Drinking water that is pathogen-free and safe for humans flows from German taps. There is no nutrient as vital to human beings as water and – at least in Germany – that is subjected to so many checks and controls. Nevertheless, access to clean drinking water is not to be taken as given. According to the latest WHO figures (2011), 768 million people do not have access to healthy drinking water, especially in some of the poorer African countries. The project “DNA-Crack” from Ulm aims to change this situation.
Bacteria can be killed by exposure to ultraviolet (UV) light. For many decades, around 50 countries have been using a method known as SODIS (solar water disinfection) for disinfecting drinking water. Contaminated surface water is poured into transparent PET bottles and exposed to the sun for at least six hours. During this time, the sun’s UVA radiation kills disease-causing pathogens. This method helps prevent diarrhoea and thereby saves lives. However, the simple procedure has its limitations; it is unsuitable for disinfecting large amounts of water and storing it for later use. Investigations undertaken by Ulm University researchers have shown that if the water is not kept in cool conditions, pathogens that survive disinfection will start to multiply again.
The Ulm researchers’ DNA cracker is a small disinfection system, equipped with a novel UVC-LED, a small ultrasound generator, a solar cell and a battery. It is able to disinfect around five litres of water per hour. Germicidal UVC light (ultraviolet light with wavelengths between 200 and 280 nm) has been used for disinfecting water for many years. Energy-rich photons are absorbed and lead to the destruction of the genetic information in the DNA of microorganisms. Irreversible DNA damage renders harmful bacteria and viruses ineffective. Michael Sift, one of the researchers from Ulm, believes that inactivating human pathogens such as E. coli as well as those that cause life-threatening diseases such as cholera, typhus and amoebic dysentery is crucial.
Mercury vapour lamps have a strong emission peak at 254 nm, which is close to the DNA absorption maximum of around 260 nm. They are therefore suitable for killing bacteria using UV light. However, they are not very environmentally friendly, have a limited lifespan and require a high voltage supply.UVC-LEDs have only recently become available as an alternative to mercury vapour lamps. UVC-LEDs have emission peaks between 240 and 280 nm. Current UVC-LEDs still have comparatively low power levels (one-digit milliwatt range) and a limited lifespan (around 1,000 hours). Based on information from his Berlin-based project partner, LED specialist EPIGAP Optronic, Prof. Dr. Martin Hessling hopes that major advances in LED technology will soon lead to high-power, long-life UVC-LEDs. The most powerful UVC-LEDs to date have a maximum wavelength of 280 nm. Relative DNA absorption is, however, lower at 280 nm than at 254 nm, which, as mentioned above, is the radiation peak of mercury lamps. On the other hand, 280 nm wavelength UVC is absorbed less strongly by water, enabling UVC light to cover a greater distance in water. Hessling and Sift believe that LED systems are particularly suitable for use in technical devices due to their small size and low operating voltage and power requirements. They also believe that they will be suitable for compact UVC-LED systems with a solar energy supply.
The biggest challenges are still to come. The DNA cracker needs to pass a practical endurance test, which means that it must be able to disinfect heavily contaminated water under extreme climatic conditions day and night. This requires a much stronger UVC-LED lamp (10 mW). In addition, the researchers will have to miniaturize the ultrasound generator and subsequently integrate it into the system. Moreover, the entire system, which combines UVC-LED and ultrasound with different voltages, must consume no more than 10 W in order to run on power generated by a pico-solar system. The DNA cracker’s solar system will be developed by fosera GmbH & Co. KG based in the city of Illerkirchberg, close to Ulm. The company run by an Ulm University of Applied Sciences graduate and experienced in Third World markets, sells 100,000 long-life solar battery LED systems per year and knows what application security is all about.
A functional DNA cracker, including a pre-filter for cleaning heavily contaminated water from puddles, waterholes or other types of surface water, will most likely be available by late 2015. However, current high LED costs prevent the system from being placed on the market, at least for the time being. Hessling and his team are well aware of this, but believe that LED technology development will make rapid progress, resulting in rapid commercialization and cost reductions. As originally planned, Hessling wants to optimize the DNA cracker for application in the field of medicine. He believes that the LED and ultrasound modules that are currently being developed are small enough for the point-of-use disinfection of surfaces in intensive care wards and dental clinics.
Sift, M.; Wagner, S.: Wasserdesinfektion: Untersuchungen zum Wirkprinzip der Solar Water Disinfection (SODIS), Master's Project Report, Ulm University of Applied Sciences, 2013 (in German only)