top of page

Water Purification Technology

This report was last updated in November 2020. It may no longer be accurate, both with respect to the evidence it presents and our assessment of the evidence. We may revise this report in the future, depending on our research capacity and research priorities. Questions and comments are welcome.


Offsets based on water purification technology rely on the assumption that the technology replaces the practice of boiling for purification. In general, all parties would agree that households receiving the filters are not currently boiling water nor do they plan to. However, most water purification offsets are granted on the theory of “suppressed demand,” which is based on the concept that households have a right to clean water, and if they were wealthy enough, they would have boiled the water in the absence of the new technology. Although endorsed by the UN’s Clean Development Mechanism (CDM), we don’t find this logic tying water purification to decreased emissions convincing, and therefore we do not recommend any offsets tied to water purification.

Offsets based on water purification are unlikely to lead to avoided emissions, and therefore we do not recommend purchasing them.

Water purification as a carbon offset

Water purification offsets rely on the fact that purifying water with purification devices like chlorine tablets requires less carbon than boiling water would. The key assumption is that providing alternatives to boiling will cause households currently boiling water to switch to less carbon-intensive water purification methods. No project first verifies that households are actually boiling water, to begin with.


There is little evidence that providing a water filtration alternative to boiling water to households has a causal impact on greenhouse gas emissions. In order to certify emissions from water purification, project implementers do not have to show that households would have been boiling water in absence of the new technology. In fact, boiling water for purification is extremely rare throughout the world, as it is very expensive to do so.

Instead, offset certifiers justify water purification offsets through the concept of “suppressed demand,” which has been endorsed by the UN’s Clean Development Mechanism (CDM) as a valid way to structure offsets. The logic behind this concept is as follows: poor people have a right to certain basic necessities, such as clean water. But in some cases, they are not accessing these necessities due to poverty. If they were not poor, they might use carbon-intensive approaches to achieve these necessities. By providing a carbon-neutral technology to provide these same necessities, we are providing human rights in a way that previously would only have been possible through emitting carbon. In other words, in the absence of the water purification technology and had they been wealthy enough to achieve their basic rights, households would have burned fuel to boil water.

This logic is problematic for a number of reasons. First, it clearly does not require any emissions to be avoided by providing the technology. This is highly problematic for an offset. Second, there is no evidence that households would indeed boil water if they had the resources to do so. There are no examples we have seen of countries in which boiling water has become a common purification practice as the country has developed. Instead, other systems (such as UV purification) are much more common.

The logic behind water purification offsets (specifically the “suppressed demand” assumption) has been challenged frequently over the years. For instance, this SSIR article called out the absurdity of the situation, and spurred some extremely interesting and heated discussion in the comments, including by the former CEO of Gold Standard. Additionally, experts agree that funding water purifiers is unlikely to decrease emissions. As described this assessment of expert opinions, the concept of suppressed demand is “considered ‘fiction’ in terms of baseline measurement” and “contradicts the goal of carbon credits that aims to reduce emissions”.

Project-level additionality

Water purification projects tend to give away purification technology, and therefore rely on donor funding to operate. If projects are funded primarily by offsets, they would likely satisfy project-level additionality. For projects that sell purification technology, additionality would be less clear.

Marginal additionality and Permanence

Since purification technology is low-cost and modular, it is likely that in a well-run project, additional offset sales would lead to additional purification technology being distributed. Therefore, they could satisfy marginal additionality.


Water purification offsets may have the co-benefit of providing households with a means to purify their water, thus improving health. This co-benefit is most likely to occur when households are not currently boiling their water, and so is unlikely to exist if the offsets work properly. If households do boil their water, however (which we believe to be an untenable overall assumption), then these offsets may have the co-benefit of limiting exposure to indoor air pollution depending on the fuel source used by households to boil their water.

Assessment of water purification projects

Overall, it is clear that purchasing offsets for water purification do not avoid emissions whatsoever, and therefore we do not recommend purchasing them.



Summers, S. K., Rainey, R., Kaur, M., & Graham, J. P. (2015). CO2 and H2O: Understanding Different Stakeholder Perspectives on the Use of Carbon Credits to Finance Household Water Treatment Projects. PloS one, 10(4), e0122894.

Starr, K. (2011). Thirty million dollars, a little bit of carbon, and a lot of hot air. Stanford Social Innovation Review Blog.

bottom of page