Emission of volatile organic compounds from composting: A review on assessment, treatment and perspectives

Science of the Total Environment 695 (2019) 133725

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Review

Emission of volatile organic compounds from composting: A review on assessment, treatment and perspectives Kondusamy Dhamodharan a, Vempalli Sudharsan Varma a, Chitraichamy Veluchamy b, Arivalagan Pugazhendhi c,⁎, Karthik Rajendran d a

Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India School of Environmental Science, University of Guelph Ridgetown Campus, Ontario, Canada Innovative Green Product Synthesis and Renewable Environment Development Research Group, Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Viet Nam d Department of Environmental Science, SRM University-AP, Amaravati, Andhra Pradesh 522 502, India b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Plausible VOC emission treatment are biofiltration, trickling filter and bioscrubbing. • Factors affecting the VOC emissions during composting were identified. • Predictive models help in better understanding of VOC emissions from composting.

a r t i c l e

i n f o

Article history: Received 31 May 2019 Received in revised form 28 July 2019 Accepted 1 August 2019 Available online 05 August 2019 Editor: Huu Hao Ngo Keywords: Composting VOCs & Odorous gas emissions Biological treatment methods Models EIA

a b s t r a c t Composting is a sustainable technology in treating organic pollutants and controlling odorous gas emissions from different organic solid waste, by reducing its size and volume. When the process parameters are handled efficiently, composting process is greatly effective than other waste treatment options in terms of operational costs, income generation out of compost, reduced air and water pollution. The successful composting operation does not count only the final product, but also the odorous gas emissions being released off to the atmosphere. Biofiltration is a relatively successful air treatment technology for polluted gases containing biodegradable compounds. By optimizing and focusing the operational parameters of biofiltration technology, 90% of treatment efficiency could be achieved with more economical advantage compared to other air treatment technologies. However, the complexity and the uncertainty measures in operating the system and understanding the process biodegradation mechanism is very crucial for the successful performance. Therefore, this review focusses and provides an assessment and treatment of different odorous gas emissions emitted during the composting processes. The recent advancements and treatment options for various volatile organic compounds (VOCs) and other odorous gas emissions during composting is updated. The advancements in bio-trickling filters,

⁎ Corresponding author at: Innovative Green Product Synthesis and Renewable Environment Development Research Group, Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Vietnam. E-mail address: [email protected] (A. Pugazhendhi).

https://doi.org/10.1016/j.scitotenv.2019.133725 0048-9697/© 2019 Elsevier B.V. All rights reserved.

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bioscrubber technology and membrane bioreactors treating VOCs has been focused. The use of different models in evaluating the process optimization and gas mitigation is also explained. Finally, the environmental impact of VOC compounds released into atmosphere from composting plants has been discussed. © 2019 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . Gas emissions from composting . . . . . . . . . . . Characterization of VOC emissions from composting . . Biological treatment of VOC emissions from composting 4.1. Biofiltration . . . . . . . . . . . . . . . . . 4.1.1. Factor's influencing the process . . . . 4.2. Bio-trickling filter. . . . . . . . . . . . . . . 4.2.1. Factor's influencing the process . . . . 4.3. Bioscrubber . . . . . . . . . . . . . . . . . 4.4. Membrane bioreactors . . . . . . . . . . . . 5. Models on VOC emission from composting . . . . . . 6. Environmental impact assessment . . . . . . . . . . 7. Perspectives and challenges . . . . . . . . . . . . . 8. Discussions . . . . . . . . . . . . . . . . . . . . . 9. Conclusion . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction The consensus of practicing composting techniques for different organic solid waste has become an environmentally friendly and sustainable alternative for landfilling and other disposal options. Composting is a natural biochemical process involving millions of indigenous microbial populations under controlled aerobic, moisture, C/N and other major controlling factors for the decomposition of organic matter to a stabilized and nutrient rich end product. The major advantage of the process is the volume reduction of mass waste and also resulting in compost for agricultural purposes (Varma and Kalamdhad, 2014). However, the gas emissions from the composting plants are of a major concern to look upon. Since majority of the emissions contribute to CO2, which is considered not to add to the global warming due to the biogenic origin of carbon. While, the other gaseous compounds such as N2O, and CH4 directly contribute to the global warming; and NH3, sulphur compounds and most of the volatile organic compounds (VOCs) emissions cause undesirable and other odor nuisances (Colón et al., 2012; Komilis et al., 2004). VOCs are emitted during the handling and decomposition stages of waste materials during composting. These VOCs are mostly biodegradable, due to biogenic origin. In addition, they have the properties of water solubility and adsorption potential, which allows use the pseudofilters for the control of emissions. Further, use of any active biological process would also be effective in treating the emissions (Fig. 1). The amount and characterization of the gases emitted from composting process vary, and are related to the initial feedstock materials composted and the composting methodology adopted. Ruggieri et al. (2009) have reported the control measures of odor nuisance associated with the composting plants. It includes the optimal combinations of the feedstock materials based on the C/N ratio and assuring the aerobic conditions of the composting mass by maintaining higher levels of porosity. Compared to the turned windrows and aerated piles, invessel composting methods enables faster treatment with small footprint and problems associated with the outdoor composting methods. The importance of gaseous emissions and odor nuisances from composting plants were reported by several researchers (Arriaga et al., 2017; Colón et al., 2012; Nasini et al., 2016). However, there are

