Phosphorus Recovery and Simultaneous Heavy Metal Removal from ISSA in a Two-Compartment Cell

Traditional acid extraction or electrodialytic remediation (EDR) is inefficient to recover phosphorus (P) from incinerated sewage sludge ash (ISSA). This study used a hybrid process including acid extraction and EDR to extract P from ISSA and remove heavy metals/metals from the P extract sequentially. Specifically, the P extract was obtained by extracting ISSA with 0.2 M H2SO4 and a two-compartment cell was applied in the following EDR process. Constant currents of 15 mA, 35 mA and 50 mA were applied for the electromigration of the heavy metals/metals. Results showed that the efficiency of heavy metals/metals removal fluctuated and was relatively low (approximately 20%) under a current of 15 mA. Increasing the current to 35 mA significantly increased the removal efficiency and that of 50 mA was conspicuous, except Fe, Al and As (<50%). Meanwhile, P gradually immigrated to the catholyte after an EDR duration of 96 h. Consistent with heavy metal/metal immigration results, the pH change and 50 mA voltage drop were dramatic (the pH change was 12 and the voltage drop was 11 V). In addition, flocculent precipitates, which were predominantly Ca, P, Al, Mg and Fe, were found in the catholyte.