Nutrient neutral assessments – calculating the right answers

2 March 2023 By 0

Excess nutrient run-offs from new developments in England are now banned. The latest challenge has been to approve a robust model for assessing how much phosphorus and nitrogen is involved that threaten biodiversity and highly-protected species. That question has now been answered.

The basic problem is that these two unwanted nutrients found in artificially-introduced fertilisers – particularly on present or former farmland – can trigger huge algae blooms and aquatic plant growth spurts that starve otherwise healthy wetlands and their biodiversity of essential oxygen.

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This “eutrophication” process can put fish, shellfish, invertebrates, and birds at severe risk. However, achieving the right balance is crucial because human population growth has also increased the pressure to find new development land for not only homes, but other infrastructure assets too.

It is vital to find an effective assessment tool that everyone recognises. But this is not the only solution. Which is why I would like to run over the whole nutrient neutral assessment process, and then show good results in practice by referring to important case studies we have conducted.

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Doing the maths

Enzygo Water Sciences uses a number of sophisticated models to determine, as the law now requires, how much of these nutrients are being, or will be, washed off the land before and after a new development is completed. We then need to work out the difference between the two.

The final answer must be zero, or less than zero. If it is not, special mitigation measures are needed as I describe in the case studies below.

Two approaches are producing good results as I hope to show. The first is SUDS (sustainable drainage systems) which mimic nature in temporarily storing and cleaning run-off. The second is floating wetlands which maximise contact between water, root systems and biofilm networks to turn pollutants into safe biomass and gases.

However, because the wellbeing of the water environment is now at significant risk, I include later details of Enzygo’s ( services portfolio in this critical sector as an environmental consultancy and planning consultancy, the work of our integrated teams of multidisciplinary specialists, plus the company’s systematic approach to solving complex water-related problems.

The nutrient neutral concept

Because of the dire need for new housing, Natural England – the Government’s natural environment advisor sponsored by Defra – is tackling the problem through the concept of nutrient-neutrality. This stipulates that total run-off and emissions from new residential sites – during construction and long-term occupancy – must be no higher than before work started.

To provide a standard assessment tool, Natural England issued a ‘Nutrient Neutrality Generic methodology’ in February 2022, and national guidance in March 2022. Several local authorities in affected catchments have also produced nutrient assessment or nutrient budget calculators for use by developers.

The upshot is that if the receiving wastewater treatment works (WwTW) cannot treat all the TN (total nitrogen) and TP (total phosphorous) load in foul flows from a development, the Nutrient Neutral Assessment will show the amount of nutrients that need on-site treatment and removal to achieve nutrient neutrality.

Some real figures

Before getting to the case studies, to show how this works in practice, the following four-stage nitrogen neutrality assessment is an example taken from a summary of real life site data (see the attached table: –

  • Stage 1 estimates the nitrogen load in primary foul effluent from a development to house 250 people as 271 kg TN/year based on 110 l/person/day water consumption
  • Stage 2 (on-site) estimates the current nitrogen load from the existing site footprint as arable farmland (using 10 years of data on fertiliser applications) as 310.9 kg TN/year
  • Stage 2 (off-site) then estimates the nitrogen load in watercourses flowing through the site from a wider catchment draining other arable land and an urban area as 9,580 kg TN/year
  • Stage 3 assesses total future nitrogen from the site, including that discounted against the existing nitrogen load and a proposed wetland treatment system to remove N
  • Stage 4 compares the nitrogen load for the proposed development against the load from the WwTW, shows that the site is nutrient neutral, and that the treatment wetland as designed exceeds the neutral requirement and reduces the nitrogen load from the total catchment by 115kg TN/year.
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Treatment wetlands can be used to ‘polish’ treated effluent from a WwTW (wastewater treatment works) designed primarily to treat biological oxygen demand (BOD), ammonia and total suspended solids (TSS). This can further reduce nutrient concentrations. In fact, several water utilities are now considering wetlands as potential sites with sufficient land for future WwTW sites.

Onsite package treatment plants (PTP) – and a combination PTPs and constructed treatment wetlands – are able to reduce nutrient concentrations. However, it is not possible, or practicable, to reduce concentrations to zero.

Other mitigation options were reviewed in my previous article (‘Floating new ideas for nutrient-neutral home-building’ –

Some complications – two case studies

Each site we have worked on has been different, and very dependent on the favourability of specific site hydrology and hydrogeology, plus former land use.

Importantly, former brownfield or commercial forestry sites that are low producers of nutrients require greater nutrient mitigation when redeveloped than, say, farmland under arable crops or various livestock grazing regimes.

Generally, nutrient neutral assessments require detailed evidence of former land use – for example 10 years of farm records describing what the land’s former uses were.

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Case study 1 – This concerned a proposed residential development on cleared forest land with artificial surface watercourses underlain by a sand and gravel aquifer.