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limited literature reports/review articles that detailed all the technical information about the gas emissions and the possible treatment methodologies. Hence, the review aims the readers in providing the collective information through the following objectives: 1. Various advancements and treatments for the various volatile organic compounds (VOCs) and other odorous gas emissions during composting. 2. The review was conducted by comparing the literatures and data related to the VOCs and gas emissions during the composting methods. 3. Latest advancements and major influencing parameters related to the emissions and composting processes compared from research & review articles, conference proceedings, web based reports. 4. Modelling of gas emissions and Environmental impact assessment. 2. Gas emissions from composting The gas emissions from the organic waste start from the storage, collections points and during the transport services due to microbial activities and putrefaction. Therefore, it is important to handle the organic waste as soon as possible after the collection. Smet et al. (1999) have reported the emissions of odors were 2 to 6 times higher after one week of materials storage, which was collected every two weeks once. The composting process when operated under controlled optimum conditions; the potential odorous gas emissions could be reduced. During composting, organic matter loss as volatile gases largely contribute to the overall volume reduction. These gases include CO2, CH4, N2O, sulphur compounds and many other volatile organic compounds (VOCs). Composting is strictly an aerobic process, but however anaerobic conditions prevail in few zones of the windrow piles which are unavoidable. These zones lead to the formation of methane due to the insufficient diffusion of oxygen from the windrow piles (Font et al., 2011). Hao et al. (2004) reported that the emissions of CO2 and CH4 are observed at higher levels from the middle zones of the windrow piles, where nil oxygen was detected. In addition, anoxic conditions could also occur in 2− composting and are available as NO− 3 and SO4 . N2O emissions are reported during windrow composting and are majorly influenced by temperature, nitrate concentration and aerobic conditions (Hellebrand,

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Fig. 1. Composting process and the cycle in atmosphere.

1998). Nag et al. (2016) reported the production of nitrous oxide due to nitrification and denitrification at different aeration conditions. The intense of gas emissions from the composting process depends on three major components, that include the feedstock materials, the composting methodology adopted and the application of final compost. These emissions could be severe if the process is not well operated with proper aeration conditions and also with the final compost, when is applied to the soil. However, the proper maintenance would majorly reduce the gas emissions. Well maintained and operated composters emit lower methane and N2O during composting process (EPA). The incorporation of compost into the soil would result in the long-term carbon sequestration depending on the soil management practices, temperature, rainfall and the feedstocks. It also results in the increased soil water holding capacity, thereby avoiding chemical fertilizers which indirectly minimize the gas emissions associated with less irrigation and the energy inputs. Therefore, the major impact of composting is the avoidance of methane from the organic waste that is landfilled. The use of appropriate bedding materials during composting has a great impact in the gas emissions. The addition of wood chips along the organic waste during composting increases the convection of air circulation within the windrow piles, thereby increasing the circulation of air. Maintaining aerobic conditions during the process largely reduces the emissions of methane and N2O, since most of the degraded carbon would be released as CO2. Among many bedding materials, wood chips provide better water holding capacity and requires less frequent

addition of bedding materials. Moreover, the wood chips are considered to be economically viable compared to other bulking agents (Hao et al., 2004; Ward et al., 2001). Theoretically, the emission from composting operational activities emit around 0.323 and 0.284ton carbon-di-oxide equivalent per ton of mixed waste (Lou, 2008; White et al., 2012). Turning of the compost piles majorly influences the odor reduction by releasing off the trapped gases out of the piles. In turn, lack of turning creates anaerobic conditions thereby increasing the odor nuisance of the piles when they are turned later. Optimization of the turning frequencies of the compost piles with respect to the different organic waste materials highly reduces the gas emissions. Hence, proper turning, height of the compost pile and proper aeration helps in maintaining a proper odor balance. 3. Characterization of VOC emissions from composting Volatile organic compounds (VOCs) belong to the several organic chemicals with higher vapor pressures (boiling points b 80 °C, except methane), while other semi-volatile compounds have boiling points within (80–180 °C). These VOCs cause odor nuisance due to their malodorous and hazardous properties (Komilis et al., 2004). Even though certain VOCs are malodorous, they are not directly associated with affecting human health, but can cause defensive reactions due to the psychological effects (Dalton, 2003). However, certain VOCs can directly affect human health, which include benzene, formaldehyde, 1,3-

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butadiene, trichloroethylene that are reported to be human carcinogens. The impact of odorous emissions caused by these VOCs from composting facilities may be spread or carried away based on the weather and topographical conditions of the location. But, generally the impact is reported to be judged as significant within 500 m of the site location (Heroux et al., 2004). During composting, high concentration of VOCs are released during the initial stages of composting i.e. hydrolytic stage when the microbial activity is higher (Schlegelmilch et al., 2005). Komilis et al. (2004) have reported that the release of VOCs during the initial period is also due to the wetting and heating factors. Generally, the composting plants emit gases that are with high flow rates and low pollutant concentration. However, the VOCs contribution in those exhaust gases remain to be the major pollutants (Pagans et al., 2006). There are N100 VOCs compounds studied during composting process, which can be categorized to individual families i.e., aldehydes, alcohols, carboxylic acids, esters, ketones, sulphides, terpenes, organosulphur compounds, and ammonia etc. (Delgado-Rodríguez et al., 2010; Font et al., 2011). The emission of the different VOCs may vary depending of the feedstock materials and composting phases i.e., (initial phase, thermophilic phase and cooling phase) of the composting (Table 1). The aeration or insufficient oxygen supply to the compost piles play a major role in the release of specific compounds. During composting aerated static piles are reported to release less odors compared to the windrows. This is due to the supply of high oxygen levels by the forced aeration, which reduces the release of odorous compounds (Mosier et al., 1977). Rosenfeld et al. (2004) have reported the positive effects of aeration in reducing the concentrations of 72% NH3, 57% - CH2O2 and 11% of CH3COOH compared to the windrow, composting method. Biofiltration is also an effective odor treatment methodology in composting facilities. The odors could be reduced by passing the polluted air or exhaust gases from compost piles through the biofilter (Fig. 2). The use of negative aeration and biofiltration in reducing odor emissions by N98% from composting facilities have been discussed in many literature reports (Amirhor et al., 1997; Devinny et al., 1998; Rosenfeld et al., 2004). Szanto et al. (2007) reported the optimization of ventilating and turning of the composting pile and other operational parameters for controlling the gaseous emissions.

