Cleaning of kitchen exhaust air

Avoid trouble with neighbours and authorities

In canteen kitchens, the combustion of animal and vegetable oils produces a large number of complex, aggressive compounds. Due to the effect of heat during cooking and frying, these gaseous compounds are transported with the air. They are often very odour-intensive and their particle size is far too small to separate them from the exhaust airflow of the canteen kitchen using conventional air filters or separators. This task requires special processes.

Kitchen exhaust, which is simply released into the open air, often causes complaints from the neighbourhood due to the unpleasant smell. In addition, the air pollution in commercial canteen kitchens and the associated high health hazard due to aggressive compounds will call the trade association into action. How can such odour nuisances and air pollution be avoided?

To answer this question, we should take the two main tasks of commercial kitchen ventilation into account at first. These are:

  • The effective capture and extraction of vapours, gases and aerosols generated during the cooking process. CFD analysis (numerical fluid mechanics) and measurements with flame ionization detectors (FID) and particle measuring devices can show that conventional kitchen exhaust systems often fail at this task and that there is no really efficient capture and extraction of the vapours, gases and aerosols released during cooking. The following video tutorial on YouTube explains in detail how such compounds can really be captured and extracted efficiently: 
  • After effective capture and extraction have been ensured, the pollutants in the kitchen exhaust air must be separated from the airflow. These are small liquid droplets, i.e. aerosols, on the one hand, and evaporated liquids, which are present in the exhaust airflow in gas form, on the other hand. In addition to this, there is the odorous matter mentioned above. Evaporated liquids can be separated from the airflow by condensation, aerosols by separation due to their mass inertia. We also show the scientific background in a video tutorial on YouTube:  

The aggressive and odour-intensive gas compounds pose a major challenge, however.

The usual methods for eliminating odorous compounds from the exhaust air of canteen kitchens are absorption by activated carbon and oxidation by means of UV rays and the ozone produced thereby. Relatively unknown is the oxidation by potassium permanganate and zeolite volcanic rock. Let us look at these methods one by one:

Activated carbon has proven its worth in the chemical and food industries and in the filtering of adhesive- and solvent-containing vapours in other branches of industry as well as in the filtering of fuel vapours at filling stations. In simple terms, the highly porous activated carbon acts like a molecular sieve and thus, also filters long-chain carbon compounds, such as solvent vapours, from the exhaust airflow. The pores of the activated carbon through which the exhaust air flows are in a size range from around 0.1 to 50 nm. As long as relatively dry exhaust air, contaminated with solvent vapours, for instance, flows through these microscopically small pores, the long-chain carbon compounds can be filtered out of the exhaust airflow. However, activated carbon is not suitable for cleaning kitchen exhaust air. In commercial canteen kitchens, there is usually a very high level of humidity in the exhaust airflow. In addition, if the aerosol separation is not efficient, there is also a high aerosol load. All these particles occurring in the exhaust air have an extremely negative effect on the microscopically small pores of the activated carbon and lead to very early saturation. Furthermore, activated carbon is combustible and can only be used at an ambient temperature of up to approximately 40 °C. At higher temperatures, there is a risk that the activated carbon will release filtered particles back into the exhaust airflow due to molecular vibrations. Moreover, activated carbon does not kill germs, it becomes sticky at high humidity and starts to release even unpleasant odours when saturated. Beyond that, activated carbon filters are classified as hazardous waste and disposal costs are high. 

The well-known disinfectant potassium permanganate (KMnO4) is an alternative to activated carbon.  In contrast to activated carbon, potassium permanganate has a germicidal effect and inhibits the growth of many bacteria. Odour molecules are effectively degraded by the chemical reaction with the active substance, remaining residual molecules are trapped by the carrier material (zeolite volcanic rock). A large part of the odorous compounds is therefore chemically oxidized and not just filtered out as in an activated carbon system. The degradation rate of odorous substances in this high-performance granulate can be up to 90 %. This property makes potassium permanganate interesting for indoor air cleaners working in recirculation mode – as well as for recirculation kitchen hoods, where the cleaned exhaust air flows back into the kitchen.

recirculation kitchen hood
recirculating kitchen exhaust hood with odor reducing filter system

Due to its numerous advantages, KMnO4 has also been used for decades for the initial disinfection of drinking water. The process has also proven its worth for exhaust air purification in the food industry. Potassium permanganate is a low-cost product and can easily be disposed of with household waste when it is depleted. A welcome side effect: With increasing oxidation, the granules change colour from violet to dark brown. Therefore, a sight glass is sufficient for status monitoring. With activated carbon, on the other hand, saturation only becomes noticeable when the carbon begins to release odorous substances into the exhaust airflow again or when the pores are blocked and no more air can flow through the system. If you use potassium permanganate beyond its service life, however, this is not dramatic: The carrier material then acts as a simple mechanical filter. The potassium permanganate is contained in filter cartridges housed in a multi-cartridge housing. This filter module is inserted into the outgoing air duct and can be easily retrofitted.

