Volume 45 Number 5
HPC Bacteria Issues and Their Effect on the POU Industry: Analysis
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
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
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
Stay tuned for future developments.
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