Utilisation of polarity inversion for phosphorus recovery in electrochemical precipitation with anaerobic digestion effluent

Phosphorus is a non-replaceable and limited resource, whose future drought is a matter of concern. Its recovery from wastewater has gained attention as a method of making the recovered phosphorus available for beneficial use. This study applied electrolysis with a platinum-coated titanium electrode to an actual anaerobic digestion effluent and investigated the phosphorus removal and precipitation characteristics with various current values. The separation of the phosphorus precipitate from the sludge residue and the usefulness of the polarity inversion to recover the phosphorus precipitate were evaluated. The availability of the recovered precipitate as a fertiliser was also demonstrated. The anaerobic digestion effluent after the centrifugal dehydration process with a coagulant was collected from a wastewater treatment plant. Phosphorus was precipitated as a form of calcium phosphate, including hydroxyapatite, which was confirmed by X-ray diffraction. The generated gases from the electrodes during the electrolysis floated and condensed the sludge residue, leading to successful separation of the phosphorus precipitate from the sludge residue. The polarity inversion successfully detached the phosphorus precipitate from the electrode, and at least 3 min polarity inversion was enough to detach the whole phosphorus precipitate. The submerging phosphorus precipitate contained abundant phosphorus (12.6%), in which citric-acid-soluble phosphorus accounted for 94.4%, and the utilisation of the precipitate as a slow-release phosphorus fertiliser was suggested. [Display omitted] •Electrochemical precipitation was applied to the actual anaerobic digestion effluent.•Precipitation of calcium phosphate, including hydroxyapatite was confirmed.•Generated gas during electrolysis separated the precipitate from the sludge residue.•A 3 min polarity inversion detached the precipitate from the electrode.•Utilisation of the precipitate as slow phosphorus release fertiliser was suggested.