COMMISSIONING AND RE-DESIGN OF A “CLASS A” THERMAL HYDROLYSIS FACILITY FOR PRE-TREATMENT OF PRIMARY AND SECONDARY SLUDGE PRIOR TO ANAEROBIC DIGESTION

Thermal hydrolysis of sewage sludge as a pretreatment technique for producing a “Class A” sludge, with existing (or new) mesophilic anaerobic digestion systems are presently being considered for wastewater treatment plants in the USA and Canada. In Europe, a number of innovative hydrolysis systems have been installed or are planned. One technology sold under the trade name CAMBI™ has been used in a number of WWTPs in Norway, Denmark, and the United Kingdom. The CAMBI™ process utilizes high temperature and pressure to hydrolyse and disrupt sludge producing a pasteurized feed to downstream digesters that is more homogeneous and more amenable to high rate mesophilic anaerobic digestion.As a result of project construction scheduling, initial commissioning of the CAMBI™ process at the Dublin WwTW was planned on primary sludge. The hydrolysis process was expected to operate at an average dry solids concentration of between 13.5% and 15%. However, a maximum dry solids concentration of only 6 – 8 % in the reaction stream was initially achievable. In addition, at these low solids concentrations, significant clogging of the single pass tube-in-tube heat exchangers occurred, resulting in random pressure spikes that were 300% of the expected values. These pressure spikes eventually resulted in daily plugging of the heat exchangers. Investigation of the plugged heat exchangers showed severe build up of a fatty hydrocarbon layer on all of the tubes and typically dewatered plugs of fibrous primary sludge.Various blends of digested sludge and hydrolyzed sludge were prepared to check flow properties. A volumetric blend of 67:33 digested:hydrolyzed sludge introduced at the head of the heat exchanger banks proved successful at eliminating the pressure spikes. In addition the formation of the hydrocarbon layer on the heat exchanger tubes has been significantly reduced. The performance of the hydrolysis and digestion processes were not compromised by the internal recycle of digested sludge as evidenced by volatile solids destruction, gas production, digester pH, and gas % methane.