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Nutrient neutrality is big news. Nutrients are focused on nitrogen and phosphorus compounds, and crop and livestock production in the UK is dependent on 174,000t of feeds and fertilisers it imports from Russia, Morocco, and China.

The availability of phosphate rock is being depleted, which means higher prices combined with geopolitical risks of supply. The fact that UK food supplies are reliant on imports is a significant issue. Inappropriate application of phosphorus fertilisers and animal waste causes ecological impacts in our rivers. The ability to close the loop capturing and recycling phosphorus, rather than importing, is the holy grail in the water sector.

In the UK, the government says that EU laws are defective and tabled amendments in the Levelling Up and Regeneration Bill (LURB) which would allow housebuilders to build in the 73 catchments that are affected. The House Builders Federation (HBF) asked the government to set up a mitigation scheme based on credits for all catchments and said developers are willing to pay. But the government went further in the amendments. It required local authorities to ignore facts and evidence. The cost of mitigation fell on the taxpayer and undermined evidence bases, and the polluter pays principle, which caused a backlash.

The move was defeated in the Lords, and at the recent Conservative Party conference levelling up secretary Michael Gove said the government will try and introduce new legislation in the next parliamentary session. Whether it will get through prior to the general election is open to question. This creates more uncertainty for investors and limbo for housebuilders. Labour says it will take a balanced approach to house building and environment.

The political problems these compounds cause is seen in the Netherlands, where a European Court of Justice case in 2019 required a 50% reduction in nitrogen emissions by 2030, necessitating a drastic reduction in livestock, and giving rise to a political party, which now has 16 seats in the senate, making it hard to effect the changes.

Numbers game

In England, wastewater treatment works (WWTW) treating foul flows amounts to 60-70% of total phosphorus loads, with agriculture contributing 25% with diffuse urban sources at 7%. There are roughly 25 million homes in England, with 171,000 new homes constructed in 2022. New homes are 0.7% of the total number of existing homes, and 0.7% of the total phosphorus load is added annually. The numbers vary between catchments. Nutrient neutrality laws came into force in England in 2019, and primarily affected the development of new homes. All phosphorus sources from new homes are 0.5%. The water industry is having to upgrade treatment works, but agriculture has been little affected so far.

The majority (55%) of river water bodies in England fail on the 'good ecological status' metric. Ellaway takes one catchment as an example. In the Wye, there has been an explosion of poultry farms, with 20m chickens within the boundaries. Nutrients in manure are spread on farmland as fertiliser. The nutrient load significantly exceeds capacity and the excess washes off into the Wye, resulting in deteriorating river quality, large algal blooms and damaged river ecology. Wye Rivers Trust estimates 72% is from agriculture and 23% from WWTW. Yet it is housebuilding which is subject to nutrient neutrality rules. Developers are paying large sums in comparison for small additional loads.

Elevated concentration of phosphorus is degrading the quality of lakes and rivers, and increasing algal blooms and decreasing oxygen concentrations in the rivers. It is smothering riverbeds. The rivers affected are designated as 'special areas of conservation'.

Other examples include the tidal mud flats of the Poole Harbour, the Solent and the Tees, where nitrogen pollution is creating smothering algae and reducing the population size of important and vulnerable bird species.

Nutrient neutrality currently applies to 27 catchments, covering 72 local authorities. Three are coastal nitrogen only; 15 are nitrogen only; 15 are freshwater and phosphorus only; 9 catchments cover phosphorus and nitrogen. Wales has 5 catchments and Scotland has 1 catchment (Loch Leven) where catchments are required to mitigate nutrient positivity 125% of nutrient load. Phosphorus is much harder to mitigate.

Natural England has published specific nutrient load calculators for catchments. Local authorities may use their own. Natural England’s calculation method is very conservative (with a 20% precautionary buffer), and the development industry thinks this is disproportionate to the relatively small annual contribution. However, developers say reducing emissions is important and are willing to pay, but want schemes quickly to continue working.

Essentially, new wastewater load and future land use loads are subtracted from the current land use load. If it is positive, mitigation is required.

Natural England has also published mitigation principles. For neutrality, developments should be highly targeted to the WWTW location for phosphorus. Nitrogen can be mitigated elsewhere in the catchment. Mitigation must be scientifically certain and in place for between 80-125 years.

Natural England and Natural Resources Wales have imposed new nitrogen and / or phosphorus neutrality requirements on all new developments containing overnight accommodation in catchments containing habitat sites in an unfavourable condition because of nutrient enrichment. Last year they introduced other sectors including industry and agriculture into the scope.


Achieving nutrient neutrality is time consuming and costly – delaying housing delivery and planning applications, pushing up land prices, reducing affordable housing, and stopping the delivery of 145,000 homes, although this number is challenged. A significant unintended consequence is to take agricultural land for mitigation (with less than 50% self-sufficiency in food).

In the current LURB there is a requirement for water companies to upgrade larger works serving over 200,000 people. They must achieve 'technically achievable limits' (TAL) of 0.25mg / phosphorus/l and 10mg/l nitrogen/l by 2030. Natural England has set up an accelerator unit to develop mitigation schemes and sell credits. One in the Tees catchment opened earlier in the year. Once the bill is in force local authorities can rely on the 2030 TAL upgrade deadline.

A smaller development will not be helped by TAL in smaller WWTW catchments, with TAL not low enough to deliver neutrality. Ellaway said it would be good if some of the nutrient headroom achieved can be used to deliver the gaps up to 2030 and stressed that developers need the schemes and solutions now.

