J Sci Food Agric 1997, 73, 437È445

Characterisation of Odorous Compounds andEmissions from Slurries Produced from WeanerPigs Fed Dry Feed and Liquid Diets

Philip J Hobbs,* Tom H Misselbrook and Brian F Pain

Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB, UK

(Received 8 September 1995 ; revised version received 13 September 1996 ; accepted 10 October 1996)

Abstract : Changes in odorous emissions were recorded from slurries produced byweaner pigs fed dry feed and feed with water added in the respective ratios of3 : 1 and 4 : 1. Slurries were placed in an environmentally controlled emissionschamber, periodic air sampling was performed to determine the olfactometricresponse as odour concentration, and the air was analysed to identify volatileorganic compounds present. Distinctive odours were produced by each slurry.However, four major groups of odorants were identiÐed as sulphides, volatilefatty acids, phenols and indoles. The odour concentration from the slurry of the4 : 1 diet was signiÐcantly less (P\ 0É05) than the odour concentration from thedry feed and 3 : 1 slurry samples. Decay of the sulphide component of the odourswas investigated and the role of methanogenesis in reducing odour production isdiscussed. While monitoring the emissions in the chamber the slurry odorantconcentrations increased by up to 50 ppm h~1.

Key words : pig slurry, odours, liquid feed, odour analysis.

INTRODUCTION

It is generally accepted that liquid feed systems improvefeed intake, growth, feed conversion and health ofweaned pigs (Partridge and Gill 1993). Cumby (1986)attributed the increasing use of liquid feed systems tothese factors and to a reduction in cost resulting fromthe use of cheap liquid by-products. The e†ect thatliquid feed systems may have on the volume or charac-teristics of the effluent produced by the pigs is less welldocumented. The health of the pig can also beneÐt ; in astudy (Russell et al 1996) L actobacillus colonised theliquid feed and was found to reduce the number of coli-forms in the diet. Kovacszomborszky et al (1994) foundthat the lactic acid concentration of the ileal contentincreased when the probiotic “LactoSaccÏ was fed. Thisreduced pH and coliform numbers, resulting inimproved ileal digestibility of nutrients and essentialamino acids.

* To whom correspondence should be addressed.

Odours from piggeries and slurry spreading oper-ations have become increasingly a source of complaintfrom the public. Intensive pig production systems resultin large volumes of slurry being stored, usually underanaerobic conditions where degradation of protein andplant Ðbre leads to the production of volatile organiccompounds, many of which are odorous (Hobson et al1974). Most odorous compounds or odorants aresequentially degraded to methane (Hobson et al 1974).An imbalance between the formation of volatile fattyacids (VFA) and methane production leads to an accu-mulation of VFA (Spoelstra 1980), which contribute tothe odour.

Better conversion of feed should result in a reductionin excretion of substrate for production of odorants.Comparing the chemical analysis of odours from pigsfed di†erent diets is exceedingly complex because over160 volatile compounds have been identiÐed in andabove pig slurry (OÏNeill and Phillips 1992). As yet nocompound has been found suitable as a chemicalmarker to predict odour consistently, probably due to

437J Sci Food Agric 0022-5142/97/$09.00 1997 SCI. Printed in Great Britain(

438 P J Hobbs, T H Misselbrook, B F Pain

the complexity of odorants present and, therefore,dynamic dilution olfactometry is commonly used tomeasure odour concentration (OC) (Pain et al 1991).However, more recently Hobbs et al (1995) demon-strated by direct chemical analysis of the slurry head-space gases that as few as 15 major odorants contributeto pig odour.

The aim of this study was to investigate the e†ect onthe odours and 15 major odorants produced from theslurry of adding water to the feed of weaner pigs. Theodours were generated in a large chamber to betterassess the types of odours and their relative concentra-tions of odorants during the passage of air over theslurry surface. Methane emission was also quantiÐed toassess the rate of methanogenesis, which may be associ-ated with odorant concentrations in the slurry.

MATERIALS AND METHODS

Animals and diets

Weaned pigs direct from the sow were fed ad libitumwith either dry pellets, 3 : 1 or 4 : 1 (w/w) water to feedrespectively. The feeds (Colborn Dawes Nutrition Ltd,Heanor, Derbyshire, UK) are described in Table 1, theywere “Easy-WeanÏ which was fed for the Ðrst 7 days,followed by “Piglets ProgressÏ which was fed for the next21 days. The pigs were housed in an environmentallycontrolled Ñat deck weaner house, comprising threepens each containing six pigs.