Volatile sulphur compounds (VSCs) are another important odorous compound belonging to the VOCs that are emitted during anaerobic or inadequate aerobic conditions of composting. These VSCs include the dimethyl sulfide (DMS), dimethyl disulfide (DMDS), hydrogen sulfide (H2S), carbonyl sulfide (OCS), carbon disulfide (CS2) and methyl mercaptan (MM) (Panetta et al., 2005). Zhang et al. (2017) reported the effects of aeration in controlling ammonia and VSCs during composting of kitchen waste. Aeration rates of 0.1, 0.2 and 0.3 L (kg DM min)−1 were studied and was found that the lower aeration was more significant than the other two treatments. Furthermore, during kitchen waste composting around 43 gaseous compounds belonging to volatile sulphur compounds (VSCs), hydrocarbons, aromatic hydrocarbons and other five odor substances were reported by Zhang et al., 2017. The study significantly correlated the loss of sulphur in the form of VSCs. 4. Biological treatment of VOC emissions from composting There are different methods to trap volatile organic carbon (VOC) emissions including incineration (Huang et al., 2014), condensation (Luengas et al., 2015), absorption, adsorption (Jo and Yang, 2009), plasma catalysis (Hoeben et al., 2012), photocatalytic oxidation (Yokosuka et al., 2009), ozone catalytic oxidation, membrane separation (Zhang et al., 2017) and biological methods. Biological methods include bio-filtration, tricking filter, bio-scrubber and membrane bioreactors. This section addresses the different biological treatment methods mentioned above that could be used to trap the volatile organic carbon (VOC) emissions. 4.1. Biofiltration Biofiltration denoted the biotechnological means of treating polluted gases from the organic waste compositing (air). Biofilters are the bioreactors made of biological material i.e. mature compost or any other packed agents (as depicted in Fig. 3), in which the indigenous microorganisms treat the polluted gases by means of diffusion mechanism in the biofilm. Kennes and Veiga (2001) reported the biological oxidation of pollutant within the reactor due to microflora. These biofiltration methods are very effective in eliminating or reducing most of the potent

Table 1 Gas emissions from different organic waste and composting methods. Reference

Composting type

Feedstock materials

Gas emissions

Hao et al., 2004

Open windrow composting

Straw bedded manure (SBM) and wood chip bedded manure (WCM)

Colón et al., 2012

Turned windrows and home composting Window composting

Source-separated organic fraction of municipal solid wastes (OFMSW) Municipal solid waste

–

Organic fraction of municipal solid wastes (OFMSW)

SBM WCM CO2–349.2 kg Mg−1; N2O – CO2–368.4 kg Mg−1; N2O – 0.077 kg N mg−1 0.084 kg N mg−1 Methane and nitrous oxide emissions between 0.34 and 4.37 kg CH4 Mg OFMSW−1 and 0.035 and 0.251 kg CH4 Mg OFMSW−1 . Methane and nitrous oxide emissions ranging from 0.02 to 1.8 kg CH4 Mg OFMSW−1 and 0.0075 and 0.252 kg CH4 Mg OFMSW−1 Emission factor values of 1.70 and 0.59 kg of VOC Mg−1 of OFMSW 0.2 and 7.3 kg VOC Mg−1 of OFMSW treated in two different full-scale composting plants using different technologies The instrument indicated a CO2 concentration that was always higher (ranging from 0.33 to 3.33%) in comparison to nearby atmospheric levels the trails, a high rate of NOx at the beginning of each trial (4.4, 5.9 and 3.9 mg Nm−3 in the first, second and third year, respectively) and limited emissions of aldehydes, ammonia, aliphatic amine and VOCs from the composting piles. Maximum NH3 release was at 2.0 mg m−2d−1 after the second/third turning events. Baseline N2O losses were below 50 mg m−2 d−1, with maximum rates close to 500 mg m−2 d−1 some days after turning works. Methane emissions were mostly below 100 mg m−2 d−1, while CO2 losses were lower than 25 g m−2 d−1. Carbon dioxide peaks (≈250 g m−2d−1) were reached after the second/third turnings. Overall, gaseous N and C losses accounted for 0.1 and 1% of the initial N and C content of the windrows, respectively.

Boldrin et al., 2009

Baky and Eriksson, 2003; Smet et al., 1999 Cadena et al., 2009

Windrow composting

Nasini et al., 2016

Static pile composting

olive mill waste and grape stalk composting for three consecutive years.

Arriaga et al., 2017

Windrow composting

livestock farms (mostly dairy and beef cattle)

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Fig. 2. Compost gas emissions and treatment through biofiltration.

odors emitted from the composting plants that includes ammonia, sulphur compounds, amines and many other VOCs. Biofiltration has the industrial success in Europe and Asia countries (López et al., 2017). Mudliar et al. (2010) reported that N600 chemical processing industries treat VOCs and odors by using biofiltration in Europe. In general, twobiofilter configuration exists; • Fig. 3a, shows the opened biofilters, where the gas flows in an upward direction and are largely installed outdoors due to large space requirement, and are unprotected to climatic conditions. • As depicted in Fig. 3b, the enclosed biofilter with less space requirement and installed within the closed room. The gas flow to the filters can be passed in either ascending or descending directions (Kennes and Veiga, 2001).

