Necessity of drinking water disinfection

The larger part of pathogenic microorganisms is removed from water during the primairy water purification steps. However, water disinfection is still necessary in order to prevent drinking water from being harmful to our health.

Microorganisms
Microorganisms can be found commonly in nature. Invisible to bare eyes, 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. The microorganisms will remain present on and in our bodies. Most microorganisms are harmless and will contribute to a number of vital processes in the human body, such as the metabolism. But there are also microorganisms which can cause disease or which are harmful to people with low resistance to disease.
Pathogenic microorganisms in the water have a number of specific properties which distinguish them from chemical contaminants. They are living organisms. They are not dissolved in water, but they will coagulte or attach to colloids and solids in water.

Types of pathogenic microorganisms
Pathogenic microorganisms in drinking water can be divided up into three types: bacteria, viruses and parasitic protozoa. Bacteria and viruses can excist in both surface water and groundwater, whereass parasitic protozoa can be found mainly in surface water.

Bacteria
Bacteria are single-cell organisms, shaped like a sphere, spiral or rod. They can excist as individual bacteria or in bacterial chains, bundles or pairs. Bacteria are the most abundant lifeform 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 bacteria to split into two independent cells. In ideal circumstances this process taken about 15 to 30 minutes.

Some types of bacteria can form spores. These spores contain a protective layer which is heat resistant and can protect bacteria from a lack of moist and food.
Bacteria play a role in various processes. Some bacteria breack down organic matter and play an important ecological role, other assist in the human metabolism.

Viruses
Viruses are organisms which can cause infections and which only reproduce in living host cells. When viruses excist outside host cells, they are inactive. Viruses contain a protective shell. They are shaped like a spear, sphere or wire and they are so small (between 0,02 and 0,09 μm) that they can slip through filters which capture bacteria.
Contrary to bacteria and parasitic protozoa, 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. When secreation takes place water can be contaminated with viruses. When the water is not thoroughly disinfected, other people can be infected with viruses.

Figure 1: three different types of viruses

Parasitic protozoa
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 take place through splitting of the cells. Various types of parasitic protozoa are spread in sleeping, protected form as a cyste or oocyste. 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 chlorine dioxide application.

The odds of infection
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 means the number of microorganisms that need to enter te 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 protozoic cysts are the most persistent ones.
Young children, elderly people and sick people have a lower resistance to disease and are therefor 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 secretion.

Secretion and sewer water
When water flows through a certain area, it collects all kinds of substances and gives these off in other areas. Microorganisms also enter the water. The larger part of microorganisms which cause waterborne diseases originate from human or animal feaces.

Figure 2: E. Coli bacteria

One drop of feaces contains millions of microorganisms. In the faeces of cattle there can be millions of E. Coli bacteria (figure 2), Giardia cysts (figure 3) and Cryptosprodium spores (figure 4). In chicken faeces pathogenic bacteria such as Salmonella and Campylobacter can be present. When fertilizers are applied to land, rain can cause bacteria to rinse out te surface water or groundwater, causing the microorganisms to contaminate water.

Figure 3: Cryptosporidium spores Figure 4: Giardia cysts

Sewer or waste water cannot be discharged into the environment untreated. The larger part of purified waste water ends up in rivers, lakes and oceans. Sometimes heavy rainfall can cause sewer systems to flood, causing untreated water to end up in surface water or groundwater. Not every country purifies water before it enters surface or groundwater. Mainly developing countries lack sanitary facilities. The water can contaminate water that is used for drinking water purposes, causing the risk of infection with diseases carried by waterbone microorganisms to become very high. this is a particular risk when drinking water is not treated at all. When septic tanks are used for waste water treatment, pathogenic microorganisms can contaminate surface water and groundwater sources.

Not all pathogenic microorganisms in water originate from faeces. Legionella (figure 5) can be found commonly in water and easily multiplies in the water distribution system. There are also other pathogenic microorganisms that can be found commonly in surface water.