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.

Sustainable crop production in nutrient-poor sandy soils requires fertilization strategies that improve nutrient uptake while reducing environmental impact. This study evaluated anaerobic cattle manure digestate and carbon dot biostimulants as alternatives to conventional mineral NPK (nitrogen–phosphorus–potassium) fertilizer for corn (Zea mays L., cv. AG 1051) during vegetative development. A randomized greenhouse experiment compared nine treatments over three successive 45-day cycles, assessing shoot-tissue macronutrient content (N, P, K) and morphological parameters (shoot dry weight, stem diameter, and plant height). Digestate delivered approximately 1.4× more phosphorus and 8.4× more potassium per pot than mineral NPK, although nitrogen inputs were matched (~77 mg pot⁻¹). Digestate-based treatments achieved shoot dry weight 132% above control and 63% above mineral fertilizer (p < 0.001), with biomass advantages sustained across all three cycles while mineral fertilizer effects dissipated entirely by Cycle 3. Phosphorus content was the strongest biomass predictor (r = 0.86, p < 0.001), and a significant nitrogen–phosphorus antagonism (r = −0.59, p < 0.001) revealed relevant nutrient interaction dynamics. The higher biomass observed under digestate-based treatments reflects both the higher total P and K inputs from digestate and the beneficial effects of organic matter on nutrient bioavailability in this phosphorus-limited system. Carbon dot biostimulants did not improve biomass when applied alone (values at or below control), but they contributed to intermediate biomass gains when combined with nutrient sources, functioning as nutrient uptake enhancers rather than standalone fertilizers. Principal component analysis (74.3% variance explained) classified the nine treatments into three distinct treatment clusters. These findings support digestate valorization as a circular-economy alternative to conventional mineral fertilization, offering higher biomass under N-equivalent application and sustained residual effects in nutrient-poor sandy soils.