Balance tanks are a necessary element in decklevel pools. They ensure that the pool water remains at the optimum level for effective surface water removal under all operating conditions. Their design and maintenance are critical. There are three operating levels that must be considered in balance tank design.
Firstly the maximum water level in the tank must be just below the base of the transfer channel around the pool. Secondly the extraction system from the balance tank should be sized to take the total circulation rate when the tank is at the maximum level during bather surge. Finally the minimum level takes into account the volume of water required for filter backwashing from the normal operating level and ensures that the pumps don’t draw air.
BATHER SURGE : Bather surge is of course affected by the number of bathers and the level of activity. But it is reasonable to base this figure on a minimum of 50% of the instantaneous bathing load (see PWTAG online code of practice) and to allow 80 litres of displacement per bather. Filter backwashing volume should be based on a backwash rate of 30m/h per m2 of filter area for at least 5-7 minutes, or until the water runs clear, whichever is first. Ideally the balance tank should be constructed in concrete as part of the pool construction. It should have an internal clearance height of 2m and be no less than 1.5m at the access from the pool surround area.
INTERNAL CLEARANCE : Allowing these heights can be a challenge – for example when retrofitting a balance tank. But the drawbacks of a height of under 1m are twofold. Cleaning – particularly the top surface – is more difficult. A tank 1m deep would need a footprint almost three times that of a 2m deep tank in order to have the same water capacity. If the tank is over 8m long, it should have an access point at each end. The access covers need to be secured but it is essential that they can be easily removed and replaced.
Balance tanks should be inspected at least once a year and cleaned as necessary. Debris should be removed and inner surfaces brushed and flushed down with 100mg/l chlorinated water (1 ml of 10% w/w sodium hypochlorite in 1 litre of tap water), which can be returned to the circulation system via the filters. For more information visit the PWTAG website www.pwtag.org.