Water Disinfection Challenges and Solutions in Aerospace Applications
Mitch Hansen - December 8, 2016
When boarding a flight, it is hard to miss the lack of drinking water dispenser. This is not through lack of on-board water supply, because commercial planes have a water tank. However, these tanks and pipework are vulnerable to contamination. The possibility of tank/pipework contamination forces airlines to provide expensive (weight and volume) bottled water to their passengers and flight crews. The ability to dispense purified drinking water would clearly save airlines time and money. In the past, a few techniques were tested to purify the water but they were found to be difficult to maintain, expensive, or too fragile to withstand multiple takeoffs and landings. An exciting new technology, utilizing the latest development in LEDs (Light Emitting Diode), might play a key role in a robust solution.
Commercial flights in the United States are held to water regulations established by the US Environmental Protection Agency (USEPA). In 2009 the USEPA published a water quality study on domestic flights within the US and Canada. The first round of the study was completed in 2004 then repeated in 2006. The results were alarming. Nearly 15% of all water tanks tested were positive for Coliform.
Across the world, airlines and government agencies have limited the use of on-board water tanks. In some countries, this water can only be used for lavatories. It is common to see signs in airplane lavatories warning passengers to avoid drinking faucet or galley water. Because of the possibility of contamination in these tanks, governments err on the side of caution and prevent the galley water to be consumed. Although the on-board water may have originated at a reliable water source, it passes through several phases before reaching its destination.
The chart below illustrates the many potential areas for pathogens to contaminate the water.
Source: EPA Aircraft Drinking Water Rules Part 1, June 2011 Issues with water tank
The USEPA found water tanks in commercial flights had consistent bacteria and debris issues. The study concluded that these tanks have residual bacteria and sediment even after the system is flushed with disinfection chemicals. Once a small amount of bacteria is present in the system it can easily grow and spread. When a system is flushed it is common for small pathogen-harboring particulates and sediment to remain.
Carbon or ceramic filters could be used to remove the sediment; however, these filters cannot remove micro organic material allowing bacteria to remain in the system. Bacteria is commonly treated with chemicals, such as chlorine, that leave byproducts in the water to be consumed by unsuspecting customers. Chlorine levels can be difficult to maintain and this is especially true for mobile systems with limited power, space, and personnel. Chlorine treatment also requires periodic testing of the water to ensure the proper level of chlorine is maintained in the water. Further problems exist with the formation of disinfection byproducts from chemical treatment and some chlorine resistant pathogens (e.g. cryptosporidium).
Ultraviolet (UV) systems paired with filters are common alternatives when chemicals are no longer an option. When combined, the filter removes sediment and while the UV light irradiates the bacteria’s DNA making the bacteria incapable of reproducing. However, conventional UV systems utilize fragile, mercury-vapor lamps which are particularly susceptible to breaking in mobile applications. If a lamp was to break during flight, mercury could be absorbed into the water supply. Mercury UV lamps also run on higher voltages and have limited on-off cycles, meaning operational constraints.
"LEDs have been a defining factor in many technological innovations, and water treatment is no exception."
UV-C LED solutions
Recently developed LEDs that produce deep ultraviolet light, affect bacteria in the same manner as conventional UV lamps, but offer all the typical benefits of semiconductor technology. UV-C LEDs provide the option for a versatile reactor design that caters to space and weight constraints of airlines. Additionally, LEDs are mercury-free and rugged enough to survive the shock specifications, without any risk of mercury contamination. The small size and instant on/off capabilities mean this technology can be placed near the water dispensing point, such as a sink or galley faucet, and switched on only when water is flowing. Disinfecting the water right before it reaches a user, blocks contamination coming from the piping and diminishes the need for residual disinfection chemicals. This new UV-C LED technology gives a new alternative to an on-going water problem the airline industry has been dealing with for decades.
A UV-C LED system is being developed under multiple space programs, including two for the International Space Station. The current ISS’s water system is only 88% efficient and uses filtration and chemical technologies. A 2010 study completed by NASA shows bacteria can still grow inside the current water treatment system and recommended an alternative be developed.
The Bio-contamination Integrated Control of Wet Systems for Space Exploration (BIOWYSE) Project was commissioned by the Horizon 2020 program to address the efficiency and disinfection issues within the ISS water treatment system. BIOWYSE Project is employing UV-C LED technology for water disinfection. This work is showing that UV-C LED systems can be implemented into aerospace applications.