Odours metrology and industrial olfactometry

Vigneron/Hcrmia/Chaouki (Eds), Characlcrization and Control of Odours and VOC in thc Process Industrics 0 1994 Elscvicr Scicncc B.V. All rights rcscrvcd.

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Odours metrology and industrial olfactometry J. Herniaa and S . Vigneronb a Professor at the Universite Catholique de Louvain (UCL), Institute of Chemical Engineering, Chairman of the Societe Belge de Filtration (SBF). Research Engineer (UCL), Head of Laboratoire de Traitement des Effluents gazeux et d’Olfactom6trie(LTEG/SBF).

1. Introduction The sense of smell is for the human being a key factor enabling him to recognize, identify and qualify elements around him. Moreover, odour reveals air quality: pleasant perfumes, nauseating emanations, toxicity alarm. Odour is also used as a warning of danger, fact illustrated by the odorous tracker added to natural gas. Today, odour has taken an connotation reflecting the quality of the environment : it is most often found at the crossroads of toxicity problems due to direct exposure and nuisances - rather indirect - linked in peculiar to air pollution. The problem of Volatile Organic Compounds emissions illustrates this phenomenon : some VOC are carcinogenic and precursors of ozone formation in the troposphere while they can also be odorous. According to the definition given by the WHO (World Health Organization), the fight against odorous emissions is an undertaking benefical for the human health : it contributes to a state of physical, mental and social welfare while simultaneously decreasing the risks of disease and physical disabilities by reducing - see eliminating - possibly toxic odorous compounds. This implies measuring an odour to assess the discomfort it causes. Objective and reliable methods available are most of the time hardly known although being the subject of normalization in several countries. The method and results, herein exposed, are based on the use of a panel of experts whose job consists in sniffing odours ( method called by the name of dynamic dilution) coupled to an accurate physico-chemical analysis. A link is established between the two results by calculating the odorous power or d o u r decibel of the analyzed sample. Odour is often found at the basis of staff preoccupations or complaints of factories riverside residents : olfactometric analysis appears thus as a useful tool for industry representatives because it allows them to obtain information linked to hygiene and environment, possibly to establish specifications for the design of facilities, while limiting itself to the original problem.

78 2. Odours metrology

2.1. Generalities Odours are linked to the presence of volatile compounds in the air; therefore the idea arose of identifying, by physico-chemical analyses techniques, the nature of these compounds as well as to quantify them and to interpret, by means of olfactory data tables giving the perception thresholds for each of them, their possible contribution in the perceived odour. Unfortunately, several arguments opposes this approach. The first, and by far the most important, is the at first unknown interaction between the compounds arriving on the mucous membrane, possibly the potential interactions between them, as well as the complexity of the gaseous mixture submitted to it. Indeed, ambient atmosphere - especially industrial - consists of tens, possibly hundreds of volatile compounds in slight traces. Yet, most of odorous compounds posess perception thresholds close to the tenth of a ppb (see figure 4). Simple mixtures have been studied and researches showed that the odorous action of compounds do not add one to another in a n arithmetical way but rather in a vectorial manner, each compound having a phase angle depending of the natures of neighbouring compounds [l]. This interpretation allows to understand the inhibiting or exacerbating effects of compounds between themselves. It is also possible to compare odours to noise and to interpret odorous power (pOU) in terms of odour decibels (dBO), like proposed by R. C . Oberthiir [2]. The second argument lies in the complex nature of gaseous mixtures, forbidding any prediction concerning the olfactory discomfort from physico-chemical analyses, the interference of compounds being almost impossible to simulate. Finally, possibilities of chemical arrangements of the molecules ,organic in peculiar, are so numerous that the help brought by the data tables remains limited. To conclude, in the current state of knowledge, an odour can only be measured by the sole available sensor: the human nose. The appreciation of an experimenter constitutes therefore the basis of developed measuring methods. Olfactometry includes measuring techniques allowing the determination of the dilution level at the olfactory perception threshold of an odorous gaseous mixture and the determination of the odorous intensity. The latter, psychophysical, suffers limitations arising from the fact that the human nose is essentially a qualitative instrument, allowing to measure in quantitative terms only by use of artifices or supplementary instruments. So,olfactory sense allows each of us to feel impressions convertible in numbers if a scale and units are fixed to it. For instance, in comparison with compounds concentration standards such as n-butanol or pyridine, an odorous intensity, expressed in ppm equivalents of these compounds, can be deduced. Moreover, this method (called supraliminal) has the inconvenient of submitting the olfactory mucosa to quite high odorous compounds concentrations, causing their fatigue or even their saturation, with as consequence a bad reproducibility of results.

