Current Technology on Nutrients Removal, Recovery and Reuse from Liquid Fraction of Digestate

Abstract

By-products of anaerobic digestion, digestate is commonly managed via several ways for its optimal transportion and application. The common practice for digestate management is through solid-liquid separation. The common use for solid fraction of digestates are either through land spreading; directly applied or after composting, as organic fertilizer. Several routes proposed for valorization of solid digestate include production of biochar, bio-fuel for domestic furnaces, bioethanol production after centrifugal milling as well as post treatments (enzymatic, thermal and alkaline) for the recovery of methane. Liquid fraction of digestates contain high concentration of nutrients; from 1.5 to 6.5 g/L total nitrogen and from 0.94 to 2.51 g/L total phosphorus (P2O5) as well as high ions concentrations from 0.5 to 3.1 g/L ammonium (NH4+), from 1.05 to 5.48 g/L potassium (K+) and from 0-2.13 g/L phosphate (PO43-). Besides it also contains other ions such as sodium, chloride, magnesium, calcium and sulfate. High nutrients concentration limits its application to land with maximum application of 60 kg/ha/y of phosphate and 100kg/ha/y of potassium. Therefore, the removal of these nutrients is important before land application or disposal. In addition, these nutrients could be marketed to regions with high demand of nutrient or to the non-agricultural sector. The opportunities for nutrients marketing from digestate are largely unexploited and the strategies for marketing is still immature. This paper reviews the current technology on the removal, recovery as well as reuse of nutrients from liquid fraction of digestate. The discussion on the removal of nutrients include ammonia stripping, anaerobic ammonium oxidation (ANNAMOX), direct contact membrane distillation, constructed wetland system and vapor pressure membrane contactor. Nutrients recovery technology discussed in this paper include vacuum evaporation, struvite recovery, vacuum thermal stripping with acid absorption, combined evaporation and reverse osmosis. Meanwhile, the current technology on nutrients reuse include cultivation with microalgal for biomass production, nutrients recycling back to digester, soil application and subsurface injection into soils.