Knowledge base – Industrial air treatment
Microorganisms cause issues in various places. In a clinical setting, bacteria can cause dangerous outbreaks and spread of multi resistant bacteria can occur. In the food industry, microorganisms shorten the shelf life of the products. Ozone can be used as a chemical disinfectant to kill bacteria and viruses with low ozone concentrations. The contact time is altered depending on the desired deactivation grade as seen in the figure below:
For many applications, a bacteria reduction of 99.99 % (which corresponds to a 4 log reduction) is sufficient. For a higher deactivation grade, the solution is easily adapted to provide higher concentrations and exposure time, and even bacteria spores can be treated. In food production, the migration of microorganism inside the plant speed up molding but with gentle ozone treatment, the molding process is easily postponed for several days without damaging the food. Furthermore, in the mdairy industry, equipment has been sanitized using ozone which performs well and eliminates the need for more harmful chemical disinfectants.
Ozone treatment of strawberries can double their life.
In contrast to the food industry where rooms are rarely treated at all, hospitals have high cleanness requirements and already employ frequent cleaning to reduce the spread of pathogens. This is commonly done by frequent wiping of surfaces with chemical disinfectants. Although this traditional method is effective, it has been shown that traditional manual cleaning and disinfection practices are often suboptimal. Reasons for failed cleaning can be due to misunderstandings, deviations from cleaning protocol, not wiping all surfaces, incorrect dilution of cleaning agents and the cleaning equipment surface structure being ineffective on certain materials. In a hospital setting, such discrepancies increase the risk of pathogen spreading between areas and patients. Automated non-touch technologies are good additions to cleaning practices as it adds another layer of cleaning. (Boyce, 2016)
Non-touch technologies include the usage of UV-lamps and chemicals dispersed as an aerosol or gas which deactivates microorganisms. Compared to other treatment methods for air disinfection, ozone can efficiently disinfect large air volumes, neutralizing micro-organisms, including viruses. This makes it ideal for use in medical applications, for example in waiting rooms or treatment rooms in hospitals or similar. An important factor that enables savings is the time the cleaning agent can actively deactivate bacteria, as compared in the figure below:
The chart shows that the contact times vary greatly, UV cleaning systems often have a very short time window to irradiate the air and therefore needs to add a lot of energy to ensure sufficient deactivation in this short time. Wiping with a cleaning solution is limited by the time it takes for the surface to dry while ozone will continue to attack bacteria until it naturally decomposes or is removed. This enables ozone solutions to increase energy saving significantly.
Ozone is produced on site when needed, thus eliminating extra resources required for handling and storing hazardous chemicals. Furthermore, no chemical residue remains after the treatment, as ozone naturally decomposes to oxygen, compared to e.g. disinfection with chloride. In terms of usage, distribution as a gas enables ozone to reach hard to sanitize surfaces, reaching spots that could be difficult to wipe. Studies have shown that low ozone concentrations are capable of deactivating bacteria and enveloped viruses by reacting with unsaturated fatty acids in the plasma membrane, certain surface proteins and DNA. However, for dealing with stable viruses and bacteria spores, higher concentrations of ozone together with air moisture is required over several hours.
In a clinical setting, ozone generators can reduce the risk of pathogens spreading when new patients enter previously occupied rooms. It can also be used after traditional cleaning to reduce the risk of diseases spreading from infectious body fluids left by the previous patient.
For clinics that are closed during the night, ozone treatment can be scheduled in waiting rooms, corridors and bathrooms giving plenty of time for treatment and ozone destruction before the first employee enters in the morning.
For surgery rooms or other areas that need a 6 log scale deactivation of bacteria, viruses and even spores, specialized solutions using high concentrations of ozone in a sealed room over several hours is necessary. The sanitation process can be started from a panel outside the room, and information regarding remaining time is displayed. Once the set time is over, the ozone is destroyed with an ozone destructor.
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Odorous compounds such as VOCs (Volatile Organic Compounds), H2S and Volatile Fatty Acids (VFAs), can be found in kitchen exhaust and a wide variety of different industries. Although the odors do not necessarily impose physical health problems, malodors disturb the surrounding, resulting in complaints towards the source.
The sources of H2S emission are similar to the ones of VOCs, with the main difference of the presence of sulfur compounds in the source substrate. For example, the organic material in food industries and wastewater treatment plants contains a significant concentration of sulfur compounds, leading to H2S emissions. H2S is also common in breweries and biogas production plants, due to the anaerobic reduction of sulfur compounds in the bioreactors or digesters.
