Comparative study of biofiltration process for treatment of VOCs emission from petroleum refinery wastewater—A review

Accepted Manuscript Comparative study of biofiltration process for treatment of VOCs emission from petroleum refinery wastewater—A review Srikumar Malakar, Papita Das Saha, Divya Baskaran, Ravi Rajamanickam

PII: DOI: Reference:

S2352-1864(17)30161-X https://doi.org/10.1016/j.eti.2017.09.007 ETI 158

To appear in:

Environmental Technology & Innovation

Received date : 22 May 2017 Revised date : 10 September 2017 Accepted date : 27 September 2017 Please cite this article as: Malakar S., Saha P.D., Baskaran D., Rajamanickam R., Comparative study of biofiltration process for treatment of VOCs emission from petroleum refinery wastewater—A review. Environmental Technology & Innovation (2017), https://doi.org/10.1016/j.eti.2017.09.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Comparative study of Biofiltration process for treatment of VOCs emission from Petroleum Refinery Wastewater - A Review Srikumar Malakara, Papita Das Sahab, Divya Baskaranc, Ravi Rajamanickamc,* a

Environment Water & Safety Division, Engineers India Limited, Gurgaon-122001, Haryana,

India b

Department of Chemical Engineering, Jadavpur University, Kolkata-700032, West Bengal,

India c

Department of Chemical Engineering, Annamalai University, Chidambaram-608002, Tamil

Nadu, India

*Corresponding Author Dr. R. Ravi Associate Professor Department of Chemical Engineering Annamalai University, Chidambaram – 608002, Tamil Nadu, India Email: [email protected]







ABSTRACT Volatile organic compounds (VOCs) and many in-organic hazardous air pollutants are generated from wastewater of petroleum refinery. Due to the regulatory norms and urgency of treating these hazardous compounds, implementation of suitable technology is necessary. Among many conventional methods, biofiltration is acquiring high importance due to its environment friendly process and cost effectiveness. Operating data, lab experiments and literature review suggests that biofiltration can easily achieve removal efficiency of >90% VOCs and odor in a controlled environment. Hence this process is widely gaining popularity across Europe, Japan, US and other countries. However, due to the complexity in biodegradation and bioactivity, a complete understanding about the process, mechanism of biofilter is needed. In laboratory VOC removal experiment may be effective, but in actual operating condition with multi-component VOCs and in-organics require proper design and optimum operating atmosphere. Also, there are many full scale biofilters installed worldwide, there are less comprehensive review on VOC removal by biofiltration generated in petroleum wastewater treatment plants. In this paper, bio-treatability of VOCs found in refinery wastewater, different bio-reactors, reaction kinetics, designing and operating parameters are discussed. It also outlines the advantages, disadvantages and problems associated with biofiltration.

Keywords: Biofiltration, Wastewater, VOC, Biofilm, Reaction Kinetics







1. Introduction In a typical petroleum refinery effluent treatment plant (ETP) act as end of the pipe treatment facility for various waste water streams emitted from the refinery process units & offsite/utility facilities. In this process ETP receives different volatile organic compounds (VOC) and in-organics like NH3, H2S and so on associated with the liquid effluents and subsequently becomes an emission source for these air substances. As the VOCs have low boiling point or higher vapor pressure at room temperature, they evaporate easily and some are hazardous to environment and humans. VOCs also generate ozone after reacting with oxides of nitrogen which in turn forms smog that is also harmful to humans and vegetation. During petroleum refining, VOCs present in crude oil, distributed to products, some released to atmosphere via leaks/venting from storage, pipes, fittings, equipments, loading/unloading facilities etc., and a part goes to liquid wastewater produced from process units & offsite facilities. Hence, VOC generation in a refinery largely depends upon the type of crude processed, refining process involved, storage commitments etc. Therefore, the total VOC emission is typically ranges from 50-1000 tonnes per million tonnes of crude oil processed (Barthe et al., 2015). Moreover, a hundreds of volatile organic compounds are found in a refinery and are classified as alkanes, alkenes, aromatics and cyclic hydrocarbons. Although there are numerous VOCs present in a typical petroleum refinery wastewater, common VOCs are 2,2,4-trimethylpentane, benzene, biphenyl, cresols, cumene, ethylbenzene, hexane, methyl tertiary-butyl ether, naphthalene, phenol, styrene, toluene, xylene, 1,3-butadiene (RTI International, 2011; Malakar and Saha, 2015). Singh et al. (2013) collected the air sample from a refinery in Indian coastal area and a measured average BTEX (benzene, toluene, ethylbenzene, m-xylene) concentration as 40.2 g m-3, 32.8 g m-3 and 51 g m-3 in process area, ETP area and product loading area respectively. Treatment of these VOCs present in wastewater off-gases is compulsory to meet statutory refinery quality standard and minimize pollution. Hence, VOC removal/control techniques are physical, chemical and biological methods. Among these, biofiltration process strongly applied to treat VOCs in industries as well as public sewer treatment plants due to its cost effectiveness while treating high volume of polluted air at low concentration. A brief description of the major treatment processes for VOC removal is given in Table 1. Though biotreatment process of VOCs is eco-friendly and less established, it is advantageous with respect to operating & capital costs and energy consumption over the traditional processes. Most bio-filters can treat odor and VOCs having removal efficiencies greater than 90% (Leson and Winer, 1991; Adler, 2001). Sometimes odor removal efficiency for biofiltration





process can go up to 99% (Mohamad et al., 2014). A wide variety of organic hydrocarbons like aliphatic hydrocarbons (methane, propane, etc.), aldehydes, aromatic hydrocarbons (benzene, phenol, toluene, etc.), esters, chlorinated hydrocarbons, amines, ketones, alcohols, terpenes, nitriles and inorganic compounds like ammonia, hydrogen sulfide, nitrogen oxide and other sulfur containing compounds can be biodegraded via biofiltration (Govind, 2009). Biofiltration or bio-reaction is a green process (CATC, 2003) which is environmentally safe and economical (Robert and Don. 1997). In biofiltration process, the polluted gas stream passed through the porous media, where the pollutants are absorbed and biodegraded by microorganisms. Microorganisms use VOCs to produce energy and metabolic intermediates and end of the product is CO2, H2O and biomass (Van Agteren et al., 1998; Schmidt, 2012). During metabolism of H2S and NH3, products like oxides of nitrogen (NOX) and oxides of sulphur (SOX) are produced (Chung et al., 2000; Malhautier et al., 2003). Biofilters primarily designed for odor control at composting facilities or municipal wastewater treatment plants. Generally, biofilters are used to control the off-gas emissions from composting operations, rendering plants, adhesive production, food and tobacco processing,

iron

and

steel

foundries,

and

other

industrial

facilities

chemical

manufacturing/operations, composting facilities, chemical storage, coca roasting, landfill gas extraction, film coating, fish frying, slaughter houses, investment foundries, flavours and fragrances, waste oil recycling, tobacco processing, coffee roasting, print shops, pet food manufacturing, industrial and municipal wastewater treatment plants in Europe (Kerry and William, 2007; Leson and Winer, 1991; Robert and Don, 1997 ). Also in US and Japan, biofilters are in widespread use. Biofiltration process can efficiently remove contaminant concentration as high as 5,000 ppmv (Govind, 2009) provided proper designing & operation parameters are followed. For industrial applications of any VOC removal/treatment system, its economic feasibility and operational simplicity are key factors. Presently in India, as VOC emission in ETP is being regulated, VOC handling and treatment systems are coming up in the refineries. Hence, there is immense opportunity to design and install biofiltration based VOC system for treating VOCs generated from ETP as this case is suitable for low VOC concentration and high volume air flow rate. However, as this is microorganism based biological process, a proper study of inlet VOC concentration, air flow rate, surrounding environmental conditions are needs to be studied before designing a biofilter. In this paper, the probable VOCs emanating from refinery wastewater, their treatability and biofiltration parameters are studied and presented which are specific to the refinery wastewater. Moreover, in-organic





compounds like ammonia and hydrogen sulphide which are generated along with VOC in ETP off-gases of refinery that needs to be treated. Though vast research done on the biofiltration, most of the research and data either based on compost based open system or laboratory based single or multi-component removal study. Research specifically to refinery wastewater VOC and its treatment by biofiltration are less. Most typically found VOC components and in-organics in refinery wastewater off-gases and their properties like vapor pressure, solubility and biodegradability are presented in Table 2. It can be noted that all the VOC components and in-organics and which are more or less bio-degradable. However, the environment should be made to sustain microorganisms to grow and thrive in the biofilter. Regular supervision is required to monitor the performance and removal efficiency of biofiltration. Biofiltration is proven technology to treat odorous or volatile organic compounds above 90% including hazardous vapors emitting from petroleum oil refinery WWTP. However, the removal efficiency depends on inlet gases characteristics and surrounding factors. Understanding the filtration of odorous or harmful gaseous contaminants in air by biological degradation is absolutely necessary to design and operate an effective VOC treatment system. Though biofiltration is green process and widely applied in Europe, US, Japan for treating compost gases and livestock odors wherever implementation of this technology in petroleum refinery industry requires technical and commercial feasibility. This paper provides a holistic review regarding thus. 2. Physico-chemical methods Many more well established and emerging VOC removal (recovery & destruction) physic-chemical processes like adsorption (by activated carbon or zeolite or polymer), thermal oxidation, catalytic oxidation, UV oxidation, plasma technology, ionization, absorption, condensation, membrane separation and phyto-remediation are implemented for industrial applications. A different physico-chemical methods used for treating VOCs containing emission as detailed in Table 1. Adsorption process by activated carbon/zeolite is widely used to fill the vessels with granular activated carbon (GAC or any other adsorbent) which is adsorb air pollutants on its surfaces. Thermal oxidation or Incineration is used for higher VOC concentrated mixture. VOC is oxidized at high temperature (~ 870 °C) and producing to CO2 & water. However, catalytic oxidation involves the addition of a catalyst to a thermal oxidizer to accelerate oxidation at relatively lower temperature (320-540 °C). VOC removal by absorption or scrubbing by liquid solvent is also popular in industry. However,





