Indicators

2.b.2 Total discharges to water bodies and share of total discharges safely treated

General information

The circular economy indicator pollutant discharges from material extraction and processing to water bodies and share safely treated is considered difficult to calculate due to data unavailability. For this reason, a proxy indicator is proposed: proportion of domestic and industrial wastewater flows safely treated, that corresponds to the SDG 6.3.1. (see Table 2. 14).

The presented methodology for this indicator is based on UNSD’s Metadata for 6.3.1 and UNEP’s Step-by-step monitoring methodology for indicator 6.3.1 (2016). SDG target 6.3. aims to improve water quality by reducing pollution, eliminating dumping and minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally.

Indicator interpretation 

This indicator tracks the proportion of wastewater flows from households, services and industrial economic activities that are safely treated at the source or through centralized wastewater treatment plants before being discharged into the environment, out of the total volume of wastewater generated.

In a circular economy, ensuring the availability and quality of water needs to improve minimizing and significantly reducing different streams of pollution into water bodies.

Wastewater data are crucial to promote strategies for sustainable and safe wastewater use or reuse to the benefit of the world’s population health and the global environment, but also to respond to growing water demands, increasing water pollution loads, and climate change impacts on water resources.

This indicator provides information to decision makers and stakeholders to make informed decisions to accelerate progress towards reducing water pollution, minimizing release of hazardous chemicals and increasing wastewater treatment and reuse.

Additional disaggregation

Wastewater can be generated through a variety of economic activities as well as through private households. The following categories of wastewater flows can be distinguished:

• Agricultural (ISIC 01-03) covers crop and animal production, hunting and related service activities; forestry and logging; and fishing and aquaculture. Wastewater generated from these activities for the most part enters the environment as non-point pollution and will not be monitored as part of indicator 6.3.1.
• Mining and quarrying (ISIC 05-09) includes the extraction of minerals occurring naturally as solids (coal and ores), liquids (petroleum) or gases (natural gas). Extraction can be achieved by different methods such as underground or surface mining, well operation, seabed mining etc.
• Manufacturing (ISIC 10-33) includes the physical or chemical transformation of materials, substances, or components into new products. The materials, substances, or components transformed are raw materials that are products of agriculture, forestry, fishing, mining or quarrying as well as products of other manufacturing activities. Substantial alteration, renovation or reconstruction of goods is generally considered to be manufacturing.
• Electricity (ISIC 35) includes electric power generation, transmission and distribution, as well as the manufacture and distribution of gas, and steam and air conditioning supply. Water used for cooling in power generation is explicitly excluded from calculations of wastewater flows.
• Construction (ISIC 41-43) includes general construction and specialized construction activities for buildings and civil engineering works. It includes new work, repair, additions and alterations, the erection of prefabricated buildings or structures on the site and also construction of a temporary nature.
• Services (ISIC 45-96) These Divisions are considered service industries and include a wide range of economic activities where water is mainly used for sanitary purposes, washing, cleaning, cooking, etc
• Wastewater can also be generated by private households, originating predominantly from the human metabolism and from household activities. A portion of the water which is brought into private households for domestic purposes (e.g. cooking, drinking, bathing, washing, ISIC division 36) exits the household as wastewater. Domestic wastewater flows are not directly covered by ISIC codes, unless the household generates water in the course of an economic activity. Note that wastewater generated by residents of communal institutions may be covered under ISIC divisions, e.g. 85 (education) or 87 (residential care activities).

Figure 2. 64 shows the different classifications used by OECD/Eurostat (left) and UNSD/UNEP (right) for generation of wastewater flows. The variables used to populate this indicator are highlighted in colours.

Differentiating between the different wastewater streams is important as policy decisions need to be guided by the polluter pays principle. However, wastewater conveyed by combined sewers usually combines both hazardous and non-hazardous substances discharged from different sources, but also runoff and urban stormwater, which cannot be separately tracked and monitored. As a consequence, although the flow of wastewater generated can be disaggregated by sources (domestic, services, industrial), the treated wastewater statistics are most commonly disaggregated by type (e.g. urban and industrial) and/or level of treatment (e.g. secondary) rather than by sources.