The odour and emissions chamber

The odour and emissions chamber (OEC) was designedso that up to 200 litres of liquid could be exposed to anenclosed atmosphere under controlled environmentalconditions (Cumby et al 1995). BrieÑy, it consisted of anenclosed system of stainless steel ducting(0É5 m ] 0É5 m internal section) whose ends were con-

TABLE 1Nutrient analysis of weaner pigs feeda

Piglets progress Easy-wean(g kg~1) (g kg~1)

Protein 220 210Oil 75 90Fibre 20 20Ash 50 50Total lysine 15 15É5

Additives (mg kg~1)Copper sulphate 175 175Avilamycin 40 40

a Plus a full compliment of minerals, trace elements and vita-mins, including selenium at 0É4 mg kg~1.

nected into a Tedlar bag (manufactured at SilsoeResearch Institute, Silsoe, Bedfordshire, UK fromTedlar supplied by Allied Signal Laminate Systems, 16CEtherow Industrial Estate, Woolley Bridge Road, Hol-lingworth, Hyde, Cheshire, UK) as shown in Fig 1. Theair in the system was pressurised by the bag beingwrapped around a roller, which moved down support-ing rails under gravity. This compensated for leakagesby maintaining a positive pressure, thus preventing dilu-tion of the OEC odour by incoming air. During a 3 hperiod the volume in the OEC can reduce from 40 m3to 15 m3. OEC air was circulated at 1É0 m3 s~1 at20¡C. The slurry was stirred at a constant rate for allsamples. The OEC was cleaned after use by Ñushingwith ambient air at 1 m s~1 containing ozone producedat 250 mg h~1 for 2 h and then with ambient air over-night.

Sampling

Sufficient quantities of slurry were collected frombeneath each of the slatted Ñoors during a 28 dayperiod for the three diets. The three samples were stored

Fig 1. Schematic representation of the odour emission chamber.

E†ect of diluting pig feed on slurry odour 439

in enclosed stainless-steel trays (2 m] 0É5 m ] 0É25 m)placed in a water bath at 15¡C for 1 month before beingplaced one at a time into the OEC. These conditionsimitate storage of the slurry, and emission rates aremore likely to be representative of those from piggerywastes. Once in the OEC the slurry was stirred at aconstant rate throughout the experimental period. Aninitial 500 cm3 slurry sample was withdrawn during theÐrst 10 min, the end of this period being regarded astime zero. Headspace samples were collected in 6 litreTeÑon FEP gas sampling bags (Adtech Polymer Engin-eering, Gloucestershire, UK) at zero (blank), 15, 30, 60,90, 120, 150 and 190 min after the slurry surface wasexposed to the circulating air in the OEC with theintention of determining the emission proÐle and sta-tistical error. Headspace samples of 20 cm3 were alsotaken for methane determination. After 190 min a Ðnalslurry sample was collected. Slurry samples were storedin enclosed sample bottles at 3¡C and analysed within 1week.

Olfactometry

Odour concentration (the number of dilutions ofodorous with odourless air at which 50% of an odourpanel can just detect an odour) was measured using twodynamic dilution olfactometers (Project ResearchAmsterdam BV, Singel 97, 1012 VG, Amsterdam,Netherlands). Equipment and procedures conformed tocurrent recommendations (Dutch Normalisation Insti-tute 1990). Each olfactometer had two sniffing ports andwas of the forced choice type, with odourless air beingpresented to the panellist through one port and dilutedodorous air through the other. Each panellist wasrequired to indicate via a keyboard which port con-tained the odorous air. A range of at least Ðve dilutionsteps, each di†ering from the next by a factor of two,was presented to the panellists in steps of ascendingconcentration, the whole range being presented twice.The 50% detection threshold was calculated accordingto Dravnieks and Prokop (1975). A 198É2 mg m~3butan-l-ol in nitrogen sample was also presented to thepanel for threshold determination on each occasion as areference standard (Dutch Normalisation Institute1990).

Headspace analysis

To identify volatile organic compounds in the head-space, silica-based (Orbo 52, Supelco Inc, Supelco Park,Bellefonte, PA, 16823-0048 USA) and carbon-based(Orbo 32) adsorbents were packed into adsorptiontubes and used to concentrate 500 ml of OEC head-space sample for analysis by GC-MS. Those volatilecompounds identiÐed by GC-MS in the headspace andknown to be odorous were analysed in the slurry. Chro-

matographic retention time and mass spectral matchingwere used to conÐrm odorant identity.

