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Discharge from rural sewage works

Bathing Water Catchment monitoring

Location: Bude Cornwall UK
Purpose:  To carry out a river survey identifying potential pollution sources and their likely impact on bathing waters downstream.


STS and the Environment Agency have, for some time, been discussing the issue of monitoring water courses which discharge directly into bathing waters. Of particular interest are water courses where known sources of potential pollution exist from sewage treatment works, storm drain overflows, agricultural activity such as slurry pits or illegal discharges.

The problem with such activity is that it is often sporadic occurring after heavy rain events or cyclical such as farms clearing yard areas to slurry pits which are leaking or consequently overflow.  Monitoring such events is therefore difficult using current techniques for identifying organic matter such as the standard 5 day BOD test. 

A regulatory officer needs to be able to rapidly identify that there is a problem likely to lead to – or already in breach of discharge consents.  Taking a sample for lab analysis may well prove that there was an incident somewhere up stream of the monitoring point but it does not enable the officer to follow that event back to source.

Further, the cost and time implications of multiple sampling and analysis often prevent root causes of pollution from being traced and dealt with at source.



The STS SMF4 portable fluorometer offers two separate solutions to this problem.  Firstly it can offer a instant result from a discrete sample taken from the water body.  The sample is placed into a UV cell, placed in the instrument, and the reading taken virtually instantly.  Being an instantaneous reading means that repeated readings can be taken to verify the initial result and to map the localisation of the polluted area.  Additionally there are no reagents required which again cuts down on the ongoing costs associated with other types of monitoring equipment.

Using this instant sampling method a series of samples can then be taken working upstream and following the polluted trail until the readings return to the background level for that watercourse.  The point of increased readings will then identify the source of the pollution for instance an outfall from a treatment plant.  This is of course a fairly obvious source of higher organic matter but the instrument will also be able to identify less clear sources such as a leaking slurry pit that may not manifest itself as a single point but as an area of higher readings.

The second solution offered by the SMF4 Fluoremeter is to carry out longer term monitoring of a suspect water course in order to capture occasional pollution events when an officer is not on site.  The SMF4 can capture over 2000 records onto its internal data logger – capturing the sample reading, date, time, location, sample reference and instrument temperature for each sample.  Sampling periods may be set between 1min and 24 hours, for example at 15 min intervals over 3 weeks of data may be captured.

To carry out such sampling a small peristaltic pump is used to draw the water through a UV flowcell housed in the instrument.  The pump may be run off a battery and the SMF4 will run on its own battery for up to 4 weeks meaning that monitoring in remote locations is straightforward.

This regime can provide excellent historical data to identify cyclical pollution events, unusual discharges occurring outside of expected time frames, and to give a general indication of the organic load over time.

Contaminated outfall to river
SMF4 Survey Map


A small river flowing out the beach at Bude was selected for the trial, the intention being to walk down the rive monitoring at intervals to determine if any there were any apparently high readings and to attempt to identify the source of the elevated readings.

The river chosen runs through a golf course into a small industrial and residential area and then out to the beach.  The weather was dry and clear with no surface run off evident.

Samples were taken from the main water body and a tributary and also from various outfalls into the main stream.  The results were categorised into 4 groups with the lowest numbers being the “cleanest” water through to the higher numbers being the less clean/polluted.


Readings taken above the sampling point no.35 (reading 175) were in the region of 125 to 155, this area is partly golf course and partly nature reserve and so we can reasonably assume that this can be taken as our background “clean” reading.

Moving downstream readings taken at points 34 (reading 510) and 32 (reading 480) were in the order of 4 times higher that those upstream.  This would seem to indicate a localised source of pollution upstream of these points but downstream of point 35.

At sampling point 34 a noticeable oily film could be seen on the water,  this part of the stream is close to some small industrial units which may be the source of the pollution.

Sampling point 33 which appears to be back to normal levels was actually a SWO discharging to the stream at a much better quality than the river water.

Sampling point 30 is the junction of a small tributary joining the main stream – the reading shown is of the tributary rather than the main stream which still showed readings of 480.

Below this point the readings gradually fall off as the pollution begins to become diluted and the eventual reading at the outfall to the beach was 307.  This was however still over twice the level seen before the first sampling point and would therefore justify further investigation into the contamination seen at points 32 & 34.

Correlation of SMF4 results to actual BOD

For further information and technical details please contact us at or call us +44 (0) 1344 483563

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