Northern Ireland Water
RPS Consulting, BSG Engineering, Doran Consulting
A new fully-configured wetland wastewater treatment works (WwTW) in Fermanagh, Northern Ireland, has recently been commissioned and hand-over trials to Northern Ireland Water (NIW) completed.
The works, in the rural village of Clabby, provide the complete treatment of sewage including solids and sludge management as well as effluent treatment to discharge consent for all critical parameters.
The system, designed by ARM Ltd/SINT and constructed through ARM's delivery partners RPS Consulting, BSG Engineering and Doran Consulting, replaces an existing conventional RBC WwTW constructed in 2002 which, with limited capacity and spatial constraints for expansion, was restricting growth within the Clabby catchment.
The design capacity for the new solution was set at a population of 750 and the solutions originally considered by NIW included an additional RBC site or a more passive wetland solution. Economics (whole life costs) and spatial requirements eliminated these options in favour of ARM's Phragmifiltre wetland technology which provided the green solution required by NIW. As with all wetland treatment technologies the system is simple and robust, has minimal maintenance and power requirements all of which give rise to low operating costs, and therefore reduce whole life costs. It also provides a high natural capital blending into the environment, adding biodiversity to the locality.
Phragmifiltre systems comprise two stages of multiple vertical-flow wetlands. The first stage consists of 3-4 reed beds, operated alternately. These trap all the solids and, through rotation of the beds, have a resting period between effluent dosing periods allowing the solids to dewater and compost/mineralise. The liquors pass down through the free-draining wetland media which removes soluble organic matter and ammonia through conventional means as an attached growth reactor. The liquors pass on to the second stage of wetlands where final polishing of the effluent occurs before discharge to the local water course. The system is very versatile and often just the first stage is installed upfront of ageing secondary treatment technologies to provide complete sludge management and secondary treatment on site taking the load off the existing biological treatment system and extending the life of a treatment works. Principally this technology eliminates sludge transportation costs as well as providing full treatment.
Each site is unique and the challenges in Clabby were primarily linked to the ground conditions. The system required a 0.2-hectare footprint and the chosen location, a greenfield site 300m from the existing works, contained dense woodland below which was a significant depth of peat. Local weather conditions also impacted on the system design as well as a tight 3mg/l ammonia discharge consent. The use of light aeration in the second stage reed beds ensured that this was comfortably achieved. The system is designed to treat all flows up to Formula A so that a stormwater management system is not required.