The Hogsmill river, a globally rare chalk river, is regularly contaminated with sewage

On the afternoon of Wednesday 24th January 2018, this happened…

…. And it’s not an isolated event. This is sewage, temporarily held in a Thames Water storm water tank in Epsom, discharging into the Green Lanes Stream and the Hogsmill river.

What are Storm Tanks and why is this happening?

The Hogsmill (Epsom and Ewell) Storm Tanks are both located in the upper reaches of the Hogsmill river. The tanks function as temporary storage for untreated sewage as it travels through the network of pipes towards the Hogsmill Sewage Treatment Works (STW). During heavy rainfall, rainwater as well as sewage fills up the system: to prevent it backing up into homes, there are temporary storage systems like these storm tanks, and sometimes overflow pipes into rivers (known as combined sewer overflows), which help to relieve the pressure.  Usually the storm tanks contain the sewage until the rain has passed and the sewage can drain back into the network to be treated at the Hogsmill STW. However, occasionally, during higher rainfall, the storm tanks fill up completely and will discharge any excess sewage to the Hogsmill river itself.

This system was designed when the population of London was much lower and the area of paved urban surfaces (which cause rain to run off rather than infiltrating into the ground) was much less. It is a consented discharge – which means it is legal – as it was originally designed to happen only very occasionally as a kind of safety valve for the system (once or twice a year we think), but with population growth, urbanisation and climate change, it now happens more frequently. Our volunteers have monitored around 12 or 14 flow events per year in recent years.

Why is all this a problem for the Hogsmill?

The Hogsmill is classified as a chalkstream, a globally rare habitat with only 200 remaining worldwide. Chalkstreams give rise to a unique set of species that depend heavily on the clean, chalk-purified water and are consequently very sensitive to any decline in water quality.  The Hogsmill Storm Tanks are therefore a real threat to the chalkstream species community, even more so because they discharge sewage into the headwaters, affecting the entire river downstream.

When the system was designed and consented, it was done so on the assumption that the sewage would be diluted as the river would have high flows from all the rain. Unfortunately two things have happened which mean there is much less water in the Hogsmill than at that time. Firstly, many areas have been paved over with urban growth, meaning rainwater runs down drains rather than soaking into the ground and feeding the river. Secondly, the groundwater aquifer from which the Hogsmill flows, is the same source of water that comes out of our taps, and all the water we use has reduced the amount available for the river, and, ultimately, dilution of the storm tank effluent.

Finally, we know that the Hogsmill is important to local people and loved by many. None of us want to see it polluted, and we are particularly worried about this pollution because many people enjoy paddling in the river in warmer months.

What can we do about it?

The South East Rivers Trust and the Hogsmill Catchment Partnership are very concerned about all these impacts and have been working with Thames Water to find a solution.

 Our network was designed to discharge here when it becomes overloaded after heavy rain.  These discharges are legal in storm conditions, but of course deeply unpleasant and undesirable.  We understand the local concerns and are currently studying ways to manage flows better across this whole catchment.  We will share the report with the Catchment Partnership when the work is complete.  In the meantime, I can confirm that a clean-up operation started today, with the aim of removing as much litter as possible” – Richard Aylard, External Affairs and Sustainability Director, Thames Water 25th January 2018

What can you do to help?

While it may seem a problem which most of us can do little about, everyone can actually be part of the solution. The Catchment Partnership will continue to work with Thames Water to find a permanent solution to the problem. In the meantime, below are just some of the ways we all impact the sewer network and how we can help to alleviate the system.

Say no to paved gardens!

Every time a garden or driveway is paved over, we are exacerbating this problem. Tarmac, pavements, decking, and other similar surfaces, are usually impermeable to water, so rain simply washes off straight down the drain. With more and more people opting for paving, we are putting an increasing demand on the sewage network, making it more likely that the tanks will fill up and over top, discharging to the Hogsmill more and more frequently and with greater volume ending up in the river.

Bin it, don’t flush it!

Walking down the Hogsmill, you may see some unexpected items such as cotton buds, wet wipes and sanitary products. These have been flushed down toilets and have reached the Hogsmill through the Storm Tanks or misconnected pipes. Within the sewer system, they can also cause blockages and when combined with fats and oils from the kitchen sinks, create giant fatbergs, which can burst sewers, over top into nearby rivers or flood people’s homes.

The only things that should be flushed down the toilet are the three Ps – pee, poo and paper. Wet wipes are often claimed to be flushable, but these are actually are one of the biggest causes of blockages. So, unless it’s one of the three Ps, bin it, don’t flush it… or you may see it again in the Hogsmill!

The overflow pipe from Ewell Storm Tanks on the Hogsmill after a discharge incident.

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