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Urban waste water treatment

Indicator Specification Created 07 Oct 2004 Published 07 Apr 2005 Last modified 04 Sep 2015, 06:59 PM
Topics: ,
Indicator codes: CSI 024 , WAT 005


Justification for indicator selection

Wastewater from households and industry represents a significant pressure on the water environment because of the loads of organic matter and nutrients as well as hazardous substances. With high levels of the population in EEA member countries living in urban agglomerations, a significant fraction of wastewater is collected by sewers connected to public wastewater treatment plants. The level of treatment before discharge and the sensitivity of the receiving waters determine the scale of impacts on aquatic ecosystems. The types of treatments and conformity with the directive are seen as proxy indicators for the level of purification and the potential improvement of the water environment.

Primary (mechanical) treatment removes part of the suspended solids, while secondary (biological) treatment uses aerobic or anaerobic micro-organisms to decompose most of the organic matter and retain some of the nutrients (around 20 - 30 %). Tertiary (advanced) treatment removes the organic matter even more efficiently. It generally includes phosphorus retention and in some cases nitrogen removal. Primary treatment alone removes no ammonium whereas secondary (biological) treatment removes around 75 %.

The indicator tracks the success of policies to reduce pollution from wastewater by describing the trends in the percentage of the population connected to public wastewater treatment plants with different levels of purification.

Scientific references

  • No rationale references available

Indicator definition

Percentage of population connected to primary, secondary and tertiary wastewater treatment plants. The indicator illustrates:

1. changes in wastewater treatment in the regions of Europe since the 1980s;

2. conformity (in terms of providing tertiary treatment) by Member States with the requirement to provide, by 31 December 1998, stringent treatment for agglomerations with population equivalent (p.e.) more than 10 000 that discharge into sensitive areas;

3. levels of urban wastewater treatment in large cities in the EU (agglomerations >150 000 p.e.).


Percentages of population connected to primary, secondary and tertiary wastewater treatment.

Policy context and targets

Context description

The Urban Wastewater Treatment Directive (UWWT) prescribes the level of treatment required before discharge. It requires Member States to provide all agglomerations of more than 2 000 population equivalents (p.e.) with collecting systems. Secondary treatment (i.e. biological treatment) must be provided for all agglomerations of more than 2 000 p.e. discharging into fresh waters. Special requirements with intermediate deadlines depending on the sensitivity of the receiving waters are placed on agglomerations of more than 10 000 p.e. with various size classes of agglomerations. The performance of the treatment is assessed using 5 different determinands (BOD, COD, TSS, Ntot and Ptot). In the EU Member States, there are about 25 000 agglomerations with more than 2 000 p.e., the population is 501 million inhabitants and the overall generated load of  621 million p.e.

For agglomerations smaller than described above and those equipped with a collecting system, the treatment must be appropriate, meaning that the discharge allows the receiving waters to meet the relevant quality objectives.
The WFD is asking for the estimation and identification of significant point and diffuse source pollution, in particular by substances listed in Annex VIII, from urban, industrial, agricultural and other installations and activities, based, inter alia, on information gathered under Articles 15 and 17 of Directive 91/271/EEC and other Directives. From the substances listed in the Annex VIII, the following are important for the indicator : substances which have an unfavourable influence on the oxygen balance (and can be measured using parameters such as BOD, COD, etc.), materials in suspension, and substances which contribute to eutrophication (in particular, nitrates and phosphates). Member States should thus take the necessary steps to build a data collection system able to provide these data, urban source being one of the sources listed. The ultimate aim of this is to reach the target of the WFD that is a good chemical and biological status for all waters in 2015, the discharge of substances being one of the major problems to face.


The Urban Waste Water Treatment Directive (UWWTD; 91/271/EEC) aims to protect the environment from the adverse effects of urban wastewater discharges. It prescribes the level of treatment required before dischargeand has to be fully implemented in the EU-15 countries by 2005 and in the ten new Member States by 2008 - 2015. The directive requires Member States to provide all agglomerations of more than 2 000 population equivalent (p.e.) with collecting systems and all wastewaters collected to be provided with appropriate treatment by 2005. Secondary treatment (i.e. biological treatment) must be provided for all agglomerations of more than 2 000 p.e. that discharge into fresh waters, while more advanced treatment (tertiary treatment) is required for discharges into sensitive areas. To help minimise pollution from various point sources, the Integrated Pollution Prevention and Control directive, which came into force 1996, has a set of common rules on permitting for industrial installations.

The achievements through the UWWTD and the IPPC directive have to be seen as an integrated part of objectives under the Water Framework Directive (WFD) which aim at a good chemical and biological status for all waters by 2015.

