Electrolyte-free electrochemical ammonia stripping integrated with flocculation for efficient nitrogen recovery from swine manure biogas slurry.

Abstract

A novel electrochemical ammonia stripping (EAS) technology integrated with flocculation was developed to effectively remove and recover ammonia nitrogen from swine manure biogas slurry (SMBS) without alkali addition or heating. The optimal operational conditions were established by the suspended solids (SS) removal rate, the ammonia nitrogen removal rate (ANRR), and recovery content. The maximum SS removal rate (66 %) was achieved by adding 4 g/L of cationic polyacrylamide at pH 7 with stirring at 1000 rpm for 20 min. The pH of SMBS in the cathode could be raised beyond 12, facilitating the ammonia nitrogen removal and its recovery as (NH₄)₂SO₄. The cathode feeding mode performed better than the anode feeding mode, increasing the ANRR from 57 % to 86 %, and a corresponding enhancement in the recovery content from 172 to 414 mg/kg. Specifically, under a 35 mA/cm² applied current density, 0.9 L/min aeration flow rate, and 40 % absorbed sulfuric acid concentration, the ANRR reached 91 %, with a recovery content of 1151 mg/kg. The electrolyte-free configuration avoids interference from competing cations, enhancing NH₄⁺ transport and improving ANRR (>90 %), with a recovery content of 1784 mg/kg at an applied current density of 25 mA/cm². The economic assessment revealed that the electrolyte-free EAS system exhibited the lowest energy consumption at 0.5 kWh/kg N and a cost of $0.06/kg N, representing only 16.5 % of the expenses associated with conventional ammonia stripping. These results confirm the potential of EAS as a sustainable and eco-friendly technology for ammonia nitrogen recovery.