The major important factor in designing the biofilter is the air flow rate, which should be calculated per unit surface area of the biofilter. The gases pass through the biofilter containing moist organic or inert material, thereby undergoes biological degradation of the odorous compounds. In general, humidified air enters into the bed of filtration unit. The material used for biofilter such as compost, soil, peat, chipped brush, and bark. Sometimes these materials are blended with a biologically inert material such as gravel in order to maintain adequate porosity for the unit. Bed depth of the biofilter typically ranges from 1.0 to 1.5 m deep. The major operational parameters influencing the biofiltration processes are discussed in the subsequent section. Some practical and operating parameters for biofilters used for VOC treatment are reported in Table 2. Biofilters are advantageous in terms of low investment, operation costs and does not require secondary treatment installments for the treating the waste gas streams. It also has the low pressure drop and is suitable for treating large volume of minimal concentrated odorants. Dissolution of gas into liquid is the rate-limiting step, hence longer gas residence times (30 to 60 s) and the specific active surface area (500–1000 m2 m−3) are required (Mudliar et al., 2010). Some disadvantages in biofiltration include the adjustments of moisture content and

pH control, special care for treating high concentrated pollutants, corrosion of the bed materials due to overload that requires periodic substitute and probable blocking of the packed materials due to particulate matter (Mudliar et al., 2010; Padhi and Gokhale, 2014). 4.1.1. Factor's influencing the process 4.1.1.1. Physical characteristics. The temperature and water content of the packed material in a bioreactor system is a critical factor which should be governed initially and in periodic intervals. The survival of the microorganisms is completely dependent on the moisture content of the biofilm for their microbial metabolism. 40–60% of moisture content is always recommended for the active process. Water content less that the mentioned range would dry out the bed, eliminating the microbial growth and leads to the lower performance of the treating efficiency. In case of higher water content, it leads to oxygen inhibition, higher solubility of hydrophobic compounds, increased backpressure and channelling of the bed. Moreover, the moisture content would also vary on the different type of filter media used. The recommended temperature of the biofilter should be in the range of 20–40 °C, due to the microbial metabolic activity temperature that is closer to the operating temperature. Majority of the active microorganisms with the system perform well at the mesophilic temperature. However, reports suggest the operating temperature of 0–70 °C (Shareefdeen and Singh, 2005). The fluctuation of temperature with the biofilter could be observed as the result of microbial exothermic reactions. The energy output of the microbial reaction could reach up to 50 kcal per hour that can provide the adequate temperature gradient within the filter bed with 2-4 °C and some time it reaching up to 10 °C for high VOC concentration of the inlet (Hwang et al., 2002). Therefore, it should be noted that the temperature gradient between the gas IN and OUT of the biofilter should be carefully designed, unless it can lead to the bed desiccation and poor performance of the system. Major requirement needed for the microorganisms such as carbon and energy would be produced by the degradation of contaminated gases. However, in addition micronutrient supplements are very essential for the active microbial metabolism that includes nitrogen,

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Fig. 3. Typical schematic sketch of biofiltration unit.

phosphates, minerals, and trace elements. Biofilters made of peat and compost naturally is loaded with higher concertation of nutrients that supports the microbial growth. Microorganisms require optimum pH between 7 and 8 for their better metabolism. The pH of the packing materials could be balanced by adding certain during the start of the preparation of bed materials, later it would be replaced with fresh material when its buffering capacity is exhausted. During the bioreactor operation, formation of byproducts such as carbon dioxide, organic acids or sulfate due to microbial degradation would tend to lower the pH. In such cases, additional buffering agents could be supplied to maintain the optimum pH of the system. Pressure is one another critical factor in the design of biofilters. Pressure drop in the filter bed is influenced by flow rate, water content and biomass density. It is important to maintain the 40–80% of inter-particle void space, so as to ensure the operating pressure drop is low and to guarantee fluent gas flow in the biofilters (Kumar et al., 2011).

Generally, biomass concentration is observed higher at the inlet portion that could affect the bed characteristics, by reducing the particle void space, and the compaction of packing material. It is likely observed in all the system, that the increased flow rate directly affected the pressure drop. In addition to these criteria, with a known flow rate, the pressure drop increases exponentially with increase in biomass and with decrease in particle size particularly b1 mm particles (Detchanamurthy and Gostomski, 2012). 4.1.1.2. Filter bed and flow rate. The filter bed supports the growth and provides the medium for the active microorganisms needed for the treatment of odorous gases. The major criteria of the active filter media should have the following criteria: ✓ Active indigenous microorganisms ✓ higher surface area for gas/biofilm exchange ✓ higher porosity

K. Dhamodharan et al. / Science of the Total Environment 695 (2019) 133725 Table 2 Typical operating parameters for biofilters used for VOC removal. Parameters

Values

Filter depth (m)

0.5–2.5 (Mostly, 1) 2–5

Life time (year) Residence time (s)

Pressure drop (m of H2O) Bed porosity Air temperature (°C) pH Specific surface area (m2 m−3) Oxygen content Velocity of air (m h−1) Air humidity (%) Moisture content (%) Pollutant concentration (mg m−3) Acclimation time (d) Performance (%)

Remarks

Optimisation between the residence time and pressure drop Based on inorganic packing materials increase the life time 15 s to several Depends on degradation kinetics minutes Alcohols Nketones Nlinear alkanes Naromatics 0.1–1.0 Variation as a function of support compressing and/or clogging 0.5–0.9 Possible bypass 10–40 Depends on microorganisms growth 5–9 300–1000 Higher value increase the mass transfer coefficient 5–15% (inlet) 100–500 Weak vales due to lower degradation kinetics 60–100 Cent percent is interesting 40–60 Higher level leads to anaerobic death zones and a transfer limitation 10–1000 Higher concentration lead to inhibitions

