Biofil­ter for the de­odour­iza­tion of in­dus­trial emis­sions

Chemical Industry Digest - - What’s In? - A Gan­gagni Rao, Bharath Gandu, Kranti Ku­ruti

- A Gan­gagni Rao, Bharat Gandu, Kranti Ku­ruti, CSIR-IICT, Hy­der­abad

Odour is one of the ma­jor prob­lems of in­dus­trial emis­sions and it could range from of­fen­sive to nox­ious. Bi­o­log­i­cal meth­ods are quite ef­fec­tive in ad­dress­ing this prob­lem com­pared to physico-chem­i­cal tech­niques as de­scribed in the ar­ti­cle.

Ab­stract

Gaseous emis­sions from var­i­ous in­dus­tries pose prob­lem to hu­man and en­vi­ron­men­tal health. Strin­gent en­vi­ron­men­tal leg­is­la­tions en­forced by govern­ment agen­cies, have led pol­lut­ing in­dus­tries to adopt ef­fec­tive air pol­lu­tion treat­ment pro­cesses to com­ply with these reg­u­la­tions. In­dus­trial waste gases are tra­di­tion­ally be­ing treated by physico-chem­i­cal meth­ods like ad­sorp­tion, scrub­bing, con­den­sa­tion, etc. Bi­o­log­i­cal waste gas treat­ment rep­re­sents a new treat­ment al­ter­na­tive. The suit­abil­ity and per­for­mance of bi­o­log­i­cal meth­ods for the treat­ment of a wide range of or­ganic and in­or­ganic com­pounds has been proven at pi­lot level and ac-cord­ingly their im­ple­men­ta­tion and use at in­dus­trial scale is cur­rently grow­ing ex­po­nen­tially com­pared to physico-chem­i­cal tech­nolo­gies. Bi­o­log­i­cal meth­ods are the most cost-ef­fec­tive and sus­tain­able tech­nolo­gies as the con­tam­i­nants are de­graded into in­nocu­ous or less con­tam­i­nat­ing prod­ucts un­like in physico-chem­i­cal meth­ods where the con­tam­i­nant is sim­ply trans­ferred from one phase to an­other. This ar­ti­cle re­views the bi­o­log­i­cal meth­ods of for the treat­ment of emis­sions caus­ing nox­ious odour.

Dr A. Gan­gagni Rao is Chief Sci­en­tist at CSIRIn­dian In­sti­tute of Chem­i­cal Tech­nol­ogy (IICT), Hy­der­abad. He has about 28 years of re­search ex­pe­ri­ence in the field of bi­o­log­i­cal waste man­age­ment (anaer­o­bic di­ges­tion) and bi­o­log­i­cal gas pu­rifi­ca­tion. The tech­nolo­gies de­vel­oped by him are com­mer­cially proven in the field and work­ing suc­cess­fully. He is re­tained as ad­vi­sory con­sul­tant by re­puted com­pa­nies and he has won sev­eral pres­ti­gious awards. He has 50 re­search pub­li­ca­tions and 4 patents to his credit.

Dr Bharath Gandu has ob­tained his Doc­toral de­gree un­der the guid­ance of Dr A Gan­gagni Rao. Presently car­ry­ing out his post-doc­toral stud­ies in Is­rael and ex­per­tise in the ar­eas of bi­o­log­i­cal gas pu­rifi­ca­tion, anaer­o­bic di­ges­tion and bio­elec­tro­chem­i­cal cells.

Kranti Ku­ruti is pur­su­ing Doc­toral stud­ies in En­gi­neer­ing sciences (AcSIR) un­der the guid­ance of Dr A Gan­gagni Rao. His ex­per-tise is in the ar­eas of bio­gas, bioethanol, bi­o­log­i­cal gas pu­rifi­ca­tion, and volatile fatty acid gen­er­a­tion from var­i­ous or­ganic sub­strates.

In­tro­duc­tion

Malodourous gases (volatile or­ganic com­pounds and volatile in­or­ganic com­pounds (VOC & VIC)) emit­ted from var­i­ous in­dus­tries pose prob­lem to hu­man and en­vi­ron­men­tal health and this af­fects the im­age of com­pany also. How­ever, this prob­lem has not re­ceived suf­fi­cient at­ten­tion till re­cently. Gaseous emis­sions hav­ing volatile or­ganic com­pounds (VOCs) and volatile in­or­ganic com­pounds (VICs) that cause odour prob­lem is en­coun­tered in var­i­ous in­dus­trial sec­tors such as re­finer­ies, latex pro­cess­ing, phar­ma­ceu­ti­cals sec­tors, tan­ner­ies, waste treat­ment plants, poul­try farms, fish pro­cess­ing fa­cil­i­ties etc. 1,2]. There­fore, gaseous emis­sion con­trol is very

[ much es­sen­tial not only due to prob­lems to pub­lic, but also from the VOCs and VICs re­moval point of view. In­dus­trial waste gases have tra­di­tion­ally been treated by physic­o­chem­i­cal tech­niques such as ab­sorp­tion, ad­sorp­tion, con­den­sa­tion, ther­mal, cat­alytic incin­er­a­tion and mem­brane sep­a­ra­tion. Ad­vanced ox­i­da­tion pro­cesses are most pop­u­lar among tech­niques. Bi­o­log­i­cal waste gas treat­ments rep­re­sent a new treat­ment al­ter­na­tive. Four ma­jor biore­ac­tor de­signs are: biofil­ter, bio trick­ling fil­ter, bio scrub­ber and mem­brane biore­ac­tors. Amongst these biofil­ter, biotrick­ling and bio­scrub­ber tech­nolo­gies have largely been ac­cepted by in­dus­try, but mem­brane biore­ac­tors are still in de­vel­op­men­tal stage.

