The Basics

Disinfection

Our natural environment contains numerous microorganisms. Most of these present no concerns. However, some—such as Giardia lamblia and various viruses, which can be present in water supplies—are extremely harmful and can cause disease in humans. These disease-causing organisms are known as pathogens.

Disinfection of drinking water is one of the major public health advances in the 20th century. One hundred years ago, typhoid and cholera epidemics were common throughout many cities. Disinfection was a major factor in reducing these epidemics, and it is an essential part of drinking water treatment today.

The primary method used for the disinfection of water in treatment systems is chlorination. Chlorine can be applied for the deactivation of most microorganisms and it is relatively cheap. There are numerous alternative disinfection processes that have been less widely used in small and very small water treatment systems, including ozonen ultraviolet irradiation (UV), chlorine dioxide, potassium permanganate, chloramines and peroxone (ozone/hydrogen peroxide).

As an individual pathogenic organism can be difficult to detect in a large volume of water or wastewater, disinfection efficacy is most often measured using "indicator organisms" that coexist in high quantities where pathogens are present. The most common indicator organism used in the evaluation of drinking water is Total Coliform (TC), unless there is a reason to focus on a specific pathogen.

However, the disinfectants themselves can react with naturally-occurring materials in the water to form unintended organic and inorganic byproducts which may pose health risks. Over the past ten years, we have also learned that there are specific microbial pathogens, such as Cryptosporidium, that are highly resistant to traditional disinfection practices. In 1993, Cryptosporidium caused 400,000 people in Milwaukee to experience intestinal illness. More than 4,000 were hospitalized, and at least 50 deaths have been attributed to the disease. There have also been cryptosporidiosis outbreaks in Nevada, Oregon, and Georgia over the past several years.

There are two kinds of disinfection: primary disinfection achieves the desired level of microorganism kill or inactivation, while secondary disinfection maintains a disinfectant residual in the finished water that prevents the regrowth of microorganisms.