Ahead of IWA’s Conference on Rainwater Harvesting & Management on 1-6 September, Mooyoung Han highlights the next evolution of WHO rainwater guidance.
In June 2026 the World Health Organization (WHO) released the updated version of the fourth edition of its Guidelines for Drinking-water Quality, incorporating the first, second and third addenda.
For water professionals, this publication is an important milestone. Over several decades, WHO has provided the scientific foundation that has helped governments, utilities and communities protect public health through safer drinking water. Among its many contributions, WHO also deserves recognition for helping establish rainwater as a legitimate source of drinking water.
In the 2008 edition, rainwater was described as relatively free from impurities, except those acquired from the atmosphere and collection surfaces. In the 2011 fourth edition, WHO took a further step by recognising rainwater as an ‘important source of drinking-water’ and acknowledging its increasing use beyond individual households. These were important advances, made at a time when the central challenge was demonstrating that rainwater could be collected and consumed safely.
For this reason, WHO guidance appropriately focused on issues such as roof contamination, bird droppings, first flush diversion, storage tank maintenance, mosquito control and microbial risks. These remain essential elements of safe rainwater management today. Yet, the world of rainwater has changed dramatically over the last two decades. Climate change, prolonged droughts, floods, growing water insecurity and the Sustainable Development Goals (SDGs) have transformed the global water agenda, and the United Nations (UN) 2023 Water Conference further accelerated this shift through the UN Water Action Agenda, which emphasises practical actions capable of accelerating progress towards SDG 6.1.
As a result, the challenge facing rainwater practitioners today is no longer simply how to collect rainwater safely. Increasingly, it is how to operate, monitor, teach and scale safe rainwater systems in ways that contribute to climate resilience, community participation and sustainable development. In this respect, a new generation of rainwater practice is beginning to emerge.
Rain Schools
In Cambodia, for example, the Rain School Initiative is demonstrating how rainwater can become more than a water supply technology. Schools are producing drinking water from rainwater that meets national drinking water standards, while simultaneously creating opportunities for education, monitoring and community engagement. Students participate in system operation and monitoring through groups known as BiTS (Bi meaning ‘rain’ in Korean, together with Teachers and Students), while school authorities, parents, local governments and experts collaborate through SkyWater Committees to support long-term sustainability.
Digital monitoring
What distinguishes this model from many conventional rainwater systems is its digital backbone. Through the Skybot platform, students and teachers record rainfall, water quantity, water quality and operational performance using mobile devices, transforming rainwater management from a one-time engineering installation into a continuous, transparent learning process.
1000 Rain Schools
Most importantly, these systems are not isolated demonstration projects. Following a Memorandum of Understanding (MoU) signed on 4 November 2024 between the Ministry of Education, Youth and Sport, the Royal Academy of Cambodia and Seoul National University, the initiative is being expanded towards a national goal of 1000 Rain Schools.
This evolution is significant. Traditional rainwater guidance has largely focused on infrastructure. The Rain School model expands the discussion to include governance, education, digital monitoring, youth engagement and long-term operation. It is not simply about collecting rainwater – it is about building a culture of water stewardship.
The purpose of highlighting these developments is not to criticise WHO. On the contrary, Rain Schools build upon the foundation that WHO has helped to establish, and without confidence in the safety of rainwater, many of these innovations would not have been possible. However, successful implementation also creates an opportunity for international guidance to evolve. The next chapter of rainwater guidance may need to address not only how rainwater can be made safe, but also how safe rainwater systems can be managed, monitored, taught and expanded through schools, communities and public policy.
The UN Water Action Agenda encourages the global water community to move from discussion to implementation, and experiences such as Cambodia’s Rain School Initiative suggest that this transition is already underway. Perhaps the future of rainwater guidance lies not in replacing the principles of first flush and water safety, but in building upon them.
The sky continues to provide water freely to every community on Earth. WHO has helped the world understand how that water can be made safe. The next challenge is learning how safe rainwater systems can help achieve SDG 6.1, strengthen climate resilience, and inspire the next generation of water stewards.
That evolution may already be beginning – from first flush to Rain Schools.
The author:
Mooyoung Han is Professor Emeritus, Seoul National University, South Korea, and Chair of IWA’s Rainwater Harvesting and Management Specialist Group
More information
Join the conversation at RWHM2026, the 4th edition of IWA’s Conference on Rainwater Harvesting & Management which will focus on ‘Rainwater for Tomorrow – Water, Climate and Culture’. Join us on 1-6 September 2026 at the Royal Academy of Cambodia (RAC), Phnom Penh, Cambodia.