Physico-chem­i­cal meth­ods

The most com­mon non-bi­o­log­i­cal treat­ment tech­nolo­gies are ab­sorp­tion, ad­sorp­tion, ox­i­da­tion and ther­mal meth­ods. These can be used as stand­alone pro­cesses or in com­bi­na­tion with bio­pro­cesses[ 3]. The phys­i­cal meth­ods in­volve trans­fer of waste gas from one phase to an­other phase such as trans­fer to a solid or liq­uid me­dia. Fol­low­ing is the brief out­line of the above pro­cesses.

Phys­i­cal treat­ment

Gen­er­ally mask­ing is done to con­trol the bad odour. Mask­ing in­volves ad­di­tion of pleas­ant smell com­pounds to over­come un­de­sir­able odour 4]. This

[ method may be ap­pli­ca­ble where area of bad odour spread­ing is small and the con­cen­tra­tion is low. In fact, mask­ing of the odor­ous com­po­nents is a tem­po­rary so­lu­tion only even for small area. Hence, mask­ing method is un­suit­able for the pu­rifi­ca­tion of the waste gases em­a­nat­ing in­dus­trial sec­tors where the quan­tity is high.

Ther­mal or Cat­alytic incin­er­a­tion

Ther­mal incin­er­a­tion aided by cat­a­lysts is very fast, takes less than a sec­ond. It en­sures 99% de­struc­tion of vir­tu­ally all or­ganic com­pounds 5]. Such sys­tems are

[ de­signed to han­dle a ca­pac­ity of 1000 to 500,000 cfm (cu­bic feet per minute) and VOCs con­cen­tra­tion ranges from 100 to 2000 ppmV. But it con­sumes large quan­tity of fuel and is there­fore an ex­pen­sive process. Since the op­er­at­ing tem­per­a­ture is 710oC to 980oC incin­er­a­tion pro­duces NOx which should be cap­tured and treated be­fore dis­pens­ing, thus adding to the ex­pen­di­ture. Halo­genated com­pounds are con­verted to their acidic coun­ter­part and it may ne­ces­si­tate the use of ex­pen­sive cor­ro­sion re­sis­tant ma­te­ri­als of construction and use of ad­di­tional acid gas con­trols such as scrub­bing as fol­low up treat­ment. In ad­di­tion, there are con­cerns re­gard­ing the for­ma­tion of diox­ins when chloro or­ganic con­tam­i­nants are in­cin­er­ated. Cat­a­lysts and heat re­cov­ery meth­ods can re­duce fuel costs but it needs greater cap­i­tal and main­te­nance costs. The method of cat­alytic com­bus­tion can only be used with well-de­fined waste gases, since poi­son­ing of cat­a­lyst is likely to take place by cer­tain com­pounds.

Ox­i­da­tion

It is one of the emerg­ing pu­rifi­ca­tion tech­niques for both waste­water and waste gases due to its ver­sa­til­ity and high ef­fec­tive­ness at low tem­per­a­tures. Ox­i­da­tion pro­cesses are ei­ther chem­i­cal or photo cat­alytic. This mech­a­nism pri­mar­ily de­pends on the char­ac­ter­is­tics of ir­ra­di­a­tion, photo cat­a­lyst and con­cen­tra­tion of the ox­i­dants 6]. Re­cent stud­ies have shown greater re­moval

[ of VOCs by a com­bined O /TiO /UV process, as ex­cess

3 2 ozone mol­e­cules could scav­enge hy­droxyl rad­i­cals pro­duced from the ex­ci­ta­tion of TiO by UV ra­di­a­tion

2

8]. The re­ac­tion in sev­eral in­stances is quite fast and [7, re­moval ef­fi­ciency of­ten ex­ceeds 90%. Chem­i­cal ox­i­da­tion is in­ef­fec­tive for hy­dro­car­bons 10].

[9,

Ab­sorp­tion or Scrub­bing

Ab­sorp­tion or scrub­bing is a dif­fu­sion mass trans­fer op­er­a­tion by which sol­u­ble gaseous pol­lu­tants are re­moved by di­rect dis­so­lu­tion in an ab­sorbent liq­uid. Ab­sorp­tion or scrub­bing is one of the most fre­quently used tech­nolo­gies for con­trol­ling the con­cen­tra­tion of VOCs and VICs (odor­ous com­pounds) be­fore they are dis­charged into the at­mos­phere 11]. It in­volves the

[ trans­fer of the pol­lu­tant from the gas phase to the liq­uid phase across the in­ter­face in re­sponse to a con­cen­tra­tion gra­di­ent with the con­cen­tra­tion de­creas­ing in the di­rec­tion of mass trans­fer. A key vari­able of this process is the se­lec­tion of a suit­able liq­uid ab­sorbent.

