How new phosphate targets may impact on your development

Phosphates - Frequently Asked Questions

Further information and questions on phosphate targets and what it means to your development

Phosphorus is an element and occurs naturally as inorganic and organic phosphorus containing compounds. Phosphate is a compound and is the most bioavailable form of phosphorus. Naturally it occurs in low levels and is an essential nutrient for all organic life.

Excess concentrations of phosphate in rivers can trigger a process known as eutrophication. This process has devastating effects to river health and aquatic ecosystems. Excess nutrients, in the form of phosphate, can trigger an algal bloom. Algal blooms stop sunlight penetrating water. Without sunlight aquatic plants cannot photosynthesise, they die as a result and microbes respire on the decomposing organic matter. During photosynthesis plants produce oxygen which aquatic life rely on to breathe. When microbes respire on dead plants and sewerage, they consume oxygen. Deoxygenated waters cause aquatic organisms such as fish, to die through suffocation. Eutrophication collapses entire ecosystems, and the result is an essentially ‘dead’ water body.

The main sources of phosphorus are the rural land use sector (fertilisers and manure) and wastewater sewerage (sewage, food waste, detergents). Dŵr Cymru Welsh Water have undertaken a review of P sources. This will help better inform our approach and where to place specific mitigation strategies. You can access the source apportionment data here.

Types of development that may be affected include (this list is not exhaustive and is subject to review):  

  • New residential units including homes, gypsy and traveller sites/pitches;
  • Tourism attractions and commercial developments where overnight accommodation is provided;
  • New large commercial or industrial developments where customers will be attracted from outside of the catchment such as large retail sites, conference facilities, or major tourist attractions;
  • Agricultural development, including prior notifications (developments that don’t require planning permission, but where the local planning authority needs to check that it is allowed) including additional barns and slurry stores likely to lead to increased herds;
  • Prior notifications for change of use of office to homes and agricultural buildings to homes.

Further guidance and advice can be found on the NRW Phosphate Planning Advice Page.

The following developments can be screened out as not likely to have a significant effect on a river SAC in relation to phosphorus inputs, as there is unlikely to be a source of additional phosphorus or pathway for impacts:

  • any development that does not increase the volume and phosphorus concentration in wastewater.
  • any development that improves existing water quality discharges by reducing the phosphorus concentration of wastewater without increasing volume or by decreasing the volume of wastewater produced without increasing the concentration of phosphorus.
  • developments intended to provide services, facilities, commercial sites, or places of employment (e.g., community buildings, schools etc.) for a local population already served by residential connections to existing public or private sewers discharging within the SAC river catchment.
  • any development that reduces the frequency, or volume of irregular phosphorus discharges within a SAC river catchment such as the erection of agricultural structures and drainage schemes to separate rainwater from manures and slurries by covering yards and existing manure/slurry stores. Note that any such development must not be linked to an increase in livestock numbers or the capacity for an increase in livestock numbers through provision of additional infrastructure.
  • private sewage treatment systems discharging domestic wastewater to ground, which are built to the relevant British Standard (BS 6297:2007+A1:2008), the maximum daily discharge rate is less than 2 cubic metres (m3) and the drainage field is located more than 40m from any surface water feature such as a river, stream, ditch or drain and located more than 50m from a SAC boundary and at least 200m from any other known discharge to ground.

Further guidance and advice can be found on the NRW Phosphate Planning Advice Page.

If your planning application is within a SAC river catchment area, you will need to:

  1. Ascertain if the development can be ‘screened out’ as not likely to have a significant effect on a river SAC in relation to phosphorus inputs. This stage is called ‘the Test of Likely Significant Effects (TLSE)’. Please refer to ‘What sort of development is affected?’ If you feel that your application will not have a significant effect, you will need to provide us with information in support of this.
  2. If the development cannot be ‘screened out’, you will need to calculate the additional phosphorous loading from the proposed development using the Nutrient Budget Calculator. There is a written instructional guide and a video to assist you in using the calculator.

  3. You will then need to submit a Mitigation Proposal that demonstrates how the additional phosphorous generated will be mitigated. Please see the Mitigation guidelines for detailed information on the most appropriate measures that could be employed within Carmarthenshire.

