Benefits of new Honey Hill sewage works for River Cam are unclear - because Anglian Water hasn’t modelled overflows, hearing learns

The benefits to the River Cam of the new sewage works proposed for Honey Hill remain unclear because Anglian Water has not modelled overflows, it emerged this week.

A hearing was held examining the water company’s application to replace the existing works in the north of Cambridge.

The new waste water treatment centre is proposed for the countryside to the north-east of the city, on land north of the A14 between Fen Ditton and Horningsea.

The move will unlock the potential for the development of a new North East Cambridge district, with around 8,000 homes around the former sewage works site.

Anglian Water has applied for a Development Consent Order for the move and the Planning Inspectorate held a hearing last week to explore the plans.

Mona Komen, representing Anglian Water, said the new plant would be better for the River Cam.

She told the hearing: “Existing modelling shows that without additional storage there are storm spills once every 10 years [for the existing facility].

“For the improved works with improved storage it is less than that, in the modelled period of 10 years there are no storm spills.

“That is a reduced number of incidents according to the modelling of storm spills to the existing facility, which can only be a benefit to the River Cam.”

But Helena Ombremski, one of the planning examiners, questioned whether the reduction could really be considered “significant” when current spills are only expected once a decade.

Ms Komen said it was a beneficial impact in itself, but there would also be benefits to the river water quality as well.

Ms Obremski asked about what work had been done to look at the impact of combined sewer overflows (CSOs) - by which diluted sewage is allowed into watercourses - from the new facility.

Initially, representatives from Anglian Water said there would be no CSOs from the new works, but when examiners identified that there would be overflow, they clarified that this was “to protect the network” and said it would be used “if necessary”.

Ms Ombremski highlighted there had been no modelling of the combined sewer overflow events - and said without this modelling it was not clear what the overall benefits for the River Cam would be.

Anglian Water representatives said they would come back to the examiner with more information on that point.

CSOs are like pressure valves, used during heavy rainfall.

They were developed because in England’s sewerage system, clean rainwater and waste water from toilets, bathrooms and kitchens are conveyed in the same pipe to a waste water treatment works. If heavy rain means the capacity of the pipes is exceeded, sewage works could be inundated and diluted sewage could back up into people’s homes and flood roads and open spaces

CSOs reduce this risk, but mean diluted sewage is allowed into rivers and the sea.

Government data shows there were nine CSO incidents involving Anglian Water in 2022, as well as 59 pollution incidents across its sewage treatment works.

Questions were also raised by Ms Ombremski at the hearing about uncertainty over river flows and quality of the water in the future, which she said made it unclear how the discharging of treated water from the new facility into the river would impact it.

Ms Komen said adaptive permits from the Environment Agency would allow them to respond to changes with the river.