@UR CONTROL METHODS

Review of Methods of Odour Control Brian Mil ls

Haden Drysys International Ltd, 1506/1508 Coventry Road, Swan Office Centre, Yardley, Birmingham B25 8AD, UK

Presented at the Filtration Society meeting on 'Pollution Control in the Foundry and Allied Industries' in Birmingham, UK on 10 May 1994

Odours are generally regarded as a nuisance. The paper examines the definition of an odour, and touches briefly on legislation, the measurement of odours, and the meaning of Odour Numbers and Odour Emission Rates. Methods of abatement are then described, including prevention, blosystems, activated carbon, wet scrubbing and thermal oxidation. The advantages and disadvantages of the various methods are compared, and the paper concludes with a table indicating the relative capital and operating costs for the systems described.

O dours arc commonly perceived to be those 'foul smel l ing subs tances ' found as fugitive emiss ions from a var ie ty of toed,

chemical or was tewa te r t r e a t m e n t operat ions. In this paI)er the a~ssumption is made t h a t any emiss ions have

been cap tu red by the correct app l ica t ion of vent i la t ion enclosures and dueling. The purpnse of the pape r is to compare the methods ,ff t r e a t m e n t once this s tage is complete.

What is an odour? An ()dour is, qui te simply, a smell wbich may be pleasant, or unpleasant . Plea~sant odours we might Iorget -- excep t t ha t some ()(Iours, for example the smell of fresh tobacco, can quickly become nauseous on prolonged exposure .

Our disl ike of had smel ls is a deeply ingra ined human defence mechanism. The human nose possesses mil l ions of receptor cells which are except iona l ly sensi t ive to odours, and which can de tec t smells a t very h)w concen t ra t ions in the a tmosphere . The typical sewage smell, skatole, ik)r e xa m p le has an odour threshold of 0.0(lt)0l par t s per mil l ion (ppm) , Other commonly accepted odour th resholds are given in Table 1.

Although not in themselves toxic, odours ('an cause nausea, s t ress and annoyance. Members of the publ ic who are subjected to unp leasan t smells ~fill becume more sensi t ive to them, whi l s t people who work in areas tha t o£Len conta in u n p l e a s a n t odours can heeonm desensi t ised. This leads to p rob lems when, for example , works managers visit a h)ng-term compla inan t ' s home to assess a problem, resul t ing in widely oppos ing views on what is an odour nuisance. Consequent ly the whole topic of odour- re la ted p rob lems ~:an appea r to be somewha t nebulous a fact reflected in the legislat ion.

Al though odour emiss ions have been subject to s t a tu to ry r e g u l a t i o n s for more t h a n a century , it is only wi th t i le in t roduct ion of the E n ~ r o n m e n t a l 15"otection Act (EPA) in 1990 in the ElK tha t a defini t ion of the problem has been a t t empted .

The EPA consol ida ted the 'Recurring Nuisance" legislat ion, and alh)wed h)cal au thor i t i e s to serve a S ta tu tory Notice when they had good reason to expec t t ha t a nu isance would recur, even if it was not p resen t a t the t ime of serving the notice. Secondly, the Act in t roduced the word 'smell ' into the legislat ion, t hus ending more than 10t) years of ,judicial wTangling and assoc ia ted legal fees. Finally, the Act also in t roduced 'Scheduled Processes', which have to be 'anthor ised ' by the local authori ty.

Al though nuisance law has been with us for more than 100 ),ears, a 'nuisance ' has never been defined. The fundamen ta l concept of nuisance is above all flexible, al lowing lbr different ideas )f acceptabi l i ty a t different, t imes and in different places, and it may be r e a d i l y a d j u s t e d to t a k e a c c o u n t of c h a n g e s in ~:ircumstarlces.

Moreow,r, the Process Guidance Notes appl ied to 'Authorisa- t:ions' refer to offensive odours as ' those which are perceived by the local au thor i ty inspec tor a t the boundary of the works' .

Odour p r o b l e m s can the re fo re be seen to be complex , apparen t ly somewha t unscient i f ical ly based, and not e a s t to define.

Measurement of odoura While it is relat ively easy (a l though expens ive) to measure the

Filtration & Separation

Table 1. Odour thresholds.

