Woolston Wastewater Treatment Works

Work is well underway on our multi-million redevelopment of the treatment works which started in September 2014.

Picture above is an artist's impression of the completed redevelopment

The existing site was built in 1966 and was in need of modernisation. The redevelopment project once completed will:

  • Reduce smells coming from the site
  • Ensure the treated wastewater leaving the site meets new, higher environmental standards
  • Ensure the redesigned works fits in with the ongoing redevelopment of the area.

The end result will be a modern wastewater treatment works that will serve the area long into the future. Our film below gives a 3D virtual tour of how the works will appear from the street or estuary when construction is complete.


Temporary works

More than half of the temporary treatment works have now been demolished and we will continue to be phased out as we progress the construction of the permanent site.



The redevelopment will significantly reduce the smells coming from the site and residents should start to see improvements as the redevelopment progresses.

While we are still using parts of the original and temporary processes, and during warmer or particularly wet weather, we’ll continue to experience some odour issues from the site, due in part to accumulation and breakdown of fats, oils, and grease (also known as FOG) in the flows arriving for treatment.

As the new site will be completed in stages we need to ensure we are able to continue cleaning your wastewater and there will be occasions where we need to do undertake activities that are normally not required. These activities could cause additional smells, but should only be temporary. We’ll also be cleaning the old tanks before they’re demolished and you may notice some additional smells during this time.

The new site will see the open air tanks, where the majority of the smells originate from, replaced with new treatment processes fully odour controlled and contained within the building.

We’ll do everything we can to keep smells to minimum during construction.


Local Engagement

An Employment and Skills plan is in place which will encourage the participation of local contractors, sub-contractors and suppliers and provide employment opportunities for Southampton residents.

We’ll be keeping residents updated on progress and plan to hold regular drop-in sessions with the project team at the Community Liaison Group meeting. The project is being carried out by contractor 4Delivery.

Community Liaison Group

As agreed with local community groups and Southampton City Council, we have created a Community Liaison Group. The aim of the group is to create a link between Southern Water, 4D and the local community. Regular meetings allow us to receive feedback from the community and provide progress updates.

The group meets every three months at the site offices and includes representatives from all parties involved.

Members of the project team will also be on hand to answer questions and discuss the ongoing activities. The chair for the group is Councillor Warwick Payne.

For anyone interested in attending or finding out more information, please call our site office number (during office hours) on 02380 420856.


For all other enquiries please see our contact us page.

Frequently asked questions


Why are you doing this?

The Environment Agency has specified tighter standards on the treatment of wastewater before it can be recycled into the environment, in line with the latest European Union requirements.

We must improve the treatment processes at the site so it can meet these new standards. We also want to reduce the smells that sometimes come from the works, which we know cause problems for residents and businesses nearby.

The best way of doing this is to completely redevelop the site. The end result will be a modern wastewater treatment works which will be able to serve Woolston long into the future.

How does the existing site work?

The sewer system which feeds into the works collects wastewater from homes and businesses in Woolston. The wastewater is treated in three stages before it is released through an outfall into the River Itchen Estuary.

The by-products produced from the treatment process are:

  • Screenings: collected at the first stage of treatment and consists of the large materials removed from the wastewater, including items such as wet wipes and sanitary products
  • Sludge: which is transferred by pump via the jetty, for removal by barge for further treatment at Millbrook Wastewater Treatment Works where it is converted into a soil conditioner.

What are you proposing?

We are proposing a redevelopment that will:

  • Completely modernise the wastewater treatment works
  • Ensure it meets new, tighter environmental standards
  • Address the odour issues at the plant.

The new treatment processes are the most efficient and cost effective and take into account the limited working space.

As well as improving the quality of the treated wastewater, one of the main goals of the redevelopment is to address the odour problem that has been affecting local residents for a long time.

What will the new site look like?

The new buildings have been designed to blend into the urban area, taking into account the new developments already under construction. Fencing, walls and trees will minimise views into the site.

See our film above for a virtual tour of the area once construction is complete.


Why not close the works and transfer waste to another site?

Originally, the Environment Agency’s National Environment Programme (NEP) specified that we transfer flows from Woolston to Peel Common Wastewater Treatment Works in Gosport.

This proposal was challenged by our regulator, Ofwat, and following an assessment it was agreed that an on-site redevelopment would be the best option.

Who is carrying out the works?

Our appointed contractor 4Delivery Limited (a consortium comprising of industry leaders Veolia WaterCostain and Stantec) has vast experience in delivering works on our behalf and is fully committed to keeping customers and stakeholders up to date.


Woolston construction

Work is under way on our redevelopment and modernisation of Woolston Wastewater Treatment Works.

We started work in September 2014 and expect the construction process to take about five years to complete. 4Delivery (4D), a consortium comprising Veolia Water, Costain and Stantec, is carrying out the work on behalf of Southern Water.

The project is being carried out in stages to make sure we can continue to treat the wastewater for the local area while the redevelopment is being done, and because of the very small area of the site which we can work in.

We built a temporary treatment works which took over much of the treatment from the existing works during the main construction phases. This enabled us to start work on the existing site by demolishing some tanks and structures.

Update – August 2018

June 2018 saw us reach our third significant milestone – starting to pass wastewater through some of the new secondary process equipment, specifically the membrane biological reactor (MBR) which is the largest wastewater MBR of its kind in the UK.