Scrub­bing with wa­ter: VOCs and VICs from air stream can be re­moved by scrub­bing with wa­ter us­ing sieve plate col­umn, spray cham­ber etc. Counter cur-

rent op­er­a­tion is most com­mon in packed scrub­bers for waste gas pu­rifi­ca­tion. Treat­ment of con­tam­i­nated wa­ter by bi­o­log­i­cal or chem­i­cal meth­ods be­fore dis­posal is re­quired that adds to the cap­i­tal and op­er­at­ing cost of the in­te­grated process. The lim­i­ta­tion of the process is that it is ap­pli­ca­ble for waste gas con­tain­ing wa­ter sol­u­ble com­pounds only 12]. [

Scrub­bing with sol­vents: The VOCs and VICs from gas stream can be scrubbed with suit­able sol­vents (Ex: Hy­dro­gen per­ox­ide, sodium hypochlo­rite, etc.) and the sol­vent can be re­gen­er­ated ap­pro­pri­ately. The costs in­volved in re­gen­er­a­tion are ex­pen­sive. The ma­jor draw­back of this tech­nol­ogy is the ne­ces­sity to dis­solve the gaseous pol­lu­tants in an aque­ous phase. This is crit­i­cal, as res­i­dence time of the gas phase in the ab­sorp­tion col­umn is short. Scrub­bing is there­fore of in­ter­est for gaseous com­pounds with a Henry’s Con­stant (or) parti-tion co­ef­fi­cient of less than 0.01. This is of ma­jor im­por­tance since most of the tar­get odours caus­ing com­pounds are volatile and poorly sol­u­ble in most of the sol­vents and wa­ter 13]. [

Mem­brane tech­nol­ogy

In a typ­i­cal mem­brane sep­a­ra­tor 10], the waste gas [ stream is fed to an ar­ray of mem­brane mod­ules, where or­ganic sol­vents pref­er­en­tially per­me­ate the mem­brane. The or­gan­ics in the per­me­ate stream are then con­densed and re­moved as liq­uid for re­cy­cle or re­cov­ery. The pu­ri­fied gas stream is re­moved as the residue. Trans­port through the mem­brane is in­duced by main­tain­ing the higher va­por pres­sure on the per­me­ate (down­stream) side of the mem­brane and lower va­por pres­sure on the feed (up­stream) side. In some cases, a vac­uum pump is re­quired on the per­me­ate side to main­tain this driv­ing force. A com­pound per­me­ates the mem­brane at a rate de­ter­mined by its per­me­abil­ity in the mem­brane ma­te­rial and par­tial pres­sure (driv­ing force). In some sys­tems, the feed stream is com­pressed on the feed side of the mem­brane to pro­vide the pres­sure drop for the mem­brane and to al­low op­er­a­tion of the sol­vent con­denser at a higher tem­per­a­ture.

Bi­o­log­i­cal meth­ods

Bi­o­log­i­cal meth­ods play a very im­por­tant role in the con­trol of VOCs and VICs gases that are emit­ted by pol­lut­ing in­dus­tries. Al­though sev­eral dif­fer­ent con­fig­u­ra­tions ex­ist, there are three ba­sic types of bi­o­log­i­cal re­ac­tor sys­tems used to treat waste gases: biofil­ters, bio trick­ling fil­ters and bio­scrub­bers 14]. These can be [ grouped into two types. In bio­scrub­bers mi­cro-or­gan­isms are dis­persed freely through­out the liq­uid phase and in biotrick­ling fil­ters, biofil­ters and mem­brane bio- re­ac­tors micro­organ­isms are im­mo­bi­lized or at­tached on a pack­ing/ car­rier ma­te­rial/mem­brane. In bio­scrub­bers and biotrick­ling fil­ters the wa­ter phase is con­tin­u­ously mov­ing, whereas in biofil­ters it is sta­tion­ary. A bio­scrub­ber con­sists of a scrub­ber unit and a re­gen­er­a­tion unit. In the scrub­ber (ab­sorp­tion col­umn), wa­ter sol­u­ble gaseous pol­lu­tants are ab­sorbed and par­tially ox­i­dized in the liq­uid phase (the cul­ture medium con­tain­ing the micro­organ­isms), which is dis­trib­uted from the top of the unit 13]. The con­tam­i­nated wa­ter is sub

[12- se­quently trans­ferred into an aer­ated stirred tank re­ac­tor (re­gen­er­a­tion unit), like an ac­ti­vated sludge unit, where the con­tam­i­nants are fully biode­graded. The re­gen­er­ated sus­pen­sion is con­tin­u­ously re-cir­cu­lated to the top of the scrub­ber sec­tion, thereby en­hanc­ing ef­fi­ciency. The pol­luted air flows through a bi­o­log­i­cally ac­tive bed, where mi­cro-or­gan­isms are at­tached in the form of a biofilm. As the gas dif­fuses through the packed bed, the pol­lut-ants are trans­ferred to the bi­o­layer and de­graded. To en­sure op­ti­mal op­er­a­tion of biofil­ters, the in­let gas usu­ally re­quires pre-treat­ment process such as par­tic­u­lar re­moval in or­der to pre­vent pos­si­ble clog­ging and sludge build up, load equal­iza­tion in case the waste gas con­cen­tra­tion is sub­ject to strong fluc­tu­a­tions, tem­per­a­ture con­trol and hu­mid­i­fi­ca­tion.

