Sewer Misconnection Survey
- Date: March 2010
- Location: Edgebaston Birmingham UK
- Purpose: To test the application of the SMF4 portable fluorimeter in the detection and quantification of organic pollution from faecal matter in Surface water Sewers.
STS was invited independently by the Environment Agency and Severn Trent water to carry out a series of monitoring operations in an area of Edgebaston which had a history of issues arising from the misconnection of waste water into a “clean” surface water sewer system.
Traditionally surveys of this type are carried out using contractors who are tasked to lift manhole covers along a predetermined route and obtain discreet samples for 5 day BOD analysis. Additionally the operators use their local knowledge and visual interpretation of the condition of the sewer and its contents to guess its apparent load.
There are clearly several issues with these methods. Obtaining samples for laboratory analysis is both time consuming and costly. The results are then not available for approx 7 days by the time the samples have been delivered to the lab, incubated and the results written up. The information given does then give a snapshot of the condition of the sewers load but this information is very limiting. Returning to the same sewer and re-sampling may return completely different results, no indication of the source is possible and a positive result requires the inspection team to return and take further samples involving more cost.
The visual inspection is also fraught with misleading conclusions and misinterpretations. An apparently “clean” flow down the sewer may be far from the reality – recent storm events may have scoured the pipe clean removing evidence of build up of waste.
Severn Trent and The EA were therefore looking for a solution which enabled them to quickly take samples analyse them in situ and repeat sample if necessary. Further the requirement was to then attempt to trace the sources of pollution back to joining smaller sewers from the main sewer and eventually back to an actual source.
This is of course possible but involves intrusive work using dye testing over a large number of residential and commercial premises to identify the actual source.
Using the SMF4 would enable tracing of the pollutant source upstream to a particular junction serving only a single or perhaps a small number of properties.
A typical area of Edgbaston was identified with a main surface water sewer running though a mostly residential area and discharging into a stream in an urban green area.
The main sewer had a number of connecting surface water sewers feeding it with connections made directly into these from properties.
The map below shows the area to be surveyed.
The surveys were carried out in overcast/light rain conditions, contractors provided by Severn Trent lifted manholes in the sewer at the marked locations for samples to be taken.
The water samples were placed into a UV Quartz cell and placed in the instrument. 5 consecutive readings were taken of each sample and the results averaged – the 5 readings took less than 1min to take and record.
This procedure was repeated at each sampling point and the results were then provided to Severn Trent by STS for their comment. A topographical map was produced showing the sewers direction of flow and branches joining the main sewer with the sample reading at each location.
Severn Trent had however previous knowledge of the area and had carried out extensive sampling and dye tracing to establish the sources of grey water pollution into the rain water sewer system.
Severn Trent was therefore able to annotate the Map of readings taken with known sources of pollution.
As can be seen the areas where high readings were taken corresponded with known upstream sources of pollution. Similarly areas where there was no known source of pollution were found to have very low readings.
The survey also shows clearly the affect of dilution that clean water joining the system has in lowering the readings further downstream. The final outfall monitored (sampling point 1) also shows the affect of a 2nd rain water sewer which joins it just before discharge causing a dilution of the pollution seen at sample point 2.
Sample points 17 & 13 although not identified on this map as having any direct misconnections were later found to have had domestic services coupled up to the rain water sewer after the Severn Trent survey had been completed.
The SMF4 provided a rapid and reliable method of determining the presence of organic pollution in the form of grey water and faecal contamination in the rain water sewer.
The instrument was able to differentiate between variable pollutant loads and was able to trace the source of the pollution back to the branch.
Most importantly the operators could have traced the pollution directly back to a particular connection, taken a sample for BOD testing and to provide permissible evidence- and directly approached the homeowner to resolve the issue.
The savings in time and cost of secondary visits, extensive dye testing and follow up are immense when this practice is scaled up across even a small town let alone such a large city as Birmingham.