Tracing Toxic Discharges to Sewers by Analysis of Biofilms (Stage 2)

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

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Report no. UWRAA 62

July 1993

Synopsis

Previous workers have already tested and verified the possibility of using analysis of biofilms in the sewers to detect the illegal discharge of heavy metals into the sewer system (UWRAA Research Report No 27) and this report is a continuation of their work.

During the establishment of laboratory conditions to ensure that the results of further experiments would be consistent with previous experiments, it was found that the processes involved in the transfer of metals from sewage to slimes were not as simple as assumed.

Particulates in solution will scavenge metal ions from the solution very effectively, a process which is well documented in scientific literature .A consequence of this is that the major transfer process of metals from solution to slimes occurs in the sequence, metal ion particulate+metal biofilm+particulate+metal, involving physical processes which are difficult to quantify or control. It was felt that with so many variables, best results would be obtained from work in the field.

The cessation of discharges from industry over the summer vacation period presented the opportunity to observe the effect on the slimes when metal discharges stopped for an extended period. It was found that almost all the metal (aluminium) was lost from the slimes within 15 days and their recovery occurred within 3 days of discharge commencing.

Further surveys were conducted over three months, where discharges of chromium and copper were known to occur, enabling a history of the changes of metals in the slimes to be compiled for analysis and application to further research and practical aspects of detecting illegal discharges.

A significant result of the surveys was that the relative concentration of metals in the sewer slimes did not necessarily correspond with the relative concentration of metals in the sewage. The association of the metals with particulates or as complexed precipitates and their physical behaviour in slime-coated sewers provided adequate explanation of the observations and results obtained.

A field experiment involving the controlled release of heavy metals in solution gave results consistent with the mechanism put forward for the mass transfer of metals from sewage to the slimes and demonstrated that metals are taken up by the slimes within minutes of a solution contacting the sewage.

An improved knowledge of the processes involved has enabled a much better understanding of the uptake and release of heavy metals by slimes, with subsequent improved interpretation of survey data.

The scavenging of metals from solution by particulates is not a novel proposition but it is the mode by which metals are largely removed in sewage treatment processes. With more attention and research into this aspect of removing pollutants, including pesticides, the efficiency of sewage treatment could be improved, reducing the load of pollutants discharged to natural waters.

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