South East Rivers Trust

Sightings from our Outfall Safaris


Sewage in our rivers is commonplace (Credit: Connect Right)

You might think that sewage flowing into our rivers is something banished to the history books. Unfortunately, in reality, this is still a widespread problem.

But thanks to our volunteers, the Beverley Brook and Wandle will be seeing some improvements.

Deciphering our Drainage

To understand how sewage is getting into our rivers, we need to go underground and think about how London has grown over the last century or more.

As a much smaller city with fewer residents, London in the early 1900s relied on a “combined sewer” where all wastewater, as well as rainfall, were channelled into one system. This area is marked in light blue on the map below.

Combined sewers frequently become overwhelmed during heavy rains, causing raw sewage to discharge into the River Thames and its tributaries through “consented sewage overflows“. With this in mind, the area marked green has a modernised system designed to better cope with a larger population and larger volumes of wastewater. The newer system has separate networks for surface water (which drains from roads and buildings as it rains) and foul water (from toilets, sinks and appliances etc.) to prevent the system polluting our rivers so frequently.

Map showing the Combined Sewer Network in London (blue) and the Separate Sewer (green).

When plumbing is connected right, the foul water is taken to a sewage treatment works where it is cleaned before being released into a nearby watercourse. The surface water drainage flows straight into a river or the sea untreated.

Correct plumbing from Connect Right

Problems arise with this system when appliances are “misconnected” and plumbed into the surface water drainage system. In these cases there is raw, untreated sewage flowing straight into our rivers, causing chronic pollution, degrading water quality and harming wildlife.

Incorrect plumbing from Connect Right

The Department for the Environment, Food and Rural Affairs has estimated that up to half a million properties in the UK are likely to have some kind of misconnection. The most commonly misconnected appliances are washing machines (35%), sinks (20%) and dishwashers (10%), but 5% of instances involve a toilet being misconnected!

When a surface water outfall is polluting a river, it is the local water company’s responsibility to trace the misconnection but it’s the property owner’s responsibility to fix it. Knowing where these problem outfalls are can be a challenge, and that’s where Outfall Safaris can play a big part in helping to fix the problem of misconnections.

What is an Outfall Safari?

Developed by ZSL and partners, an Outfall Safari is an innovative citizen science method for locating, assessing the impact of, and reporting on these polluted surface water outfalls. It was first put into practice on the River Crane, but has since been applied to many other rivers all over the country.

In February and March 2019, we worked with our volunteers to conduct the first ever Outfall Safaris on the Beverley Brook and Wandle.

Each outfall observed was scored from 0 (not polluting) to 20 (severely polluting) based on whether there were signs of pollution and what length of the river the outfall was impacting.

Signs of pollution include discoloured water, gross smell, foam or scum, sewage fungus and sewage litter aka “rag” (debris such as wet wipes that have been flushed down the toilet).

Signs of pollution

The survey has to be completed in dry weather to ensure that these signs of pollution have not been washed away by recent rains. Although we started off well, with an unseasonably warm spell in February, we quickly faced weather more characteristic of the time of year, posing more of a challenge to our volunteers.

Despite this, the team did fantastically well and we had 28 volunteers dedicate over 200 hours to help us to survey over 30 km of river. A massive well done and thank you to everyone who contributed!

What did we see on safari?

We recorded the locations of 325 outfalls along the two rivers: 129 on the Wandle and 196 on the Beverley Brook.

Of these, 73 were polluting (scoring a 2 or more); only 16 on the Wandle (12.5% of outfalls recorded on the river) but a massive 57 on the Beverley Brook (29% of recorded outfalls!), despite its shorter length. As you can see from the map below, many of the polluting outfalls were located on the Pyl Brook, a tributary feeding into the main Beverley Brook at New Malden.

Map of polluted outfalls on the Wandle and Beverley Brook

This section of the river has almost no public access meaning that there are fewer people able to see these problems and report them to the Environment Agency or Thames Water to be fixed. This is why the Outfall Safari has provided such valuable insight; making sure less of our rivers are “out of sight, out of mind” when it comes to pollution.

Outfalls scoring more than 10 were considered to be serious sources of pollution. Along the Wandle, where many people spend time and are on hand to spot bad pollution sources, there were only 3 outfalls scoring higher than a 10.

On the Beverley Brook, the problem was much worse with 13 seriously polluting outfalls being recorded.

Outfalls showing signs of serious pollution

What next?

Information on the offending outfalls have been submitted to Thames Water and the Environment Agency.

Any polluting outfall that is a Thames Water asset will be investigated as part of the Surface Water Outfall Programme. This uses various techniques, including placing cages down manholes and using CCTV, to trace the misconnections to specific properties. The SWOP process is outlined below.

Thames Water Surface Water Outfall Programme

An outfall could be receiving water from a catchment with only a few dozen houses, or it could be thousands of properties! This means that even once an outfall makes it off the waiting list, finding these misconnections is not always a quick job and some can take years to clean up and get signed off.

Any outfall that is privately owned is passed to the Environment Agency to pursue rectification.

We will be in regular contact with Thames Water and the Environment Agency to find out which outfalls are on the waiting list, which are under live investigation and which have been signed off. We’ll then monitor these newly signed off outfalls to make sure any re-occurrences of pollution are investigated.

We’ll be doing another Outfall Safari on the Wandle and Beverley Brook in 3-4 years time to update the findings.

In the spring of 2020 we will be updating the Hogsmill Outfall Safari results after the first survey there in 2016. Join our mailing list to be kept informed of up and coming training sessions and other ways to get involved.

Thanks to all the dedicated volunteers that went on an Outfall Safari. Without your input all these polluting outfalls would have remained unknown to Thames Water and the Environment Agency, but now we’ll begin to see improvement.

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[…] Outfall safaris on urban rivers: a walkthrough with the South East Rivers Trust […]

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