The largest part of pathogenic microorganisms is removed from water during the primary water purification steps, flocculation, coagulation and filtration. However, water disinfection is still necessary in order to prevent drinking water from being harmful to our health.
Microorganisms can be found commonly in nature. Invisible to the naked eye, microorganisms are present in soils, air, food and water. Before humans are born, we are free from microorganisms. Through consumption of food and air we are exposed to microorganisms soon after we are born, and they will remain present on and in our bodies.
Most microorganisms are harmless and contribute to a number of vital processes in the human body, such as metabolism and the development of our immune system. But there are also pathogenic microorganisms that can cause disease or can be harmful to people with low resistance to disease.
Pathogenic microorganisms in the water have a number of specific properties that distinguish them from chemical contaminants. They are living organisms. They are not dissolved in water, but they will coagulate or attach to colloids and solids in water.
In drinking water, they can be divided up into three types: bacteria, viruses and parasitic protozoa. Bacteria and viruses can exist in both surface water and groundwater, whereas, parasitic protozoa can be found mainly in surface water.
Bacteria are single-cell organisms, shaped like a sphere, spiral or rod. They can exist as individual bacteria or in bacterial chains, bundles, pairs or biofilms. Bacteria are the most abundant life form on earth. They are between 0.4 and 14 μm in length and about 0.2 to 12 μm in width. Consequentially, they can only be viewed under a microscope.
Bacteria feed on fluid nutrients. They can reproduce by means of DNA replication, causing a bacterium to split into two independent cells, which, in ideal circumstances, can take about 15 to 30 minutes. Some types of bacteria can form spores. These spores contain a protective layer that is heat-resistant and can protect bacteria from a lack of moisture and food. Bacteria play a role in various processes. Some bacteria break down organic matter and play an important ecological role, while others assist in the human metabolism.
Viruses are organisms that can cause infections and can only reproduce in living host cells. When viruses exist outside host cells, they are inactive. They are so small (between 0.02 and 0.09 μm) that they can slip through filters that capture bacteria. Viruses contain only one type of nucleic acid (RNA or DNA). They cannot reproduce, but instead take over the metabolism of the host cell and make sure the DNA is copied in the host cell, causing new viruses to develop.
Contrary to bacteria, viruses are not naturally present in the human body. When people are infected with a virus, it usually leaves the body through secretion of waste. When the secretion takes place, water can be contaminated with viruses. When the water is not thoroughly disinfected, other people can be infected with viruses.
Parasitic protozoa are single-cell organisms. They have a very complex metabolism and feed on solid nutrients, algae and bacteria, which are present in multiple-cell organisms, such as humans and animals. Multiplication takes place through splitting of the cells. Various types of parasitic protozoa are spread in a protected form, such as a cyst or oocyst. Oocysts of Cryptosporidium and cysts of Giardia can be found in waters throughout the world as a consequence of fecal pollution. As cysts, the pathogens are resistant to chlorine disinfection. Parasitic protozoa can be removed by means of filtration or a chlorine dioxide application.
The odds of infection depend upon the type of pathogen, the way in which it is transferred, the infective dose and persistence of the microorganism and the resistance of the person that is infected.
The infective dose refers to the number of microorganisms that need to enter the body before the disease occurs. This dose is very low for viruses and parasitic protozoa. The persistence of a microorganism depends upon the viable time of the microorganism, when it is not present in a human host. Bacteria are commonly the least persistent microorganisms, and protozoa cysts are the most persistent ones.
Young children, elderly people and sick people have a lower resistance to disease and are therefore more fragile. When a person is infected, the pathogens multiply within the host, causing the risk of illness to rise. Not every person that is infected with a pathogen falls ill. People that do become ill will spread a disease easily, mainly through secretions.
Why is it important to disinfect drinking water? Not only because it is the law, but because it is the right and safe thing to do for the consuming public. The 461 public water systems that produce and/or distribute safe, reliable drinking water are regulated by the Kentucky Division of Water (DOW) and the U.S. Environmental Protection Agency through the Safe Drinking Water Act. Kentucky’s drinking water is well-treated and safe to consume. One indicator of continued improvement in Kentucky’s drinking water is the steady, five-year decline in the number of violations issued by the Kentucky DOW to public water systems. This is significant because it shows that the public water systems continue to operate effectively and safely, while making adjustments in procedures and techniques to make sure they are adhering to the new regulations.