EcoloxTech, Norwegian Cruise Lines and AquaMetrix
We make tough probes for tough applications—like wastewater treatment, chemical plating, and pulp-and-paper. But every now and then one of our customers puts our probes in some place unique. This includes Yellowstone National Park, Antarctica, Kwajalein Islands and Orlando water parks. But Norwegian Cruise Lines is a new one.
First, let’s start with a primer on that chemical that comes in several formulations used for disinfection but that is colloquially known as “chlorine.” Since Covid heightened the world’s proclivity for sanitizing interiors and sodium Hypochlorite, NaOCl, aka “bleach”, has become the standard disinfectant. NaOCl dissolves in water to make the hypochlorite anion, OCl–, and cation, Na+. The OCl– is the disinfecting species. For commercial use it comes in concentration solutions that must be diluted. Add acid and the H+ ions convert hypochlorite ions to hypochlorous acid, HOCl. HOCl is about a hundred times more effective at killing most pathogens so it is a better disinfectant. Most water treatment plants still use HOCl for disinfection and most do it by passing very deadly chlorine gas through water:
Cl2 + H2O –> HOCl + HCl
HOCl remains the most prevalent disinfectant today but there is a safer and more consistent method for making it: Dissolve salt (NaCl) in water and pass electricity through the solution generates chlorine and hydrogen gas:
2 NaCl + 2 H2O –> 2 NaOH + 2 H2 + 2 Cl2
The chlorine gas dissolves in water just as it does in most treatment plants according to the prior reaction, to create HOCl.
The difference, of course, is that the chlorine gas is made on the spot rather than trucked in and stored. Note also that the production of hydrochloric acid, HCl, along with HOCl, ensures that the pH of the solution is acidic. This guarantees that HOCl concentration exceeds the OCl– concentration.
Eliminating the need to transport and store deadly chlorine does not eliminate the need to ensure that the chlorine gas that is generated onsite makes HOCl and does not escape. If that were to happen on a cruise ship the results could be catastrophic.
That’s where good engineering comes in and why EcoloxTech’s onsite chlorine generator, has thrived. Their system is an elegant, simple-to-use cabinet system that produces what they call “electrolyzed oxidizing water.” The company claims a 3-log (99.9%) reduction in most microbial species, including E. Coli, salmonella, listeria, SARS-COV-2, and the scourge of cruise ships—norovirus. You can read about it on NCL’s website: https://www.ncl.com/travel-blog/new-sanitizing-technology-on-norwegian-cruise-line-ships.
What makes the EcoloxTech machine so effective? The answer to any tried-and-true technology is always the same—superior engineering. Most onsite chlorine generators use a Na+-permeable membrane to separate the oxidation and reduction chambers. The resulting system is the size of a refrigerator. The EcoloxTech uses a single chamber so that its machine is a fraction of the size of competing systems—24”x8”x17”. The consistency of the process enables the output to vary between 50 ppm—the standard concentration for food safety—and 200 ppm—the highest concentration for killing Covid-19 viruses in seconds. Twenty of these small machines service an entire cruise ship.
No matter how you make HOCl, you need to ascertain that you are making it in the correct concentration. The obvious approach is to measure HOCl concentration directly—either through a colorimetric (DPD) test kit or an amperometric probe. The former does not provide real time measurements and requires the use of expensive DPD reagent. The latter requires an expensive probe that is highly pH dependent and requires a constant flow across the front end, which means side stream plumbing with flow regulation. Fortunately, the EcoloxTech chemistry is so consistent that a surrogate measurement can be made.
Since HOCl is a strong oxidizing agent its concentration correlates with ORP. An ORP probe is inexpensive, easy to install, robust and sensitive. For monitoring disinfecting solutions that are based on a single oxidizing agent they are a good substitute for the so-called free chlorine measurement. ORP measurements can even be superior for measuring very low concentration of oxidants but the flip side of this is that they are very insensitive to changes of very high concentrations of oxidants like HOCl. Their sweet spot is the range of 0 to 10 ppm typical of water treatment. But the operating range of 50 to 200 ppm used for industrial hygeine renders the ORP probe a bad fit.
The EPA is very clear on reporting requirements: Either a colorimetric DPD test or an amperometric probe must be used to report free chlorine measurements. There’s no getting around that but, fortunately, one needs only to take measurements intermittently. For cruise ships that’s every four hours. For this purpose, test strips, like the ones used for swimming pools (albeit for higher concentration ranges), suffice. For continuous measurement EcoloxTech resorts to a characteristic of chlorine solutions), which is that they are a mixture of HOCl and OCl–, The sum of both is called “free chlorine” (to distinguish it from combined chlorine, which is mostly chloramine, NH3Cl). The fraction of free chlorine that is the more efficacious HOCl is highly dependent on pH. As Figure 2 shows, at pH 8.5, 90% of free chlorine is OCl– is and, at pH 6.5, 90% of free chlorine is HOCl. The EcoloxTech machine uses a pH probe to ensure that the pH stays below 5.5, for which 99% of free chlorine is HOCl.
To ensure that the EcoloxTech system is operating at peak efficiency a pH probe is inserted into the reaction chamber. If the pH drifts higher than 5.5 more brine (NaCl) is injected into the generator. If the pH falls below 5.5 the brine supply slows down.The solution is simple enough but, finding a probe that withstands the harsh chemistry of onsite chlorine generation is not so simple. Morton Larson, owner of EcoloxTech, tried several different industrial-strength probes but none lasted more than a few weeks. One probe from a premium manufacturer in Europe corroded quickly in the highly oxidative environment. Morton tried the flagship differential pH probe from AquaMetrix, the P65R5, and the probe went the distance. This model probe outputs a 4-20 mA signal, which goes to a PLC that controls the injection of NaCl brine into the system. It’s differential design means that it is robust even in corrosive environments and can be rejuvenated when the reference solution inevitably becomes contaminated with the process. It also resists stray voltages that are common in electrically charged environments and that change the output of traditional combination probes.
To find out more about the EcoloxTech disinfecting system go to https://www.ecoloxtech.com/industries/electrolyzedwater/.
To find out more about our differential probes go to https://www.wateranalytics.net/home/products-2/ph-orp-2/
To check out the EcoloxTech system in person book a cruise at www.ncl.co.