ANAEROBIC DIGESTION AND THERMAL HYDROLYSIS TO REDUCE PRODUCTION OF SLUDGE IN WWTPs
Mesophilic and thermophilic digestion of primary and activated sludge was investigated in lab scale reactors. Mixed primary and activated sludge was digested in one experiment and activated sludge was digesred alone in another. The same gas yield, COD and dry solids reducrion was obtained with both mesophilic and thermophilic digestion. However, more dissolved COD was found afrer thermophilic digesrion. A gas yield of370 and 225 1/kg dry solids was found for mixed sludge and activated sludge respectively. The filterability of the sludge decreased after thermophilic digestion, bur a. pre-treatment with addirion offerric chloride or elutriation with water prior to addition of polymer provided an acceptable filterability. Thermal hydrolysis at 180°C was tested as a pre-trearment prior to the digestion. This increased the gas yield by 22 % for mixed sludge and 24 % for activated sludge. Thermal hydrolysis further seemed to improve the dewaterability of the sludge after digestion.