disposal of spent catalyst/ absorbent is a problem. Membrane separation through selective permeable membrane, condensing the VOC laden mixture to liquid phase in sub-zero environment by cooling or compression and using UV irradiation to directly oxidize VOC are many methods that are used for treating VOCs. Among the physico-chemical treatments, VOC destruction method by thermal oxidation or bacterial oxidation are deemed to be most suited as VOC are degraded and there is no scope of re-emission to atmosphere. However, most of the VOC treatment system are granular activated carbon (GAC) based adsorption system. As the quantity of VOC generated in ETP may fluctuate with respect to the inlet wastewater quality and quantity, the exhaustion time of GAC media bed may get reduced or increased. Also the GAC adsorption process is exothermic and generates heat during the process. A heat quenching system and variable air flow control may be required to regulate inlet flow and bed temperature. Moreover, during regeneration of GAC bed, the adsorbed VOC again emitted back to atmosphere as most of cases recovery of VOCs is not economical due to low concentration of adsorbed VOCs. These factors makes GAC based VOC system complex, costlier and sometimes unreliable. 3. Biofiltration Biofiltration generally refers to biological treatment or transformation of organics or in-organic contaminants into harmless compounds whether air or gas phase. Though biodegradation in area of wastewater treatment and bio-remediation techniques are widely applied for treatment of soil and ground water, only in last decades biofiltration is emerging as treatment for VOC removal technique for VOC removal & industrial application. Most of the biofilters built as an open single bed systems (Leson and Winer, 1991) for treating compost gas or odor and gradually emerging as VOC removal technique for industrial application. In the biofiltration, contaminated stream is moistened and pumped to bio filter bed. While the stream slowly flows through the filter media, the contaminants in the air stream are absorbed and metabolized. Biological oxidation by microorganisms can be written as follows: Organic pollutant + O2 + Microorganisms

O2 + H2O + Heat + Biomass

Biofiltration is favored over the conventional control methods because of its lower capital & operating cost, low chemical usage, flexibility in design, can remove a wide range of contaminants, high treatment efficiency and can be tailor made. However, as this is living




processs it is very sensitive to surroundiings and caannot handle high flucctuation in load or process lik extremee weather. Several facctors contriibute to thee overall biofiltration b ke filter media, temperaturee, humidity, pH, resideence time, pressure p dro op, nutrient etc. Biofilttration is efficiennt for the removal r or destructionn of much h type of off-gas o pollu lutants, partticularly organicc compoundds including g inorganicc compound ds such as H2S and N NH3 (Adlerr, 2001). Biofiltration is veery much effective e annd econom mical when the air voolume is high h and w. Merits and demerits oof biofilter have been shown s in Taable 3. contaminant is low Primarily, contaminan c nts that can be treated by biofiltraation shall have the fo ollowing V Kumaar et al., 201 13; Frederickson et al., 2013): characteeristics (Addler, 2001; Vijay (1) Highh water soluubility - Co ompounds hhaving high water solub bility diffusses to waterr layer at biofilm more easiily which is required further forr its biodeg gradation. C Compoundss having v presssure exhibiit higher higher vvalue of Heenry’s consttant (H, mool m-3 Pa-1) and lower vapour water soolubility. (2) Reaady biodegraadability - After A the coompounds absorbed a intto the waterr film (biofiilm), the same iss needed to be biodegrraded, otherrwise the co oncentration n of the sam me will inccrease in biofilm which maay be detrim mental to m microorganiisms and also a the furrther diffusion into biofilm may be decreased d. Compouunds havin ng lower water soluubility and d lower biodegrradability likke halogenaated hydroccarbons etc. can also bee treated in biofilter. However, H the treaatment efficiiency depen nds on desiggning particcular type an nd environm ment for thee same. Removal of contaminaants in a bioofilter is a multi-step m process p starrting from diffusion d hase/biofilm m and thenn cell surrface of of conntaminants from gas phase to liquid ph microorrganisms annd finally biodegradati b ion by micrroorganisms (mainly bbacteria and d fungi). Major pprocesses involved i in n biofiltratioon are illusstrated as follows f (Thhakur et all., 2011; Govindd, 2009; Fredderickson et al., 2013; Aydin et all., 2012):

As the air passes p throu ugh the paccking bed, contaminant c ts are transfferred from m the gas stream to the wateer in the biiofilm. Oncce adsorbed d in the biofilm layer oor dissolved in the water laayer aroundd the biofilm m, organic ccompound is available as food for the microo organism metabolism, servinng as carbon and eneergy source to sustain microbiaal life and growth. Contam minants mayy also be directly addsorbed at the surfacce of the packing and then





transported to microbial cells and degraded. For highly soluble compounds, major removal occurs in water dissolved form, whereas for hydrophobic contaminants, the major removal mechanism may be adsorption on the surface of the medium (Frederickson et al., 2013) and subsequent biodegradation within the microorganism cells. Biological decomposition rate depends on concentration of biomass and specific growth rate coefficient. Governing control parameters in bioreactors are: nature of packing material, inlet flow rate, temperature, pH, humidity and inorganic nutrients. Although in biofiltration system, both bacteria and fungi used and sometimes specialized bacterial or fungal culture is applied, most of biofiltration studies are based on microbial multiplication which is commonly found in biofilters (Francisco, 2014). 3.1. Types of biofiltration process Type of biofiltration process classified based on the design of bioreactor which includes biofilter, biotrickling filter and bioscrubber (Kerry and William; Kauselya et al., 2015; Soccol et al., 2003; Francisco, 2014). Though the basic pollutant removal mechanisms in all types of bio-reactors are almost similar, they are different in microorganism structure (suspended in liquid or may be immobilized in biofilm), packing media type and inlet pollutant concentration. These processes have unique advantages which make them proper techniques to biologically remove variety of compounds. Sometimes hybrid of these processes may be employed depending on the actual treatment requirements and inlet VOC requirements. 3.1.1. Biofilters Biofilters are the simplest and oldest reactor system among the three vapor-phase bioreactors for the treatment of VOCs (Kerry and William, 2007). VOC laden waste gas is passed through a biofilm immobilized support media and are converted by the microorganisms into carbon dioxide, water and additional biomass. Waste gas is humidified prior to routing to bioreactor that is important for treatment. Generally media bed has organic packing material like compost, peat, wood or bark chips etc. or a synthetic like plastic, ceramic etc. Microorganisms develop as biofilm onto this packing media support, which (organic media) can also be source for extra nutrients to support microbial growth. When a synthetic media bed is used, nutrients should be added for microbial growth. For organic packing also nutrients are needed to be supply after some period as the nutrients in media started to deplete. Nutrients are generally sprayed intermittently on the packing material.




Sprinkling of water over the filter media is also applied intermittently to ensure suitable moisture content inside the packing media and enable the wash-out of degradation byproducts (Francisco, 2014). In biofilter, inlet loading of VOC or odor generally is <1 g m-3 (Frederickson et al., 2013). Though biofilters exhibit odor reduction efficiencies always greater than 80%, VOC removal efficiencies are 20-90% (Francisco, 2014). Hence, the operational parameters should be maintained safely to ensure effective removal of VOCs. 3.1.2. Biotrickling filters Biotrickling filters are similar in structure to biofilters with the exception that instead of pre-humidification, there is liquid medium continuously re-circulating through the filter (co-currently or counter-currently) and the use of synthetic packing material (Kerry and William, 2007; Francisco, 2014). Biotrickling filters can handle higher inlet concentrations of contaminants (Soccol et al., 2003). Plastics, ceramics, resins, granular activated carbon, rocks and etc. are generally used as packing media. Due to the inert property of the packing material, biotrickling filters need to be inoculated as the packing is non-organic and in order to maintain microbial activity addition of nutrient solution is also required. Microorganisms grow primarily on synthetic bed as a fixed film, organisms also present in the liquid phase. Contaminants are transferred from the gas phase to the liquid phase and form the biofilm formation for subsequent degradation. As a liquid flow phase exists in biotrickling filter, it offers the advantages to control pH, nutrient concentrations and washing out the degradation by-products. Though biotrickling filters are good for treatment of readily water soluble contaminants but not suitable for poorly soluble contaminants (Francisco, 2014). In biotrickling filter, generally inlet VOC or odor loading is <0.5 g m-3 (Frederickson et al., 2013). 3.1.3. Bioscrubbers In bioscrubber, the contaminated gas is treated via two ways (Kerry and William, 2007; Francisco, 2014; Soccol et al., 2003). In first step, the contaminated air stream is committed with water in a reactor packed with inert media, resulting in absorption of contaminant to the liquid phase. The liquid is then directed into an activated sludge reactor or any biological unit where the contaminants are biologically degraded. The treated liquid effluent from the second bio-reactor (after clarification) is re-circulated to the first reactor (Francisco, 2014). Hence, the second reactor allows the bioscrubber to treat higher concentrations of VOCs than biofilters. Since, absorption and biodegradation occur in two





