Greywater use in anaerobic digestion of cattle manure: Advancing biogas yield, energy efficiency, and digestate valorization for environmental sustainability in semi-arid regions

Brazil's semi-arid region faces acute water scarcity challenges affecting 28 million inhabitants, with rural communities particularly vulnerable due to inadequate sanitation infrastructure (75% lack proper sewage treatment) and limited access to reliable water sources. The main scientific challenge lies in developing integrated waste-to-energy solutions that can operate effectively under water-constrained conditions while addressing both energy security and environmental sustainability. The objective of this study was to evaluate the feasibility of substituting conventional water with domestic greywater (GW) in anaerobic co-digestion of cattle manure (CM) under semi-arid conditions. Bench-scale bioreactors (350 mL working volume) were operated under mesophilic conditions (36 ± 2 °C) with systematic evaluation of varying GW dilution ratios (25%, 50%, 75%, and 100% v/v) over 76 days. Biogas production kinetics were analyzed using the modified Gompertz equation, and comprehensive physicochemical characterization was performed using standard analytical methods. Physicochemical characterization revealed CM had high nutrient density and organic matter content, while GW exhibited low total solids and chemical oxygen demand, positioning it as a viable co-substrate. Maximum biogas production increased from 951.9 ± 55.7 mL g VS^-1 (control) to 1235.0 ± 43.8 mL g VS^-1 (100% GW). Wobbe Index values improved from 31.10 ± 1.65 MJ m^-3 (control) to 53.29 ± 2.76 MJ m^-3 (25% GW), confirming enhanced biogas quality. Multielemental analysis by inductively coupled plasma optical emission spectrometry identified low concentrations of inhibitory trace metals in GW. Digestate phytotoxicity assessment using Lactuca sativa germination assays demonstrated germination indices above 100% at 10% dilution for most treatments, indicating biostimulant potential. The results demonstrate that GW-assisted anaerobic digestion significantly enhances biogas yield (up to 30% increase) while producing nutrient-rich digestates suitable for agricultural application, offering a sustainable solution for integrated waste and water management in water-scarce rural regions.