Related policy documents

  • COM(2004) 248 final
    Implementation of Council Directive 91/271/EEC of 21 May 1991 concerning urban waste water treatment, as amended by Commission Directive 98/15/EC of 27 February 1998
  • Urban wastewater treatment summary report
    Summary report on: the identification of sensitive areas by the Member States; the measures implemented by the Member States with the view to the deadline of 31 December 1998; wastewater treatment in major cities; verification of the identification of sensitive areas by the Commission.

Key policy question

How effective are existing policies in reducing loading discharges of nutrients and organic matter?

Specific policy question

What is the level of the Urban Waste Water Treatment Directive (91/271/ECC) implementation in large cities of the EU Member States?


Methodology for indicator calculation

Eurostat data are used and aggregated in groups of countries. Average of percentages connected to each treatment type, weighted by total population in each country:


National resident population = "WW0_1 National resident population" if available or else "Table: pjan, unit: inhabitant, Population by sex and age on 1. January of each year"

% of population connected to a type of treatment for one area = sum of (National resident population X "WW0_2_1 Urban waste water treatment of primary treatment"or

"WW0_2_2 Urban waste water treatment of secondary treatment" or

"WW0_2_3 Urban waste water treatment of tertiary treatment") or

"WW0 5: Urban wastewater collecting system without treatment"

EU aggregated and national data are used as presented on DG Environment's web page and reports.

EU aggregated values representing the type of treatment applied in the wastewater treatment plants of 586 big cities/big dischargers (250,2 mil. p.e.) reported in 2011 by the Member States and Norway were retrieved from the latest version of Waterbase  via a series of subsequent queries linking information on agglomerations with total generated load higher than 150 000 p.e., treatment plants serving those agglomerations and the information on the percentage of total generated load treated in particular treatment plants. First, the sequence of scripts retrieved disaggregated information at the big city/big discharger level. Disaggregated information was then aggregated at the national level.

Methodology for gap filling

For the Eurostat dataset, gap fillings were made, by using figures provided by the respective NFPs during the review process or by replacing data available in Newcronos.

Methodology references

No methodology references available.

Data specifications

EEA data references

  • No datasets have been specified here.

Data sources in latest figures


Methodology uncertainty

For the assessment shown in Figure 1, countries have been grouped to show the relative contribution on a larger statistical basis and to overcome the incomplete nature of the data. Complete dataset (ie data available for all years ad all countries) is not available for either of the geographic groups except for South-eastern countries (since the beginning of 2000 ies). Aggregated values are calculated only for the years for which the most complete dataset (with regard to number of responding countries) is available.

Data sets uncertainty


Rationale uncertainty

Data gained from the UWWTD focuses on the performance of the treatment plant alone. But wastewater treatment systems could also include sewer networks with storm water overflows and storages which are complex and whose overall performance is difficult to assess. In addition to the treatments covered by the UWWTD there are other possible treatments, mostly industrial, but also independent treatments of smaller settlements outside urban agglomerations not included in UWWTD reporting. Compliance with the levels defined in the directive therefore does not guarantee that there is no pollution due to urban wastewater.

In addition Urban Waste Water Treatment is the main waste water treatment used across the EEA area, but there are other possible treatments that are treatment in Other Waste Water Treatment, mostly industrial, or in independent treatments. Furthermore there are differences in how countries have interpreted and implemented the Directive leading to differences in the data reported. There are especially variations in the definitions of different classes of treatment between countries (classes based on performance or on design capacity and tertiary treatment for N, P or organic matter) that in turn lead to differences in the level of purification attributed by the countries to the classes. These differences all the more emphasize the problem of using types of treatment plant as a proxy for the level of purification. Member States have also been taken to court by the European Commission because of insufficient application of the Directive.

Further work

Short term work

Work specified here requires to be completed within 1 year from now.

Work description

Data should progressively become available from the last two years, with a better comparability due to the providing by Eurostat of a Manual to help the filling of the questionnaire. Improvement also of the past series is expected. A sub-indicator on emissions of the 5 mandatory parameters will be possible when the data on performance will be made available though the EU database.

Resource needs

No resource needs have been specified


Not started


2006/01/01 00:00:00 GMT+1

Work description

Data from  the DGEnv database on Urban Waste Water treatment plants will improve in the coming years. More detailed information on P and/or N removal would give a better indication of the quality of purification.

Resource needs

No resource needs have been specified


Not started


2008/01/01 00:00:00 GMT+1

Long term work

Work specified here will require more than 1 year (from now) to be completed.

General metadata

Responsibility and ownership

EEA Contact Info

Peter Kristensen


European Environment Agency (EEA)


Indicator code
CSI 024
WAT 005
Version id: 1
Primary theme: Water Water


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Frequency of updates

Updates are scheduled every 2 years


DPSIR: Response
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)

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