Horizontal flow wetlands - Nutrient neutrality private mitigations

A horizontal flow (HF) wetland is a system where water enters the system at one end, flows through the gravel media, and is collected on the opposite end of the wetland prior to being discharged. The wetland is ‘self-contained’ from the surrounding land with a layer of impermeable liner. As the wetland is saturated with water an anoxic environment is created with minimal flow.

These types of wetlands are used for secondary-tertiary treatment, therefore they require primary treatment before water is fed to the wetlands.

• Reuses water
• Can be fed by gravity (low energy use)
• Biodiversity enhancement
• Carbon sequestration
• Of aesthetic value

• Requires primary treatment
• Produces methane
• Health and safety concerns around algal blooms prohibit public access

1. Phosphorus
2. Ammonia
3. Nitrogen
4. Coliforms
5. BOD/COD
6. Suspended solids

• Short term energy requirements for earth works
• HF wetlands act as a net carbon sink

• Gravel filtration will immediately remove suspended sediment
• Removal of nutrients / organic matter will first need vegetation to become established

• Bioclogging is the major concern, so silt needs to be removed from wetland as required, may require replanting
• Gravel needs to be renewed as required
• Suggested cutting back of plants and removal of litter as required

• Performance can be kept at optimum with well a well-established maintenance schedule
• Performance fluctuates with seasonal changes; lower in autumn/winter, highest in spring/summer

The size of the wetland is correlated to the load of nutrient removal. Design criteria to meet:

• Monitoring required of the inlet and outlet concentrations and a Population Equivalent must be evidenced
• Hydraulic and mass loading will directly impact sizing, aspect ratio, and depth
• Must take into account climate factors and climate change, such as droughts and dry weather, or water gains from rainfall, etc. As well seasonal dynamics and differences in flows through different seasons
• Ecology and plant community; what plant species to use

Alexandros Stefanakis, C. S. (2014). Vertical Flow Constructed Wetlands. Eco-engineering Systems for Wastewater and Sludge Treatment.

Dotro, G., G Langergraber, Pascal Molle, Nivala, J., Jaume Puigagut Juárez, Stein, O.R., Marcos Von Sperling and Iwa. (2017). Task Group On Mainstreaming The Use Of Treatment Wetlands . Treatment wetlands. London: Iwa Publishing.

ICRA. (n.d.). Horizontal Flow Wetlands. 

Ricardo for Herefordshire Council. (2021). Interim Phosphate Delivery Plan Stage 2, Mitigation options for phosphate removal in the Wye Catchment.

Silviya Lavrova, B. K. (2013). Nutrients and Organic Matter Removal in a Vertical-Flow Constructed Wetland. Applied Bioremediation.

Wetland Engineering. (n.d.). Treatment Wetlands. Retrieved from https://wetlandengineering.co.uk/treatment-wetlands