C.M.R. 10, 144, ch. 202, app 144-202-B - WATER REPLACEMENT STANDARDS

This procedure provides requirements of how to determine the frequency for which spa and hot tub water should be replaced.

Test the TDS (Total Dissolved Solids) and calculate the WRI (Water Replacement Interval) to determine when your spa needs to be drained. Drain the spa completely, clean it thoroughly, and refill it with source water when either of the following conditions is met:

1. The Total Dissolved Solids (TDS) in the spa water exceeds the source water TDS by 1,500 ppm or more; or

2. The Water Replacement Interval (WRI) is less than or equal to the number of days since the last time the water was drained. WRI is calculated as shown in the formula and examples below.

WRI, days = (1/3)(Spa Volume, U.S. Gallons)/(Number of Bathers/Day)

EXAMPLE 1: The TDS of the original source water was measured and recorded to be 800 ppm. The TDS of the spa water is now reading 2,500 ppm. The difference is greater than 1,500 ppm (2,500 ppm - 800 ppm = 1,700 ppm) and therefore the spa should be drained immediately.

EXAMPLE 2: Consider a 600-gallon spa that was last drained and refilled Sunday evening. Each day the operator estimates the number of bathers that used the spa that day and calculates the WRI. This calculated value is then rounded off to yield a whole number. Referring to the table below, the operator would have calculated based on 85 bathers on Monday:

Water Replacement Interval, days = (1/3) x 600 / 85 = 200 / 85 = 2.4 or about 2 days.

The spa water was replaced Tuesday evening because the Monday bather load (85) dictated that a 2- day Water Replacement Interval was required.

Mon	Tue	Wed	Thur	Fri	Sat	Sun
Bathers/day	85	2	19	20	105	100	<50
Calculated days to replace water	2.4	100	10.5	10	1.9	2	>4
Days to replace water (WRI)	2	100	11	10	2	2	>4
Drain the spa"	No	Yes	No	No	Yes	No	Yes

EXAMPLE 3: Referring to the table above, the WRI values for Wednesday and Thursday did not indicate a need to drain the spa. The water was replaced on Friday evening because the Friday bather load (105) dictated a 2-day Water Replacement Interval (Friday is already 3 days after Tuesday).

EXAMPLE 4: The water was replaced again Sunday evening because of the Saturday bather load (100). The bather load on Sunday was irrelevant because the spa would be drained regardless due to the high bather load on Sunday.

Notes

C.M.R. 10, 144, ch. 202, app 144-202-B