How new phosphate targets may impact on your development

Page updated on: 31/08/2023


In January 2021 Natural Resources Wales (NRW) published new targets to reduce the concentration of phosphorus in Special Areas of Conservation (SAC) across Wales.

The revised targets followed evidence from the Joint Nature Conservation Committee that warmer and drier weather, predicted as a result of climate change, could reduce river flows during the summer and, therefore, increase phosphate concentrations. It is also based on new evidence about the damaging effect of excess phosphate to water ecosystems and species. The importance of this is partially recognised through the declaration of a Nature Emergency by Carmarthenshire County Council in 2022.

NRW Guidance states that Local Authorities are responsible for upholding the Conservation of Habitats and Species Regulations 2017 through monitoring development within SACs.

At present, over 60% of waterbodies in Wales fail against the tighter targets, and Local Planning Authorities are being asked to take more action to avoid further deterioration of the environment. This means all future development proposals within failing SAC River Catchments  that will generate an increase in the volume of wastewater must now prove that the ensuing development will not contribute to increased phosphorus levels.

In Carmarthenshire, the Afon Teifi, Afon Tywi, River Wye, River Usk, and Afonydd Cleddau are designated as riverine SACs. At present, Afon Teifi, River Wye, River Usk, and Afonydd Cleddau are failing against NRW’s targets, whereas Afon Tywi is passing although with limited headroom. Further details on status against these targets can be found within The NRW Compliance Assessment report of Welsh River SACs against Phosphorus Targets Report

Developments in proximity to these rivers may have limited capacity to connect to the public sewerage system and alternative solutions must be found that will meet the new targets, either by meeting nutrient neutrality or betterment.

What action have we taken?

We have written to Welsh Government outlining our concerns about the impact on development and the necessity to find a solution as quickly as possible. We have also taken the following proactive steps:

  • We have established a Nutrient Management Board (NMB) for the River Tywi. This board is responsible for producing a Nutrient Management Plan to improve the condition of the river and to facilitate nutrient neutral development to proceed. Further details of this Board, including a webpage displaying live updates will be posted on this page when it becomes available. We have also helped set up the Teifi NMB and the Cleddau NMB.
  • We have produced the West Wales Calculator to help developers calculate the level of nutrients their development will generate. In practise, the application of the calculator will span three counties (Carmarthenshire, Pembrokeshire, and Ceredigion) and river catchments (Tywi, Cleddau and Teifi) under the supervision of the newly established NMBs.
  • We have produced comprehensive Mitigation Guidance that explains the most effective types of mitigation that could be utilised in Carmarthenshire.
  • In recognition of the leadership demonstrated in Carmarthenshire, we were invited to the Phosphate Solutions Summit held in July 2022 by the First Minister at the Royal Welsh Show. In March 2023, the second Phosphate Solutions Summit, again attended by representatives from the Local Authority, further highlighted the need for a River Pollution Action Plan and to develop solutions with haste.

West Wales Calculator 

Our West Wales Calculator is now live.

Carmarthenshire’s original Nutrient Budget Calculator, the first in Wales, has been developed to form the West Wales Nutrient Budget Calculator. This calculator functions in the same way as the previous one but has additional features and has undergone refinement to cater to all three SAC catchments. This is an important first step in allowing nutrient neutral development to proceed in all catchments within West Wales.

This is a free resource, designed specifically for the Tywi, Teifi and Cleddau SAC catchments. It will support you to understand the impact of your development by confirming the proposed development’s total phosphorous budget, enabling you to consider the mitigation requirements. We have produced a Calculator Guidance document and a Calculator Technical Review as an instructional guide to help you use the calculator which also explains where data has been gathered from and how calculations have been deduced. Separate mitigation guidelines have been produced for each river catchment. These can be accessed here:

West Wales Mitigation Guidelines  

Although it is not required that you use this calculator, we strongly encourage you to do so.  Any applications submitted using alternative calculators will be subject to additional scrutiny to determine their relevance to conditions in the county.

If the answer is yes to either:
2 and 3
2 and 4

Please use the West Wales nutrient budget calculator to assess whether development will increase nutrient loading to a European site/SAC and whether the development is within a catchment that drains to an affected European site.

