Anaerobic digestion can adapt to free ammonia to a certain extent. During the anaerobic digestion of slaughterhouse waste, however, an ammonia concentration of up to 15 g N l−1 can be reached in the sludge liquid and this will even inhibit adapted sludge. To lower this concentration, a fraction of the digester liquid must therefore be continuously separated from the digested sludge and the free ammonia stripped before the liquid is recycled to the digester. A mesophilic laboratory digester was successfully operated with an ammonium concentration of 4–5 g l−1 and a pH of 8.0–8.4. After free ammonia stripping, the excess liquid was treated in a laboratory SBR for nitrogen and phosphorus removal before being added to the receiving water. The effluent had no toxic effect on daphnia and algae.

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