Tracing Toxic Discharges to Sewers by Analysis of Biofilms

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Report no. UWRAA 27

May 1991

Synopsis

The collection and analysis of grab samples of sewage has been found to be an inadequate method for identifying industries which discharge metal-laden waste. The quality and quantity of wastewater streams are highly variable. Furthermore, detailed knowledge of the industrial processes is required to be able to collect grab samples of sewage coincident with the discharge of wastewater containing high metal concentrations. This information is often difficult to obtain from the industries involved. The aim of this project was to determine the effectiveness of sewer slime analysis as an alternative technique for identifying dischargers of heavy metals.

Part One of Section One details the findings from a survey of metal concentration in slime collected from different sites in the metropolitan Adelaide sewerage system. During the survey, techniques for sampling, pre-digestion treatment, and digestion of slimes were developed. The results from the survey showed significant differences in metal concentrations in slime collected from different sites. Metals found in high concentrations in industrial slime corresponded with those used by the specific industries being studied.

In the tracing trial (Part Two) a strong correlation was demonstrated between the metal concentration in slime from different sites and the distance from the main source of metals, an electroplating site. This was particularly obvious for lead and copper, the main metals at the electroplating site.

Sewer slime analysis appears to be a very effective technique for tracing the source of a metal discharge. However, more knowledge is required about the uptake of each metal by the sewer slime and any factors that may influence uptake, in order to determine whether the metal ions are accumulated within the slime or whether the uptake process is due entirely to physiochemical adsorption to the slime surface.

In Section two of the project it was originally intended to study the uptake and release characteristics of sewer slimes with respect to a range of metals and certain organic compounds. However, time limitations restricted the study to one metal only. Cadmium was chosen for this purpose, because it has no known useful biological function.

The uptake experiments were designed to simulate exposure of slime in a sewer pipe to periodic discharges of cadmium and also to the discharge of wastewater containing no metals. From these experiments alone, it was difficult to determine the mechanism associated with the uptake of cadmium to the slime surface. However the data indicate that the metal ions are adsorbed onto the slime layer. Further work would be necessary to confirm this.

The experimental findings suggest that the initial uptake of cadmium by the sewer slime is a very rapid process. Equilibrium was reached after approximately eight hours and the cadmium concentration in the sewer slime stabilised at a level which did not change significantly during the remainder of the study. There was a small, but short-lived disruption to the cadmium adsorption directly after the simulated release of metal-free wastewater into the system.

The concentration of cadmium in the slime appears to be proportional to the amount of cadmium available in the synthetic sewage. This could provide the basis for the possible development of a method to deduce the concentrations of illegal substances in industrial waste.

The data obtained in this study supports the use of sewer biofilm analysis to trace illegal dischargers of toxic heavy metals into the sewerage system. However, desorption studies should be carried out to confirm that the rate of metal ion desorption is not too rapid after the discharge of metal containing wastewater stops.

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