SEARCH WITHIN CONTENT
Citation Information : Architecture, Civil Engineering, Environment. Volume 14, Issue 3, Pages 89-101, DOI: https://doi.org/10.21307/ACEE-2021-025
License : (CC-BY-NC-ND 4.0)
Received Date : 02-August-2021 / Accepted: 30-August-2021 / Published Online: 08-October-2021
Biotrickling filters are one of the most effective methods of air bio-purification, this bioremediation process if of high efficiency in pollution reduction. It is an eco-friendly process and economically viable. The technology of biotrickling filters includes Compact Trickle Bed Bioreactors (CTBB), which are currently used in an increasingly wide range. The aim of this work will be an objective assessment of the implementation potential of the CTBB technology to various industries, including the municipal sector. The paper briefly discusses the characteristics and operating parameters of biotrickling filters, a review of their applications as an effective method of VOC and odor removal including sources of their emissions, as well as the characteristics of CTBB and implementation possibilities to various industries. It was concluded that CTBB are promising solution for the future, as it combines the high degradation efficiency of a wide range of pollutants with cost-effectiveness and ecology. According to the analyzed data and results, this technology can be successfully used to remove VOCs and odors from various industries.
The rapid development of industry in the recent decades, and their consequential effect to the degradation of the environment, has led to the intensification of the progress of innovative technologies to limit their negative impact. The quality of the air we breathe with is one of the most important aspects in our everyday life. Air pollution affects not only the health of our respiratory system, but also our nervous system and mental health [1, 2]. Volatile organic compounds (VOCs) such as benzene, toluene, acetone, xylene, butyl acetate, vinyl acetate and styrene are emitted as a result of different production processes in many industries: paint industry (including automotive companies), chemical industry (e.g. production of solvents, fertilizers), production of varnishes, glues, paints, mining industry, petrochemical industry. The emission of odors are also extremely burdensome, including some VOCs, e.g. thiols, sulfides, disulfides, but also bioaerosols contain microorganisms present in wastewater, inorganic compounds such as ammonia and ammonium compounds, hydrogen sulfide emitted by municipal sector (wastewater treatment plants, waste management plants) food processing plants (sugar factories, breweries) agriculture and animal husbandry farms and slaughterhouses of poultry, cattle and pigs . The emission of bioaerosols is dangerous due to the presence of pathogenic microorganisms present in the mentioned industries. These microorganisms can be transported over long distances with the wind, which makes it particularly harmful for close neighborhoods.
Overview of each sector’s contribution to total emissions for NMVOCs in the European Union (28 countries) for 2018 can be found in Fig. 1. The manufacturing and extractive industries were the largest contributor to NMVOC with the agriculture sector following as the second largest contributor . The harmfulness of VOCs and odors are due to their toxicities and their high reactivity within the air, which can result in the formation of even more harmful products of complex transformations, as is the case with ozone, which is formed in the atmosphere by chemical reactions caused by solar radiation with the participation of VOC and NOx . The World Health Organization (WHO) divides VOCs into very volatile organic compounds with a boiling point below 50–100°C (e.g., propane, butane), volatile organic compounds with a boiling point between 50–100°C and 240–260°C (e.g. formaldehyde, acetone, ethanol, toluene), and semi-volatile organic compounds with a boiling point between 240–260°C and 380–400°C (e.g. pesticides, phthalates). The lower the boiling point of such a compound (higher volatility) the more likely it is to be emitted from surfaces into the air. VOCs and odors are carcinogenic and can cause diseases of the nervous, immune, and respiratory systems . Due to the negative impact of VOCs and odors, it is important to manage air quality in a sustainable way, which includes, limiting air pollution, effective and ecological methods of purification and deodorization as well as effective systems of air quality control and monitoring. Due to the continuous development of industry and economy, methods of cleaning polluted air are an extremely important tool, which can be divided into four main groups: combustion, adsorption, absorption and biological methods . The use of an appropriate air purification technology is conditioned by several factors: the type and concentration of pollutants, and the intensity of emissions. Increasing emphasis on sustainable development will further stimulate the use of biological methods of air purification, due to their high efficiency in biodegradation of pollutants, environmental friendliness and economic viability. The development of biotechnology has resulted in significant advances in biological air purification methods. Initially based mainly on the technology of basic biofilters, it has evolved over time to maximize the efficiency to various configurations and hybrids of several technologies. One of such hybrids include the Compact Trickle Bed Bioreactors (CTBB), which belongs to the biotrickling filters group . In recent years, this technology has been widely developed and implemented in various industries and is worth taking a closer look at. Development of a technology capable and suitable for implementation in various industries that are sources of different groups of pollutants, both organic and inorganic, would be extremely valuable. There are many technologies with high efficiencies in removing pollutants from the air, however each has their limitations: the economics of the technology (e.g. catalytic combustion), the size of the plant (e.g. biofiltration), secondary waste utilization (e.g. adsorption, biofiltration), the degradation efficiency of the pollutants, and the ability to treat pollutants from different sources and different chemical compositions. A solution is being sought that is adaptable to different industries. This work, therefore, aims to analyze the capability and deployment potential of Compact Trickle Bed Bioreactors to treat air with pollutants of dissimilar origin, VOCs and odors (including H2S) from different industries with different characteristics of plant performance and gas emissions.
