THERMAL HYDROLYSIS OF A SLUDGE PRIOR TO ANAEROBIC DIGESTION

When Black & Veatch submitted a bid for the Design, Construct and Operate plant for Dublin Eire, the sludge produced by the secondary treatment plant had to comply with EU directives for the disposal. This can be compared with a Class A sludge in the US. Twenty year life cycle costs had to be considered. The inaccessibility of the site on a Peninsula in the harbor of Dublin required minimization of the mass of final product. Drying to 95% was required. The selected system of treatment consisting of grease and grit removal, lamella primary sedimentation and secondary treatment, produced the following quantities of sludge: Fast and grease 12 t/d; Primary sludge 54 tonnes/d; Secondary sludge 55 t/d. The specifications required that the sludge volume be reduced and the sludge be dried to 92% dry solids, bagged and be fit for unrestricted use. The Cambit thermal hydrolysis process was selected for hydrolyzing the combined sludge prior to anaerobic digestion. This was to be followed by thermal drying of the final product. The bid price was on life cycle costs over a period of 20 years. This dominated all process decisions. Energy recovery was a prominent factor in the choice of process.