Residents from Corfe Castle in Dorset were treated to a tour of their local sewage works and were able to quiz Wessex Water officials about why sewage still gets into Corfe River.
As part of a series of Around The Bend tours organised by Wessex Water across its region, Corfe Castle water recycling centre opened its doors to the public on Thursday 28th September 2023.
The Corfe Castle sewage works is near Purbeck Park at Norden
Traditional water recycling centre built in the 60s
The sewage works, built in 1961, is located to the north of Corfe Castle near to Purbeck Park at Norden and treats sewage from a population of around 2,000 living in and around the village.
One of the members of Wessex Water staff showing groups of people round was area scientist for Blandford and Purbeck Danielle Wallin. She said:
“Our aim is to make sure that the effluent that is pumped into the Corfe Castle water recycling centre, is turned into the highest quality of water when it goes out, causing no negative effects to Corfe River and the Poole Harbour shellfish industry.
“While the Corfe Castle works is a traditional water recycling centre – built more than 60 years ago – it’s been updated over the years, with the latest addition being the UV treatment process.
“This disinfects the water and that’s really important when it flows into Poole Harbour and for the shellfish that live there.”
Wessex Water mechanical and electrical manager for West Dorset and Purbeck Jack Taylor shows what the water looks like before it is treated
The first step in the process is to remove all the debris like wipes, plastic and paper. It’s then washed, chopped up and dried before being put in a skip and taken to be composted
With the debris removed, the sewage starts to be filtered. If there’s too much sewage coming into the works, typically during heavy rainfall, the excess sewage goes down the overflow pipe pictured at the front of the photo, to a storage tank
Discharge of untreated sewage
The key question that was asked by those visiting the works was, ‘Why does Wessex Water allow untreated sewage to sometimes spill into the River Corfe and how can that possibly be legal?’
The Wessex Water staff explained that discharge of untreated sewage can occur when it rains heavily and the water off roofs and road surfaces pours into the drainage system, along with the waste from toilets.
This is the way the drainage in most older homes was designed with rainwater from the gutters and toilet waste going into the same pipe. That’s not allowed in newly built properties now.
Reducing phosphorus from the sewage is really important for keeping watercourses oxygenated for wildlife. This is done by adding ferric sulphate
Once the sludge has been removed and tankered away, the sewage, which is becoming increasingly cleaner, passes through one of the two large biological filterbeds at Corfe Castle. This process reduces the amount of bacteria in the water
There are also two newer, more compact, moving bed biofilm reactors (MBBRs) which also remove bacteria as well as oxygenating the water
Water storage tank is overwhelmed
This sudden surge of storm sewage can overwhelm the water recycling works and rather than allow it to back up in the pipes, at Corfe Castle it overflows into a large water storage tank.
Once it has stopped raining, the content of the tank is pumped back to the start of the treatment process and is filtered until it is clean.
However sometimes there’s too much water and even the overflow tank overflows. This is when some untreated sewage goes straight into the River Corfe, although it’s heavily diluted by the rainwater.
On 22 occasions this happened at Corfe Castle works in 2022. This is legally allowed to happen because otherwise the water would flood back into homes.
It is only illegal when untreated sewage goes straight into the River Corfe, when it hasn’t rained heavily and this is called a dry spill. This hasn’t occurred recently at Corfe Castle.
Although most people only worry about poo in sewage, one of the biggest concerns for the environment is phosphorus, which is present in some washing powders and fertilisers.
If this is not removed during the process and is released in to a river it boosts the growth of weeds which reduces the level of oxygen in the water and can cause fish and other wildlife to die.
The Corfe Castle operations hub which allows the works to be automated
The ultraviolet treatment process is a new development which makes the water discharged at the end of the process, of a much higher quality than in the past, by disinfecting it
Here the sewage has been turned into clean water and will be tested to ensure it is up to the high standard required
Wessex Water action to prevent storm overflows
However the Wessex Water team told those on the tour, that while a storm sewage spill is mostly rainwater, the company agrees with the general public that releasing any untreated sewage directly into a water course has no place in the 21st century. The big question is how to achieve that?
Wessex Water is currently spending £3 million a month to reduce overflows across its region which includes most of Somerset and Wiltshire as well as Dorset.
The main solution is to build bigger water storage tanks to hold more water when it rains and that’s already happening in Bournemouth and Bristol and is likely to be a solution for Corfe Castle, as well as Swanage sewage works.
A concrete tank stores sewage when there’s too much coming into the works during heavy rainfall. It then gets pumped back to the start of the process once the system can cope again
Occasionally, during persistant rainfall, even the giant storage tank can’t store all the sewage, so it is discharged, although diluted by the rain, directly into the River Corfe
The River Corfe is classified as having poor ecological status by the Department for the Environment but that is attributed to the agricultural sector as well as the water industry
There are also nature-based solutions like wetlands or reedbeds that can be introduced in rural areas, which are more environmentally friendly than building more concrete tanks.
The target for Corfe Castle in the future is to reduce the number of storm overflows to 10 a year – roughly half the number that occurred in 2022. Any solution will be subject to regulatory approval.
One of the areas for discussion was around what we can do to our property, to help reduce the storm overflows. While it can be quite costly for a homeowner to split their rainwater drainage from their toilet waste drainage, it’s not impossible.
However other measures like installing water butts to collect rainwater and not covering front gardens with impermeable hard surfaces, can be cheaper and more easily achieved.
Quizzing the Wessex Water employees
Wessex Water area scientist for Blandford and Purbeck Danielle Wallin with a sample of the clean water produced at the end of the process
“I don’t think people generally appreciate how complex the process is”
Danielle Wallin added:
“This is the first time that I’ve done an Around the Bend tour with the general public – it’s great for people to see what we do and to get to ask questions. I’d love more schools and young people to come and visit too.
“I don’t think people generally appreciate how complex the process is or understand how many filters the water goes through. People seem quite surprised how clean the water is at the end!”
- Wessex Water information on sewage discharge and storm overflows