Revealing the synergy mechanisms of organic components anaerobic co-digestion from the prevailing tendency of endogenous inhibitors

The anaerobic co-digestion (AcoD) test among three organic components (model compounds) under high feeding load was carried out in this study, then the dynamic change of intermediate metabolic inhibitors was detected and analyzed to identify the interaction mechanism among organic components from the prevailing tendency of endogenous inhibitors. Carbohydrate and protein show significant two-way synergistic effects for methane production. The addition of protein can supplement additional alkalinity to alleviate the sharp drop of pH (from 4.56 to 6.73) and avoid the acidification caused by high concentration of carbohydrate. Additionally, volatile fatty acids (VFAs) produced by carbohydrate can transform free ammonia to ammonium ion with the largest drop of 54%, thus reducing ammonia toxicity. Carbohydrate and lipid show one-way synergistic effects, namely, lipid can reduce VFAs accumulation by 49% through long chain fatty acids (LCFAs) inhibiting action, and then avoid complete rancidity of carbohydrate fermentation at high concentration. However, protein and lipid show serious two-way antagonistic effects, especially under high load digestion. On one hand, LCFAs would weaken ammonia volatilization and thus aggravate ammonia inhibition; On the other hand, protein would not only accelerate the formation of LCFAs inhibition by promoting lipid hydrolysis, but also transform the degradation pathway of oleic acid (unsaturated LCFAs) and then increase the proportion of stearic acid from 5.17% to 40.33%, which is of higher toxicity than palmitic acid.