In bi­o­log­i­cal trick­ling fil­ters the packed beds con­sist only of in­ert ma­te­ri­als (glass, ceramics, and plas­tics) while the liq­uid phase, con­tain­ing in­or­ganic nu­tri­ents, flows with the con­tami-nated gaseous stream and is con­tin­u­ously re cir­cu­lated through the biore­ac­tor. Bio­scrub­bers and biotrick­ling fil­ters are ap­pli­ca­ble mainly to the treat­ment of waste gases con­tain­ing good or mod­er­ately wa­ter-sol­u­ble com­pounds, whereas biofil­ters, due to the large sur­face area avail­able for mass trans­fer, are also suited to treat poorly wa­ter sol­u­ble com­pounds. More­over, due to their high re­ac­tion se­lec­tiv­ity, biofil­ters are par­tic­u­larly suit­able for treat­ing large vol­umes of air con­tain­ing eas­ily degrad­able pol­lu­tants with rel­a­tively low con­cen­tra­tions, typ­i­cally 1,000 ppm. Com­pared with the other bi­o­log­i­cal sys­tems, biofil­ters have the widest ap­pli­ca­tion be­cause they are easy to op­er­ate, sim­ply struc­tured, and im­ply low in­stal­la­tion and op­er­at­ing/main­te­nance costs. Also, the re­li­a­bil­ity of biofil­ter op­er­a­tion is higher than that of bio­scrub­bers, where the risk ex­ists of wash­ing away the ac­tive micro­organ­isms. More­over, the pres­ence of a large amount of pack­ing ma­te­rial with a buffer­ing ca­pac­ity di­min­ishes the sen­si­tiv­ity of biofil­ters to dif­fer­ent kinds of fluc­tu­a­tions. Be­cause the ma­jor dis­ad­van­tage is the dif­fi­cult con­trol of pa­ram­e­ters like

pH, tem­per­a­ture and nu­tri­ent sup­ply, biofil­ters may be un­suit­able for de­grad­ing halo­genated com­pounds (as acid me­tab­o­lites are pro­duced) and treat­ing gas streams con­tain­ing high con­cen­tra­tions of VOC’s, un­less long res­i­dence times or large bed vol­umes are ap­plied 14]. Biotrick­ling fil­ters and Biofil­ters are cur­rently

[ uti­lized mainly in com­post pro­duc­tion plants, sewage treat­ment plant, and agri­cul­ture, whereas biofil-ters and bio­scrub­bers are pre­ferred in in­dus­trial ap­pli­ca­tions. The com­par­a­tive de­tails are shown Ta­ble 1. Biofil­ter

The odour con­trol us­ing biofil­ter tech­nol­ogy is rapidly gain­ing pop­u­lar­ity around the world. The in­creased use of this tech­nol­ogy is a re­sult of new lev­els of un­der­stand­ing and the cost ad­van­tages of the tech­nol­ogy over the life of the equip­ment. Biofil­tra­tion is now re­garded as a ma­ture tech­nol­ogy rather than a new process. Biofil­tra­tion is a rel­a­tively new odour (VOCs and VICs) con­trol tech­nol­ogy. It was first used for the treat­ment of off gases from waste­water of chem­i­cal man­u­fac­tur­ing fa­cil­i­ties, solid waste pro­cess­ing plants, com­post­ing op­er­a­tions etc. The schematic flow di­a­gram of biofil­ter is shown in Fig. 1.

In the biofil­ter, the volatile or­ganic or odour laden gases are passed through a bi­o­log­i­cally ac­tive por­ous me­dia. The de­com­po­si­tion of the pol­lu­tants is car­ried out by micro­organ­isms grow­ing on the solid car­rier, which forms the por­ous me­dia. Sol­u­ble com­pounds in the gas stream par­ti­tion into a liq­uid film (bi­o­layer) sur­round­ing the me­dia. The com­pounds in the liq­uid film are avail­able for biodegra­da­tion by a res­i­dent mi­cro­bial pop­u­la­tion. The mi­cro­bial pop­u­la­tion mo­bi­lizes the hy­dro­car­bons mainly to CO and H O.

2 2 Com­pounds shown to be de­graded in a biofil­ter in­cludes ben­zene, toluene, hy­dro­gen sul­fide, car­bon

[3] disul­fide, mer­cap­tans, dimethyl sul­fide, dimethyl disul­fide, am­mo­nia, methanol, ethanol, propanol, bu­tanol, alde­hy­des, bu­tyralde­hyde, pyridines ace­tone, styrene, xy­lene, methy­lene chlo­ride, di and tri chloro­meth­ane, tri and tetra chloroethene, ni­tro­gen ox­ides, isopen­tenyl, gaso­line de­rived VOC’s, tri­ethyl­amine, etc. 18]. Biofil­tra­tion in its sim­plest form in­volves the

[15- pass­ing of air through a bi­o­log­i­cally ac­tive fil­ter ma­te­rial to be cleaned through bi­o­log­i­cal ox­i­da­tion pro­cesses. The fil­ter ma­te­rial used may have vir­tu­ally any com­po­si­tion as long as it sup­ports bi­o­log­i­cal ac­tiv­ity.

