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.