Nutrient Neutrality

Nutrient neutrality is a significant area of concern to many of our clients, local authorities and of course Natural England, with many areas such as Poole Harbour and the Somerset Levels & Moors subject to strict requirements to ensure that downstream water bodies do not receive any increase in nutrient levels. The main nutrients under regulation are phosphorus and nitrogen, both essential elements in agriculture and plant growth. However, high levels of these nutrients in water bodies can lead to accelerated algal growth or ‘eutrophication’ and the degradation of aquatic ecosystems.

The nutrient neutrality obligations require developers to calculate a ‘budget’ based on the nutrients produced by the existing site, the proposed development, and any mitigation provided by SuDS. This is represented in the equation below, measured in kilograms of nutrient produced annually:

+ Wastewater Nutrients + Proposed land use runoff Nutrients – Existing land use Nutrients – Nutrients removed by SuDS = Net Nutrient Budget

Typically, the most significant factor in the equation above is the ‘Wastewater Nutrients’ section, which is why many schemes are being required to treat foul flows on site via package treatment plants. When developers cannot achieve a Net Nutrient Budget of zero or below, they are typically asked to offset the remainder through a credit system, which can be very costly. It’s therefore beneficial to developers, and the wider environment, to implement effective SuDS to reduce the amount of nutrients present in surface water runoff and consequently reduce the net nutrient budget for the site.

Nitrogen is notoriously difficult to remove through SuDS measures, but for phosphorus, it is slightly easier. AWP have worked on various sites in catchments sensitive to phosphorus and have developed a methodology to calculate the amount of phosphorus removed by any SuDS ‘treatment train’, based on CIRIA guidance C808F ‘Using SuDS to reduce phosphorus in surface water runoff’.

Whilst developing this methodology, AWP’s Ben Fenton worked collaboratively with CIRIA to share the results of his work and his recommendations for how the guidance could be clarified to assist others in the wider industry. Jo Bradley, one of the authors of the CIRIA document had the following to say of Ben’s work:

‘The work that Ben did assessing CIRIA C808 was incredibly useful to the team. It is invaluable to have an industry practitioner with an analytical mind to identify errors in the document, but also to suggest improvements and amendments. By helping us to update the guidance, Ben has driven forward the ability of designers to create drainage schemes and treatment systems to reduce phosphorus pollution from new developments. By acting as a ‘critical friend’ and having the courage and the tenacity to contact the authorship team with suggested improvements, he saved us all time and made sure that a better document will be delivered.’ - Jo Bradley

AWP are pleased to be able to advise clients on nutrient neutrality for both phosphorus and nitrogen. We are informed on how the design and sequencing of SuDS can reduce the amount of credits required,   and are happy to assist in completing Natural England Nutrient Neutrality calculators. Through our expertise, we can achieve better outcomes, both for out clients, and the environment.

Pictured: Algal Bloom at Slapton Ley Nature Reserve.  Credit: Adrian Davies Imaging, July 2014