Effect of ammoniacal nitrogen on one-stage and two-stage anaerobic digestion of food waste

•Almost 100% of the biomethane potential of food waste was recovered during AD in a two-stage CSTR.•Recirculation of the liquid fraction of the digestate provided the necessary buffer in the AD reactors.•A higher OLR (0.9gVS/L·d) led to higher accumulation of TAN, which caused more toxicity.•A two-stage reactor is more sensitive to elevated concentrations of ammonia.•The IC50 of TAN for the AD of food waste amounts to 3.8g/L. This research compares the operation of one-stage and two-stage anaerobic continuously stirred tank reactor (CSTR) systems fed semi-continuously with food waste. The main purpose was to investigate the effects of ammoniacal nitrogen on the anaerobic digestion process. The two-stage system gave more reliable operation compared to one-stage due to: (i) a better pH self-adjusting capacity; (ii) a higher resistance to organic loading shocks; and (iii) a higher conversion rate of organic substrate to biomethane. Also a small amount of biohydrogen was detected from the first stage of the two-stage reactor making this system attractive for biohythane production. As the digestate contains ammoniacal nitrogen, re-circulating it provided the necessary alkalinity in the systems, thus preventing an eventual failure by volatile fatty acids (VFA) accumulation. However, re-circulation also resulted in an ammonium accumulation, yielding a lower biomethane production. Based on the batch experimental results the 50% inhibitory concentration of total ammoniacal nitrogen on the methanogenic activities was calculated as 3.8g/L, corresponding to 146mg/L free ammonia for the inoculum used for this research. The two-stage system was affected by the inhibition more than the one-stage system, as it requires less alkalinity and the physically separated methanogens are more sensitive to inhibitory factors, such as ammonium and propionic acid.