Urban waste water treatment in Europe

Assessment versions

Published (reviewed and quality assured)

• No published assessments

Rationale

Justification for indicator selection

Waste water from households and industry represents a significant pressure on the aquatic 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 settlements, a significant fraction of urban waste water is collected by sewers connected to public waste water treatment plants. The level of treatment before discharge and the sensitivity of the receiving waters determine the scale of the impacts on aquatic ecosystems. The proportion of the population connected to urban waste water treatment plants and the types of treatments used are seen as proxy indicators of the level of purification and the potential for improvement of the water environment.

Primary (mechanical) treatment removes some of the suspended solids, while secondary (biological) treatment uses aerobic or anaerobic microorganisms to decompose most of the organic matter and retain some of the nutrients (around 20-30 %). Tertiary (advanced) treatment removes 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 % of ammonium.

This indicator tracks the success of policies aimed at reducing pollution from waste water by describing the trends in the percentage of the population connected to urban waste water treatment plants with different levels of purification.

Scientific references

• No rationale references available

Indicator definition

The indicator on urban waste water treatment, WAT005, collects data on the percentage of the population connected to sewage collection systems, as well as on the prevalence of primary, secondary and tertiary urban waste water treatment plants.

The amount of urban waste water treated from 'big cities' is expressed as population equivalents (p.e.).

The indicator illustrates:

1. urban waste water collection and treatment in Europe in 2017;
2. the development of more stringent (tertiary) urban waste water treatment practice;
3. the level of urban waste water treatment in 'big cities (agglomerations of > 150 000 p.e.) in the EU.

Units

The percentages of the population connected to primary, secondary and tertiary urban waste water treatment facilities (Figures 1 and 2).

The percentage of population equivalent (p.e.) was used for “big city” treatment (Figure 3).

Policy context and targets

Context description

The main objective of the Urban Waste Water Treatment Directive (91/271/EEC) (UWWTD), and equivalent national legislation for non-EU countries, is to protect surface waters from the adverse effects of waste water discharges. The UWWTD prescribes the level of treatment required before discharge to surface waters. It requires Member States to provide all urban settlements (called 'agglomerations' in the UWWTD) of more than 2 000 p.e. with collecting systems. Primary (mechanical) and secondary (i.e. biological) treatments must be provided for all agglomerations of more than 2 000 p.e. that discharge into fresh waters. Special requirements, with intermediate deadlines depending on the sensitivity of the receiving waters, are placed on urban settlements of more than 10 000 p.e., with various size classes. The performance of the treatment is assessed using several determinands (biochemical oxygen demand (BOD) and chemical oxygen demand (COD); plus total nitrogen and total phosphorus in the case of more stringent treatment).

For urban settlements smaller than those described above and equipped with a collecting system, the treatment must be 'appropriate', meaning that the discharge must allow the receiving waters to meet the relevant quality standards.

The UWWTD, adopted in 1991, is also a basic measure under the Water Framework Directive (WFD). The WFD requires the estimation and identification of significant point- and diffuse-source pollution, in particular by the substances listed in Annex VIII, from urban, industrial, agricultural and other installations and activities, based, inter alia, on information gathered, for instance, under Articles 15 and 17 of the UWWTD. Based on the substances listed in Annex VIII WFD, the following are important for this indicator:

• substances that have an unfavourable influence on oxygen balance (and can be measured using parameters such as BOD, COD, etc.);
• materials in suspension;
• substances that contribute to eutrophication (in particular nitrates and phosphates).
  
  

Member States should thus take the necessary steps to collect these data. Reducing pollutants stemming from waste water is one of the key challenges of reaching good ecological and good chemical status of surface waters, as required by the WFD.

Collecting and treating waste water has required huge investment across Europe in recent decades. The kinds of new challenges facing urban waste water treatment, such as climate change, resource efficiency and improved environmental protection, are set out in the EEA briefing Urban waste water treatment for 21st century challenges.

Further information on emissions from industry to water, including to urban waste water treatment plants, is available in Industrial waste water treatment - pressures on Europe's environment.

