Upgrading an old septic tank or a poorly performing package treatment plant (PTP) is a recognised method of nutrient mitigation for small housing developments. When done correctly, it can significantly reduce the amount of phosphorus entering local water catchments.
To estimate how much phosphorus can be saved, we use the Natural England Nutrient Budget Calculator. This tool assumes a default water use of 120 litres per person per day (l/p/d) for new homes.
However, for existing dwellings (typically those built five years ago or more), a higher rate of water use is more realistic. After reviewing local consumption data from United Utilities, we now apply an average of 140 litres per person per day when calculating nutrient loads for these properties.
A single home using a compliant septic tank with an average occupancy of 2.2 people and water usage of 140 l/p/d produces approximately 1.3 kg of total phosphorus per year (Figure 1).
Figure 1: Nutrient load for a single dwelling on a septic tank
Replacing the septic tank with a high-performance biological PTP (with a phosphorus discharge limit of 1.6 mg/l) reduces this to: 0.18 kg of total phosphorus per year (Figure 2)
Figure 2: Nutrient load for a single dwelling on a biological PTP
Saving: 1.30 kg – 0.18 kg = 1.12 kg of phosphorus saved per year
Please note: The 140 l/p/d figure only applies to existing homes. For new dwellings, continue to use the default 120 l/p/d assumption, which reflects modern water efficiency standards and government targets.
For tailored advice on 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.
