09.012019

Tobago’s Reverse Osmosis Desalination plants with effective disinfection of Legionella bacteria



In 1976 in the Bellevue-Stratford Hotel (Philadelphia, Pennsylvania) the annual convention for war veterans of the American Legion was held. During next week after the end of the convention 149 Legionnaries and 33 other people, associated with the hotel, became sick with similar symptoms, 29 of them had died. Just after 6 months the reason of this tragedy – bacteria from the cooling tower of the hotel’s air conditioning system was finally identified. Because of the American Legion convention this bacterium got its name Legionella.


These bacteria cause Legionnaires’ diseases such as pneumonic Legionellosis and Pontiac fever (second one is less dangerous and nonfatal). Legionellosisis a common form of severe pneumonia that can be fatal if it is not promptly and correctly diagnosed and treated.


Legionella may be free-living or living within biofilms that protect it from most chemical disinfectants and all disinfection technologies that do not use chemicals.

Most people become infected with Legionellosis by inhaling or aspirating aerosolized microscopic water droplets contaminated with Legionella. It cannot be spread from person to person or got from contaminated water that enters stomach in the normal way. Older adults (over 50 years), who have smoking and drinking habits, and people with weakened immune systems are those who especially vulnerable to disease. About 25000 cases occur each year and cause more than 4000 deaths in U.S.


The sources of the bacteria are water systems, such as cooling towers, swimming pools, hot water systems and showers, fountains and industrial coolant. Conditions that favor the growth of Legionella are water temperatures between 20 and 50 °C and the presence of sources of nutriment of the bacteria. But it also can survive at temperatures below 20 °C.

Ideal growth conditions are in warm water between 35 and 46 °C.  High relative humidity, typical for Italian region, increases the viability of Legionella species in contaminated aerosols. Due to its wide distribution and the impact on the human health many countries have issued recommendations and guidelines regarding the problem.

Many different physical and chemical disinfection methods including thermal treatment, oxidizing agents (such as chlorine, chlorine dioxide, monochloramine and ozone), copper/silver ionization (CSI), ultraviolet light (UV) to control Legionella, especially in man-made water systems, have been applied. Some of these treatments kill bacteria as consequence of irreversible damages to cellular structures or functions vital for the bacteria (bactericidal effect), while others only inhibit the growth of bacteria without killing them (bacteriostatic effect).


Disinfectants can be applied at high doses for shorter time (shock treatment) or at low concentrations for a long time (continuous treatment). Disinfectants used for drinking water must be safe for human consumption in terms of both their chemical nature and concentration.

There is a lack of available information on many disinfectants regarding to effective doses and exposure times, action mechanisms, secondary effects, resistances, activity in biofilms, etc. Bacteria exposed to bacteriostatic disinfectants or to an insufficient concentration of bactericidal products can remain alive and retain the potential to become resistant or promulgate any resistance selected by the exposure to the disinfectant.

Several oxidizing agents are widely and successfully used to disinfect potable water, cooling water, swimming pools and other water systems. Among these oxidizing agents, chlorine is the best-known and the most effective disinfectant against Legionella. Chlorine can be added to water using chlorine gas or hypochlorite salts (i.e., sodium or calcium hypochlorite). It is applied in shock or continuous treatments, following the recommendations of different guides.


In water chlorine exists as hypochlorous acid. Shock hyperchlorination (periodic chlorination at an elevated chlorine dosage, 20 – 50 mg/L) can be applied for cleaning purposes followed by replacing the water in the system after 1 – 2 h with fresh water and maintaining around 1 mg/L of chlorine concentration in the water. There are three main issues associated with chlorination: long-term effectiveness, corrosiveness, chlorination by-products and chlorine toxicity. Chlorination can suppress but rarely definitively eliminate Legionella.


The corrosiveness of chlorine must be considered with respect to the pipes and materials of construction used in the chlorinated water systems. Due to concerns about harmful disinfection by-products (DBPs) associated with chlorine (e.g., trihalomethanes such as chloroform), other disinfectants (e.g., other chlorine compounds with less harmful disinfection by-products, ozone) and/or lower chlorine dosages are used for treating potable water.


A determination of best suited technology for a particular water system is important in part due to the complex and diverse nature of different water systems. Typical uses of the disinfectants that may be applied in water treatment, their advantages and disadvantages must be accurately evaluated.


Here in Tobago the Reverse Osmosis Desalination plant can be combined with electrochemical system, which allows to produce hypochlorous acid solution by the electrolysis of brine (sea water). This colorless solution has no harmful chemical by-products, as do biocidal agents, and is simple to use for disinfection and control of Legionella bacteria.




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