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A smart toilet design introduced in rural Cambodia was supposed to change lives—keeping families safe and protecting the environment.

However, while households reported that they liked the new system, a crucial piece was missing: using it correctly.

James Harper, an assistant teaching professor in the��Paul M. Rady Department of Mechanical Engineering at �鶹��Ѱ�����Boulder,��. The work, conducted through Harper’s own research consultancy�� in partnership with international development organization��, assessed rural households using a backyard sanitation system designed for families to treat human waste themselves.

“Here in the States, we don’t need to think about clean water coming from our tap or having a toilet that flushes, but there are plenty of places around the world that do,” said Harper. “Cambodia is a country of about 17 million people where sanitation systems are lacking. There have been a lot of innovations to help bring new infrastructure and practices to rural areas of the country, and this study examined one of those innovations from a behavioral perspective.”

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A man digging a pit for one of the alternating dual-pit latrines (ADP) in rural Cambodia.

An easy, effective sanitation solution

iDE partnered with local business owners to promote, sell, and install alternating dual-pit latrines (ADP) across six Cambodian provinces. These pour-flush toilet systems are most commonly used in off-grid locations and are an adaptation of single-pit latrines like those commonly found in U.S. national parks.

Since 2009, iDE has facilitated the implementation of over 410,000 basic, pour-flush latrines, representing nearly one in five toilets in all of rural Cambodia.

The ADP is essentially a new second pit and is sold as an upgrade to an existing single-pit latrine that is currently full of waste. When one pit fills—usually after two to five years of regular use by a household of five—users can switch to using the empty second pit.��

When the initial installation is performed, the service provider mixes hydrated lime, a common soil conditioner used globally in agriculture, into the first pit to speed up the process of treating the waste and kill pathogens that endanger humans.The toilet is then connected to the new second pit.��

After the second pit fills, users are encouraged to call their original service provider or they can empty the first pit safely using shovels and buckets, and then reconnect their toilet back to the first pit. The lime mixture and storage time remove most pathogens after two years, making the treated waste easy and safer to compost, allowing the cycle to repeat indefinitely.

It’s not the most glamorous engineering project, but Harper says ADPs are extremely effective at killing off dangerous pathogens, especially in areas with limited infrastructure. The waste they produce can be used, somewhat surprisingly, for agricultural purposes, as well.��

Human waste is replete with nutrients that plants love, so as long as pathogens are removed, human waste can be a great fertilizer.

“Lime is a cheap agricultural amendment that’s commonly used around the world to change the pH of soil to improve crop production,” Harper said. “We use it to raise the pH in a full pit to kill pathogens that endanger humans. Then, that human waste can be used as a fertilizer to make crops grow better.”

The interesting, important results

From a theoretical standpoint, ADP latrine implementation in Cambodia was sound. It was safe, cost-effective and came with a number of potential benefits for the area.��

However, the team recognized early that long-term use of the system depended on the engagement and behavior of the households themselves.��

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A man putting the finishing touches on an ADP in rural Cambodia.

After multiple training sessions and allowing families to manage their ADPs themselves, iDE and Harper surveyed 765 households to see how well people were using the toilets and following the recommended guidelines, particularly those about waiting 2 years before emptying and switching their pit connections. Despite a general understanding of the required steps and expectations, they found that follow-through among households was inconsistent.

According to the survey, only 40% of people could recall certain details of recommended ADP maintenance, including how long before emptying a pit can be done safely. In fact, not one household in the entire study waited the recommended two-year period before emptying, potentially exposing themselves and others in their community to health risks via pathogen exposure.

Harper says there were clear motivations for the results. Some families opted to empty pits early before hosting guests during cultural events. Others simply reconnected their old pits to their toilets without emptying the dried, compacted waste. But overall, households just did not want to manage their own waste—for obvious reasons.

“Ultimately, what the study highlighted was the difference between theory and practice,” said Harper. “We learned that households can’t—and actually shouldn’t—be relied upon to completely manage their own waste, and that’s a reasonable finding. We don’t manage our own waste in the States; we just flush and forget. So, why would we expect something different from people in Cambodia?��

While the ADP could be a great product, its use in the real world is different than we assumed during design, and that’s why the design must be improved going forward—to meet the need while working within the real-world constraints of our users.”

iDE will use the results of this study to pivot and experiment with other solutions. They are currently testing and scaling up a service-based approach where waste can be disposed of safely on the household premises by a trained service provider. All a household needs to do is call when they need help emptying their pits.

But Harper and his iDE colleague Tyler Kozole also hope the study can be a moment of learning that all engineers across any discipline can benefit from, too.

“We recognize that this research indicates what could be viewed as a type of failure in our work,” said Kozole, director of iDE Cambodia’s Wash, Sanitation and Hygiene (WASH) program. “We’re broadcasting this research to advocate for embracing and learning from failures. If the whole sector embraced this mindset, we’d solve problems faster, use our resources more efficiently and ultimately make the kind of impact we’re trying to see in the world.”