Nitrogen (N) and phosphorus (P) are nutrients essential for plant growth, but too much nutrient in waterbodies can cause excessive plant and algae growth (‘eutrophication’). Phosphorus is the most important nutrient in freshwater systems and nitrogen is more important in marine systems.

Eutrophication can then result in dramatic reductions in light levels and oxygen concentrations in the water causing damaging effects to water life and dependent fish populations with a detrimental knock-on effect on birds and other species that depend on fish as a food source.

Waterbodies protected as designated sites (or habitats sites) are managed by Natural England. Where development is in the catchment of statutory nationally designated sites (e.g., Sites of Special Scientific Interest (SSSIs), Special Areas of Conservation (SAC)); or internationally designated Convention on Wetlands (Ramsar 1971) sites, Natural England requires the development to be nutrient neutral, so the development land parcel must produce no more nutrients post-development than it did pre-development.

Development generates nutrients from two sources, primary and secondary:

Primary sources: Foul waste (‘Black Water’ and ‘Grey Water’) from residential property and commercial amenities, usually discharged to foul sewer and treated by a wastewater treatment works (WwTW). Water consumption generates foul/grey water at a rate of around 125 litres/per person per day (Building Regulations average use), containing on average 27.7 mg/litre Total Nitrogen (TN) and 1.59 mg/litre Total Phosphorus (TP).

Secondary sources of nutrients are form the nutrient load of storm runoff from residential gardens or landscaped areas where fertilisers are applied and include nutrients from pet dog and cat faeces and urine. These sources can be captured by development Sustainable Drainage Schemes (SuDS) and treated to reduce nutrients released to the wider environment.

Natural England issued a ‘Nutrient Neutrality Generic methodology’ in February 2022 and national guidance in March 2022. Concurrently, several local authorities in affected catchments have produced nutrient assessment calculators for use by developers.

If the WwTW cannot treat all the TN and TP in foul flows from the development, the Nutrient Neutral assessment will indicate the amount of nutrients that require on site treatment and removal to ensure nutrient neutrality.

An example 4-stage nitrogen neutrality assessment using real life site data.

• Stage 1 estimates the nitrogen load in primary foul effluent from a development housing 250 persons as 271 kg TN/year, based on 110 l/person/day water consumption.

• Stage 2 (on site) estimates the current nitrogen load from the development footprint land use- arable farmland (10 years of data on fertiliser applications) as 310.9 kg TN/year

• Stage 2 (off site) estimates the nitrogen load in watercourses flowing through the site from the 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 discounting against 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 and shows that the site is nutrient neutral, and that the treatment wetland as designed goes beyond the neutral requirement and reduces the nitrogen load from the total catchment by 115kg TN/year.

In situations where there is no watercourse, or where existing emissions of nutrients are low treatment wetlands cannot be used as the only other constant source of water would be treated effluent from a package treatment plant (PTP) and a combination PTP and constructed treatment wetland will be unable to reduce N concentrations to zero. However, it may be possible to send foul to sewer and construct a treatment wetland on another watercourse provided it is in the same catchment. Other mitigation options were reviewed in my previous article here [link]

Enzygo Water Sciences undertakes nutrient neutral assessment, carries out water quality and nutrient load assessments on watercourses and works with its partners on design and construction of small footprint treatment wetlands

I would welcome comments and would be happy to discuss your projects, please message me directly.

Dr Paul Hardwick, Technical Director Water Sciences, Enzygo Ltd

Paul.Hardwick@enzygo.com.

Visit www.enzygo.com