The issue – Why is this relevant to The Rivers Trust?

Constructed Wetlands are artificially created wetlands that use natural processes to treat pollutants in water. Pollutants, such as nutrients, pathogens, and heavy metals, are absorbed and stored through uptake by wetland plants, sedimentation, adsorption [1], physical filtering, and microbial activity, thereby reducing the load entering the river/receiving waterway. Constructed Wetlands can address pollution from sewage treatment works, farm run-off, road-runoff and more. They can also provide wider benefits for biodiversity, climate mitigation and adaptation, and increase access to nature for communities.

The role Constructed Wetlands can play in tackling nutrient pollution is particularly pertinent, as pollution by phosphorous and nitrogen is one of the main reasons waterbodies are failing to receive clean bills of health. For example, Constructed Wetlands can contribute to Catchment Nutrient Balancing approaches that maximise nutrient load reduction through a combination of water company sewage treatment work upgrades and land use change and nature-based solutions implemented by land managers. [2]

The Rivers Trust uses evidence-based and effective methods to protect and restore our rivers and their catchments, working collaboratively across sectors to ensure we implement the right solutions in the right places. There are various stakeholders on the topic of Constructed Wetlands, not least professional bodies such as the Constructed Wetland Association, environmental experts and organisations, regulators and policymakers, and community and campaign groups. The Rivers Trust is committed to working together to ensure Constructed Wetlands deliver for nature and people.

What are the contentious aspects?

Constructed Wetlands have come under criticism due to claims that they are not delivering the water quality benefits, particularly phosphorous removal, at water company sewage treatment works as required. Water companies are subject to a legal target to reduce phosphorous from treated wastewater by 80% by 2038 under the Environment Act 2021, therefore their plans and approach to achieving reductions are under significant scrutiny. Furthermore, this question around effectiveness then raises concerns around potential greenwashing, if water companies are using Constructed Wetlands to demonstrate that they are taking action but, in reality, are not effectively addressing their pollution impacts.

It is correct that, if not designed for phosphorous removal, implemented in unsuitable locations, or left without proper maintenance, Constructed Wetlands will not deliver intended treatment outcomes. However, this is equally the case with traditional, engineered infrastructure, which requires proper installation and maintenance. The solution to this issue is not to disregard Constructed Wetlands, but rather to strengthen the governance, standardisation, and monitoring frameworks around them.

It is also worth noting that most Constructed Wetlands are not designed to achieve 80% phosphorous reductions; to achieve this target, they would be used in combination with other, traditional or engineered approaches. As long as water quality results are accurately accounted for, Constructed Wetlands can still make a valuable contribution to phosphorous removal.

What is our position?

Based on the current, peer-reviewed evidence base, and our decades of experience delivering river and catchment restoration, The Rivers Trust is clear that Constructed Wetlands do have a role to play in tackling water quality issues. Constructed Wetlands that are well designed and maintained, and located in the right place, have the potential to make a modest but significant contribution to catchment water quality. Additionally, like other nature-based solutions, Constructed Wetlands will provide additional benefits for climate mitigation and adaptation, habitat and biodiversity improvement, and community health and wellbeing.

Like all nature-based solutions, Constructed Wetlands must be assessed as part of an integrated approach to catchment management. Constructed Wetlands must be considered alongside engineered solutions, taking into account all catchment conditions, to find the best available solutions to improve our water environment for people and planet.

It is essential that governance, standardization, and monitoring rules around Constructed Wetlands are strengthened and fully implemented by Government and regulators. The Rivers Trust has been concerned that unrealistic claims about the effectiveness of Constructed Wetlands and a naïve market for nutrient credits could drive a rush to build wetlands that have little, or no, water quality benefits. Therefore, The Rivers Trust is committed to helping develop guidance and frameworks around Constructed Wetlands in order to facilitate their proper use for water quality benefits.

Of course, treating existing pollution is not a sustainable or efficient solution to improving the health of our rivers and environment long-term. The Rivers Trust works on the principle that the most effective way to improve water quality is to reduce pollutants as close to the source as possible, through effective regulation that takes a precautionary approach and prevents harm as far as possible. We continue to work with our local Member Trusts and alongside a wide range of partners across catchments to make this a reality.

What are we doing about this issue?

The Rivers Trust engages closely with environmental groups, technical experts, and regulators, to share evidence and learning about Constructed Wetlands and ensure that environmental expertise drives the design, implementation, and maintenance of these wetlands. We aim to work with a wide range of stakeholders to foster evidence-based consensus on this topic.

Our concern around lack of governance and standardisation of the approach to Constructed Wetlands led us to work on the Constructed Wetland Detailed Design Guide with various technical experts, commissioned by Natural England. The Guide is built on clear principles, whereby Constructed Wetlands can only claim nutrient credits if we can quantify the removal efficiency based on rigorous, industry standard design calculations. If this is not possible, credits can only be claimed if monitoring proves nutrients are being removed. Additionally, the Guide will stress that effective maintenance is fundamental to the success of a Constructed Wetland. The Guide should be published shortly and is part of series of resources, including the Constructed Wetland Explorer and Wetland Mitigation Framework, that The Rivers Trust have developed and hosted on the Constructed Wetland Hub to support the delivery and maintenance of effective constructed wetlands.

[1] Adsorption is a process that results in the transfer of a molecule from a fluid onto a solid surface. Adsorption is a key removal mechanism for Phosphorus in Constructed Wetlands.
[2] Transforming the River Petteril