Developing Nutrient Mitigation Solutions

Explore the resources to help you create a mitigation strategy for your development

Replacing a septic tank or PTP with a more efficient system

The new system should be on-site and connected to the development, or if off-site, upstream of the development.
The new system should have a British Water Industry certification (showing mg/l of Phosphorus).
The output of the old and new tanks must be input into the appropriate catchment calculator to determine the phosphorus savings.
The tank must be secured through a Section 106 agreement for the duration of the development. (Learn more about Section 106 here)

Changing use of land

The site should be either on-site and connected to the development, or if off-site, upstream of the development.
The proposed land-use change must be signed off by Natural England. See the guidance on nature-based solutions as nutrient mitigation.
You must put the output of the old and new land uses into the appropriate catchment calculator to work out the phosphorus savings.
Ideally, any land-use change should also consider co-benefits with projects that contribute to wider nature recovery goals.
Along with your proposal, you must provide a full ecological assessment of the site, habitat mapping and all condition assessments. Ecological reports must by carried out by a suitably experienced ecologist and consider the presence of protected and priority species.
You must provide detailed plans for management and monitoring with your proposal.
The site must be secured through a Section 106 agreement or Conservation Covenant for the duration of the development.

Lower water usage

You may wish to consider ways to reduce the water usage in your development below 120 litres per person per day (the rate used in the catchment calculators).

You’ll need to provide full evidence for your lower water usage rates and submit it with your completed budget calculator.

Sustainable Urban Drainage Systems (SUDs)

SUDs capture and remove nitrogen from surface water runoff.

The 2023 Construction Industry Research and Information Association (CIRIA) guide provides design criteria for effective SUDs schemes in nutrient neutrality areas.

Upgrading a septic tank

How to calculate phosphorus savings from upgrading a septic tank

Upgrading an old septic tank or a poorly performing package treatment plant (PTP) is a way for small housing developments to stop nutrients from getting into rivers.

When done in the right way, these upgrades can prevent a lot of phosphorus from entering local water catchments.

How we work out phosphorus savings

To estimate phosphorus savings, we use the Natural England Nutrient Budget Calculator.

For new homes (built in the last five years), we assume each person uses 120 litres of water a day, on average. This reflects modern water efficiency standards and government targets.

For existing homes (built more than five years ago), we use a higher average of 140 litres per person per day. This is based on United Utilities’ data on local consumption.

Example calculation

Before the upgrade

Let’s consider a single existing home where 2.2 people live (the average) and water usage of 140 litres per person per day. The home has a compliant septic tank.

This home produces approximately 1.3kg of phosphorus per year.

After the upgrade

Now we replace the septic tank with a high-performance biological PTP that has a phosphorus discharge limit of 1.6mg per litre.

This reduces the total phosphorus produced to 0.18kg per year.

This reduces the total phosphorus produced to 0.18kg per year

Saving: 1.30kg – 0.18kg = 1.12kg of phosphorus saved per year.

For help with using these calculations in your planning application, please get in touch with the relevant planning authority: Westmorland and Furness Council, Cumberland Council or Lake District National Park Authority.