Bio-electrochemically assisted sulfide, phosphorus, and nitrogen remediation in continuous anaerobic digestion of dairy manure with improved biogas production.

Abstract

Anaerobic digestion (AD) is an industrial practice to properly manage and valorize dairy manure, whereas impurities in biogas and excessive nutrients in digestate always require post-treatment. In this study, integration of bio-electrochemical (BEC) treatment with AD of dairy manure was proposed to simultaneously improve biogas production, reduce hydrogen sulfide (H₂S) release, and remediate nutrients in digestate. A continuous stirred tank reactor (CSTR) and a BEC unit using stainless steel mesh electrodes at applied voltages of 0.5-0.8 V were integrated for continuous AD treatment of liquid dairy manure. At a relatively short hydraulic retention time of 20 d and a high voltage of 0.8 V, the biogas production of CSTR-BEC significantly outperformed that of the control operated in an open circuit mode. The methane (CH₄) content in the biogas from CSTR-BEC at 0.8 V reached 71.1%, leading to a specific CH₄ yield of CSTR-BEC (238.6 mL/gVS) higher by 42.5% than that of the control. The higher applied voltage of 0.8 V in CSTR-BEC also secured significant aqueous sulfide and gaseous H₂S removals of 58.6% and 89%, respectively. Meanwhile, stronger electrochemical reactions in CSTR-BEC resulted in efficient removals of soluble and total phosphorus from dairy manure at a range of 49.5-63.7%. The compositional analysis of cathode precipitates implies that the release of iron ions from the sacrificial anode for further precipitation and adsorption might be the main route for sulfide and phosphorus removal. The average power consumption of the BEC unit (1.024 kWh/m³/d) at 0.8 V was 7.9-fold that at 0.5 V, whereas the net energy gain of CSTR-BEC (7.42 MJ/m³/d) was still comparable to that of the control because of the improved CH₄ production. This bio-electrochemically assisted AD system offers a promising perspective in cleaner bioenergy production with concurrent considerable contaminants remediation from dairy manure.