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Nutrient offsetting catchment study

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Mott MacDonald Bentley was commissioned to undertake a catchment management study to reduce total phosphorus (TP) loading to a river in North Wales.

This study constitutes a catchment management nutrient offsetting investigation to understand whether the wastewater treatment works (WwTW) could offset a proportion of the TP load reduction under the proposed permit by working with other sectors (e.g. agriculture). This drives benefit for the environment, local community and economy.

Study area

Under the Water Framework Directive (WFD), the river is classified as ‘Moderate’ due to phosphorus and fish (salmon, eel and stone loach) drivers. Natural Resources Wales has issued a proposed final effluent TP discharge permit to the WwTW (serving ~500 people) in the catchment to meet WFD ‘Good’ status. DCWW is considering alternative, innovative solutions that can provide a sustainable approach to wastewater treatment.

What is catchment management?

Catchment management focuses on delivering environmental and social gain through a holistic view of natural processes in a catchment. Catchment management aims to improve water quality and can achieve multiple and wide-reaching benefits compared to traditional engineering solutions, as exemplified in Figure 1. There has been a movement within the water industry towards nature-based and ‘capitals’ (such as natural capital) solutions.

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Study approach

The conceptualisation, literature study and ecosystem services were used to identify contributors to the in-river TP load (Table 1 and Figure 2) and define the catchment intervention scenarios to be modelled as part of this study to address the following objectives:

1.  What are the sources of phosphorus in the river catchment?

2.  What is the proportion of in-river phosphorus load from WwTW?

3.  Could a catchment management solution be an alternative to introducing conventional phosphorus removal treatment methods at the WwTW and, if so, how effective could it be?

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Modelling solutions

Four catchment intervention scenarios were modelled in SIMCAT:

Baseline ‘do nothing’ (Figure 2) - no catchment interventions or additional phosphorus treatment at WwTW (max. permitted discharge).

Proposed consent – “business as usual” scenario where no catchment interventions are applied and the WwTW achieves the proposed permit by conventional treatment methods.

Top 5 measures - top 5 on-farm phosphorus management measures with the greatest phosphorus reduction for each farm type with no additional WwTW treatment.

Optimise maximum - optimal group of on-farm phosphorus management measures aiming to achieve the maximum phosphorus reduction for each farm type with no additional WwTW treatment



•The proposed consent scenario achieves WFD ‘Good’ status (Figure 3).

•The agricultural scenarios do not achieve WFD ‘Good’ status (Figure 3). On-farm phosphorus management measures alone cannot offset the required improvement in discharge quality.

•Combining a less stringent permit with catchment management may meet the required water quality at a lower cost with greater environmental benefits than the proposed consent. All sectors would need to reduce their TP input to the river to achieve WFD ‘Good’ status.

Want to learn more? Speak to Hannah Simpson @ Mott MacDonald