Volatile Organic Compounds (VOCs), commonly found in the atmosphere, are odorous compounds with negative effects on human and environment. VOCs and odours emitted from industrial sources have been demonstrated as hazardous and annoying compounds which may cause negative effects on humans and environment.
The control of these compounds is therefore a key action by the plant managers in order to avoid complaints and negative impacts. In this study, microalgae and bacteria were implemented in a vertical tubular photobioreactor for the biodegradation of toluene, used as model VOC. Chlorella vulgaris strain was chosen as photosynthetic platform due to their high adaptability to adverse environmental conditions.
2. Environmental Engineering Program, National Graduate School of Engineering, University of the Philippines, Diliman, Quezon City, Philippines.
3. Inter-University Centre for Prediction and Prevention of Relevant Hazards (Centro Universitario per la Previsione e Prevenzione Grandi Rischi, C.U.G.RI.), Via Giovanni Paolo II, Fisciano (SA), Italy.
4. Department of Chemical Engineering, University of the Philippines, Diliman, Quezon City, 1101 Philippines.
Competing interests:The author has declared that no competing interests exist.
Academic editor: Carloz Diaz
Content quality: This paper has been peer-reviewed by at least two reviewers. See scientific committee here
Citation: R. Pahunang, G. Oliva, V. Senatore, T. Zarra, A. Buonerba, V. Belgiorno, and F. Ballesteros Jr., V. Naddeo, 2021. Advanced photo-biotechnology for the simultaneous control of VOCs, odours and GHGs emissions in municipal solid waste treatment plants, 9th IWA Odour& VOC/Air Emission Conference, Bilbao, Spain, www.olores.org.
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Keyword: algal-bacterial photobioreactor, toluene biodegradation, algal-bacterial symbiosis.
Volatile Organic Compounds (VOCs), commonly found in the atmosphere, are odorous compounds with negative effects on human and environment. VOCs and odours emitted from industrial sources have been demonstrated as hazardous and annoying compounds which may cause negative effects on humans and environment. The control of these compounds is therefore a key action by the plant managers in order to avoid complaints and negative impacts. In this study, microalgae and bacteria were implemented in a vertical tubular photobioreactor for the biodegradation of toluene, used as model VOC. Chlorella vulgaris strain was chosen as photosynthetic platform due to their high adaptability to adverse environmental conditions. Active secondary sludge collected from a large scale civil wastewater treatment plant was used for the inoculation of bacteria consortium. Two stages have been performed by gradually increasing the toluene inlet load. Results showed that the system can efficiently biodegrade toluene with removal yields up to 98%. Overall, this study offers an alternative configuration of photo-bioreactor, culturing algae and bacteria for an efficient biodegradation of toluene and a simultaneous CO2 uptake.
Volatile organic compounds (VOCs) are among gases that can be found in the atmosphere which can be detrimental to human and the environment (Hsu et al. 2007), since some VOCs have been identified as toxic and carcinogenic and responsible for photochemical pollution. Due to their volatility, the VOCs are also responsible for odor pollution that can affect the quality of life of the adjacent population (Pahunang et al. 2021). Among BTEX (benzene, toluene, ethylbenzene, and xylene), toluene is considered among the pollutants commonly present in the atmosphere (Hu et al. 2020). Toluene is hence commercially and industrially used as a solvent in paints, lacquers, thinners, among others, and it is also used in crude oil production (Fisher et al. 2017).
As a consequence of the reaction that can occur with other compounds such as hydroxyl radicals (OH-), NO3- radical, and O3 (Atkinson 2000), VOCs, including toluene, are considered one of the main responsible in the production of urban petrochemical smog (Warneke et al. 2001). With these effects to environment and human, the emission of these compounds should be effectively mitigated.
Conventional method for the degradation of VOCs involves physical and chemical processes. However, the economic aspects in terms of chemical and energy consumptions are the major drawbacks of these processes (Lebrero et al. 2016). The use of this kind of processes often entails the need of the treatment of solid of aqueous phases in which the contamination is transferred (Oliva et al. 2018). Conversely, biological process has been considered a promising methods for the treatment of organic gaseous contaminants (Senatore et al. 2020). Symbiotic relationship of microalgae and bacteria, where microalgae gives oxygen (O2) to bacteria and bacteria provides carbon dioxide (CO2) in return, is a significant way for the effective biodegradation of VOCs (Anbalagan et al. 2017; Lebrero et al. 2016; Oliva et al. 2019). The extracellular products produced by microalgae during photosynthesis enhances the degradation potential of bacteria to degrade organic compounds and accelerates the growth rate of bacteria (Vermi et al. 2021). Microalgae are also capable of removing organic contaminants with bio-adsorption processes (Senatore et al. 2021b).
In this study, a novel configuration of algal-bacterial photo-bioreactor (PBR) has been developed. The results demonstrated that the investigated solutions may represent an efficient and sustainable alternative to the conventional method for the treatment of VOCs due to the possible recovery of algal biomass to produce valuable compounds (e.g. biofuels, cosmetics, animal feed, etc) besides reducing GHGs (Greenhouse Gases) (Lebrero et al. 2016; Senatore et al. 2021a).
2. Materials and methods
2.1 Microorganisms preparation and inoculum
The Chlorella vulgaris (C. Vulgaris CCAP 211/11B) acquired from the Culture Collection of Algae and Protozoa (CCAP), Dunberg, Scottland was pre-inoculated with modified Bold Basal Medium (mBBM). The bacterial sludge (0.5 L), on the other hand, was collected from the wastewater treatment plant of Battipaglia, Salerno, Italy. The activated sludge was centrifuged at 9000 rpm in 10 mins and was decanted before the inoculation in the PBR. The C. vulgaris (1.5 L) and sludge were suspended in the reactor with mBBM (Bischoff and Bold 1963). Pre-culture condition of C.Vulgaris and composition of mBBM are reported in previous study of Senatore et al. (2021a).
