The impact of sewer misconnection effluents on diatom communities

Sewer misconnections lead to direct discharge of foul sewage to surface waters, which conflicts with one of the fundamental aims of a separate sewer system - to prevent discharge of foul sewage to surface waters. Though misconnections are often thought to be a threat to receiving water bodies, and to achieving Water Framework Directive targets, they have received little study to date, and their ecological impacts remain unknown. Review of current knowledge of misconnections and their potential impacts in receiving waters identified key areas of research which are required to improve understanding of the misconnection problem. To fully understand the impacts of misconnection effluents in receiving waters, their impacts in biological communities must be considered. Diatoms, a group of organisms which are commonly used for ecological assessments, play an important role in freshwater ecosystems and exhibit many characteristics which make them ideal for use in investigating response to intermittent stresses. This thesis investigates the impacts of misconnection effluents in the diatom communities of receiving waters.

Current methods to identify misconnection effluents in sewer systems lack sensitivity to misconnection effluents specifically, and suffer from issues of low sampling frequency. An inexpensive, passive, method for detecting misconnection effluents in sewer systems using optical brighteners produced very promising results, and is highly appropriate for use by the Environment Agency and Water companies, pending further validation.

Exposure of diatom communities to detergent effluents in a microcosm study show that detergents cause significant decrease of algal total abundance and notable change in community composition a closed laboratory system. These effects correlate well with high surfactant concentration and alkalinity in the detergents, leading to the conclusion that these are key drivers of responses in the diatom community. Specific diatom species are identified as potential indicators of detergent pollution, showing strong increases in abundance when exposed to high detergent concentrations.

Diatom communities exposed to misconnection effluents in the field in one catchment showed a shift in community composition toward more motile species, indicative of exposure to organic pollution discharged from the misconnected outfalls, however communities in three other catchments did not show any such response. Common diatom based monitoring measures do not respond to effluents from misconnected sewer outfalls, and diatom communities do not show the decrease in abundance or shifts in species composition which were observed in the laboratory.

Misconnection effluents have potential to cause significant impacts in diatom communities. However, low frequency of discharge, short exposure period, and high dilution of effluents, prevent significant responses in the diatom community. In the upland streams investigated, misconnections do not pose a significant threat to diatom communities.