different reactors, optimization of these reactors can be done (Kerry and William, 2007). Though biotrickling filters and bioscrubbers are offer greater operator control over pH, nutrient and wash out of degradation by-products because of presence of liquid phase. Main disadvantage of bioscrubber over biofilter is disposal of excess sludge/ effluent. As the system relies on absorption, highly water soluble pollutants can only be efficiently treated. Since large part of the VOCs present in ETP or STP emissions are moderately hydrophobic in nature, bioscrubbing then become less popular (Francisco, 2014). In bioscrubber, the inlet VOC or odor loading generally is <5 g m-3 (Frederickson et al., 2013). Characteristics, advantages and disadvantages of biofilter, biotrickling filter and bioscrubber was given in Table 4. A schematic diagram of biofilter, biotrickling filter and bioscrubber was shown in Figure 1. Rene at al. (2012) studied the BTEX removal in a fungi dominated biofilter and overall removal efficiency found to be 35-97% under different operating conditions. Zamir et al. (2011) conducted a compost biofilter experiment with toluene after inoculation with a special type of white-rot fungus Phanerochaete chrysosporium and found to be 92% of reduction efficiency. Li et al. (2012) developed a styrene biofilter experiment containing PU foam as media and the removal efficiency was >96%. Chen et al. (2015) designed a biofilter for toluene removal acquiring suspended biofilm. The removal efficiency obtained was >90.2% under 14 days of start-up time and 128 days of operating time. Rene et al. (2015) conducted a compost biofilter experiment with benzene and toluene loading by varying inlet concentrations and the removal was 72.7% for benzene and 81.1% for toluene respectively. Natarajan et al. (2017) were handling an ethylbenzene and xylene mixture in biofilter having mixed microbial culture with tree bark media. The removal efficiency of 58-78% and 68-89% were recorded for xylene and ethylbenzene for a continuous 96 days of biofilter operation. Rahul et al. (2013) conducted a biofilter operation with corn-cob as filter media with BTEX and observed more than 99% of removal. Gallastegui et al. (2011) operated a toluene and p-xylene biofilter packed with inert material. They observed inhibition of p-xylene in presence of toluene while the presence of pxylene enhanced the toluene removal. Li et al. (2012) designed an integrated bioreactor system that is a gas separation membrane module installed after a control biofilter. Due to combining, the removal of styrene was enhanced and the overall system efficiently handled for the fluctuating inlet load. Also, in the membrane module, styrene was condensed and recovered back to biofilter which in turn extend the retention time. Schiavon et al. (2017) recently conducted the experiment with Non Thermal Plasma (NTP) at upstream of biofilter for removal of mixed VOCs. The NTP using the different specific energy densities reduced





the VOC concentrations down to optimum level. Additionally, plasma treatment converted non-water soluble VOCs to more soluble compounds. They used NTP successfully for pretreatment before biofiltration. In the subsequent sections, governing physical, chemical and biological parameters and operating parameters which are integral part of a biofiltration process are discussed: 3.2. Microorganisms Biofilm contains a mixture of fungi, bacteria, ciliated protozoa, algae, yeasts, amoebae and nematodes. However, in biofilter, bacteria and fungi are mostly found microorganisms (Frederickson et al., 2013). As microorganisms are responsible for degradation or destruction of VOCs, they are the most critical part of a biofilter (Soccol et al., 2003). For microbial population and activity, the most suitable environment (Karl et al., 2002) were sufficient oxygen supply, absence of toxic materials, ample inorganic nutrients, optimum moisture content, moderate temperatures, and neutral pH range should exist in biofilter. All above parameters need to be controlled in biofilter in ensure successful removal of VOCs. The following microorganisms are frequently identified in biofilters ( Robert and Don, 1997; Ottengraf, 1987) like bacteria species: Actinomyces globisporus, Micrococcus albus, Proteus vulgaris, Bacillus cereus, Micromonospora vulgaris, Streptomyces sp., Pseudomonas

sp.,

Corynebacterium

Alcaligenes

jeikeium

A,

xylosoxidans, Corynebacterium

Rhodococcus nitrilophilus,

sp.,

Exophiala

Micrococcus

sp.,

luteus,

Sphingobacterium thalphophilum, Turicellaotitidis, Exophiala sp.; Fungi species: Penicillium sp., Mucor sp., Circinella sp.,

Cephalosporium sp., Cephalotecium sp., Ovularia sp.,

Stemphilium sp., Paecilomyces variotti, Exophiala oligosperma. Microorganisms identified in research experiments for degrading BTEX in biofiltration process have been detailed in Table-5. Microorganism developed in biofilter is dependent on the type of incoming VOCs and other inorganic contaminants to be destroyed, hence treating a wide range of VOCs results in diverse ecology of fungal and bacterial populations (Aydin et al., 2012). Start-up of a biofiltration process always require some acclimatization time (Karl et al., 2002) which allows the microorganisms to develop acceptance or tolerance for VOCs to be treated or component that might be toxic to them. Acclimatization time is approximately 10 days (Karl et al., 2002) for easily degradable substances and for more complex compounds or for mixtures, this acclimatization period may be longer, sometimes it takes months. Microorganisms act as bio-catalysts in biodegradation of VOCs. Researchers have indicated






that heterotrophic bacteria and fungi are the main microorganisms used in degradation of VOCs (Thakur et al., 2011; Aydin et al., 2012). Generally, heterotrophic microbial strains metabolize VOCs via these two or any of pathways (Aydin et al., 2012): (i) Catabolic pathway of consuming organic compounds for energy (ii) Anabolic pathway of using VOCs as a carbon source Sometimes, biofilters are employed intermittently or seasonally, depending on the requirement. In case of shut down of biofilter, biomass can sustain weeks when nutrient and oxygen are supplied. As the substrate is consumed by bacteria then the growth consists of four phases (Metcalf and Eddy, 2007): (1) Lag Phase: Upon introduction of the biomass to substrate (VOCs), organisms require time acclimate to their new environment before multiplication of cells and biomass production occur. (2) Exponential growth phase: During this phase, bacterial cells are multiplying in their maximum rate, as there is unlimited supply of substrate and nutrients. Only temperature affects the rate of exponential growth (Metcalf and Eddy, 2007). (3) The stationary phase: During this phase, biomass concentration remains almost constant with time and bacterial growth is balanced by the death of cells. (4) The death phase: In this phase, as the substrate has been utilised, there is no biomass growth and decrease in biomass is because of cell death. Natural microorganisms are usually proper for biofiltration process, as these are adapted to the contaminant and operation for some specific contaminants, and especially cultured microbes or genetically engineered cultures (Soccol et al., 2003) may be used. Sometimes fungal biofilter is favored (Aydin et al., 2012) over bacterial biofilter for treatment of hydrophobic pollutants because of their ability to degrade contaminants with wide variations in pH, temperatures, low water content and limited nutrient supply. It must be noted that to achieve higher bio-conversion efficiency, the bio-filter should be exposed to low VOC concentrations initially to allow culture adaption for utilising VOCs as a carbon source and then the organic loading can be increased over time (Aydin et al., 2012). 3.3. Biofilm Microorganisms are attached themselves on the rough surface of the packing media and form a viscous slime layer of biological film called as biofilm. Destruction of contaminants in biofilter happens onto the biofilm. Biofilm development may take days or months depending on the media bed and microorganism population. Water soluble





contaminants diffuse into the biofilm, where attached microbes destruct the organic compounds in liquid biofilm. Biofilm thickness generally averages around 1,000 mm but ranges from 10 to 10,000 mm (Mohamad, et al., 2014; Thakur et al., 2011) and for successful operation of biofilter a healthy biomass on media should be maintained. During the biofiltration process, thickness of the biofilm increases. Above certain thickness called active thickness, supply of nutrients becomes a limiting property (Thakur et al., 2011). There are three main processes that occur in biofiltration systems (Thakur et al., 2011; Aydin et al., 2012; Durgananda et al., 2003): (i) Attachment of microorganisms, (ii) Growth of microorganisms and Decay (iii) Detachment of microorganisms. The attachment of microorganisms onto the filter media surface and forming biofilm is complex process and the mechanism by which microorganisms can attach and colonize on the surface of biofilter media are (Durgananda et al., 2003; Kandasamy et al., 2009) (i) transportation, (ii) initial adhesion, (iii) firm attachment, and (iv) colonization. Several forces are involved in microbial attachment to media surface like electrostatic interaction, hydrophobic interactions and covalent bond formation (Durgananda et al., 2003; Thakur et al., 2011). Biofilm mechanism and substrate transport is illustrated in Figure 2. The strength of the attachment relies on various conditions like air flow rate, inlet VOC concentration, oxygen & nutrient availability, type of microorganism and surface properties. Detachment of biomass from bed may occur due the following reasons: erosion due to fluid shear, sloughing of large patches of dead biomass, abrasion by external particle present in inlet air, grazing or predation by protozoa and filter backwashing or air scouring. Though a small portion of biomass may be lost due to backwashing or air scouring, effective biomass responsible for biodegrading organic compounds is not lost after filter backwash (Durgananda et al., 2003). 3.3.1. Substrate transfer and utilization As biofilm is attached onto media surface, supply of carbon source to the microorganisms must be made available for its metabolism. Transport of substrate to microorganisms occurs in bulk phase through biofilm surface. The process involving the transport and substrate utilization within biofilm is (Durgananda et al., 2003; Thakur et al., 2011): (i) Transport of substrate from gas phase to bulk liquid phase:







If a continuous equilibrium is supposed to exist between gas phase and liquid phase in the system, the distribution of the volatile compound over both phases is given by Henry’s law (Ottengraf et al., 1986; Aydin et al., 2012):     

(1)

Where,  is partial pressure of VOC in the gas phase,  is concentration of VOC in liquid phase and H is Henry’s law constant. (ii) Substrate mass transfer from bulk liquid to surface of biofilm: The substrate transport from the bulk liquid phase to the outer surface of biofilm can be best described by Fick’s first law under steady state. Diffusion flux of VOC across biofilm width (direction
) is expressed by (Durgananda et al., 2003; Robert, 1981):  

  

(2)



where,  is diffusion co-efficient or diffusivity in biofilm. (iii) Diffusion of substrate into the biofilm: The carbon source transport to the microorganism inside biofilm by molecular diffusion can be described by Fick’s second law. Continuity equation while doing material balance for a component of the fluid applicable to a differential volume considering solution density is constant and diffusion across biofilm width (direction ), it reduces to Fick’s second law. The mass balance equations around the liquid-phase biolayer in the biofilm are: at an instant of time Continuity equation steady state diffusion with Homogeneous chemical reaction are as follows: differential control volume stationary medium. Changes in VOC concentration with respect to time can be manipulated as (Durgananda et al., 2003; Sivasankari and Rajendran, 2013; Robert, 1981):   

 


  

(3)

 

Having the following boundary conditions: at   ,

  

at   ;

  



(iv) Substrate utilization kinetics within biofilm: The substrate utilization rate can be described by the Monod expression. According to this equation, VOC utilization rate accounted as follows (Frederickson et al., 2013; Durgananda et al., 2003; Reza et al., 2012):

  

  



 




 

 



(4)

Biomass concentration at any time t is expressed as (Ottengraf et al., 1986):





 !   "




#

When oxygen depletes which affects the biodegradation rate, then the growth rate accounted by interactive model. Interactive model from Monod kinetics (Sivasankari and Rajendran, 2013; Dolloff and Rakesh, 2005):  !

