Roles of dehydration conditioners on the formation of apatite phosphorus during the pyrolysis of various sludge

Abstract

Sludge contains a high content of phosphorus (P) which makes the recovery of P from this material attractive. Inorganic apatite phosphorus (AP) could be utilized as a slow-release P fertilizer in a long term, thus transforming P-containing compounds into AP is greatly attractive. Herein, the migration and transformation behaviors of P from two municipal sewage sludge and a food-processing sludge were investigated during pyrolysis, and the effects of two calcium-based conditioners (CaO and CaCl2) on the formation of AP were explored. The results showed that nearly all P remained in sludge below 650 °C but around 22.5–49.0% P released from that at 850 °C, respectively. CaO helped retain phosphorus in sewage sludge at 850 °C, while CaCl2 facilitates its volatilization. Even though, the generation of AP was still improved under the effect of CaCl2. AP amount increased by 7.1–37.5% in food sludge derived char while AP amount increased by 7.3% to 21 times in sewage sludge derived char because of the two calcium-based conditioners. Model experiments showed that CaO could react with non-apatite inorganic phosphorus (NAIP) to generate calcium pyrophosphate and calcium orthophosphate, and CaCl2 could react with NAIP to form calcium pyrophosphate, calcium chlorapatite and phosphide. In general, CaCl2 exhibited a more excellent effect on the formation of AP than CaO while CaO showed more stable performance on the retention of TP.