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

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 trans-forming P-containing compounds into AP is greatly attractive. Herein, the migration and transformation be-haviors 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 degrees C but around 22.5-49.0% P released from that at 850 degrees C, respectively. CaO helped retain phosphorus in sewage sludge at 850 degrees 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 ortho-phosphate, and CaCl2 could react with NAIP to form calcium pyrophosphate, calcium chlorapatite and phos-phide. 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.

Automated summary

The study investigated the migration and transformation behaviors of phosphorus in sludge during pyrolysis, as well as the effects of calcium-based conditioners on the formation of inorganic apatite phosphorus (AP). It was found that CaCl2 showed a more excellent effect on the formation of AP than CaO, while CaO exhibited more stable performance on the retention of total phosphorus (TP).