ANAEROBIC TREATMENT OF KRAFT PULP-MILL WASTE ACTIVATED-SLUDGE - GAS-PRODUCTION AND SOLIDS REDUCTION

Anaerobic digestion of kraft pulp-mill waste activated-sludge was studied in a pilot-scale unit. The anaerobic digester was operated for 21 months over a wide range of volatile solids (VS) loading rates (1.5-5.2 kg VS/m3 day; Hydraulic Retention Time, HRT, 24-8 days) to establish the optimum operational conditions for solids reduction and biogas production. The process started up in 6 weeks using a mixture of municipal-digester sludge and sediment slurry, which was acclimated to kraft pulp-mill effluent, as seed material. Digestion of sludge containing about 38% lignin showed the median VS removal of 40% with the median biogas production of 220 litres/kg VS added or 570 litres/kg VS removed. The respective 90 percentiles were 60% for VS reduction and 350 litres/kg VS added or 780 litres/kg VS removed for biogas production. Optimal process performance was obtained at the VS loading of 2.2 kg VS/m3 day. Alkali addition (13 g NaOH/kg VS) to feed sludge, and sludge recycle (with a ratio of 0.25) were required for stable operation at VS loadings exceeding 1.5 kg VS/m3 day. The anaerobic process remained stable even at the VS loading of 5.2 kg/m3 day.