By turning the flows into this new secondary treatment process which completes the second stage of cleaning the wastewater, we’re now able to move on to decommissioning, cleaning, and removal of some of the temporary treatment works from now on. Demolition will be required over the course of the next few months.

We’re well underway with erecting the steel framework that will support the new buildings. Cladding to the building framework starts in late summer and will be finished in winter 2018. These superstructures enclose the primary, secondary and sludge treatment processes and form an odour and sound barrier between us and our neighbours.

Update - November 2018

We have diverted incoming wastewater through the next new stage of process. The full flows are now processed entirely through the new plant and local residents will benefit from a reduction in odour sources because of a more efficient screening and treatment process.

Construction of the sludge treatment area has made speedy progress, with the building’s steel frame being erected during August and the concrete bases made ready for installation of the sludge process plant and storage tanks during the next few weeks.

Our specialist contractors will continue to build the steel framework for the sludge building, moving on to start placing the roof and wall cladding on the main buildings during October.

We’ve been making steady progress removing redundant pipework and equipment from the enabling works temporary treatment works area.

We will move on to demolition of the redundant tanks and structures in the original treatment works site once they have been cleaned. The cleaning operations are likely to generate odours when residual sludge or screenings are disturbed – please bear with us whilst we carry out this essential activity.

The new sludge treatment process is expected to be up and running during spring 2019.

Recent changes


In January 2018, as part of bringing the plant into service, we ‘turned flows’ and started to pass wastewater through new treatment processes. The means you may hear new noises coming from site – don’t worry, we are also monitoring this.

What is the noise?

In January 2018, as part of bringing the plant into service, we ‘turned flows’ and started to pass wastewater through the new primary treatment process. The means you may hear new noises coming from site – don’t worry, we are also monitoring this.

What are you doing about it?

Where possible, we have been using additional acoustic screening alongside the manufacturer’s noise covers. We’ll continue to monitor the sound experienced at the site boundary and do what we can to try and reduce these.

Is it going to change again?

Over the next few months we’ll be continuing with the commissioning and optimisation of the new equipment, which will cause some fluctuations in the noises you hear from the site.

We apologise for any inconvenience this may cause and thank you for your patience. As more of the new site is brought into operation over the coming months, we anticipate there will be more changes to the noises you might hear, alongside construction noise.

Will it get better?

When the site is finished in Summer 2019, the new process equipment will be contained within a new building, which is due to be completed during Spring 2019.

This building will significantly reduce the amount of noise you hear from the process plant.

What can I do?

We’re doing everything we can to minimise disruption during this redevelopment. Things on site are progressing quickly and we’ll continue to monitor the impact of these changes on our neighbours.

We’d like to thank you for your patience during these works and welcome any feedback you might have on how we could do better, please use the contact details below or speak to one of the site team directly.

How can I contact you?

If you have any further questions, please contact Southern Water’s customer call centre on 0330 303 0368, quoting the site PRN reference 539389. Alternatively, you can contact the 4D Woolston Site Office on 023 8042 0856.

Construction FAQs

Will there be an increase in traffic?

There will be temporary increases in the number of vehicles travelling to and from the site, but traffic management measures will be put in place to ensure that this causes as little disturbance as possible. We will consult with other developers working in the area to ensure that there are no conflicts with traffic.

During construction, access to the site will be from Victoria Road, either through the existing compound entrance next to Crest Nicholson, or via the new entrance to the treatment works opposite Swift Road junction.

How much noise will there be?

We’ll do all we can to keep noise to a minimum. The noisiest part of the scheme is likely to be the demolition of the redundant temporary treatment works.

Hydraulically-operated plant will be used where possible to minimise vibrations arising from sheet piling works, however, if we meet ground conditions that do not allow us to use this method, we will need to use other techniques.

Bored screw piling is the preferred installation method where ground conditions allow. Again, where there are technical reasons why this is unsuitable, we will need to look at alternative methods.

Will the work cause any dust?

Any dust coming from activities on site will be reduced using water sprays, the wheels of vehicles leaving site will be washed and road surfaces cleaned to keep the surrounding area as clean as possible.

When will deliveries arrive at site?

All works relating to construction of the development will only take place between the following hours:

  • Monday to Friday 8.00am–6.00pm
  • Saturdays 9.00am–1.00pm
  • At no time on Sundays and recognised public holidays.

Deliveries to the site during the construction phase shall avoid peak traffic hours of 7.30-9.30am and 4-6pm unless agreed with Southampton City Council.

Will you have to access my home at any time?

We will not need to enter your home at any point during these works. Should someone come to your address claiming to be from the works and requiring access, please close the door and call the police on 999. Remember to always know the identity of doorstep callers.

Works connected with the ongoing operation of the treatment works will continue as usual and there may be activity out of normal working hours arising from emergencies on site.

Where will your staff park?

Parking for vehicles connected with the works will be provided in the currently unused section of the Victoria Road car park. Provision for any additional parking will be agreed with Southampton City Council.

How will you protect the environment during construction?

We have liaised closely with Natural England and the Environment Agency and have carried out environmental surveys to assess impact on wildlife.

Robust mitigation measures will be put in place during construction and these are included in the planning consent.

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