PHMB (polyhexamethylene biguanide) is the active ingredient in both Softswim, a trademark of BioLab, and Baquacil, a trademark of Zeneca. Originally, developed as a hospital sanitizer, PHMB is considered an effective pool and spa sanitizer, although questions remain unanswered regarding its efficacy against certain pathogens, particularly giardia and cryptosporidium. However, chlorine is not very good at killing these, and they are uncommon in residential pools in any case. More significantly, unlike chlorine, PHMB does not seem to be effective against viruses.
Given these weaknesses alone, I'm not enthusiastic about PHMB. I rarely recommend PHMB and am providing this guide primarily as a means of responding to the many questions I receive about PHMB pool sanitation.
Pool owners should understand that, whatever else is true about PHMB systems, it is also true that dealer and manufacturer gross profit margins on PHMB are much higher than on conventional chlorine chemicals. You may judge for yourself how this is likely to effect a dealer's enthusiasm for the products. It is not disputed by BioLab or Zeneca that treating pools with PHMB is at least 200% more expensive than chlorine. My own limited experience suggests that this may be conservative.
PHMB does have one outstanding advantage over chlorine: because it is not an oxidizer, and is not significantly affected by sunlight, PHMB residuals are much more stable in both pools and spas than chlorine.
Another advantage of PHMB is that neither it, nor the other required chemicals (shock and algicide) are irritating to eyes and skin, at the levels that are maintained in pools. By contrast, badly managed chlorine pools can be very irritating and smelly. I'm not sure whether a well managed chlorine pool is more irritating than PHMB. My guess that is that there is some difference, but not much.
However, judging by the email questions I receive, many poolowners do not initially understand that PHMB alone does not replace chlorine. Chlorine is a sanitizer, as is PHMB. However, chlorine is also an oxidizer and an algicide. Both of these functions are essential in pools. PHMB is neither. In other words, neither Softswim(tm) nor Baquacil(tm) replace chlorine. Rather, the Softswim(tm) and Baquacil(tm) systems, which involve multiple chemicals besides PHMB, replace chlorine.
In pool treatment systems utilizing PHMB, the oxidizer and algicidal functions must be replaced by other chemicals. Normally, 27% hydrogen peroxide - a hazardous chemical that is extremely irritating on skin contact - is used to provide oxidation; a blend of foaming linear quatenary ammonia compounds are used to provide algae control. Neither of these chemicals offer the level of stability that PHMB does, and hydrogen peroxide is destroyed by sunlight. (That's why the 3% drugstore solutions come in dark brown bottles.)
Thus, the apparent benefits of PHMB pool sanitation are somewhat negated by the necessity of using these additional chemicals. Also, because both PHMB and the linear quats share the problem of foaming, pool water which has been freshly dosed with both, will tend to be somewhat foamy. Apparently, poolowners, wishing to avoid the foam, sometimes underdose their pools, resulting in problems.
Perhaps due to underdosing, such as just described, some PHMB treated pools acquire 'infections' with 'pink' algae or 'water mold'. Apparently, once this problem arises, it is very difficult to remedy. I was astounded to learn that BioLab occasionally recommends doses of chlorine to help solve the problem, since chlorine destroys PHMB
Although PHMB is not affected by sunlight, it is broken down by many other chemicals . . . and when it breaks down,it makes a sticky, sometimes yellow, mess. PHMB is incompatible with most other pool chemicals (a great benefit to PHMB dealers!).
Perhaps the worst problems arise from PHMB's incompatibility with metals. Low levels of copper and iron are commonly present in homeowner tap and well water. While these metals can cause problems for all poolowners, they are disastrous for pools using PHMB, since PHMB is catalytically degraded by their presence, even in minute quantities. Extreme care must be used when adding water to a PHMB treated pool. Failure to do so is apparently a common source of problems.
For this reason, and for others, owners of PHMB treated swimming pools should plan for a much higher level of filter maintenance than owners of chlorine treated pools. Although Zeneca disputes this, it is a common opinion among pool service people that both cartridges and sand in filters on PHMB pools must be replace annually. Horror stories of the condition of filter media on PHMB pools are also a frequent topic of conversation.
My general opinion is that PHMB is not better than chlorine, only different, and more expensive. There are a few, rare, circumstances in which it would be my treatement of choice. However, avoiding the problems that can arise with chlorine sanitation, is much easier, in my opinion, than avoiding those that come with PHMB.
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