Redesigning wastewater treatment works with river health in mind

30 November 2025

The CIWEM East Midlands Branch organises a visit to Toton Sewage Treatment Works to understand a capital project to improve water quality on the River Erewash

The Attenborough Nature Reserve is an important overwintering site for wildfowl including shovelers and diving ducks. Yet parts of this Nottinghamshire wetland, which is a site of special scientific interest (SSSI), as well as the River Erewash, which feeds the reserve, have an excess of phosphorus. An essential nutrient for plant growth, phosphorus causes algal blooms with potentially devastating consequences for freshwater ecosystems as it inhibits the growth of other plants and the fish and insects that depend on the oxygen they create.

Aware of the risks that phosphorus poses to freshwater ecosystems, the Environment Agency (EA) has tightened up permit requirements for phosphorus at Toton to just 1mg/l. To meet this requirement, and generally reduce levels of phosphorus entering the Erewash,, Severn Trent Water have completed a programme of work under the Water Framework Directive which included a new outfall location on the River Trent and the installation of a rising main transfer pipeline with a series of watercourse and rail crossings. By moving the outfall to a place where the phosphorus in the wastewater is more diluted by the higher volumes of river water, Severn Trent avoids the need for complex tertiary treatment.

CIWEM East Midlands Branch arranged a site tour of Toton SWT this past June to inform members of the basic principles of how the sewage works operates and explain the current capital investment work. Around 40 people attended from as far away as Manchester, including prospective and recent graduates, modellers, contractors, academics and retirees. Dave Stone, the site manager for the works, led the tour supported by operational technician Allan Nicholls, John Cordoon, the STW project manager for the capital investment and our contractors Coffrey Construction.

We started off looking at the sewer inlet of the works where all of the flows come into the site. This included a set of Archimedes screw pumps (basically a corkscrew), to raise the incoming flows to a suitable height as to be able to gravitate through the treatment process. The works is located to the west of Nottingham, taking in the town of Long Eaton and the Chilwell and Toton areas of Nottingham. The population equivalent (PE) – the maximum load of domestic and industrial sewage a works can process, expressed as a number of people – at Toton STW is currently 70,114. It receives a peak flow of 532l/s in dry weather and a storm flow of around 1,200l/s.

The inlet screening building, where the grit and large debris is screened and removed, was unfortunately out of bounds due to roof repairs, but these processes were explained from a distance. Toton STW has been designed with built-in resilience in that it has two different process streams: biological filter beds and activated sludge plant. Most sewage works have just one form of treatment.

We first viewed the treatment pathway of the primary settlement tanks (PSTs), this removes finer solids that have passed through the initial inlet screening by settling the flow, allowing them to gravitate to the bottom of the tank. This sludge is then pumped away into storage tanks to be collected and sent away to biological digestion sites. The wastewater gravitates to a distribution chamber, from where 60% of the flow goes to the filter beds, where the settled sewage is distributed over the filter media. This media has a layer of microbial film which treats the settled sewage. We learned about some of the positives of this method, such as the fact that it is low-energy, self-propelled and tried and tested. There are disadvantages too: the bacteria struggle in colder weather and weeds grow on the beds in the summer. Lots of questions were asked about the processes.

The remaining 40% of the settled sewage is sent to the activated sludge plant. The process begins with an aeration tank, where air is injected into the wastewater. This is followed by a settling tank to allow the sludge to settle and separate from the clear treated water. Part of the waste sludge is recycled to the aeration tank and the remaining waste sludge is removed for further treatment and ultimate disposal.

We then went to look at a recently constructed 12-metre deep storm tank capable of storing 1,050 cubic metres. It was constructed as part of the capital project to ensure that storage was keeping up with growth in the catchment. This was designed by engineering consultancy COWI – several of the company’s designers attended the site visit and explained some of the issues faced, such as a risk that high groundwater might cause the tank to float. Some modellers on the visit said how the event brought to life what they often see on models.

We visited the final effluent monitoring station to see how on-site gauges and alarmed telemetry systems measure turbidity, ammonia and phosphates in wastewater before it is discharged to the River Trent. We also got to see the brand-new pumping station and rising main that has recently been going through testing and commissioning. One of the visitors commented that it was great to see such a large rising main project and all the complexities with regard to design and buildability associated with proximity to a railway, a watercourse and an already operational STW. We then returned via the alternate treatment process of the activated sludge plant. We heard from the site manager and operator how air is blown into the process to stimulate bug growth – this provides an improved process yet is very energy intensive.

The CIWEM site visit was a success, both in terms of seeing capital investments underway and giving visitors a better understanding of the processes and challenges of operating a STW of this scale. This should help graduates and early career modellers in applying what they have seen to their models, guiding them to ask the right questions and increasing both their awareness and understanding of the field.

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Andrew Warren is a wastewater network operations specialist at Severn Trent Water, chartered member of CIWEM and chair of the East Midlands Branch.