Effect of ammonia and nitrate on biogas production from food waste via anaerobic digestion

•We examine the effect of ammonia on anaerobic digestion performance of food waste.•We evaluate the effect of NO3–N at a TAN addition of 1 g l−1 on methane production.•No inhibitory effect on methane production at added TAN concentration below 1.1 g l−1.•Added NO3–N concentrations below 750 mg l−1 cause increase in methane production.•Nitrification may be effective for removing ammonia below 2.29 g l−1 from digestate.

Ammonia accumulation is potentially encountered in anaerobic digestion of food waste, restricting its application in industrial biogas plants. This bench scale study investigated the suitability of nitrification process for ammonia removal from food waste digestate in the recirculated anaerobic digestion system. The effect of ammonia, in terms of total ammonia nitrogen (TAN) on anaerobic digestion of food waste was evaluated over an added concentration range of 0–4.5 g l−1. It was observed that lower TAN concentrations (<1.54 g l−1) were beneficial to anaerobic digestion, while higher TAN concentrations (>3.78 g l−1) caused an excessive inhibition of methanogenesis. In an attempt to simulate conditions of recycling digestate after nitrification treatment into the recirculated anaerobic digestion system, the impact of nitrification products on anaerobic digestion performance was investigated by employing the nitrate as a variable compound with an added TAN concentration of 1.0 g l−1. The results showed that no inhibition of methane production occurred at added NO3–N concentrations below 0.75 g l−1. A maximum methane yield of 314.7 ml g−1 VSadded was obtained at added NO3–N concentration of 0.5 g l−1, which was 11.8% higher than the yield when no nitrate was added. However, methane yield was reduced by 50.8% when added NO3–N concentration exceeded 1.5 g l−1. The results suggest that the nitrification process can be potentially suitable for ammonia removal when the TAN and NO3–N concentrations in the anaerobic reactor after digestate recirculation were less than 1.54 g l−1 and 0.75 g l−1, respectively.