The human nose and odor perception
The human nose has been evolving over the millennials developing different sensitivities for various compounds. For instance, the nose has a very high sensitivity of compounds emitted from decomposing organic matter. This way, the ingestion of rotting food could be avoided during the course of history, preventing the associated diseases. Therefore, it is not surprising that the human nose can detect trace concentrations of these compounds as shown in the table below. On the other hand, the nose is less sensitive to compounds not emitted naturally such as toluene.
Examples of odorous compounds, character and odor threshold in ppm
|Compound||Character||Odor Threshold [ppm]|
|Methyl mercaptan||Decayed cabbage, garlic||0,002|
|Hexyl acetate||Fruity green apple or banana sweet||0,12|
As shown in the table above, the concentrations at which odors are a problem is in the order of magnitude of parts-per-million (ppm), and in some cases even lower. Therefore, odor emissions are particularly difficult to treat, as the purification system needs to have a very high performance. Furthermore, when the odor is emitted in an open space, the issue is more complex, since the odor values are affected by external agents, e.g. atmospheric conditions such as humidity and wind.
We offer many types of air analyses aimed at identifying the odor concentrations. For more information, contact us today!
The term VOC (Volatile Organic Compounds) refers to a broad range of various chemical compounds. All VOCs are organic chemicals (i.e. containing carbon) with a high vapor pressure at room temperature. This group of compounds is divided into categories such as aromatics, aldehydes or hydrocarbons. Each category has different chemical properties, possibly leading to various health and environmental issues.
VOCs are emitted by various sources throughout the European region. The most common ones are the “industrial processes” and the “commercial, institutional and household fuel consumptions”. Only those two sources combined emit two thirds of the total emissions in Europe, so they will, therefore, be the focus in this text.
The emission of VOC in the atmosphere causes environmental issues. Some VOCs may cause odor problems due to their high odor intensity. Such cases are often encountered in food processing industries and wastewater treatment plants. Toxicity is another issue related to VOCs since some VOCs can be directly toxic to humans and animals. Even if the use of the most toxic compounds have been strictly limited, the emissions of high concentrations may contribute to a toxic environment. For this reason, regulations are in place to reduce this hazard. A typical example is the solvent emission from industries such as the pharmaceutical, textile, paint and coating industries.
Examples of VOC often found in industrial and commercial applications are presented below.
Acetaldehyde belongs to the category of aldehydes. It occurs naturally in coffee, bread, and ripe fruit.
Acetaldehyde is one of the most important air pollutants to treat because it is toxic, irritating and carcinogenic. Acetaldehyde emissions may also cause odor problems, especially from commercial kitchens, and food processing industries.
Benzene is one of the most well-known aromatic compounds. It has been widely used as a solvent and it is currently an important intermediate in the chemical industry.
Due to the known carcinogenicity, the use of benzene as a solvent has been widely replaced. The industrial emissions from industries such as petrochemical and chemical production need to be carefully monitored.
Acetone belongs to the category of ketones. It is widely used as a solvent in many industrial processes.
Skatol är kopplat till kategorin “aromatiska föreningar”. Bildas vid naturlig nedbrytning av proteiner. Kan användas som parfym vid låga koncentrationer.
Xylenes belong to the category of aromatics. They are important chemical intermediates used in the production of plastic PET bottles.
Air emission of xylenes is often associated with odor problems , due to the low odor threshold. They are also toxic if emitted at high concentrations. Xylenes are emitted by biogas production plants and in many chemical industries.
Limonene is one of the most common terpenes. It is the major component in oil of citrus fruit peels, so it takes its name from the word “Lemon”.
We are specialists in analyzing the type and concentration of VOC in every seperate case. Read more here.
Directives and regulations on VOC emission
Due to the environmental issues caused by VOC, several directives and regulations has been enforced, both at the European and national level. Considering the European directives, two important milestones were set by the 2001/81/EC and the 2016/2284. The first directive sets national emissions ceilings for the VOC emission from all sources to be reached by the year 2010. The second directive specifies the percentage reduction for the VOC emissions, both for the individual countries and for the whole EU-area. Since 2001, the EU has been very active for reducing the VOC emissions, setting high reduction goals as shown in the Figure below.
The 2016/2284 EU directive was transposed in Sweden by the Swedish parliament with the regulation SFS 2017:418.
VOC removal for specific industries
Besides the directives regarding the overall VOC reduction, specific actions were taken to limit the emissions from certain industries and sectors. One example is the directive 2010/75/EU regulating combustion plants, incineration plants, production of titanium dioxide and use of organic solvents. For this last sector, many limits for the emission concentrations in waste gases were defined, according to the type and size of the industry using solvents. Most of these limits vary between 20 and 100 mg organic carbon/Nm3. As for all the EU directives, the 2010/75/EU was transposed in Sweden with the regulations SFS 2013:254 and SFS 2014:20.
