Tracing Toxic Discharges to Sewers by Analysis ofBiofilms (Stage 2)

Report No WSAA 62

July1993

 

SYNOPSIS

 

Previousworkers have already tested and verified the possibility of using analysis ofbiofilms in the sewers to detect the illegal discharge of heavy metals into thesewer system (UWRAA Research Report No 27) and this report is a continuation oftheir work.

 

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

 

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

 

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

 

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

 

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

 

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

 

An improved knowledge of the processes involved has enabled a muchbetter understanding of the uptake and release of heavy metals by slimes, withsubsequent improved interpretation of survey data.

 

The scavenging of metals from solution by particulates is not a novelproposition but it is the mode by which metals are largely removed in sewagetreatment processes. With more attention and research into this aspect ofremoving pollutants, including pesticides, the efficiency of sewage treatmentcould be improved, reducing the load of pollutants discharged to naturalwaters.

 

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