10–30 90–99.9

Depends on utilization of inoculum Depends on VOC

✓ good water retention capacity ✓ reserve of essential nutrients

Generally, recommended bed materials for biofiltration include peats, soils, composts, and smaller wood chips or barks due to their low cost and easy availability. Normally peats and compost are widely used compared to soils, wood chips and/or barks. Delhoménie and Heitz (2005) reported the use of wood parks in biofiltration units in addition to peat and compost due to its longer life time. Limestone and activated carbon are also recommended for the pH buffering and adsorbing characteristics. The adsorption coefficient of the filter bed is very important in determining the efficiency of a biofiltration system in treating the pollutant gases. Higher porosity of the bed helps in maintaining constant flow rate and helps in avoiding the pressure drop. Bed porosity ranging between 35 and 40% is recommended for soil medium used in the biofiltration system (Delhoménie and Heitz, 2005). The two-major physio-chemical mechanism i.e. the pollutant transmission rate from the polluted gas and the biodegradation rate defines the overall treatment efficiency. The flow rate which is the ratio of bed volume to the residence time signifies the overall performance of the biodegradation. Hence, the active degradation is highly dependent on the above-mentioned parameters. Kissel et al. (1984) reported higher degradation of the pollutants having higher diffusion mechanisms. Therefore, to improve the biofiltration performance, the time required to diffusion processes should be less than EBRT that is the low operating flow rate. Hence the larger filter bed volume is required to maintain the long EBRT. In case of higher flow rates, the degradation efficiency could be minimized due to the lower contact time of pollutant gases with the microorganisms. Moisture content of the filter bed should also be checked, where the higher gas flow into the system could dry out the water. The recommended residence time ranges from 15 s to several minutes. 4.1.1.3. Microbiology and aerobic conditions. Maintaining complete aerobic condition inside the system is very adequate for the survival of the microorganisms and to achieve higher efficiency of the unit. Oxygen concentration of 5–15% is required for the survival of aerobic heterotrophs to avoid oxygen depletion within the unit. Generally, most of

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the systems satisfy the aerobic conditions by the normal inflow of the airstream. However, care should be taken in the case of higher inflow rate that results in the acidic environment and other byproducts. Active microbial populations are very essential for the effective operation of biofiltration. Among the millions of indigenous microflorae, heterotrophic organisms are observed to be more dominant that includes most of the bacteria and fungi. Generally, bacteria and fungi populations in the biofilters will be observed in the range of 106 to 1010 and 103 to 106 CFU/g of bed. The survival of the microbes within the biofilters could be observed for a longer time, even when the filters are not in operation (Delhoménie and Heitz, 2005). The gas transfer rate from the influent pollutant concentration to the biofilm is very important in determining the overall biofiltration efficiency and pollutant degradability. The gas transfer rate is influenced by the inlet flow rate and other physical conditions of the filter bed, particularly pH, water content, temperature and the pollutant load. Fig. 4 depicts the overview of treating soluble compounds at lower concentration. However, the compounds with higher concertation are transferred at higher rate to the biofilm. Generally, it is recommended that the inlet VOC concentration should not exceed 5000 mg per cubic meter in biofilters. In case of higher values, it would tend to lower the efficiency of the biofilter performance, by affecting the microbial activity. It is also reported that the oxygenated hydrocarbons are considered to be easily biodegradable when compared to alkanes and other aromatic hydrocarbons (Deshusses and Gabriel, 2005). 4.2. Bio-trickling filter Bio-trickling filters are advanced biological techniques for odor and VOC control, through which the liquid is trickled providing a controlled pH, salt concentration, and other required nutrients for the process (Lebrero et al., 2014). In biotrickling filters, contaminated air is allowed to pass through the packed bed of inert materials, which is continuously supplied with the liquid containing the essential nutrients for the active microorganisms. The degradation of the pollutants occurs as the contaminated gases pass though the packed materials via absorption onto the biofilm. The schematic description of a typical biotrickling filter is shown in Fig. 5. These filters are primarily used to remove gases with acidic components. Cox and Deshusses (1998) reported the process of co- or counter current configuration for water or gaseous phases does not influence the trickling filter performance. Table 3 gives a range of operating conditions for biotrickling filters. 4.2.1. Factor's influencing the process 4.2.1.1. Filter bed and flow rate. Packing material of the filter bed is one of the important factors deciding the performance of biotrickling filters. The packing bed is supported by the wall and geometry for the resistance to corrosion and thermal insulation. Packing materials enable flow through the bed and the gas/biofilm transfers, pollutant removal, supports microflora growth, and provides resistance over crushing and compaction. To improve the efficiency, the bedding materials such as structured plastics, random dump plastics, resins, ceramics, celite, polyurethane foam, lava rock are suggested. Most of these materials have surface areas between 100 and 300 m2 m−3 or with some cases b600 m2 m−3. Due to the inert nature, these materials do not contain any indigenous microbial population. In other cases, filter beds inoculated with activated sludge, compost extract, enriched cultures etc. could also be practiced. Practically, it could be suggested that using 0.02 to 0.1 m3 of activated sludge per m3 of bed, or 0.2–1 kg of dry cells per cubic meter of bed. A fundamental characteristic of biotrickling filters is the recirculation of the liquid phase on the filling bed. The recommended trickling velocity ranges from 0.01 to 10 m h−1. The optimum trickling flow rate depends on the process and the required rate of degrading polluted gases. Therefore, experimentations with varying flow rate is always

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Fig. 4. Schematic representation of oxygen and pollutant concentration profile in the biofilm.

required to optimize the running conditions and system performance. Mudliar et al. (2010) reported that a balance between the liquid flow rate and the related energy consumption is crucial in view of optimisation of the system in terms of degradation rate and energy costs. In order to survive and oxidize the VOC, a minimum of 5–15% of O2 is required for the aerobic heterotrophic microorganisms. 4.3. Bioscrubber Biotreatment methods are demonstrated as one of the effective treatment methods for reducing VOC concentrations from polluted under ambient temperatures and relative humidity varying from 5 to

15% (Deshusses, 1997). Bioscrubbing method has been considered as an appropriate technology for the degradation of vaporizing solvent pollutants from coating facilities, carboxyl acids, esters, heterocyclic sulphur and nitrogen components, mercaptans, phenols and sulphides. The bioscrubber technology maintains stability during operation and the factors affecting the process include pH, nutrient concentration and operational parameter such as low pressure drops, which could be controlled (Mudliar et al., 2010). Bioscrubbers are highly potential in treating higher concentration of polluted gases compared to other biofilter systems, its capacity ranges from 3000 to 4000 m3/m2 h (Kennes et al., 2009). Le Cloirec et al. (2001) reported 90.1–100% removal of ethanol concentration from polluted gas streams at liquid/air

Fig. 5. Schematic description of biotrickling filter.

