- 20th September 2016
- Posted by: Joanne Woodhouse
- Category: Projects
The existing Manx Utilities Authority (MUA) sewage treatment works is serving a residential population of 839. It consists of two units, an above ground level “package” Extended Aeration Sewage Works and a below ground level RBC (Rotating Biological Contactor) located at the base of a tree lined embankment. The works are hydraulically overloaded, producing a poor quality of final effluent discharging to the River Dhoo and are at the end of their service lives.
The Department of Environment, Food and Agriculture has set a revised discharge standard for the river of 7mg/l Biochemical Oxygen Demand (BOD), 60mg/l Suspended Solids (SS) and 4mg/l Ammonia. We are tasked with designing a new treatment works within the existing site area while maintaining the operation of the existing treatment works. The new works to be sized for a future population estimated for the year 2033, allow for high levels of ground water infiltration, allow for potential light industry within the catchment and include two hours of storage. In the initial assessment we identified an additional 69 properties that could be diverted into the Ballagarey drainage catchment at very low cost. This was approved by MUA. The new works requires an Environmental Impact Assessment and Planning Approval as the site is located in a rural area and alongside the old steam rail line Heritage Trail.
We have prepared a design based upon a future population of 910 for the installation of two integral RBC designed by specialists Tuke and Bell Ltd where the primary settlement tank is integral with the Biozone RBC and final settlement tank. The BioDisk rotates continuously requiring very low power consumption. With the restricted site the plan area of the tanks ensures the foot print of the new works is kept to a minimum, though with deep excavations. This will require the construction of a cofferdam and well point de-watering. Due to the sloping ground the new works would be set at 1m above ground level at the front of the works and digging 1m at the rear of the works. The treatment works are fully covered, containing noise and odours and less intrusive in the surrounding rural environment. An area of the site has also been identified for any required future UV tertiary treatment of the effluent from the integral RBCs. The final effluent discharges via the existing outfall culvert to the River Dhoo.
The new works form part of the ongoing strategy by the MUA for the improvement of sewage treatment for villages and towns throughout the island and in particular within the Central Valley of the island discharging to river.
For more information: John Sullivan