Heavy metals partition between leachate and solids in the course of municipal sewage treatment

Abstract

Measurements of heavy metals contents (Cu, Zn, Pb, Cd) were carried out in liquid and solid fractions of sewage and sewage sludge after stabilization as well as dewatering. Investigation was carried out in municipal WWTP ,,Wschod" in Gdansk. The flow of sewage was equal to 88 000 m3 /d. The treatment consists of two stages: the first stage - mechanical (4screens, 2 sandtraps, 4 primary sedimentation tanks) and the second stage - advanced biological MUCT system (6 reactors - each incorporating the following chambers: dephosphatation, pre-denitrification, denitrification and nitrification followed by 2 secondary sedimentation tanks) modified by introduction of an additional chamber which secures dioxidation of sewage recirculated between nitrification and denitrification chambers. Sewage sludge (primary and secondary) is stabilized by fermentation. The last step of sludge utilization is mechanical dewatering. Samples of sewage were collected at the following points of the WWTP: at the beginning - raw sewage, mechanically treated sewage and biologically treated sewage. Samples of reject waters after thickening and after mechanical dewatering of sludge were collected too. The following types of sewage sludge were investigated: primary, secondary, thickened (on the Klein press), digested and mechanically dewatered (Noxon centrifuges). In the sampling points samples were collected twice per month during the 6 months period in the years 2000-2001. Inflowing sewage carried the following loads of heavy metals: Zn - 27.2 kg/d, Cu - 5.4 kg/d, Pb - 1.3 kg/d and Cd - 0.4 kg/d.  Basing on balances of heavy metals loads it was proved that 87.0% Zn and 83.0% Cu were kept in sludge. Among analysed sewage sludge samples, the highest contents of heavy metals was found in sludge after mechanical dewatering and in the mixture of primary and secondary sludge (Zn - 780.4 mg/kg d.m., Cu - 144.6 mg/kg d.m., Pb - 5.4 mg/kg d.m. and Cd - 2.4 mg/kg d.m. The contents of heavy metals in examined sludge were compared with Polish admissible loads of metals applied with sludge to soil fertilization. Basing on the findings, admissible loads of sewage sludge to agricultural land were not exceeded.