AEROBIC THERMOPHILIC WASTE SLUDGE TREATMENT

The expansion in both industrial and municipal wastewater treatment in recent years has resulted in a major increase in the quantities of by-product sludge produced and has exacerbated problems of waste sludge treatment and/or disposal. The traditional method for waste sewage sludge treatment is anaerobic mesophilic digestion for sludges that are to be disposed of on agricultural land. Other disposal methods for untreated sludges include incineration, an option receiving increased interest, and ocean dumping, an option that is environmentally incompatable and used much less frequently today compared with 10 years ago. Sewage sludge can be considered to be a resource as far as its mineral nutrient composition is concerned. However, this resource cannot be exploited in agriculture because sludges are inevitably contaminated with noxious chemicals that partition into the sludge during either primary or secondary wastewater treatment. In the case sewage sludge, pathogenic and potentially pathogenic organisms are also present unless such contaminants are removed during treatment. Traditional mesophilic treatment under anaerobic conditions does not remove either noxious chemicals such as detergent residues or pathogenic organisms to a satisfactory degree. During the past decade, autothermal aerobic thermophilic pretreatment processes have been introduced as a complementary sludge treatment stage. Such aerobic pretreatment processes allow both the biodegradation of chemicals that are recalcitrant to anaerobic treatment and the thermal inactivation of pathogenic organisms. However, their introduction as a total treatment process for sludges is inhibited by their relatively poor conversion efficiencies as far as mineralization is concerned. In this contribution the biodegradation bacteria under aerobic thermophilic conditions will be described and discussed and concepts for biomass yield coefficient reduction that could enhance aerobic thermophilic sludge treatment process effectiveness will be introduced.