Assessing the use of crude glycerol from biodiesel production as an alternative to boost methane generation by anaerobic co-digestion of sewage sludge

In this work, the effect of the addition of crude glycerol (GL) (1 and 3% (v v−1)) from biodiesel production on anaerobic digestion of sewage primary sludge (PS), was evaluated through biochemical methane potential tests. Parameters such as pH, volatile acid/alkalinity ratio, chemical oxygen demand, volatile solids (VS) and carbon/nitrogen (C/N) ratio were monitored to assess process stability and performance. The methane yield from the co-digestion experiments of the binary mixture comprising PS and GL corresponded 223.8 mL CH4 g−1 VS and 368.8 mL CH4 g−1 VS for the tests supplemented with 1 and 3% GL, respectively, which represent an increase in methane yield of 61% and 167% compared to the control test conducted only with PS (138.2 mL CH4 g−1 VS). For each percentage increase in the applied VS load, the percentage increase in methane yield was found to be 4.7% and 5.7% for the tests with 1% and 3% glycerol, respectively. At the highest GL content, the maximum daily biogas generation rate was reached (i.e., 56.8 mL g−1 VS d−1). However, for this particular condition, an initial instability period with methanogenesis inhibition of 5.8 days was noticed, likely related to the accumulation of intermediate volatile acids. Overall, the results suggest that the use of crude glycerol as a co-substrate for the digestion of sewage sludge is interesting alternative to maximize the generation of biogas and methane, although its quantity should be controlled to avoid overload conditions which may compromise the stability of the system. •Effect of biodiesel-derived glycerol on anaerobic digestion of sludge was assessed.•Addition of 1% and 3% (v/v) of GL led to 62% and 167% methane yield increase.•An initial period of methanogenesis inhibition (5.8 days) was noticed at 3% glycerol.•Each percentage increase in VS load led to 5.7% CH4 yield increase for 3% glycerol.•Glycerol feed rates must be controlled so that process instability is avoided.