Biomethane Production from the Two-Stage Anaerobic Co-Digestion of Cow Manure: Residual Edible Oil with Two Qualities of Waste-Activated Sludge

Wastewater treatment systems produce large volumes of sludge which is not used; its final disposal is in soil or landfill. This sludge represents a biomethane-energy alternative through anaerobic co-digestion, contributing to reducing the environmental impacts caused by their inadequate disposal. Biomethane production by the two-stage production method in batch digesters with pH and temperature control was evaluated by two qualities of waste-activated sludge (SLB50 and SLB90) and with a mixture of two co-substrates: cow manure (CEV50 and CEV90) and residual edible oil (CAV50 and CAV90). Bacteria in good-quality sludge (SLB90) showed a faster adaptation of 2 days than those in low-quality sludge (SLB50), with a 25-day lag phase. The highest CH4 production was for SLB90 (303.99 cm3 d-1) compared to SLB50 (4.33 cm3 d-1). However, the cow manure-sludge mixture (CEV90) contributed to the increased production of CH4 (42,422.8 cm3 d-1) compared to CEV50 (12,881.45 cm3 CH4 d-1); for CAV90 and CAV50, these were 767.32 cm3 d-1 and 211.42 cm3 d-1, respectively. The addition of sludge co-substrates improves the nutrient balance and C/N ratio; consequently, methane production improves. This methodology could be integrated into concepts of the circular economy.