Legionella contamination via water purification installations
Water purification installations have made use of a biological purification method based on so-called “activated sludge” since the 1950s. This involves mixing waste water with a population of specific microorganisms. These microorganisms have a cleansing effect, since they feed on organic material in the water. Subsequently, the sludge is removed before the water receives further treatment using additional purification techniques.
Alarm in Boxtel and Son
Until recently, there was no reason to believe that purification using activated sludge could involve any risk to public health, but some recent events have revealed that this is in fact the case. In the Dutch towns of Boxtel and Son, a number of people living close to a water purification installation fell victim to Legionella contamination. Subsequent research showed that in both cases the bacteria were spread by aerosols released by activated sludge from a nearby water purification plant.
Airborne transmission
This indicates that the presence of Legionella bacteria in sludge can indeed be problematic if they become airborne through the formation of aerosols. Once airborne, the bacteria can be inhaled by employees or nearby residents leading to illness and, in exceptional cases, even death. It is therefore very important to eliminate this airborne transmission of the Legionella bacteria as much as possible.
Contamination risk increases with excessive aerosol emissions
Activated sludge can only perform its cleansing function effectively if there is sufficient oxygen in the water. This is why traditional water purification installations extract oxygen from the surrounding air, adding this to the water by means of injectors. However, outside air is only 20% oxygen, making it a very inefficient process. Nitrogen, which forms the greatest part of the injected air by far (80% by volume), is not needed in the process.
Excessive emissions of aerosols occur precisely because the injected air is five times greater in volume than necessary. Not only is the process lacking in sustainability due to the large amounts of energy used to inject superfluous gas, the marked increase in aerosol production can also put public health at risk.
Greater efficiency, fewer aerosols
Air Liquide has developed an innovative new method - now in use in 16 water purification plants in the Benelux alone - based on injecting pure oxygen into the reservoir. This reduces aerosol emissions by a factor of at least 100 compared to traditional installations. Henry’s Law on the solubility of gas in a liquid helps explain the phenomenon: “At a constant temperature, the quantity of gas that can be dissolved in a liquid is directly proportional to the partial pressure of that gas in comparison to that liquid”. Gas solubility depends on multiple factors, including the solubility coefficient of the gas used, the viscosity of the liquid, temperature, duration of contact and the contact surface (small gas bubbles increase solubility).
When injecting air (including outside air), the partial pressure is 0.20 for the O₂ component. If we inject 100% pure O₂, however, the partial pressure rises to 100%. This means that using outside air leads to competition between the 20% O₂ and the 80% N₂, allowing dissolution of only 8 mg/l of O₂ (at 1 bar and a temperature of 28°C). However, using pure oxygen under the same conditions leads to a dissolution of 40 mg/l, five times greater.
Stripping effect
“The difference when using air as opposed to pure oxygen in this application is known as the ‘stripping effect’,” explains Rudy Lamond, Water Treatment Growth & Innovation Leader at Air Liquide.
“With the use of pure oxygen, 80-97% of the bubbles are absorbed by the activated sludge. When using outside air, however, the N₂ present in it will seek equilibrium with the atmosphere as quickly as possible. In doing so, the N₂ also carries O₂ bubbles to the surface of the liquid, as well as VOCs, Legionella and H₂S particles. The increased aerosol formation results in a significantly higher risk of Legionella contamination, in addition to odour and foaming issues due to the H₂S.”
“If we assume a transfer efficiency of 10%, this requires 42 kg of outside air to transfer a certain volume of O₂ to the bacteria in the activated sludge, as opposed to just 1 kg of pure oxygen. It follows that the application based on pure oxygen results in 90% optimisation of aerosol emissions.”
“Use of pure oxygen could even render expensive reservoir covers unnecessary. Unless there’s a zero tolerance policy put into effect, of course. In that case however, the cost of the extraction system will still be significantly lower than when using outside air. Moreover, pure oxygen can be converted into ozone to combat the extracted Legionella.”
Other advantages
As well as the indisputable benefits in relation to Legionella, an installation based on pure oxygen offers many other advantages. For example, energy consumption is much lower (> 14 kg O2/kWh) and the oxy-injectors can serve as dynamic mixers in addition to their use in oxygen injection. The submerged oxy-injectors also function soundlessly.
Want more info? Contact Rudy Lamond.
