Electrodialytic treatment of sewage sludge ash for the recovery of phosphorous and separation of heavy metals

Mobilization and extraction of both phosphorus (P) and heavy metals (HM) from sewage sludge ash through means of acidification has been studied extensively. However, separation of both P and HM after mobilization to provide reusable materials has proven to be challenging. This study presents a combination of acidification and electrodialytic separation (EDS) to mobilize and separate P and HM from sewage sludge ash (SSA). The EDS experimental setup consists of three compartments, separated by ion exchange membranes which are located at either side of the stirred ash suspension. Through application of a direct current to electrodes in the outer compartments, ionic complexes migrate and concentrate in the electrode compartments in accordance to their charge. Application of both EDS and
acidification of the ash resulted in an increased release of phosphorus from the ash, but did not always result in separation from the ash suspension to the anode compartment. Although 96% of the recovered P was mobilized, only 55% was separated from the ash suspension. Less mobilization (m), but better separation (s), from the ash and ash suspension was observed for heavy metals, 78% (m) 69% (s) for Cd; 24% (m) 7% (s) for Cr; 76% (m) 72% (s) for Cu; 23% (m) 20% (s) for Ni; 12% (m) 10% (s) for Pb; and 59% (m) 56% (s) for Zn.