You could withdraw your application and wait for further progress on solutions or speak to your planning officer to agree an extension to your application.

You could also exercise your right of appeal if no decision has been made after eight weeks from when your application was registered/validated, however the Planning Inspectorate will also consider the impacts of the development on phosphate levels.

We also advise you to keep an eye on your application progress on our system and sign up for updates. 

Domestic extensions can provide increased living space within existing properties. They may not result in a change in the number of occupants and, in our opinion, it would appear reasonable for domestic extensions to be screened out at the test of likely significant effect.  Our view is that unless the proposal would result in the creation of independent living accommodation, a separate planning unit and/or a change in use, where it can no longer be said to be ancillary to the main residence, such developments are unlikely to lead to significant effects on a SAC through changes in discharge of wastewater. However, proposals that lead to the creation of independent living accommodation as a separate planning unit may lead to an increase in occupancy by residents from outside a SAC river catchment, and in these instances, proposals require further assessment.


It is not normally considered environmentally acceptable to install a private sewerage treatment facility in areas where there are main sewers because there is greater risk of failure, which could lead to pollution.

Welsh Government planning guidance states that, where possible, new development foul drainage should be discharged into a public sewer.

If, because of cost and/or practicability, it can be demonstrated that connection to a public sewer is not feasible, then a private treatment plant - or non-mains foul sewage disposal - could be considered.

Please note that you must have an environmental permit, or register an exemption with Natural Resources Wales, to operate a private drainage system. NRW will not normally grant a discharge permit for a private sewerage treatment system where it is reasonable to connect to the public foul sewer.

For more information on the use of  private treatment plants in relation to phosphate removal please refer to the NRW Phosphate Planning Advice Page.

When there is a change from existing commercial (or other non-residential) use to residential use, it is assumed that this will result in more wastewater being produced and therefore more nutrient discharges to wastewater treatment plants.

Such plants have a capacity, and when reached there is no easy way to create more space. This creates greater risk of overloading which could increase nutrient discharges into watercourses.

This type of change of use will therefore require a Habitats Regulations Assessment (HRA) and mitigation.

Healthy rivers support social, economic, and environmental resilience and the capacity to adapt to change. We want to find solutions that enhance biodiverse natural environments and promote healthy functioning ecosystems. We want to collaborate with environmental and river groups in finding solutions and would welcome your involvement. Please contact our Nutrient Management Officer at

Mitigation simply means finding a way to prevent excess phosphate from entering the protected waterways.  Measures may include the control of nutrient sources, restoration of damaged ecosystems, catchment management, and features such as Sustainable Drainage Systems and river buffer zones that can be used to reduce run-off. Mitigation closest to the source of nutrients is preferable.

Offsetting simply means taking action to reduce phosphorus production to compensate for increased levels elsewhere. However this must be localised.

Using agriculture as an example - phosphate is produced by the spreading of fertiliser and livestock manure on agricultural land. Rainwater carries these nutrients to water courses, particularly on sloping land. Taking a section of agricultural land out of production could halt the addition of phosphates creating an off-set.

The area of land needed to offset new homes would depend on the lithology and gradient of the land, and how intensively it was farmed.

This phrase refers to the removal of nutrients by developers or local planning authorities to create capacity for growth without causing additional nutrient input. Principles of nutrient neutrality in relation to development or water discharge permit proposals are described by NRW here.

Yes, and they are primarily nature based. The issue can be complex and a range of measures (such as removal of phosphate at source, mitigation, and offsetting) are currently being explored on a catchment-wide and site-specific basis. Please see our mitigation guidance to explore potential solutions.

New agricultural developments involving the storage, management and spreading of fertiliser slurry and manure within the protected riverine SAC catchment has the potential to contribute excess nutrients to the watercourse. It is likely that such developments will be affected and need to provide mitigating measures.

There are nutrient management, land, manure, and habitats management measures that can be put in place to reduce the amount of phosphate entering rivers from diffuse sources. For example riparian buffer strips, wooded/vegetated wetland, and drainage pools and ditches. There are also immediate changes that can be made, for example erecting fencing along riverbanks to prevent livestock entering the rivers.

