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.

The quality of waste activated sludge produced in urban wastewater treatment systems varies according to the efficiency of the operation of treatment units along the content of microflora and organic carbon. However, these represent a biogas-energy alternative through anaerobic co-digestion, contributing to reducing the environmental impacts caused by their inadequate disposal. Biogas production by the Two-stage production method in batch digesters was evaluated by two qualities of waste activated sludge (SLB50 y SLB90) and with a mixture of two co-substrates: cattle manure (CEV50 y CEV90) and residual edible oil with mixing, pH and temperature control. Bacteria in good quality sludge (SLB90) showed a faster adaptation of 2 days than those in low quality (SLB50) with 25 days lag phase. The highest CH4 production was for SLB90 (303.99cm3 d-1) compared to SLB50 (4.33 cm3 d-1); while cow manure sludge mixture (CEV90) contributed to increasing production of CH4 (42 422.8 cm3 d-1) compared to CEV50 (12 881.45 cm3 CH4 d-1), while in residual edible oil mixture they were 767.32 cm3 d-1 and 211.42 cm3 d-1 for CAV90 y CAV50, respectively. The addition of sludge co-substrates improves the nutrient balance, C/N rate and micro flora diversity; consequently methane production improves too; this methodology could be integrated into concepts of Circular economy.