Clearing up a cloudy issue concerning pool water

Chlorine: Upside/downside

Significant, but almost lost in the high visibility of this report, was the detection in some samples of parasite cyst fingerprints. In these instances, even the best chlorine sanitizing system is not capable of inactivating Cryptosporidium (Crypto) oocysts. These samples are more likely to have a human feces origin in the pool water, and almost certainly would still be alive and infectious in the filter. Of course, filtering them out and regularly backwashing into the septic outflow is another component of ongoing effective pool management; however, in these cases, there is a higher risk to bathers and there is much less certainty with respect to the filter medium’s ability to capture and eventually remove oocysts as these microbes measure only a few microns (µm), and, therefore, are able to pass through the pore tunnels of commonly used filter media beds.

These microbes are a real RWI hazard and one with explosive potential as they are highly infectious. In serious cases, a large pool with enough chlorine-resistant cysts is likely to ensure a mouthful of water would contain an infectious dose. With this in mind, there is good reason to take the presence of Crypto in backwash samples, even at a low rate (the CDC study found Crypto oocysts in less than two per cent of the samples) as a serious indicator of potential problems.

This risk is not readily diminished by adopting sanitizing alternatives to chlorine. For all its downsides, including objectionable smell, taste, eye and skin irritation, and disinfection byproduct generation, amongst others, chlorine serves up an unmatched array of advantages in maintaining safe pool water, and is deserving of its centrepiece role in recreational water sanitation. The fact chlorine does not cope with the toughness of Crypto oocysts, in addition to some other shortcomings, has led the industry to look for alternatives. However, this has not proven easy, and while practical alternatives are available, they bring their own weaknesses to the challenge. Chemical agents with oocyst-inactivating power such as chlorine dioxide (ClO₂) and ozone (O₃) pose more operational demands on users than chlorine, and cannot compare in their inability to sustain biocidal activity in the main body of pool water that is readily accomplished with chlorine residuals.

Ultraviolet (UV) irradiation can destroy the infectivity of oocysts; however, it too does not offer any residual protection to the recirculating water and it must operate via line-of-sight exposure to the inactivating rays. Any obstruction in the form of turbidity in pools with heavy bather loads can undermine the advantages of UV. Further, its inactivating power must wait upon the arrival of germs over the course of the pool’s turnover; in the meantime, they are free to wreak havoc.

Clearer is safer

This leads to the final piece of the framework for this perspective on the CDC report—water clarity as a contributor to pool water safety, regardless of the method of disinfection being deployed.

Turbidity is the enemy of effective water safety management and efficient filtration is the way to counter it. Turbidity interferes with UV, consumes disinfectants (i.e. its ‘demand’ for halogen [e.g. chlorine, bromine (Br), etc.], meaning less is available for sanitation), and it contributes to the risk of drowning by obscuring the view of bathers who get into trouble and sink out of sight.

Filter backwashes should contain evidence for what the medium is removing from the water—just as CDC researchers found—but helping that process by flocculating suspended particles is a worthy goal, and it can be achieved as a part of regular pool management. By using a natural biopolymer formulation, specifically aimed at improving filtration efficiency, Crypto oocysts and other microbes become enmeshed in large floccs (shown in Figure 1). This allows pool filters to more readily trap these microbes, reducing the chance they remain in the recirculating body of water and cause RWI outbreaks. With this method of treatment, microbes as small as 2 µm long and 0.5 µm in diameter can be aggregated into clumps (shown in Figure 2), which end up being trapped by, in this case, a flat filter membrane with 5 µm pores; without treatment, these microbes simply pass through the filter. Then, proper pool sanitation can be counted on to inactivate most other disease-causing agents.

This treatment is also effective at trapping other waterborne microbes (e.g. Giardia, E. coli), in addition to algae. For large pools, polymer treatments can be accomplished via controlled metering, while manual treatments, at properly timed intervals, are also practical for smaller scale operations. The polymers used in this treatment are biodegradable and safe for bather exposure, and the concentrations required are extremely low (typically in the range of parts per billion [ppb]).