Ensuring Nutrient Neutrality in protected water catchment areas

06 December 2022

Developers considering new projects in protected water catchment areas must be aware of their responsibility to ensure they meet their Nutrient Neutrality obligations. Ridge Civil Engineering Associate Martyn Grant here looks at the problem of nutrient overload, the various means of addressing it and the ways in which Ridge assists clients in formulating a Nutrient Neutrality strategy.

The problem

Nitrogen and phosphorus occur naturally in the atmosphere and waterways. Living organisms need these chemical elements to grow but too much can be damaging. Nutrient excess causes water pollution. Eutrophication (overload of nutrients) can be caused by an excess of:

  • Nitrates,
  • Phosphorous
  • Ammonia
  • Biochemical oxygen demand (BOD) and chemical oxygen demand (COD)

Nutrients result naturally and from agriculture (fertilisers and animal waste) or sewage treatment works effluent or CSOs (untreated or diluted treated sewage), including surface water. Pollution can either be point source or non-point source.

  • Point source – discharge pipe from waste water treatment works (sewage)
  • Non–point source – runoff generated through farming

Nutrient overload, particularly of phosphates and nitrates, can lead to harmful algae blooms (HAB) – the sudden proliferation of phytoplankton, primarily dinoflagellates. Some phytoplankton species produce neurotoxins, and, in large enough numbers, these organisms can collectively cause negative and sometimes fatal effects in fish, birds, marine mammals and even humans. Oxygen levels for fish and other aquatic life can be reduced significantly.

The regulatory response

Following the Dutch Nitrogen Case judgment by the Court of Justice of the European Union, developers in certain water catchment areas must pass a site integrity test, or an ‘Appropriate Assessment’ (AA). Local Planning Authorities (LPAs) are affected to different degrees – some will have only a small part of their area affected, while others will be impacted to a much greater extent.

In general, a buffer area does not need to be applied for a river catchment. If a development is located outside of the sensitive river catchment and does not drain (run-off or wastewater) via surface water or groundwater pathways into the sensitive catchment, then the application can be screened out.

If a development is located within an affected catchment but is served by a wastewater treatment works that discharges outside of the sensitive catchment, the wastewater element of the proposed development can be excluded. The drainage of surface water will still need to be considered though.

If a development is located outside an affected catchment but is served by a wastewater treatment works that discharges within the catchment, a habitats regulations assessment (HRA) will be required. Such habitat sites include Special Areas of Conservation (SAC) / Ramsar and Special Protection Areas (SPA) designated under Habitats Regulations.

Nutrient Neutrality is achieved when the nutrient load from a development is mitigated. A new development calculates its nutrient impact from wastewater and counterbalances it through mitigation. Natural England’s advice is that any neutrality measures relied on in an Appropriate Assessment (AA) should:

  • Have scientific certainty that the measures at the time of the AA will deliver the required reduction to make the plan or project ‘neutral’
  • Have practical certainty that the measures will be secured for the lifetime of the development’s effects and in place before the new development causes an effect, e.g., occupation (at the time when the HRA is undertaken)
  • Be preventive in nature to avoid effects in the first place rather than offset or compensate for damage
  • Be carefully justified together with calculations of the change in the nutrient contribution before and after the development, taking account of any mitigation on land outside the development.

Mitigation can be provided on a case-by-case basis by the developer, strategically by the LPA or else by third parties, e.g., Wildlife Trust. It needs to be suitably located to remove the effect of new development within the same catchment as the development, upstream of the habitat site and / or the development.

An amendment to the current Levelling Up and Regeneration Bill proposes a statutory duty on water companies in England to upgrade wastewater treatment works to the highest technically achievable levels for nutrient removal in designated catchments by 1 April 2030. These upgrades will be considered for Habitats Regulations Assessments, meaning lower levels of mitigation will be required from development, reducing costs for developers.


Mitigation strategies include:

  • On-site measures such as sustainable urban drainage systems (SuDS) which can also be retrofitted into existing developments
  • Agricultural land use change (e.g., intensive agriculture to woodland)
  • Treatment wetlands (wastewater treatment works, package treatment plants)
  • Interceptor Wetlands (agricultural run-off)
  • Replacement of inefficient package treatment plants / septic tanks

The concept of ‘stacking’ benefits is being discussed and is an opportunity to secure nutrient mitigation in the longer term by linking with other approaches to delivering environmental services and nature recovery, e.g., Biodiversity Net Gain (BNG) through woodland or wetland creation.

Wetlands are often described as “filtering out” pollutants from water, acting as “nature’s kidneys.” Wetlands can remove nitrogen and phosphorus through a combination of physical, chemical and biological processes.

