AN EXAMINATION OF THERMOPHILIC ANAEROBIC-DIGESTION AS THE FIRST STAGEIN DUAL DIGESTION

Three 51 completely stirred tank reactors (CSTRs) were used to examine the thermophilic anaerobic digestion of waste activated sludge. The o bjective of the study was to determine the most appropriate hydraulic retention time for the use of such a system as the first stage of a th ermophilic/mesophilic dual digestion process. The hydraulic retention times used ranged from 48 hours to 8 hours. The results showed that, w ith hydraulic retention times in the range 18-48 hours, the digesters functioned as single-stage units with little accumulation of volatile fatty acids (VFAs). At lower hydraulic retention times, acids did accu mulate. Gas production showed a similar pattern except that the critic al retention time was 14 hours. The results show that a thermophilic C STR can be used as the first stage of a dual digestion process with lo w hydraulic retention times and, therefore, with a compact design.