Encyclopedia of Environmental Science and Engineering, Volume I and II

156 BIOLOGICAL TREATMENT OF WASTEWATER

Depending on pH, ammonia can be toxic to anaerobic

bacteria and free ammonia is more toxic. If concentration

of free ammonia exceeds 150 mg/L, severe toxicity will

result, whereas the concentration of ammonium ions must be

greater than 3000 mg/L to have the same effect. At a concen-

tration of 1600 mg/L as N, ammonia can upset the process.^20

The volatile acids cause little inhibition in anaerobic reac-

tors at neutral pH.^21 Operating parameters of conventional

anaerobic digesters are shown in Table 4.

6. NUTRIENT REMOVAL

Biological nitrification and denitrification is one of the

common methods for nitrogen removal from wastewaters.

In warmer climates, nitrification may occur to a consider-

able degree in conventional aerobic biological treatment

processes, followed by serious adverse effects of denitrifica-

tion in settling tanks and/or the receiving bodies of water. In

northern cold climates, below 18°C, a three-stage biological

system as shown in Figure 22 is considered necessary for

nutrient removal.^46 In the first stage, carbonaceous BOD is

reduced to a level below 50 mg/L. In the second stage, the

ammonia, present in effluent from the first stage, is oxidized

to nitrites and nitrates by nitrosomonas and nitrobacters,

respectively, as shown below:

23 2 2

2

NH O NO H O 4H

NO O

42

Nitrosomonas

22

22

Nitrobacte





⎯→⎯⎯⎯⎯

rr

⎯→⎯⎯⎯ 2 NO 3



The third stage accomplished denitrification–conversion of

nitrites and nitrates to atmospheric nitrogen under anaerobic

conditions:

32

2

NO CH OH 3NO CO H O

2NO CH OH N CO H O 2OH

33 222

23 222





→

→

PHOSPHORUS AND BOD REMOVAL NITRIFICATION DENITRIFICATION

Coagulating Chemical Application (Optional Points)

Air Air Methanol

Raw

Wastewater

Settling Settling Settling Reaction Settling

Ta n k

Aeration

Ta n k

Waste Sludge

Waste Sludge Waste Sludge Waste Sludge

Return

Sludge

Return

Sludge

Return

Sludge

Aeration Effluent

Tank

FIGURE 22 Typical three-stage treatment process for nutrient removal.

TABLE 4

Operating parameters of conventional anaerobic digesters

Parameters Unmixed Mixed

Bv – Loading Rate, 0.02–0.05 0.1–0.3

1b VSS/day/cubic ft

kg VSS/day/cubic metre

0.32–0.80 1.6–3.2

t ̄ – Detention time, days 30–90 10–15

E – Volatile Solids

Reduction percent

50–70 50

Mixing Absent Present

pH 6.8–7.4 6.8–7.4

Temperature, °C 30–35 30–35

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