Effect of C/N ratio and ammonia-N accumulation in a pilot-scale thermophilic dry anaerobic digester

The effect of ammonia-N accumulation in a dry anaerobic digestion was studied effectively using pilot-scale thermophilic reactor. Two simulations were prepared to attain C/N ratio 27 and C/N ratio 32 using bio-degradable feedstocks such as food waste, fruit and vegetable waste, green waste and paper waste. Organic loading rates and digestate recirculation rates were varied during different time intervals and the performance was evaluated using parameters like pH, VFA, Alkalinity, ammonia-N and biogas yield. Results showed that the simulation with C/N ratio 32 had about 30% less ammonia in digestate as compared to that with C/N ratio 27. The system performed well up to organic loading rate (OLR) 7–10 kgVS/m3 d and retention time up to 19 days, with surplus energy production of 50–73%. Moreover, a free ammonia accumulation/inhibition effect was documented and methods to overcome the adverse effects were discussed.