Optimizing the H3PO4 leaching conditions of post-precipitated sewage sludge using response surface methodology

Abstract

The leaching procedure of post-precipitated aluminium phosphate sludge with dilute phosphoric acid was developed. The leaching offers a route to recover both critical phosphorous from sewage sludge and the metal used in precipitation. Using phosphoric acid as leaching solution makes it possible to continue the recovery process without the need to remove chloride or sulfate anions. The optimization of the leaching was based on experimental three-level-four-variable central composite face-centered design. The four variables included were acid concentration, volume of acid, temperature and time of leaching. The leaching was conducted for dewatered and water-containing sludge (total solid content 3–4%) and for both second-order regression models were obtained. For water-containing sludge optimal conditions for leaching are solid to liquid ratio (S/L) 400 gL-1, a temperature of 60 °C and a leaching time of 6 h. For the dewatered sludge, optimal leaching is attained when S/L ratio 119 gL-1 with 2 M acid is used at a temperature of 20 °C. The obtained results enable the developing of full-scale process where phosphate in the sludge is refined to phosphorous acid and metal used in sludge production recycled back to precipitation.