Storm overflows are receiving a lot of public attention these days. There are now many groups doing citizen science and volunteering to monitor overflows of sewage into their local favourite river. There is much campaigning, by the Lib Dems and by Feargal Sharkey, to stop sewage flowing into rivers by banning the annual bonuses of the water company CEOs if they do not stop them. But there is also a lot of confusion about where rainwater goes and what to do to improve drainage on this rainy island.
Where does all the water go?
We have inherited a drainage infrastructure consisting of an underground network of terracotta pipes (sewerage). Property owners are responsible for the pipes and drainage functions within their boundaries. The underground pipes take surface water (mostly fresh rainwater) to the public surface water pipes. The foul water pipes, usually collected from toilets, bathrooms and kitchen drains, connect to the deeper public foul water system.
Public surface water pipes then carry rainwater to local water courses. The public foul water pipes take the mucky liquid (sewage) to the wastewater treatment plants (WWTPs). There it is filtered then put into settling tanks, chemicals are added and it is checked for safety, before being discharged into the watercourses.
Overwhelmed wastewater treatment plants
But all too often, the WWTPs are overwhelmed with too much rainwater coming into the system for this slow, safe process. When this happens, the water companies are allowed to use a quick-release bypass pipe, which discharges the overflowing liquid (rainwater contaminated by sewage) into the natural watercourse without full processing. These diagrams show how excessive rain can trigger storm overflow into the sea or local river.
Protesters insinuate that the water companies have been shirking because they have not invested enough in infrastructure to cope with storm discharges.
The huge cost of replacing old drainage infrastructure
But what exactly is this infrastructure? A tour around the local WWTP revealed the enormous size of each settling tank. But building new tanks requires a lot of concrete, an expensive carbon-emitting construction. An increase in chemicals for treating increased volumes of stormwater is also not planet-friendly.
So, if not new tanks, what might better manage the stormwater flow?
Storm water is currently carried through terracotta pipes of 150 to 225mm diameter. Some of these are 100 to 150 years old. Some are combined sewers, taking both foul water and surface rainwater. Regulations to separate sewerage only came in the 1960s. And in areas of Kent, like Whitstable, the combined sewers have been a cause of storm overflows.
Of the 100,000km of combined sewerage in the UK, Southern Water reckons we have 40,000km in the South East. It would be a huge infrastructure expenditure to change all of these into separate sewerage systems and extremely disruptive where this would involve digging up urban roads, especially as the sewers are at the deepest level under them and often in the centre.
Benefits of combined sewer system pipes
Interestingly, I have found some research illustrated by diagrams which show that old, large, combined sewer pipes may contribute less to stormwater overflows than the smaller diameter foul water-only pipes. This is because the biggest cause of storm overflow is cracking in the pipes where rainwater enters. In excessively wet weather, a larger diameter pipe, even with rainwater leaking into it, is less prone to overflow or back up. This means it does not trigger the storm overflow system.
New plastic pipes aren’t the answer
Whatever the size of the pipe, cracks are inevitable because the ground and soil constantly shift, putting pressure on the old terracotta. Rainwater soaking into the earth contributes to this process. The geology of the South East has much permeable chalk with water flowing through it. This leaks through the cracks around the ceiling of old pipes.
Would using plastic pipes address this problem? They can be more flexible and, therefore, less prone to cracking; are cheaper, lighter, easier to install, and resist corrosion. But, there are concerns about emissions from plastic production and plastic waste. Also, certain chemicals in wastewater might degrade plastic pipes over time. There are further fears that microplastics might leach from the pipes into wastewater and so, ultimately, into watercourses.
New technology for the old pipe network
Southern Water recently found a techno-fix for cracks in terracotta pipes. They are now experimenting with a resin which they can pump into the pipes to reline them. This means they do not have to dig up roads to mend the old leaky sewerage. Good news, but remember they have 40,000km of sewers, so there are probably many years of resin work ahead.
How do SuDS work?
Another solution is SuDS. However, we must be more aware of how SuDS operate in practice.
Before a property owner or developer can get permission to build, they must get their designs signed off for SuDS. The local water company must be consulted if any pipes will be connected to the public system, and they cannot refuse an application. In other words, the assumption was that water companies just had to plan for future increased demand for sewerage. Often, this is raised when campaigners protest about new housing. It is a highly emotive topic, with people visualising all those extra ‘poopies’ flowing to the local WWTP and on into their local river.
Local authority SuDS offices: still awaiting implementation
It is not the extra flushing of toilets that causes overflow. All those new houses require roads and parking spaces. That means concreting over the green acres that used to soak away the excess rainwater. which causes it to gush into the surface drains or the old combined pipes.
Regulations about SuDS are to be found in the 2010 Water Act at Schedule 3. This states that as far as possible, new builds must not increase the flow through the public pipes. Each new build has to use nature-based solutions, such as ponds, soakaways, ditches, and tree planting.
The Schedule assumes that each local authority will set up an office to handle SuDS applications. The problem is that this was supposed to be implemented in 2024, but the new government have not yet brought Schedule 3 into operation. Why not? At a guess, it is because it may have to wait until the massive reorganisation of local government is undertaken.
A mixed picture of SuDS, and it’s the residents who pay
However, putting the onus and cost of SuDS infrastructure on to the builder is being implemented. I can see it in many places around Ashford, where there are ponds and trees near new builds. But there are some places where it is being badly done. For example, recent news from Allingham near Maidstone reveals that nine owners of new builds suffer frequently from floods. This is because the pumping station that was installed for SuDS does not work properly. This is despite it being passed as adequate and put into their property deeds as something they had to maintain.
Our water bills contain a proportional charge for surface water drainage. If a property does not drain into the public system, for example, a farm where the rainwater from the roofs of barns and all buildings drains into a pond, then the water bill should not contain this charge. I wonder if the aggrieved residents at Allingham are having to pay for the water company charge and their specific pumping system?
