Removing nutrients from wastewater - FAQ

Housing development in our region can affect the levels of nitrogen and phosphate in the wastewater we treat. Nutrient removal is governed by strict permits from our regulators. So we must consider the impact any local development will have on our ability to operate to the expected standards.

Frequently asked questions

We’ve prepared the below questions and answers to help builders and developers understand how the removal of nutrients from effluent is regulated, the scope of our permits and the capacity of our wastewater treatment works (WWTWs).

Is Southern Water currently open to agreements to increase the nitrogen or phosphate removal rate at receiving Wastewater Treatment Works beyond consented levels, to provide mitigation for local housing development?

Within the existing environmental and financial regulatory framework of the water industry, this is not feasible. Southern Water operates its Wastewater Treatment Works (WWTWs) in accordance with permitted discharge and quality limits which are issued by, monitored and reported to the Environment Agency.

Our WWTWs are designed, constructed and upgraded when necessary to achieve the permit limits in force at the time and to any changes to these during the lifespan of the WWTW. The Environment Agency has, in its Technical Guidance Note (June 2019), confirmed that following recent investigations carried out through the Water Industry National Environment Programme, no further investment is needed to treat wastewater to a tighter nitrogen limit at any of the WWTWs in the Solent area in the next Business Plan 2020-2025.

In order to voluntarily increase nitrogen or phosphate (nutrient) removal beyond a WWTW’s existing permit, or to introduce nutrient removal at a WWTW where it does not presently exist, significant investment would be required, particularly in the latter case. This would be financed through our General Charges Income (GCI), collected from existing customers.

Southern Water would need to seek the approval of Ofwat, the water industry’s economic regulator, through the five-yearly price review process, to use GCI to invest in upgrades to our WWTWs to meet tighter discharge standards. It is very unlikely that it would be possible to justify this cost, and pass it on to customers, when investigation work carried out by our environmental regulator has already shown that this is not currently necessary, and where our economic regulator is also looking to protect the interests of customers, and ensure operational efficiency.

Where developer contributions to maintain an increase in nutrient removal at a WWTW are suggested, this risks contravening Financial Competition Rules and the requirement to maintain a level playing field between all developers and other agencies operating in our region. Southern Water would be in a difficult situation in accepting a financial contribution to amend a WWTW which is already operating to acceptable standards and would be at direct risk of future challenge by accepting such a contribution should the present situation be resolved by a change in the present advice and position of Natural England.

Nutrients include both Nitrogen (N) and Phosphorous (P) and WWTWs tend to have either a P or N permit, rather than both. What is the difference?

Environmental permits for discharges from WWTWs are determined based on statutory water quality standards and objectives. These standards and objectives differ depending on whether the receiving watercourse is inland or coastal and also depending on its quality, amenity and whether there are any protected sites located nearby.

For inland receiving water, the assessment will take into account the immediate receiving water and main receiving river. In general, P is believed to be the primary limiting nutrient for UK inland waters, hence for inland waters discharges only P removal is required. There are exceptional cases, but these will be linked to specific factors.

What is the capacity of works? Is there enough capacity for the proposed development?

Growth within each WWTW catchment is monitored through population forecasts, as well as through Local Plan housing allocations, and monitoring of local authorities housing supply and delivery. Where this monitoring identifies a risk that capacity at a WWTW is likely to be met within the next five-year investment period, known as an Asset Management Programme (AMP), we would make a case in our business plan for new infrastructure to support the projected growth in population. Once our business plan is agreed, we can then work towards delivering the required additional capacity to accommodate projected growth.

The Environment Agency is the water industry's environmental regulator and defines the environmental permits that water companies are required to meet. These permits are designed to protect the environment and ensure that water quality objectives are met.

Southern Water therefore operates its WWTWs in accordance with environmental permits issued and enforced by the Environment Agency. The permits set the maximum volume of treated wastewater that the company is permitted to recycle to the environment in terms of Dry Weather Flow (DWF). They also define the standards of treatment that must be met in order to protect water quality objectives.

If the future release of treated wastewater at a WWTW is anticipated to exceed the maximum allowed by the environmental permit (as a result of new development), Southern Water could apply to the Environment Agency for a new or amended permit. This would increase the volumetric permit headroom above that which is currently available. The Environment Agency would normally permit increased flows provided the treatment standards are tightened so that the total load (e.g. of nitrogen or phosphates) to the environment is not increased. This is in line with the "no deterioration" principle.

Fundamentally wastewater treatment capacity is not a constraint to future new development even if investment requirements are significant. Southern Water has a statutory obligation to find solutions and provide infrastructure to serve new development. Local Plan periods generally run for 15 years so there are repeated opportunities through the water industry's five-yearly price review process to investigate and implement solutions.

In the absence of a Nitrogen or Phosphate permit level, are you able to provide details of the nitrogen/phosphate effluent levels for WWTW?

Where there is no Nitrogen (N) or Phosphorous (P) permit in place, Southern Water does not monitor N or P levels in the final effluent discharging from that WWTW. It is therefore not possible to state what levels of N or P are in the discharges from a WWTW that has no N or P permit.

