When special conditions apply
Wide variations in flow, extreme weather conditions and nutrient removal are the key challenges for the new wastewater treatment plant at Scotland's Cairngorm ski resort.On-site wastewater treatment for small, isolated communities is not new. But when the location is of great ecological sensitivity, has extreme weather conditions and large fluctuations in usage, meeting effluent quality standards can be a major challenge. The problem is exacerbated if the site demands more stringent discharge standards than those imposed on larger municipal STWs.
One such location is the Cairngorm ski resort, which needed a new wastewater treatment plant as part of a redevelopment programme which includes a new funicular railway with en-route restaurant and toilets.
Usage patterns of the new facilities mean the treatment plant has to cope with large, sudden increases in flow at peak times of day, as well as periodic peaks and troughs caused by changing weekly and seasonal use. The site is so ecologically sensitive that to ensure no imbalances were created, any excavated material or vegetation had to be stored and reinstated in the order in which it was removed. Extreme weather conditions and the large distances between the facilities and the treatment plant posed additional design challenges.
Nutrient removal was key to meeting the required quality standards. The Highlands and Islands Enterprise Board (HIEB) engaged consulting engineers AF Cruden of Inverness to specify the requirements for the site and KEE Process of Aston Clinton, Buckinghamshire, was appointed to evaluate, design and install the STW. The plant is currently being commissioned and the site will open to visitors on November 1, 2000.
The flow rate and character of the wastewater entering the treatment plant is very inconsistent, with half of the week's flow sometimes being generated at the weekend. Flows can be as low as 30m3 per day during the summer rising to 110m3 per day during the peak winter skiing season.
Other conditions affecting system requirements are;
- water is supplied to the site from two to three sources with large variations in pH and alkalinity,
- ambient temperatures that at peak times of the year fall well below -20°C,
- snowfall variations in excess of 3m,
- flow generated from various locations - one location is several kilometres from the treatment plant, leading to potential loss of valuable heat from the wastewater,
- minimal availability of on-site operative and maintenance personnel.
The plant has a number of features which ensure quality standards are met throughout the year with the plant operating efficiently and placing minimum demands on resources.
Process stability is achieved using rotating biological contactor (RBC) units and pH correction is handled by automated alkali dosing. Multiple-stream units are used to ensure consistent treatment regardless of demands on the plant. KEE's managed flow system overcomes variations in flow with flow balancing and constant forward feed to the secondary treatment stage.
To cope with extreme drops in temperature the plant uses insulated GRP covers and space heating in the RBC unit. Ortho-phosphorus removal is achieved by using coagulant/flocculant dosing controlled by the RBC's managed flow system.
To avoid eutrophication of the receiving water, nutrient removal is a critical factor when effluent is discharged into static or slow-flowing water. The Cairngorm site is a long way from the sea and final effluent is discharged into a local burn. The chemical dosing which removes phosphates is also controlled by the managed flow system.
Level-sensing probes in the primary chamber of the RBCs control chemical dosing. The secondary RBC chamber is automatically dosed with coagulants and the solids are flocculated, enabling the phosphate compounds to settle with any other remaining solids in the secondary clarifier before final discharge.
Managed flow technology and other processes used in the Cairngorm project have been applied in many situations where wide variations in flow are a feature. Flow rates need to be streamlined in order to take out the peaks and troughs, unifying the strength and composition of wastewater entering the RBC. To achieve this, the RBC is split into two chambers. In the first chamber the RBC is linked hydraulically to the primary clarifier, and these chambers act together to provide balancing. The load level in the first RBC chamber varies in line with the level in the primary clarifier. A bucket-feed mechanism, mounted on the first stage of the RBC, forward feeds partially-treated wastewater into the second RBC chamber thus ensuring there is always water in the secondary RBC chamber.
The system is designed to ensure optimum performance and consistent effluent discharge quality. KEE Process holds the international patent for the managed flow system which has been successfully used for wastewater treatment plants throughout the world.