Sources of heavy metals in urban wastewater in Stockholm

The sources of heavy metals to a wastewater treatment plant was investigated. Sources can be actual goods, e.g. runoff from roofs, wear of tires, food, or activities, e.g. large enterprises, car washes. The sources were identified by knowing the metals content in various goods and the emissions from goods to sewage or stormwater. The sources of sewage water and stormwater were categorized to enable comparison with other research and measurements. The categories were households, drainage water, businesses, pipe sediment (all transported in sewage water), atmospheric deposition, traffic, building materials and pipe sediment (transported in stormwater). Results show that it was possible to track the sources of heavy metals for some metals such as Cu and Zn (110 and 100% found, respectively) as well as Ni and Hg (70% found). Other metals sources are still poorly understood or underestimated (Cd 60%, Pb 50%, Cr 20% known). The largest sources of Cu were tap water and roofs. For Zn the largest sources were galvanized material and car washes. In the case of Ni, the largest sources were chemicals used in the WTP and drinking water itself. And finally, for Hg the most dominant emission source was the amalgam in teeth. For Pb, Cr and Cd, where sources were more poorly understood, the largest contributors for all were car washes. Estimated results of sources from this study were compared with previously done measurements. The comparison shows that measured contribution from households is higher than that estimated (except Hg), leading to the conclusion that the sources of sewage water from households are still poorly understood or that known sources are underestimated. In the case of stormwater, the estimated contributions are rather well in agreement with measured contributions, although uncertainties are large for both estimations and measurements. Existing pipe sediments in the plumbing system, which release Hg and Pb, could be one explanation for the missing amount of these metals. Large enterprises were found to make a very small contribution, 4% or less for all metals studied. Smaller enterprises (with the exception of car washes) have been shown to make a small contribution in another city; the contribution in this case study is still unknown.