Site investigations showed that most rainfall infiltrates to ground, with off-site runoff only flowing through site watercourses during storm events.

Connection to foul sewer could be made to treat primary nutrients in human waste. But the nutrient budget suggested that onsite treatment and mitigation of secondary nutrients would be required to achieve nutrient neutrality. In this case, infiltration of nutrients to the aquifer is unlikely to reduce their concentration in groundwater draining rapidly to surface watercourses and designated sites.

An onsite package treatment plant could avoid the need for sewer connections and would provide enough nutrients to maintain a SUDS (sustainable drainage systems) treatment wetland, as explained below. In this instance, a traditional pipe surface water drainage connection to a carefully profiled attenuation pond drainage system would allow the wetland to treat nutrients in any surface runoff.

Cleaned runoff could be discharged to an onward watercourse. However, the cost of a treatment wetland to reduce nutrients to acceptable levels would be prohibitive, even for a large residential development.

Case study 2  In this case a nutrient budget for a proposed critical extension to a residential development currently connected to foul sewer showed that a treatment wetland for secondary runoff with discharge to watercourse could ensure the extension was nutrient neutral.

However, the receiving WwTW does not plan to reduce the current nutrient concentration in its consented discharge until at least AMP9 (2030-2035).

As a result, the developer is considering an onsite package treatment plant and treatment wetland such that the proposed discharge concentration of nutrients to receiving watercourses and designated sites will be 75% lower than that discharged by the WwTW. This will provide betterment and ensure that the development is nutrient neutral.

Other critical issues

Wetland treatment of water is a natural bioremediation process subject to changes in a large number of variables. They include flow rate, wetland geometry, water surface area and depth, and hence residence time in the wetland. Also, water and air temperature, sunlight, plant type, respiration, age, replacement rate. And the wetland management regime.

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Globally, metadata studies have shown that natural and artificial wetlands achieve on average a 50% reduction in the nutrient concentration flowing through them. However, it is unlikely that ‘spot’ measurements of wetland performance inflow and outflow nutrient concentrations will be consistent on short timescales.

Instead, continuous monitoring of these variables, and controls on many of them, are ideally required to provide performance metrics for the regulators. Without those metrics, treatment wetlands are unlikely to gain approval from the regulator for construction and use. This puts us into a classic Catch-22 dilemma.

Letting nature do the work

With all potential ‘total nitrogen’ and ‘total phosphorous’ sources minimised, including broken sewer pipes, we can move on to active sustainable solutions such as SUDS.

SUDS work well where ground can be profiled into vegetated swales, reed beds and retention ponds to slow down, store and clean run-off and rainwater while it seeps away. They do not need artificial energy sources, and solutions are robust, attractive, biodiversity-friendly and create local amenities.

Pioneering floating wetland technology.

This works on the same principle as conventional SUDS, with the crucial difference that using floating wetlands increases exposure. Although a minimum depth of water is needed to maximise treatment efficiency, floating minimises the risk of drowning out vegetation. Also, unlike conventional ground-based wetlands, the flow of water is unimpeded.

Enzygo (

As promised above, I should briefly mention the wider context of Enzygo’s work in environmental services. Water is one of our primary areas of specialisation. While we have a detailed understanding of water supply and wastewater disposal planning and environmental permitting regulations, we also regularly resolve water-related issues that can affect neutrality.

Our specific aim is to help developers structure the requirements, opportunities, and delay-risks of both water and nutrient neutrality into their early stage thinking so that optimum solutions become part of all local plans, neighbourhood plans, and development management planning applications.

As such, we manage planning applications from first inception to consent, liaise closely with local planning authority officers at every stage, and respond to all requests for information from consultees to avoid negative impacts on neighbouring residents.

Successful projects regularly include major infrastructure proposals, residential housing developments, renewable energy programmes, and water environment challenges. Much of our work integrates planning with hydrology & drainage, permitting & regulation, landscape, ecology, transport, geo-environmental & hydrogeology, noise & vibration, air quality and arboriculture.

In practical terms, this often involves providing environmental audits and environmental impact assessments (EIA), planning practice guidance, environmental management systems, plus advice on environmental permitting regulations, with a strong focus on ecological issues to minimise any potential disturbance to vulnerable plant or animal habitats and encourage biodiversity.

Our work onsite routinely includes ground investigations, flood risk assessments, landscape character assessments, transport assessments, ecological impact assessments and habitat surveys.

Tailored water services

As a specialist arm of Enzygo, Enzygo Water Sciences undertakes nutrient neutral assessments, water quality and nutrient load assessments on watercourses. We also collaborate with our partners on the design and construction of small footprint treatment wetlands.

I would welcome comments and am happy to discuss individual projects. Please feel free to contact me.

Dr Paul Hardwick, Consultant Specialist – Water Environment Sciences – For Enzygo Ltd