Water sustains life, both plant and animal, on earth. Though we could not live without water, there is a dearth of information regarding water quality at the global scale (see Box 8: Challenges of Measuring Global Water Quality). Insufficient comparable data across countries and the importance of landscape-level factors in determining water quality, among other challenges, restricts us from including a direct output measure that assesses how countries maintain water quality.

Sustainable Development Goal (SDG) 6, which “ensure[s] access to water and sanitation for all,” requires that innovations in data technology and transparency be applied to measuring and reporting water quality (see Box 9: Water Ripples Through the Sustainable Development Goals). Water targets are included in five SDGs, covering a range of water-related issues like pollution, use efficiency, disease, disaster, and wastewater treatment.[footnote 2] Indicators and data that measure progress toward these goals at a global scale, however, are elusive.

As a second-best metric, we rely on drivers of water quality, specifically the treatment of wastewater, as a key component of overall management. More than 80 percent of the world’s discharged wastewater is untreated when it is released into the environment.[footnote 3] Untreated wastewater leads to high pollution levels, eutrophication of water bodies, high coliform bacteria counts, and hypoxia and fish-kills. It is also a waste of water. The Wastewater Treatment indicator is a measure of treatment at the municipal level, weighting the results by the sewerage network’s coverage. The indicator is distinct from other, related metrics, such as JMP’s “Access to Sanitation” measures, which survey latrine access at a basic level and do not describe water quality or ecosystem health.

Wastewater is the water that has been used by households and industrial facilities that, without treatment, no longer serves a useful purpose. Graywater, blackwater, and the slurry of industrial and agricultural wastewater flows into natural water systems, if untreated, carries harmful chemicals into the environment, damaging ecosystems and threatening human health. Sound wastewater management requires a system for collection — normally through sewage pipes — and treatment at different stages, which are described below.[footnote 4] Treatment plants can be public or private utilities that serve a given municipality or community.

Wastewater treatment plants, even when properly located, often do not have the capacity to treat all the water collected. Overburden can occur when the population of a city outpaces the development of new treatment facilities, due in large part to insufficient funding.[footnote 5] Wastewater treatment facilities discharge excess wastewater directly into the waterways when they do not have room to handle it all,[footnote 6] and in some cases, treatment plants discharge effluent because they are dysfunctional.

Wastewater pollution leads to eutrophication and algal blooms, which occurs when a body of water is enriched with chemical nutrients, causing certain plant species such as algae to proliferate at the expense of others. Eutrophication can cause fish die-offs as some types of algae deplete the water of oxygen. Killing fish harms ecosystem health and also causes economic hardship for human communities that subsist on aquatic resources.[footnote 22] Untreated wastewater also leads to toxin buildup in shellfish as these filter feeding organisms accumulate chemical and biological.[footnote 23] The presence of pharmaceutical residues[footnote 24] and other chemicals in waterways has unseen harmful biological effects including faunal and human endocrine-disruption.

A host of bacterial, viral, and protozoan organisms persist in human waste and fecal matter, most notably the bacterium Escherichia coli (or E. coli),[footnote 25] which causes diarrheal diseases. This waste is often home to the bacteria Vibrio cholerae, Shigella spp., and Campylobacter spp., as well as noroviruses and rotaviruses, a cocktail of pathogens that cause terrible human diseases such as bancroftian filariasis, and worm-borne schistosomiasis.[footnote 26] Many of these problems can be ameliorated by sound wastewater treatment that reduce pathogen concentrations to levels safe for human consumption.[footnote 25]

Treatment is completed in sequential steps with differing levels of complexity depending on available resources. The typical range of treatment options includes primary, secondary, and tertiary stages.[footnote 27] Primary treatment uses basic processes including settlement tanks to remove suspended solids from water and to reduce the biochemical oxygen demand (BOD). Secondary treatment involves biological degradation that allows bacteria to decompose elements in the wastewater, further reducing nutrient levels and BOD. Tertiary treatment encompasses any process that goes beyond the previous steps and can include the use of advanced technology to remove remnant contaminants (see Wastewater Treatment Infographic). Tertiary treatment is typically employed to remove phosphorous or nitrogen content, major causes of eutrophication.[footnote 28] 

Data availability limits the Wastewater Treatment indicator to examine only the wastewater that receives “at least primary treatment” because this distinction is the only common definition for globally comparable measurement. Water and sanitation policies in many nations have in the past decade focused on wastewater treatment – more than ever before, signaling a shift toward including water quality as well as water access in performance metrics.[footnote 29] There remains, however, a pressing global need for more and better data on wastewater generation, treatment, and use.

The EPI team worked to update and improve the wastewater indicator, creating an interactive map to crowdsource feedback from experts worldwide[footnote 30]. The Water SDG (SDG-6) includes a target to improve water quality, in part by “halving the proportion of untreated wastewater” by 2030.[footnote 31][footnote 32] This international goal will encourage improved wastewater treatment and should result in better data for future monitoring. An ideal wastewater indicator for SDGs will include a distinction between primary and secondary types of wastewater treatment,[footnote 4] but this level of data is lacking for most countries, although the European Environment Agency does collect and report these data for most countries in Europe (Figure 13). EPI’s data collection efforts are an important step to provide a baseline for countries to gauge where they stand. As the world urbanizes, improving wastewater treatment is powerful investment in building healthy societies, as well as in individual health, especially in countries where infrastructure improvements struggle to keep pace with demand for services.

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