Comparison of mesophilic and thermophilic upflow anaerobic sludge blanket reactors treating instant coffee production wastewater

Stable anaerobic digestion of settled instant coffee wastewater was achieved for over 100 days in mesophilic (35°C) and thermophilic (55°C) UASB reactors. Thermophilic upflow anaerobic sludge blankets (UASBs) were seeded with mesophilic granules and converted to thermophilic operation by raising the temperature to 55°C in one step. Successful thermophilic operation was achieved within 28 days. Both mesophilic and thermophilic UASBs achieved stable digestion at organic loading rates (OLR) of up to 10 kg COD m^-3d^-1 (hydraulic retention times (HRT) 24 h). Higher OLRs to the mesophilic resulted in reactor failure due to increasing total volatile fatty acid (TVFA) levels. The thermophilic UASB achieved stable operation at an OLR of 11.4 kg COD m^-3d^-1 (21 h HRT) but an OLR of 13.3 kg COD m^-3d^-1 (18 h HRT) saw a rise in TVFA from 80 to 600 mg l^-1. COD reduction in the thermophilic UASB was slightly lower at all OLRs, achieving a 70% COD reduction compared to a 78% COD reduction in the mesophilic UASB. The average TVFA levels were low, but somewhat higher in the thermophilic UASB than the mesophilic UASB (100 mg l^-1 compared to 25 mg l^-1). It is concluded that either mesophilic or thermophilic digestion could be used successfully to treat settled instant coffee production wastewater in UASB reactors.