The bi­o­log­i­cal pro­cesses in a biofil­ter sys­tem take place in the wa­ter com­po­nent of the fil­ter ma­te­rial 19].

[ All ac­tiv­ity oc­curs in the bi­o­layer or biofilm sur­round­ing the in­ert sup­port. The heart of the process is the bi­o­layer. The bi­o­layer is the bi­o­log­i­cally ac­tive wa­ter layer that ex­ists within the ma­trix of the fil­ter ma­te­rial.

As the odor­ous com­pounds pass through the fil­ter ma­te­rial they are ab­sorbed into the bi­o­layer. The micro­organ­isms present in the sys­tem use these odor-

ous com­pounds as part of their food source for en­ergy pro­duc­tion and re­pro­duc­tion. The com­pounds taken up by the micro­organ­isms are bi­o­log­i­cally de­graded to CO2 and H2 O [20].

The bi­o­layer has sev­eral roles in Biofil­tra­tion in­clud­ing:

• Sup­ply­ing the aque­ous en­vi­ron­ment for bac­te­rial life.

• Sup­ply­ing the nu­tri­ents for bi­o­log­i­cal ac­tiv­ity.

• Act­ing as the wa­ter/air in­ter­face for trans­port of the air com­po­nents to be treated.

• Act­ing as the re­cip­i­ent of the by-prod­ucts of re­ac­tion.

Dif­fer­ent con­fig­u­ra­tions of biofil­ters are be­ing em­ployed de­pend­ing upon the ap­pli­ca­tion and per­for­mance re­quire­ments tak­ing into con­sid­er­a­tion the techno eco­nom­ics. The de­tails are shown Ta­ble 2.

Mech­a­nism of Biofil­ter: Biofil­ter is a two-phase process con­sist­ing of:

1. The trans­fer of the com­pounds from the gas phase to wa­ter phase (Biofilm phase)

The speed of this process is de­pen­dent on the solu- bil­ity and par­tial pres­sure of the com-pound and is best es­ti­mated us­ing Henry’s law con­stant for the com­pound.

2. The ox­i­da­tion of the ab­sorbed com­pound by the bac­te­rial species present in the fil­ter.

The ki­net­ics of this is based on the en­zy­matic ca­pac­ity of the bac­te­ria to use the com­pound as a food or en­ergy source. A fur­ther com­pli­ca­tion is the abil­ity of the bi­o­layer to elim­i­nate the byprod­ucts of the re­ac­tions in or­der to pre­vent end prod­uct in­hi­bi­tion.

The prin­ci­ple is like con­ven­tional biofilm pro­cesses and is shown schemat­i­cally in the Fig. 2. First, a con­stituent com­pound in the gas phase crosses the in­ter­face be­tween gas flow­ing in the pore space and the aque­ous film sur­round­ing the solid mat­ter. Then it dif­fuses to a con­sor­tium of ac­cli­ma­tized micro­organ­isms. Fi­nally, the micro­organ­isms ob­tain en­ergy from ox­i­da­tion of the com­pound as a pri­mary sub­strate or it is co-me­tab­o­lized via non­spe­cific en­zymes. Si­mul­ta­ne­ously, there is dif­fu­sion and up­take of nu­tri­ents such as ni­tro­gen and phos­pho­rous in avail­able forms from the fil­ter me­dia and oxy­gen from the gas.

A prop­erly de­signed and op­er­ated biofil­ter con­tin­u­ously maintains con­cen­tra­tion gra­di­ent and driv­ing dif­fu­sive trans­port in the biofilm 14,21]. The

[ volatile or­ganic com­pounds present in the waste gas as well as oxy­gen, are par­tially dis­solved in the liq­uid phase of the bi­o­layer and are de­graded or con­sumed by aer­o­bic mi­cro­bial ac­tiv­ity. In this way a con­cen­tra­tion gra­di­ent is cre­ated in the bi­o­layer, which maintains a con­tin­u­ous mass flow of the com­po­nent from the gas to the wet bi­o­layer. The volatile meta­bolic prod­ucts like CO dif­fuses to the

2 gas phase and are trans­ported in the ax­ial flow di­rec­tion and leave the bed with the exit gas 21]. The

[ or­ganic nu­tri­ents are nec­es­sary for mi­cro­bial life. These nu­tri­ents are trans­ported by dif­fu­sion from the fil­ter me­dia ma­te­rial to the micro­organ­isms. Nat­u­ral ma­te­ri­als such as hu­mus, com­post, peat, wood chips, rice husk, co­conut coir, pith and other re­lated sub­stances gen­er­ally con­tain these nu­tri­ents in suf­fi­cient quan­tity. These ma-

teri­als also pos­sess buffer­ing ca­pac­ity for neu­tral­iz­ing acid­ity or al­ka­lin­ity formed by ox­i­da­tion. The el­e­men­tary nu­tri­ents are sub­jected to a re­cy­cling dur­ing the op­er­a­tion of the biofil­ter af­ter the dy­ing off the mi­crobes. Min­er­al­i­sa­tion pro­cesses lib­er­ate these nu­tri­ents. As the ef­fi­ciency of re­cy­cling is less than 100%, the me­dia ma­te­rial will be even­tu­ally be­ing ex­hausted must gen­er­ally be re­newed af­ter sev­eral years of op­er­a­tion 21]. Due to the small size of the par­ti­cles (few