Chemical Threshold, parts per million

Ethanol 0.6 Chlorine 0.05 Sulphur dioxide 0,6 Butyric acid (BO) 0.08 Ammonia 1.5 Methylamine 0,04 Trimethylamine 0.00003 Hydrogen sulphide 0.0004 Methyl mercaptan 0.00007 Mercapta ns 0.000003 Dimethyl sulphide 0.003 Skatole and indole 0.00001

concent ra t ion of individual eomponen t s in an odour emission, this chemica l knowledge is of dubious value in a s sess ing odnul prohlems.

Because the u l t ima te a rb i te r of the presence of an odour is the human nose, the general ly accepted method of moni to r ing odours is based on a 'smell pane l ' or dynamic olfactomctry, comhined with the concept of Odour Numbers.

Each odorous compound has a threshold of smell a l read) referred to in Table 1. Any given air s t ream conta in ing an odour mus t he d i lu ted at. leas t to th is th resho ld to e l imina te the smell. The number of d i lu t ions required l.o reach the th resho ld is termed the Total Odour Number (TON), and is usual ly expressed as Odour [ ;ni ts per cubic metre (OU/m:~).

Total ()dour Numbers are usual ly de t e rmined by dynamic olfactometry, where in a panel of usual ly 6 - 8 people are a l ternate ly subjected to ()dour-free ai r and a d i lu ted sample of the odorous air. The poin t a t which 50% of the pane] cannot detect the odour is the threshold, and the number of d i lu t ions is the TON This pe rhaps e x p l a i n s why there is a wide var ia t ion in repor ted values of odour thresholds , but it is the best we have at the moment.

Mult iplying the TON by the air flowrate gives the odour emiss ion rate.

Evalua t ion of a s i te problem involves col lect ing a sample of odorous a i r in a Tedlar p las t ic bag, and t r anspor t ing it to a labora tory specifically set up for odour th reshoh i testing. Test ing is carr ied out wi th in 24-- 30 hours of t ak ing the sample.

While the TON does not es tab l i sh a direct measure of the odour emission, it does give an ind ica t ion of the magn i tude of the problem. TONs for p l an t emiss ions can be several million. The need to reduce th is value to the region of 100-- 500 gives some idea of the problem.

Full desc r ip t ions of the me thod of odour m e a s u r e m e n t can be

February 1995 0015-1882/95/US$7.00 d) 1995 Elsevier Science Ltd 147

CONTROL METHODS

found in the re levant s tandards . H''~] A European s t a n d a r d for ()dour m e a s u r e m e n t is p l anned to be issued dur ing 1995.

Methods of control There are several me thods avai lable to control odours -- all wi th a variety of advantages , d i sadvan tages and degrees of cost-effective- B e s s .

The following me t hods are commonly applied:

~] Prevention.

[ ] Biofi l trat ion and bioscrubbing.

[ ] Act ivated carbon.

[ ] Wet chemical scrubbing.

[ ] Thermal oxidat ion.

Since prevent ion is be t t e r t han cure, th is is by far the bes t s t r a t e ~ , if it can be implemented . It may cost r a the r more than some of the a b a t e m e n t techniques, but normal ly has spinoffs of be t te r process control and operat ion, an improved working env i ronment and o ther p roduc t benefits.

Dosing of chemica ls into process l iquors or s ludges can ofl, en achieve the required aba tement .

The a b a t e m e n t processes all require a process modif icat ion and /o r add i t iona l equipment , increas ing the complex i ty of the manufac tur ing operat ion.

However, a b a t e m e n t m u s t not be confused with t ransference of the problem from one s t r eam to another . The In tegra ted Pol lut ion Control r equ i rement of the Envi ronmenta l Protect ion Act 1990 s ta tes tha t merely t ransfer r ing the problem from one phase to m o t h e r ( a i r / w a t e r / l a n d ) is not desirable.

Of the a b a t e m e n t techniques , only the rmal ox ida t ion (with or wi thout a cata lys t ) destroys the odorous componen t s complete ly whils t ensur ing efficient d i spers ion of the products of combust ion. Changes in odour concent ra t ion or compnsi t ion do not deflect I:hermal oxid isers from the i r p r ime function of the sa t is factory a b a t e m e n t of the odours.

