How to use laser to respond to the global water crisis?

The supply of clean drinking water has always been regarded as a basic human right in many regions, but the situation in many parts of the world is very different. The third world has been struggling for clean drinking water. A new type of laser-based aluminum sanitation system can help developing countries provide clean water and save lives.

A research team led by Subhash Singh of the Institute of Optics at the University of Rochester in New York believes that solar-based water sanitation may be the key to providing sustainable clean water to the rapidly growing world population. The results of the study were published in the journal Nature Sustainability.

The World Health Organization has listed ultraviolet radiation to sun-sterilize transparent bottles and heat-sterilize opaque containers as methods to clean water over a period of time. There are also many systems on the market that use sunlight to purify water. But these systems have obvious flaws: scalability issues. A system that can disinfect a small group of people or a family may not be very useful when disinfecting an entire town or village. Coupled with the related costs of the water disinfection system, one in nine people (approximately 785 million people) cannot get clean water. Among them, 1 million people die every year due to lack of clean water.

The research team discovered several problems with the current water sanitation system, especially the problem of solar-driven interface evaporation. These include, but are not limited to: the inability to control the solar tracking limit of the interface evaporator, which means that when the angle of the sun changes, the optical density will decrease, resulting in a decrease in disinfection capacity. Another problem is the blocking effect that severely reduces equipment efficiency.

Since the current solar sterilization system uses a "bottom-up" heating system, most of the energy generated will be lost through the evaporation of water and air. Some systems solve this efficiency problem by introducing a solar thermal energy generation system at the location of the air/water interface. Since this surface is floating on the water, it cannot be accurately positioned to face the incident light, and the clogging problem will be exacerbated by contact with the core water.

In addition, there is a very important problem: the current solar purifiers can only eliminate biological pollution. There are also a large number of other pollutants, especially heavy metals, that have not been treated.

The team suggested using a surface with super water absorption and super light absorption to sanitize water with solar energy. This surface with super water absorption and super light absorption is composed of aluminum, which is easy to obtain and low in cost.

However, traditional untreated aluminum does not have the porous properties required for water absorption. In addition, its surface is reflective, so it does not absorb light. To solve this problem, the research team lasered aluminum with femtosecond laser pulses. The porous surface created by this laser treatment allows water to run at a steady speed of approximately 2 millimeters per second under the influence of gravity. This means that water can be transported to the solar absorption surface for disinfection.

Therefore, this porous surface can be installed on the floating platform at any angle to increase the incidence of solar rays. This also means that this panel is connected to the sun tracking system to track the sun's trajectory and keep the disinfection level in the best condition.

The researchers also focused on the shape of this porous surface and how it dissipates heat through water. They found that the U-shaped surface can minimize contact with the water surface, making it the most energy-efficient model currently. So far, the evaporation rate the team has recorded even exceeds the ideal equipment in a 100% efficient operating state.

The research team not only tested biologically contaminated water, but also tested water contaminated by other pollutants such as metals, household and agricultural wastes. Their research found that the density of water dropped by 4-5 orders of magnitude. Meet the WHO standard for safe drinking water.