BEHAVIOR OF TRACE-METALS IN SEWAGE-SLUDGE AFTER DISPERSAL IN SEAWATER

In recent years, many countries have moved to stop disposal of sewage sludge to sea, on the grounds of possible adverse environmental effects. However, available information on the behaviour of sludge contaminants following disposal by this route remains limited. In this study the desorption of trace metals from digested sewage sludge after dispersal at sea was investigated. Two experiments were carried out in the dark, at a constant temperature and under constant aeration. Initially a single sludge was incubated for 76 days at three different dilutions (1:75, 1:500 and 1:2500) with sea water. Results showed that for cadmium and copper there was adsorption from the water at first followed by desorption after approximately 5 days. The degree of adsorption varied with dilution, with the lowest dilution producing the greatest initial adsorption and subsequent desorption. Nickel showed an initial immediate increase in dissolved phase concentrations which was though to result from the presence of soluble nickel sulphide complexes in the undiluted sewage sludge. This was then followed by further slow release from the particulate phase over a similar timescae to that for cadmium and copper. After around 40 days all metals appeared to be reasonably close to equilibrium with respect to solid solution partitioning. Comparison of the release of trace metals from sludges obtained from five different sources and incubated for 36 days at a dilution of 1:500 showed essentially similar behaviour in all cases. Solid:solution phase distribution coefficients at the end of the 36 day equilibration period also showed reasonable agreement between the different sources. It was concluded that copper and cadmium were initially adsorbed from the seawater onto the sludge particulate phase by complexation with organics. With time it was shown that the concentration of suspended solids in the water reduced by 50 %, presumably through microbial degradation. As a result this lead to the dissolution of copper and cadmium. For nickel which exhibits a lower affinity for complexation by organics, there was an initial rapid partitioning into the dissolved phase, driven by inorganic processes, followed by a slower release as the solids were degraded.