Effluents (digestates) resulting from anaerobic digestion of wet organic wastes can berecycled as fertilizers. Separation of such digestates into a wet and a dry fraction is common.The solid fraction is often sold as a soil enhancement product while the liquid fraction isusually discarded as wastewater. A large portion of the digestate nutrients is dissolved as ionsin the liquid. The aim of this study is to establish an efficient way to convert this into a liquid"organic fertilizer". Enhancement of the nutrient concentration is necessary in order to makethe final product commercially acceptable. Direct evaporative concentration is not suitable asit would lead to a significant loss of ammonia due to the fact that most of the availablenitrogen in these digestates is present in the ammonium/ammonia form. Thus stabilizing theproduct by partial nitrification prior to evaporation is proposed based on a series ofexperiments conducted to evaluate the appropriateness of this approach.The ammonium-N concentration of the digestate used was 1.7 g/L. The nitrification does notappear to be inhibited by ammonia, nitrate or nitrite accumulation, except when pH > 7.5, asituation that can be avoided by regulating the feeding rate based on pH measurements.Significant nitrite accumulation was not observed either. Nitrification is however consistentlylimited by alkalinity so that only about 75% of the ammonia can be converted to nitratewithout alkalinity supplementation. The nitrification brings the pH down below 5.0 where theremaining ammonia is present as> 99% NH/. At this condition the nitrified digestate can beevaporated without a significant nitrogen loss and the product is recognized to be a highquality liquid fertilizer. It is suggested that the production of concentrated partially nitrifiedorganic fertilizers can become a sustainable way of nutrient recovery from anaerobicdigestates originating from municipal organic wastes.
Green chemistry of nutrient recovery: From mechanochemistry to reduced nitrogen loss
