Valorization of Cheese Whey with Dairy Manure: Optimal Ratio Determination and Kinetic Analysis

Abstract

Cheese whey and dairy manure represent two of the largest organic effluents from the dairy sector, both associated with severe environmental externalities yet enriched with bioenergy potential. Their joint valorization via anaerobic co-digestion (AcoD) remains insufficiently characterized in terms of systematic substrate gradients and kinetic interpretation. This study performed controlled mesophilic batch assays spanning manure mono-digestion to whey loadings of 10–50% (volatile solids basis), replicated in triplicate to ensure statistical robustness. Results revealed a pronounced synergism, with cumulative methane yields increasing from 180 mL CH₄ g⁻¹ VS (manure only) to a peak of 350 mL CH₄ g⁻¹ VS at 40% whey inclusion. This enhancement was accompanied by accelerated microbial activation, as lag phases declined from 5.6 to <2.5 days and maximum methane production rates nearly doubled (µmax = 22.7 mL CH₄ g⁻¹ VS d⁻¹). However, 50% whey supplementation induced process destabilization through volatile fatty acid accumulation, delineating a critical threshold for sustainable co-digestion. Kinetic modeling with five empirical formulations underscored the predominance of sigmoidal functions. The Modified Gompertz model achieved superior fits (R² > 0.997; RMSE < 5), though Logistic functions more accurately captured ultimate methane yields under inhibition-prone conditions. Information-theoretic criteria (AIC, BIC) confirmed that non-sigmoidal models (First-Order, Cone, Feller) lack predictive reliability in multi-phase co-digestion systems. This work advances the mechanistic understanding of whey–manure synergism by defining optimal blending ratios and kinetic boundaries. The findings provide actionable guidelines for decentralized biogas systems, directly supporting renewable energy expansion, nutrient recycling, and circular bioeconomy transitions.