K. Dhamodharan et al. / Science of the Total Environment 695 (2019) 133725 Table 3 Typical operating parameters for biotrickling filters. Parameters

Values

Filter depth (m) 10–15 Packing Lava rocks: 5–30 materials mm diameter Random dump packing: 10–100 mm Open-pore foam: 4–10 pores cm−1 Bed porosity 0.5–0.95 Residence time Generally, 10–40 (s) In some case as low as 2–3 Specific surface 100–400 2 area (m m−3) Pressure drop 1–5 cm of water column Air temperature Generally, 10–30 (°C) In some cases as high as 60–70 Air velocity (m 100–1000 h−1) Liquid velocity 0.01–10 (m h−1) Liquid holdup b5 (%) pH VOC control: 6–7 H2S removal: 2–3 Oxygen content 5–15 (%) Moisture 60–80 content (%) Lag time (d) 5–210

Remarks Light packing material allows high column Highly variable

High value avoid the clogging Residence time lower than for biofilter

Lower compared to biofilter

Treatment with high temperature could be efficient with thermophilic populations. Limited by clogging because of microorganisms growth Compromise between the mass transfer in water, flooding point, and the biomass loss

Easy to control

Higher level leads to anaerobic death zones and a transfer limitation Depends on the function of biodegradability of the molecules

ratio of 0.6 × 10−3 and 2 × 10−3. Aerobic bioscrubbers are not fully established or utilized during the biotreatment systems due to its high energy consumption during operation. However, anaerobic bioscrubbers could be positively utilized for transforming the polluted gases to bioenergy to create a positive energy balance. Anaerobic bioscrubbers could be a proper technology for controlling VOC emissions from the flexographic sector. Pilot scale studies with packed scrubber along with an expanded granular sludge bed reactor were reported to remove higher concentrations of VOC with relatively optimized control of pressure drop (Bravo et al., 2017).

4.4. Membrane bioreactors Membrane bioreactors (MBRs) are equipped with biofilm of active microorganisms growing over the membrane surface, which enables the treatment of contaminant gases as they travel through the reactors (Van Langenhove et al., 2004). MBRs possess higher high permeability and affinity of specific membranes towards specific hydrophobic pollutants that helps to overcome the mass transfer limitations during the treatment process (Kumar et al., 2008). The treatment efficiency of MBRs are based on the type of membrane material used, that includes polydimethylsiloxane (PDMS), polypropylene (PP), polyethylene (PE), polyvinylidene difluoride (PVDF) (Kumar et al., 2008) and also its membrane configuration type could be plate and frame, spiral wounded, tubular, capillary or hollow fiber modules (Mulder, 1996). MBRs are highly advantageous over the traditional treatment systems, that include optimal moistening of biomass, removal of degradation products and limiting the decay of biomass MBR configurations of different models have been reported on lab scale studies, that includes hollow fiber (i.e. b 0.5 mm), capillary (0.5 mm b i.e. b 10 mm), tubular (i.e. N 10 mm), flat sheet and spiral-wounded membrane type reactors (Mulder, 1996).

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5. Models on VOC emission from composting Modelling of gas treatment through biofiltration unit simplifies and explains the phenomena of the degradation pattern occurring within the system. Generally, the treatment of polluted gas undergoes physical, chemical, and biological transformation by the action of microflora and other physical conditions of the bed, which is complex and integrated process. However, the mathematical models can explain the complex process occurring within the system in a simpler way. Such explanations will be very much helpful in understanding basic science of the process and also in the development of large scale engineered reactor design, scale up and process optimization. This engineered design of large-scale biofiltration systems needs mathematical models to assess the effect of operational parameters such as flowrate, pollutant concentration, nutrients effects etc. These models are also handy in the development of process tools to explore the performance and prediction of the system. The process models express the behavior of system performance in terms of equations which could be mathematically solved. In the case of biofilters and biotrickling filters modelling, it requires the mass balance of the contaminants, oxygen, by products of the degradation, and biomass growth. This comprises the gas phase, solid phase, biofilm and liquid phase for biotrickling filters. The model concepts and assumptions would vary depending on the process and the final equation would include sum of all products such as odors, VOCs, oxygen, mass transfer, diffusion rates, adsorption mechanisms, biomass growth and all other physical conditions and by-products in all phases. The difference between the several models proposed in the literature reports vary based on the assumptions by the users to establish the mass balance and equations. The complexity of the models is often substantially reduced by using assumptions such as (a) equilibrium between biofilm and/or air interface, (b) ideal mixing in liquid phase, (c) plug flow for air phase, (d) constant biofilm thickness, (e) steady- state conditions, (f) availability of excess oxygen, and (g) use of simple kinetics for biological oxidation. These assumptions help in making the predictive models as simple as possible and also help in doing the valuable correlations by the engineers, consultants and researchers to calculate design variables, to demonstrate the conditions of the process. Sometimes, these models based on the assumptions would also lead to unrealistic and errors in predicting the design parameters. Since many models have been comprehended in biofiltration systems, these models could describe both the steady-state and the transient behavior of the system. Ottengraf and Van Den Oever (1983) literature reports were very first and the most widely used models on the biofiltration system. This model was developed on assuming the plug flow regime in the gas phase, flat geometry for the biofilm and pollutant equilibrium state at the air/biofilm interface. The assumptions of the model also neglect the mass transfer resistance between the gas and biofilm. For the study of biodegradation pattern within the biofilm, the user should start with the Michaelis-Menten expression (no growth assumed), and the limiting cases of zero and first order kinetics were presented. These models are very simple in understanding the process and were used by many researchers (Hodge and Devinny, 1995; Park et al., 2004; Shareefdeen and Baltzis, 1994). First-Order model
 C out −EBRTAS De ϕ tanhϕ ¼ exp C in mδ