One common feature of all of the methods of odour control is 1hal of ox ida t ion of the odour component , a~s s h o ~ l in Table 2 -- ,rely the t empe ra tu r e at which it is achieved changes. However, if :he ox ida t ion energy is not suppl ied by the rma l means, i t m u s t ,:ome by ano the r route.

Biological sys t ems Biological a b a t e m e n t systems rely on a d i sperse med ium to adsorb con t aminan t s into a wet environment . Selected bac te r ia then decompose the organic compounds by us ing them as feed mater ia l . The bac te r ia are encouraged to grow on pea t or hea the r con ta ined in a. large box or on packing in a tower.

Biofilters Biofilters are formed from large beds of na tu ra l med ia such as peat or heather , which provide large surface a reas and exce l len t breeding condi t ions for the bac te r i a (Figm-e 1).

When fed on a cer ta in d ie t of organic compounds the bac te r ia become acc l imat i sed to the compounds and the concentra t ion, but a sudden or large change in concentra t ion, composi t ion or flow can stop the bac te r ia working.

Decomposi t ion p roduc t s are usual ly acidic, and the fi l ters require regular f lushing with wa te r to ma in t a in near -neu t ra l , 'ondi t ions in the bed.

This wash wa te r then becomes an effluent requir ing disposal , a l though in smal l volumes. Throughput veloci t ies are in the ! ) .01- 0.03 m/s range, resu l t ing in large cross-sect ional areas.

Bioscrubber$ Bioscrubbers work in a s imi la r way to biofllters, excep t t h a t there is

Table 2. Oxidation of odours.

Process Temperature

Thermal oxidation 800-1100°C Catalytic oxidation 300- 650°C Wet scrubbing/chemical reaction 20--80°C Biological 20--38°C

FLUSHING WATER

HUMPOIF YING SPRAYS

OOOROUS AIR

DISPERSED CLEANED AiR

COMPOST BED

IO DRAIN

Figure 1. Typical biofilter.

a con t inuous reci rcula t ion of wa te r a round the packed tower (Figure 2). They are su i tab le for h igher concen t ra t ions of odours, and thus have a h igher blowdown volume than biofilters.

Opera t ing costs are very low lbr both units, bu t the ground area r equ i r ed for a b io f i l t e r can be very large. For h igh in le t concentra t ions , a combina t ion un i t of a b ioscrubber topped wi th a Biofilter can be used.

The efficiency of removal is be t t e r for biofil ters t han for bioscrubbers, but changes in flo~Tate or inlet ,)dour compos i t ion can h inder the i r effectiveness.

The b iomedium will need to be replaced about every live years and mus t be d i sposed of as a waste-product .

Activated carbon Solvents, volat i le organic compounds (VOCs), odours or gaseous componen t s general ly may be adsorbed onto ac t iva ted carbon (and also molecu la r s ieves) . However, th is is usua l ly a means of concent ra t ing the pol lutant . Generally it has to be recovered from the ac t iva ted carbon bed and t rea ted further, or the con tamina ted carbon d isposed of as a waste-product .

There are two major types of un i t used for ac t iva ted carbon systems:

Fixed bed system For vent a i r s t r eams th i s would comprise a s ingle vessel, with the carbon being changed when spen t and d isposed of as a waste- product.

For larger ins ta l l a t ions the equ ipmen t compr ises two vessels con ta in ing ac t iva ted carbon (in e i ther g r anu la r or pel le t form),

MAKE-UP

LIQUOR

PUMP

CLEAN AIR OUTLET

t

i x x x x x )a

- - REC;RCUL ATION SUMP

DROPLET SEPARATOR

LIQUOR SPRAYS

TOWER PACKING

TO DRAIN

Figure 2. Typical bioscrubber.

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Figure 3. Fixed-bed activated carbon adsorption regeneration process.

interconnected so that they may be used alternately. Contaminated air is passed through the first vessel, to adsorb the contaminants, with the clean air then discharged to atmosphere. When the first vessel is fully charged with solvent, the vessels change over such that the second becl adsorbs the pollutant whilst the first bed is regenerated. This regeneration can be effected either by hot inert gases or by steam. Air cannot be used, because of the risk of flammability or explosion.