The process of biogas production is a series of complex stages aimed at producing methane from waste. Traditional biogas plants can use different types of waste as feedstock, e.g. biomass. Since the feedstock is renewable, biogas plants produce green energy, contributing to a circular economy. According to the World Bioenergy Association, the sector of biogas production is one of the fastest growing among all biofuel sectors. The average growth of biogas production was 11.2 % in 2014, reaching an overall production of 58.7 billion Nm3. Almost half of this amount was produced in Europe where approximately 17000 biogas plants are in operation. Germany is the leading country in terms of the number of biogas plants, with a total of 11000 installations, followed by Italy with 1600 and France with 800.
Sulfur – A Common Issue
In each stage of the biogas production process, sulfur compounds and ions are commonly present. These compounds are often responsible for a variety of issues affecting the overall performance. For instance, high amounts of sulfur ions in the feedstock enhance the activity of sulfur reducing bacteria (SRB) in the digester, inhibiting the activity of methane-producing microorganisms such as archaea. As a result, the yield of methane production is reduced while the production of reduced-sulfur compounds, such as H2S, is favored. When high concentrations of H2S are released from the digester, the process may suffer from odor and corrosion problems. In particular, odor problems are common in biogas plants, since H2S has one of the lowest known odor thresholds, making the human nose very sensitive even to trace concentrations.
Prior to the anaerobic digestion stage, a pretreatment step is often implemented. The purpose of this step is reducing the workload of the hydrolytic fermentative bacteria by making the substrate more easily biodegradable. This includes properties such as increasing the surface area, dissolving complex matter, reducing crystallinity in polymers such as cellulose etc. The most common types of pretreatment applied today are summarized in the tables below.
Common pre-treatments for the feedstock prior to anaerobic digestion.
|Mechanical||Milling to reduce particle size, increasing biomass availability. Leads to increased complexity increasing process sensitivity and cost.|
|Thermal||Heating (200 oC), disrupting hydrogen bonds (chemical macro-structures) and increasing biomass availability. Leads to high energy demand.|
|Alkali||Alkali treatment over several weeks, easing the degradation of ligno-cellulosic compounds. Requires large amounts of chemicals and chemical handling which leads to a slow process.|
|Ozonation||Inline ozonation, significantly increasing the biodegradability of stable organic matter, potentially tripling biogas production.|
|Microbiological||Composting, an aerobic pretreatment step forming hydrolytic enzymes, facilitating the first step of the anaerobic digestion.|
|Fungal||Pure cultures of an aerobic fungi during a 4 day incubation time has shown results with up to 40 % more biogas and an increase in the grade.|
After the pretreatment stage, the feedstock enters the digester for the biochemical digestion. In this step, several types of microorganisms react with the feedstock in different stages. All stages are anaerobic i.e. with an absence of oxygen. These are summarized in the table below.
The steps in an anaerobic digestion stage.
|Hydrolys||Breaking down of large substrates, such as cellulose and proteins to glucose and amino acids.|
|Fermentation||Formation of volatile organic acids and alcohols.|
|Ättiksyrabildning||Formation of acetate, carbon dioxide and hydrogen.|
|Metanbildning||Formation of methane and carbon dioxide.|
The methane production through anaerobic digestion may be performed with a single-stage or two-stage process. In the first case, all the reaction steps above are carried out in the same reactor. The substrate is converted into methane with a concentration of 50-55 % in the biogas, depending on the type of feedstock. In the two-stage process, only the reaction steps before the acetogenesis are carried out in the first digester. The following methane production is performed in a second stage, called methane upgrading stage. By splitting the process into two stages, it is possible to increase the methane concentration in the biogas with up to 70 %. Therefore, the system becomes more efficient and the costs for the following biogas purification are reduced.
Biogas after treatment
In the biogas after the methane upgrading, high concentrations of hydrogen sulfide, H2S, are often present, creating issues for the following process steps. Corrosion is one of the issues to be considered, since very high H2S levels may corrode pipes and process instrumentation, resulting in costs up to several thousands of euros per year. Odor is also one of the major problems related to the methane upgrading off-gas. This is due to the very high sensitivity of the human nose to hydrogen sulfide, since the odor receptors are triggered for concentrations in the part-per-billion (ppb) range. Therefore, even a small leak in the process lines or an opening in a process step may create an odor issue for a large area, since the emitted gas needs to be diluted up to 200 000 times before the odor is masked.
Reuse of liquid effluent
After the production of biogas, the liquid effluent still has a high content of nitrogenous compounds which constitute an important resource. These can be reused in agriculture as crop fertilizers, thus providing important economic savings. However, further treatment is needed, prior effective reuse, removing emitted odors and unpleasant colors.