Mercury contamination in the sludge of drinking water treatment plants dumping into a reservoir in Rio de Janeiro, Brazil

Abstract

Although sludge piles from drinking water treatment plants can contain harmful substances, in many countries, their disposal methods are still unregulated. Besides aluminum, which is a major constituent in these residues, many other contaminants—like trace metals—can be present and may result from the quality of the raw materials used for water treatment. The application of these chemicals for the treatment of drinking water can generate toxic sludge and contaminate the produced water. In the present work, mercury contamination in the sludge piles of two drinking water treatment plants located along the margins of the Juturnaíba Reservoir, Southeast Brazil, was evaluated to verify whether contaminants are incorporated during water treatment. In the summer 2012, five cores were collected from the piles, and were analyzed for Eh, granulometry, total carbon, total nitrogen, and total mercury. The results indicated an anoxic environment, reflecting composition of the suspended matter. Carbon and nitrogen presented elevated concentrations, but also seemed to reproduce the characteristics of the suspended matter in the raw water. The concentrations of mercury were extremely variable but presented unexpectedly high values in some of the layers, reaching 18,484 ng g⁻¹. On the other hand, concentrations ten times lower than those observed in the natural system (8 ng g⁻¹) could be observed. It was concluded that the only possible source for the contamination of the sludge was the chemicals used for water treatment.