VOLATILE FATTY-ACID PRODUCTION IN AEROBIC THERMOPHILIC PRETREATMENT OF PRIMARY SLUDGE

The efficiency of volatile fatty acid (VFA) production in a thermophil ic aerobic digester (TAD) process using primary sludge was studied und er two oxygenation states and detentions times ranging from 12 to 24 h ours. The highest VFA concentration increase occurred in the 18 hour a naerobic aerated digester (O-2 flow rate: 0.025 m(3)/m(3) . h; ORP: le ss than -300 mV) from 0.047 mg HAc/mg VSS in the feed to 0.106 mg HAc/ mg VSS in the effluent. The anoxic condition (O-2 flow rate: 0.14 m(3) /m(3).h; ORP: between 0 to -225 mV) resulted in VFA utilisation and a final concentration of only 0.001 mg HAc/mg VSS, Under the anaerobic a erated condition, acetic acid constituted the largest fraction of shor t short chain volatile fatty acids at an average of 60.4% and propioni c trailed at 19.3%. The fraction of butyric and valeric acids were 12. 2% and 8.1% respectively. It was found that with a decreasing oxygen s upply and a decreasing detention time (HRT), the soluble organic carbo n to ammonia ratio (SOC:NH3) increased. (C) 1997 IAWQ. Published by El sevier Science Ltd.