Electro-Oxidation of Anaerobic Digester Effluent Results in Solids Separation by Electro-Flotation Combined with COD and Ammonia Oxidation

Abstract

Electrochemical oxidation can provide a post-treatment alternativeto remove recalcitrant organics, ammonia and disinfect effluents from anaerobic digestion facilities. In this work we examined the electrochemical oxidation of a raw anaerobic digester effluent by using Boron Dopped Diamond (BDD) anode and Ti/Pt cathode at different current densities (25, 50 and 70 mA/cm2). The digester effluent was characterized by a chemical oxygen demand (COD) concentration of 23 g/L, total suspended solids 15 g/L, ammonia nitrogen 1800 mg/L and electrical conductivity 14 mS/cm. Mass balance calculations revealed that the percentage of COD transferred to the foam increased from 41 to 58 and 72% with increasing current density from 25 to 50 and 70 mA/cm2. The remaining liquid after 4.5 h electro-processing time was characterized by low COD (between 1.8 and 4.2 g/L) and ammonia nitrogen concentration (between 48 to 540 mg/L). This is in accordance with the Hellenic digestate reuse standards in agriculture (as an organic fertilizer) where COD concentrations should be less than 4.5 g/L. The results from this study demonstrate that electro-separation of digestate solids is favoured at high current densities followed by oxidation of organics and ammonia.