Waste Water Treatment and Its Implications

Why waste water needs treatment

Water leaving a kitchen, bathroom, farm or factory can contain much more than water molecules. Urban lifestyles and industrial processes produce large volumes of waste water, which must be treated before being released into the environment. Without treatment, pollutants can damage ecosystems, harm plants and animals, and contaminate water supplies.

The treatment needed depends on what the water contains. Sewage contains human waste and other waste from homes and buildings. Sewage and agricultural waste water, such as water contaminated with animal waste, require the removal of organic matter and harmful microbes. Organic matter is material originating from living organisms, such as faeces and food waste. Harmful microbes can cause disease.

Industrial waste water may require the removal of organic matter and harmful chemicals. For example, some industrial processes produce waste water containing toxic metal compounds. Removing visible solids alone would not remove these dissolved pollutants, so treatment must be suited to the contaminants present.

Removing large waste and grit

Sewage treatment begins with screening and grit removal. Screens catch large insoluble objects, such as pieces of plastic and twigs. Grit removal separates small, heavy particles such as sand.

These processes remove physical debris, but they do not make the water clean enough to release. Smaller suspended particles, organic matter and harmful microbes can remain, so further stages are needed.

Sedimentation separates sludge from effluent

The sewage enters a settlement tank, where suspended solids are given time to settle under gravity. This process is called sedimentation.

The solid-rich material collecting at the bottom is sewage sludge. The liquid above it is effluent. Effluent is not clean drinking water: it still contains organic matter that needs treatment.

Sedimentation creates two separate treatment routes. Sludge and effluent are treated differently because one is concentrated solid-rich material and the other is mainly liquid.

Sludge is digested without oxygen

Sewage sludge undergoes anaerobic digestion. Microorganisms break down its organic matter without oxygen: anaerobic means that oxygen is not used.

This digestion produces methane, which can be used as an energy source. Recovering energy from the sludge is a useful benefit of treatment, rather than simply disposing of all the waste.

Effluent is treated using oxygen

Effluent undergoes aerobic biological treatment. Air is pumped into the liquid to supply oxygen, allowing aerobic microorganisms to break down the remaining organic matter. Here, aerobic means that oxygen is used, and biological means that living organisms carry out the treatment.

The contrast is important: sludge is digested without oxygen, while effluent is treated with oxygen. Both routes use microorganisms, but under different conditions.

The purpose of waste water treatment is to make the water suitable for release into the environment. That is a different standard from making water potable. Water that has passed through sewage treatment must not automatically be regarded as safe to drink.

Weighing the benefits and costs of treatment

Water treatment has implications for individuals, communities, the economy and the environment. These are connected rather than four unrelated lists.

For an individual, safe drinking water and effective sewage treatment reduce exposure to harmful microbes. For society, treating large volumes of waste water protects shared water supplies and the rivers and seas used by whole communities. These benefits matter even to people who live far from the treatment works because water moves through the environment.

However, treatment works require equipment, energy and continued operation, all of which cost money. Pumping air into effluent is one example of an energy demand. These costs must be weighed against the benefits of preventing pollution and protecting health. Methane recovered from sludge can provide useful energy, but this does not mean the entire treatment process has no costs.

Environmentally, removing pollutants protects organisms and their habitats. There is also an environmental cost to supplying energy for treatment, particularly where that energy comes from burning fuels. An evaluation should consider both the pollution prevented and the resources used, rather than describing treatment as either completely beneficial or completely harmful.

Choosing a source of potable water

The same balancing approach applies when deciding where drinking water should come from. Potable water is water that is safe to drink, not necessarily chemically pure water.

A suitable fresh-water source, including ground water, usually has relatively low levels of dissolved substances. Filtration and sterilisation can make such water potable, so an available, suitable fresh-water supply is generally easier to treat than heavily contaminated waste water. Waste water requires additional stages to deal with organic matter, harmful microbes and, where present, harmful chemicals. Treating it protects the quality of water sources downstream, even when the treated water is not supplied directly for drinking.

Where fresh water is scarce, desalination can provide water from salty water or sea water. Distillation and reverse osmosis require energy, which adds economic and environmental costs. Nevertheless, those costs may be justified where there is no adequate fresh-water supply: a community still needs reliable drinking water.

A justified conclusion therefore depends on local conditions. Where suitable fresh water is available, using it may reduce treatment demands. Where it is limited, more demanding methods may be necessary. In either situation, waste water must be treated appropriately before release; avoiding the treatment cost would leave the health and environmental risks unresolved.

What must be removed?

• Sewage and agricultural waste water: organic matter and harmful microbes.
• Industrial waste water: organic matter and harmful chemicals may need removing too.
• Treatment before release protects ecosystems and water supplies.

Sewage treatment sequence

1. Screening and grit removal: remove large objects and grit.
2. Sedimentation: solids settle, separating sludge from liquid .