The water company warned that extreme rainfall could still overwhelm Portsmouth sewers

Despite tens of millions of pounds being invested in projects across Portsmouth, Southern Water has said there will be “occasions where we cannot cope” with the amount of rain water.

The water company, which handles wastewater treatment in the city, outlined the work during a Portsmouth City Council scrutiny panel examining sewage discharges.

It presented plans for 43 surface water reduction projects alongside broader infrastructure investments across Portsmouth and beyond, amounting to £79.77m.

Sewage discharges, or storm overflows, are mainly caused when rainwater overwhelms the city’s Victorian combined sewer system, which carries both foul and surface water in the same pipes.

However, newer developments have separated surface water from foul water, with work also being carried out to retrofit this separation on existing buildings or developments.

In a bid to reduce storm overflows, Southern Water has proposed 43 projects aimed at removing 50 hectares of land that does not absorb water in Portsmouth, known as impermeable surfaces.

Separation projects

These projects aim to separate surface run-off water from foul water in large buildings across the city where this currently does not occur.

They include Portsmouth Cathedral, Portsmouth Grammar School, Portsmouth Museums and Records Office and “multiple large buildings and car parks”.

There are also plans to address areas where water has already been separated, “but you follow the route down and eventually it goes back into the wastewater system”.

Sites including Salisbury Road Allotments, Pier Road in Southsea and Guildhall Walk have “historic misconnections” which are being corrected in phases.

Fitzherbert Road, Drayton

There is 15 hectares of impermeable area within Fitzherbert Industrial Estate which currently drains directly into the combined sewer network.

The proposal is for a new surface water network discharging into Langstone Harbour via a new pumping station and 800m pipeline.

An alternative “nature-based solution” to rehydrate Farlington Triangle is currently being assessed.

Sewer sealing at Spice Island

On Spice Island in Old Portsmouth, 300 litres per second of seawater flows into the sewer twice a day at high tide, which “overwhelms the system” and leads to storm overflow releases.

The solution is to seal 2.8km of public and private sewers to stop seawater from entering.

Work started in December last year and is anticipated to end in December this year.

The Western Interceptor

The Western Interceptor is a deep trunk sewer designed to divert flow from the existing sewer to Henderson Road pumping station.It is proposed to extend the pipe, which could reduce 120 storm overflow releases across six overflows.

The project would involve a 900m sewer extension delivered using a “trenchless installation method via three launch and reception pits”.

Highways sustainable drainage systems (SuDS)

SuDS is to soak up run-off, prevent flooding and stop water reaching the sewers.

Working in partnership with the council, the delivery of SuDS should improve flood resilience, create greener and more attractive streets, support biodiversity and provide urban cooling.

There are currently plans across 40 roads to deliver sustainable drainage covering 7.66 hectares of highways.

The work is being implemented across 16 roads in Cosham and Drayton, seven roads in North End, eight in Stamshaw and seven in Southsea, alongside a completed project in Talbot and Bramble roads.

Five Portsmouth schools are also receiving SuDS projects through a £1.37m investment from Southern Water and the Government.

A further £913,000 investment is planned for four schools in 2027.SuDS projects include slow-draining water butts, rain garden planters, soakaways and permeable paving.

Investment into Budds Farm Wastewater Treatment Works

Budds Farm Wastewater Treatment Works treats wastewater from Portsmouth and some flows from Havant.

Southern Water is investing £58m between 2025 and 2030 to increase storm overflow capacity.

This includes upgraded pumps, two additional primary settlement tanks, three additional final settlement tanks and upgraded effluent pumps which will “ensure there are less storm overflow releases”.

Questions over future capacity

Councillor Richard Adair said “there’s no doubt our climate is changing”, anticipating wetter winters, and asked Southern Water whether it felt prepared for them “even with the measures you’re taking”.

In response, a spokesperson said: “You’re right, the conditions are potentially becoming more exceptional”.

They added that separating the water is a more sustainable approach than building “bigger and bigger” pipes and tanks “which have a finite life”.

“The question is what’s the new design horizon, how tough is it going to get, how much more do we need to separate.

“Some of these conditions are going to be more exceptional and there will still be occasions where we cannot cope with the amount of water.”

They added that the company is building towards a target of no more than ten storm overflows per year on average.

Dry day spills were also addressed, where discharges occur during seemingly dry weather due to operational issues such as blockages, power cuts and groundwater pressure.

On blockages, a company spokesperson said they “welcome the government is making steps to ban wet wipes because they are behind 75 per cent of our network blockages – they’re made of plastic so we want to see the end of those one day.”