Salix were invited by Anglian Water’s One Alliance delivery team to bid on for a constructed wetland on a design and build basis.
A 2000m2 reedbed involved a composite approach using both a Sub Surface Flow (SSF) gravel reedbed and a large floating/rafted reedbed in order to meet the treatment criteria and retention times within a limited land footprint. As SSF reedbeds can blind over a relatively short period of time the reedbed was designed as an overland flow wetland so that performance would be guaranteed longer term.
A deeper cell area with a cover of 70% floating wetlands increased retention time and sediment storage capacity. This approach represented the first time that a large-scale floating reedbed has been used for tertiary treatment within a sewage treatment works.
Over 800m2 of floating reedbed was installed over a 1m deep lagoon. All reed plants came from Salix’s in-house nurseries, as did the floating wetlands. 13 years on and the floating wetland cells are still performing as designed and have capacity to store further sediments due to the deep-water storage capacity. The floating island cells also incorporated open water habitat to ensure hydraulic functionality and to allow access in the future to pump out sediments as required.