  

!




$

 

  

%$

   

%

(5)

However, we are assuming that there is sufficient oxygen is available for the biomass and hence oxygen limitation may be ignored. where, 
! VOC utilization rate (g m-3 h-1)  ! Concentration of VOC in the biofilm (g m-3)  ! Initial concentration of VOC in the biofilm (g m-3)  ! Concentration of VOC in the biofilm at time t (g m-3) 

! Maximum specific growth rate (g m-3 h-1)


! Biomass yield coefficient (g g-1)  ! Biomass concentration (g m-3)  ! Biomass concentration at t=0 (g m-3)  ! Saturation constant or Michaelis-Menten constant (g m-3) For this steady-state model, the kinetics parameters "



   ) are assumed to be

constant throughout the reactor with respect to time. Bacteria populations grow at their maximum rate when the substrate utilization rate was maximized. The maximum growth rate of bacteria is related to the high specific substrate utilization rate as: !



where, k is maximum specific substrate utilization rate and is summarized as the pollutant utilization rate constant as follows:  !   Therefore, Equation (1) can be rewritten as follows:   

! 

   

(6)

where,
 ! substrate utilization rate for VOC, (g m-3 h-1)  ! maximum specific substrate utilization rate constant (h-1) When the substrate concentration is much smaller than the KS value (i.e. when C<



 

"



where  #

# 



(7)

is the first-order reaction rate constant (h-1) when the rate of reaction is



proportional to the substrate concentration. Integration of equation (7) with the boundary condition Cv=Cvo at t=0 & Cv=Cvf at time t gives 

 !

! 

!

 when (

!

#



!

! !



# 
  (8)

) is plotted against t, a straight line should be obtained and the first-order reaction

constant k1 can be determined from the slope of the line. This condition is generally expressed as diffusion rate limited. In this case, biofilm layer is not fully active the reaction rate is controlled by the rate of diffusion of the VOC into the biofilm (Ottengraf and Van den Oever, 1983; Robert and Don, 1997). The substrate concentration is much larger than the value of KS (i.e. when C>> KS), by defining the new constant. Equation can be simplified to a zero order reaction rate as follows: !

"

# 



where,  #

 

(9)


is the zero-order reaction rate constant (h-1) when the rate of reaction is

independent of substrate concentration. Integration of equation (9) with the boundary condition Cv=Cvo at t=0 & Cv=Cvf at time t gives !



!



 # 
 

 "
 #



(10)

When (Cvo-Cvf) is plotted against t, a straight line shall be obtained and the zero-order reaction constant ko can be determined from the slope of the line. This condition is generally expressed as Reaction rate limited. In this case the biofilm layer is fully active the conversion rate is only controlled by the reaction rate (Robert and Don, 1997; Ottengraf and Vanden Oever, 1983). 3.4. Operating parameters As bioreactors use living cultures, they are limited by many parameters in their environment. Efficiency of biofiltration depends on optimizing several variables that enable to maintain healthy microbial community for degrading VOCs within the biofilm. Below are variable parameters that limit the performance of biofiltration:






3.4.1. Packing bed media Filter bed is main support for the microbial population where selecting filter medium is one of the most significant decisions as filter types that can significantly vary in performance, cost and longevity (Frederickson et al., 2013). The most generally used materials in BF beds are peat, compost, soil, compost and wood chips for their stability, low cost and effectiveness. Other media like coconut fibres, lava rock, activated carbon, synthetic materials, paper and ceramic are also frequently used in biofilters. Different media bed has some advantages as well as few drawbacks. Soil has a rich microflora, but it contains very less amount of intrinsic nutrients, low specific area and high pressure drops across beds due to less porosity (Kauselya et al., 2015; Thakur et al., 2011). Peat material contains high specific area, better water retention capacity and permeability. Hence, peat lacks high levels of mineral nutrients and dense indigenous microflora as present in soil or compost (Kauselya et al., 2015; Thakur et al., 2011). Compost is most widely used media; it has a dense and more micro/macro organisms, good water retain ability, good permeability and has high amount of intrinsic nutrients (Thakur et al., 2011). Compost biofilters are very efficient in treating low concentration contaminants of <25 ppmv (Govind, 2009), though treating higher contaminants it is not suitable. Compost media faces problem like treating acidic gases, shorter life, drying, is less stable and high pressure drops (Mohamad, et al., 2014; Kauselya et al., 2015). In biotrickling filters synthetic packing like plastics, ceramic and lava rocks etc. are increasing used. In some biofilters, granular activated carbon used as supporting material to enhance the removal efficiency which also directly adsorbs the contaminant gases. Biofiltration media bed shall have the following characteristics (Aydin et al., 2012; Kauselya et al., 2015; Robert and Don, 1997; Mohamad, et al., 2014; Govind, 2009; Thakur et al., 2011):  High specific surface area for development of microbial biofilm and mass transfer from gas to biofilm.  High porosity for proper distribution of gases. Though porosity is an intrinsic property of media, porosity of bed shall preferably be 35-40%.  High water retention capacity to prevent bed drying.  Good bed drainage to wash out reaction by-products.  Availability of intrinsic nutrients and indigenous microbes. In cases of long-term operation, inorganic nutrients can be periodically added to the bed.  Good buffering capacity in order to maintain a pH of at least 3 which is vital when inorganic contaminants are treated in biofilter.





 Media material shall not have objectionable odor.  Low gas-phase pressure drop across the biofilter bed.  Hydrophilic surface, to allow better water retain ability which is very important to maintain water in the biofilms.  Low cost of biofilter media.  Long life of media. Supporting materials used in biofiltration can be divided into two categories: organic and inorganic. Organic materials including composts and sludge have been used in many studies. The main advantage is organic media that they contain the natural micro-organisms and inoculation may not be needed. This is containing some nutrients necessary for the growth of micro-organisms and easily available. Natural inorganic or synthetic materials like rocks, ceramics, glass, plastics are used in biofiltration, sometimes mixed with organic material. Synthetic packings are sounder and geometrically structured. But for synthetic material, inoculation of bed and regular nutrient dosing are necessary. Though plastic media is costly they can handle the risks of compaction and channelling. From then on, organic material is better than the inorganic material. 3.4.2. Effect of temperature Temperature is one of the most limited factors in biofilter performance as the operating temperature of biofiltration is governed by the inlet gas temperature. Microorganisms responsible for degrading organic compounds within biofilms are strongly influenced by temperature (Frederickson et al., 2013). The biofiltration process is divided into mesophilic (20-45 °C) and thermophilic (45-122 °C) region (Mohamad, et al., 2014) based on the inlet gas temperature. However, the optimum temperature range in biofilter reported in different literature and lab experiments vary. Normally the microorganisms which effectively degrade VOCs in biofilter are mesothermic and their optimum activity temperatures are between 30 and 40 °C (Aydin et al., 2012). In most laboratory studies or industrial applications, the BF unit was run under normal temperature between 15 and 30 °C (Mohamad, et al., 2014). The recommended operating temperature range for high destruction efficiency is between 20-40 °C, with an optimum temperature of 37 °C (98 °F) (Robert and Don, 1997). The microorganism’s activity and growth is optimal in a temperature range of 10-40 °C (Karl et al., 2002). However, industrial polluted air has temperatures more than 30 °C, then it is recommended to cool down the polluted before routing to the biological treatment. The optimum temperature is 20-30 ºC, though mesothermic microorganism temperature is 30-40




ºC (Kauselya et al., 2015). It is suggested that to achieve optimum microorganism performance within a biofilter it should be operating between 30 and 40°C (Frederickson et al., 2013). However, if ammonia is present in the waste gas stream, then taking into account the surviving temperature of nitrifying bacteria (25-30°C) would be recommended. In Some cases increase in temperature enhances the biodegradation (Kauselya et al., 2015) but temperature shall be kept below 40 °C to avoid starting of decay in microorganism and reduction in biofilter efficiency (Kauselya et al., 2015; Frederickson et al., 2013). Heating or cooling of inlet air stream to biofilter might be necessary when the temperature is not falling between 15-40 °C. However, temperature adjustment can be a cost factor in operation of a biofilter. In fact, the proper temperature increases the rate of biofilm development and biomass accumulation. Under optimum range of operating temperature, the degradation performance on biofilter can increase 2 fold by 10 °C increase in temperature (Aydin et al., 2012). The most feasible explanation is that as biofiltration process is exothermic and solubility decreases with increasing temperature for the majority of gases in accordance with Henry‘s law, therefore increasing temperature further leads to VOCs and O2 evolution rather than absorption and diffusion transfer increases. 4.4.3. Effect of pH A pH of the medium in biofilter is very important, which affects the biodegradation efficiency. Microbial growth can be easily affected when biofiltration processes not operated in optimum pH range. As most of microorganisms existing in biofilter exhibit neutrophillic behavior, maximum degradation of VOCs would usually be achieved at neutral pH of around 7 (Aydin et al., 2012). Though fungi can thrive under neutral as well as acidic conditions and are active over a wide pH range between approximately 2 and 7 (Aydin et al., 2012; Karl et al., 2002), bacteria are usually considered to be less tolerant to pH fluctuations (Aydin et al., 2012) and require near neutral pH range shall be in range of 7 to 8 (Karl et al., 2002). The most suitable pH range for biofilter is reported as 7.5 - 8 (Mohamad, et al., 2014; Kauselya et al., 2015; Frederickson et al., 2013; Aydin et al., 2012), though ideal pH of the biofilter medium depend on the contaminants being treated and the characteristics of microbial ecosystem. When inorganic gases like ammonia, hydrogen sulphide and halogenated organic compounds being treated, inorganic acids (H2SO4, HCl, HNO etc.) along with CO2 may be produced, which further lower the pH of media (Karl et al., 2002; CATC, 2003). If filter media not adequate to counter pH drop, lime or soda ash solution may need to be added to the biofilter for adjustment of pH (Kauselya et al., 2015; CATC, 2003; Karl et al., 2002). 4.4.4. Moisture content





