Storm Water and Poo – the View From Southern Water

Floodwater pouring into a Storm Water Drain.
Storm Water Drain – picture: gettyimages-1021169416_a5_200dpi

Presentation to KCC About Storm Water (and Poo)

Southern Water were recently invited to give a presentation to Kent County councillors. The topic was storm water and poo, not only about poo. It was about the risk of floods, and the need for Combined Sewer Overflows (CSO). 

It is amazing how complex decisions about water can be. Southern Water, together with other water companies, are working on scenario planning for 2050 and 2080. In order to plan for these decades ahead, they are feeding into the calculations predictions on the following variables:

  • Population increase
  • Economics (demand for food and energy)
  • Technical improvements (eg in pipe joint materials)
  • Water infrastructure
  • Global Climate change (drier? wetter?)
  • Environment including agriculture
  • Social, cultural and ethical changes
  • Institutional changes
  • Political – change in governance

“Variable” means that the number put in this part of the calculation can be widely different depending on the assumptions made. Yet it is on these calculations that investment decisions have to be made decades in advance of the effect they will have on water supply and drainage. It is not as simple as just choosing to fix the poo problem this year to avoid fines.

Please Change the Subject

When I rang up the media office to talk about poo, what they wanted to talk about first was storm water. The prime risk that preoccupies those in charge is stopping floods.

These can be internal – when sewage wells up the internal drains of the house, and that is poo coming up out of your toilet or sludge up your kitchen drain. Or they can be external, flooding over your driveway or over the road.

Graphic depicting how combined sewers and overflows work to prevent foul water entering the environment.
Slide #9 from Southern Water presentation
Graphic depicting the arrangement of wastewater treatment and overflow tanks.
Slide #10 from Southern Water presentation

Last year, I encountered one of these on the Bethersden road. Fortunately the guys in hazard jackets had just cleared it and it was passable again. Sometimes tanker lorries are sent along to suck up the discharge.

Nine-Thousand Kilometres Of Sewers to Look After

It is a big job to maintain the infrastructure in each river catchment area:

Sewage catchments7721
Wastewater Treatment plants6921
Pumping Stations635392
km of sewers40205325

On their website, Southern Water  gives this explanation about effluents:

Ashford WWTW serves just over 84 000 people and it is permitted to discharge 24 000m3 per day of recycled water into the Great Stour.

Canterbury WWTW is downstream of Ashford WWTW and serves just under 60 000 people in the city. The flow from the works goes into the Great Stour and is permitted to release 20 176m3 of recycled water per day.

The Environment Agency (EA) sets limits on the quality and quantity of recycled water (known as effluent) that can be discharged from WTWs. The EA issues discharge permits to ensure the recycled water released from WTWs complies with three main legal provisions

(i) The Water Resources Act (WRA) 1991;
(ii) The Environmental Permitting (England and Wales) Regulations 2010 and
(iii) The Urban Wastewater Treatment Regulations (UWWTR) 1994.

The permits ensure that the quality of the receiving water (ie the river or the sea) is protected and that the discharges do not cause an unacceptable impact on the environment. The flow that may be discharged (released) in dry weather is one of the limits set by permits. Our 21 WTWs operate Drainage and Wastewater Management Plan River Stour Catchment in accordance with their permits and recycle the wastewater to the specifications set out by the EA to ensure it is safe and clean to be released back into the rivers and streams or directly to the sea.

Under heavy storm conditions, rainfall can enter the sewerage systems and significantly increase the flow in the system. The flow of water arriving at the WTWs can exceed the recycling capacity of the works, so any excess water is temporarily stored in large storm tanks. If these tanks ever fill to capacity, then they would discharge water into the rivers or sea through storm overflows. Our aim is to prevent any discharge of water that has not been fully recycled to the required standards. 

Any water released from storm tanks is screened to remove items such as wet wipes and solids. This control mechanism is required to prevent the backing up of water within the sewers and putting homes at risk of flooding and these discharges are permitted by our regulator and monitored carefully.

So that explains why, when I wanted to talk about poo, the media officer wanted to tell me facts about storm discharge. It is not as simple as just building larger storage tanks. They recently spent £2.5m building one, and then the manager reported that it filled up in seven minutes in the latest storm.  

Global climate change is having local effects. Our summers in Kent are drier and our winters have heavier deluges. The system may have been planned to sustain a one in 30 years storm. But what if these are coming every year now?

Getting into local Kent details, there are some pie charts which show the source of storm overflow in various towns:

Tonbridge – 80% from the roads

Tunbridge Wells – 40% from roads, 34% from roofs and 26% from green spaces

Areas at high risk of blockages are Margate and Broadstairs, and Weatherlees which gets blockages due to nappies and fat thrown down the domestic drains.

Across the region, some 20 000 blockages are dealt with each year. There are detailed charts of the various risks for each wastewater treatment area, for instance for the Stour. There is also an opportunity to join in the public consultations about planning and investment of the local water infrastructure.


  1. A solution for roads runoff is to build a dyke alongside the road
    I am keen to investigate this further, as Kent has many roads that are probably sending traffic pollution of  micro-particles into our water courses. There are highway gullies which are managed by Highways England, but blockages are not acted on quickly enough.
  2. Build more sustainable houses
    Current designs of houses send too much storm water into the sewers. More sustainable designs would send only 13% into the sewers (from toilets and kitchen) while 87% (rainwater from roofs and paths) would go into the environment and filter naturally into the water courses.
  3. More public education to prevent blockages, and also to prevent urban creep of conservatories and paved-over gardens that send more rain into the sewers rather than into the soil.
  4. Plan for nature-based drainage of rain gardens and wetlands on public land
    A good example of a recent project (funded by the European Union) can be found at St George’s park in Margate.
  5. Plant more trees and have more green roofs.
    In South Africa, I once redirected rainwater from some gutters to drain under a large avocado tree. The tree thrived, and the rising damp in an out-building ceased. We got lots of huge avocados. There is a task-force currently identifying sites in Kent (specifically in Deal, Margate and Swalecliff) where tree-planting can soak up excess stormwater, thus preventing the sewer overflows that cause poo in waterways.