Biotrickling filters in simplification, are a combination of biofilter and bioscrubber technologies. These are apparatuses filled with a solid, inert packing material (e.g. polypropylene rings, ceramic rings, activated carbon or glass beads) inoculated with microorganisms capable of metabolizing pollutants. The gaseous pollutants introduced into the apparatus are absorbed into the circulating liquid phase, which is a solution of mineral salts, necessary for the proper functioning of microorganisms, that populate the biofilm formed on the bioreactor bed . The biotrickling filters technology is characterized by high stability of the processes taking place within the reactor and the ability to control them.
Biological methods of air purification utilize the natural metabolic activities o microorganisms to decompose pollutants by using them as a carbon source. In industrial practice, biological methods of pollutant degradation are carried out in 3 types of devices: biofilters, bioscrubbers and biotrickling filters, also known as Trickle Bed Bioreactors. These technologies are based on the following processes: absorption of gaseous pollutants into the liquid phase, their assimilation and degradation by microorganisms. However, they differ in the location of the inoculated microorganisms and the type of filling and mobile phases .
In biotrickling filters, the contaminated gas entering the reactor, flows co-currently or counter currently to the liquid phase, then absorption of the gaseous pollutants into the liquid phase take place. Therefore, it is important to control the process in terms of the degree of dissolution of the pollutants introduced into the apparatus . The constantly recirculated liquid flows in a thin film on the surface of the bed with microorganisms, providing them with the nutrients necessary for proper development. The pollutants absorbed into the liquid phase diffuse into the biofilm formed on the filling, where the metabolic activity of microorganisms eliminate the pollutants for example to carbon dioxide and water in the case of VOC degradation. Absorption and regeneration of pollutants takes place in one device, which is an indisputable advantage . In addition, the entire process, running at a moderate temperature of 25–30°C and atmospheric pressure, does not generate additional operating costs (Table 1).
Excessive growth of biomass in biotrickling filters, in addition to the risk of clogging the bed and an increase in the flow resistance of the liquid phase through the bed, may cause the accumulation of intermediate decomposition products of pollutants in the circulating liquid over a longer period . If this product is a toxic substance, it may inhibit microorganisms and adversely affect the bio-purification process, so it will require an additional degree of solution purification. However, there are several methods of counteracting this. When the biofilm growth on the surface of the bed is too high, it is advisable to increase the flow rate of the liquid phase, which will separate the excess biomass with a large liquid stream. A good solution is also the use of temporary or periodic backwashing. This will increase the specific surface area of the biofilm attached to the packing material . It’s also possible to temporarily limit and reduce the dosage of mineral salts as well as to introduce protozoa into the system, that feed on biofilm-forming bacteria . Removal of the excess accumulated biomass is necessary to obtain long-term and stable operation of higher loaded biotrickling filters.
the appropriate concentration of oxygen supplied to the microorganisms
selection of process parameters: the flow rate of the liquid and gas phase, maintaining the optimal pH for the microorganisms used, temperature and concentration of mineral salts determining the appropriate environment for their development
controlling the presence of compounds inhibiting the growth of biomass, supplied with polluted air or formed as secondary products of the process
the appropriate degree of gas wetting and the solubility of VOCs and odors in the liquid phase
selection of the packing material, both in terms of the microorganisms used and the type of contamination.