Microorganisms always require water to perform their normal metabolic reactions (Frederickson et al., 2013; CATC, 2003; Thakur et al., 2011), sufficient bed and optimum moisture content. Also water presence is required for gas to water to biofilm transportation of substrate and oxygen and detachment of by-products. Biodegradation rate may be hampered due to deficiency of water in microorganisms and as the air stream evaporate the moisture of bed, non-sufficient moisture in inlet gas stream can create drying of bed which subsequently cracks the solid bed causing channeling and high pressure drop resulting in reduction of biofilter efficiency. In case of bed flooding, gas flow to biofilm will be impeded and biological activity will decrease. Too much water can cause flooding and problems such as anaerobic zones, channeling of water & gas, creation of odorous compound, increased in the differential pressure head across bed and will also hinder the transfer of hydrophobic contaminants and oxygen to biofilm (Thakur et al., 2011; CATC, 2003; Adler, 2001; Frederickson et al., 2013; Karl et al., 2002). Moisture content of filter media for optimal operation of biofilter should be 30-60%, 20-60%, 40-60% and 40-70% by weight respectively (Karl et al., 2002). However, optimum moisture content also depends on type of filtering medium to be used, porosity and surface area. Moisture can be added to biofilters by directly pre-humidification of the gas stream before entering biofilter or spraying on the media bed surface. For pre-humidification, relative humidity of inlet gas stream should be 90-95% (Kauselya et al., 2015), 99%-100% (optimum) (Karl et al., 2002; CATC, 2003). Surface sprays to the biofilter surface used and for spraying; water droplets should be kept small in size to prevent compaction of bed. 4.4.5. Nutrients Along with optimum temperature and moist environment, microorganisms need a substrate or food and oxygen for metabolism and inorganic or mineral nutrients (such as nitrogen phosphorous, potassium, calcium, magnesium, sulphur, sodium and iron) to sustain a healthy growth and propagate. Among these, nitrogen phosphorous, potassium is the most vital for microbial cell built up. For aerobic microorganisms, the O/N/P ratio is estimated at 100/5/1 (Karl et al., 2002). If a media bed becomes deficient in certain nutrients, growth of microorganisms can be hindered and could begin to die. Organic support media provides some amounts of intrinsic nutrients, but over the period it depletes and a steady supply of nutrients is needed. For inorganic or synthetic support media generally have no or very limited amount of inherent nutrients, and also needs to be supplied. Common nutrients identified and mostly used in lab experiments and industrial uses are: KH2PO4, NaxH(2-x), PO4, KNO3, (NH4)2SO4, NH4Cl, NH4HCO3, CaCl2, MgSO4, MnSO4, FeSO4, Na2MoO4, and





vitamin B1 (Frederickson et al., 2013; Karl et al., 2002; CATC, 2003; Thakur et al., 2011). 4.4.6. Oxygen content Oxygen is one of the vital parameter because of most VOC or odor reducing microorganisms are aerobic in nature. In inlet air stream, oxygen ought to be at least 5-15% (Frederickson et al., 2013) for requirement of aerobic microorganisms present in biofilms. Oxygen scarcity is not desirable as anaerobic zone inside the bed may be created where reaction can be by-products forming within the biofilm, such as carboxylic acids and aldehydes, which can cause odor. Oxygen content within biofilter shall be adequately supplied by evenly distribution of air over media and oxygen should not be a limiting factor for microorganisms inside biofilm. VOC laden gas shall be diluted and mixed adequately before routing to biofilter. In biofilter, minimum 100 parts of oxygen (Karl et al., 2002) per part of contaminant gas is to be supplied. 4.4.7. Empty bed residence time (EBRT) EBRT is a critical design and operating parameter which is defined as the total filter bed volume divided by the air flow rate. A sufficient EBRT is required for transfer of substrate and O2 to biofilm and bio-oxidation of contaminants to occur. The actual gas residence time is the time available for adsorption and some extent of biodegradation of contaminants in the biofilter and defined as EBRT divided by the air
filled porosity available for gas flow (Joseph et al., 1998). Biofilters treating different contaminants have different absorbing and degradation characteristics and need different EBRTs to be effectively degraded and also depends on reactor volume and gas flow rate (Kauselya et al., 2015). EBRT is also related to moisture content in media and inlet contaminant loading. Higher water retains ability and lower pollutant loading results the shorter EBRT. Biofilters are typically designed to have EBRTs in the range of 15 to 60 seconds (Adler, 2001; Frederickson et al., 2013). Minimum gas residence time suggested is 30 seconds and 1 minute for soil (compost) media. For inorganic gases slightly longer residence times may be required (CATC, 2003). At high concentration, the EBRT is kept high to complete the biodegradation (Kauselya et al., 2015). Overall longer EBRT promotes better removal of pollutant from gas phase. 4.4.8. Pressure drop Pressure drop is a key parameter in cost effective biofilter design. Hence, pressure drop across the given biofilter depends on filter geometry like porosity and media pore size, airflow rate, type of media and moisture content. While geometry and air flow controlled directly, material flow resistance is fixed once the choice of material has been made. Porosity





of the packing can decrease if the bed gets compacted. Pressure drop is increased when the biofilm grows very thick and porosity of filter bed reduced. Hence, a higher inlet pressure is necessary to overcome back pressure of the filter bed. When particle size of filter bed is high, the resistance of filter bed decreases towards gas flow and thus pressure drop decreases. In general, pressure drop across the biofilter ranges from 2-8 hpa (Kauselya et al., 2015) or 1-3 inches of water (CATC, 2003). During operation the pressure drop has been observed to increase as a result of dust and biomass accumulation and filter medium compaction (Andreasen et al., 2012). Increasing in pressure drop in turn increases the overall power requirement. 4.4.9. Inlet flow rate The air flow rate is an important parameter that effects transfer rate of the VOCs from gas phase to biofilm and biodegradation rate of VOCs. Rates of VOCs transfer to the biofilm and degradations decrease with increasing the flow rate as the contact time between microorganisms and gases decreases and biodegradation reaction cannot be completed (Aydin et al., 2012). Pollutant loading rate also determines the size of the biofilter. The typical range of contaminant gas is 0.3-1.6 m3 min-1 (CATC, 2003). However, very high pollutant loading rate can result in overloading the bed and contaminants may pass bed without proper treatment. 4.5. Common biofilter issues & prevention Based on the present studies, biological methods (biofilter, biotrickling filter and bioscrubber) can be used as potential methods for VOCs removal as compared to the other available elimination techniques. However, uses of biological systems to remove VOCs still have limitations and challenges. For effective and consistent biofilter operation, regular maintenance is required which involves media replacement, moisture control of the filter bed, pH control, flow rate control, hardware upkeep and special requirements (nutrient supply etc.). While treating multi component VOC mixture, interaction phenomena like Cometabolism, Cross-inhibition, Vertical stratification shall be taken care of while designing a biofilter system. Biofiltration system should be designed or operated with due considerations to the following limitations (Thakur et al., 2011; Adler, 2001; Mohamad, et al., 2014; Frederickson et al., 2013; Aydin et al., 2012):  Variable pollutant loading Biofilter is less effective for controlling high concentrations of pollutants. Biofilter efficiency does affected when the inlet concentration of pollutants vary. During sudden






increase in inlet concentration, removal efficiency decreases due to lowering of residence time. When the inlet concentration of VOC is less and variable, the removal efficiency also reduces due to the non availability of food to microorganisms over the time. However, after starvation (or shut down) periods removal efficiencies can recover quickly, suggesting brief starvation periods are not critical for the efficiency performance of biofilters (Frederickson et al., 2013). Also certain pollutants like acidic gases, can affect the filter media. If the level of acidic pollutants is very high, the pH of the media can drop to less than 3 and microbial activity decreases which trigger need for alkaline wash (Adler, 2001). However, when the level of acidic pollutants is not high, the bacterial culture in the filter bed can adjust or acclimate itself thereafter.  Clogging of bed Biomass accumulates in a biofilter when growth of inactive layer biomass increased and the removal efficiency decreased due to the difficulty in penetrating to the active biolayer. This excess accumulation may clog the filter bed which in turn creates high pressure drop across bed and channeling. Additionally, back pressure in a biofiltration system can also cause excessive wear and tear on blowing equipment and air channeling will reduce the contact time between inlet air and filter medium affecting removal efficiencies. Also particulate present in the inlet VOC streams tends to compact bed which also causes clogging of the biofilter and decrease in bio-activity due to blocking biofilm. Particulate loadings should preferably be kept below 10 mg m-3 and particulate removal might be necessary for high particulate solid containing gas before routing to biofilters. Control strategies to rectify or prevent blockages and biofilm clogging can be done by (Frederickson et al., 2013): (1) Physical method like periodically mixing the support media or backwashing with water at a high flow rate. (2) Chemical method like washing with chemical solutions or limiting substrate & nutrient to starve the microorganisms. (3) Biological methods like adding higher trophic level organisms such as protozoa, metazoa, and nematodes that can graze on microorganisms and consume dead cells.  Generation of secondary waste stream In biotrickling filter waste stream is generated comprising of contaminants, microorganisms and nutrient. However, quantity this stream is very less and can be treated in existing aeration tank or any biological unit inside ETP/ STP.  Media poisoning by acidifying or bactericidal compounds







