Lin S.D. Water and Wastewater Calculations Manual

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Wastewater Engineering 675

The process has been developed in an attempt to match oxygen supply

and oxygen demand and to use a high-rate process through mainte-

nance of high concentrations of biomass. The major components of the

process are an oxygen generator, using high-purity oxygen in lieu of

air, a specially compartmented aeration basin, a clarifier, pumps for

recirculating activated sludge, and sludge disposal facilities. Oxygen is

generated by manufacturing liquid oxygen cryogenically on-site for large

plants and by pressure swing adsorption for small plants. Standard

cryogenic air separation involves liquefaction of air followed by fractional

distillation to separate the major components of oxygen and nitrogen.

The aeration basin is divided into compartments by baffles and is

covered with a gastight cover. An agitator is included in each compart-

ment (Fig. 6.24). High-purity oxygen is fed concurrently with waste-

water flow. Influent wastewater, return activated sludge, and oxygen gas

under slight pressure are introduced to the first compartment. A dis-

solved oxygen concentration of 4 to 10 mg/L is normally maintained in

the mixed liquor. Successive aeration compartments are connected to

each other through submerged ports. Exhausted waste gas is a mixture

of nitrogen, carbon dioxide, and approximately 10% of oxygen supplied

and vented from the last stage of the system. Effluent mixed liquor is

then settled in a secondary clarifier. Settled activated sludge is recir-

culated to the aeration basin, with some wasted. The process is used for

general application with high-strength wastes with limited space.

Design parameters of high-purity oxygen systems are: BOD

loading ⫽

1.6 to 3.2 kg/(m

⭈ d) (100 to 200 lb/(1000 ft

⭈ d)), MLSS ⫽ 3000 to 8000

mg/L, F/M ⫽ 0.25 to 1.0 per day, HRT ⫽ 1 to 3 days, u

⫽ 8 to 20 days,

and Q

/Q ⫽ 0.25 to 0.5. The BOD

removal efficiency for the process is

85% to 95% (WEF and ASCE, 1992).

Figure 6.24 Schematic flow diagram of a high-purity activated-sludge system.

676 Chapter 6

Advantages of the high-purity oxygen activated-sludge process include

higher oxygen transfer gradient and rates, smaller reaction chambers,

improved biokinetics, the ability to treat high-strength soluble waste-

water, greater tolerance for peak organic loadings, reduced energy

requirements, reduction in bulking problems, a better settling sludge

(1% to 2% solids), and effective odor control.

In a comparison of air and pure oxygen systems, researchers reported

little or no significant difference in carbonaceous BOD

removal kinet-

ics with DO concentration about 1 to 2 mg/L. Lower pH values (6.0 to 6.5)

occur with high-purity oxygen and loss of alkalinity, with nitrate pro-

duction. Covered aeration tanks have a potential explosion possibility.

Example: The high-purity oxygen activated-sludge process is selected for

treating a high-strength industrial wastewater due to limited space.

Determine the volume of the aeration tank and check with design parame-

ters, using the following data.

solution:

Step 1. Determine the tank volume

Using Eq. (6.88)

F/M ⫽ 0.7/day

where X ⫽ 5500 mg/L ⫻ 0.8 ⫽ 4400 mg/L

8.34 ⫽ conversion factor, lb/(Mgal ⭈ (mg/L))

Therefore V 5

1 Mgal/d 3 280 mg/L 3 8.34

s0.7/dayd 3 4400 mg/L 3 8.34

V 5

sF/Md X

F/M 5

Design average flow 1 Mgal/d (3785 m

/d)

Influent BOD 280 mg/L

Influent TSS 250 mg/L

F/M 0.70 lb BOD applied/(lb MLVSS ⭈ d)

MLSS 5500 mg/L

VSS/TSS 0.8

Maximum volumetric BOD loading 200 lb/(1000 ft

⭈ d)

⫽ 3.2 kg/(m

⭈ d)