Removal efficiency (RE) [%] expresses the percentage ratio of the difference in pollutant concentrations in the bioreactor inlet and outlet gases to the pollutant concentration in the inlet gases
Mass Loading Rate (MS) [g/(m3 h)] expresses the ratio of concentration in inlet gas to residence time τg [h]
Vr - bioreactor volume [m3],
Vg - gas phase flow [m3/h],
CgIN - substance concentration at the inlet to bioreactor [g/m3],
τg - residence time [h].
– Elimination capacity (EC) [g/(m3 h)] expresses the ratio of the concentration difference in inlet and outlet gases of the bioreactor to the residence time of the gas phase in the bioreactor - it determines the amount of pollutants that are decomposed in a unit of time and the volume of the filter material.
– Efficiency of the biotrickling filter - presents the dependence of removal efficiency (RE) on the bioreactor load with pollutant load (MS). For low MS values, RE usually reaches 100%, but as MS increases, it is often the case that the biotrickling filter is not able to completely remove the contamination. The load value at which the ratio of EC to MS changes linearly is defined by the critical load (MSe). For MS > MSe, despite the increase in the MS load value, the EC is essentially constant. In the case of very high MS loads, the EC reaches its maximum ECmax value.
The review of the current state of knowledge shows that the biotrickling filters technology can be used for bio-purification of air from various emission sources. It is possible with the appropriate selection of parameters such as the type and adaptation of microorganisms appropriately selected for the degradation of the selected type of contamination, or the type of packing materials . The correct selection of microorganisms for specific pollutants determines the maintenance of certain conditions inside the apparatus, i.e. pH, temperature, composition of the mineral salt solution. Zhou et al. 2016  tested the removal efficiency of chlorobenzene in biotrickling filters using Ralstonia picketti strain, the obtained results of chlorobenzene air biopurification efficiency are in the range of 88-90%. Cheng et al. 2017  conducted a study to evaluate the potential of a consortium of Ralstonia picketti and Trichoderma viride to enhance the biodegradation of monochlorobenzene. The results of this study showed an increased degradation rate for the used consortia of microorganisms compared to the degradation rate of monochlorobenzene for each strain individually, suggesting that the fungal-bacterial consortium can be effectively used to completely biodegrade this pollutant. The study of Lebrero et al. 2012  confirmed the high potential of biotrickling filter to treat moderately hydrophobic VOCs and sulfur compounds reaching more than 95% removal efficiency for methyl mercaptan, toluene, alpha-pinene, while for hexane the removal efficiency was 70%. High microbial diversity was also observed during long-term operation of the biotrickling filter, with the main phyla being Proteobacteria, Actinobacteria, Nitrospira, Chloroflexi and Gemmatimonadertes. Tu et al. 2015  conducted experiments to apply saponins to enhance the removal of hexane in a biotrickling filter. The results showed that the addition of saponins to the biotrickling filter increased the hexane removal efficiency of 91.3% while the sample without saponins achieved an efficiency of 62.8%. As the concentration of hexane increased, the efficiency decreased as well as the gas empty bed contact time. Moreover, it was found that saponins could also decreased the biomass accumulation rate within the packing material. The recently published results of Rybarczyk et al. 2021  on the removal of cyclohexane and ethanol from air in biotrickling filters prove that not only bacteria are capable of efficient degradation, but also, Candida albicans and Candida subhashii fungal strains indicated their good ability to utilize cyclohexane and ethanol as a carbon source and achieve 95–99% biodegradation efficiency of these compounds. Moreover, it was found that the presence of ethanol increases the efficiency of removing cyclohexane from the air. Wan et al. 2011  investigated the removal efficiency of trimethylamine from waste gas in a biotrickling filter whose fill was ceramic particles and inoculated with microorganisms. The study showed 100% removal efficiency for an empty bed retention time greater than 110 s and an inlet trimethylamine concentration of 0.3 mg/l but halving the retention time to 55 s reduced the efficiency to 64.7%. In contrast, 100% degradation efficiency was maintained at a retention time of 83 s and a trimethylamine concentration in the inlet gas <0.2 mg/l.
