Physicochemical conditions necessary to anaerobic digestion

The anaerobic digestion can be carried out only under the following conditions:

• absence of oxygen, nitrates or sulphates (Degremont, 2004).

• pH close to neutrality: optimum 6,8 — 7,5 (Moletta, 2002)

• concentration in volatile fatty acid (VFA) lower than 2 — 3 g/l (McCarty, 2001).

• a partial hydrogen pressure: 10 — 20 Pa to the maximum (Trably, 2002)

• a potential of oxydoreduction lower than -300 mV (Suh & Roussaux, 2002)

• absence of inhibiting elements: agent chlorinated, antibiotic,…

• an optimal stable temperature for micro-organisms (Bitton, 1994)

3. Advantages and disadvantages of anaerobic digestion

The advantages of anaerobic digestion are:

• A reduction of about 50% of the dry waste;

• A production of a Biogas which may undergo beneficiation in the form of energy (heating, cogeneration of electricity);

• A reduction of the number of pathogenic micro-organisms;

• An agronomic interest, related to a significant phosphate and ammoniacal nitrogen concentration (NH4 + (PO4 3) due to the lysis of the organic matter (Munch & Greenfield, 1998);

• Request lower energy compared with aerobic processes ;

• the possibility of treating high organic loads: from 2 to more than 80 kg of COD per cubic meter of digester and per days, with a treatment rate from 80 to 98%

However, it has also certain disadvantages:

• High sensitivity to the toxic compounds (Schnurer & al., 1999);

• A slower degradation compared with aerobic processes (Bitton, 1994);

• Significant capital costs ;

• The growth kinetics of bacteria is low, the pretraitement kinetics is also low and the time needed for the treatment is relatively long;

• the microbial populations are sensitive to the disturbances, in particular with oxygen and with heavy metals (OTV, 1997);

• the treatment by anaerobic digestion is often insufficient to directly reject the effluents in the natural environment: an aerobic postprocessing is necessary to complete the elimination of carbon and possibly nitrogen and phosphorus.

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