cycle 5

175 kg/h 3 18 min/cycle

60 min/h

Wastewater Engineering 811

solution:

Step 1. Calculate the mass of water plus polymer, W

W ⫽ 50 gal ⫻ 8.34 lb/gal ⫹ 2 lb

⫽ 419 lb

Step 2. Calculate the strength of polymer solution

Example 2: Determine the amount of dry polymer needed to prepare a 0.12%

solution in a 1000-gal (3.78 m

) tank. If 8 gal/min of this 0.12% polymer solu-

tion is added to a waste activated sludge flow of 88 gal/min (333 L/min) with 8600

mg/L solids concentration, compute the dosage of polymer per ton of sludge.

solution:

Step 1. Compute dry polymer required for 1000-gal solution

Polymer ⫽ 1000 gal ⫻ 8.34 lb/gal ⫻ (0.12%/100%)

⫽ 10.0 lb

Step 2. Compute the dosage in lb/ton of sludge

Sludge stabilization. After sludge has thickened, it requires stabiliza-

tion to convert the organic solids to a more refractory or inert form.

Thus the sludge can be handled or used as a soil conditioner without

causing a nuisance or health hazard. The purposes of sludge stabiliza-

tion are to reduce pathogens, eliminate odor-causing materials, and to

inhibit, reduce, and eliminate the potential for putrefaction.

Treatment processes commonly used for stabilization of wastewater

sludges include anaerobic digestion, aerobic digestion, chemical (lime,

disinfectants) stabilization, and composting.

Anaerobic digestion. Anaerobic sludge digestion is the biochemical degra-

dation (oxidation) of complex organic substances in the absence of free

oxygen. The process is one of the oldest and most widely used methods.

5 25.4 lb polymer/ton dry sluge solid

8 gal/min 3 8.34 lb/gal 3 0.12% 3 2000 lb/ton

88 gal/min 3 8.34 lb/gal 3 0.86%

Dosage 5

lb polymer added

lb dry solid

2000 lb

ton

Solids at 8600 mg/L 5 0.86%

Polymer strength 5

mass of polymer

total mass

2 lb

419 lb

3 100% 5 0.48%

812 Chapter 6

During anaerobic digestion, energy is released, and much of the volatile

organic matter is converted to methane, carbon dioxide, and water. Thus

little carbon and energy are available to sustain further biological activ-

ity, and the remaining residuals are rendered stable.

Anaerobic digestion involves three basic successive phases of fer-

mentation: hydrolysis, acid formation, and methane formation (WEF

and ASCE, 1996b; US EPA, 1991). In the first phase of digestion, extra-

cellular enzymes (enzymes operating outside the cells) break down com-

plex organic substances (proteins, cellulose, lignins, lipids) into soluble

organic fatty acids, alcohols, carbon dioxide, and ammonia.

In the second phase, acid-forming bacteria, including facultative bac-

teria, convert the products of the first stage into short-chain organic

acids such as acetic acid, propionic acid, other low molecular weight

organic acids, carbon dioxide, and hydrogen. These volatile organic acids

tend to reduce the pH, although alkalinity buffering materials are also

produced. Organic matter is converted into a form suitable for break-

down by the second group of bacteria.

The third phase, strictly anaerobic, involves two groups of methane-

forming bacteria (methanogens). One group converts carbon and hydro-

gen to methane. The other group converts acetate to methane, carbon

dioxide, and other trace gases. Both groups of bacteria are anaerobic.

Closed digesters are used for anaerobic digestion. It should be noted that

many authors consider only two phases (excluding phase 1 above).

The most important factors affecting the performance of anaerobic

digesters are solids residence time, hydraulic residence time, tempera-

ture, pH, and toxic materials. Methanogens are very sensitive to envi-

ronmental conditions. Anaerobic digesters are usually heated to

maintain a temperature of 34 to 36⬚C (94 to 97⬚F). The methane bacte-

ria are active in the mesophilic range (27 to 43⬚C, 80 to 110⬚F). They have

a slower growth rate than the acid formers and are very specific in food

supply requirements. The anaerobic digester (two-stage) usually pro-

vides 10 to 20 days’ detention of sludge (WEF and ASCE, 1996b).

Optimum methane production typically occurs when the pH is main-

tained between 6.8 and 6.2. If the temperature falls below the operat-

ing range and/or the digestion times falls below 15 days, the digester

may become upset and require close monitoring and attention.

If concentrations of certain materials such as ammonia, sulfide, light

metal cations, and heavy metals increase significantly in anaerobic

digesters, they can inhibit or upset the process of performance.

The key parameter for digester sizing is solids residence time. For

digester systems without recycle, there is no difference between SRT and

HRT. Volatile solids loading rate is also used frequently as a basis for

design. Typically, design SRT values are from 30 to 60 days for low-rate

digesters and 10 to 20 days for high-rate digesters (WEF and ASCE, 1991b).

Wastewater Engineering 813

Volatile solids loading criteria are generally based on sustained load-

ing conditions (typically peak month or peak week solids production),

with provisions for avoiding excessive loading during shorter time peri-

ods. A typical design sustained peak volatile solids loading rate is 1.9 to

2.5 kg VS/(m

⭈ d) (0.12 to 0.16 lb VS/(ft

⭈ d)). A maximum limit of 3.2 kg

VS/(m

⭈ d) (0.20 lb VS/(ft

⭈ d)) is often used (WEF and ASCE, 1991b).

The configuration of an anaerobic digester is typically a single-stage

or two-stage process (Fig. 6.60). For the low rate, single-stage digester,

three separate layers (scum, supernatant, and sludge layers) form as

decomposition occurs. The stabilized (digested) sludge settles at the

bottom of the digester. The supernatant is usually returned to the plant

influent. In a single-stage high rate process, the digester is heated and

814 Chapter 6

Figure 6.60 Configuration of (a) single-stage and (b) two-stage anaerobic digesters

(source: US EPA, 1991).