Water pollution in the UK: the causes and effects

Water pollution is a huge challenge for freshwater in the UK, impacting our rivers, streams and lakes and the wildlife that call them home.

What are nitrates and phosphates and why are they a problem?

Nitrates and phosphates are nutrients that occur naturally in the environment. Plants need them to grow. Healthy freshwater ecosystems usually have small amounts of these nutrients helping aquatic plants to survive.

However, human activity on land can cause unnaturally high levels of nutrients – and other pollutants – to enter our rivers, streams and lakes resulting in nutrient pollution.

What are the causes of water pollution?

Agriculture

The modernisation and intensification of agriculture during and after World War II resulted in too much nitrate entering our environment. The manufacture and application of artificial fertilisers since the 1940s has increased the amount of surplus nitrate.

Artificial fertilisers dissolve easily in water, and unlike natural fertilisers, add no organic content to the soil. When artificial fertilisers are used on land, rainfall washes them into ditches and rivers. They also drain from the soil into groundwater (underground water in the cracks and spaces in soil, sand and rock).

This makes agricultural areas a historical source of diffuse water pollution. Diffuse pollution is where small amounts of contaminants build up from multiple sources across a large area. This is in contrast to pollutants that enter watercourses from a specific point, such as a pipe or outflow.

Although the amounts vary depending how local land is used, the Environment Agency estimates that agriculture is responsible for 50-60% of nitrate entering waterways. With phosphates, agriculture contributes 20% of the total phosphorous entering our watercourses.

Sewage and wastewater

The majority of phosphate pollution (around 80%) comes from urban wastewater. As for nitrates, sewage effluent (liquid waste) contributes about 25-30% nationally.

Urban wastewater, which is generally called sewage, is a mixture of domestic wastewater from baths, sinks, washing machines and toilets.

It also includes wastewater from industry, and rainwater washing off roads and other surfaced areas (called surface run-off). Over 300 different chemicals are released from car tyres wearing down on roads.

Every day, 347,000 kilometres of sewers collect over 11 billion litres of wastewater. This goes through sewage treatment works before being released back to inland waters, estuaries and the sea. The treatment cleans and recycles the water by removing organic substances.

Solid matter is filtered out (primary treatment) and then bacteria is used to ‘digest’ and break down the organic substances (secondary treatment). Finally, nitrates and phosphates are removed (tertiary treatment) with various methods including sand filtration, activated carbon filtration, and chemical oxidation.

During heavy rainfall, the sewers can fill up and overflow.  This can result in the inundation of sewage works and the potential for dirty water to back up and flood peoples’ homes, roads and open spaces.

To prevent this, combined sewer overflows were developed with overflow valves to reduce the risk of sewage backing up during heavy rainfall. However, this results in untreated sewage being sent to our rivers and other freshwater bodies instead, causing water pollution.

What are the effects of water pollution?

High levels of nutrients trigger a process called eutrophication. This is when excessive plant and algal growth (caused by the extra nutrients) leads to high levels of bacteria. The bacteria decrease oxygen levels in the water, killing plants and animals.

Lots of algae on the water’s surface can also choke waterways preventing wildlife from feeding, and block sunlight causing further problems for aquatic plants.

Our rivers are being put under pressure by historical and current intensive farming. Sewage discharge is making things even worse. The high nitrate levels are driving the poor ecological state of freshwater across the UK, with phosphate levels adding to the dire mix.