[ mm) and the com­pounds to be trans­ferred is gen­er­ally wa­ter in­sol­u­ble, the mass trans­fer re­sis­tance in the gas phase can gen­er­ally be ne­glected. Dur­ing the elim­i­na­tion of VOC, het­erotrophic mi­cro-or­gan­ism­sare pre­dom­i­nant com­par­a­tively au­totrophic micro­organ­isms, most of­ten be­ing bac­te­ria or fungi. The bed in­oc­u­la­tion de­pends on both the na­ture of the fil­ter­ing ma­te­ri­als and the VOC biodegrad­abil­ity level. Many re­views have sug­gested tak­ing ad­van­tage of the ecosys­tems in­dige­nous to the beds 24]. Af­ter an ac­clima­ti­za­tion

[22- pe­riod, the most re­sis­tant pop­u­la­tions are nat­u­rally se­lected and a mi­cro­bial hi­er­ar­chy is es­tab­lished in the bed. In many other cases (ma­te­ri­als with low biomass den­sity, re­cal­ci­trant VOC, re­duc­tion of ac­clima­ti­za­tion pe­riod), re­searchers in­oc­u­late the beds with con­sor­tia, ex­tracted from sewage sludge, for ex­am­ple, or strains de­rived from ei­ther com­mer­cial sources or iso­lated from pre­vi­ously op­er­ated biofil­ters. Biofil­tra­tion is ef­fec­tive in re­mov­ing haz­ardous com­pounds like ac­etalde­hyde, bu­ta­di­ene, cresols, ethyl­ben­zene, formalde­hyde, methanol, styrene with high biodegrad­abil­ity and ace­toni­trile, ben­zene, car­bon disul­phide, hex­ane, methy­lene chlo­ride, methyl ethyl ke­tone, phe­nol, toluene, xy­lene with medium biodegrad­abil­ity.

The ad­van­tages of biofil­tra­tion are that it is very cost ef­fec­tive and ef­fi­cient method to elim­i­nate odor­ous con­tam­i­nants and other VOCs, which are present in low con­cen­tra­tion in the waste gas stream. This method of­fers com­plete de­struc­tion of con­tam­i­nants rather than trans­fer­ring them to an­other me­dia. This method can be used for both or­ganic as well as in­or­ganic com­pounds 3].

[

Types of fil­ter ma­te­rial: The fil­ter ma­trix of a biofil­ter has been con­structed from many ma­te­ri­als over the past cen­tury 26]. Ex­am­ples of me­dia in­clude the

[25, fol­low­ing: soil mix­tures, com­post, bark, co­conut coir pith, peat, car­bons and mix­tures of the above. All of these types of me­dia have been suc­cess­ful to some ex­tent. The biofil­ter bed ma­te­rial im­proves in terms of the biofilm in­tegrity and sur­face area, then the biofil­ter ef­fi­ciency in­creases and ac­cord­ingly size of the biofil­ter de­creases for sim­i­lar ap­pli­ca­tions 27]. An ef­fec­tive

[ biofil­ter medium should have the fol­low­ing char­ac­ter­is­tics: high spe­cific sur­face area for de­vel­op­ment of a mi­cro­bial biofilm and gas-biofilm mass trans­fer, high poros­ity to fa­cil­i­tate ho­mo­ge­neous distri­bu­tion of gases, a good wa­ter re­ten­tion ca­pac­ity to avoid bed dry­ing, pres­ence and avail­abil­ity of in­trin­sic nu­tri­ents, and pres­ence of a dense and di­verse in­dige­nous mi­croflora.

Biotrick­ling fil­ter

Bi­o­log­i­cal trick­ling fil­ters (BTFs) com­bine pol­lu­tant ab­sorp­tion and biodegra­da­tion in the same re­ac­tor. Pol­lu­tant de­grad­ing bac­te­ria are nat­u­rally im­mo­bi­lized on a pack­ing ma­te­rial which is ei­ther a ran­dom pack­ing or a three-di­men­sional struc­ture. In biotrick­ling fil­ter, the gas is car­ried through a packed bed, which is con­tin­u­ously ir­ri­gated with an aque­ous so­lu­tion con­tain­ing es­sen­tial nu­tri­ents re­quired by the bi­o­log­i­cal sys­tem. Sev­eral stud­ies have shown that the choice of a co or counter cur­rent con­fig­u­ra­tion for liq­uid and gaseous phases does not in­flu­ence the biodeg-ra­da­tion per­for­mance 28]. Micro­organ­isms grow on the pack­ing

