A laboratory hot tub model for disinfectant efficacy evaluation

This paper describes a novel laboratory hot tub (LHT) apparatus and associated standard operating procedure (SOP) designed to reproduce the key biological, chemical, and engineering parameters associated with recreational and therapeutic hot tubs. Efficacy, as measured quantitatively by log reduction values, was determined against both biofilm and planktonic bacteria. When the LHT was run according to the SOP, with no antimicrobial treatment, a consistent level of bacterial contamination occurred. The means of log10 viable cell densities (+/- the repeatability standard deviation of log densities) were 7.2 (+/-0.31) for the bulk water (density in units of cfu ml-1), 5.3 (+/-0.56) for the coupons (density in units of cfu cm-2), and 6.6 (+/-0.50) for the filters (density in units of cfu cm-2). When control and chlorine treated LHTs were run in parallel, the log reduction increased significantly with chlorine concentration for samples of planktonic bacteria in the bulk water (p=0.016), biofilm bacteria on the coupons (p=0.09) and biofilm bacteria on the filter (p=0.005), indicating that the method was sensitive to chlorine concentration. The method also displayed sensitivity by differentiating between chlorine and bromine treatments; in every case, chlorine produced a greater log reduction than did the same concentration of bromine. The model and SOP were shown to be rugged with respect to slight changes in fluid mixing intensity, water chemistry (saturation index), inoculum size, and organic loading. The LHT and associated SOP provide a reliable second tier in a three-tiered testing process, in which the first tier is a suspension test and the final tier is a field test.