Influences of electrode distance and electrolysis time on phosphorus precipitation and composition during electrolysis of anaerobic digestion effluent

Simultaneous phosphorus precipitation and sludge thickening by electrolysis with an anode covered by bivalve shells

A wastewater treatment plant with a large inflow of phosphorus (P) is a potential P source that can act as an alternative to phosphate rocks and a renewable source of P. During electrolysis with inert electrodes, hydroxide ions generated from the cathode cause calcium phosphate (CaP) precipitation, and oxygen and hydrogen generated from the electrodes cause thickening of the sludge by electroflotation in sludge treatment streams. However, these two effects have not been achieved simultaneously because the precipitation of CaP requires much more time than that required for thickening by electroflotation. In this study, an electrolysis system that used an anode covered with bivalve shells was used. Batch experiments were conducted and the results demonstrated that covering the anode with shells resulted in their dissolution and that the calcium ions provided by this process considerably enhanced P removal in the form of CaP, thereby shortening the time required for CaP precipitation. In continuous experiments with excess sludge, electrolysis with shells accomplished sludge thickening by electroflotation (the thickened sludge had 5.5 times the total solids in the original excess sludge) and low relative phosphate-P concentrations (0.0545-0.0812) in the effluent compared to the influent. This effect is attributed to CaP precipitation. Additional mixing of the CaP precipitates in the effluent enhanced their settleability. The results demonstrate that electrolysis using an anode covered with bivalve shells simultaneously achieved CaP precipitation and sludge thickening.

Enhanced electrochemical precipitation of phosphorus in wastewater by the addition of drifting Corbicula shells

Phosphorus (P) is a finite and essential resource, and its linear movement from mines to waste streams may result in shortages. This has encouraged efforts to recover P from sewage systems for reuse. This study developed a new electrochemical P precipitation system for the subnatant of the sludge flotation thickening process, in which drifting Corbicula shells are added to provide a supply of calcium ions (Ca²⁺) to promote P precipitation. However, adding Corbicula shells to coexisting suspended solids (SS) and coagulant resulted in adsorption of the shells in the neutralized and hydrophobized floc clusters, which limited their electrochemical dissolution. Adding Corbicula shells after SS removal by flotation with electrochemically generated gases resulted in their successful electrochemical dissolution, which enhanced phosphate-P removal. Increasing the amount of Corbicula shells enhanced the phosphate-P removal to a point, after which further addition simply increased Ca²⁺. The consumption of H⁺ generated near the anode for the dissolution of Corbicula shells increased the pH of the bulk solution, which enabled P precipitation not only onto the cathode but also in the bulk solution. Analysis of chemical composition in the generated particles suggests that they can be used as a slow P-release fertilizer and soil conditioner.