Improving methane production during the codigestion of waste-activated sludge and fatty wastewater: Impact of thermo-alkaline pretreatment on batch and semi-continuous processes

► Pretreatments were applied to a mixture of sludge and fatty wastewater before codigestion. ► Recalcitrant compounds were formed during 170°C pretreatment of fatty wastewaters. ► Thermo-alkaline pretreatments increased the rate during batch anaerobic digestion. ► Semi-continuous methane production was 58% higher after pretreatment at 80°C, pH=8. ► Semi-continuous digestion of a high lipid content mixed waste (73% COD) was stable. With the objective of maximising methane production from waste-activated sludge (WAS), codigestion with fatty wastewater (FW) collected from restaurants was studied and the application of both a 170°C thermal pretreatment and thermo-alkaline/saponification to mixed WAS/FW waste was evaluated. Batch anaerobic digestion tests showed that recalcitrant compounds were formed during the 170°C pretreatment of fatty wastewater and thermo-alkaline pretreatments (80°C or 120°C, pH 8, 9 or 10) led to an increased initial methane production rate and to a very slight impact on the methane potential of the mixed waste (+4–7%). Semi-continuous anaerobic digesters were fed over 4months with WAS, two mixed substrates WAS/FW 90/10 and WAS/FW 60/40, and with WAS/FW 90/10 pretreated at softer conditions (80°C and pH=8, 0.14gKOHgVS-1). This pretreatment led to a significant increase in the methane production of semi-continuous reactors (+58%). Finally, this study showed the feasibility of the codigestion of waste activated sludge with highly concentrated fatty wastewater (40% in volume, 49% in VS and 73% in COD, equivalent to 7gL−1 of lipids). It resulted in specific methane production of 362mLCH4g−1 VS whereas WAS alone produced only 116mLCH4g−1 VS.