A Hewlett-Packard (HP) (HP Ltd, Heathside ParkRoad, Cheadle Heath, Stockport, Cheshire, UK)GC-MS system consisting of a 5890 II Series gas chro-matograph and a 5972A mass selective detector (MSDII) was used for analysis. A 25 m fused silica (cross-linked methyl siloxane) HP-1 column with an internaldiameter (id) of 0É2 mm and a 0É34 km Ðlm with a 1 mdeactivated fused silica guard column (0É25 mm id) wereused. The Ñow rate of helium (the eluting gas) was0É40 ml min~1. The Optic temperature programmableinjector (Ai Cambridge Ltd, Pampisford, Cambridge,UK) was used to desorb headspace samples from theadsorbents and was initially at 30¡C and heated to250¡C at 16¡C s~1 for 1 min for headspace samples. Anelectronic pressure controller was used to o†set peakpressure broadening with increasing GC column tem-perature. The GC oven conditions were an initial tem-perature of 27¡C, then to 220¡C at 15¡C min~1 andremaining at 220¡C for 1 min. The GC-MS interfacewas at 280¡C. The mass spectrometer scanned from 35to 250 mass units every 0É2 s to give responses in theppb range. Even so only the sulphides were analyseddirectly as the concentrations of the other odorantswere too low and required preconcentration on theadsorbents before analysis.

Sulphides were analysed directly by injection of0É2 ml of headspace gas sample into the GC-MS with aglass inlet at 110¡C and using the same GC conditionsas described in the thermal desorption technique men-tioned above.

Headspace methane concentration was determinedusing a HP 5890 series II GC Ðtted with an FID and a2 m stainless-steel Haysep “QÏ column. Operating tem-peratures were : detector 250¡C, column 40¡C and injec-tor 100¡C.

Slurry analysis

Slurry samples were analysed for odorant concentra-tion, pH, total solids, ammonium-N, nitrate-N and totalN contents. Total solids were determined by drying toconstant weight at 90¡C. Ammonium-N and nitrate-Nwere determined after extracting the slurry with 1 M

potassium chloride. Ammonium-N was determined byreaction with sodium dichloroisocyanurate, sodium sali-cylate and sodium nitroprusside to form an indophenoldye (Searle 1984). Nitrate-N was determined byreduction to nitrite with hydrazine sulphate, diazo-tisation with sulphanilamide and coupling with N-1-naphthylethylenediamine dihydrochloride to form anazo dye (Kamphake et al 1967). Total N content wasdetermined using the Kjeldahl technique (Tecator 1987).

Direct quantitative analysis of the odorants proveddifficult because of the low concentrations in the head-space and they were therefore analysed in the slurry,

440 P J Hobbs, T H Misselbrook, B F Pain

TABLE 2Odorants identiÐed in the OEC during the experiments with

maximum concentrations given in mg m~3

Odorant Dry feed 3 : 1 4 : 1

Hydrogen sulphide 15 240 3Methanethiol 36 NDa NDDimethyl sulphide 14 ND NDDimethyl disulphide 12 ND NDDimethyl trisulphide 5 ND NDAcetic acid 4É7 2É3 1É8Propanoic acid 2É5 0É07 0É022-Methyl propanoic acid 0É2 ND NDButanoic acid 1É1 4 ND3-Methyl butanoic acid 1É1 0É8 NDPentanoic acid 0É2 ND NDPhenol 4É8 3É7 4É34-Methyl phenol 7É0 5É8 4É64-Ethyl phenol 4É9 0É5 0É5Indole 0É1 0É5 0É33-Methyl indole 0É1 0É4 0É4

a ND, not detected (See Table 3 for odour detection thresh-olds ; threshold values for 2-methyl propanoic acid are notknown).

with the exception of the sulphides. Slurries were pre-pared for odorant quantiÐcation by centrifuging at5000 ] g for 30 min and injecting 2 kl of supernatantinto the GC-MS system. Temperature programmingwas the same as for headspace samples, except that the

TABLE 3Odour detection thresholds for odorants present in pig slurrya

Compound Odour detectionthreshold(mg m~1)

Acetic acid 25È10 000Propanoic acid 3È890Butanoic 4È30003-Methyl butanoic acid 5Pentanoic acid 0É8È70Phenol 22È40004-Methyl phenol 0É22È35Indole 0É63-Methyl indole 0É4È0É8Methanethiol 0É5Dimethyl sulphide 2È30Dimethyl disulphide 3È14Dimethyl trisulphide 7É3Hydrogen sulphide 0É1È180

a After van Gemert and Nettenbreijer (1977).

initial temperature of the programmable injector was60¡C.

RESULTS

The changing character of pig odours in the air in theOEC was demonstrated using slurries produced fromweaner pigs fed on di†erent diets and showed that odor-

Fig 2. Volatile fatty acid concentrations in the slurry samples from weaned pigs given dry feed, and water to feed in the ratios 3 : 1and 4 : 1.

