An integrated process for struvite electrochemical precipitation and ammonia oxidation of sludge alkaline hydrolysis supernatant.

This study reported two-phase electrochemical processes, including struvite electrochemical precipitation and ammonia electrooxidation, for the treatment of supernatant from the hydrolysis sludge. The results showed that in phase I, the removal efficiencies of 92.3% PO43--P and 50.1% NH4+-N could be achieved in electrochemical precipitation with magnesium sacrificial anode at pH 9.0 and 40 mA after 120-min electrolysis, and slightly increased to 95.1% and 57.3%, respectively, when current further increased to 120 mA, while the energy consumption (ECS, from 0.6 to 6.7 kWh m-3) and specific energy consumption [SECS, from 2.7 to 29.9 Wh g (PO43--P)-1] sharply increased. In phase II, the residual NH4+-N is further indirectly electrooxidized to nitrogen with modified Ti anode (Ti/SnO2-Sb-Pd). With the generation of active chloride, about 83.2% NH4+-N was removed with the molar ratio of Cl/N 5:1 at 50 mA after 120-min treatment, and slightly increased to 92.2%, when current increased to 125 mA, while SECS significantly increased [from 0.027 to 0.117 kWh g (NH4+-N)-1]. The results indicated that current were the crucial factors; meanwhile, lower current and longer reaction time may be the optimal options in electrochemical process with higher efficiency and lower energy consumption. Finally, the integrated process was conducted at the optimal conditions (pH = 9.0, I = 40 mA in phase I; Cl/N = 5, I = 50 mA in phase II) with the supernatant of the alkaline hydrolysis sludge. Removal of ammonia nitrogen (79.3%) and removal of phosphorus (94.3%) were achieved, confirming the feasibility of practical application for the simultaneous phosphorus recovery and ammonia removal.