ð1Þ

where sffiffiffiffiffiffi K ϕ¼δ De when the pollutant concentration reaches high, the expression rate

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becomes zero in the substrate. Hence in such situations two conditions were established by Ottengraf and Van Den Oever (1983), which include the reaction limitation and diffusion limitation. Zero-Order reaction limited model C out K O AS δEBRT ¼ 1− C in C in

ð2Þ

Zero-order diffusion limitation model 0 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 C out @ K O De ¼ 1−EBRTAS Þ C in 2mC in

ð3Þ

where AS:biofilm surface area, Cin: concentration at the inlet stream, Cout:concentration a the outlet stream, De: effective diffusion coefficient, K: first-order rate constant, Ko:zero-order rate constant, EBRT:empty bed residence time, m: air/biofilm distribution coefficient, δ: biofilm thickness. In reaction-limited model, the biofilm is fully active, whereas in diffusion-limited model, biofilm is not fully active. Therefore, diffusion depth of the pollutant seems smaller than that of the actual thickness in the biofilm. This model is limited to single pollutant. In order to use for mixed pollutants, various researchers have proposed to use expression for the limiting concentration as expressed below 0

C limit

sffiffiffiffiffiffiffiffiffiffiffiffi2 1 mK o ¼@ Þ AS 2De

ð4Þ

Table 4 describes the development of mathematical model in biofiltration system. The modelling work on biofilter and biotrickling filter has shown significant progress in very first model were introduced. The developments have resulted in a better understanding of the biofiltration and/or biotrickling filtration process as well as increase in personal computing tools. In recent times representation such as actual pollutant, fluid dynamics, oxygen mass transfer, and microbial phenomena has been started to include in the realistic model approach. Although these remains an area of intense discussion and disagreement. 6. Environmental impact assessment Environmental impact assessment (EIA) quantifies the consequences of a project that helps in decision-making to the government, industries or policymakers. Composting is often reported as a clean technology that could handle solid organic wastes, however, the associated greenhouse gas (GHG) emissions are not considered into the assessments. The by-product of composting usually is carbon dioxide and other gases that when releases GHG that favors climate change and global warming. For the same, environmental impact assessment of composting process is necessary that accounts the gas emissions released during the composting process. Preferably, capturing carbon dioxide produced during the composting process and utilizing it for the production of other value-added products helps in reducing GHG emissions. The other technologies that could be considered including the power to gas where excess electricity is converted to hydrogen and the carbon dioxide from the composting process could act upon and produce methane through the Sabatier reaction (Eq. (1)) (Gotz et al., 2016). In addition, composting is an aerobic process that consumes energy which needs to be accounted as well to calculate the environmental impact assessments. CO2 ðgÞ þ 4H2 ðgÞ→CH4 ðgÞ þ 2H2 O ðgÞ ΔH ¼ −165:0 kJ=mole

ð5Þ

Emissions from composting could be distinguished into direct and avoided emissions. The direct emissions include fuel consumption during transportation, the energy requirement for the process of

Table 4 Characteristics of selected biofiltration mathematical models. Model description

Results

Reference

Steady-state model used for methanol vapor biofiltration, considering the oxygen limitation and substrate inhibition in their kinetic expression. Steady state model used for treating mixture of pollutants, considering the Monod's degradation kinetics.

Effective in removing methanol at the rate up to 113 g h−1 m−3 of packing. Model suggest that biofiltration was limited by oxygen diffusion and degradation kinetics. Model accounts for potential kinetics interactions among the pollutants, effects of oxygen availability on biodegradation and diversification in the filter bed. Dealing with the transient behaviour of biofilters, the effects of adsorption were separated from the biodegradation.

Shareefdeen et al., 1993.

Mathematical models for steady state and non-steady-state regimes that describes the basic transport and biological process for biofilter. A mathematical model for Development of three phase, taking into account steady state and of pollutant adsorption at transient conditions. the gas/solid interface. This could considered the oxygen limitation and substrate inhibition. The pressure drops in the A model based on Ergun equation to predict the biofilter are due to the accumulated biomass on pressure drop and the bed pellet surface, and biomass thickness. the biofilm thickness can reach several hundred μm after several weeks of operation. All these transient regime Transient model, models describes the proposed a complex complex interaction generalised form of mechanisms such as Monod expression for adsorption, micro substrate kinetics, mass transfer biodegradation. and gas flow patterns. Established transient model take into account of heterogeneity of the packing materials, adsorption phenomena on the pellets surface. It assumes that both biofilm thickness and biomass density remain constant during the operation. At the end, biomass growth are counter balanced by death and maintenance.

Baltzis et al., 1997; Deshusses and Hamer, 1993.

Hodge and Devinny, 1995.

Shareefdeen and Baltzis, 1994; Gerrard et al., 2010.

Morgan-Sagastume et al., 2001; and Delhoménie and Heitz, 2005.