Minimum aeration time 2 h

Minimum mean cell residence time 3 days

Wastewater Engineering 677

⫽ 0.0909 Mgal

⫽ 344 m

⫽ 12,150 ft

Step 2. Check the BOD loading

⫽ 192 lb/(1000 ft

⭈ d)

⬍ 200 lb/(1000 ft

⭈ d) (OK)

⫽ 3.08 kg/(m

⭈ d)

Step 3. Check the aeration time

⫽ 0.0909 day ⫻ 24 h/d

⫽ 2.2 h

⬎ 2.0 h (OK)

Step 4. Check mean cell residence time (sludge age)

Influent VSS X

⫽ 0.8 ⫻ 250 mg/L

⫽ 200 mg/L

⫽ 2.0 days

⬍ 3.0 days (OK)

Oxidation ditch.

The oxidation ditch process was developed by Pasveer

(1960) in Holland and is a modification of the conventional activated-

sludge plug-flow process. The system is especially applicable to small

communities needing low-cost treatment. The oxidation ditch is typically

an extended aeration mode. It consists typically of a single or closed-loop

elongated oval channel with a liquid depth of 1.2 to 1.8 m (4 to 6 ft) and

45-degree sloping sidewalls (Fig. 6.25).

Wastewater is given preliminary treatment such as screening, com-

minution, or grit removal (usually without primary treatment). The

wastewater is introduced into the ditch and is aerated using mechanical

aerator(s) (Kessener brush) which are mounted across the channel for

an extended period of time with long HRT of 8 to 36 h (typically 24 h)

and SRT (solids residence time) of 20 to 30 days (WEF and ASCE, 1992).

0.0909 Mgal 3 4400 mg/L

1 Mgal/d 3 200 mg/L

HRT 5

0.0909 day 3 24 h/d

BOD loading 5

1.0 Mgal/d 3 280 mg/L 3 8.34 lb/sMgal

smg/Ldd

12.15 3 1000 ft

678 Chapter 6

The aerators provide mixing and circulation in the ditch, and oxygen

transfer. The mechanical brush operates in the 60 to 110 rev/min range

and keeps the liquid in motion at velocities from 0.24 to 0.37 m/s (0.8 to

1.2 ft/s), sufficient to prevent deposition of solids. A large fraction of the

required oxygen is supplied by transfer through the free surface of the

wastewater rather than by aeration. As the wastewater passes the aer-

ator, the DO concentration rises and then falls while the wastewater

traverses the circuit.

The BOD

loading in the oxidation ditch is usually as low as 0.08 to

0.48 kg/(m

⭈ d) (5 to 30 lb/(1000 ft

⭈ d)). The F/M ratios are 0.05 to 0.30

per day. The MLSS are in the range of 1500 to 5000 mg/L. The activated

sludge in the oxidation ditch removes the organic matter and converts

it to cell protoplasm. This biomass will be degraded for long solids res-

idence time.

Separate or interchannel clarifiers are used for separating and return-

ing MLSS to the ditch. Recommendations for unit size and power

requirements are available from equipment manufacturers.

The oxidation ditch is popular because of its high BOD

removal effi-

ciency (85% to 95%) and easy operation. Nitrification occurs also in the

system. The oxidation ditch is a flexible process and is used for small

communities where large land area is available. However, the surface

Figure 6.25 Schematic diagram of oxidation ditch activated-sludge process.

Wastewater Engineering 679

aerators may become iced during the winter months. Electrical heating

must be installed at the critical area. During cold weather the opera-

tion must prevent loss of efficiency, mechanical damage, and danger to

the operators.

Example 6: Estimate the effluent BOD

to be expected of an oxidation ditch

treating a municipal wastewater with an influent BOD of 220 mg/L. Use the

following typical kinetic coefficients: k ⫽ 5/d; K

⫽ 60 mg/L of BOD; Y ⫽ 0.6 mg

VSS/mg BOD; k

⫽ 0.06/d; and u

⫽ 20 d.

solution:

Step 1. Estimate effluent substrate (soluble BOD

) concentration

From Eq. (6.80)

⫽ 2.3 mg/L

Step 2. Estimate TSS in the effluent

Using Eq. (6.78)

⫽ 59.4 mg/L

Step 3. Estimate the effluent BOD

It is commonly used that 0.63 mg BOD will be exerted (used) for each mg of

TSS. Then, the effluent total BOD is

BOD ⫽ 2.3 mg/L ⫹ 0.63 ⫻ 59.4 mg/L

⫽ 39.7 mg/L

Deep shaft reactor.

