Occurence of Metals in Total Suspended Solids in Urban Waters Determined by Sequential Extraction

The binding of metals to total suspended solids (TSS) and their speciation in urban wastewater drastically influences the efficiency of capturing metals in sewage sludge in the framework of wastewater treatment plant technology. In this article, a modified sequential extraction procedure of the Bureau Communautaire de reference (BCR) was applied to TSS from urban wastewater and digested sludge samples. Samples were collected from combined sewer system and the Central Wastewater Treatment Plant in Ostrava, Czech Republic. The collection of wastewater samples and digested sludge was performed April to June 2013. Altogether, 42 samples of urban wastewater and 3 samples of digested sludge were obtained. The sequential extraction of TSS has proven that the highest proportions of As, Cr, Cu, Ni and Fe are associated with the residual fraction (61-75%). Zn and Mn are bound to the reducible fraction (64-65%). Finally, Pb and Cd are bound predominantly to the oxidizable fraction (56%). The results show that general trend proportions of metals from TSS in the oxidizable fraction is Cd similar to Pb > Cu > Zn > Fe > Ni > Cr > As > Mn. The distribution of metals between fractions was highly variable in digested sludge. The smallest amounts of all metals in digested sludge are associated with the exchangeable fraction (Ni 5.8%; other metals < 3.7%). Cadmium, Zn and Mn are predominantly bound within the reducible fraction (68%, 37% and 35%). In the case of the oxidizable fraction, the contents of all metals are relatively low (max 20.3% of Fe). Except Cd, the predominant portions of all metals in digested sludge are bound to residual fraction (45.8-99.7%). The comparison of distribution metals in the individual fractions from TSS and digested sludge shows that the portions of Cd, Cu, Cr, Mn, Pb, Zn and Fe are significantly different. The results indicated that Cu, Cd, Pb, Zn and Mn in digested sludge are bound in silicates well as to the phosphates.