What are the current N and P permit levels at WWTWs in the Solent area?

Please see the following table:

 WWTW name, location  P permit (mg/l)  N permit (mg/l)
 Ashlett Creek, New Forest District  -  -
 Bishops Waltham, Winchester District  -  15
 Bosham, Chichester District  -  10
 Budds Farm, Havant Borough  -  9.7
 Chichester, Chichester District  -  9
 Milford Road Pennington,
 New Forest District
 -  9.5
 Millbrook, City of Southampton  -  10
 Peel Common, Fareham District  -  9
 Portswood, City of Southampton  -  -
 Sidlesham, Chichester District  1 15
 Slowhill Copse, New Forest District  - 14
 Thornham, Chichester District  - 10
 Woolston, City of Southampton  - 15


Why do some WWTWs not have a P or N permit?

The substances and concentrations controlled by a permit are assessed and determined by the Environment Agency based on the water quality objectives of any given waterbody into which our assets discharge.

It’s been suggested that one solution may be to treat wastewater on site before discharging to the public sewer – is this feasible?

This solution would not achieve a reduction in nutrients being discharged from the receiving WWTW. Once the treated wastewater is discharged into the public sewer, it will mix with other ‘raw’ wastewater within the sewer network, negating the effects of any preliminary treatment. It will then arrive at the WWTW and be treated according to the permit requirements of the receiving WWTW.

Will Southern Water provide and maintain an increase in nutrient offsetting from catchment management measures (this may include mini-farm interceptor wetlands)?

The majority of Southern Water’s landholdings are required for current or future operational purposes and therefore there is limited scope to provide land for nutrient offsetting of this type. However, we are committed to working with other agencies and stakeholders to identify and help implement, where possible, viable catchment management solutions.

Can you provide an update on your plans regarding tree planting?

As part of the Water UK Public Interest Commitment the water industry has pledged to achieve net zero carbon by 2030. As part of this there is a commitment to plant 11 million trees by 2030. Southern Water is currently developing its thinking around how to achieve net zero and what part carbon offsetting plays in this.

We recognise that tree planting forms only part of the solution and that the ‘right tree in the right place’ principle must be followed to protect natural capital assets and support protected landscapes such as the South Downs National Park.

We are working with our environmental stakeholders to understand how and where we might invest in carbon offsetting whilst delivering wider ecosystem benefits e.g. we are supporting the Sussex IFCA & Sussex Wildlife Trust project to ‘protect our kelp’. This work is in its early stages. However, Southern Water would be open to conversations about how this might have the potential to tie in with nutrient offsetting, in the right location.

What are the costs of upgrading WWTWs to incorporate nutrient removal?

The costs of upgrading WWTWs are wide ranging and will depend on factors such as the existing treatment process & capacity (can it be modified? Or is a brand new process required?), what the new standard to be met is, whether or not new land would need to be purchased. All these variables would need to be calculated on a case by case basis.

How long would it take to plan and construct a new WWTW?

Where a WWTW is required to increase the quality but not the quantity of wastewater it treats this requirement should be promoted by the Environment Agency and/or Natural England and included in water company business plans which are delivered over a 5 year period. The next water company investment period commences on 1st April 2020 and ends on 31st March 2025, planning for an investment period starts 2 to 3 years before the delivery period commences.

No requirements for new N limits have been requested for the next investment period although some investigations maybe undertaken to inform requirements for the investment period commencing 1st April 2025. Depending on the size of the WWTW design and construction normally takes around 3 to 5 years.

Is there a way this process could be accelerated?

Where a requirement for an improvement at a WWTW was not promoted in the business planning process a request can be made to swap a new requirement into the plan and swap other requirements, of equal or less value, out. This process ensures the overall value of the business plan, and therefore the cost of customer bills, remains unchanged.

Who decides what standards a WWTW has to meet for its effluent discharge?

The Environment Agency sets the standards, following requirements that are currently driven by European legislation (such as the Urban Wastewater Treatment Directive 1991 and the Water Framework Directive, 2000) and UK legislation such as the Water Resources Act 1991 and Environmental Permitting Regulations 2016. The Water Industry National Environment Programme (WINEP) sets out actions that the Environment Agency has requested water companies to carry out in their next business plan period, in consultation with Natural England. Follow the link to download WINEP data for 2020-2025.

What can do to enable growth in a region without having P or N permits imposed or tightened at WWTW?

We continue to work with both the Environment Agency and Natural England to plan for any asset improvements required in future investment periods. In the meantime we can look to support developers by giving advice to landowners on how they can reduce nutrient inputs by changing land use and working practices and working with landowners to create wetlands fed by our existing treated effluent discharges to the environment. Wetlands can help reduce nutrient concentrations discharged to water bodies and offer a range of other benefits such as increasing biodiversity and providing carbon sequestration.


We are also working with Defra on their ambition to create a nutrient trading platform for the Solent. For any further queries, please contact Southern Water’s Regional Planning team at

Still have questions?

For any further queries, please contact Southern Water’s Regional Planning team at

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