Phosphorus recovery from sewage sludge hydrochar: process optimization by response surface methodology

Hydrothermal carbonization can play an innovative role in sewage sludge (SS) treatment and valorization, as well as in phosphorus recovery. In this study, leaching tests using nitric acid were performed on hydrochar from SS and the influence of pH (1–3.5), leaching time (30–240 min), and solid/liquid (S/L) ratio (5–20 wt%) was analyzed and optimized according to the Design of Experiments method, under the Response Surface Methodology approach. The highest phosphorus extraction yield (59.57%) was achieved at the lowest pH and the lowest S/L ratio, while an increase in temperature from 20 to 60 °C negatively affected the phosphorus recovery. Quadratic models, with the addition of semi-cubic terms, were found to best represent both phosphorus yield and ash content of the hydrochar after leaching. As observed by 3-dimensional surface responses, phosphorus yield increases as the pH decreases. The pH is the factor that most influences this response, while time has little influence. At pH 1, the yield increases as the S/L ratio decreases, while the S/L ratio only slightly affects the response at pH 3.5. At an S/L ratio of 12.5%, multi-objective optimization indicates that pH 1 and a leaching time of 135 min are the parameters that allow both maximum phosphorus yield and minimum ash content.