Extraction of raw sewage sludge containing iron phosphate for phosphorus recovery

► Raw sewage sludge is more effective than incinerated ash for phosphorous extraction using alkali. ► An unheated method would be superior for FePO4-containing sewage sludge. ► The alkali extraction using FePO4-containing sewage sludge was allowed to extract phosphorus selectively. The objective of the present study was to establish an alkali extraction technology for FePO4-containing sewage sludge obtained from a wastewater treatment system that includes phosphorous removal by iron electrolysis. By clarifying the extraction properties of phosphorous, organic matter, and inorganic matter, conditions for alkali extraction were optimized. As a result, it was suggested that unheated phosphorous extraction would be superior for FePO4-containing sewage sludge. And, extraction methods and sewage sludge properties were also compared, and the noteworthy result that extraction of metals can be suppressed to extremely low amounts with alkali extraction as compared with acid extraction was obtained. A new insight was also gained that, as compared with the use of incinerated ash reported in previous studies, alkali extraction was more efficient when raw sewage sludge was used.