The World Health Organization (WHO) says that contaminated water refers to water whose composition has changed to an unusable level. In other words, it is toxic water that cannot be drunk or used for basic purposes such as agriculture. It can also cause diseases such as diarrhea, cholera, dysentery, typhoid fever and polio, causing more than 500,000 deaths worldwide each year.

The main water pollutants include bacteria, viruses, parasites, fertilizers, pesticides, pharmaceutical products, nitrates, phosphates, plastics, feces and even radioactive materials. These substances do not always change the color of water, which means they are usually invisible contaminants. This is why a small amount of water and aquatic organisms are tested to determine water quality.

Deteriorating water quality is destroying the environment, health and the global economy. World Bank President David Malpass warned of the economic impact: "Deteriorating water quality is hindering economic growth in many countries and exacerbating poverty." The explanation is that when the biological oxygen demand—a measure of organic pollution found in the water— exceeds a certain threshold, the gross domestic product (GDP) growth in the relevant watershed drops by one-third. In addition, there are some other consequences:

Destroy biodiversity. Water pollution has depleted the aquatic ecosystem, and has caused the unscrupulous proliferation of phytoplankton to the lake-eutrophication.

Food chain contamination. Fishing in contaminated waters and using waste water for animal husbandry and agriculture can bring toxins into food, which are harmful to our health when consumed.

Lack of drinking water. The United Nations stated that billions of people around the world do not have access to clean drinking water or sanitation, especially in rural areas.

disease. The World Health Organization estimates that about 2 billion people have no choice but to drink water contaminated with excrement, exposing them to diseases such as cholera, hepatitis A and dysentery.

Infant mortality rate. According to the United Nations, diarrhoeal disease, which is related to lack of sanitation, kills about 1,000 children worldwide every day.