**4.2 Anaerobic digestion**

Anaerobic digestion (AD) is used for the stabilization of particulate organic matter. An anaerobic digester is well mixed with no liquid-solids separation. Consequently, the bioreactor can be treated as a continuous stirred tank reactor (CSTR) in which the HRT and SRT are identical. An SRT of 15 to 20 days is typically used, although SRTs as low as 10 days have been used successfully and longer SRTs are employed when greater waste stabilization is required [8].

For several cannery waste streams, the recovery of useful by-products could be achieved by anaerobic digestion. High COD content fruit and vegetable wastes (>50,000 mg/l) have been treated successfully by AD using a HRT of 10 days and a sludge age of 80 days. For elder sludges, the SS build-up within the reactor reached 30,000 mg/l, but at higher concentrations settling became a problem. Generally, successful treatment of food processing wastes could be achieved using AD with a retention time greater than 10 days and gas production of up to 0.75 m3 /kg volatile solids [9, 11].
