The stillage that cannot be used as animal feed and that has high BOD and COD values can be treated by anaerobic digestion. During this process, the transforma­tion of the organic matter by a mixed bacterial culture is carried out in such a way that the BOD is reduced and the resulting sludge can be more easily disposed of, i. e., it is stabilized. Wilkie et al. (2000) have reviewed the conditions for accom­plishing the anaerobic digestion of different stillage types under both mesophilic and thermophilic conditions and have reported the main configurations of the equipment required by this process. COD reductions of 97% for corn stillage in an up-flow anaerobic sludge blanket (UASB) reactor, as well as 94% reduction of the soluble COD of sugarcane stillage, have been achieved. For stillage derived from wheat starch, some studies at pilot plant scale have shown 89 to 92% reduc­tions of COD (Nguyen, 2003).

The production of sludge by anaerobic digestion is about 10% lower than the sludge produced by aerobic digestion. This anaerobic sludge can be used as a feed­stock for producing components for balanced animal feed (Wilkie et al., 2000). The effluents from anaerobic digestion contain plant macronutrients (nitrogen, phosphorous, and potassium), as well as micronutrients (iron, zinc, manganese, copper, and magnesium), so they could be applied to plantations ensuring an appropriate dosage. These nutrients should be removed when the liquid efflu­ents are to be directly discharged into the natural water streams. If they are not removed, eutrophication problems can arise. These nutrients can be removed through the cultivation of algae in special ponds containing these effluents. In general, the stillage treated by anaerobic digestion should undergo color removal through different methods, including flocculation and coagulation, photovoltaic removal, or microbial removal (Wilkie et al., 2000). One alternative for degrada­tion of the organic matter contained in the stillage is the use of aerobic processes like high-rate oxidation ponds for those cases when low cost lands are available (Olguin et al., 1995).