Urban waste water
Urban Waste Water Treatment Directive
The Urban Waste Water Treatment Directive (91/271/EEC) (‘the Directive’) is one of a number of European Union (EU) directives which are intended to protect the water environment for the animals and plants that live in and around water, as well as for recreational purposes and use as a resource for drinking water, sanitation, industry and commerce. The Directive was adopted on 21 May 1991. EU member states are required to implement the Directive through their national legislation.
What is ‘urban waste water’?
'Urban waste water' is defined in the Directive as the mixture of domestic waste water from kitchens, bathrooms and toilets, the waste water from industries discharging to sewers, and rainwater run-off from roads and other impermeable surfaces such as roofs, pavements and roads draining to sewers. Urban waste water is often referred to as ‘sewage’.
Why treat urban waste water?
Although untreated waste water is mostly water (in general, less than 0.1% is solid material), without treatment the waste water produced every day would cause significant damage to the environment. The impacts of untreated waste water include the following:
- Chronic ecosystem damage, due to oxygen depletion in receiving waters from the biodegradation of organic matter
- Ecosystem damage or eutrophication of waters, resulting from excessive inputs of nutrients present in waste water
- Potential health risks from water-borne pathogens, from discharges into waters used for recreational activities, such as swimming and canoeing
Environmental impacts
Untreated waste water also contains sewage litter and other sewage solids that can have an impact on the environment – for example, through the smothering of river beds or as a hazard risk if ingested by wildlife. Sewage solids can also damage commerce by making beach and riverside resorts unattractive to potential visitors.
Waste water collection
Before waste water can be treated it needs to be collected. Every day in Wales, over 18,000 kilometres of sewers collect waste water from homes, municipal, commercial and industrial
premises and rainwater run-off from roads and other impermeable surfaces.
Collections systems
There are three main types of collection system:
- Surface water drainage, collecting rainwater run-off from roads and urban areas and discharge direct to local waters
- Combined sewerage that collects rainwater run-off and waste water from domestic, industrial, commercial and other premises
- Foul drainage that collects domestic waste water from premises (no rainwater is collected)
Transfer of private sewers in England and Wales
As of 1 October 2011, ownership of private sewers transferred from homeowners to English and Welsh sewerage companies. Responsibility for the repair of collapsed and blocked formerly-private sewers now rests with sewerage companies.
Waste water treatment levels
The treatment provided at waste water treatment plants can involve the following:
- Preliminary treatment, to remove grit and gravel and to screen large solids
- Primary treatment, to settle larger suspended – generally organic – matter
- Secondary treatment, to biologically break down and reduce residual organic matter
- Tertiary treatment, to address different pollutants using different treatment processes
Monitoring waste water discharges to the environment
Natural Resources Wales is the regulatory body tasked by the Welsh Government to oversee the implementation of urban waste water regulations and associated reporting. The regulatory role covers continuous discharges from the water industry, other industrial sectors and private discharges as well as intermittent discharges, such as those from combined sewer overflows or emergency overflows.
Discharge authorisation
Typically, any discharge to controlled waters requires a discharge authorisation, which sets out standards for the monitoring of effluent from treatment plants. For intermittent discharges, an authorisation may specify the volume of permitted discharges or it may specify a need to screen discharges to remove sewage litter. Discharge authorisations are referred to as ‘Environmental Permits’ in Wales and England.