Clarifying the Role of Free Ammonia in the Production of Short-Chain Fatty Acids from Waste Activated Sludge Anaerobic Fermentation

Free ammonia (FA) could accumulate at high levels in the sludge anaerobic fermentation, especially under alkaline fermentation conditions, which might significantly affect the anaerobic fermentation. However, its role in the sludge fermentation process has not been revealed fundamentally. This work therefore aims to fill the knowledge gap through the integration of experimental and mathematical approaches. Experimental results showed that when the initial ammonium concentration increased from 20 to 300 mg/L, the maximal short-chain fatty acid (SCFA) yield from fermentation systems with different pH values varied from 91.2 to 296.7 mg of chemical oxygen demand/g volatile suspended solids (VSS). The increasing SCFA production was observed to correlate with the FA level rather than the ammonium level, suggesting that FA, instead of ammonium, is likely the true contributor to enhance SCFA production. Batch tests confirmed that ammonium in the fermentation-strength range (e.g., 0–300 mg/L) did not affect any process of sludge fermentation, but all the processes were affected significantly by FA, pH, or combined FA-pH. It was found that FA facilitated sludge disintegration but inhibited the processes of hydrolysis, acidification, and methanogenesis. When FA and alkaline conditions were combined, synergistic effects on all these processes were observed. The significant contribution of FA to SCFA production was finally confirmed by a sludge fermentation mathematical model proposed recently. The findings reported here revealed the actually existing, yet previously unrecognized contributor to the sludge fermentation, which help engineers better understand the role of FA in sludge anaerobic fermentation.