Regeneration of the bed merely produces a more concentrated stream of solvent in the inert gas. This can be destroyed by feeding it to an incinerator, where the heat of combustion of the solvent or pollutant (:an be sufficient to maintain the incineration process without the need for an additional fuel source. Partial cooling of ~.he off:gases provides the inert gas for the activated carbon regeneration (Figure 3).

Rotary wheels .~m alternative to the fixed bed system is the rotary wheel using a carbon matrix. Rotary wheels are typically 450 mm deep, and have a range of diameters from 1.5 m upwards.

Contaminated air is passed through a segment comprising about 90% of the wheel, which is ro ta t ing slowly. As the ('.ontaminated air passes through the active carbon, the pollutant is adsorbed onto the carbon, with the clean air discharged to atmosphere.

By m e a n s of a ser ies of specially des igned seals, hot regenerating gas is passed countercurrently through the other 10% segment; the pollutant adsorbed onto the wheel is thus now driven off fl'om the active carbon.

The pollutant (now concentrated in the regenerative gas) can be passed to an incinerator in the same way as previously described I'or the fixed bed sYstem (Figure 4).

Special act ivated carbons For special applications active carbons are available impregnated with chemicals to adsorb and destroy specific compounds, in particular hydrogen sulphide. These carbons are normally used in single tower systems.

Figure 4 (above). Rotary wheel activated carbon adsorption system with regenerative incineration.

Figure S (right). Venturi scrubber, showing typical flow.

Carbon disposal In the absence of a means of recovering the solvent or oxidising il thermally, then a means must be found for disposal of the active carbon contaminated with the odorous compounds, or in the case of impregnated carbons, the mixed carbon and adsorbed products

Wet scrubbing What is probably regarded as the s implest and cheapest option is to absorb the pol lutant in a wet scrubber. However, once absorbed the question remains - what then? The problem is transfem-ed to the solvent, which in most cases is water.

If the concentrat ions of organics are high, it is possible to recover the pollutant by distillation. However, in the majority ot cases the pollutant mus t be destroyed by chemical reaction with a variety of oxidising agents, such as sodium hy-pochlorite or hydrogen peroxide.

Because odours normally contain a wide spectrum of different compounds, it is often necessary to utilise three scrubbers in series. with the first using an acid, the second the oxidising agent in the presence of alkali, and the third as a final clean-up using sodium hydroxide solution.

Each scrubbing system requires pH or redox control (or both), together with bulk storage and handling for the chemicals, dosing p u m p s and sometimes an effluent t rea tment system for the blowdown liquor (assuming that a disposal route cannot be found).

From the above it can be seen tha t care mus t be taken to consider the total system, and not jus t the simple scrubber.

The basic requirement of the wet scrubber is to provide a convenient and efficient means of bringing air and scrubbing liquor into contact. For absorpt ion of gaseous components the most common types of scrubber are as follows:

Venturi scrubbers In the venturi scrubber a liquor sprayed into the throat of a venturi normally creates sufficient pressure differential to ventilate the process operation. Vigorous contact is made between the liquor spray and the odorous air.

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@UR CONTROL METHODS

- - -F

D CLEAN ~R OUT

DROPLET SEPARATOR

LIQUOR SPRAYS

TOWER PACKING

ODOROUS ~ I ~R IN

pH CONTROL

REDOX CONTROL

PUMPING TANK

CHEMICAL i DOSING

LIQUOR BLOWD='OWN

REGIRCULATION PUMP

Figure 6. Packed tower scrubber.

After absorpt ion of the contaminant, the liquid descends into a holding tank before being reeirculated back to the venturi spray. A ciclone separation is often required to separate water droplets from the discharge air s t ream (Figure 5).

Packed towers A packed absorpt ion tower is commonly a tall cylinder comprising the liquor recirculation tank, air inlet, packing suppor t grid, a packed section of tower, liquor distribution, a droplet separa tor and the clean air discharge (Figure 6). Contact is countercurrent between liquor falling down the tower and air passing up the tower. The function of the packing is to provide a large surface area for this contact.

The diameter and height are dependent on air th roughput and the required efficiency, but range from 1 to 5 m in diameter and from 10 to 25 m in height.

In operation the odorous component is absorbed into the circulating liquor. The contaminant then needs to be neutralised or oxidised. As a general guide, compounds containing nitrogen (for example, ammonia or amines) will be alkaline in nature, and will Iherefore require an acid scrubbing medium. Sulphur-containing v.ompounds are most often acidic in nature, and hence require an alkaline scrubbing medium. Both systems can require hypochlorite (tor example) as an oxidising agent.