Some developers will fail to meet standards even with an onsite WWTW, with TAL not low enough to deliver neutrality for all sites. Developers are sceptical that affected WWTWs will be upgraded by 2030, with the water companies facing financial and technical challenges. Phosphorus schemes take much longer.

Optimising the load calculations

Actions can be taken to lessen the problems. Ellaway stressed that it is critically important to optimise the nutrient load calculation and improve Natural England’s conservative default values and focus on key attributes. Natural England has accepted the use of alternative occupancy rates. This requires a socio-economic assessment, support from the local planning authority and reducing default water usage.

In the default calculation method, the nitrogen export rates for new residential land use assumes 80% impermeability for phosphorus and 100% for nitrogen. Surface water drainage strategies often indicate the actual 'percentage imperviousness' (PIMP) value is 40-50%. Applying an accurate PIMP value for nitrogen for urban land use is 45-50%, plus a 10% uplift. Accurate PIMP calculations can reduce nutrient load by 50%.

Ellaway provided an example of the value of optimisation at a 600-home development in the Tees catchment, where a nitrogen load of 700kg/yr was reduced to 340kg/yr using a worked occupancy rate and a calculated PIMP value.

Mitigation options

Having optimised the load there are a wide range of mitigation options available.

Onsite WWTW discharge limits can deliver discharge limits of 0.15mg/l for phosphorus and 5 mg/l for nitrogen – beating TAL early and a great option for new developments. If existing houses are near a poorly performing WWTW, if possible they can be linked to new works which can generate significant savings.

A small nutrient system for 180 homes would be approximately £1.25m (+0.5m for maintenance), for 1,800 units it would be £5.25m (with sewerage charges covering operations). Ellaway said challenges developing an onsite works include site and pipes’ location and odour buffers, all of which can reduce site housing yield. A discharge consent is needed from the Environment Agency which can take 1-3 years. But for large multi-phase housing developments they are a strong part of nutrient mitigation strategy.

Organica Water’s botanical WWTW is an option for high-end developments and easier on the eye. Organica Water’s systems can deliver 0.2mg for phosphorus and 10mg for nitrogen, costing £2.5m for 1,000 homes and £3.5m for 5,000 (although the cost is rising).

Comprehensive SuDS treatment trains are part of the mitigation mix and should consist of source controls: filter stops, swales, and ponds. Guidance on the design can be gleaned from the 'CIRIA C808 on SuDS for Phosphorus' guidance, published in January.

In rural areas, a package treatment plant optimised for nutrient reduction may be beneficial, such as Klargester-run systems. Also, septic tanks can be replaced as a mitigation measure.

Integrated constructed wetlands can provide significant reduction for nitrogen mitigation, but require a large water input. Wetlands can also act as a reactive filter media to boost retention of phosphorus. Natural England has provided design guidance and will require post-commissioning monitoring, but requires a high degree of confidence in the flow to claim 100% of the mitigation.

Reactive filter media can be used with constructed wetland to polish effluent. The company behind Polonite Nordic claims it can get rid of 90-95% of phosphorus, and can apply recovered materials as fertiliser.

Depending on land availability, another solution would be to convert existing land to woodland or conservation meadow, but this only applies to sites with nitrogen mitigation.

Offsite mitigation interventions count. Developers can buy farms or a series of farms. One example is in the Hampshire Avon where one developer has purchased fish farms and a watercress farm, and is selling excess credits for £75,000 per kg. This can also include buying up and closing high-polluting businesses such as pig and dairy farms.

In terms of reduced water use, Natural England has accepted that social tenant houses can be retrofitted with non-removable plumbed in flow control devices, but can only be used where WWTW has a nutrient discharge consent limit.

Natural England or commercial schemes can be used to purchase mitigation credits; this solution avoids delivery risks. Nitrogen mitigation costs £1,825 to £3,500 kg/n which reflects the land use price. Phosphorus is £14,000 to £75,000 kg/p. The Natural England scheme is concentrating on catchments with the greatest housing pressure, and is looking at wetland sites in Eden, the Broads and Stodmarsh catchments. Phosphorus credits are not available in some catchments until 2025 or later, but they are coming.

Ellaway provided an example of mitigation options for a 4,000 unit development on a 270ha site with 50% green space and occupancy rate of 2.4 per unit, resulting in a baseline nutrient load of 600 kg/p year and over 7,000kg/n. Mitigation options are: 15,000ha of arable conversion (£375m), mitigation credits of £30m or 30ha wetland – and not feasible for phosphorus – at £6m.

Instead, the solution was load optimisation and mitigation. This entailed a locally specific occupancy rate of 1.88/unit, reduced water use at 110l/pp/day, a development specific PIMP value and a SuDS nutrient removal efficiency rate of 95% with infiltration, a cutting edge onsite WWTW (£8m+) and treatment of existing foul flows for 125 existing homes. This provides phosphorus and nitrogen neutrality – with the resulting nutrient load of -1.0kgTP/yr and -4.250kg/TN/yr.


Ellaway said that delivery of nutrient neutrality strategy requires an experienced multidisciplinary team, involving water quality specialists, engineers, hydrogeologists, flood risk analysts and SuDS practitioners. Early advice is critical to confirm viability as mitigation solutions, such as SuDS, wetlands and WWTWs, have impacts on the developable area.

Challenges are multi-faceted and complex, but solutions are available to deliver nutrient neutrality for most sites, although this is harder for smaller schemes.


Watch on-demand

Watch this Expert Talk, 'Nutrogeddon: Delivering Nutrient Neutrality in the Housing Development Sector', with James Ellaway, technical director, water division at WSP, on-demand for free.

Watch on-demand