2.2 Experimental design
The schematic layout of the experiment is shown in Figure 1. The bioreactor was made of glass with an internal height of 50 cm and an inner diameter of 15 cm. The working volume is 8.50 L. The liquid broth was continuously mixed using a magnetic mixer (IKA® C- MAG HS 10). Pure liquid toluene was feed in the system through a syringe pump (model No. 300 from New Era Pump Systems, Inc.) and was diluted with ambient air via air compressor (COMPACT 120, Fiac Air Compressor) where the air flow rate was controlled using a flowmeter (Platon NG, Roxspur Measurement & Control). One sampling port was located in the air line before the insufflation of the contaminated stream at the bottom of the vertical column via metallic diffuser. The other sampling port is located at the outlet. The bioreactor was illuminated by three white light emitting diode (LEDs) corresponding to a light intensity of 5620 Lux. This experiment was operated for 30 days to determine the capacity of the system to biodegrade the toluene. The specific operating conditions of the parameters at different stages are described in Table 1.
Figure 1. Schematic layout of the experimental set-up
Table 1. Operating conditions of the parameters at different stages
2.3 Extraction and analysis of chlorophyll-a
The extraction of chlorophyll-a was based on the study of Sun et al., 2018 and was performed two times a week. The concentration of the chlorophyll-a was calculated according to the standard methods for the biological examination of plankton in water and wastewater (APHA, AWWA, and WEF 2017).
2.4 Analytical methods
The liquid temperature, pH, and DO were daily measured using a multi-parameter probe (HI 9829 from HANNA Instruments). The inlet and outlet toluene concentration were directly measured using a GC-PID (Gas chromatography -Photo Ionization Detector, Ion Science). The total biomass concentration was measured in terms of total suspended solids (TSS) according to Standard Method for the Examination of Water and Wastewater (Rice, Baird, and Eaton 2017).
3. Results and discussion
3.1 Algal-bacterial biodegradation efficiency
The time series of the biodegradation of toluene are shown in Figure 2, where the vertical lines represented the operational stages as indicated in the upper part of the graph. Stage I showed an increasing RE from 20.26% to 98.27%, with stabilization occurred from day 9 to day 15. Stage II, on the other hand, showed an unexpected drop of the RE to 84.90% on day 22. The unexpected decrease of the RE in Stage II may be attributed to the attachment of the biomass on the walls of the PBR, which blocked the light diffraction and hindered the light exposure of the biomass. Consequently, this condition could lessen the production of reactive oxygen species (ROS) able to enhance the biodegradation and oxidation of the toluene and intermediates (Li et al. 2015; Rajendran et al. 2019). This result is also supported by the study of Lebrero et al. (2016) in which the RE is associated with the suspended biomass concentration in the reactor.
In Stage II, it can also be observed that the inlet load and the elimination capacity have an average difference (IL-EC) of 52.31 ± 1.08 mg m-3 h-1, with removal yields up 98%.
Figure 2. Time series of the (A) C7H8 concentration (inlet and outlet), and removal efficiency of the algal-bacterial vertical tubular photobioreactor; (B) inlet load and elimination capacity of the system.
3.2 Biomass quantification
The amount of biomass was determined by measuring the TSS and chlorophyll-a concentration. The TSS increased from day 1 to day 18, from 340 mg L-1 to 1370 mg L-1, while during days 20-30 decreased from 1170 mg L-1 to 1270 mg L-1 as depicted in Figure 3. The chlorophyll-a concentration increased from day 1 to day 16, from 3.74 mg L-1 to 10.57 mg L-1 and declined from day 18-30.
The fluctuation of the TSS can be attributed to the attachment of the biomass on the wall of the reactor and to the increased liquid renewal. The renewal of the mineral medium was hence increased in Stage II from 250 mL d-1 to 750 mL d-1. The continuous declining amount of chlorophyll-a could also be linked to nutrients starvation (Kim et al. 2020) or possibly due to the increase of the toluene intermediates concentration in liquid phase which could negatively affect the algal photosynthetic metabolism (Lin et al. 2005; Peng et al. 2015). Nonetheless, the dissolved oxygen (DO) was not affected by the declining concentration of chlorophyll-a, that was also observed from the previous studies of Perner- Nochta et al., (2007), and Eriksen et al., (2007). Moreover, with the daily liquid renewal of 750 mL d-1, it has been possible to recover up to 825 mg of algal biomass per day, to use for producing value-added biomass, while maintaining sufficient biomass concentration to promote biodegradation.
Figure 3. Total suspended solids (TSS) and chlorophyll-a measured throughout the duration of the experiments.
The results obtained in the present work confirmed the high performance of microalgae and bacteria in a vertical tubular photobioreactor for the biodegradation of toluene laden gas mixture. The biodegradation efficiency of the system reached 98% in terms of removal yields, for inlet load of almost 3 grams of toluene per cubic meter per hour.
The performance of the system has been evaluated also in terms of biomass production, measured in terms of total suspended solid concentrations. The TSS reached a maximum of almost 1400 mg L-1. Increasing the daily liquid renewal, the TSS concentrations decreased while the degradation potential of the system resulted quite constant during the time. The maximum biomass recovery has been obtained at the end of Stage 2, in which more than 0.8 grams of biomass was daily recovered by the system.
Overall, the investigated system has been demonstrated as an effective solution for the treatment of VOCs and the simultaneous capture of the correlated GHGs, besides the possible recovery of significant amount of exploitable valuable biomass.
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