IMPACT OF ORGANIC NITROGEN LOADING ON THE ANAEROBIC DIGESTION PROCESS

Inhibition or toxicity thresholds for ammonia in anaerobic digesters are reported over a broad range in the literature. EBMUD conducted pilot studies at bench- and full-scale in mesophilic and thermophilic digesters to evaluate ammonia toxicity resulting from high organic nitrogen loads. Organic nitrogen loading was gradually increased while monitoring digester stability on the basis of total volatile acids, total alkalinity, volatile solids reduction, and fraction of methane in the digester gas. Based on monitoring these parameters it was observed that thermophilic digesters were relatively less stable than mesophilic digesters. Thermophilic digesters showed higher total volatile acid concentrations and greater volatility in total volatile acids to total alkalinity ratio. However, with gradual acclimation of the methanogens to the higher concentrations of ammonia, it was found that total ammonia concentrations of 4,000 mg/L-N could be tolerated in both thermophilic and mesophilic digesters prior to the inhibition of methanogenesis. It may be possible to operate mesophilic digesters at higher total ammonia concentrations.