Disinfection by Chemical and Mechanical Means


The disinfection of water by chemical or mechanical means that began around the turn of the 20th century has resulted in the end of epidemics like cholera, dysentery, and typhoid fever. These techniques neutralize the waterborne microscopic pathogens that can cause often-fatal diseases. Countless lives have been saved.

What these forms of purification do not do is to remove all contaminants or even, necessarily, kill all microorganisms. They are ineffective against inorganic chemicals like pesticides or heavy metals, both of which can cause long-term health problems. Still, the widespread use of disinfectants essentially ended the fear of a quick death from drinking water, something of obvious importance and great value for society.

The three main forms of drinking water disinfectants are chlorination, ozonation and ultraviolet, or UV, light. The first two are chemical; the third is mechanical. All three have advantages and disadvantages, and, when it can be done, a combination of these techniques yields the safest drinking water possible.

Chlorination has been in use in the United States since around the turn of the 20th century. Chlorine or a chlorine derivative (such as chloramine or chlorine dioxide) is added in very small quantities to drinking water. The chlorine mixes rapidly with the water and creates a reaction which destroys the structure of any bacteria and viruses found there.

Drinking water utilities use chlorination for many reasons. It is highly effective with consistent outcomes, inexpensive, relatively easy to use (so costly staff isn't needed), does not require the construction or maintenance of complex infrastructure, and chlorine is widely available. The chlorine also stays in the drinking water for at least several hours after it is added, and the purification process continues long after first application offering an additional layer of safety.

But there are negatives, too. Aside from legitimate concerns about operator safety in transporting or handling the chlorine, the largest worry is that when chlorine mixes with organic matter in the drinking water supply (like bits of dead leaves), disinfection byproducts (DBPs) are created. Several of these DBPs have

been linked to cancer, especially bladder cancer, but also interference with the proper functioning of the liver, kidneys, central nervous system, and/or reproductive systems. As a result, utilities making use of chlorine need to balance the risks of pathogenic microorganisms that must be eliminated against the potential longer-term risks to public health. Drinking water disinfection must stop what can kill people fast, but that disinfection shouldn't become a public health menace either.

Much less frequently used techniques are ozonation and the use of ultra-violet light. Neither of these more recently adopted methods are deployed by even 10 percent of utilities. Many utilities are satisfied with the outcomes provided by chlorination and see no need to replace it or to augment it as a disinfectant tool.

Ozonation mixes ozone – a toxic gas – with the drinking water to disinfect it in a way similar to chlorination. As one benefit over chlorination, ozonation is also effective in water of widely differing temperatures and acidity levels. Thus, constant modifications of chlorine levels are not required. Ozonation also helps to improve the taste and smell of drinking water making it more appealing.

As with chlorination, there are negatives to ozonation. The ozone used is created at the point of use via an energy-intensive process. That makes energy costs high and may add to use of carbon fuels. It isn't effective against inorganic chemicals or heavy metals and some microorganisms. Aside from the expense of the creation of the ozone, ozonation also requires greater budget resources to build, buy and maintain the equipment that produces the ozone gas. Ozone gas dissipates very rapidly and there is no ongoing disinfection value as with chlorination. Most troubling of all, when ozone mixes with bromide found in some drinking water, it can produce bromite. Bromite has been found to be a carcinogen in laboratory animals.

In ultra-violet (UV) disinfection, light wavelengths are absorbed by the DNA in microorganisms, rendering them unable to reproduce. While still alive, the pathogens pose no threat as disease results when the microorganisms in the human host begin to rapidly reproduce. In addition, while chlorination and ozonation aren't effective against Cryptosporidium, Giardia, and other protozoa, UV is.

UV pulses are applied to drinking water as it flows through a UV reactor. Because this is a mechanical process requiring no chemicals, there are no disinfection byproducts potentially causing health concerns or residual chemicals left in the water. In addition, once the UV equipment is installed, it is easy to operate the necessary equipment.

Negatives of UV are few other than the energy cost, the expense of installation, and the risk of the system going offline for even a few moments in the event of a power surge. Most concerning is that there is no residual disinfection from the use of UV. This has led many of the utilities that use UV to follow the light treatment by adding in small amounts of chlorine after UV treatment to keep the water free of microorganisms for as long as possible.

Seth Siegel