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As soon as 1982, and in cooperation with other laboratories taking part at the same research program, a method known as dynamic dilution at the perception threshold was chosen. This method minimizes subjectivity in odour evaluation because it expects from the panel members a “yes or no” binary answer. Since then, use of this method tends to be generalized [3]. More than a measuring technique, olfactometry applied to industry is an approach capable of categorizing environmental problems linked to gaseous effluents. This approach includes several steps going from preliminary expertise to the reception of an abatement technique including characterization steps of effluents to be treated, possibly repercussions study (dispersion calculations). 2.2. Prelimina y expertise

Preliminary expertise is important since the emission problem is faced by an expert, exterior to the considered site, who acquired some systematics about the possible sources. The expert report already includes a concise inventory of sources and compounds possibly present. This first step is used in particular for the choice, possibly the elaboration of techniques to be used for the collection of gaseous samples and their subsequent analysis. Though intervening at an ulterior step of the study, the establishment of an exhaustive inventory of emissions, possibly the process study (comprehension, spottings, balance sheets) will be classified in this step; such an approach being the monopoly of industrial plants. 2.3. Sampling Sampling requires peculiar precautions and must be adapted to the nature of the present compounds as well as to the configuration of the source. Following the compounds nature, methods of sampling using bags, bubbling in a solution (basic for carboxylic acids), adsorbing or reacting cartridge (e.g. formaldehyde) can be applied. Specific sensors can also be used for specific analyses. Olfactometry requires a collection of at least 15 liters of gaseous sample and thus always uses the bag sampling method; for chemical analyses, this method is, if necessary, coupled with another technique, the most adequate for the targeted compounds. The sampling technique cannot alter the effluent which is to be analyzed; therefore it has to use the most inert materials and techniques.

-

Fig. 1 Sampling technique

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The chosen sampling method is based on allowing low vacuum into a barrel containing the sampling bag which is put in contact with the external source (figure 1);by this way, the bag fills up without any external mechanical help. Moreover, the choice of pipes material can be made among non-adsorbing materials such as TEFLON. Especially, the bag has to be made in non-adsorbing material : TEDLAR@appears to be the material giving the best guarantees. 2.4. Measure of odour level according to the method of dilution at the threshold This method consists in determining the perception threshold of a gas, the latter being defined as the dilution level with pure air to be obtained so that only 50 OO/ of a panel perceives or not the odour. Dilution factor (f) obtained by the means of the olfactometer is expressed as :

DA being the flow of pure air supplied by the olfactometer, possibly epurated on activated charcoal, and DGO the flow of added odorous gas. The odour curve (figure 2) displays the percentage of the panel perceiving the odour as a function of the dilution of the gas; this s-shaped curve exhibits two plateaus characterizing the saturation and absence of perception levels of the human nose reached for respectively very low and very high dilutions. Between these two limits, the curve can be exploited and statistically adjusted; the perception threshold at 50 Y' O can thus be calculated as being the most probable (probability level : 1 - cr = 0,95) by using the approximation of the PROBIT [4] analysis method using the observed frequences : p = r/n with r = amount of "yes" and n = total amount of answers. Statistically, it is common knowledge that the choice of a panel can be made without any previous selection if it includes at least 9 persons. Moreover, a maximum accuracy is obtained with n = 18 (by submitting three times the same sample to a 6 persons panel) [5]; the mean standard error (asE)being :

where oo2is the variance obtained on a member of the panel [5]. The option chosen in the laboratory was to constitute a panel of 4 persons, which requires a previous selection excluding extremes, i.e. persons displaying ab- or subnormal perceptions in a general way or for some compounds in particular. A same dilution level is submitted 4 times; the total amount of answers, n, is equal to

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16, which gives a highly satisfactory accuracy to the results. The trust interval around d is also determined and validity tests are carried out following the X 2 method.