A new variation of the activated-sludge process,

developed in England, the deep shaft reactor is used where costs of land

are high. It is used for general application with high-strength wastes.

There are a number of installations in Japan. It has been marketed in

the United States and Canada.

The deep shaft reactor consists of a vertical shaft about 120 to 150 m

(400 to 500 ft) deep, utilizing a U-tube aeration system (Fig. 6.26). The

20 days 3 0.6s220 2 2.3d mg/L

20 dayss1 1 0.06/day 3 20 daysd

X 5

YsS

2 Sd

us1 1 k

60 mg/Ls1 1 20 days 3 0.06/dayd

20 dayss0.6 3 5/days 2 0.06/dayd 2 1

S 5

s1 1 u

sYk 2 k

d 2 1

680 Chapter 6

shaft replaces the primary clarifiers and aeration tanks. The deep

shaft is lined with a steel shell and fitted with a concentric pipe to form

an annular reactor. Wastewater, return activated sludge, and air is

forced down the center of the shaft and allowed to rise upward through

the annulus (riser). The higher pressures obtained in the deep shaft

improve the oxygen transfer rate and more oxygen can be dissolved,

with beneficial effects on substrate utilization. DO concentration can

range from 25 to 60 mg/L (Droste, 1997). Microbial reaction rates are

not affected by the high pressures. Temperature in the system is fairly

constant throughout the year. The flow in the shaft is a plug-flow

mode. The F/M ratios are 0.5 to 5.0 per day and the HRT ranges from

0.5 to 5 h.

A flotation tank serves for the separation of the biomass (solids) from

the supernatant. Most of the solids are recirculated to the deep shaft

reactor, and some are wasted to an aerobic digester. The effluent of the

reactor is supersaturated with air when exposed to atmospheric pres-

sure. The release of dissolved air from the mixed liquor forms bubbles

that float suspended solids to the surface of the flotation tank.

Figure 6.26 Schematic diagram of deep shaft activated-sludge system.

Wastewater Engineering 681

The advantages of the deep shaft reactor include lower construction

and operational costs, less land requirement, capability to handle big

organic loads, and suitability for all climatic conditions.

Biological nitriﬁcation. Conversion of ammonia to nitrite and nitrate can

be achieved by biological nitrification. Biological nitrification is performed

either in single-stage activated-sludge carbon oxidation and ammonia

reduction or in separate continuous-flow stir-tank reactors or plug-flow.

The single-stage nitrification process is used for general application

for nitrogen control where inhibitory industrial wastes are not present.

The process also removes 85% to 95% of BOD.

The design criteria for single-stage activated-sludge nitrification are:

volumetric BOD loading ⫽ 0.08 to 0.32 kg/(m

⭈ d) (5 to 20 lb/(1000

⭈ d)); MLSS ⫽ 1500 to 3500 mg/L; u ⫽ 6 to 15 h; u

⫽ 8 to 12 days;

F/M ⫽ 0.10 to 0.25 per day; and Q

/Q ⫽ 0.50 to 1.50 (Metcalf and Eddy,

Inc. 1991; WEF and ASCE, 1991a).

The advantage of the separate-stage suspended-growth nitrification

system is its flexibility to be optimized to conform to the nitrification

needs. The separate-stage process is used to update existing systems

where nitrogen standards are stringent or where inhibitory industrial

wastes are present and can not be removed in the previous stages.

