Watering Systems

There are many things to consider when using chlorinated water for laboratory animals.

It’s no secret that contaminated drinking water can threaten the health of your laboratory animals and the validity of your research. To maintain clean, microbiological-free water, facilities commonly use a residual disinfectant such as chlorine in their automated watering systems to kill harmful bacteria potentially found in the drinking water.¹ Chlorine also helps minimize the growth of biofilm in the watering system. But how much chlorine is effective? Is there such a thing as too much chlorine? How can you ensure the watering system is maintaining the correct level of chlorine?

Maintaining the proper amount of chlorine in your system is important. If there is not enough chlorine available, bacteria in the water will not be destroyed. On the other hand, if there is too much chlorine present, it can damage the watering system by causing corrosion of stainless steel drinking valves, manifolds, and room distribution piping. Although over chlorination has not been shown to cause any real health effects in animals in concentrations well in excess of 250 parts per million (ppm),² animals may have an aversion to drinking water with a high chlorine concentration due to the foul smell and taste. This usually begins to become an issue as the chlorine concentration reaches approximately 50 ppm.

Chlorination byproducts, on the other hand, can have adverse health effects in animals. These chemicals result when chlorine reacts with organic substances in the water. Trihalomethanes (THMs) are one class of disinfection byproducts found in nearly every chlorinated public water supply, to some extent. The most prevalent is chloroform (trichloromethane), a THM which is carcinogenic to rats and mice.² It is important to note that chlorination byproducts are likely already present in the municipal tap water supplied to an animal facility. Chlorination byproducts can be removed by activated carbon adsorption. This process also helps eliminate the formation of byproducts downstream by removing the organic material that reacts with chlorine to form byproducts such as THMs.

Reverse osmosis (RO) will further enhance the quality of the animal drinking water by removing greater than 99% of most contaminants. A chlorinated watering system using reverse osmosis with carbon filtration pre-treatment will provide a better level of protection for animal health and better control over experimental variables than a non-chlorinated watering system.¹

Types of Chlorine
The three terms commonly used to describe chlorine are free, total, and combined. Free chlorine is the sum of the concentrations of dissolved chlorine gas (Cl2), hypochlorous acid (HOCl), and hypochlorite ion (OCl-).

The three forms of free chlorine exist together in equilibrium. Their relative proportions are determined by the pH value and temperature of the water. Because of its small size (low molecular weight) and the absence of an electrical charge, hypochlorous acid requires little effort to penetrate cell walls, making it approximately one hundred times more powerful of an oxidant and disinfectant than hypochlorite ion. Consequently, free chlorine is most effective at pH 4.0 to 7.0 where HOCl is the predominant form. The effectiveness declines as the pH moves out of this range.³