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Imagine sitting next to the river watching crystal clear water flowing over clean gravel whilst the first mayflies emerge from the water surface. You sit back breathing in fresh air as the sunshine dapples the flowers and reeds along a thriving river system.

Unfortunately this is often not the case.

As we discussed on Wednesday 30th November 2022 at the Water Quality Forum, rivers across the United Kingdom are facing unprecedented issues as pollution impacts river water quality and so are the species that thrive there too. There were presentations from the Environment Agency, Affinity Water, Thames Water and Groundwork East which introduced us to the Watford Quality issue here in Watford and the surrounding areas which the River Colne is facing.

The River Colne is impacted by pollution from a variety of diffuse and point sources; these include waste water from the water industry, as well as pollution related to agriculture and urban and transport infrastructure. A major cause of pollution of the Colne, is raw sewage entering the river via drains and sewers that are meant to only carry rainwater.

Like much of the country, the Watford area has two separate drainage networks with different functions:

a) A Foul system which transports wastewater from household appliances (e.g. toilets washing machines) to the sewage treatment works

b) A Surface water system which sends rainwater from roofs and roads, directly to the river system.

Unfortunately, when property owners and professionals carry out repairs and improvements to homes and workplaces, wastewater pipes can be misconnected to the surface water sewers, either due to ignorance or, corner cutting. In parts of the UK, it is estimated that as many as one in five properties have misconnections that discharge effluent into rivers[1].

The surface water pipes discharge to the rivers through outfalls located along river banks. If wastewater from e.g. toilets, showers and washing machines, is incorrectly connected to the surface water system, it goes straight into the river with no form of treatment.


These issues create negative impacts with higher levels of phosphate and ammonia going into the river system which unbalances the ecosystem and causes eutrophication.  Eutrophication is when there is too much nutrient in the water course resulting in excessive growth of algae and plants. This increases the prevalence of toxic algal blooms, shifting from sensitive plants in the river to nutrient tolerant species which in turn reduce oxygen levels. All this negatively impacts invertebrate and fish life species.

When large discharges or continuous pollution enters the river through misconnections there is the visible evidence in the river of sewage fungus, a grey filamentous fungus in the water. This is  normally found below the outfall tangled with a range of different rags such as tampons and wet wipes (wet wipes are for the bin not the toilet!). This is very bad for the river, it’s wildlife and our enjoyment of the watercourses as smelly wafts of sewage is not ideal for a summer picnic.

But there is hope!

Local volunteers have been out completing outfall safaris assessing outfalls into the River Colne to identify misconnections. Several were found and reported to Thames Water, a key project partner, who have been working to resolve these misconnections and reduce the amount of pollution entering the river. Once a polluting outfall has been reported there is a process to locate the source and resolve the issue using a range of techniques to trace the pollution such as visual inspections, cctv work and dye tracing.

There have been 4 major outfalls reported in Watford along the River Colne with misconnections which Thames Water has been working to resolve. Each outfall has several different misconnections which are sending pollutants straight into the River Colne. To illustrate the complexity, there were 8 properties with misconnections which had 11 different misconnected appliances (see table below) carrying foul untreated water straight into the River Colne.

But there is good news!  Of the 8 households with misconnections, Thames Water successfully rectified 7 to reduce the amount of pollutants entering the River Colne. The final investigation and resolution of the last misconnected property is still ongoing to identify a solution.

The mission to protect our rivers doesn’t just stop there, we can all do our bit to stop the misconnections happening in the first place and protect our local rivers. 

1. Check your homes and pipes to ensure they are accurately connected (Source: A Guide to Running an Outfall Safari January 2019, ZSL and The Rivers Trust) 

2. Ensure to use licensed contractors when completing any development work in your home, for more details, please visit Thames Wate's website

3. Report pollution incidents to the EA & Thames Water. You can also add it to the local website here:






Environment Agency incident hotline - 0800 80 70 60

Thames Water incident hotline - 0800 316 9800


4. Volunteer and help monitor the health of the river through a variety of different activities, the details are available on out 'Get Involved' page. 

Source: Volunteers being trained to identify polluting outfalls during an evening activity. Groundwork East

Water is vital for all life to thrive, today and in the future. By working together we can secure a healthy waterway for future generations of all species to enjoy. 

Source: WBC - The River Colne flowing through Knutsford Playing Fields.

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