Metal removal, partitioning and phase distributions in the wastewater and sludge: Performance evaluation of conventional, upflow anaerobic sludge blanket and downflow hanging sponge treatment systems

Metals pollution pose a serious problem to environmental and human health, if not effectively removed from wastewater using different state-of-the-art treatment technologies. This study investigated the phase distribution of copper (Cu), lead (Pb), zinc (Zn), chromium (Cr) and cadmium (Cd) in wastewater and sludge samples collected from domestic wastewater treatment plant (DWTP), upflow anaerobic sludge blanket (UASB), and downflow hanging sponge (DHS) reactor systems. Sludge from the UASB and DHS contained predominantly reducible fractions of Zn and Cd and oxidizable fractions of Cu, Cr, and Pb. A characteristic bioavailability sequence was Zn > Cu > Cr > Pb > Cd, with significant differences in dissolved fractions among operational modes. Metal concentrations (solid fractions) were substantially high in the DWTP water, and in UASB and DHS systems (dissolved fractions). Pb, Cr and Cd were strongly associated with the solid phase in the final polishing units (FPU) of the DHS system after treatment, indicating binding and removal (∼75–90%) of metals with particulates. MINTEQA2 calculations indicated that Zn was the most readily available metal in free form in all of the systems, with a general order of Zn<inf>DWTP</inf> > Zn<inf>UASB-DHS</inf> > Pb<inf>DWTP</inf> > Cd<inf>UASB-DHS</inf> > Pb<inf>UASB-DHS</inf> > Cu<inf>DWTP</inf> > Cu<inf>UASB-DHS.</inf> Overall, metals in the UASB-DHS showed higher binding potential with coexisting ions, mainly anions like carbonates, hydroxyls, and bicarbonates. Non-anion fractions were more prevalent in DWTP samples. Sorption coefficients indicated that metal bioavailability decreases during the treatment in sludge samples, but increases in case of the wastewater samples, which means the treatment process affects metal phase distribution. In general, the selection of wastewater treatment processes should not be solely based on total metal removal efficiency, but also on their potential to promote the most desirable phase distributions.