[Effects of Hydrothermal Treatment Time on the Transformations of N, P, K and Heavy Metals in Sewage Sludge]

Hydrothermal treatment (HTT) of sewage sludge was conducted, focusing on the influence of HTT time on the dewaterability of sludge and transformations of elements N, P, K and heavy metals. The results showed that at a hydrotherma temperature of 160°C, with HTT time increasing from 30 to 120 min, the sludge dewatering performance was significantly improved. The transfer rate of N element in the sludge transferring to aqueous product increased gradually. Almost all of P element remained in the solid phase, and most of K element (57%-62%) was still in the solid phase although it was more easily transferred to the liquid phase than P element. The transferring behavior of heavy metals during the HTT related to their own properties, and their transferring behaviors were different with the increase of HTT time. Compared with the raw sludge, the contents of Cu, Zn, Cr and Pb in the dewatered sludge increased significantly, As increased slowly, while Ni and Cd were first lower than those in raw sludge, and then increased with the prolonging HTT time.