Effect of external field on the migration and transformation of copper in sludge fermentation

The surplus sludge (SS) with 88.5% moisture content of the sewage treatment plant was selected as the raw materials. The electricity assisted anaerobic fermentation (EAAF) system was used to explore the changes of heavy metals (HMs) copper (Cu) speciation which was exogenous added in the SS. What's more, the migration and transformation of Cu in the EAAF was simulated by the COMSOL Multiphysics (CM). Results showed that the exchangeable Cu (F1 Cu) content was first increased to the highest then decreased in different zones, which was fluctuated in the range of 0–100 mg/kg during the EAAF. The carbonate bound state Cu (F2 Cu) content was fluctuated in a small range at 0–25 mg/kg and the iron manganese oxidation state Cu (F3 Cu) content was only fluctuated in the range of 0–40 mg/kg in the SS. Moreover, it was confirmed that the Cu ions in anode area were enriched in the middle and cathode area through electro-migration under the action of the electric field (EF). In addition, the higher voltage was applied, the more obvious the Cu ion was migrated. The organic and sulfide-bound Cu (F4 Cu) content was increased slightly resulted from the complexation and chelation of Cu ions with small molecule organic matter. The simulation results showed that the longer the power-on time, the higher the voltage and the larger the porosity could promote the migration and transformation of Cu, which could provide a reference to improve the efficiency of the biogas production meanwhile realizing the removal of HMs in the fermentation process.