Insights on anaerobic co-digestion of carbohydrate and protein at high load: synergy evolution and inhibition transfer mechanism

The anaerobic co-digestion (AcoD) of carbon-rich and nitrogen-rich substrates has significant synergistic effect in methane production, but its synergy mechanism is still unclear, which hinders the in-depth understanding of the synergistic effect in AcoD and the establishment of scientific proportioning rule based on organic components. This study determined the synergy evolution and revealed the two-way synergistic mechanism between carbohydrate and protein in AcoD system. The addition of certain protein can avoid the acidification of AD process with high-concentration carbohydrate by providing additional alkalinity to neutralize H⁺. However, the increased dosage of protein (≥60 %) would change the acid inhibition gradually to ammonia inhibition. The optimal dosage of protein was 40 % of carbohydrate with the highest methane conversion efficiency of 0.93, where that of carbohydrate mono-digestion was only 0.16. On the other hand, low proportion of carbohydrate can reduce ammonia inhibition during the AD process of high-concentration protein through promoting the conversion of free ammonia to ammonium ion, but the excessive addition (≥60 %) would significantly inhibit the volatilization of ammonia nitrogen and then aggravate the ammonia inhibition. The suitable dosage of carbohydrate was 40 %–50 % of protein with the optimal methane conversion efficiency of 0.68–0.71, where that of protein mono-digestion was only 0.55. These results provide a deep insight in the two-way synergistic mechanism between carbohydrate and protein in the AcoD process, and promote the concrete implementation of AcoD with multiple substrates.