The design criteria for the separate-stage nitrification process are: vol-

umetric BOD loading ⫽ 0.05 to 0.14 kg/(m

⭈ d) (3 to 9 lb/1000 (ft

⭈ d)),

very low; F/M ⫽ 0.05 to 0.2 per day; MLSS ⫽ 1500 to 3500 mg/L; HRT ⫽

3 to 6 h; u

⫽ 15 to 100 days (very long); and Q

/Q ⫽ 0.50 to 2.00. More

details on single-stage and separate-stage biological nitrification are

given under advanced treatment for nitrogen removal in a later section.

21.8 Aeration and mixing systems

The treatment efficiency of an activated-sludge process depends on the

oxygen dissolved in the aeration tank and the transfer of oxygen supply

from the air and from pure oxygen. Oxygen is transferred to the mixed

liquor in an aeration tank by dispersing compressed air bubbles through

submerged diffusers with compressors or by entraining air into the

mixed liquor by surface aerators. A combination of the above has also

been used. An air diffusion system consists of air filters and other

conditioning equipment, blowers, air piping, and diffusers, including dif-

fuser cleaning devices.

A large number of combinations of mixing/aeration equipment and

tank configuration have been developed.

Oxygen transfer and utilization. Oxygen transfer is a two-step process.

Gaseous oxygen is first dissolved in the mixed liquor by diffusion or

682 Chapter 6

mechanical aeration or both. The dissolved oxygen is utilized by the

microorganisms in the process of metabolism of the organic matter.

When the rate of oxygen utilization exceeds the rate of oxygen dissolu-

tion, the dissolved oxygen (DO) in the mixed liquor will be depleted. This

should be avoided. DO should be measured periodically, especially

during the period of peak loading, and the air supply then adjusted

accordingly.

The rate of oxygen transfer from air bubbles into solution is written

as (Hammer, 1986)

r ⫽ k(bC

– C

) (6.113)

where r ⫽ rate of oxygen transfer from air to liquid, mg/(L ⭈ h)

k ⫽ transfer coefficient, per h

b ⫽ oxygen saturation coefficient of the wastewater

⫽ 0.8 to 0.9

⫽ DO concentration at saturation of top water

(Table 1.2), mg/L

⫽ DO concentration in mixed liquor, mg/L

– C

⫽ DO deficit, mg/L

The greater the DO deficit, the higher the rate of oxygen transfer. The

oxygen transfer coefficient k depends on wastewater characteristics

and, more importantly, on the physical features of the aeration, liquid

depth of the aeration tank, mixing turbulence, and tank configuration.

The rate of DO utilization is essentially a function of F/M ratio and

liquid temperature. The biological uptake of DO is approximately

30 mg/(L ⭈ h) for the conventional activated-sludge process, generally less

than 10 mg/(L ⭈ h) for extended aeration, and as high as 100 mg/(L ⭈ h)

for high-rate aeration. Aerobic biological reaction is independent of DO

above a minimum critical concentration. However, if DO is below the

critical value, the metabolism of microorganisms is limited by reduced

oxygen supply. Critical concentrations reported for various activated-

sludge systems range from 0.2 to 2.0 mg/L, the most commonly being

0.5 mg/L (Hummer, 1986). Ten States Standards stipulates that aera-

tion equipment shall be capable of maintaining a minimum of 2.0 mg/L

of DO in mixed liquor at all times and provide thorough mixing of the

mixed liquor (GLUMRB, 1996).

Oxygen requirements generally depend on maximum diurnal organic

loading (design peak hourly BOD

), degree of treatment, and MLSS. The

Ten States Standards (GLUMRS, 1996) further recommends that the

normal design air requirement for all activated-sludge processes except

extended aeration shall provide 1500 ft

/lb of BOD

loading (94 m

/kg

BOD

). For extended aeration the value shall be 2050 ft

/lb of BOD

(128 m

/kg of BOD

). The design oxygen requirement for all activated-

sludge processes shall be 1.1 lb oxygen/lb design peak hourly BOD

applied.

