A Quantitative Analysis of Swimming Pool Recirculation System Efficiency in Returning Water to the Treatment System
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Abstract
While chemical reactors have been studied extensively, little work had been done in understanding the recirculation system efficiency of swimming pools. It was proposed by Gage and Bidwell (1926) that recirculation in swimming pools was described by their "Law of Dilution". They stated that the recirculation efficiency (or contaminant removal efficiency) in a swimming pool followed an ideal exponential decay model. The recirculation efficiency is defined as the percentage of the pool volume and contaminant load that reaches the treatment system during a "turnover" period. Following the Gage and Bidwell model, a pool would only remove approximately 63% of the initial contaminant concentration during any turnover period when using a 100% efficient filter since only 63% of the water is filtered per turnover period. Recent research has shown that sand and cartridge filtration are only able to achieve Cryptosporidium removal rates of 25 to 50% under normal US operating conditions. This has led designers and regulators to look more closely at newer treatment options like UV, membranes, and regenerative media filters that boast of removal efficiencies from 99 to 99.9999+%. Despite significant increases in both cost and complexity for many new treatment technologies, the recirculation system efficiency would limit the removal per turnover to 63% for a perfect (100% efficient) filter system according to Gage and Bidwell. Quantifying the recirculation efficiency of swimming pools allows for a more efficient overall design of pools as well as accurate prediction of contaminant removal over time.Two bench-scale swimming pools were systematically evaluated using dye studies and salt tracer experiments. Each pool was investigated using a two-phased approach. The first phase included triplicate non-recirculating salt tracer studies in order to calculate the residence time distribution and hydraulic characteristics of each pool. Short-circuiting and mixing behavior were also visually assessed via dye studies. Triplicate salt tracer studies were performed with alternate pool flowrates and/or flow patterns to assess changes in the recirculation efficiency. In the second phase, triplicate salt tracer experiments were performed while operating the system in a recirculating mode like normal swimming pools. Pools were allowed to come to a steady-state condition to quantify the initial mixing.Non-recirculating salt tracer studies indicated that in all experimental operating conditions salt tracer removal trends agreed with the Gage and Bidwell model with approximately 63% of the salt being removed during the first turnover. Regardless of the internal flow pattern and/or turnover rate, the hydraulic efficiency was not significantly altered. In all cases while operating the system in a non-recirculating mode, greater than 98% of the salt was removed from the system and/or conductivity detection limits were reached within 4 turnover periods. In operational modes with 1 hour or 6 hour turnovers, the exit age distribution followed a predictable exponential decay model as described by Gage and Bidwell. The exponential decay of the salt removal was approximately proportional to the flowrate divided by the system volume, multiplied by a fitting parameter of 1.00 ± 0.11.In characterizing the bench-scale swimming pools, it was also important to characterize the salt tracer distribution. Describing the time and uniformity of tracer distribution emulates the time to distribute chlorine in an actual pool system. In all cases, the time to distribute the salt tracer was less than 12% of the turnover period. During a standard 6 hour turnover period operations, the tracer typically reached peak concentration in less than 30 minutes. While operating the systems in recirculation mode, in all cases (n=6) a steady state condition was reached within 10% of the turnover period (or less than 36 minutes for a 6 hour turnover). All dye studies were performed using a standard 6 hour turnover period. Dye studies showed initial short-circuiting and uneven initial mixing. However, in all cases (n=6) the pool reached a uniform dye saturation within 2% of the turnover interval or 7 minutes. A model was developed to determine the time needed to reach a pathogen removal goal. Pool flowrate, volume, and treatment system efficiency were used to predict the time required to obtain a specific removal percentage goal. Combined filter and UV systems of 1 to 4 log10 (90%, 99%, and 99.99%,) efficiency were found to reach 3 log10 pathogen removal at 48 hours ± 3 hours when operated using a 6 hour turnover period. This indicates that increasing the filter and UV system efficiency between 90 and 99.9% has almost no effect on overall pathogen removal efficiency for a pool despite significant increases in cost. The overall results demonstrate that bench-scale pools recirculate in a predictable manner that is controlled by the pool’s volume and flowrate. The recirculation efficiency is a bottleneck that controls the treatment system rate of removal. The required time for any given removal goal can be calculated using the recirculation model and treatment system efficiency. Pool treatment systems (e.g., UV, Ozone, and filtration) with an efficiency of greater than 90% are unlikely to justify the cost of the upgraded treatment system since changing from 90% efficiency to 99.99% or greater would only decrease time required for the entire pool to reach 99.9% removal goal approximately 10%. In terms of practical recommendations for future pool designers, the design of inlets and outlets has little effect on recirculation system efficiency or overall contaminant removal rates, as long as current U.S. or European design standards are met. The combined efficiencies of filtration and disinfection systems of greater than 90% (1log10) have little impact on contaminant removal rates due to inherent inefficiencies of the recirculation systems. Increasing the removal efficiency of current technologies such as sand and cartridge filters from 25% to 90% would provide valuable improvements. Decreasing pool turnover times (as opposed to more efficient disinfection or filter systems) appears to be the most practical means of increasing the rate of contaminant removal from pools using existing technology with removal rates of at least 90% efficiency.