Thermophilic anaerobic digestion and pasteurisation. Practical experience from Danish wastewater treatment plants

Increasing sludge disposal costs have highly intensified the interest in re ducing the sludge quantities from Danish wastewater treatment plants. By up grading existing mesophilic digesters to the thermophilic temperature range , the retention time can be halved and many digesters designed only for pri mary sludge will have sufficient capacity to treat also the biological exce ss sludge. At the moment, eight full-scale thermophilic digesters are in operation in Denmark and five are under construction. This paper describes the full-scal e experience gained from digestion of biological excess sludge as well as a mixture of primary and biological sludge. Thermophilic digestion has proven to be a good and stable process for solid s reduction and pathogen removal. The digested sludge can be dewatered to a high solids content and thereby the sludge quantity for disposal can be re duced by 30-40% depending on the type of wastewater treatment plant. A draw back of the process is that the polymer costs for sludge dewatering may be increased depending on the sludge type.