Pennington sewage works spilled for 922 hours before its £17m fix

Southern Water has started a £17 million upgrade of Pennington Wastewater Treatment Works, which it says treats wastewater from thousands of homes and businesses near Lymington. The company announced the work on 15 September.

What the announcement does not say is how often that works has been discharging into the Solent. Its regulator publishes the figure, and it is the highest in the district.

The storm tank overflow at Pennington discharged for 922 hours across 66 separate spills in 2025. That is 38 days and 10 hours of discharge in a single year, from one outfall, on the edge of Lymington. The figure comes from the Environment Agency’s Event Duration Monitoring storm overflow annual returns, which every water company in England must file.

No other treatment works in the district came within 300 hours of it.

What the £17 million buys

The works stands off Milford Road at Pennington, west of Lymington. Southern Water says the scheme includes:

• an increase in treatment capacity from 375 litres per second to 567, a rise of more than half
• a new ultraviolet disinfection system to treat the water before it is returned to the environment
• an additional settlement tank
• upgrades to pipework, pumping stations and electrical infrastructure across the site

The company expects to finish in September 2028, two years from now.

Jorge Rodriguez, the project manager, said in the announcement: “By increasing treatment capacity by more than 50% and introducing new UV technology, we’re improving the site’s ability to treat wastewater to an even higher standard.”

Southern Water says the benefit will fall to Avon Water, which reaches the Solent near Keyhaven. It also says it has spent £63 million on water quality work in Southampton Water in recent years, and a further £50 million on New Forest treatment works in the last three years, including upgrades at East Boldre and Lyndhurst.

The announcement makes no claim about storm overflow spills, and does not mention them.

The record the announcement leaves out

A storm overflow is the relief valve on a sewage works. When rain or groundwater pushes more flow through the site than it can treat, the excess is screened and released rather than backing up into homes and streets. The Environment Agency requires a monitor on every one of them, and the readings are published once a year.

Pennington’s readings for the last five years look like this.

Share this chartFacebookWhatsAppXCopy linkEmbed

2025 was the second lowest of the five years, and it was still 38 days of discharge. 2023 was the worst at 2,311 hours, or 96 days, more than a quarter of the year.

Two details in the 2025 return are worth pulling out.

The first is the long-term average. The Environment Agency calculates one from every validated year of monitoring at each overflow, and Pennington’s is 68.89 spills a year. The 66 spills recorded in 2025 are not an unusual year. They are the normal one.

The second is the regulatory status. Against Pennington, Southern Water recorded a Urban Waste Water Treatment Regulations investigation as ongoing, and, in the column for improvement activity, “no improvement action in the reporting period”. The investigation was running and the work had not started. It started this month.

A second, separate overflow at the same site, on the inlet, discharged for six and a half hours in 2025 on one occasion. That one is not the problem.

Where it goes

The Environment Agency’s consents database records the receiving water for Pennington’s storm overflow as the Solent, in the Solent water body. It also records three shellfish waters the outfall sits within:

• Lymington and Sowley
• Pennington
• Yarmouth, across the water on the Isle of Wight

Shellfish waters matter here because they are classified on bacterial contamination, and the new ultraviolet system is the part of this scheme aimed squarely at bacteria in treated effluent.

How the rest of the district compares

Nine treatment works inside the district reported storm tank discharges in 2025. These are their hours, ranked.

A tenth works, Millbrook, sits on the Test estuary right on the district boundary and spilled for 439 hours across 28 spills. It takes its name from the Southampton suburb across the water, and it is left out of the table above for that reason.

Brockenhurst is the one to watch after Pennington. Its storm tank spilled 52 times in 2025, which is below its own long-term average of 86.78 spills a year, the highest average of any New Forest overflow in the return.

The west of the district is served by Wessex Water, not Southern Water, and its figures are thinner. Wessex reported 23 spills at Ringwood sewage treatment works and 21 at Fordingbridge in 2025, both into the Hampshire Avon, but left the duration column blank for both. There is no published total of hours for either town.

Fordingbridge has one more entry that stands out. A combined sewer overflow on Salisbury Street spilled 31 times for 25 hours in 2025, and the Environment Agency records its receiving water as a designated bathing water, the River Avon at Fordingbridge. Five other New Forest overflows list a bathing water, at Calshot, Lepe, Christchurch Bay, Highcliffe and Milford on Sea. All five are on the coast. Fordingbridge is the only one on a river.

What it means for you

• If you swim, paddle, sail or walk dogs off Pennington and Keyhaven, the overflow that discharges there did so on 66 occasions last year. An annual table is the wrong tool for deciding about today. Southern Water publishes near real time release information for every storm overflow outfall in its region on Rivers and Seas Watch, the service it used to call Beachbuoy.
• If you buy local shellfish, the beds at Lymington and Sowley and at Pennington sit inside the area this outfall discharges to. They are separately classified and monitored for bacteria.
• If you live near the works, the construction runs until September 2028. Southern Water has not published a traffic management plan for the site, and none of the current Hampshire road closures name it. We list the ones that do affect the district on our New Forest roadworks and travel page, which already carries a separate Southern Water water main replacement at Long Copse, Fawley.
• If you want to check any of this yourself, the annual return is a public spreadsheet. Pennington’s rows are under permit reference A00665 on the Southern Water sheet.

The numbers here are a measure of how often untreated flows are released, not of how much was released or what was in it. Volume is not monitored at these outfalls, and no figure for it exists. The monitor at Pennington was working for 99.99 per cent of 2025, so the gaps in the record are not the reason the hours are high.

Sources

• Southern Water, “Southern Water begins major work to improve water quality in the New Forest”, 15 September 2026, for the £17 million figure, the capacity increase from 375 to 567 litres per second, the ultraviolet system, the settlement tank, the September 2028 completion date, the Avon Water benefit, the £63 million and £50 million of earlier spending and the quotation from Jorge Rodriguez.
• Environment Agency, Event Duration Monitoring, Storm Overflow Annual Returns, 2021 to 2025 returns, Southern Water and Wessex Water sheets, for every spill hour, spill count, long-term average, receiving water, shellfish water, bathing water and investigation status quoted above. Sites were matched to the New Forest district using their published grid references and the Office for National Statistics local authority boundary.
• Environment Agency, EDM 2025 Storm Overflow Annual Return dataset guide, for the definitions of total duration, the 12 to 24 hour spill counting method, the long-term average spill count and the Urban Waste Water Treatment Regulations investigation category.