Wastewater Engineering 683

Example 1: Compare the rate of oxygen transfer when the existing DOs in

the mixed liquor are 3.0 and 0.5 mg/L. Assume the transfer coefficient is

2.8/h and oxygen saturation coefficient is 0.88 for wastewater at 15.5⬚C.

solution:

Step 1. Find C

at 15.5⬚C

From Table 1.2

⫽ 9.92 mg/L

Step 2. Calculate oxygen transfer rates, using Eq. (6.113)

At DO ⫽ 3.0 mg/L

r ⫽ k( bC

– C

)

⫽ (2.8/h) (0.88 ⫻ 9.92 ⫺ 3.0) mg/L

⫽ 16.0 mg/(L ⭈ h)

At DO ⫽ 0.5 mg/L

r ⫽ (2.8/h) (0.88 ⫻ 9.92 ⫺ 0.5) mg/L

⫽ 23.0 mg/(L ⭈ h)

Note: The higher the DO deficit, the greater the oxygen transfer rate.

Example 2: Determine the oxygen transfer efficiency of a diffused aeration

system providing 60 m

/kg BOD

applied (960 ft

/lb BOD

). Assume that the aer-

ation system is capable of transferring 1.1 kg of atmospheric oxygen to DO in

the mixed liquor per kg BOD

applied, and that 1 m

of air at standard conditions

(20⬚C, 760 mm Hg, and 36% relative humidity) contains 0.279 kg of oxygen.

solution:

Step 1. Calculate oxygen provided per kg BOD

Oxygen provided ⫽ 60 m

/kg BOD ⫻ 0.279 kg/m

⫽ 16.7 kg/kg BOD

Step 2. Determine oxygen transfer efficiency e

e ⫽ oxygen transferred/oxygen provided

⫽ (1.1 kg/kg BOD)/(16.7 kg/kg BOD)

⫽ 0.066

⫽ 6.6%

Diffused air aeration.

A diffused air aeration system consists of diffusers

submerged in the mixed liquor, header pipes, air mains, blowers (air

compressors), and appurtenances through which the air passes. Ceramic

diffusers (porous and nonporous) are manufactured as tubes, domes, or

plates. Other diffusers, such as jets or U-tubes, may be used. Compressed

684 Chapter 6

air forced through the diffusers is released as fine bubbles. The oxygen

is then introduced to the liquid.

The oxygen requirement for the aeration can be calculated using

Eq. (6.94) or (6.95). In practice, the efficiency of oxygen transfer of a

diffusion and sparging device seldom exceeds 8%. The oxygen trans-

fer efficiency for various bubble sizes is available from the manu-

facturer’s data. The diffuser system should be capable of providing

for 200% of the designed average daily oxygen demand (GLUMRB,

1996).

Air piping. As compressed air flows through a pipe, its volume changes

according to the pressure drop. The total headloss from the blower (or

silencer outlet) to the diffuser inlet shall not exceed 0.5 lb/in

(3.4 kPa)

under average operational conditions (GLUMRB, 1996). The pressure

drop in a pipeline carrying compressed air can be calculated by the

Darcy–Weisbach formula (Eq. (4.17)).

The value of friction factor f can be determined from the Moody dia-

gram. However, as an approximation, the friction factor of a steel pipe

carrying compressed air is expressed as (Steel and McGhee, 1979)

f ⫽ 0.029D

0.027

0.148

(SI units) (6.114)

f ⫽ 0.013D

0.027

0.148

(US customary units) (6.115)

where f ⫽ friction factor

D ⫽ diameter of pipe, m or ft

Q ⫽ flow of air, m

/min or gal/min

The headloss in a straight pipe is calculated from

H ⫽ f(L/D)h

(6.116)

where H ⫽ headloss in pipe, mm or in

L ⫽ length of pipe, m or ft

D ⫽ diameter of pipe, m or ft

⫽ velocity head, mm or in

Another approximation of the estimation of headloss in a pipe can be

expressed as

H ⫽ 9.82 ⫻ 10

–8

(fLTQ

)/(PD

) (6.117)

where T ⫽ temperature of air, K

P ⫽ absolute pressure, atm

Other terms are as given above.