Data availability

Total flows of wastewater generated and treated are reported by countries to UNSD and OECD/Eurostat databases. Eurostat deals with Member States of the European Union (EU) and the European Free Trade Association (EFTA) as well as the respective candidate countries. OECD works with all its Member States not contacted by Eurostat. UNSD sends the UNSD/UNEP Questionnaire to the rest of the world (approx. 165 countries). However, the response rate for the UNSD/UNEP questionnaire is around 50% and data completeness and quality remain a challenge, especially for developing countries.

The WHO/UNICEF Joint Monitoring Programme for Water Supply, Sanitation and Hygiene (JMP) collects and compiles national data related to use of sanitation services including wastewater treatment, for calculation of SDG indicator 6.2.1a “proportion of the population using safely managed sanitation services.”

These databases rely on a comparable harmonized terminology for water statistics. Wastewater data are nonetheless still relatively sparse on a global scale.

Outside these databases, data on wastewater generation and treatment are not widely available, and existing data may not align with international definitions and classifications.

At national level, data are collected from National Statistical Offices, ministries of environment, municipalities or regulatory authorities.

Limitations in the use of the indicator

There is a relative lack of knowledge about the volumes of wastewater generated and treated, because wastewater statistics are in an early stage of development in many countries and not regularly produced or reported. Monitoring is relatively complex, costly, and data are not systematically aggregated to the national level and/or accessible; especially industrial wastewater data which are in general poorly monitored and seldom aggregated at national level.

To some extent, this may be explained by the fact that a large proportion of the industrial water requirements are covered by the use of private systems using non-public/drinking water supply (groundwater, rivers and wells) which are not systematically included in national statistics.

Diffused pollution from non-point sources such as runoff from urban and agricultural land can contribute significantly to wastewater flows. Presently, it cannot be monitored at source and its impact on ambient water quality can only be monitored indirectly, for example under indicator SDG 6.3.2 on the proportion of bodies of water with good ambient water quality.

Different types of wastewaters have different degrees of contamination and pose different levels of threat to the environment and public health. Some data exist on the pollutant loading in terms of BOD5 and COD (kg O2/day), but these are not as widely available as data on volumes.

Finally, whether wastewater is classified as safely treated or not depends on the wastewater treatment plant’s compliance rate to the effluent standards (i.e. performance). Many wastewater plants produce effluent which does not meet quality standards, due to improper design or loading. Effluent standards rely on both national and local requirements, as well as on specific water uses and potential reuse options, so that this approach may not provide strictly comparable variables between countries.

Definitions

“Domestic wastewater is wastewater from residential settlements which originates predominantly from the human metabolism and from household activities” (United Nations, SDGs metadata).

“Independent treatment corresponds to facilities for preliminary treatment, treatment, infiltration or discharge of domestic wastewater from dwellings generally between 1 and 50 population equivalents, not connected to an urban wastewater collecting system. Examples of such systems are septic tanks. Excluded are systems with storage tanks from which the wastewater is transported periodically by trucks to an urban wastewater treatment plant” (United Nations, SDGs metadata).

“Industrial (process) wastewater corresponds to water discharged after being used in, or produced by, industrial production processes and which is of no further immediate value to these processes. Where process water recycling systems have been installed, process wastewater is the final discharge from these circuits. To meet quality standards for eventual discharge into public sewers, this process wastewater is understood to be subjected to ex-process in-plant treatment. Cooling water is not considered here. Sanitary wastewater and surface runoff from industries are also excluded here” (United Nations, SDGs metadata).

“Non-treated wastewater is wastewater which doesn’t undergo any form of treatment before discharge to the environment” (United Nations, SDGs metadata).

“Other wastewater treatment corresponds to treatment of wastewater in any non-public treatment plant, i.e., Industrial Wastewater Treatment Plants (IWWTPs). Excluded from "other wastewater treatment" is the treatment in septic tanks. IWWTPs may also be classified under ISIC 37 (Sewerage) or under the main activity class of the industrial establishment they belong to” (United Nations, SDGs metadata).

“Primary wastewater treatment implies the treatment of wastewater by a physical and/or chemical process involving settlement of suspended solids, or other process in which the Biochemical Oxygen Demand (BOD5) of the incoming wastewater is reduced by at least 20% before discharge and the total suspended solids of the incoming wastewater are reduced by at least 50%. To avoid double counting, water subjected to more than one type of treatment should be reported under the highest level of treatment only” (United Nations, SDGs metadata).