Ingress or presence of heavy metals like As, CN- in media bed is detrimental to microbial health. Also presence of compounds in media bed that causes either pH decrease or poisoning of microorganisms causes loss of biodegradation activity. Sometimes inlet substrate or intermediate substrate product which takes longer time to decompose, accumulate in bed and may also act as hindrance for food & oxygen supply to microorganisms. Hence proper design and control along with pre-conditioning of inlet air stream becomes necessary.  Large size Large size of bioreactor is a challenging factor generally for open compost/ soil based biofilter. However, for industrial purpose biofilter can be tailor made considering the space constraints and specific treatment requirements.  Limited life of packing Life of compost or organic media is less. However, life of synthetic media like plastic / PVC media can lose up to 10 years.  Treating of hydrophobic compounds Treatment of hydrophobic compounds such as alkanes, alkenes and aromatics by biofiltration leads to problems, due to low solubility in water. In this condition, fungal biofilter is useful as fungi biolayer can decompose hydrophobic compounds directly (Mohamad, et al., 2014). Elimination capacity of the pollutants increases while using fungi bio-filter and has 10 times more than bacterial bio-filter (Kandasamy et al., 2009). However, biofilter with only selected fungi species is comparatively new approach and they are yet to be installed on commercial scale.  Complexity in construct and operation As biofiltration process comprises of pre-conditioning, humidification, biological decomposition, nutrient dosing, venting treated air, design of biofilter sometimes becomes complicated. During operation also, operator needs to monitor moisture content, inlet flow rate, temperature, pH, nutrient dosing rate. Though maintenance & operation of biofilter is be monitored and recorded on daily basis, until there is no huge fluctuation in the above parameters envisaged, the operation is quite simple. The development of bioreactor should take care of the above problems associated with biofilter and a pilot-scale testing is generally recommended to accurately size a biofilter bed for a multi-component waste gas stream like VOCs generated in refinery wastewater treatment plant.







4. Conclusions and future challenges The biofiltration processes in various forms have been applied worldwide to remove VOCs from contaminated air stream which is environment friendly technologies and destroy contaminants like VOCs, inorganic compounds and other HAPs without generating zero or very less by-products. Biofiltration offers less expansive and a cost-competitive and passable alternative technology for the treatment of pollutant streams containing VOCs, air toxics, and odors. Its success has been proven in Europe and widely being applied in the U.S. More than 7,500 biofiltration reactors have been already installed in European countries to treat polluted air, odor from the wastewater. Based on the present study, bioreactors (biofilter, biotrickling filter and bioscrubber) can be used as potential technology for removal of VOCs removal as compared to the other available elimination techniques. However, use of biological systems to remove VOCs still has limitations and challenges. These systems face problems in handling sudden loading fluctuations in inlet load and changes in surrounding conditions. Due to detrimental nature of VOCs, a more practical and more efficient VOC removal techniques are in demand. A combination of existing treatments can be considered as another approach which may help to increase VOCs treatment effectiveness at higher concentrations, whilst reducing costs e.g., biofiltration as a post-treatment to adsorption. The future of biofiltration depends primarily on the regulatory or statutory requirements imposed on industry and it is expected to grow rapidly in near future. In U.S. alone, biofiltration market was estimated as $100 million in 2000. Biofiltration is most adoptable technology for treatment of organic and inorganic emissions from waste streams and a variety of industrial processes including ETP in petroleum refinery. Design of biofiltration system shall be cleverly done to minimize capital cost and operation cost and the system should not be complex to operate and maintain. Successful biofiltration requires a design to ensure a proper environment for the microorganisms. This includes being able to control and monitor such parameters as moisture content, pH, temperature, and nutrient supply. This review outlines of the biofiltration process used for waste gas treatment, drawbacks of bio-reactors, design aspects and attentions required. There is a necessory to work on innovative technologies such as pretreatment of VOCs to enhance biodegradability and to treat multiple pollutant mixtures & complicated contaminant airstreams. Understanding of the fundamental principles and process requirement of biofiltration process is needed as there are cases of failure in large scale BFs. The operating parameters such as gas flow, media type, moisture content, liquid flow and gas velocity, gas residence time and





pressure drop over the system needs to be carefully selected for optimization of bioreactor. Design of full-fledged modern bioreactor remains in high priority as single prototype bioreactor is not suitable for treating multi-component VOC mixtures and associated inorganic vapors as in this case. Understanding the fundamentals of the bioprocess and biofilters may be helpful to find a logistic, innovative, inclusive and focused approach in bioreactor design and enhance its performance. Based on this review, aspects for future research on this field are proposed as follows: (1)

As the wastewater from ETP fluctuate both in terms of flow rate and quality, accurate estimation of wastewater flow rate is necessary.

(2)

By available estimation tool like WATER9, volatile organic compounds can be predicted based on maximum wastewater generated.

(3)

A number of biofiltration study has been done based on compost gases, livestock gases, landfill gases or food processing industry but few have been done for petroleum industry wastewater off-gases.

(4)

Climatic parameters affecting removal efficiency needs to be studied continuously for at least a year on pilot scale including the cost factors.

(5)

Biofiltration experiment shall be done & recorded based on natural long lasting media like structured wood, loofa sponge etc. instead of just compost or plastic media.

(6)

While designing a biofiltration system factors like usage of less power, less footprint, operational simplicity and uses of suitable hybrid systems needs to be considered.

(7)

Existing bioreactors in ETP like aeration tank or bio-tower or SBR/MBR can be integrated with biofiltration for futuristic design.

(8)

Locally acclimatized culture may be more usefull in degrading VOCs than foreign cultured microorganisms.

(10) Regular monitoring and sampling facilities are to be considered while designing a full scale biofiltration system integrated with online instrumentation & control system like PLC or DCS with recorder & control facilities. Acknowledgement Authors would like to acknowledge all members of Department of Chemical Engineering, Jadavpur University and Department of Chemical Engineering, Annamalai University for their utmost cooperation and support throughout the study.






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Rahul, M.A.K., Balomajumder, C. 2013. Biological treatment and modeling aspect of BTEX abatement process in a biofilter. Bioresource Technology 142, 9-17. Rene, E.R., Balsam, T.M., Maria, C.V., Kennes, C. 2012. Biodegradation of BTEX in a fungal biofilter: influence of operational parameters, effect of shock-loads and substrate stratification. Bioresource Technology 116, 204-213. Rene, E.R., Kar, S., Krishnan, J., Pakshirajan, K., Lopez, M.E., Murthy, D.V.S., Swaminathan, T. 2015. Start-up, performance and optimization of a compost biofilter treating gas-phase mixture of benzene and toluene. Bioresour. Technol. 190, 529-535. Reza, D., Babak, R., Mahzar, A., Mohammad, F., Ahmad, A., 2012. Confirmation of Monod Model for Biofiltration of Styrene Vapors from Waste Flue Gas. Health Promotion Perspectives 2, 236-243. Robert, E.T. Mass Transfer Operations, 3rd Edition, McGraw-Hill. Inc. Singapore, International Edition 1981, pages. 800. Robert, H.P., Don, W.G. 1997. Perry’s Chemical Engineers’ Handbook, Seventh Edition, McGraw-Hill Publishing Company Limited, 456-457. RTI International, 2011. Emission Estimation Protocol for Petroleum Refineries, Version 2.1.1, Final ICR Version – Corrected, Version 2.1: Final ICR Version, Submitted to: Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. 7, 10-11. Sander, R. 2015. Compilation of Henry's law constants (version 4.0) for water as solvent. Atmospheric Chemistry & Physics 15, 4399. Schiavon, M., Schiorlin, M., Torretta, V., Brandenburg, R., Ragazzi, M. 2017. Non-thermal plasma assisting the biofiltration of volatile organic compounds. Journal of Cleaner Production 148, 498-508. Schmidt, M. Synthetic Biology: Industrial and Environmental Applications, Wiley-Blackwell; 2012.

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Sivasankari, M., Rajendran, L., 2013, Analytical Expressions of Concentration of VOC and Oxygen in Steady-State in Biofilteration Model. Applied Mathematics 4, 314. Soccol, C.R., Woiciechowski, A.L., Vandenberghe, L.P., Soares, M., Neto, G.K., ThomazSoccol, V. 2003. Biofiltration: an emerging technology. Indian Journal of biotechnology 2, 396-410. Strauss, J.M., Plessis, C.A.D., Riedel, K.H.J. 2000. Empirical model for biofiltration of toluene. Journal of environmental engineering 126, 644-648. Thakur, P.K., Rahul, Mathur, A.K., Balomajumdar, C. 2011. Biofiltration of Volatile Organic Compounds (VOCs) - An Overview. Research Journal of Chemical Sciences 1, 83-92. USEPA, 1995. Compilation of Air Pollutant Emission Factors. Volume 1: Stationary Point and Area Sources, AP 42, Fifth Edition. USEPA, 2011. Refinery wastewater emissions tool spreadsheet - March 2011. https://www.epa.gov/ttn/chief/efpac/protocol/index.html Van Agteren, M.H., Keuning, S., Janssen, D. Handbook on Biodegradation and Biological Treatment of Hazardous Organic Compounds. Kluwer, Dordrecht, The Netherlands; 1998.

