Performance comparison of phosphorus recovery from different sludges in sewage treatment plants through pyrolysis

Abstract

Recycling of phosphorus (P) from pyrolysis sewage sludge is a good methodology to deal with the deficit of P resources and safely dispose the sewage sludge. However, it is essential to determine the type of sludge in sewage treatment plants (STPs) along with the most appropriate pyrolysis temperature. These factors affect the form and bioavailability of recoverable P. Thus, four kinds of sludge in a typical STP were selected, and three pyrolysis temperatures including 300, 500, and 700 °C were set to find a suitable strategy for P recovery. The results indicated that with increasing pyrolysis temperature, more organic phosphorus (OP) was transformed into inorganic phosphorus (IP) and more non-apatite inorganic phosphorus (NAIP) was converted to apatite phosphorus (AP, mainly Ca3(PO4)2, Ca(H2PO4)2, Mg3(PO4)2, Ca3Mg3(PO4)4, and Ca5(PO4)3OH). Compared to other sludges, dewatered cake sludge (CS) pyrolyzed at 700 °C had the highest AP content, about 1.1–3.6 times that in other pyrolysis products, and the highest Ca5(PO4)3OH content containing 35.8% of total phosphorus (TP). The contents of P-2% citric acid (CA) and neutral ammonium citrate (NAC) in CS obtained by pyrolysis at 500 °C reached 45.6 and 49.6 mg/g, respectively, which were 1.1–3.0 and 1.1–2.8 times that of P-CA and P-NAC in other sludges and their biochars, respectively. Moreover, the diethylenetriaminepentaacetic acid extraction analysis indicated that the good stability of heavy metals in CS-derived biochar was better than that in the feedstock. Hence, to ensure the efficiency and safety of P, CS is used as raw material in STPs to recover P resources. CS-500 can be used as the direct P source of land owing to the highest concentration of bioavailable P (P-CA and P-NAC), while CS-700 can be used to represent a slow-release P fertilizer because it contains the highest concentration of AP.