Enhanced Biohydrogen Production with Increasing Ammonia Nitrogen during Co-digestion of Swine Wastewater and Food Waste leachate

This study investigated the effects of ammonia nitrogen (NH3–N) concentration and the COD/NH3–N ratio on biohydrogen production during the acidogenesis phase of co-digestion with swine wastewater and food waste leachate. Acidogenic sludge obtained from a full scale co-digestor treating real swine wastewater (S) and food waste leachate (F) was used as the inoculum, and substrate mixing ratios of 8:2 (SF82) and 6:4 (SF64) were prepared. The fed-batch experiments were conducted after the ammonia concentration was adjusted to 2.1–5.2g NH3–N/L, corresponding to COD/NH3–N ratios of 10–40. The results showed that increasing ammonia concentration led to enhanced maximum hydrogen production (up to 31.6% for SF82 and 26.0% for SF64), hydrogen production rate (up to 75.3%), and hydrogen yield (up to 108.0%) under all conditions. Volatile fatty acid analysis indicated that hydrogen was mainly produced during lactate production and the lactate-to-butyrate conversion reaction. At lower ammonia concentrations, lactate accumulation delayed hydrogen production, while operation under weak acidic conditions (pH 5.8–6.0) suppressed methanogenic activity, thereby inhibiting methane production. In contrast, glucose fermentation used to elucidate the underlying mechanisms showed reduced hydrogen production when NH3–N exceeded 3.5g/L, confirming that enhanced hydrogen production at high ammonia levels is not attributed to direct stimulation of glucose fermentation. Overall, this study demonstrates that maintaining pH below 6.0, optimizing substrate mixing ratios, and high ammonia nitrogen concentration are critical factors for enhancing hydrogen production during the acidogenesis stage of anaerobic co-digestion of swine wastewater and food waste leachate.