Deshusses and Hamer, 1993; Hwang et al., 2002; Rene et al., 2017; Savvas et al., 2017. Lim and Park, 2006. Zarook et al., 1997; Song and Kinney, 2002.

pretreatment/composting (Bong et al., 2017). IPCC (2006) recognizes the carbon dioxide generated from the composting as biogenic and it does not count as emission, while during the composting process other gases including methane and NOx are considered as GHG. The avoided emissions include transporting the waste to a landfill, emissions from landfilling as it is an avoided process, and replacing mineral fertilizer that avoids the need for its production (Saer et al., 2013). The GHG emissions from the home composting are lower than centralized systems because of the ability to avoid transportation of waste or feedstock and in addition, it is cost-effective. Dissolved organic carbon and nitrogen in a substrate increases the GHG emissions during composting due to the reason it is readily available for degradation hence it is lost to the atmosphere easily (Martínez-Blanco et al., 2010). Some contrasting

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views were being reported on global warming potential (GWP) of composting process. For instance, Quiros et al. (2014) reported higher emissions when no mixing was employed while turning frequently reduced the emission by N5 times. A contrary result was obtained from the studies by Andersen et al. (2012) shows that the emission would be different for different substrates under different processing conditions. It is equally important that the environmental impacts from one study need to be comparable with the other and for the same, standardization of environmental impacts based on regions, processes and calculating methods is essential. Composting food waste resulted in a GWP of 123 kg CO2 eq./ton (Kim and Kim, 2010) while that of mixed waste was 300 kg CO2 eq./ ton (Couth and Trois, 2012). Comparing with landfilling (1010 kg CO2 eq./ton), composting reduced the emission by 70%, however combining technologies such as anaerobic digestion alongside composting and capturing carbon from these processes yield negative emissions (Bong et al., 2017). Ammonia emission from the organic fraction of the municipal solid waste (OFMSW) under thermophilic conditions ranged between 0.34 and 8.63 kg NH3/ton (Cerda et al., 2018). To reduce the emissions from composting process, certain strategies could be employed including adding a bulking agent, modifying aeration system, and pre-composting. Pre-composting the duck manure using vermicomposting reduced the methane and nitrous oxide emissions by 84.2% and 80.9% respectively (Wang et al., 2014). 7. Perspectives and challenges This section provides the common practices for a composting facility related to the emission of odors. Table 5 provides the six key features practiced in most of the composting facilities. Out of which, the steps involved during the handling of materials and composting piles emit large amounts of odors. Hence odor treatment could be targeted effectively to these steps for the overall reduction of odors. Winges (2011) reviewed a critical contribution of odors from a composting facility at Cedar Grove Composting (Seattle, WA), in which the relative percentage of odor flux per unit operation was reported to be 32.9 and 48% (total OU/s) from handling of materials and composting piles. Another case study from the residential area of Augustenborg, southern Sweden consisting Table 5 Key features for a composting facility.

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of 1631 households was analyzed for the environmental impacts from incineration, decentralized composting and centralized anaerobic digestion using EASEWASTE LCA-tool by Bernstad and Jansen (2011). The results supported the lowest greenhouse gas emissions by combining both anaerobic digestion and composting methods. Another approach for the reduction of odors from composting facilities of organic waste treatment could be the preprocessing of waste by anaerobic digestion. This anaerobic digestion step allows the material to undergo an acidic-hydrolysis step, thereby suppressing the odor releasing compounds to a major extent by the production of energy. Later the materials could also be followed (preprocessed waste) by composting processes for nutrient rich end product. The combined treatment of anaerobic digestion followed by composting was reported to reduce seven times lesser ammonia and VOC from the facility (Mata-Alvarez et al., 2000). It could also be suggested that the combined set up of anaerobic digestion and composting facility could be cost intensive. However, proper management and utilization of energy production from anaerobic digestion and profits from mature compost would be potentially economical and environmentally feasible. 8. Discussions The review critically covered the major influencing parameters for the gas emissions during composting and the possible treatment methodologies for those odorous compounds. Also the possible use of modelling concepts and environmental impact assessment of these gaseous compounds. The gaseous/odor emissions during composting or handling of the waste materials is inevitable, while the option of choosing the appropriate composting methodology and the time factor of treating the waste within the shorter time of waste generation could be the best suitable option for reducing the intensive gas emissions. Although the technical uncertainties/maintenance issues of the composting process i.e. aeration/turning of the composting materials has major impact on producing the greenhouse gas emissions. The aerobic condition of the composting process should be monitored as primary factor in controlling the anaerobic zones within the composting mass, which avoids major odouors compounds. The aeration to the composting system could be enabled by forced aerating blowers or by the mechanical turning. The temperature of the composting system should also be given a priority check, where the organic nitrogen would be volatilized at higher rate in thermophilic temperatures. Hence controlling the above mentioned parameters would help in controlling the methane and other anaerobically produced gases which are more potent than CO2 as greenhouse gas emissions. Then, choosing the appropriate biofilter system for the composting materials should be based on the type of waste material composted and the characteristics of the gas emissions/odorous compounds emitted from the system. Since, N100 VOCs are emitted during the composting process, selecting an appropriate biofilter system targeting all the compounds still remains limited. Even though most of the literature reports have listed different system reporting highest treatment efficiency have either used single odors compounds or combination of few, but not for N100 in the laboratory experiment designs. However, priority should be given to the VOCs which are emitted at higher concentration and the relative odor activity value (OAV), which represents the real odor nuisance or GHG emissions impact. These factors and considerations while treating the waste through composting mechanism and odor treatment by biofilter designs would be useful for planning and attaining higher treatment efficiency. 9. Conclusion VOC emissions during the composting methods and their treatment options using biological methods have been reviewed and discussed in detail with respect to the recent and emerging approaches. The paper discussed the treatment options, factors affecting the process, optimized

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