The PROBIT statistical adjustement method is especially used for observations of biological events based on the count of answers or ascertainments as ”having occured” or “having not occured”, which is a type of answer in which the dilution factor determination at the olfactory perception threshold can be classified. By definition, it is difficult for the observer to state positively with certainty, despite a positive or negative answer from the panel, that the odour was indeed perceivable or not. 2.5. Description of the

olfactorneter used

Fig. 2 -Typical odour curve and PROBIT regression line

The determination of this curve is made with the help of an olfactometer, device designed to produce easily, with accuracy and reproducdeter mined ti b i 1it y, mixtures and to submit them to the appreciation of the panel of experts. The olfactometer used is a dynamic dilution device developed by the Warren Spring Laboratory 3835 from Prosser Scientific Instru-

ments Ltd, PSI). The device scheme is given in figure 3; in this device, an odourless air flow (240 litres/min.) - ambient air epurated on activated charcoal - is combined with a controlled flow of odorous sample; the mixture is submitted simultaneously to all members of the panel. Dilutions obtained by means of the PSI olfactometer range between 25 and 250000; a primary dilution of the studied gaseous sample allows of course to increase as much as needed this ratio. The step between successive dilutions has been chosen in order to respond to a geometrical progression of ratio 1O0~*(1,585); this step is the minimum required by the human nose to establish a difference between two dilutions. In the PSI olfactometer, dilution is carried out in two steps; the first covers a dilution range between 25 et 2 500 while the second allows, with addition of a

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supplementary dilution factor of 100, to cover the rest of the range between 2500 and 250000.

Sniffing doors

Activated filter

Flow regulation and measurement

Fig. 3 - Scheme of the PSI olfactometer

2.6. Chemical analyses For volatile organic compounds, chemical analyses are generally carried out in gas phase chromatography coupled with mass spectrometry. Mass spectrometry is assisted by a computerized treatment system allowing compounds identification through a data bank. Quantitative analysis is partially automated thanks to the software allowing the measure of targeted compounds concentrations. In some cases, more simple and more specific detectors can be used : FID (Flame Ionization Detector), for organic gaseous mixture of which the composition is either known or not too complex, FPD (Flame Photometric Detector) for sulphurated compounds and specific tools like electrode for ammonia. 2.7. Odorous powers A simple but quite useful presentation of results can be obtained by expressing the odorous power annoted of one of the compounds by the following relation [6]:

pOU = log(Ci/di)

[dB01

where Ci and di are respectively the concentration and perception threshold expressed in bg/SCM of the detected compound i, the perception threshold being accessible by means of tables for most of widespread volatile compounds. If Ci is lower than di, odorous power is given a zero value. Odorous power expressed in odour decibel allows to visualize easily the relative contribution of a compound or a family of compounds in the perceived odour. This approach allows to catch sight of substitution methods of odorous compounds (which compound to be preferantially substituted) and to catch sight of the best abatement technique (varying with the family of compounds contributing mainly to the odour). By globally adding the obtained pOU, an image of the gaseous sample odorous

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power appears and the importance of one source with regard to another is relativized (see expo works 3,9,10). 2.8. Odourflows and dispersion In order to estimate the impact of an odorous emission in the environment, determining the dilution level is not enough; the calculation of the odorous flow of the source (Q,) is required, the latter being the product of the gaseous flow of the source, expressed in Nm3/h (Q),by the dilution level at the perception threshold (f): Qo = f*Q [o.u./h]