Targets

The UWWT Directive (91/271/EEC) aims to protect the environment from the adverse effects of urban waste water discharges. It prescribes the level of treatment required before discharge and should have been fully implemented in the EU-15 countries by 2005. For the newer Member States (i.e. the EU-13), staged transition periods were set within the Accession Treaties which, in principle, did not extend beyond 2015. However, in Romania, smaller agglomerations (with less than 10 000 p.e.) should have complied with the directive by the end of 2018, and Croatia has different transition periods, from 2018 to 2023.

Under the directive, EU-15 Member States were required to provide all urban settlements of more than 2 000 p.e. with collecting systems and all waste waters collected had to be provided with appropriate treatment by 2005. Secondary treatment (i.e. biological treatment) must be provided for all urban settlements 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.

Related policy documents

• Commission implementation reports of the Urban Waste Water Treatment Directive
  provided by Directorate-General for Environment
• Council Directive (91/271/EEC) of 21 May 1991
  Council Directive of 21 May 1991 concerning urban waste water treatment (91/271/EEC)

Methodology

Methodology for indicator calculation

The indicator is based on data from Eurostat, which show the percentage of the population connected to each treatment type. Compared with previous versions of the indicator, Figure 1 now shows the treatment level in EEA member and cooperating countries in 2017, rather than grouping countries according to region.

A p.e. of 1 is equivalent to an organic biodegradable load having a 5-day BOD of 60 g per day.

Tertiary treatment is known in the UWWT Directive as 'treatment more stringent than secondary' and includes the application of secondary treatment.

'Big cities' is a term used in the UWWT Directive for cities of at least 150 000 p.e. or agglomerations responsible for large waste water discharges. Countries themselves identify their 'big cities'.

Methodology for gap filling

Gap filling was undertaken on the basis that once an urban waste water infrastructure had been put in place, it was likely to be used in subsequent years. Therefore, any gaps were filled with data from the most recent year reported, e.g. 2016 data carried forward to 2017. This approach was used for up to 5 years of gap filling, i.e. 2012 data could be carried forward to up to 2017.

Methodology references

• Eurostat Water statistics on national level (env_nwat) Yearly data on freshwater resources, water abstraction and use, connection rates of resident population to wastewater treatment, sewage sludge production and disposal, generation and discharge of wastewater collected biennially by means of the OECD/Eurostat Joint Questionnaire - Inland Waters. Data aggregation: national territories.

Data specifications

EEA data references

• Waterbase - UWWTD: Urban Waste Water Treatment Directive – reported data provided by Directorate-General for Environment (DG ENV) , European Environment Agency (EEA)

External data references

• Water statistics (Eurostat)

Data sources in latest figures

Uncertainties

Methodology uncertainty

The main uncertainties relate to data reported to Eurostat. In 2005, treatment for 24 % of the population was 'unknown', which fell to 15 % in 2017.  The second area of uncertainty relates to numbers of inhabitants, as these do not necessarily align with reported population data, though error here is generally small.

The 'EU-27' value is strongly influenced by the situation in Member States with large populations (France, Germany, Italy and Spain).  

At country level, the population figure and the generated p.e. load are not usually the same, as there can be other sources of organic pollution, such as food industry and temporal changes in population owing e.g. to tourism.

Data sets uncertainty

Data reported to Eurostat sometimes provide an incomplete picture of inhabitants connected to waste water treatment (e.g. the percentage of the population connected to urban waste water systems is given, but the percentage for which the waste water is collected without treatment is missing).

'Big cities' data sometimes include data from agglomerations that are much smaller than 150 000 p.e. However, the impact of such errors on the final percentages is rather small.

Rationale uncertainty

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

In addition, urban waste water treatment (primary, secondary or tertiary, as described above) is the main waste water treatment used across the EEA area, but there are other possible treatments classified as 'Other Waste Water Treatment', which are mostly industrial or independent treatments. Furthermore, there are differences in how countries have interpreted the definitions of different classes of treatment (classes based on performance or design capacity and tertiary treatment for nitrogen, phosphorus or organic matter) that, in turn, lead to differences in the level of purification attributed by the countries to the different classes. These differences emphasise the problem of using types of treatment plant as a proxy for the level of purification.