Published by Australian Academy of Science
Cleaner production – a solution to pollution? Box 1 | What is pollution?

Pollution can be defined as any physical phenomenon added to the environment that causes unwanted or harmful effects. People talk of air, water and soil pollution, visual pollution, noise pollution, thermal pollution, electromagnetic pollution, and so on. Here, we concentrate on air and water pollution.

Air pollution

Nearly all of us are affected by air pollution. It may not have a major effect on most of us immediately, but it may damage our health in the long term. In addition, it can affect soil and water through the deposition of substances from the air.

About 99 per cent of the entire atmosphere of the Earth is made up of two gaseous molecules nitrogen (N₂) and oxygen (O₂). The remaining 1 per cent consists of carbon dioxide (CO₂), a range of rare gases such as argon, neon and helium, and trace gases such as carbon monoxide (CO), nitrogen dioxide (NO₂), ammonia (NH₄), ozone (O₃) and sulfur-containing compounds such as sulfur dioxide (SO₂).

What we call pollution occurs when the concentrations of certain trace gases increase significantly. For example, sulfur dioxide can be produced by the burning of fossil fuels, particularly coal, as well as by the smelting of metallic ores such as zinc.

In the atmosphere, sulfur dioxide can be oxidised to form sulfur trioxide (SO₃). This reacts with water droplets to form sulfuric acid (H₂SO₄). Sulfuric acid is highly corrosive. Having formed in the atmosphere, it falls to Earth, leading to the phenomenon of 'acid rain' when concentrations are high enough. Acid rain has been blamed in Europe and North America for declines in freshwater fish populations and in the health of forests.

Water pollution

Fresh water – the water that fills our reservoirs, rivers and lakes – is increasingly being viewed as a valuable and rare resource. In Australia, industry and agriculture are the main causes of freshwater pollution, although human sewage may play a part. Nitrates and phosphates from fertilisers and from farm animal droppings are the biggest pollutants. Pesticides and chemical wastes from industrial processes come next.

Fresh water is usually a rich cocktail of substances, including dissolved gases such as oxygen and carbon dioxide, a variety of cations (such as sodium, potassium, magnesium, calcium and iron), anions (such as chloride, sulfate and hydrogencarbonate), and particles of soil and organic matter.

Despite its abundance in air, the concentration of oxygen in water is usually quite low, at just a few parts per million. Nevertheless, this concentration is a good indicator of water quality. If it is too low, many organisms, including fish, will die.

Aerobic bacteria use dissolved oxygen to oxidise – or decompose – biodegradable organic matter in water into products such as carbon dioxide, water, nitrates, sulfates and phosphates. If there is too much organic matter in the water, the concentration of dissolved oxygen in the water may fall to a level at which the aerobic bacteria cannot survive. The decomposition process may then be taken over by anaerobic bacteria, which produce gases such as methane (CH₄), hydrogen sulphide (H₂S) and ammonia (NH₃) (note that none of these contain oxygen) – these are the rotten-smelling gases often indicative of polluted waters.

Biochemical oxygen demand

The amount of oxygen needed to decompose all the biodegradable organic matter in water is known as the biochemical oxygen demand (BOD). The BOD is measured by taking a water sample and assessing the concentration of dissolved oxygen. A second sample taken at the same time is held in a sealed container at a constant temperature for 5 days and then assessed for dissolved oxygen. BOD is simply the difference between initial and final dissolved oxygen concentrations. It is a useful measure of pollution, since a high reading would suggest that the concentration of oxygen might fall to a level at which many aquatic organisms will not survive.