It is on the base of the odorous flow that intervention priorities on complex industrial sites will be decided: indeed, the site can be mapped by department, workshops or activity sectors and assessment of the relative importance of the flows emitted by each of them can be made. On the other hand, this data will also serve as basis for dispersion calculations used for the evaluation of the olfactory discomfort perceived in the neighbourhood of the emitting site. 3. Norms, recommandations and regulations

Currently, to our knowledge, there is no project about norms concerning odours at the level of the European Community for emissions in the environment nor at the level of work regulation. Nevertheless, several countries or regions have acquired a kind of lead in this field and provided themselves with norms concerning measurements, emissions and tolerated immissions. The most advanced countries are the Netherlands and France; across the Atlantic it is to be noted that the Quebecer government provided itself with specific regulations about odours emissions while the urban community of Montreal has its own olfactometry laboratory. In France, a few norms exist : AFNOR NF X 43-101 (determination of the dilution factor at the perception threshold), AFNOR NF X 43-103 (supraliminary method) and AFNOR NF X 43-104 (sampling methods). Three main texts regulating odorous emissions exist : D the French technical instruction of june 27,1977 relative to quartering [7]; D the Quebecer regulation about air quality (law on the quality of environment in enforcement since december 1,1980); Cl the Dutch indicative program about air (IMP-L) of 1985-1989 [8]. The first two texts restrict odours at the source, quantified in terms of odour units or odour flow, while the third imposes an olfactory discomfort tolerable in places around the factory; it proposes, if the discomfort is judged as too important, to calculate the admissible odour flow-rates by means of an atmospherical dispersion model and to carry out the required modifications on the industrial premises.

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France recently issued a ministerial decree restricting, as a general rule, the maximum odour flow to lo7 (o.u./h). Finally, one cannot forget the German TA-luft of 1986 [9]refering, though timidly, to recommandations concerning odours emissions.

3.2. The French technical instruction Though relative only to quartering, this technical instruction is especially interesting because it fixes limit values for cold gaseous exhausts (storage...) as well as hot ones (coming from the cooking pot). Several parameters are considered : Q dilution level at the perception threshold of effluents must be inferior to 200; Q odour flow is set at 1 million cubic meters per hour; Q minimum epuration yields are fixed at :

I

Corn pounds ~~

reduced sulphur

I

amrnonidamines

I

aldehydeslketones

~

Required yield

>98 %

>98%

>95%

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Following the discomfort really measured by the inhabitants of a given region, the tolerable odour flow-rate will be calculated with a long term distribution model. 3.4. The German TA-luft Paragraf 3.1.9 relative to odorous substances recommends methods and precautions needed to limit and disperse odours but point out that, if the limitations on some substances or groups of substances are not sufficient, the limit has to be stated by means of an olfactory abatement efficiency of the epuration installations. It also tells that, in case of odour levels superior to 100000, the technically feasable epuration yields can be superior to 99 %. Finally, concerning the limits for atmospherical exhausts of organic substances, the classification of substances in three suggested categories is also carried out bearing in mind the olfactory aspect (paragraph 3.1.7). 4. Industrial olfactometry

4.1. Perception thresholds per activity sectors Table 3, on the next page, gives the odour levels reached in industry, for several sectors and several places of measurements (work stations and environment). As for the hygiene at the work stations, the thresholds reached in steel industry (coking plants : ovens floors, charging and discharging of the oven; tapping floor of blast furnaces, cold rolling mills) and in the coating industry (painting workshops) will be noted. Perception thresholds expressed in odour units are often superior or equal to 200 O.U. It is to be noted that considerable improvements can be obtained. The first example concerns the coking plants where new facilities (built after 1980) exhibit at the ovens floor an odour level reduced by 57 % by comparison with the old facilities for which few actions had been taken against emissions. Painting workshops are another example : indeed, the use of paints poor in organic solvents reduces drastically the volatile organic compounds emissions and subsequent odours. An important comment, from the environmental point of view, would be that the respect of norms, even the most severe ones, concerning the emitted volatile compounds concentrations does not always guarantee the respect of foreign norms (considered to be acting as recommandations in Belgium) on odours (for instance the thermal incinerators in the coating industry).

