Turnover Times

TURNOVER times shall be calculated based solely on the flow rate through the filtration system.

The recommended design TURNOVER time can then be calculated by dividing the volume by the recommended flow. This procedure can be performed for individual sections of a POOL or the entire POOL depending on the number of zones, which are based on depth of the water. Adjustments can then be made to this calculation to account for extraordinary conditions. For example, since a SPA has higher water temperature than a POOL, a PATRON would be expected to sweat more; an indoor POOL might experience less contamination from pollen, dust, and rain than an equivalent outdoor POOL; and a POOL filled with diaper-age children would be considered an increased-risk POOL requiring more aggressive treatment. Aquatic facilities that enforce showering prior to POOL entry could reduce the organic load on the POOL by 35-60% with showers lasting only 17 seconds¹⁴². The BATHER LOAD calculation based on surface area of the POOL has been proposed by PWTAG¹⁴³ in 1999 and has influenced the CODES proposed by the World Health Organization¹⁴⁴ and Australia¹⁴⁵. This approach has been adapted for use in the U.S. by slightly increasing the area recommended per BATHER in shallow waters and decreasing the area in deep POOLS to account for the intensity of deep water activities, the relatively low surface area to volume ratios of deep waters relative to shallow waters, the typically poorer mixing efficiency in deeper water, the increased amount of time typically spent underwater in deeper water, and the larger average size of bathers commonly found in deeper water. These values were empirically derived for the MAHC to match typical U.S. practices at the time of this writing and can be changed as necessary to achieve the desired water quality goals. Effectively handling BATHER COUNT in terms of pathogen removals and CHLORINE demand is a paramount concern for which the above calculations should provide some science-based guidance. However, there are other factors that must be considered when selecting a recirculation rate for an AQUATIC VENUE. For example, effectively distributing treated water to avoid dead spots recommends minimum water velocities to reach the POOL center and extremities. Similarly, effective surface skimming recommends adequate velocities at the surface of the POOL to remove floating CONTAMINANTS. Due to the kinetics of DISINFECTION and CHLORINE decay, CHLORINE must be replenished at some minimum intervals to maintain the recommended FREE CHLORINE RESIDUAL. For these reasons, MAHC Table 4.7.1.10 was developed to provide some maximum TURNOVER time limits for AQUATIC VENUES that are not dominantly influenced by BATHER LOAD to help ensure proper physical transport of CONTAMINANTS and DISINFECTANT. Values in this table are derived from historical practice and design experience worldwide. All AQUATIC VENUES must be designed to meet the lesser of the two maximum TURNOVER times.