AEROBIC THERMOPHILIC TREATMENT OF SEWAGE-SLUDGE AT PILOT-PLANT SCALE .1. OPERATING-CONDITIONS

The aerobic thermophilic treatment process of sewage sludge was studie d at different bioreactor scales in a pilot plant installation. Since, for a satisfactory sludge disinfection, the Swiss legislation require s minimal incubation times of all volume elements, the bioreactors wer e operated in repetitive batch mode (draw and fill). Different retenti on times and frequencies of the volume changes were applied in order t o prove the capability of the particular operation modes in assuring h igh degradative potential. The main enzymatic activity involved during the aerobic treatment was proteolysis: the RQ values ranged between 0 .8 and 0.9 depending on the applied operating conditions. Although not in a linear manner, the efficiency of the microflora decreased as the bioreactor scale increased, when this increase corresponded with a re duction of the specific power input. The sludge oxidation rates can be tuned by some process operating conditions such as the volume change frequency, the changed volume quantities and the retention times. It w as possible to improve the microbial degradative efficiency by an incr eased frequency of the changes, while the mean retention time influenc ed in particular the ultimate product quality, described as residual o rganic matter content of the sludge. The microflora present was also s atisfactorily active at mean hydraulic retention times of less than 10 h. The organic matter concentration of the inlet sewage sludge plays an important role: it influences the aerobic degradation process posit ively.