Several reasons can be put forward to justify the lower performances of abatement techniques on odours with regard to the concentrations. The main one is the relatively low perception thresholds of some compounds, possibly reaching the tenth of 0,l ppb (about 0,l pg/SCM) : such residual concentrations are difficult to obtain by whatever kind of abatement technique. More specifically for gaseous effluents epuration techniques involving incineration (thermal or catalytic), high molecular weight compounds are partially destroyed while having relatively more important perception thresholds. Finally, abatement techniques can give birth to byproducts even more odorous : partial oxidation, by thermal or catalytic way, of

m _. o\

Sector of activity Steel industry : coking plants < 1980 : ovens floors > 1980 : ovens floors Charging in oven Discharging from oven Treatment of cokemaking plant gas Steel industry : blast furnaces Tapping .~ floors Granulation Steel industry : cold rolling mills v of n o n s

+%g%GGj

Painting workshops and annexes idem idem Ovens outlet Thermal incineratorsoutlet Polyester fabric enduction PU~D enduction Automobile industry Decooconing tunnel Printing house Rotary presses outlet Adsorbing unit outlet Polypropylene drying ovens Electronic industry Printedcircuits stripping Household waste treatment Pits

Rubbish d u r n gas Food industry (yeast) Crystalkation

Compounds Families

O.U.

338 145 717 236 36099 1968 196275 191 1430

444... 68 410 320 13313 1373 450 983 131

4498 6657 7664 2

1

17172 47922 2

5,7 ...8,7 1.9 73 >15;,8

>IJ% 10

-

I

m

Aromatic compounds (among which BTX) and plycyclic unds among others H alkanes, phenols, sulphuratedco Nitrogenous compounds (among%ers

Oil emanations :carboxvlicacids. aldehvdes,VOC

so2 ,voc

27880...74535 Paints with 60 8 solvent : VOC High solids paints : VOC 5128 Water-based paints : water + VOC 23987 VOC >I gNm3 VOC 22217 Methacrylate resin :carboxylicacids,VOC 2410 260000...5 82OO( Aldehydes, VOC 97oooo

>375 133 4050

%%%,Ketones, Aromatic compounds,Acetates I?

Halogenated comunds. aldehvdes. VOC

233 Halogenated compounds,VOC

204 77775

1890 lo900

348650 425930

Carboxylic acids, Aldehydes,VOC

Table 3 Perceptionthresholds, odorous powers and concentrations of volatile compounds by sectors of activity [3,lO,ll]

2s)9

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alcohols in aldehydes, a hundred times more odorous, production of thiols in a washing column treating sulphurated compounds ... The production of NOx also contributes to a relatively lower odours abatement efficiency in comparison to the one reached for volatile organic compounds treated by thermal incineration. 4.2. Perception thresholds of some representative compounds

The figure 4 sums up the perception thresholds values expressed in pg/SCM of some of the main compounds and the evolution tendency within the main COV families. For an equal number of carbon atoms, the hierarchy of decreasing perception thresholds can be established as follows (approximatevalues) : Alkenes > Alkanes Ketones = Alkylbenzenes> Alcohols > Aldehydes = Carboxylic acids 1o8

lo7 lo6 n

a

-

w

3

10'

lo=

lo2 10'

1oo

-

Fig. 4 Perception thresholds values (d: pg/scm) for several volatile compounds and families [3,12,13]

Within a given chemical family, the perception threshold increases if the molecular weight decreases. Halogenated compounds are not very odorous compared (in the increasing order) to nitrogenous and sulphurated ones. In these two last families, the quite low perception thresholds of nitrogen oxides and sulphur dioxides are to be noted. 5. CONCLUSIONS

Odour appears at first as a very subjective concept that olfactometry intends to measure objectively. The determination of the dilution flow at the perception threshold of a gaseous sample allows to establish odour concentrations free from any subjective limitation.

