Why waste water must be treated

Waste water from homes, farms and industry contains unwanted materials. If it is released directly into rivers or other parts of the environment, these materials can damage ecosystems or harm human health. Treatment removes or reduces contaminants before the water is discharged.

Sewage from homes and agricultural waste water commonly contain large amounts of organic matter and harmful microorganisms. When microorganisms break down organic matter in water, they use dissolved oxygen. This can leave too little oxygen for aquatic organisms. Some microorganisms in the waste water can also cause disease, so treatment reduces organic pollution and removes disease-causing microbes.

Industrial waste water may contain organic matter as well, but it can also contain harmful chemicals. The chemicals present depend on the industrial activity, so the treatment needed depends on the contamination. Waste water from different sources does not all require the same treatment: treatment is designed to deal with the unwanted materials present before discharge.

Ease of obtaining potable water from different sources

Potable water is water that is safe to drink. It does not have to be pure, but harmful substances and microorganisms must be reduced to safe levels. The treatment needed depends on what is in the water before treatment begins.

Comparing water sources

Ground water is usually the easiest source to make potable. As water passes through soil and rock, it is naturally filtered, removing some particles and microorganisms. It may therefore need less treatment than other sources. However, it can still contain microbes or dissolved substances, including minerals picked up from rocks, so its suitability must be checked.

Waste water is harder to make potable because it can contain sewage, organic matter, microorganisms and other contaminants. These must be removed or reduced through substantial treatment before the water is safe to drink.

Salt water is generally the hardest source to use. In addition to treating any other impurities, the dissolved salts must be removed by desalination. Desalination requires a lot of energy and is expensive.

The ease of using a source therefore depends on both its starting contamination and the amount of treatment needed to make it safe.

Sewage treatment process

Sewage treatment removes solid material and reduces biological pollution before treated outputs are released. It separates raw sewage into solid and liquid portions, then uses microorganisms to break down organic matter in each portion.

Screening and sedimentation

First, screening removes large objects from the sewage. Grit removal takes out heavy particles such as grit. Removing these materials prevents them from interfering with later treatment.

The sewage is then left to settle in a process called sedimentation. Solid material settles out as sewage sludge, leaving a liquid called effluent. The sludge and effluent are treated separately.

Treatment of sludge and effluent

The sewage sludge is treated by anaerobic digestion. Microorganisms break down organic matter in the sludge without oxygen.

The effluent is treated aerobically. Microorganisms use oxygen to break down remaining organic matter in the liquid.

By removing solids and breaking down organic matter, these stages produce treated outputs that are less harmful to release.