It is not uncommon, therefore, to see chemical scrubbers as three-stage units, in order to control a mixture of odours.

Horizon ta l s c rubbers One novel method of achieving this staging is to use horizontal scrubbing. An example of this is the Neutraman unit, where the packing is replaced by contact screens.

Polluted air flows horizontally through the unit, and meets a series of contact screens fixed perpendicular to the air flow. Absorption liquor is sprayed at the screens, to ensure effective contact between air and liquor. Odours such as ammonia, amines and hydrogen sulphide (among others) are efficiently absorbed and neutralised.

The screens, mounted vertically in the air stream, comprise two layers of horizontal wedge-shaped bars tapering in the direction of

/

q

Figure 7. Neutraman horizontal scrubber.

flow. Liquid, sprayed onto the screen cocurrently with the air flow, forms a cont inuous film over the entire surface. In addition to the conventional surface film absorpt ion found with all packings, gas/ liquid turbulence in the space between the bars provides increased absorpt ion of the unwanted components.

The liquid film is continuously replenished as the scrubbing medium flows down the screen. Several screens, each supplied with fresh liquor, are fixed in series in the air pathway. This multiple contact, together with the fresh liquor to each screen, enables a very close approach to equilibrium to be achieved at each contact in series, thus ensuring a high efficiency of absorpt ion across the total unit.

The low operat ing height of the Neutraman unit gives a low pumping head for the liquor which, together with the low pressure loss on the air side, gives reduced energy consumption.

Each section is completely self-contained, so that the air s t ream may be scrubbed by several different liquid media in series in one horizontal unit (Figure 7).

Typical operating data are given in Table 3, highlighting the low concentrat ions achieved by this scrubber, and which are necessary to overcome odour problems.

Thermal o x i d a t i o n The most secure method available for dealing with dilute organic pollutants is that of thermal oxidation (with or without catalysts).

Thermal oxidisers are basically large heat-exchangers with a small combust ion chamber between the heating and cooling stages of the heat-exchanger.

In operat ion contaminated air is passed through a heat- exchanger (either a recuperator or regenerator) to heat up the waste air. This air reaches the combustion chamber, where a flame generated by suppor t fuel (if necessary) effects near-complete oxidation of the pollutant species. The hot exhaus t gases then pass back through the heat-exchanger, heating up the incoming waste air.

Table 3. Per formance of the Neut raman scrubber.

Concentration, ppm

Odour component Inlet Outlet

Ammonia 0.26 0.016 Trimethylamine 0.22 0.0021 Hydrogen sulphide 2.2 < 0.001 Methyl mercaptan 0.04 < 0.001 Dimethyl sulphide 0.11 0.0014 Dimethyl disulphide 0.00085 0.00056

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Figure 8. Thermal oxidation using a recuperator. Figure 9. Thermal oxidation using regenerative incineration.

Heat e x c h a n g e may be effected by t u b u l a r i n t e r c h a n g e (recuperator , see Figure 8) or by a number of beds of ceramic balls ( regenerator , see Figure 9).

Thermal ox ida t ion may be effected in the presence of a catalyst , in which case a lower t e m p e r a t u r e is required to oxid ise the pol lu tants . This would indica te lower opera t ing costs for the catalyt ic incinerator , but since a i r / h o t e x h a u s t gas hea t -exchangers are used on both catalyt ic and non-catalyt ic systems, the factor control l ing fuel usage is the t e m p e r a t u r e difference between the inlet air and the hot e x h a u s t gas. All o ther hea t r equ i remen t s are taken care of by hea t in terchange.

Since th is t e m p e r a t u r e difference is l ikely to be s imi la r on both .~5~stems, t imre is l i t t le fuel cost differential between them. The pr imary hea t in te rchange is between the inlet pol lu ted air and the hot p roduc t gases.

Secondary hea t recovery can be effected if required as shown in 'Fable 4.

Thermal ox ida t ion provides a gua ran teed method of solving VOC a b a t e m e n t and odour problems. In a well des igned system, the a b a t e m e n t process is in tegra ted into the main process, and is not seen merely as an 'end of pipe ' solution. Indeed, careful select ion of

p r imary and secondary hea t recovery is necessary to ensure that the op t i mum system is des igned in order to min imise cap i ta l and opera t ing costs.