[ ma­te­rial as biofilm. The pol­lu­tant to be treated is ini­tially ab­sorbed by the aque­ous film that sur­rounds the bi film, and then the biodegra­da­tion takes place within the biofilm. The fil­ter­ing ma­te­rial used in a biotrick­ling fil­ter has to fa­cil­i­tate the gas and liq­uid flows through the bed, favour the de­vel­op­ment of the mi­cro flora, and should re­sist crush­ing and com­paction. Biotrick­ling fil­ter pack­ing that best meet these spec­i­fi­ca­tions are made from in­ert ma­te­ri­als such as resins, ceramics, polyurethane foam etc. As they are made from in­ert or syn­thetic ma­te­rial, biotrick­ling fil­ters need to be in­oc­u­lated with suit­able mi­cro­bial cul­ture 29]. The use of ac

[

ti­vated sludge as ini­tial mi­cro­bial in­ocu­lums has been ex­ten­sively re­ported. The schematic flow di­a­gram of biotrick­ling fil­ter is shown in Fig.3.

In biotrick­ling fil­ters, the con­tact be­tween the micro­organ­isms and the pol­lu­tants oc­curs af­ter the VOC dif­fu­sion in the liq­uid film, the liq­uid flow rate and the re­cy­cling rate are rec­og­nized to be crit­i­cal pa­ram­e­ters for BTF op­er­a­tion. Stud­ies are re­vealed that an in­crease in the liq­uid flow rate should re­sult in pro­por­tional in­crease in the ac­tive ex­change sur­face for gas liq­uid mass trans­fer, and then im­prove the degra­da­tion rate 30]. Some re­searchers have shown that main­tain­ing [ min­i­mum wa­ter and nu­tri­ent sup­ply is suf­fi­cient to achieve good per­for­mance 32]. In ad­di­tion, as the dis

[31, tri­b­u­tion and the re­cy­cling of nu­tri­ent so­lu­tions add to en­ergy costs, other stud­ies sug­gest that the op­ti­mum re­cy­cling and distri­bu­tion flow rates have to be found ex­per­i­men­tally and on a case-by-case ba­sis 33]. BTFs

[ find wide ap­pli­ca­tion in VOC and odour treat­ment. As com­pared to con­ven­tional com­post or soil bed biofil­ters which are gen­er­ally lim­ited to the elim­i­na­tion of odor­ous com­pounds and no chlo­ri­nated volatile or­ganic com­pounds, a wider range of pol­lu­tants can po­ten­tially be treated in BTFs. This is be­cause, en­vi­ron­men­tal con­di­tions can be bet­ter con­trolled in the BTFs and po­ten­tially toxic dead-end me­tab­o­lites can be purged out of the sys­tem. The ma­jor draw­back of biotrick­ling fil­ters is the ac­cu­mu­la­tion of ex­cess bio- mass in the fil­ter bed. Some re­views have demon­strated that, in the course of the process, the biofilm thick­ness can achieve sev­eral mil­lime­tres

35], which can cause prob­lems that lead to per[34, for­mance loss 30]: pres­sure drop in­creases, bed

[ chan­nelling, and the cre­ation of anaer­o­bic zones. Ac­cu­mu­la­tion re-moved by the back wash­ings with wa­ter are the most ef­fi­cient and cer­tainly the least dras­tic for the ecosys­tem [36]. Nev­er­the­less, the biotrick­ling fil­ter tech­nol­ogy is still em­ployed to a lesser ex­tent than biofil­tra­tion, which is cer­tainly re­lated to its more con­se­quen­tial op­er­at­ing costs and to the VOC sol­u­bil­ity re­stric­tions. Bio­scrub­ber

Bio­scrub­bers are re­ac­tors in which the gaseous pol­lu­tants are first ab­sorbed in a free liq­uid phase prior to biodegra­da­tion by ei­ther sus­pended or im­mo­bi­lized micro­organ­isms. The mi­cro­bial process oc­curs ei­ther in the ab­sorber or in a sep­a­rate biore­ac­tor af­ter ab­sorp­tion of the pol­lu­tants 13]. Bio­scrub­bing con­sists of the ab­sorp

[ tion of a pol­lu­tant in an aque­ous phase, which is then treated bi­o­log­i­cally in a sec­ond stage in a liq­uid phase biore­ac­tor. The ef­flu­ent treated in the liq­uid phase re­ac­tor is re­cal­cu­lated to the ab­sorp­tion col­umn. This tech­nol­ogy al­lows for good gas clean­ing when the gaseous pol­lu­tants are highly wa­ter-sol­u­ble. If the ab­sorp­tion so­lu­tion is wa­ter then one can say that it is a bi­o­log­i­cal process, but all the com­pounds of waste air or gas are not sol­u­ble in wa­ter 23]. Only

[ some com­pounds in waste gas are sol­u­ble in wa­ter and some other are partly sol­u­ble. Dif­fer­ent type of ab­sorp­tion so­lu­tions are to be used in these sys­tems. At this stage, if the ab­sorp­tion so­lu­tion used for scrub­bing is other than wa­ter, then the process may be called as bio­chem­i­cal method. It is a com­bi­na­tion of both chem­i­cal and bi­o­log­i­cal meth­ods. Ab­sorp­tion is one of the most fre­quently used tech­niques for con­trol­ling the con­cen­tra­tions of gaseous pol­lu­tants be­fore they are dis­charged into the at­mos­phere. It in­volves the trans­fer of the pol­lu­tant from the gas phase to the liq­uid phase across the in­ter­face in re­sponse to a con­cen­tra­tion gra­di­ent; with con­cen­tra­tion de­creas­ing in the di­rec­tion of mass trans­fer 12]. The schematic flow di­a­gram of bio

[ scrub­ber is shown in Fig.4.

