COMPARATIVE PERFORMANCE OF ANAEROBIC DIGESTERS OPERATING ON ICE-CREAMWASTE-WATER

Pilot-scale anaerobic digesters were operated on ice-cream wastewater for over three years. The performance of four reactor designs, an anae robic filter, contact process and UASB of capacity 5 m(3), and a 0.5 m (3) fluidised bed, was compared. The anaerobic filter, with a 3.3 m(3) Pall ring bed, operated stably at organic loading rates (B-v) around 6 kg COD m(-3) d(-1), giving total COD removals around 67%. The contac t process gave consistently good total COD removals of 80%, but was li mited by the poor performance of the settling compartment. Mixed liquo r suspended solids did not rise above 3 kg m(-3), and this reactor ope rated at B-v of 1 kg COD m(-3) d(-1). The fluidised bed reactor, opera ted on sand or granular activated carbon support media at B-v of 4 and 2 kg COD m(-3) d(-1) respectively, gave about 60% total COD removal, but difficulty was experienced with GAC break-up. The UASB reactor gav e the poorest performance, (approximately 50% total COD removal at a B -v of 2 kg COD m(-3) d(-1)) since successful granulation was not achie ved. The anaerobic filter was also operated for 9 months after removin g half of its packing material, but the former loading rate could not be achieved without instability. Poor biomass retention in all reactor s, possibly related to the fat content of the wastewater, was the limi ting factor in performance. Using experience gained at pilot scale, a full-scale upflow anaerobic filter was subsequently installed on an ic e-cream factory site. This reactor showed similar performance to the p ilot-scale filter.