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Small Scale Thresholds (Phosphate & Nitrogen) 

For approx. £900 + VAT we can carried out Small Scale Threshold Checks for your project foul drainage.

If you are looking for a Phosphate Assessment please visits our Phosphate Assessment Page.

This page is relevant if you have been asked for a "Nutrient Neutrality Assessment and  Mitigation Strategy (NNAMS)", as per latest validation requirements from Somerset District Councils such as Mendip, the requirement has also spread to Cornwall, Wiltshire and many other places across the UK.

The Principal

For large developments, or developments that are not "small scale" developers may well be required to plant trees, or install reed beds and other mitigation measures so as to meet nutrient neutrality requirements, for sites in Somerset, Cornwall, South Wales, Wiltshire or elsewhere.

The Small Scale Thresholds are a set of criteria that if met result in a simplified private drainage solution of site. Typically consisting of a package treatment plant draining to a soak-away.

The Criteria

The criteria differ from region to region but in general they focus these key principals:

(a) there being no connectivity or pathway between the plan or project and the sensitive designated site interest features, and therefore the nutrients cannot impact the site interest features, or

(b) there being a pathway, but either the plan or project will not increase nutrient levels or would reduce them from the existing situation, and maintaining the current or reduced nutrient levels would not undermine the objective of restoring the site to favorable conservation status, or

(c) there is a pathway, but any additional nutrient contribution will clearly be insignificant alone and in combination (see suggested thresholds below).

The criteria, differ from region to region but we have included a list below, with example answers this particular example is from Wiltshire. We are quite happy for anyone to copy and re-use this text in conjunction with planning application, but will enforce copyright for anyone using it on websites or  other publications.

Example / Template

Stage 1

Does your development drain to the mains network or a cess pit?

No, the development will not drains to mains sewer or cess pit.

Stage 2

Does the package treatment plant or septic tank discharge to a watercourse? No, it is proposed to use a Soak-Away.  

 Stage 3

The package treatment plant or septic tank drains to the ground.   Is the discharge quantity less than  2m3/day?

The average person in the UK uses 141 litres of water a day[1]. Based on a notional occupancy of 5 persons. We might expect a daily foul flow of 705 l/day. This converts to 0.7m3 / day. Well under the 2m3 threshold.

 Stage 4

Is the drainage field is more than 50m from the designated site boundary (or sensitive feature)? Yes. River Avon Special Area of Conservation is many miles from the application site.

Stage 5

Please find drainage plan included within Appendix 1.

Rule B

The drainage field is more than 40m from any surface water feature e.g. ditch, drain, watercourse? The nearest water feature included on 1:1250 OS Mapping is 279m to the SW. 5.5.2       

Rule C

The drainage field in an area with a slope no greater than 15%? The ground in the vicinity of the drainage field fall approx. 20cm in 10 meters. This equates to a gradient of 1:50 or 2%. 5.5.3        Rule D The drainage field is in an area where the high-water table groundwater depth is at least 2m below the surface at all times? Legacy Borehole BGS ID: 397140, shows ground water levels 5mBGL.

Stage 6

Applicant to provide supporting evidence for thresholds. 5.7.1       

Rule E

The drainage field will not be subject to significant flooding, e.g. it is not in flood zone 2 or 3? Flood map for planning shows site in Zone 1. 5.7.2       

Rule F

There are no other known factors which would expedite the transport of  phosphorus for example fissured geology, insufficient soil below the drainage  pipes, known sewer flooding, conditions in the soil/geology that would cause  remobilisation phosphorus, presence of mineshafts. Legacy Borehole BGS ID: 397140, shows silts over approx.  5.0 meter thickness of “highly weathered” chalk. This is sometimes referred to as putty chalk, and is soft in nature, the structure of the chalk has broken down and no fissuring is possible. The site is in a non coal mining area.

Rule G

To ensure that there is no significant in combination effect, the discharge to ground should be at least 200m from any other discharge to ground. Records search show no recorded discharges to ground water within 200m of site. 5.7.4       

Rule H

The percolation test has been performed of the proposed location of the drainage field with the resulting value lying within the required range under the Building Regulations 2010, which specify an average Vp value of between 12 and 100. We would consider that given the site geology in with 2.5 meters thickness of silt, with flints to provide good soakage rates. The site is also large, and can accommodate a large soak-away if required. We can see no technical reason as to why soak-away drainage could not be provided on this site, considering the scale of the development and the area available for use as soak-away, whether it be drainage field or drainage mound. [1] https://waterwise.org.uk/wp-content/uploads/2019/10/WWT-Report-.pdf

Small Scale Threshold Consultancy Service

If you would rather have us write this up for you please call or email, and we will write you a report super quickly and keep your project moving forward.