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Figure 2: Biofilm mechanism and phenomena

Figure 1: Schematic diagram for types of biofiltration system

Figure Captions







Figure 1: Schem matic diagram fo for types of bioffiltration system m (Soccol et al., 2003; Vijay Ku umar et al., 2013 3)













Figure 2: Bioffilm mechanism m and phenomen na (Robert, 19881; Thakur et all., 2011; Sivasan nkari and Rajendran, 2013; Mohamad M et al., 2014)




Table 5: Microorganisms for degrading BTEX found in biofiltration process

Table 4: Details of three types of biofiltration processes

Table 3: Merits and de-merits of biofiltration system

Table 2: Details of solubility & biodegradability of VOCs and in-organics

Table 1: Different VOC removal processes description

Table Captions





Vessels are filled with granular activated carbon which is adsorbing air pollutants on its surfaces.
Exhausted GAC can be regenerated and restored adsorption capacity.
The pollutants are not destroyed in the carbon adsorption

Adsorption (By activated carbon or zeolite or polymer)




Process description

VOC removal method

<10 g m-3 (Thakur et al., 2011) 100-5000 ppmv (Edward, 2002) 700-10,000 ppm (<25% LEL) (Faisal and Aloke, 2000) 1-10 g m-3 (Govind, 2009)

Inlet concentration of VOC 5-50,000 m3 h-1 (Thakur et al., 2011) 100-6,000 cfm (Faisal and Aloke, 2000) 100-10,000 m3 h-1 (regenerative adsorption) (Govind, 2009) 10-60 m3 h-1 (nonregenerative adsorption) (Govind, 2009)

Waste gas flow rate Advantages

Drawbacks





Adsorption is exothermic process. For high VOC inlet concentration, temperature of 80-90% (GAC)
Proven and bed may 90-96% (zeolite) widely used. excessively (Faisal and Aloke,
Efficient for rise. 2000) broad range of
The pollutants VOCs. are not destroyed in the carbon adsorption process; they are just transferred to another phase.

VOC removal Efficiency

Table 1: Different VOC removal processes description

process; they are just transferred to another phase.

Process description





VOC is oxidized at high temperature (~ 870 °C) producing CO2 & H2O.
Destruction of Thermal oxidation halogenated or Thermal organics may incineration require higher temperature.
Waste heat from incineration process can be recovered & utilized.

VOC removal method

2-90 g m-3 (Thakur et al., 2011) 10-60% of LEL (Edward, 2002) 100-2,000 ppm (Faisal and Aloke, 2000) 8-140 g m-3 (Govind, 2009)

Inlet concentration of VOC

>10,000 m3 h-1 (Thakur et al., 2011) 2,000-5,00,000 scfm (Edward, 2002) 1,000-5,00,000 cfm (Faisal and Aloke, 2000) 10,000-1,00,000 m3 h-1 (Govind, 2009)

Waste gas flow rate

95%-99% (Faisal and Aloke, 2000)

VOC removal Efficiency


Efficient for high VOC concentrated gas mixture.

Advantages






Oxidation is deemed for low and inefficient for low VOC concentrations .
High energy consumer.


During regeneration process VOC components reemitted to atmosphere or burnt out.

Drawbacks




UV Oxidation

Catalytic oxidation

VOC removal method


Catalytic oxidation involves the addition of a catalyst to a thermal oxidizer to accelerate oxidation at relatively lower temperature (320-540 °C).
UV irradiation is used to directly oxidize VOC with or without photocatalyst.
Highly efficient for much diluted VOC stream.

Process description

-

2-90 g m-3 (Thakur et al., 2011) 25% of LEL (Edward, 2002) 100-2,000 ppm (Faisal and Aloke, 2000) 1-10 g m-3 (Govind, 2009)

Inlet concentration of VOC

20,000-90,000 scfm (Edward, 2002)

>10,000 m3 h-1 (Thakur et al., 2011) <75,000 scfm (Edward, 2002) 1,000-1,00,000 cfm (Faisal and Aloke, 2000) 10,000-1,00,000 m3 h-1 (Govind, 2009)

Waste gas flow rate Advantages

-


VOC removal without any chemical/ consumables.

90%-98% (Faisal
Energy and Aloke, 2000) efficient.

VOC removal Efficiency





Exposure to UV radiations


Process efficiency is sensitive to temperature & other operating conditions.
Disposal of spent catalyst is problem.

Drawbacks

Condensation





Transforming VOC vapor to liquid phase in sub zero environment by

Absorption or Scrubbing

Process description


VOC in air stream gets absorbed in liquid solvent.
The technology mostly used for high concentration of water soluble organics.

VOC removal method

>60 g m-3 (Thakur et al., 2011) >1000 ppmv (Edward, 2002) 5,000-10,000 ppm

8-50 g m-3 (Thakur et al., 2011) >200 ppmv (Edward, 2002) 500-15,000 ppm (Faisal and Aloke, 2000) 10-40 g m-3 (Govind, 2009)

Inlet concentration of VOC

100-10,000 m3 h-1 (Thakur et al., 2011) 100-20,000 cfm (Faisal and Aloke,

100-60,000 m3 h-1 (Thakur et al., 2011) 2,000-1,00,000 cfm (Faisal and Aloke, 2000) 200-20,000 m3 h-1 (Govind, 2009)

Waste gas flow rate Advantages

Drawbacks






Selection of suitable solvent is required.
The pollutants are not destroyed by the scrubbing
Recovery of process, 95-98% (Faisal VOC from transfer to and Aloke, 2000) solvent can liquid phase happen creating another pollution problem.
Disposal of absorbed solvent is problem.
Recovery of
Maintenance VOC from is high. 70-85% (Faisal solvent can
Disposal of and Aloke, 2000) happen spent coolant
VOC removal is problem.

VOC removal Efficiency




Bio-filtration

Membrane separation

VOC removal method


Separation through selective or semi permeable membrane.
VOC is passed through membrane and condensed to liquid for recovery/ reuse.
Under aerobic environment, microorganisms present in

cooling or compression.

Process description




without any chemical/ consumables.

Advantages

Drawbacks





<14,000 cfm
Can handle
Environment (Faisal and Aloke, only air stream 60-95% (Faisal friendly 2000) with lesser and Aloke, 2000)
Cost effective 60-3,00,000 m3 h-1 concentration
Efficient (Compost of VOC.

<5000 ppm (Faisal and Aloke, 2000) <1-25 ppmv (Compost


Membranes are costly.
Economically
Efficient and 90-99% (Faisal viable for low can be used on and Aloke, 2000) flow & high modular basis. concentrated VOC stream.

VOC removal Efficiency

200-1,500 cfm (Faisal and Aloke, 2000)

2000) <3,000 scfm (Edward, 2002) 200-20,000 m3 h-1 (Govind, 2009)

Waste gas flow rate

>50 g m-3 (Thakur et al., 2011) >5000 ppmv (Edward, 2002)

Inlet concentration of VOC (Faisal and Aloke, 2000) 50-200 g m-3 (Govind, 2009)




VOC removal method

packed bed metabolized VOC & other inorganics to water, CO2, inorganic compounds & biomass.

Process description

Inlet concentration of VOC Biofiltration) (Govind, 2009) 20-5,000 ppmv (Compost Biofiltration) (Govind, 2009) <1,000 ppmv (Edward, 2002) Biofiltration) (Govind, 2009) 10-3,00,000 m3 h-1 (Compost Biofiltration) (Govind, 2009) <1,00,000 scfm (Edward, 2002)

Waste gas flow rate

VOC removal Efficiency Advantages






Removal efficiency can fluctuate depending upon VOC constituents, temperature etc.

Drawbacks




Benzene

Biphenyl

Cresols

2

3

Compound

1

Sl. No.

0.3 (USEPA, 2011)

0.00964 (USEPA, 2011)

95.2 (USEPA,1995)

Vapor pressure at 25 OC (mm Hg)

10.0 (p-Cresol) (Sander, 2015)

7.9 (m-Cresol) (Sander, 2015)

4.2 (o-Cresol) (Sander, 2015)

6.09 (USEPA, 2011)

3.5247 X 10 (USEPA, 2011)

-2

3.6 X 10-2 (Sander, 2015)

1.769 X 10 (USEPA, 2011)

-3

1.7 X 10-3 (Sander, 2015)

Henry’s law constant (mol m-3.Pa-1)

Threshold Limit Value (Lide, 2003)

19000 (pCresol) (ICSC, 2016)

24000 (at 20 ° C) (m-Cresol) (ICSC, 2016)

25000 (oCresol) (ICSC, 2016) TWA: 5 ppm

TWA: 0.2 ppm 7.48 (Mackay and Shiu, 1981)

TWA#: 0.5 ppm 1780 (Mackay and Shiu, 1981) STEL@: 2.5 ppm

Solubility at 25 °C (g m-3)

Table2: Details of solubility & biodegradability of VOCs and in-organics

Some (Kristiansen et al., 2011; Kennes and Veiga, 2004)

Some (WHO, 1999; Morris and Lester, 1994)

Moderate (Joseph et al., 1998)

Biodegradability








186 (USEPA,1995)

Methyl tertiarybutyl ether (MTBE)

Naphthalene

Phenol

7

8

9

0.34

0.23 (USEPA,1995)

150 (USEPA,1995)

Hexane(-n)

6

10 (USEPA,1995)

Ethylbenzene

2.8 X 101 (Sander,

2.0433 X 10-2 (USEPA, 2011)

2.1 X 10-2 (Sander, 2015)

1.7778 X 10-2 (USEPA, 2011)

1.7 X 10-2 (Sander, 2015)

8.09 X 10 (USEPA, 2011)

-5

6.1 X 10-6 (Sander, 2015)

1.252 X 10 (USEPA, 2011)

-3

1.4 X 10-3 (Sander, 2015)

Cumene (Iso 4.6 (USEPA,1995) Propyle Benzene) 6.76 X 10-4 (USEPA, 2011)

5

4

1.2 X 10-3 (Sander, 2015) TWA: 50 ppm

TWA: 5 ppm

66666.7

TWA: 5 ppm

TWA: 10 ppm 34.4 (Mackay and Shiu, 1981) STEL: 15 ppm

42000 (at 20 °C) (ICSC, 2016)

TWA: 50 ppm 9.5 (Mackay and Shiu, 1981)

TWA: 100 ppm 152 (Mackay and Shiu, 1981) STEL: 125 ppm

2000 (at 20 °C) (ICSC, 2016)

Good (Joseph et al.,

Low (Thakur et al., 2011)

Low (Joseph et al., 1998)

Moderate (Joseph et al., 1998)

Good (Joseph et al., 1998)

Some (Shahi et al., 2016)







Styrene (Ethenylbenzene)

