Domestic sewage is a natural bye-product of life; it is the wastewater produced from toilets, baths, showers and sinks in every home. It is collected in pipes and treated either by a private septic tank (less than 10% of Scotland’s population) or by a Wastewater Treatment Works (there are 1972 Scottish Water Wastewater Treatment Works (WwTW) in Scotland). Treated wastewater must comply with quality standards set by SEPA prior to being discharged to a watercourse.

As domestic sewage collects in the sewerage system it is combined with rainwater run-off from urban and agricultural land, effluent from traders and infiltration from other surface waters. The resulting mix is a complex effluent with the potential to cause significant environmental damage if discharged without prior treatment.

Oxygen Depleting Substances and Inorganic Nutrients
The discharge of organic matter to surface waters raises the Biological Oxygen Demand (BOD), causing the depletion of oxygen available to aquatic life. The discharge of nutrients (e.g. nitrates and phosphates) can cause enrichment – also known as eutrophication – enabling the growth of bacteria and algae, again leading to the depletion of oxygen and the loss of larger aquatic plants which are an important food source and habitat for other aquatic life. ‘Sewage fungus’ - seen as slime on a river bed around outfalls – is a sign of untreated or poorly treated sewage effluent.

Toxic Substances
In addition to oxygen depleting substances, sewage effluent contains toxic substances. Ammonia is a main component of sewage, which is directly toxic to aquatic life. Other toxic substances present on roads and yards, such as hydrocarbons, heavy metals, pesticides and herbicides are transported to the sewerage system by rainfall events. These are present in much smaller quantities but are just as harmful and can be more difficult to treat and remove.

Pathogens
High levels of bacteria, viruses and parasites from human and animal sources pose a threat to the recreational use of water bodies, as defined by the EC Bathing Waters Directive.

Sanitary waste flushed down toilets blocks drains and damages screening equipment at treatment plants, resulting in the littering of river banks and beaches around Scotland’s coast. Please bag and bin sanitary waste rather than flushing it away and help to keep Scotland’s coast clean.

Sewage is still responsible for downgrading more watercourses than any other type of pollution in Scotland. To address this, Scottish Water has identified a capital investment programme of £1.8 billion and improvements to the sewerage and wastewater treatment system are currently underway to meet the requirements of the EC Urban Waste Water Treatment Directive.

Seven villages in the Deeside Wastewater Treatment Improvement Scheme will benefit from new wastewater treatment works between 2002-2006 for an investment of over £11m. Go to Scottish Water’s website and type ‘Deeside’ in the search box for more information.