BACK

Stan Pickens, Ph.D.

Why Are There So Many Recommended TDS (Total Dissolved Solids) Limits? Why Were TDS Limits Established?

A state health official recently wrote to APSP with such a question. He had been researching the issue and noticed a wide range of state standards. Some states had no TDS limits for pools. Some had limits ranging up to 3000 PPM TDS. He had also noticed that some ANSI/APSP standards gave TDS limits of 1500 PPM above start up TDS levels. He wanted to know the basis for TDS limits, so that he could make a more informed recommendation on changes to his own state code. The following is a summary of our response to him.

As pool or spa water is used, bather waste and other contaminants accumulate in the water. (These contaminants come from sweat, dead skin cells, decaying leaves and bugs, and other organic matter.) Not all of this waste is readily oxidized by chlorine, bromine, peroxide, or other common oxidizers. These wastes can serve as food for microbes. The wastes also contribute to poor water clarity, bad odors, and irritation. Dissolved ionic solids can also contribute to corrosion. But these are less important considerations than the public health considerations.

Water treatment experts wanted a way to measure these waste contaminants in the water. Total dissolved and dispersed solids would be a reasonable indicator of such debris in the water. ¹
The sum of total dissolved solids and dispersed solids could be measured by

• weighing a water sample,
• evaporating all of the water from the sample,
• weighing the residue and
• calculating the weight ratio of residue to the weight of the original water sample. ²

The results could be expressed in parts per million (PPM) or in milligrams per liter. However, such a procedure would be too awkward to be practical for field tests in most cases. There would also be plenty of opportunity for error in the measurements or the calculation.

But there are indirect methods for estimating the concentration of total dissolved solids in the water. The most common is to measure the conductivity of the water. The conductivity is roughly proportional to the concentration of ions in the water. ³ It is assumed that the concentration of ions in the water is proportional to the total amount of matter dissolved in the water. This is not strictly true, but the estimate is close enough for most work. So the measurement of conductivity became a practical way of estimating how contaminated pool water had become. (There are other methods and test kits for "measuring" TDS. Because of the speed and convenience, conductivity has become the most common method.)

Thus TDS became a common method for estimating general water quality. It is usually measured with a hand-held conductivity meter with a TDS read-out. It was common to set an upper limit of a few thousand PPM (or milligrams per liter) TDS for pools and spas. Appendix A of various ANSI/APSP standards contained such a TDS limit. ⁴ When the TDS limit was exceeded water would be drained and replaced so that the contaminants would be diluted. The standard did not apply well to salt water pools, however.

Around 1998 the RWQ (formerly the Chemical Treatment and Process Committee) was revising standards. Members of the committee were concerned that the fixed TDS limit of 3000 PPM was inappropriate for pools using electrolytic chlorine generators. In-line or in-pool chlorine generators require at least a few thousand PPM of salt in the water to function well. Thus we changed the TDS limit from a fixed limit of 3000 to a relative limit of 1500 PPM above start-up level. (The startup TDS level is measured on the initial fill after adding the salt for the chlorine generator.) This would allow for the addition of intended dissolved solids—sodium chloride salt—in the water. But it would draw attention to increases in TDS from the gradual accumulation of impurities (such as from bather waste) in the water. The old wording was in ANSI/APSP standards through part of 1999. When ANSI/NSPI-4 1999 was issued, it had the new TDS wording in Appendix A, Section B. 3. It gave no minimum or ideal TDS. It did provide a maximum of 1500 greater than pool start up and gave reasons for concern over a substantial increase in TDS over start-up conditions.

In summary, increases in TDS can be an indicator of a decline in water quality. This led to the establishment of limits for TDS. As a concession to salt water pools APSP and others changed from fixed TDS limits to relative increases in TDS above start-up.

Stan Pickens is an inorganic chemist and a Senior Research Associate in the Calcium Hypochlorite Business Group of PPG Industries, Inc. He has served as a member of the Recreational Water Quality Committee (formerly the Chemical Treatment and Process Committee) of APSP since 1997. Stan is one of many experts who advise APSP on a variety of technical issues related to standards and educational materials for recreational water.

1 See introduction to standard method 2540A, A.D. Eaton et al., "Standard Methods for the Examination of Water and Wastewater",    19th edition, Washington, DC: American Public Health Association, 1995. (The original edition was copyrighted in 1917.)
2 See for example, standard method #2540B in "Standard Methods", above.
3 Ibid, Standard method 1030F.  
4. Appendix A, "Chemical Operational Parameters", section B. 3. of ANSI/NSPI-1 1991 "Standard for Public Swimming Pools", for instance, gave TDS limits of 300 PPM minimum, 1000–2000 ideal and 3000 maximum. The comments section indicated reasons for concern with high TDS levels in pools.   

© 2007 by The Association of Pool & Spa Professionals. All rights reserved. No part of this publication may be reproduced or transmitted without written permission.