We would like to explore catchment management ideas with the farming community. Please contact our Nutrient Management Officer at

Percolation testing is now a requirement as per the latest Natural Resources Wales (NRW) advice for planning applications proposing private sewage treatment systems that discharge (less than 2m3 per day) to a drainage field. This is to ensure that the waste effluent from the private sewage treatment system can infiltrate the soil matrix (to avoid pooling) and percolate at an appropriate rate to minimise risk of groundwater table and/or surface water contamination.

Infiltration is the downward entry of water into the surface of soil and rock. When rain hits the ground, water may either infiltrate into the soil or run-off across the ground. Generally, infiltration is a faster movement of water. The infiltration rate is the rate at which the soil absorbs water. The infiltration rate is measured in millimetres (mm) per hour. 

Percolation is the downward movement of water through soil and rock. Gravity and capillary forces are the multiple factors that drive percolation. After infiltration, water starts to move downward through the soil and rock passing different layers of the ground. The initial surface entry of water is called infiltration. Percolation occurs underground and concerns the underground downward movement of water.

Below is the recommended method as outlined in Part B6.5 Natural Resources Wales Environmental Permitting Regulations guidance to determine the potential drainage field’s percolation value (Vp).

  • Avoid carrying out this test in extreme weather conditions such as drought, frost and heavy rain.
  • Excavate at least two holes 300mm2 to a depth 300mm below the proposed invert level (bottom of pipe) of the infiltration pipe and space them evenly along the proposed line of the subsurface irrigation system.
  • Fill each hole with water to a depth of at least 300mm and allow to seep away overnight.
  • Next day, refill each hole with water to a depth of at least 300mm and observe the time in seconds for the water to seep away from 75% full to 25% full (i.e. a depth of 150mm).
  • Divide this time by 150. This answer gives the average time in seconds (Vp) required for the water to drop 1mm.
  • The test should be carried out at least three times with at least two trial holes. The mean average from the tests should be recorded.

This is the percolation value Vp (in seconds)

  • The average figure for the percolation value (Vp) is obtained by summing all the values and dividing by the number of values used.
  • Drainage field disposals should only be used when percolation tests indicate average values of Vp between 15 and 100 and the preliminary assessment of the trial hole tests has been favourable.
  • The minimum value of 15 ensures that untreated effluent cannot percolate too rapidly into groundwater.
  • Where Vp is above the limit of 100, effective treatment is unlikely to take place in a drainage field as there will be inefficient soakage in this location which may lead to sewage ponding on the surface.

Provide the surface area of your infiltration system. For domestic premises, the floor area of the drainage field (A in square metres m2 ) required may be calculated from:

  • A = p × Vp x 0.25 for septic tanks
  • A = p × Vp x 0.20 for package sewage treatment plants where p is the number of people served by the tank (this should be the maximum number of people that could live in the house).

Vp is the percolation value described above. If in doubt, consult your professional advisor or local authority building control officer for advice.

Use the following calculations:

  • For sewage treatment plant:
    • Vp (percolation value) × P (number of inhabitants) × 0.20 = surface area
  • For septic tank:
    • Vp (percolation value) × P (number of inhabitants) × 0.25 = surface area

The method set by NRW must be the one followed but to consolidate understanding of the method and processes surrounding percolation and infiltration testing, resources including instructional videos can be found online.


Unfortunately, Yes. The revision of the Local Development Plan was initially delayed to re-assess and address the implications of allocations within phosphate sensitive catchments.


We have recently held a public consultation upon our second Deposit revised Local Development Plan and, in line with the Delivery Agreement confirmed by Welsh Government, we hope to adopt it in late 2024. In addition to phosphate guidance, it reflects and responds to Covid-19 Recovery, the declared Climate and Nature Emergencies, new Technical Advice Note 15 Revised Flood Maps, and Future Wales: The National Plan 2040.

Certain areas, (known as European sites) are protected by the Conservation of Habitats and Species Regulations 2017 as amended (known as the Habitats Regulations).

These areas include Special Areas of Conservation (SACs) and Special Protection Areas (SPAs).

If a development is proposed in such an area, we must carry out an assessment under the Habitats Regulations, known as the Habitats Regulations Assessment (HRA), to test if whether the proposal could significantly harm the designated features of the site.

The ‘appropriate assessment’ would look at the potential adverse effects of a plan or project (in combination with other plans or projects).

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