Levels of heavy metals in selected waste stabilization ponds in Dar es Salaam

This study was done in order to determine the concentrations of heavy metals (Pb, Fe, Cu, Zn, Mn and Cr) in influents, effluents and sludge of waste stabilization ponds (WSPs) at Mabibo, Mikocheni, UDSM and Vingunguti as well as the influence of pH on the concentrations of heavy metals. The samples of influents and effluents were respectively taken at inlets and outlets of the WSPs. They were digested with nitric acid and the supernatants analyzed using AAS. Sludge samples were taken from anaerobic and /or primary facultative pond (PFP) of the WSPs, oven dried and portions digested with aqua regia, filtered and analyzed also using AAS.The concentrations of heavy metals (mg/L) in influents of the WSPs ranged from 0.05(Mabibo)–0.10 (vingunguti) Pb, 1.5 (UDSM)–7.1 (Mikocheni) Fe, 0.006 (UDSM)–0.080(Mikocheni) Cu, 0.071 (UDSM)–0.084 (Vingunguti) Zn, and 0.25 (Mabibo)–0.47 (Mikocheni) Mn. Similarly the concentrations in effluents of the studied WSPs ranged from 0.027 (Vingunguti)–0.072 (Mabibo) Pb, 0.140 (UDSM)–0.430 (Vingunguti) Fe, BDL (UDSM)–0.023 (Vingunguti) Cu, 0.001 (UDSM)–0.088 (Vingunguti) Zn, and 0.06 (Mabibo)–0.27 (Mikocheni) Mn. Cr recorded 0.002 mg/L in influents at Mikocheni and BDL in influents and effluents of other WSPs. The concentrations of all metals in effluents complied well with the TBS standard values for wastewater discharges to receiving environment.The concentrations of heavy metals (mg/kg-dw) in the sludge from WSPs ranged from, 601.5 (UDSM)–8.75 (Vingunguti residential) Pb; 233000 (UDSM)–2893 (Mikocheni) Fe; (2756.5) (UDSM)–(88.3) (Vingunguti residential) Cu; 14900 (UDSM)–286 (Vingunguti residential) Zn; 7774.5 (UDSM)–350 (Vingunguti residential) Mn; and 740.2 (UDSM)–16.01 (Vingunguti residential) Cr. These levels (except UDSM) complied with the EPA and EU permissible limits for sludge reuse on soil. In pseudo-effluents, the concentration of heavy metals decreased with increasing pH to the point at which no further decrease in the metal concentration takes place, a point at which some metals (Zn and Cu) tended to redissolve. The present results indicate that heavy metals are reduced in wastewaters (except Fe in influents at Mikocheni WSPs) and sludge (except UDSM) of the WSPs to the levels permissible for discharge to receiving environments. Since the concentrations of metals were highest from UDSM WSPs sludge we strongly recommend such sludge not to be used or disposed on the soil unless treatment measures are done.