Influences of electrode distance and electrolysis time on phosphorus precipitation and composition during electrolysis of anaerobic digestion effluent
2022
Takabe, Yugo | Fujiyama, Masaya | Yamasaki, Yukiyo | Masuda, Takanori
Global demand for phosphorus (P) is increasing, which has led to concerns over future drought and has driven efforts to recover P from wastewater streams for reuse. In this study, platinum-coated titanium electrodes were applied to the electrochemical precipitation of P from anaerobic digestion effluent that was collected from a domestic wastewater treatment plant. The influence of the electrode distance on P removal and precipitation was investigated. In addition, the influence of the electrolysis time on the chemical structure and composition of the P precipitate was evaluated from the viewpoint of utilising the P precipitate as fertiliser. Regardless of the electrode distance (10, 5 and 1 mm), PO₄³⁻, Ca²⁺ and Mg²⁺ were removed. The bulk solution pH increased during electrolysis because of the consumption of generated H⁺ as HCO₃⁻ transitioned to H₂CO₃ near the anode. A greater increase in the bulk solution pH was observed when the electrode distance was narrowed because of the enhanced H⁺ consumption. Narrowing the electrode distance reduced the energy consumption for P precipitation. The increase in the bulk solution pH with the narrowing electrode distance changed the dominant P precipitation pathway from onto the cathode to in the bulk solution. X-ray diffraction spectra of the precipitates showed that increasing the electrolysis time transformed amorphous P to hydroxyapatite and struvite. Most P existed in a citric acid-soluble form, which is recommended for use as a slow P release fertiliser. There were no significant changes in the citric acid-soluble P content of the precipitates with increasing electrolysis time. Therefore, increasing the electrolysis time has little influence on the suitability of the precipitate as a slow P release fertiliser.
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