Polyhexamethylene biguanide
Polyhexamethylene biguanide (also known as PHMB or biguanide) is a non-chlorine, non-bromine sanitizer. Since it is not an oxidizer, a separate oxidizer must be added to eliminate contaminants that are not removed by the filter. Hydrogen peroxide (H2O2) is typically used in these systems.
Polyhexamethylene biguanide maintains its activity over a very broad pH range (much broader than pool and spa/hot tub operating pH values). Its activity is also not affected by sunlight, bather waste such as urea, or by stabilizers such as cyanuric acid. However, it is incompatible with some common pool chemicals, including:
- Metal ions (e.g. copper algaecides, ionizers, metal cartridge systems, etc.);
- Phosphates and phosphonates (e.g. many chemicals that are used to prevent staining and scaling in pools and spas/hot tubs); and
- Chlorine and bromine (e.g. chlorine shocks should never be used in polyhexamethylene biguanide pools).
Metal systems
Some metal systems are registered as sanitizers by the EPA and some are not, so care must be used in choosing a metal sanitizing system. Most systems are based on the use of silver (Ag) and copper (Cu). The efficacy of these metals is extremely dependent on water parameters, which can cause them to precipitate into insoluble forms that are not effective and can cause stains. For instance, both can be made insoluble at high pH. Silver also becomes insoluble in the presence of chloride (i.e. what is left after active chlorine does its job), while both can precipitate with phosphate (a breakdown product of phosphonate-based metal control agents). Another factor to remember is the kill rate of metals is much slower than chlorine or bromine, as shown in Figure 3 below. This is why metal systems typically require the presence of at least 0.5 ppm of chlorine.
No matter which sanitizing system is used, it is important to follow the label directions for that particular product.
Figure 3: CT values |
||
|---|---|---|
| Organism | Chlorine, typical CT* values, ppm min | Silver CT values, ppm min |
| Bacteria | 0.02-1 | E. coli 2-45 Legionella 115 |
| Virus | 0.5-30 | >5000 |
| * being the disinfectant concentration multiplied by the contact time | ||
Summing things up
The two most important ways to keep a pool and/or spa/hot tub clean and healthy are to minimize contamination by encouraging proper bather hygiene and to maintain a residual of a PMRA/EPA-registered sanitizer in all parts of the pool and/or spa/hot tub at all times.
Ellen Meyer is technical service manager with Lonza, a global supplier of pool and spa/hot tub chemicals. She has chaired the recreational water quality committee of Association of Pool & Spa Professionals (APSP), and currently serves on the technical review committee of the Model Aquatic Health Code (MAHC), and is a member of the NSF International task group for pool chemical evaluation. She can be reached via e-mail at ellen.meyer@lonza.com.