In addition to effectively treating VOCs, there are many scientific papers describing the possibilities of using biotrickling filters to bio-purify the air from pollutants emitted by municipal sector, commonly known under the general term of odors. Particular troublesome and harmful representatives of this group of compounds include ammonia (NH3) and hydrogen sulphide (H2S), due to e.g. a very low detection threshold (mainly in the case of H2S) but mainly to their toxic effects and very negative impact on humans. In most wastewater treatment plants, these compounds have the highest concentrations of odor compounds in the emitted gases [27, 28]. There is a lot of work on maximizing biotrickling filters performance by selecting the right packing material. This is a key design parameter that can determine the proper performance of the system, and also its cost-effectiveness. The type, composition, structure and shape of the filling used determine the ability and stability of the development of microorganisms and the formation of a biofilm on its surface . The packing material used must also meet other requirements, such as: the appropriate weight properly adapted to the size of the installation, or the material from which it was made - resistant to biofilm (including products of microbial metabolism), chemical compounds (organic and inorganic, including: dissolved in the liquid phase contaminants, buffer solutions regulating pH), temperature. In addition, the surface of the packing material, which is inoculated with microorganisms, is also important, the larger the mass transfer surface, the more effective the bio-cleaning process . The research shows the possibility of using a wide range of packing material, both organic and inorganic - compost, wood chips, activated carbon, perlite, lava rock, glass balls, polyurethane foam, ceramic shapes, polypropylene rings.
The following Table 2 shows the results of recent biotrickling filter performance studies using different packing material.
The cited works confirmed that using combined packing materials effectively improved conditions for the growth of microorganisms and, as a result, to improve the efficiency of pollution degradation.
An example of a biotrickling filter is the Compact Trickle Bed Bioreactors, which have been tested to remove VOCs and odors (including H2S). CTBB reactors are derived from biotrckling filters. However, they are different in full automation of operation, compactness of their construction, absence of side leachate, which are their undoubted advantages. In CTBB reactors, important factors for the course of the air biopurification process, similarly as in biotrickling filters, are the selection of carefully selected group of microorganisms for a specific group of pollutants and providing appropriate conditions for the growth and development of these organisms, such as: pH, temperature, composition of the solution with nutrients (mineral salts). In addition, it is very important to determine the optimum conditions for the entire process, adjusted to the concentration of contaminants in the inlet gases and the gas flow rate - liquid phase flow, gas phase flow and type of packing material . The Compact Trickle Bed Bioreactors have been investigated so far by Kasperczyk [13, 18, 39, 40, 41, 42] for the removal of VOCs and odors - including styrene, ethanol, dimethyl sulfide, vinyl acetate, benzene, acetone, dimethyl disulfide, xylene, H2S - from a variety of sources, studies have been conducted in both laboratory and industrial settings. Fig. 2 shows a CTBB model, illustrating the downward co-current flow of the gas and liquid phases in a thin film through a bed made of polyurethane rings, inoculated with coculture of microorganisms, which use VOCs and odors as the only carbon source.
Kasperczyk et al. 2014  examined the use of Compact Trickle Bed Bioreactor to the biodegradation of H2S and VOC mixture present in the air in a copper-ore mine at a depth of 1000 m underground. The efficiency of H2S removal was at the level of 80–99% - when the concentration of H2S was below 38 ppm, when an increase in concentration to 40–60 ppm was recorded, the efficiency of H2S removal decreased to 60–80%. The VOC removal efficiency was 90–100%. The publication of Kasperczyk and Urbaniec 2015  presented the results of laboratory tests preceding the implementation of CTBB in a copper ore mine. The biodegradation of a 4-component VOC mixture consisting of acetone, benzene, styrene and vinyl acetate and an 8-component mixture of impurities in which xylene, H2S, dimethyl disulfide and dimethyl sulfide were added to the previous 4-component mixture was investigated. Over 80% of pollutant removal efficiency was achieved at a specific pollutant load up to 40 g/m3h, except for benzene, which caused biomass poisoning at a specific load above 5 g/m3h. Bąk et al. 2017  also examined the potential of CTBB in the biodegradation of gaseous streams containing styrene, dimethyl sulfide and ethanol mixture. The average process efficiency for the 3-component VOC mixture was above 95% in the lower concentration ranges, 80% in the middle concentration ranges and above 55% in the highest concentration ranges of the pollutant (the concentration ranges were: ethanol 0.045–0.178 g/m3; dimethyl sulfide 0.027–0.061 g/m3, styrene 0.236–1.993 g/m3). Moreover, almost 100% efficiency of ethanol biodegradation was observed in the whole tested range of mass load. The possibility of using CTBB for the treatment of VOCs generated in the PKN Orlen SA wastewater treatment plant was also analyzed, the results of which were described by Kamiński and Koziczyński 2017 . The study obtained 85-99% biodegradation of VOCs, whose concentrations were in the range of - 0.06–1.75 g/m3 and 65-88% for the range 1.7–2.5 g/m3. The reduction of NH3 in the entire concentration range of 0.6–5.5 mg/m3 was above 99%, and the reduction of H2S in the concentration range of 0.1–137 mg/m3 was also above 99%. One of the latest published studies by Kasperczyk et al. 2019  confirms the use of CTBB to remove VOC and H2S emitted by a municipal wastewater treatment plant. The research on the efficiency of ventilation air purification contaminated with H2S and VOC was conducted for 60 days, during which fluctuations in the air flow intensity from 2 to 30 m3/h were recorded. The minimum and maximum concentrations of H2S were 2 ppm and 660 ppm, but most of the time they remained in the range of 50–440 ppm, as shown in Fig. 3.