kitchen exhaust odor cleaning with potassium permanganate in exhaust duct line
kitchen exhaust odor cleaning with potassium permanganate

For some years now, UV and ozone systems have also been promoted for the elimination of organic odour molecules. They use ozone gas, which allegedly oxidizes greasy aerosols to white ash and water by “cold combustion”. These odour eliminators are often also offered as plasma systems. Scientific studies, like that of the American Air & Waste Management Association, show, however, that the process is not very effective and does not eliminate separated aerosols in any way. The study investigated the effectiveness of these systems, also using sophisticated measuring techniques such as mass spectrometry, and examined to what extent ozone can really decompose airborne oil aerosols. In this context, it was also checked whether ozone can crack the long-chain carbon compounds. The US study concludes, among other things, that environmental pollution caused by the released ozone is much more likely than significant cracking of long-chain compounds.

kitchen hood with UV system direct behind separators
kitchen hood with UV system direct behind separators

Solutions with UV systems also require complex health protection and safety measures, since even a low ozone concentration can cause considerable damage to health. Therefore, indoor and outgoing air must be ozone-free. Until now, a limit value of 0.2 mg/m³ indoor air was valid for interior rooms. This WEL was recently reduced to zero because ozone gases are suspected of causing cancer. For this reason, the kitchen exhaust air must be checked for ozone traces and the result must be recorded during the acceptance and commissioning of a UV system or an ozone generator. The measuring technology required for this has been available for decades, but very few manufacturers of UV and ozone systems for odour elimination in commercial kitchen exhaust air really use it. 

It is also important that plastic components in the exhaust air system that come into contact with ozone are UV-resistant. Otherwise, ozone exposure renders them brittle within a few months.

Another disadvantage of UV systems is that the exhaust air needs to be pre-cleaned. If this preliminary purification is not provided for, the sensitive UV tubes become greasy within a short time and must be removed and cleaned weekly. Otherwise, they become completely ineffective as they cannot produce ozone due to the contamination of the tube glass. This circumstance is often concealed in advertising.

The advantage of the UV solution is that the tubes are space-saving and can be quickly installed in the hood. This makes planning, construction, retrofitting and thus sales easy. Two clamping springs per tube and the power connection are sufficient. In addition, the air resistance is lower than with other variants. All things considered, one can say that the UV solution is a simple technique for small budgets and less demanding projects.

However, the high health hazard by UV radiation and ozone is all too often concealed and unfortunately only very rarely discussed. The operators of commercial canteen kitchens with ozone-generating UV systems and the kitchen staff are often not informed in any way about the risks or sensitised to them. In this case, the only thing that separates the kitchen staff from the very dangerous UV radiation and the ozone gases are the separators of the kitchen hoods and the exhaust airflows through these separators. 

When you inform the kitchen staff about the function of these bluish UV tubes and explain that the radiation emitted by the tubes is hardly inferior to X-rays in terms of hazard potential and is so rich in energy that it can generate ozone gas from oxygen atoms, you often look into bewildered faces. If you also explain that this gas, which smells like “swimming pool and chlorine”, is suspected of being carcinogenic, the kitchen operators, as well as the employees of the designing offices, are horrified and they rightly criticize that the manufacturers did not properly inform them about these potential hazards!

exhaust air cleaning in exhaust duct with UV system
exhaust air cleaning in exhaust duct with UV system

Therefore, we recommend using these systems, if at all, only for the reduction of odour pollution! If this is the case, the UV systems should not be fitted into the kitchen hood. The proper place for installation is the exhaust duct and they should only be fitted there. The relatively new DIN EN 16282 devotes an entire chapter to this topic. Part 8 with the title “Installations for treatment of aerosol” deals exclusively with such systems!

What role do electrostatic air filters play? In the process industry, especially in machine tool construction for machining processes, where non-water-soluble coolants and lubricants are used, they represent a proven and widespread technology for exhaust air purification. However, they are not suitable for the removal of organic odorous substances at high humidity because ozone is also involved here: Electrostatic precipitators only provide for odour elimination if they generate large amounts of ozone. They then work like an ozone generator. In addition to the aforementioned safety measures, a high degree of pre-separation of large grease particles is also required, as otherwise the sensitive ionisation wires and collector plates, which are under high voltage, quickly become contaminated and inefficient. Electrostatic precipitators are not recommended for high humidity, as is often the case with kitchen exhaust air because there is a risk of voltage flashovers. Reven tests this possible solution beforehand with a laboratory test device at the user’s premises.

Conclusion: There is no all-round solution for the effective elimination of odours. Of all sorbents, potassium permanganate is the most effective and easiest to handle. In some cases, a quite sophisticated periphery might be required for this solution, however. According to Reven, the elimination of gaseous impurities will become more and more important and is currently the subject of research. In this connection, researchers also experiment with air washers integrated into ventilation hoods.

kitchen exhaust odor cleaning with water wash spray system in exhaust duct line
kitchen exhaust odor cleaning with water wash spray system in exhaust duct line