Phosphorus recovery from sewage sludge : a waste to resource approach

Abstract

Phosphorus (P) is an essential for all forms of life on Earth. The potential shortage of P is a major concern across the world. To meet the rising demand for P in response to the worldwide population explosion, phosphate rock, a critical source of P, is being mined at an alarming pace. Since natural phosphate reserves are declining, retrieving P from secondary sources, such as sewage sludge (SS), is a sustainable alternative. In recent years, various strategies for recovering P from SS have been discussed. However, there is an increasing interest in acidic and alkaline leaching as an efficient, cost-effective, technically feasible, scalable, and long-term approach.

In this study, both acidic and alkaline approaches for P recovery from dried SS were investigated. Optimization studies were carried out to determine the optimal leaching conditions for each approach. The results showed that a concentration of 0.5 mol L-1, a leaching duration of 120 min, and a liquid-to-solid (L/S) ratio of 20:1 (mL g-1) were the optimal leaching conditions for both acidic and alkaline approaches. Under optimal conditions, comparative studies were conducted to determine the best acidic, and alkaline leaching agents for extracting P from SS. Comparative studies about the leaching performances of H2SO4, HNO3, HCl, C2H2O4 (oxalic acid), and C6H8O7 (citric acid) showed that H2SO4 was the most effective leaching agent. The average P extraction efficiencies of H2SO4, HNO3, HCl, C2H2O4, and C6H8O7 were 89% (25.88 mg g-1 SS), 62% (18.2 mg g-1 SS), 65% (19.05 mg g-1 SS), 65% (19.37 mg g-1 SS), and 58% (16.83 mg g-1 SS) respectively. A comparison of NaOH and KOH showed that the NaOH had an average P extraction efficiency of 83% (24.32 mg g-1 SS), while the KOH had 82% (24.1 mg g-1 SS).

In addition to P, the dissolution of metal ions such as Al, Fe, Ca, Mg, Cu, Ni, and Zn were also investigated in this study. Substantial dissolutions of metal ions were observed during acidic leaching, whereas almost no dissolutions of metal ions (except for Al) were observed during alkaline leaching. A fixed bed cation exchange method was employed to eliminate metal ions from leachates. The results showed the efficient removal of metal ions from acidic leachate with a minor (about 3%) loss of P, while a substantial (about 55%) loss of P was detected when purifying alkaline leachate. Purified acidic leachates were used to assess the possibility of recovering P as struvite. Findings indicate that struvite recovery is feasible. Because of the significant loss of P from alkaline leachate during cation exchange, the use of alkaline leachate without purification as a source of aluminum phosphate has been proposed.