Digestate Nitrification for Nutrient Recovery

ABSTRACT

A large portion of nutrients is dissolved in the liquid fraction of the effluents (digestates) resulting from anaerobic digestion (AD) of wet organic wastes. The aim of this study is to establish an efficient way of converting such digestates into liquid “organic fertilizer”. Enhancement of the nutrient concentration is necessary in order to make the final product commercially acceptable. Direct evaporative concentration is not suitable, as it would lead to a significant loss of ammonia. Thus, stabilizing the product by nitrification prior to evaporation was proposed, and a series of experiments was conducted to evaluate the appropriateness of this approach. The study was conducted using the digestate (containing 1.7 g/L NH₄-N) from a full-scale biogas plant in Norway. The process was further analyzed by modelling and simulations using ASM 3, which was found to be an appropriate biochemical model for designing such digestate nitrification plants. The digestate was successfully nitrified to achieve above 75 % NH₄-N conversions without any addition of extra alkalinity. The nitrification brings the pH down to below 5.0 where the remaining ammonia is present as > 99 % NH₄⁺. In this condition the nitrified digestate can be evaporated without significant nitrogen (NH₃ gas) loss. The toxic metal content of the nitrified liquid fertilizer is much lower than that of the original digestate. The nitrified digestate gained superior aesthetic quality as an almost translucent and odourless liquid. It is concluded that effluents from anaerobic digesters operating on municipal organic wastes can successfully be converted into a high quality commercial grade liquid fertilizer through post anaerobic nitrification.