“Proportion of domestic and industrial wastewater flows safely treated is the proportion of all wastewater generated that is safely treated at source or through centralized wastewater treatment plants before it is discharged into the environment impacting ambient water quality, with implications on human and ecosystem health” (UNEP, Step-by-step monitoring methodology for indicator 6.3.1)“Total wastewater generation and treatment can be quantified at the national level, and wastewater can also be disaggregated into different types of flows, based on ISIC categories. Domestic wastewater generated by private households, as well as wastewater generated by economic activities covered by ISIC categories, may or may not be pre-treated on premises before discharge to either the sewer for further treatment or directly to the environment, as shown in Figure 2. 65.” (United Nations, SDGs metadata).

“The main sources of wastewater include wastewater from households, services and industries, i.e. point sources of one or more pollutant(s) that can be geographically located and represented as a point on a map. Diffused pollution from non-point sources such as runoff from urban and agricultural land can contribute quite significantly to wastewater flows (Figure 2. 65), and therefore its progressive inclusion in the monitoring framework will be important” (United Nations, SDGs metadata).

“Secondary wastewater treatment is a post-primary treatment of wastewater by a process generally involving biological treatment with a secondary settlement or other process, resulting in a Biochemical oxygen demand (BOD) removal of at least 70% and a Chemical Oxygen Demand (COD) removal of at least 75%. Natural biological treatment processes are also considered under secondary treatment if the constituents of the effluents from this type of treatment are similar to the conventional secondary treatment. To avoid double counting, water subjected to more than one type of treatment should be reported under the highest level of treatment only” (United Nations, SDGs metadata).

“Tertiary wastewater treatment is a treatment (additional to secondary treatment) of nitrogen and/or phosphorous and/or any other pollutant affecting the quality or a specific use of water: microbiological pollution, colour etc. The different possible treatment efficiencies ('organic pollution removal' of at least 95% for BOD5, 85% for COD, 'nitrogen removal' of at least 70%, 'phosphorous removal' of at least 80% and 'microbiological removal') cannot be added and are exclusive. To avoid double counting, water subjected to more than one type of treatment should be reported under the highest level of treatment only” (United Nations, SDGs metadata).

“Total wastewater generated is the total volume of wastewater generated by economic activities (agriculture, forestry and fishing; mining and quarrying; manufacturing; electricity, gas, steam and air conditioning supply; and other economic activities) and households. Cooling water is excluded” (United Nations, SDGs metadata).

“Urban wastewater is domestic wastewater or the mixture of domestic wastewater with industrial wastewater and/or runoff rainwater” (United Nations, SDGs metadata).

“Urban wastewater treatment is all treatment of wastewater in Urban Wastewater Treatment Plants (UWWTP’s). UWWTP’s are usually operated by public authorities or by private companies working by order of public authorities. It includes wastewater delivered to treatment plants by trucks. UWWTP's are classified under ISIC 37 (Sewerage)” (United Nations, SDGs metadata).

“Wastewater is water which is of no further value to the purpose for which it was used because of its quality, quantity or time of occurrence. Cooling water is not considered here” (United Nations, SDGs metadata).

“Wastewater discharge is the amount of water (in m3) or substance (in kg BOD/d or comparable) added/leached to a water body (Fresh or non-fresh) from a point source” (United Nations, SDGs metadata).

“Wastewater treatment is the process to render wastewater fit to meet applicable environmental standards or other quality norms for recycling or reuse” (United Nations, SDGs metadata). Wastewater treatment can be classified into three main categories primary, secondary and tertiary. OECD/Eurostat disaggregate the flow of treated wastewater by type (e.g. urban and industrial discharges), whereas UNSD/UNEP reports the flow of wastewater treated in other treatment plants and in urban wastewater treatment plants by level of treatment (primary, secondary and tertiary). Figure 2. 66 shows both classifications and the variables required for the indicator’s calculation are highlighted in colour.

Total wastewater generated

The amount of wastewater generated is calculated by summing wastewater generated by different economic activities and households. Wastewater flows are expressed in units of 1,000 m³/day, although some data sources may use other units that require conversion.

