Report No FR0084



Oct 1990



To demonstrate the use of bioassays to locate the source of toxic components in sewage.


Utility discharges may be monitored for Red List substances and control may require tracing the sources and eliminating them.


  1. Sewage sampled at a number of points in the Weymouth sewerage system during the first pilot study was found to be toxic in the Microtox bioassay test.
  2. The most toxic sample was from a sewer receiving effluent from industrial and housing areas. The toxicity of this sample could not be solely linked with heavy metal concentrations or the typical components of sewage such as ammonia or BOD: there was also a distinctive organic odour associated with the sample.
  3. On the second sampling occasion at Weymouth the sewage was found to be non-toxic in the Microtox bioassay but differences in toxicity, as determined by the oyster larval bioassay, were found between samples.
  4. Toxicity, in the second series of samples, was found to be significantly correlated (positively at p<O.05) with ammonia concentrations and alkalinity in the sewage. There was no significant relationship between sewage metal concentrations and toxicity.


Two studies have been undertaken on the Weymouth sewerage system with the aim of tracing the source of any toxic components within the sewage. Composite and single discrete samples were taken at eight sites covering both purely domestic sewage and also sewage with some industrial components. The Microtox and the oyster larval development bioassays have been used to test the sewage samples: the latter was shown to be more sensitive than the former. Differences in toxicity were found not only between samples taken during the same survey but also between the two surveys. On the second survey all of the sewage samples were found to be non-toxic to the Microtox test but toxic to the oyster larval test.


  1. Bioassays should be carried out on filtered rather than unfiltered sewage samples, as suspended solids in unfiltered samples appear to interfere with both light measurement in the Microtox test and the development of oyster larvae.
  2. Further work should initially continue to compare the Microtox and oyster larval bioassay, until it is established whether the former is sensitive enough for this particular application.
  3. Composite rather than single point discrete samples should be taken.
  4. A sewerage system with a higher proportion of potentially toxic industrial inputs than the Weymouth system should be selected for a future case study.

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