F e a t u r e s of t h e v a r i o u s p r o c e s s e s The above gives an out l ine of several different processes for odoul cont ro l . Whi le a l l o d o u r a b a t e m e n t i n s t a l l a t i o n s m u s t be cons ide red in the c o n t e x t of specif ic a p p l i c a t i o n s and site l imi ta t ions , Table 5 s u m m a r i s e s the advan tages and d i sadvan tages of the var ious processes.

C o s t s of o d o u r a b a t e m e n t e q u i p m e n t Capi ta l costs for odour a b a t e m e n t e q u i p m e n t are very much control led by specific appl ica t ions , volume t h r o u g h p u t and site, condi t ions. I t should be noted t h a t the costs of suppor t ing services. -- such as power, the control system, d a t a monitoring, ductwork and civil engineer ing - can be a subs t an t i a l p ropor t ion of the overall project costs.

However, Table 6 gives some relat ive cap i ta l and opera t ing costs for the var ious me thods d iscussed in th is paper.

Table 4. M e t h o d s of s e c o n d a r y heat recovery .

Type of secondary recovery Characteristics When is it economical?

Direct oxidiser exhaust Can be used for furnace, oven • At operating temperatures or press make-up air above 100°C

Waste heat boiler

Hot thermal oil

Plant make-up air

Absorption chilling

Can produce about 1 kg/hr of steam for every 3.5 Nm3/hr of process exhaust

Can be used for high- temperature processes

Requires separate heat- exchanger

Can provide chilled water

• When operating for at least 2000 hr/year

• When the process is close to the oxidiser

• When steam is used for process heating

• When operating for at least 2000 hrlyear

• When the process is distant from the oxidiser

• When operating for at least 2000 hrlyear

• When replacing electric space heaters

• When heating is required for at least 6 months of the year

• When processing chilling or air conditioning is required for at least 6 months of the year

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Table 5. Advantages and disadvantages of the various odour control processes.

Advantages Disadvantages

• Bfoffllerl:

Low operating costs Medium capital cost

Media last for approx. 5 years

Large ground area Prone to upset

Requires steady continuous feed Dislikes changes in diet or concentration

• B i o s c r u b b e r s :

Low operating costs Medium capital costs

Accepts surges of high concentrations Can handle air near dew-point

Relatively low efficiency Acidic effluent liquor

Wet, cold air discharge

• Aotiveted carbon: Adsorbs majority of components

Large volume accommodates surges Large size of equipment

High capital cost High replacement cost Unsuitable for wet air

Potential disposal problem of spent carbon

• Wet scrubbers: Medium capital cost

Collects gases and particles Minimal explosion or fire hazard

Can handle gaSes at or near dew-point

Liquid effluent is produced Corrosion-resistant materials are required

May require multiple stages, depending on odour mix Wet, cold air discharge

• TIl~rrtwl o/klal lon: Final solution

Accepts variations in flow, concentration and species Excellent dispersion Medium capital cost

Catalytic units costly to replace

Tab le 6. Re la t i ve costs of odour abatement equipment.

Equipment Typical volume, ma/hr Capital costs, £/m3/hr Operating costs

Biofilters 200-10,000 8 -15 Very low Bioscrubbers 200-10,000 10-18 Medium Activated carbon 500-50,000 7 - 1 3 High Wet chemical scrubbers 1000-100,000 5 -12 Medium Thermal oxidisers 2000 - 200,000 8-- 15 Medium - high

Acknowledgments The au tho r t h a n k s the Directors of Haden Drysys In te rna t iona l Ltd for permiss ion to publ i sh th i s paper. The views expressed here are those of the author , and not Haden DrysTs In t e rna t iona l Ltd. Thanks are due to David Yardley of NCE Ltd for providing mate r ia l on biofil ters and bioscrubbers.

R e f e r e n c e s 1 'Sensory ()dour measurements using an olfactometer', incorpurating comments and changes to May 1992. Draft Dutch Preliminary Norm 2820.

2 'Odour sensing test - The three-odour-bag comparison method' Environment Agency, Prime Minister's Office of Japan,

152 February 1995 Filtration & Separation