By Peter Cartwright, P.E., CWS-VI
As so eloquently described in Dr. Kelly Reynolds’ March 2003 “On Tap” column, it appears as though the latest scientific conclusions regarding the health effects of high concentrations of heterotrophic plate count (HPC) bacteria in drinking water are, regardless of the concentration, these bacteria are unlikely to affect human health.
There even appears to be a consensus among experts that high concentrations of HPC bacteria will inhibit the growth of pathogenic bacteria, although this may not be the case with pathogenic viruses. So, expanding on this school of thought, it’s possible to conclude that from a microbiological perspective, it may be safer to actually encourage the growth of HPC bacteria in drinking water supplies. Does this mean the end of disinfectant technologies such as ultraviolet (UV) irradiation and ozonation? That’s questionable.
First, some background, though.
Behind the studies
Arguably, the leading microbiologist in the point-of-use/point-of-entry (POU/POE) segment of the water treatment industry, University of Arizona professor Charles Gerba, is currently investigating these issues. (Gerba presented his most recent findings at the WQA Las Vegas convention in March 2003). He has indicated that the inhibitory effect of HPC bacteria on pathogenic bacteria is so significant that HPC bacteria proliferation shouldn’t be discouraged. Obviously, for semiconductor rinsing, pharmaceutical manufacturing, hemodialysis and other applications requiring microorganism-free water, this isn’t an option—but, for most drinking water uses and many other applications, it is.
This revelation also has implications regarding membrane filtration, namely the phenomenon of bacteria “grow-through.” Bacteria are viable (alive and constantly multiplying) and will grow in virtually any environment. As such, they apparently will grow through any membrane, regardless of its pore size. This phenomenon is controversial and the exact mechanism isn’t fully understood. The evidence, however, is very clear: if water sits stagnant on the feed side of a reverse osmosis (RO) membrane—or any membrane for that matter—for as short a time as several hours, HPC bacteria will appear on the permeate side.
The bottom line is that the storage tanks of virtually all residential RO systems are full of HPC bacteria in much higher concentrations than in the raw water supplies. There are exceptions of course. CTA (cellulose triacetate) membranes that allow some of the municipal chlorine to pass into the permeate stream and those units equipped with UV lights. Still, neither approach will produce completely bacteria-free water and, given the environmental conditions of these storage tanks, some bacteria species will replicate as frequently as every 20 minutes! Of course, this revelation indicates that the high concentrations of HPC bacteria may actually make this stored water safer and should be encouraged (or, at least, not discouraged), as long as there’s no aesthetic effect on the water.
Now, regarding the other types of common waterborne pathogens, viruses and protozoan cysts, even a sub micron-rated filter should remove the cysts, and RO membranes have sufficiently small pore sizes to prevent the passage of viruses. Many of these pathogens require a human host to multiply, so once they’re removed from the water supply, short of recontamination, the treated water should be relatively free of them.
Open-ended questions remain
So, back to our question regarding the future of POU disinfection technologies. While it is certainly too early to definitively answer that question, it does bring other questions to mind:
- Are there only certain kinds of HPC bacteria that inhibit pathogenic bacteria growth?
- What concentrations of HPC bacteria are required to affect this inhibition activity?
- Are there certain water conditions/parameters that inhibit HPC bacteria growth?
- What about the biofilm formed by these high concentrations of HPC bacteria?
- Will biofilms protect pathogenic organisms from the inhibiting effect of HPC bacteria?
- Do these conclusions include the immunocompromised population?
- What about the taste, odor and other aesthetic effects of high concentrations of HPC bacteria?
In addition to these technical questions, we must consider the most important factor of all—consumer perception. In spite of all the scientific evidence, the concept of consumers knowingly drinking water laden with bacteria, benign or not, may not be a realistic expectation at this time.
What about municipally chlorinated water supplies? After all, chlorine has an outstanding record of inactivating bacterial and viral pathogens. On the other hand, the levels of chlorine used in municipal treatment are basically ineffective against Cryptosporidium parvum—as well as many species of HPC bacteria—and to address this problem, the U.S. Environmental Protection Agency (USEPA) has promulgated the “multiple barrier” approach to protect surface water supplies with strong evidence that the same treatment scenario will be required for many groundwater supplies in the future.
Too early to test the wind
So, where am I going with all of this? Well, it appears to me that the revelation that HPC bacteria are non-pathogenic and will likely not require regulation is very interesting. Yet, beyond this, I don’t see much of an effect on the POU/POE industry. We still have to address the other kinds of pathogenic microorganisms. Plus, don’t forget what is arguably the most important issue of all—how consumers would react to the idea of consuming bacteria-laden drinking water. For example, remember the reaction to San Diego’s ill-fated “Toilet to the Tap” water recycling program. The “Yuck!” factor in public perception should not be taken for granted.
So, what’s next? Here are few things to consider:
- Certainly, more scientific data are required, but will these change our conclusions? Perhaps.
- Will the final conclusions have an effect on the POU/POE industry? Most likely.
- Should the industry do anything now? I don’t think so.
Stay tuned for future developments.
About the author
Peter S. Cartwright, president of Cartwright Consulting Co., Minneapolis, is a registered professional engineer in several states. He has been in the water treatment industry since 1974 and has published more than 100 papers and articles on related issues. Cartwright has been chairman of several WQA committees and task forces has received the organization’s Award of Merit. A member of the WC&P Technical Review Committee since 1996, his expertise includes such high technology separation processes as reverse osmosis, ultrafiltration, microfiltration, electrodialysis, deionization, carbon adsorption, ozonation and distillation. He can be reached at (952) 854-4911, (952) 854-6964 (fax) or email: CartwrightConsul@cs.com.