  1. Whether the development is within a catchment that drains to an affected European site.
  2. Whether the receiving Wastewater Treatment Works discharges to an affected European site.
  3. Whether the development will lead to an increase in ‘overnight stays’.
  4. Whether the development will lead to an increase in the number of customers/users or employees coming into the catchment of the SAC river from outside of the catchment to work (in this context a user is defined as a person who may use a service or facilities provided by the development without being directly considered as a customer).

West Wales Calculator     Mitigation Guidelines

Calculator Guidance     Calculator Technical review

How it works

Simply input the information specific to your development, and site, into the calculator and work through the stages - instructions have been provided to help you. 

The calculator will produce a value of total phosphorus in kg per annum. This end value can be used to help you consider your phosphate mitigation options. However, the calculator will not estimate the amount of land required for any form of mitigation due to the numerous variables in nature-based solutions that may vary depending on proposed maintenance, maturity, and site-specific conditions.


The Local Authority does not accept liability for any direct or indirect damage, loss or inconvenience caused by the downloading and/or use of the Nutrient Budget Calculator.

The calculator will be subject to periodic reviews and updates to ensure it is based on the best available evidence and accurately reflects the local conditions. Please refer to the Calculator Guidance for an explanation of the data sets and methodology used in the calculator. If you have any comments, please let us know by emailing

Mitigation Measures

Our Mitigation Guidance outlines potential options for phosphate mitigation, and includes Wetlands and Buffer strips. Such measures will need to reflect site-specific circumstances.

Any mitigation measures intended to avoid or mitigate potential phosphorus impacts must demonstrate that they are based on the ‘best available evidence’, will be effective ‘beyond reasonable doubt’, are based on estimates that are ‘precautionary’, and can be secured ‘in perpetuity’ (80-125 years).

The proposed measures must also be legally enforced.

For each measure, we need to receive information:

  • detailing how the measure(s) would avoid or reduce adverse effects on the SAC (considering the predicted duration of the effects);
  • demonstrating how the measure(s) would achieve nutrient neutrality;
  • confirming how the measure(s) will be implemented, and by whom;
  • detailing how the measure will be maintained and who will be responsible for maintenance;
  • showing how the measure will be monitored to ensure it is effective.

Further information on mitigation measures can be found in the latest NRW Phosphate Planning Advice.

Next Steps

  • We are establishing a Technical Advice Group (TAG) to provide evidence review, scenario modelling and proposals that will be presented to the Nutrient Management Boards for approval.
  • We are in the process of establishing a River Stakeholder Group. This will operate as one cross-region group. Membership of this group is open, and we welcome all contributions to help facilitate constructive and lasting change for river health. Further details, including joining information will be posted on this page when it becomes available.
  • The challenges of addressing excess phosphate are complex, and there is no easy solution. Long lasting solutions will require collaboration with neighbouring Local Authorities, Dŵr Cymru Welsh Water, the rural land use sector, NRW, Welsh Government and other key stakeholders. Active collaboration and information sharing amongst all parties will be undertaken on a regular basis.
  • A Strategic Mitigation programme that maximises mitigation and delivers the benefit of that via a credit exchange is being explored. This will remove many of the mitigation barriers that developers are having to address.

We recognise that this situation is frustrating for developers - we want to work with you to find solutions that improve the condition of our rivers, that are feasible and can be implemented quickly and successfully. We want to engage with a wide range of people representing homebuilders, employers, the rural land use sector, residents, environmental groups and more to discuss these issues.

Carmarthenshire County Council is working proactively to minimise disruption and provide a solution to this unprecedented issue. We are operating at the forefront, planning strategically and recognise the necessity for fast and effective action, setting an example for the rest of Wales.

NRW have started issuing their review of wastewater treatment works (WwTW) permits. A backstop P limit is being applied to sites that previously had none in the past. Tighter P limits may be placed on WwTW permits where a limit is already present. There may be no change to existing P limits at certain sites. The new backstop limit of 5mg/l applies to sites with a dry weather flow of <20m3/day.


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 millimeters (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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