TracingToxic Discharges to Sewers by Analysis of Biofilms

ReportNo WSAA 27

May 1991




Thecollection and analysis of grab samples of sewage has been found to be aninadequate 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 beable to collect grab samples of sewage coincident with the discharge ofwastewater containing high metal concentrations. This information is oftendifficult to obtain from the industries involved. The aim of this project wasto determine the effectiveness of sewer slime analysis as an alternativetechnique for identifying dischargers of heavy metals.


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


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


Sewerslime analysis appears to be a very effective technique for tracing the sourceof a metal discharge. However, more knowledge is required about the uptake ofeach metal by the sewer slime and any factors that may influence uptake, inorder to determine whether the metal ions are accumulated within the slime orwhether the uptake process is due entirely to physiochemical adsorption to theslime surface.


InSection two of the project it was originally intended to study the uptake andrelease characteristics of sewer slimes with respect to a range of metals andcertain organic compounds. However, time limitations restricted the study toone metal only. Cadmium was chosen for this purpose, because it has no knownuseful biological function.


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


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


Theconcentration of cadmium in the slime appears to be proportional to the amountof cadmium available in the synthetic sewage. This could provide the basis forthe possible development of a method to deduce the concentrations of illegalsubstances in industrial waste.


Thedata obtained in this study supports the use of sewer biofilm analysis to traceillegal dischargers of toxic heavy metals into the sewerage system. However,desorption studies should be carried out to confirm that the rate of metal iondesorption is not too rapid after the discharge of metal containing wastewaterstops.


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