Bio­scrub­bers be­ing op­er­ated presently use ac­ti­vated sludge de­rived from waste­water treat­ment plants as in ocu­lums[ 37,38]. In some cases, biore­ac­tors are di­rectly in­oc­u­lated with spe­cific de­grad­ing strains. The res­i­dence time for such biore­ac­tors range be­tween 20 and 40 days and these are op­er­ated prac­ti­cally as ac-

ti­vated sludge pro­cesses in­clud­ing re­cy­cle of sludge. Part of the treated so­lu­tion is re­cy­cled for ab­sorp­tion of VOCs to the ab­sorp­tion unit. Sub­stan­tial mod­i­fi­ca­tions in bio­scrub­ber de­sign have been done in the re­cent past to en­hance their per­for­mance for VOC and odour treat­ment. Some mod­i­fied bio­scrub­bers are sorp­tive slurry bio­scrub­ber, Anoxic bio­scrub­ber, Two-liq­uid phase bio­scrub­ber, Air­lift bio­scrub­ber and Spray col­umn bio­scrub­bers. Mem­brane biore­ac­tors

Mem­brane biore­ac­tors were de­signed as al­ter­na­tive to con­ven­tional biore­ac­tors for waste gas treat­ment. The mem­brane biore­ac­tor al­lows the se­lec­tive per­me­ation of the pol­lu­tant, which is not al­lowed in any of the re­ac­tors dis­cussed pre­vi­ously. The con­cen­tra­tion dif­fer­ence be­tween the gas phase and the biofilm phase pro­vides the driv­ing force for dif­fu­sion across the mem­brane. The driv­ing force de­pends strongly on the air wa­ter par­ti­tion co­ef­fi­cient of the dif­fus­ing volatile com­po­nent. For com­po­nents with a high par­ti­tion co­ef­fi­cient the driv­ing force for mass trans­fer is small 39]. The schematic flow di­a­gram of mem­brane bioreac

[ tor is shown in Fig. 5.

In a mem­brane biore­ac­tor, the mem­brane serves as the in­ter­face be­tween the gas phase and the liq­uid phase (Fig. 5)[ 39]. The gas–liq­uid in­ter­face thus cre­ated (e.g. in hol­low fi­bre re­ac­tors) is larger than in other types of gas–liq­uid con­tac­tors 40]. Two types of mem

[ brane ma­te­ri­als have been used to pre­vent mix­ing of the gas and liq­uid phases and si­mul­ta­ne­ous trans­fer of volatile com­po­nents. These types are hy­dropho­bic mi­cro por­ous mem­brane and dense mem­brane. All stud­ies car­ried out on mem­brane re­ac­tors are lab­o­ra­tory scale ex­per­i­ments. To the best of our knowl­edge, no re­ports are avail­able on pi­lot plant in­ves­ti­ga­tions or fullscale ap­pli­ca­tions of mem­brane re­ac­tors in bi­o­log­i­cal waste gas treat­ment. Mem­brane mod­ules ap­pear rel­a­tively easy to scale up given their mo­du­lar na­ture 41];

[ how­ever, an ex­ten­sive long-term per­for­mance test­ing is nec­es­sary be­fore they can be ap­plied on full scale.

Bi­o­log­i­cal pu­rifi­ca­tion of in­dus­trial gaseous emis­sions

As al­ready men­tioned, odour elim­i­na­tion was the ini­tial aim of waste gas treat­ment. Pre­vi­ously, bi­o­log­i­cal pro­cesses for the re­moval of mal­odor­ous com­pounds were widely used in only a few de­vel­oped coun­tries, but due to its ad­van­tages, re­cently, it is spread­ing to de­vel­op­ing coun­tries also. The most ex­ten­sively stud­ied com­pounds are sul­phur and ni­tro­gen con­tain­ing com­pounds. The re­moval of odours from waste wa­ter treat­ment plants was first in­stalled in 1923 and the ear-li­est patent was prob­a­bly ob­tained in 1934 42]. It was re­ported that elim­i­na­tion ca­pac­i­ties

[ vary from few grams to more than 200 g/m3/h for sev­eral VICs with re­moval ef­fi­cien­cies of­ten above 90%[ 43]. VOC emis­sions com­prise a wider range of pos­si­ble con­tam­i­nat­ing com­pounds than the VICs. Many VOCs are re­leased from in­dus­trial ac­tiv­i­ties as well as from the treat­ment of solid or liq­uid wastes and in soil re­me­di­a­tion also. VOCs in­clude halo­genated and non­halo­genated aliphatic and aro­matic pol­lu­tants. The

Fig. 2: Pol­lu­tant Pen­e­tra­tion and Degra­da­tion mech­a­nism in Bio fil­ter Ci — Ini­tial Con­cen­tra­tion of Pol­lu­tant Co — Out­let Con­cen­tra­tion of Pol­lu­tant

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