01 Feb Emergency Water Disinfection
There are essentially two approaches to the problem of emergency water disinfection for Foodservice operations: ultra-violet (UV) irradiation combined with fine-filtration to remove any parasites, and chemical disinfection combined with fine-filtration. Each has its own advantages and disadvantages.
UV and Fine-Filtration: Light energy with wavelengths near 254 nm produces resonance effects in the DNA of living cells which cause breaks and kinks, leading to cell death. Class A systems supply a minimum dosage of 38,000 uWsec/cm2and are effective against bacteria and viruses. Parasites such as Cryptosporidium and Giardia cysts/oocysts are not reliably killed by UV, so fine-filtration to remove them is required.
The Everpure MC and MH filter cartridges, which foodservice operators already buy, are NSF-Certified for cyst reduction. The main advantage of UV is simplicity. It’s all electrical, with no moving parts, and the necessary contact time of about 10seconds does not require much space. The main disadvantages are cost and upkeep. Some of the materials and components are expensive and delicate, and the optical surfaces are subject to fouling. If a UV system is not cleaned regularly during use, and more importantly, after use and before storage until the next emergency, it may become damaged.
Chemical Disinfection and Fine-Filtration: Chemical disinfection requires the use of either high chlorine, ozone or chlorine dioxide concentrations for a short time or low concentrations for along time. It is more cost-effective for small systems to feed high levels of chlorine than to provide large contact tanks such as with “superchlorination – dechlorination,” in which a large excess of chlorine is used for a short time and then removed or reduced to palatable levels. If the dechlorinating filter is also a fine-filter certified to remove cysts and other parasites, complete disinfection can be achieved with modest space and equipment.
A system comprised of a coarse prefilter, chlorine feeder pump, chlorine solution reservoir, 80-gallon retention tank plumbed upflow with at least a two foot depth of pea-sized gravel in the bottom, and a bank of QCi7-MC filters will provide disinfected water at a flow rate of about 5 GPM. Such a system could be mounted on one pallet. The main disadvantage of such systems is that they require rather careful control by the operator—the chlorinator output must be adjusted in accordance with chemical tests for chlorine residual.
EMERGENCY TREATMENT RATIONALE
Everpure, Inc. has been producing disinfection systems for Individual Water Supplies for more than 30 years. They are based on the “Superchlorination-Dechlorination” process plus the CT concept originally developed by Prof. E. Robert Baumann and associates at Iowa State University in the early 1960s (and presently in use for the “Surface Water Treatment Rule.”)
“CT” means “Concentration” (of free chlorine in ppm or mg/L) multiplied by “Time” (in minutes of contact). The current requirement, given in the Guidance Manual to EPA’s Surface Water Treatment Rule, specifies a minimum CT of 6 ppm-min., for 99.99% inactivation of Hepatitis A Virus at pH 6-9 and 10ºC. (Do not confuse this with the overall CT for systems that have to kill cysts with chemical disinfectants. Any parasites will be physically removed by Everpure precoat filters, which are NSF-Certified for Cyst Reduction and also have NSF-Certified filtration ratings of 99.9% removal of all particles ½ micron and larger in size.)
Dr. Baumann’s research, supported by Everpure grants, established that retention tanks require upflow plumbing and either special baffles or a layer of “pea gravel” to facilitate mixing of the injected chlorine solution. Otherwise, the contact time implied by the tank size and flow rate will not be achieved. As it is, use of pea-sized gravel confers a mixing efficiency of only 28%. Thus, an 82-gal. tank with pea-sized gravel will provide 4.6 minutes of contact time at a flow rate of 5 gal/min., and a chlorine residual of at least 3.0 mg/L free chlorine at the outlet of the tank will produce a CT of at least 13.8 ppm-min. That figure provides a substantial safety factor of approximately 100% compared to the CT presently required by the Surface Water Treatment Rule for viruses.
Baumann, E.R. 1961. “Should Small Water Supplies be Superchlorinated?” Water &
Sewage Works, Dec. 1961 (Part 1) and Jan. 1962 (Part 2).
Baumann, E.R. 1 964. “Safe Disinfection for Household Water Systems.” Public Works, May
Baumann, E.R. and D.D. Ludwig. 1962a. “Free Available Chlorine Residuals for Small
Nonpublic Water Supplies.” Journal AWWA, Nov. 1962.
Baumann, E.R. and D.D. Ludwig. 1 962b. “Retention of Water for Disinfection in Chlorinated
Small Water Supplies.” Engineering Report 36, Iowa Engrg. Expt. Sta., Ames, Iowa