Urban waste water treatment

What is urban waste water treatment?

Urban waste water is any domestic waste water, a mixture of domestic and industrial waste water, and/or run-off or rainwater.  We monitor the collection, treatment and discharge of urban waste water, and the treatment and discharge of waste water from industrial sites.

Waste water contains organic materials that are broken down by microorganisms, which use up oxygen. The amount of oxygen consumed by these organisms in breaking down the waste is called biochemical oxygen demand or BOD. Waste water can undergo two treatment processes.

Primary Treatment

Primary treatment is a chemical or physical process which reduces the BOD by at least 20% before it is discharged. This is important because if more oxygen is used up than is produced, dissolved oxygen levels fall and sensitive species could die. Primary Treatment also reduces the amount of solids that cannot pass through a filter by at least 50%.

We identify sensitive areas which we show on the map, and where waste water is released into these areas, it must undergo a secondary treatment which causes less impact on the environment.

Secondary treatment is a biological process where bacteria is used to break down the biodegradable matter in waste water.

Sensitive areas

Sensitive areas are:

• Natural freshwaters, other freshwater bodies, estuaries and coastal waters which have a high level of nitrates. These can cause a high growth of algae and other plants which can affect species living in the water, and the quality of the water overall. These lakes are called eutrophic. This also covers areas which could become eutrophic if we take no action. These are designated Sensitive Areas (Eutrophic).
• Surface freshwaters used for drinking water that could contain more nitrates than allowed by EC directives 75/440/EEC of 16 June 1975. These are designated Sensitive Areas (Nitrate).
• Areas where the water needs treatment further to secondary treatment to fulfill the requirements of other EC directives. These are designated Sensitive Areas (Bathing Waters) and Sensitive Areas (Shellfish waters).

What is the Urban Waste Water Treatment Directive?

The aim of the Urban waste water directive (91/271/EEC) is to protect the environment from the adverse effect of waste water discharges. It sets out guidelines and legislation on how we collect, treat and discharge urban waste water.

The Urban waste water directive sets out a series of deadlines for European member states. These state that urban waste water must undergo secondary treatment or an equivalent. The deadlines are as follows:

• 31 December 2005 for all discharges from agglomerations of more than 15,000 population equivalent (known as pe, meaning a biodegradable load which has a five day BOD of 60g of oxygen per day.)
• 31 December 2005 for discharges to freshwater and estuaries from agglomerations between 2000 and 10,000 pe.