Wastewater, or sewage, is water from domestic sources such as baths, toilets and washing machines; industrial waste; and surface water from roofs, roads and other hard-surfaced, impermeable areas.

There are several systems in place for treating wastewater:

• Surface water sewers take surface water from roads, yards and roofs, often releasing it without treatment back into the environment
• Combined sewers carry sewage, industrial waste and surface water to Wastewater Treatment Works, such as the one on the south east side of Brechin. Once treated, the water is safely released back into the environment
• Many rural houses have their own private systems such as septic tanks with soakaways or secondary treatment systems, both of which release wastewater back into the environment safely.

Combined sewers have ‘storm overflows’ which act as relief valves when there is a risk that the sewage system will become overwhelmed, for example when rainwater quickly enters the drains and the sewers during heavy rainfall. This could lead to flooding and worst-case scenario, backing up into people’s homes. To prevent this from happening, storm overflows release surplus rainwater and wastewater into rivers or seas without being treated. Untreated wastewater may also be released via storm overflows if there are blockages in the sewer or if there is a technical failure at wastewater treatment works.

All systems which release wastewater into the environment are permitted by SEPA under the Environmental Authorisation (Scotland) Regulations 2018 (EASR).

Scottish Water has responsibility for collecting and treating wastewater, while smaller treatment systems for individual properties, are the responsibility of the homeowner. Local authorities are responsible for the drainage systems that collect surface water from roads.

In 2022 Scottish Water invested £886 million, however with challenges such as an increasing population and extreme weather events caused by climate change, further considerable investment is required.