Encyclopedia of Environmental Science and Engineering, Volume I and II

the conventional process becomes equal to the solid retention

time, u x. Recently, several modifications have been made in

the conventional anaerobic treatment process. McCarty^44 has

grouped the basic anaerobic process designs into Conventional

Process, Anaerobic Activated Sludge Process, and Anaerobic

Filter Process. Operating conditions of these process designs

are shown in Figure 21. It is suggested that the conventional

process be used for concentrated wastes like sludges where

economical treatment can be obtained by keeping hydraulic

detention time, - t equal to the desired solid retention time, ux.

The economic treatment of diluted wastes, however, requires

hydraulic detention time, t-, much below the desired solid

retention time, ux , and thus, anaerobic contact processes

become more applicable.^44

Anaerobic treatment processes are more sensitive to

operating parameters and their environments as compared

to aerobic processes. The best parameter for controlling the

operation of anaerobic treatment is the biological retention

time or solid retention time, SRT. A minimum SRT exists

below which the critical methane producing bacteria are

removed from the system faster than they can reproduce

themselves. In practice, SRT values of two to ten times

this minimum value are used. Thus, the hydraulic deten-

tion time and solid retention time maintained in anaerobic

treatment processes are very high and the net growth of

biological solids becomes very low due to significant decay

as given by Eq. 12.

Mixing of the digester content is becoming a common

practice. The advantages of mixing are better contact

between food and microorganisms, uniform temperature,

reduction in scum formation, accelerated digestion and dis-

tribution of metabolic inhibitors.

Certain cations, such as sodium, potassium, calcium, or

magnesium show a toxic or inhibitory effect on anaerobic

treatment when present in high concentrations, as shown

in Table 3.^45

Soluble sulfides exhibit toxicity because only they are

available to the cells. If the concentration of soluble sulfides

exceeds 200 mg/L, then the metabolic activity of methano-

genic population will be strongly inhibited leading to the

process failure.^21 Concentrations up to 100 mg/L can be toler-

ated without acclimation and sulfide concentrations between

100 and 200 mg/L can be tolerated after acclimation.

CONTACT MEDIA

MIXED

LIQUOR

RETURN

MIXING

MIXING

WASTE ORGANISMS

EFFLUENT (Q 1 Le, ∆S/∆T)

EFFLUENT (Q 1 Le, ∆S/∆T)

EFFLUENT (Q 1 Le)

INFLUENT (Q 1 Le)

INFLUENT (Q 1 Le)

INFLUENT (Q 1 Le)

ANAEROBIC FILTER PROCESS

ANAEROBIC ACTIVATED SLUDGE PROCESS

CONVENTIONAL PROCESS

CH 4 + CO 2

CH 4 + CO 2

CH 4 + CO 2

∆S/∆T

∀, L, S

∀, L, S

1 L

FIGURE 21 Basic anaerobic process designs.

Complex Organic

Material

(Proteins, Fats,

Carbohydrates)

Acid Producing Methane Producing

Bacteria

Bacterial Cells

CH 4 + CO 2 +

Matter

Bacteria

Organic Acid

(Acetic Acid,

Propionic Acid,....)

+

Bacterial Cells

+

CO 2 + H 2 O

+ H 2 S + N 2

+ H 2 O + Humus

FIGURE 20 Sequential mechanism of anaerobic waste treatment.

BIOLOGICAL TREATMENT OF WASTEWATER 155

TABLE 3

Stimulatory and inhibitory concentrations of light metal cations to

anaerobic processes

Cation Stimulatory Con., mg/L Strong Inhibitory Con., mg/L

Sodium 100–200 8000

Potassium 200–40 12000

Calcium 100–200 8000

Magnesium 75–150 3000

C002_001_r03.indd 155C002_001_r03.indd 155 11/18/2005 10:15:54 AM11/18/2005 10:15:54 AM