Severn Trent is rolling out pioneering ‘portable sewage treatment systems’ as part of a £450m programme to reduce storm overflow spills.

The water company is boosting river health by increasing its use of specially designed SAF units - Submerged Aerated Filter - which will increase treatment capacity at small to medium sites across our region.

Severn Trent’s Martyn Farrington, Capital Programme Specialist, came up with the unique concept for the new agile units, which can be more quickly deployed.

Larger SAFs have previously been used in the water industry. But cranes were often needed to deliver them to sewage treatment sites, which can be costly and time consuming and they were not suitable for all sites, given access and space issues.

Martyn came up with the idea of smaller SAFs, along with the time saving idea of having the units pre-prepared at our Brancote sewage treatment works in Staffordshire, by batch pre-seeding the units.  The seeding is designed to help us grow the right bacteria which can treat the sewage, a process which can take between six to eight weeks. The SAFs can then be dispatched to suitable sites and plugged into the network to boost capacity. 

Martyn, who has worked at Severn Trent since 1998, came up with the agile SAFs concept 18 months ago. The dad of two, who has worked at the company since 1998, said: “I was sitting at my computer one day looking at roll-on/roll-off (RO/RO) skips and thought ‘why don’t we do that with SAFs?’

“Many of the SAF units on the market needed cranes to deliver and could take two months for the seeding process to be completed.

“We needed something far more agile and quicker, so I came up with the original design of a RO/RO SAF unit and of having a seeding hub.

“My idea won our annual innovation competition, The Challenge Cup, and the company has since funded the scheme.”

The SAF tanks whilst agile are quite complex. They can be delivered and deployed at ground level on a suitable base, with oxygen pumped in to boost the healthy bacteria and settlement zones, to aid the sewage treatment process.

Martyn said: “Adding oxygen creates more of the good bacteria. They feed on the sewage and break it down and you eventually end up with the sludge and a much-improved effluent which then moves on to the next stage of treatment.

“The benefit of these units is that they help us treat more sewage flow and they can boost capacity at some sites by 50 per cent or more.”

Severn Trent is proposing to install over 25 SAF units across the region.

Qualified hiker and mountaineer Martyn was modest about his part in helping cut spills and boost river health. “I’m very proud and did not expect my concept to take off so quickly and to such an extent,” he said.

“I do believe the SAF units will have a very positive impact and help us do even more to protect the environment.

“As a dad and a mountaineer and hiker, that is hugely important to me, so I’m delighted to have played my part.”

Holly Thomson is Assistant Project Manager on the spills programme to reduce overflow spills, having joined Severn Trent as an apprentice in 2021.  She said SAFs were “a very clever piece of kit… helping us treat more flow.

“Given the SAF unit is purpose built and seeded at specific hubs, they don’t need a massive building project, which reduces downtime and carbon impacts.  Prior to getting on site, we undertake detailed modelling to check if the site is right for a SAF, following which we get the site ready, making sure we have a suitable location and associated infrastructure. 

“All the preparation work allows us to plug the SAF units at pace on site, supercharging the local Waste Treatment Plant. Post installation, we introduce a programme of maintenance to ensure the seeding is optimal and the unit is working effectively.

“Overall, this means the works can deal with more wastewater, which is great news for communities and great news for rivers.”

The SAFs are among a range of solutions being used by Severn Trent to reduce storm overflow spills within a £450m programme of investment, where the company is targeting over 900 enhancements.

Other measures include installing additional storage solutions - with one of the biggest being at Stroud, Gloucestershire – plus planting filtering reed beds and installing water butts in catchments where we have larger roof areas.

To inform communities about the rapid progress being made with projects across the Midlands a dedicated YouTube playlist has been created, with some 50 videos now on display.