Experimental assessment of phosphorus recovery from sewage sludge ash: most relevant influencing factors besides leaching conditions

Phosphorus is an essential element for life, and it is present in all living organisms. As it is a non-renewable and limited resource, the increasing demand of fertilizers is gradually depleting the reserves of phosphate rocks. To tackle this issue, phosphorus recovery from wastewater is possible, because it concentrates in the sewage sludge. Thus, the ashes derived from mono-incineration plants are one of the main secondary phosphorus resources, thanks to their high phosphorus content (5-11%). In this paper, the most influencing parameters (combustion technology, wastewater treatment layout, ash chemical composition and morphology) on phosphorus recovery by wet chemical extraction are investigated. The experimental work was performed on six samples of sewage sludge ashes derived from pilot-scale plant grate furnace. The first step involves the acid leaching (operating conditions: 0.2 M H2SO4, liquid to solid ratio of 20:1, contact of 2 h, room temperature) followed by subsequent precipitation (precipitating agents Ca(OH)2, NaOH, KOH; pH 2.5, 3.5, 5, 8; sedimentation time: 2 h). The overall findings showed a slightly lower phosphorus content and lower level of toxicity for bottom ash compared to cyclone ash. High aluminium content in the ash limited the extraction efficiency to a maximum of 83%. Particle size dimension below 0.212 mm ensured a sufficient contact surface between solid particles and leaching solution. Overall, the recovered product presented a P2O5 content of 38%, thus aligned with common phosphatic fertilizers. However, due to the co-dissolution of toxic compounds during the leaching step, the purity of the recovered material should be enhanced