Author: Jiyoon Ahn Edited by: Inês Barreiros
Many of us take clean drinking water for granted. In the UK, and in many developed countries, public water treatment plants have been in use since the early 19th century. Modern water treatment process is often a complex procedure involving chlorination, coagulation, sedimentation, filtration and disinfection. As a consequence, much of the developing world still suffers from the challenges of providing water that is safe for human consumption. In fact, the WHO estimates that 1 in 3 people lack safe drinking water at home. Each year, about 1.8 million deaths are caused by diarrhoea, a disease often directly related to water and sanitation, with over 90% of them among children under five. Diarrheal diseases caused by drinking contaminated water include cholera, Guinea worm disease, typhoid and dysentery. In addition, millions of people are in danger from arsenic poisoning, which could cause skin lesions and result in skin, lungs, bladder and kidney cancers. Long-term exposure to heavy metals and inorganic compounds in contaminated water also adversely affect human health. To address water contamination in developing countries, the United Nations has set clean water as one of its Sustainable Development Goals: ”to ensure the availability and sustainable management of water and sanitation for all”. To this end, many companies and non-governmental organisations are working to provide clean water through various methods. Below you can find out about a few of these innovative methods.
Use of sunlight for water purification
Solar water disinfection (SoDis) is a simple yet effective method and is suitable for use in developing countries due to its low cost. Exposure to sunlight inactivates and kills bacteria in contaminated water. The simplest SoDis involves filling a plastic bottle with water and exposing it to sunlight for several hours. SoDis can be applied at the household level and has been recommended by the World Health Organisation as a viable method for water treatment.
SoDis in a plastic bottle utilises UV rays from the sun, which only make up 4% of the sun’s total energy. Researchers at Stanford University have created a tiny nano-structured device that can disinfect water much faster by also utilising the visible spectrum of sunlight. The device is covered in molybdenum disulphide, which reacts with sunlight and water to form hydrogen peroxide and other reactive oxygen species (ROS), which inactivate pathogens including bacteria and viruses. The chemicals then dissipate, leaving clean water behind. Although initial lab tests look promising, more research is required to determine which strains of bacteria can be eliminated with this device.
Another way to utilise sunlight for water purification is by using a solar still. Solar stills have been around for millennia and their work of principle is simple. A solar still uses the heat from the sun to evaporate, cool and then collect water. Most pollutants do not evaporate, so they are left behind. However, much of the solar energy is usually wasted in the slow heating of water. To improve solar stills, efficient light-absorbing nanomaterials have been tested, but these are expensive and unsuitable for widespread use in developing countries where the technology is required the most. Researchers at the State University of New York have recently published their study about cheaper alternatives using carbon-coated paper. Their solar stills cost roughly $1.6/square metre which can purify 1 litre of water per hour, and are 88% efficient at utilising energy from sunlight into water evaporation. This new technology, however, is yet to be commercialised and tested in the field.
Filtration is a simple and attractive method for water purification. Most filtration methods remove contaminating microorganisms by size exclusion. One method is the use of ceramic filters. They are made from clay, which can be readily accessed worldwide, and can remove bacteria and protozoa. In addition, silver is applied to the filters, which acts as a biocide to inactivate bacteria and viruses. The silver can also reduce the growth of bacteria on the filter itself. In 2007, UNICEF and the Water and Sanitation Program worked together to provide Cambodia with ceramic water filters as part of a pilot project. The results from the project showed that the use of ceramic filters resulted in a 99.99% reduction in E. coli in treated water and a 50% drop in diarrhoeal diseases in Cambodia. Following other successful pilot studies, 24,000 filters have been produced every year since 2007. It should be noted that an important part of this project was the establishment of a distribution network through schools, communities and local businesses.
Nanotechnology could also play an important role in water filtration. Filters made from carbon nanotubes, which have smaller pores than conventional filters, can remove not just bacteria, but even arsenic and other toxic elements. Although their pores are significantly smaller, carbon nanotubes have been shown to have an equal or a faster flow rate as compared to larger pores. The idea to use graphene – a one-atom thick form of carbon – for water purification has been around for a few years.
Another way to filter water is through membranes, as is done for converting seawater to drinkable water. The process works through desalination. A common method of desalination is reverse osmosis, which uses membranes to filter out the salt. However, since salt molecules are much smaller than bacteria, desalination requires extremely high pressure, and hence energy, to force the water through the membrane. This makes desalination too expensive for developing countries. A graphene system would require much lower pressure since the filter would only be one-atom thick. The size of the pores in the graphene filter need to be precisely controlled so that they are small enough to block salt molecules, but not too small to also block water molecules. Recently, graphene oxide membranes made by a group at the University of Manchester were shown to filter out 97% of salt molecules. Hence, these membranes have the potential to revolutionise water filtration in developing countries.
In some countries rainwater and fog harvesting are used as a source of fresh water. Water-harvesting technologies are relatively simple to set up and operate. However, they are limited to high-moisture conditions and can be unreliable. To address this issue, researchers at MIT and UC Berkeley came up with a device that can harvest water from the air, powered by sunlight alone. This device uses a new Metal-Organic Framework (MOF) material which can capture large amounts of water in its pores. The material provides a huge surface area for bonding with water at night, and then during the day, sunlight provides energy to evaporate the water which is then collected by a condenser within the device. Their study showed that the device was capable of harvesting 2.8 litres of water per kilogram of MOF. The size of the device means that it could be used at a household level to reliably provide clean water to families. However, this technology is yet to be mass-produced and tested in appropriate communities.
As outlined in this article, many innovative methods and devices are being developed to address the issue of water sanitation. Some of these are based on ancient water purification methods (e.g. SoDis, filtration), but are being made more efficient by incorporating material science or nanotechnology. With any type of clean water technology, it is important to block other routes of transmission, for example by washing hands. Therefore, effective water management not only requires innovative technology, but also the participation of a range of stakeholders to address the issue through policy changes, and to support and educate local communities.