Some countries do not separately report the volume of wastewater generated by households. In the absence of reported data on domestic wastewater generation, an estimate of the wastewater generated at the household level can be made. It can be estimated that 80% of the water supply which enters private households will subsequently exit the household as wastewater. Therefore, if data are available on per capita water consumption, these can be used to estimate domestic wastewater generation. If data on per capita water consumption are not available, data from household surveys and censuses can be used to indicate the proportion of the population which has water supplies available on premises (e.g. municipal piped water, private boreholes with overhead tanks) and the proportion of the population which collects water from off-premises sources (e.g. communal standposts, community boreholes). In the absence of other data on domestic water consumption, it can be estimated that households with on-premises water supply consume approximately 120 liters per capita per day, and therefore generate 96 liters of wastewater per capita per day; those with off-premises water supply are assumed to consume approximately 20 liters per capita per day, and therefore generate 16 liters of wastewater per capita per day.

The specific methodology for wastewater generated by households, hazardous industries, non-hazardous industries and commercial establishments can be found in the UN Water Draft step by step monitoring methodology for SDG indicator 6.3.1 on wastewater treatment³³.

The Wastewater Generation tab can be used to calculate the total volume of wastewater generated. The Wastewater Generation tab compiles information using the sector categories found in the UNSD/UNEP questionnaire on Environment Statistics (see Figure 2. 67 and Figure 2. 68).

Users will insert data on wastewater generation by sector. The tab is embedded with formulae that will automatically calculate the total volume of wastewater generated.

Total wastewater treated

The amount of wastewater safely treated is calculated by summing wastewater flows which receive treatment considered equivalent to secondary treatment or better. This wastewater flow is expressed in units of 1,000 m³/day, although some data sources may use other units that require conversion.

Domestic wastewater which enters sewage lines can be assumed to reach centralized wastewater treatment plants, unless national data is available about leakage from sewage lines. The volume of domestic wastewater estimated to reach treatment plants should be compared against the volume of wastewater reported to be received at wastewater plants, and the volume reportedly received will be taken as an upper limit to the amount of domestic wastewater which receives off-site treatment. If data are available on the proportion of wastewater flows received by centralized treatment plants which receive secondary treatment or better, this proportion can be assumed to apply equally to the flows generated by households, industries, and services which discharge into public sewers.

Domestic wastewater which enters on-site storage and treatment systems such as septic tanks can be assumed to be safely treated if national data on compliance of on-site wastewater treatment systems to relevant standards are available. In the absence of such data, half of the wastewater discharged into on-site storage and treatment systems should be considered to receive safe treatment.

The specific methodology for wastewater treated (generated by households, hazardous industries, non-hazardous industries and commercial establishments) can be found in the UN Water Draft step by step monitoring methodology for SDG indicator 6.3.1 on wastewater treatment³⁴.

Given the data limitations, especially on non-household wastewater, data currently available on compliance with discharge permits could be used to better estimate the industrial flows treated.

The Wastewater Treatment tab can be used to calculate the total volume of wastewater treated. The Wastewater Treatment tab compiles information using the categories found in the UNSD/UNEP questionnaire on Environment Statistics (see Figure 2. 69).

Users will insert data on wastewater treated by type of plans and treatment. The tab is embedded with formulae that will automatically calculate the total volume of wastewater generated. The tab has also the possibility to enter information about the volume of wastewater not treated and the quantity of sewage sludge produced each year.

Proportion of wastewater safely treated

The proportion of domestic and industrial wastewater flows safely treated is the proportion of wastewater generated by households and by economic activities that is safely treated compared to total wastewater generated by households and economic activities. The proportion of wastewater flows which are safely treated is calculated as a ratio of the amount of wastewater safely treated to the amount of wastewater generated.

Once users enter information in wastewater generated and wastewater treated tabs, the proportion of safely treated wastewater will automatically appear in proportion of safely treated wastewater tab.

According to the SDG 6.3.1 metadata, only treatments nominally classified as secondary or better (or equivalent) are used as a proxy for safe treatment. In case, national data about the compliance of treatment to relevant effluent standards or targets are available, the volume of wastewater safely treated can be entered directly in the tab, and the proportion of wastewater safely treated will be automatically recalculated according to this information.

This tab shows data about wastewater generated, wastewater treated, and wastewater not treated.

Note: when data is inserted, some cells might turn red. When cells turn red, it is because some inserted data is not correct.

In addition to this indicator, the households wastewater safely treated tab allows to calculate the proportion of wastewater safely treated only for the households sector. Enter the required data and the proportion of households wastewater safely treated will appear automatically (Figure 2.72).

Note: when data is inserted, some cells might turn red. When cells turn red, it is because some inserted data is not correct.