For H2S concentration of approximately 200 ppm, the removal efficiency was above 97%. A several-hour increase in the concentration of H2S to 600 ppmv caused poisoning of the bioreactor, the stable operation of which was achieved within 3 hours from the moment of restoring the original concentration of H2S. The efficiency of H2S and VOC removal for pollutant loads up to 20 g/m3h was over 95%. The subject of the latest publication by Kasperczyk et al. 2021  was the implementation of the CTBB developed from a pilot scale to a full industrial scale, to the automotive painting industry. Figure 4 shows the results of industrial research conducted at CTBB on a pilot scale. At 170-180 ppm VOC concentrations in the CTBB inlet ventilation air, the outlet concentration did not exceed 20 ppm, which gives a conversion rate of 85–90%. The line indicating the upper limit of the VOC concentration in the CTBB exhaust gases, as per the relevant environmental permit, is also marked in Fig. 4. The limit was set at 50 ppm, while the VOC concentration in the clean gases was a maximum of 20 ppm - well below the specified limit.
Fig. 5 shows the VOC concentration results at the inlet and outlet of CTBB tests in full scale. The graph shows significant variations in the VOC concentration at the inlet to CTBB occurring in the range of 100–1800 ppm, indicating the high flexibility of the CTBB installation. Despite significant increases in concentrations (1700–1800 ppm), the VOC biodegradation efficiency remains at the level of 85–90%, while the average VOC bio-purification efficiency in the period in question is 95–99%. The results showed that the VOC biodegradation coefficient for concentrations in CTBB inlet gases 200 ppm is in the range of 85–99%, while the paint shop ventilation air flow rate is 1–10 m3/h. The VOC biodegradation index of 85–99% was also obtained with a gas flow rate of up to 6000 m3/h using a full-size CTBB adapted to the automotive painting conditions (Fig. 6).
The results of the cited publication prove the possibility of efficient upscaling of the CTBB technology to degrade high concentrations of VOC in a wide range of polluted air flow rates.
According to the research carried out so far and the implementation of CTBB in copper ore mines, wastewater treatment plants and the varnish industry, it can be concluded that the tested technology has a very large implementation potential in a wide range of various industries, both for the degradation of VOCs and odors, including inorganic H2S and NH3.
The main advantages of using the CTBB technology are:
High efficiency of degradation of VOCs and odors
No secondary contamination
No need to replace the packing material and utilize hazardous pollutants
Operation of the technology at 25°C and atmospheric pressure, resulting in lower energy consumption
The compactness of the CTBB (Figure 4) enables its adaptation to various conditions in industrial factories, which greatly increases the implementation potential of this technology.
In the future and further development plans for the CTBB technology, it will be important to research new groups of pollutants emitted by burdensome and so far unknown sources of VOC and odors, such as: animal farms - poultry farms, pigs or fur animal farms, fish processing, food industry - sugar factories, distilling, brewing and wood industry - producers of plywood, furniture boards, laminated boards. This will allow to check the efficiency of the CTBB technology and the implementation potential in order to enter new implementation directions. Like any innovative topic, it is associated with challenges and difficulties that may arise during the research. It will be necessary to select: a special group of microorganisms capable of degrading new types of pollutants, nutrient composition, pH, temperature, and the formation of secondary products of the pollutant biodegradation process. Research into the selection of materials and the type of filling used will also be important in order to avoid negative consequences such as corrosion of used materials. There is a solid basis for further research and broadening the spectrum of new industries where CTBB technology could be applied. This technology has great research, development and implementation potential, due to the fact that is ecological, fully controlled and economically attractive.