Pilot-scale anaerobic co-digestion of municipal wastewater sludge with restaurant grease trap waste

• A test and a control pilot-scale (1200 L) anaerobic digester were operated.
• The maximum grease loading amounted to 23% of the 1.58 kg VS/(m3 d) loading.
• This loading enhanced biogas production by 67% relative to the control digester.
• VS removals at this loading increased 1.53 fold relative to the control digester.
• Biogas production declined when the grease was increased to 30% of the VS loading.

The maximum feasible loading rate of grease trap waste (GTW) to the municipal wastewater sludge (MWS) was investigated using two 1300 L pilot-scale (1200 L active volume) digesters under mesophilic conditions at a 20 day solids retention time. During the co-digestion, the test reactor received a mixture of GTW and MWS while the control reactor received only MWS. The test digester loading was increased incrementally to a maximum of 280% of the control digester COD loading. The highest feasible GTW loading was determined to be 23% and 58% in terms of its total 1.58 kg VS/(m3 d) and 3.99 kg COD/(m3 d) loadings, respectively. This test digester COD loading represented 240% of the control digester COD loading. At this loading, test digester biogas production was 67% greater than that of the control. During the test digester quasi steady state loading period when VS from GTW represented 19% of its total VS loading, the test digester COD and VS removal rates were 2.5 and 1.5 fold those of the control digester, respectively. The test digester biogas production declined markedly when the percentage of VS from GTW in its feed was increased to 30% of its total VS loading. Causes of the reduced biogas production were investigated and attributed to inhibition due to long chain fatty acid accumulation.