Scalable Sludge-Biochar Matrix Relieves Ammonia and Foaming in the Anaerobic Treatment of Dairy Wastewater: Experiments and Mass–Energy–Economic Evaluation

Abstract This study investigates the effect of biochar on enhancing the stability and methane production of upflow anaerobic sludge blanket (UASB) reactors treating dairy wastewater. Two anaerobic reactors, one control and one amended with biochar as a sludge-biochar matrix, were operated under increasing organic loading rates. The control reactor exhibited excessive buildup of free ammonia nitrogen to 185 mg/L and persistent surface foaming, both signs of microbial stress and process inhibition. These issues corresponded with lower specific methane yields (as low as 87 N mL/g COD) and unstable pH levels (up to 8.3), especially under higher organic loading conditions. In contrast, the biochar-mediated reactor maintained low free ammonia levels (<15 mg/L), avoided persistent foaming, and maintained a stable pH range (7.1–7.4). As a result, methane yields increased to 308 N mL/g COD on the 24th day, with the COD-to-methane conversion efficiency improving to 88%. A scale-up assessment for a 500 m3/day dairy plant demonstrated a 68% potential to replace furnace oil (compared to a 52% baseline), with a 29.5% internal rate of return over 25 years. Monte Carlo uncertainty analysis revealed a higher net present value (NPV) variance associated with biochar, despite its superior deterministic economics, indicating that commercial viability requires risk mitigation through policy instruments, supply chain development, and extended operational validation. This work establishes the technical feasibility and economic potential of biochar-mediated ammonia control for decarbonizing the dairy sector and identifies critical implementation barriers that require stakeholder attention.