Toluene

Xylene

10

11

12

8.04 (USEPA, 2011)

30 (USEPA,1995)

7.3 (USEPA,1995)

(USEPA,1995)

1.9 X 10-3 (p-Xylene) (Sander, 2015)

1.4 X 10 (m-Xylene) (Sander, 2015)

-3

2.4 X 10-3 (o-Xylene) (Sander, 2015)

1.634 X 10-3 (USEPA, 2011)

1.486 X 10 (USEPA, 2011)

-3

1.5 X 10-3 (Sander, 2015)

3.589 X 10 (USEPA, 2011)

-3

2.7 X 10-3 (Sander, 2015)

24.8 (USEPA, 2011)

2015)

STEL: 40 ppm

TWA: 20 ppm

185 (p-Xylene) (Mackay and Shiu, 1981)

162 (m-Xylene) TWA: 100 ppm (Mackay and STEL: 150 ppm Shiu, 1981)

175 (o-Xylene) (Mackay and Shiu, 1981)

TWA: 50 ppm 515 (Mackay and Shiu, 1981)

300 (at 20 °C) (ICSC, 2016)

(NCBI, 2016)

Moderate (Joseph et al., 1998)

Good (Joseph et al., 1998)

Moderate (Joseph et al., 1998)

1998)








15200 (USEPA,1995)

Hydrogen Sulphide

16

1 X 10-3 (Sander, 2015)

5.9 X 101 (Sander, 2015)

3.26 X 10-6 (USEPA, 2011)

3 X 10-6 (Sander, 2015)

1.34 X 10 (USEPA, 2011)

-4

5000 (at 20 °C) TWA: 10 ppm (ICSC, 2016) STEL: 15 ppm

STEL: 35 ppm

Good (Joseph et al., 1998)

Good (Joseph et al., 1998)

TWA: 25 ppm

540000 (at 20 °C) (ICSC, 2016)

Some (Chou and Lu, 1998

Some (Liu et al., 2009)

TWA: 2 ppm

2.44 (Mackay TWA: 300 ppm and Shiu, 1981) (ICSC, 2016)

1000 (ICSC, 2016)

@ Short-term exposure limit (STEL), which should not be exceeded for more than 15 min.

# Time-weighted average (TWA) concentration for a normal 8 h workday and 40 h work week.

7470 (USEPA,1995)

Ammonia

15

2,2,449.172 (USEPA, Trimethylpentane 2011)

14

2110 (USEPA, 2011)

1,3-Butadiene

13

1.3 X 10-4 (Sander, 2015)




 Clogging of the medium due to particulate medium.

 Biofiltration is versatile enough to treat odors, toxic




 Biofilter media life is about five years. There are several beneficial uses for spent compost media: landfill cap material, farm application, fill material, etc.

industrial settings.

 Biofiltration units can be designed to physically fit into most

conditions for many emission points

 Possibility of different media, microorganisms and operational

the industrial unit setting, optimizing spaces

 Biofiltration units can be designed to fit in shape and size to

 No combustion source

 Lower chemical usage

 Lower capital and operational costs

for low concentrations of contaminants (<1000 ppm).

 The treatment efficiencies of these constituents are above 90%




 In some cases it is required to regulate pH of the filter media by the addition of agents such as calcium carbonate, dolomite or sulphur.

nutrients for the microorganisms.

 Additions of nutrients. Nitrogen and phosphorus addition may be warranted as

Therefore, the system will not operate at peak efficiencies during this time period.

 Acclimation of the microbial population may take a few weeks to a few months.

microbial population, affecting the overall efficiency.

 Sources with emissions that fluctuate severely can be detrimental to the biofilter’s

 Low temperatures may slow or stop degradation.

rates, mainly chlorinated VOCs.

 System is not fitted for compounds which have low adsorption and degradation

detrimental to the of a biofilter’s microbial population and overall performance.

 Sources with emissions that fluctuate severely or produce large spikes can be

units or open areas to install a biofiltration system.

 Contaminant sources with high chemical emissions would require large biofilter

 Packing has a limited life.

 No unnecessary waste streams are produced.

compounds, and VOCs.

 Close control of operating conditions is required.

 Extremely large size of bioreactor challenges space constraints.

 Less treatment efficiency at high concentrations of pollutants.

De-merits

 Easy to operate and maintain.

concentration

 Degrades a wide range of compound with different

Merits

Table 3: Merits and de-merits of biofiltration system




Advantages

Application

Characteristics/

Catagory

 Mobile water phase  Single reactor  Low / medium VOC

 Immobile water phase

 Single reactor

 Removal of odour and low VOCs

 Degrades a wide range of components

 Does not produce waste

constraints

 Less operating and capital





 Able to deal with high flow rates

 Does not face drying

 Does not face plugging

 Hot polluted air is treatable

 High durability of synthetic

 Does not need inoculation beds

 Low pressure drop

 Does not produce wastewater

 Using natural matters as biofilter bed

 Nutrients need not be provided

 Compact equipment

conditions

 Better control of reaction

5 g m-3

 High efficiency

growing microorganisms

 Better retention to slow

less than 0.5 g m-3

 Target VOC concentration is

 Target VOC concentration less than

 Low/medium VOC concentrations

 Two reactors

 Mobile water phase

 Suspended biomass

Bioscrubber

 Low capital and operation costs

 Easy operation and start-up

 High gas-phase surface area

1 g m-3

 Target VOC concentration is less than

concentrations

 Immobilized biomass

 Immobilized biomass

concentrations

Biotrickling filter

Biofilter

Table 4: Details of three types of biofiltration processes




Disadvantages

operation

 Complexity in construct and

 Less treatment efficiency at high

concentrations of pollutants

fluctuating concentration

 Requirement of design for

biomass in the filter bed

 Accumulation of excess

expensive

materials are expensive Very

 Problems in treating acidic gases

 High-pressure drops

channeling

 compaction of natural bed and

drying

 BFs faces problems such as plugging,

matters nutrients Synthetic supporting




 Can be complicated to operate and

compounds

 Treats only water soluble

 Complex operation

 Expensive

 Produces wastewater

operational costs.

 High investments, maintenance and

degradation rates

 Extra air supply needed at high

 Needs preparation of water and

biofilter bed in comparison to synthetic

 Difficult start-up procedures

 Complex operation

 Short durability of natural matters as

 Disposal of excess sludge

microorganisms

 Wash out of low growing

 Low surface area for mass transfer

requirements

 Relatively smaller space

 Relatively low pressure drop

control of operating parameters

 Operational stability and better

and severe fluctuations

 High operation cost

 Difficult start-up procedures

transfer

 Low surface area for mass

degradation product of VOCs

 Capability to treat acid

through put

 Less relation time / high volume

 Large area required

concentration

 Slow adaptation to variation in gas

 Poor control of operation conditions

 Low pressure drop

produced

 No unnecessary waste streams are

 Easy to operate and maintain




particulate medium

 Clogging of the medium due to

 Packing has a limited life  Secondary waste stream

 Generation of liquid waste

disposal

 Excess sludge will require to





 Extra air supply may be needed

maintain




Ethylbenzene

Benzene

Target VOC

Exophiala spp.

Rhodococcus sp.

Alcaligenesxylosoxidans

Pseudomonas sp.

Bacteria

Fungi

Paecilomycesvariotti

Paecilomycesvariotti

Microorganism

Vermiculite

Biofilter with BTEX

Perlite

PU foam

Biofilter with Benzene & Toluene

Bio-trickling filter with BTEX

Vermiculite

Sugarcane Bagasse & glass beads

Media type

Biofilter with BTEX

Bioscrubber (Twophase partitioning bioreactor)

Type of biofilter

Table 5: Microorganisms for degrading BTEX found in biofiltration process

90-100%

20-32%

90-95% (avg.)

12-30%

95% (avg.)

21-near to 100%

Removal efficiency





Mohammed et al., 2007

Garcia-Pena et al., 2008

Lee et al., 2009

Garcia-Pena et al., 2008

Davidson and Daugulis, 2003

Sene et al., 2002

Reference




Toluene

Target VOC

Rhodococcus sp.

Pseudomonas strain,Bacillus

Corynebacterium jeikeium A, Corynebacterium nitrilophilus, Micrococcus luteus, Pseudomonas mendocina, Sphingobacteriumthalphop hilum, Turicellaotitidis

Bacteria

Fungi

Paecilomycesvariotti

Exophialaoligosperma or Paecilomycesvariotii

Microorganism

Vermiculite

PU foam

Biofilter with Benzene & Toluene

Perlite

Composted pine bark

Media type

Biofilter with BTEX

Biofilter

Type of biofilter

93-94%

17-31%

>99%

>90%

Removal efficiency

Lee et al., 2009





Garcia-Pena et al., 2008

Estevaz et al., 2005

Strauss et al., 2000

Reference




Xylene

Target VOC

Exophiala spp.

Bacteria

Fungi

Paecilomycesvariotti

Microorganism

Biofilter with BTEX

Bio-trickling filter with BTEX

Type of biofilter

Vermiculite

Perlite

Media type

m & p-xylene: (21-30%)

(17-31%);

o-xylene:

33-47% (pxylene)

Removal efficiency





Garcia-Pena et al., 2008

Mohammed et al., 2007

Reference

HIGHLIGHTS


Properties and biodegradability of volatile organic compounds present in petroleum refinery wastewater are summarized.




Types, treatability, advantage and disadvantage of biofiltration for removal of odorous and VOC components are summarized and reviewed.




Substrate transfer from gas phase to liquid phase, from bulk liquid to surface of biofilm reviewed.




Substrate utilization by microorganisms and kinetics within biofilm reviewed.




Design and operational parameters like packing media, temperature, ph, moisture content, nutrients, oxygen content, EBRT, pressure drop, inlet flow rate are reviewed.




Common biofilter issues & prevention during design and operation were reviewed.




Future challenges in biofiltration process for treatment of VOCs emanating from Petroleum Refinery Wastewater are suggested.