Compared with anaerobic sludge treatment, anaerobic wastewater treatment allows us to increase the concentration of bacteria remarkably, resulting in an increase in reaction rate (high-rate processes). Three different mechanisms are effective at increasing the bacterial concentration:

• Sedimentation of floc-forming bacteria and recycling of the thickened sludge into the bioreactor.

• Sedimentation and suspension of particles formed from bacteria and minerals inside the bioreactor.

• Immobilizing the bacteria at the surface of either fixed or suspended solid materials, as well as at rotating solid plates.

Comparing the specific maximum removal rate:

rsmax Umax

for an aerobic and an anaerobic process (degradation of acetate by methanogenic bacteria) and using mean values for pmax and YX/S from Table 6.2 (aerobic bacteria) and Table 8.2 (methanogenic bacteria) for unlimited substrate removal, we obtain for aerobics:

X YX/S 0.6 and for anaerobics:

rsmax umax 0.2

X YX/S 0.04 This real difference of only a factor of four is remarkably low because:

• The higher sludge density results in a higher bacterial concentration inside anaerobic reactors.

• There is less influence from mass transfer and diffusion than in aerobic aerated reactors, which can be limited by low dissolved oxygen levels.

Some effective and economical high-rate anaerobic treatment processes which take advantage of these benefits have gone into operation over recent decades (Dauber 1993).

In all these high-rate processes, the temperature is increased to about 35 °C. It is therefore absolutely necessary to construct the bioreactor with a thermal insulation and a preheating system for the inflowing and recycled wastewater and the settled sludge, especially in northern climates. The biogas produced should be used nearly completely and only a very low amount should be burned off in a bypass flame.

The lowest substrate concentrations which can be reduced anaerobically in an economical way are about S0 = 2000-3000 mg L-1 COD.

About 80-90% of the BOD5 can be removed:

If it is intended to discharge the treated water into surface water, it must be treated aerobically in a second stage to adhere to local legislation.

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