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This concept, parallel with the dynamic dilution olfactometer measuring method presented here, tends to be generalized on an international scale. Establishing normalized measuring methods and reference units already allowed several countries to regulate odours emissions. Let us stress the fact that these regulations aim at limiting and not supress odours emissions. As we saw, compounds perception thresholds expressed in pg/scm can be quite low and most of the time compounds present in slight traces turn out to bring the more discomfort. From all of this, several important conclusions can be drawn for the people in charge of these problems: given the very low concentrations at which volatile compounds can be perceived, odour does not automatically associate with the notion of toxicity. Therefore, before any further approach in this way, olfactometric analysis turns out to be most useful in order to define the real problem. As for environment, it is also to be concluded that the most volatile compounds (lowest molecular weights) are not necessarily the most odorous; on the other hand, combustion oxides, most often considered as odourless, are to be distrusted. Olfactometry, when including a physico-chemical analysis section as complete as the one presented here, is not only a measuring tool for assessing the perceived discomfort : it is firstly a method proposed to the industry to solve its odours and air pollution problems. Indeed it gives information about compounds and families of compounds responsible for the discomfort and allows to choose with more assurance a solution for emissions reduction, which compound substitution to carry out, which abatement technique will be the most efficient, where does the problem come from. Used abreviations:

-

BTX: Benzene-Toluene-Xylenes SCM: Standard cubic meter

Bibliography LAFFORT P., Synergie et inhibition en olfaction, extract from ODEURS b disodorisation dans l’environnement, TEC & DOC LAVOISIER, 1991. OBERTH- R. C., dB (Garuch), GE/m3, GE-Welche Dimension hat die Reizgroh 1, Geriiche, Stand der Erkenntnisse zur Ermittlung von Belastung WI Belastingung, Band 12, Schuiftenreihe, VDI-RdL, Workshop 28, November 1989, Dusseldorf, germany, pp 3 5 4 . GILLARD F., EnquPte, inventaire et prhention des polluants olfactifs dans les industries sidhrgiques et c o k i h - Etudes rt la source, aux posies de travail et dans I’environnement, CECA research no 725776/382/02, final report, Soci6t6 Belge de Filtration, Louvain-La-Neuve,April 1984, Belgique. FINNEY D.J.,Probit analysis, Third, CambridgeUniversity Press, 1971, Great Britain. BEDBOROUGH D.R. and TROT P.E.,Thesensory measurement of odours by dynamic dilution, Warren Spring Laboratory, Stevenage, 1979, Great Britain. HOSHIKA Y., NIMEI Y. et NUTOG, Simple Circular Chart of Characterization of Trace Amount of Odorant dicharged from Thirteen Odor Sources,, J. Chromatogr. Sci., 1981, Vol. 19, pp. 200-214. Installations classies pour la protection de I’environnement, Brochure 1001, Journal Officiel de la Rbpublique Francaise, 1977, Fifth edition Indicatief Meerjarenprogramma Lucht (IMP-L) 1985-1989, Nederlandse Tweede Kamer, vergadejaar 1984

89 [9] lnstruction technique pour le maintien de la puretC de l’air, February, 23, 1986 - TA. LUFT, CITEPA, translation of official german texts, Paris, France. [lo] VIGNERON S., Les nuisances olfactives de l’industrie sidirurgique et coki2re : ttude des dthodes de prhention et des techniques d’abattement, CECA research no 7261-01/409/02, final report, Societk Belge de Filtration, Louvain-La-Neuve, May 1989, Belgique. Ill] VIGNERON S., Les nuisances olfactives duns les ateliers de la sidhrgie afroid : Ctude des mtthodes de prhention et des techniques d‘abattement, CECA research no 9261-04/441/02, final report, Societ6 Belge d e Filtration, Louvain-La-Neuve, April 1992, Belgique. [12] VAN GEMERT L.J. and NE’ITENBREIJER A.H., Compilation ofodour threshold values in air and water, Central Institute for Nutrition and Food Research TNO (CIVO) and National Instiute for Water (RID),June 1977, The Netherlands. [13] VAN GEMERT L.J.,Compilation ofodour threshold values in air, supplement V, Central Institute for Nutrition and Food Research TNO (CIVO), August 1984, The Netherlands.