Effect of Inoculation Ratio on the Anaerobic Co-Digestion of Intensive Dairy Farm Wastewater and Sewage Sludge: Gas Generation, VFA Composition, and Process Stability

Abstract

Intensive dairy farm wastewater (DFW) poses significant environmental challenges due to its high organic loading and complex composition. Anaerobic co-digestion with sewage sludge (SS) offers a promising strategy for simultaneous pollutant removal and bioenergy recovery. However, the optimal inoculation ratio for maximizing both methane production and volatile fatty acid (VFA) accumulation remains unclear for liquid DFW following a solid–liquid separation. This study investigated the effects of three SS addition ratios (0%, 15%, and 45%) on anaerobic co-digestion performance. Daily methane production, cumulative yield, VFA composition, pH, electrical conductivity (EC), ammonium nitrogen (NH4+-N), and chemical oxygen demand (COD) were monitored over 26 days. The 45% SS treatment (S45) achieved the highest cumulative methane yield (2882.60 mL), representing 35.1% and 7.2% increases over S0 and S15. Modified Gompertz modeling confirmed S45 attained the highest methane potential (3056.8 mL) and production rate (569.8 mL/d), with the shortest lag phase (14.47 d). S45 also reached the highest total VFAs peak (2209.02 mg/L) on day 3, advancing acidification by 3 days. Process stability was maintained across all treatments (pH 6.94–8.44), with S45 showing the earliest pH recovery and lowest NH4+-N accumulation. COD removal in S45 exceeded S0 by 25.1% at day 26. These findings indicate that 45% SS addition optimally balances methanogenic performance, acidogenic efficiency, and process stability in DFW anaerobic co-digestion.