The main physical processes of nutrient removal are particle settling (sedimentation), volatilisation (releasing as a gas into the atmosphere) and sorption. Sorption can involve a nutrient adhering to a solid (adsorption) or diffusing into another liquid or solid (absorption).

Chemical processes include transformations of nutrient forms and chemical precipitation, in which a solid compound is formed from a liquid through a chemical reaction. The main biological processes are uptake (or assimilation) by plants, algae and bacteria and transformation processes conducted by microbes. All these processes occur throughout different wetland compartments, which include water, biota (plants, algae, and bacteria), litter and soil. Nitrogen and phosphorus can be present in many forms (particulate, dissolved, organic, inorganic, etc.)

While the dominant removal processes for nitrogen and phosphorus are different, both nutrients are used by wetland biota. Wetland plants uptake inorganic nitrogen and phosphorus forms (i.e., nitrate, ammonia, and soluble reactive phosphate) through their roots and/or foliage during the spring and summer and convert them into organic compounds for growth.

Nitrogen removal involves a large suite of bacteria (or microbes) that mediate or conduct numerous chemical reactions. These microbes are found on solid surfaces within the wetland, such as soil, litter and submerged plant stems and leaves. The main transformation processes are ammonification (organic nitrogen to ammonia), nitrification (ammonia to nitrate or nitrite), and denitrification, where nitrate (NO3) is converted to harmless nitrogen gas (N2), which composes 85% of our atmosphere.

The dissolved form of phosphorus (phosphate) accumulates quickly in sediments by sorption (to aluminium and iron oxides and hydroxides) and precipitation (to form aluminium, iron and calcium phosphates). The growth, or accretion, of new material (remnant plant stems, leaves, root debris, and undecomposable parts of dead algae, bacteria, fungi and invertebrates) in the wetland is the only sustainable removal and storage process for phosphorus.

The previous Natural England advice has been updated and the methodology is now based on updated water / agricultural land use / urban run-off figures, agricultural land use modelling, more recent agricultural censuses and data changes. There are also changes to how Wastewater Treatment Works (without permits) figures are agreed. Other changes are about making the approach more specific, so not using an average for a whole catchment but more site-specific.

Ridge Nutrient Neutrality Studies

Our studies can provide a robust assessment of the impact of development on nitrogen (N) and phosphorous (P) losses into a river catchment and demonstrate how, through the application of appropriate mitigation measures, a new development can deliver nutrient neutrality. The findings of such studies support the local competent authority under the Habitat Regulations, in the completion of an AA of whether the scheme adversely affects the integrity of the European Designated Sites associated with certain rivers.

Our reports provide summaries of the evidence gathered to support the Nutrient Neutrality Study together with the neutrality calculations for the scheme and effect of appropriate mitigation measures proposed for inclusion within the scheme. In the absence of mitigation measures, we look for a post-development net change first, in terms of N kg N/yr (deficit) and P/yr (deficit).

We proposed the following mitigation measures to deliver nutrient neutrality on one recent Outline Planning Application (OPA) scheme:

1. On-site SuDS to attenuate stormwater flow from highways and urban areas, which will be optimised for nutrient removal
2. Offsetting nutrient losses from land changes associated with land within the applicant’s control in addition to the land under application
3. Offsetting through the establishment of on-site constructed wetlands to remove nutrients from existing watercourses in the River Itchen Catchment
4. Offsetting through a purchase of nutrient credits from EBC.

Proposed mitigation measures 1 to 3 will improve the impact of the OPA scheme. However, there remains an overall increase in N loading kg N/yr (deficit) following their application. These three mitigation measures do have the potential to deliver P neutrality, depending on the final delivery of the on-site constructed wetlands.

Consequently, mitigation measure 4, namely offsetting of the residual N impact through the purchase of N credits from the local authority, will be necessary to deliver N-neutrality for this scheme, requiring a total of around 2,882 ‘N credits’ (i.e. x kg N/yr). The LPA has confirmed the credits to be available for the scheme to purchase, with sufficient additional credits available should the constructed wetlands result in a shortfall to that identified. Similarly, the LPA has P credits available to deliver neutrality for P should the constructed wetlands result in a shortfall to that identified. As both opportunities could deliver neutrality in isolation, this is unlikely to be necessary.

Our Nutrient Neutrality Studies are produced to accompany an application for Outline Planning Permission and to provide sufficient evidence to the LPA and Natural England that nitrogen (N) and P neutrality can be achieved as part of the development proposals, avoiding any detrimental impacts on downstream European Designated Sites.

For more information on Nutrient Neutrality, please contact

Martyn Grant
Associate (Civil Engineering)