E†ect of diluting pig feed on slurry odour 441

Fig 3. Phenol and indole concentrations in the slurry samples from weaned pigs given dry feed, and water to feed in the ratios 3 : 1and 4 : 1.

ants were present in the headspace at very low concen-trations (Table 2). Their odour thresholds are shown inTable 3, although other less signiÐcant or non-odorouscomponents may have contributed by modifying odourcharacter either synergistically or by chemical e†ect.

The odorants identiÐed in the headspace were quanti-Ðed in the slurry, with the exception of the sulphideswhich were not at sufficient concentrations in the slurryto be analysed (due to their hydrophobic nature). Sul-phides were analysed quantitatively by headspaceanalysis. VFA were present in the greatest amounts (Fig2), with highest quantities in the slurry from the dry feeddiet and the lowest from the 4 : 1 diet. Acetic acid wasthe predominant VFA, with 4-methyl phenol, phenol,4-ethyl phenol, indole and 3-methyl indole present inprogressively lower concentrations (Fig 3) (standarderrors increased from 5 to 25% as the odorant concen-

trations decreased). Other odorants were identiÐed inthe slurry, but were present at less than 0É5 kg ml~1.These minor odorants included 2-methyl propanoic, 2-methyl butanoic and hexanoic acids. These were notpresent at sufficient concentrations to be detected in theheadspace. When the total odorant concentrations inthe slurries were compared, the 3 : 1 and 4 : 1 diets con-tained 23% and 8% of that from the dry feed diet,respectively. Greater volumes of slurry were producedby weaner pigs fed the diluted feed and this has to beconsidered when determining the total odorants pro-duced from each diet. The nitrogen content was deter-mined using the slurry volumes which were 138, 183and 227 litres for dry feed, 3 : 1 and 4 : 1 diets, respec-tively, and the results are presented in Table 4. Despitethis, odorant quantities were 31% and 13% of thoseproduced from the dry feed slurry for the 3 : 1 and 4 : 1

TABLE 4Total volume, solids and nitrogen present and pH of slurries before OEC run

V olume T otal solids pHa Ammonium-N Nitrate-N T otal N(litres) (kg) (g) (g) (g)

Dry feed 138 5É93 7É1 (7É9) 458 4É97 8003 : 1 183 6É04 7É5 (8É5) 556 2É20 8584 : 1 227 4É77 7É6 (8É4) 502 5É90 785

a Bracketed values were determined after the OEC run.

442 P J Hobbs, T H Misselbrook, B F Pain

Fig 4. Odorant concentration changes in the slurry samples from weaned pigs given dry feed, and water to feed in the ratios 3 : 1and 4 : 1 occurring during the OEC experimental sampling period. Acetic acid concentration decreased by 400 g m~3 for the dry

feed slurry sample.

diets respectively. Chemical analysis revealed decreasedtotal solids for the 4 : 1 diet slurry only and an increas-ing pH when increasing the ratio of water to feed (Table4).

The slurry composition changed during the OECexperiments with the odorant concentrations increasingmore for the wet feed diets during the 190 min period asshown in Fig 4. Of the other parameters, only the pH

increased in all the samples during the experimentalperiod in the chamber (Table 4).

Olfactometric analysis demonstrated signiÐcantlyhigher OC in the headspace of the pig slurry from thedry feed and 3 : 1 diluted diets than from the 4 : 1 diet(P\ 0É05) for equal volumes of slurry when usinganalysis of variance. The latter two samples also showeddecreasing OC after reaching a maximum, after about

Fig 5. Methane emitted from the slurry samples from weaned pigs given dry feed, and water to feed in the ratios 3 : 1 and 4 : 1.

E†ect of diluting pig feed on slurry odour 443

one hour. The behaviour after 1 h is described by eqns(1) and (2) for the respective slurry samples, which di†erfrom the OC curve for the slurry from the 4 : 1 feedwhich increased from time zero and is described by eqn(3), and is included so that changes in odorant concen-tration can be compared.

OCdry feed\ [473t ] 7 ] 106 (1)

OC3>1\ [445t ] 6 ] 106 (2)

OC4>1\ 77.17t (3)

Analysis gave r2\ 0É7417, 0É7994 and 0É8206 for thedry feed, 3 : 1 and 4 : 1 slurries for the relationshipsbetween headspace OC and time t (s), respectively.Parallel regression analysis revealed no di†erence inOC, or in the rate of decline of the OC (P\ 0É05)between the slurry headspaces of the dry feed and 3 : 1diets. When the OC results were adjusted to allow fortheir slurry volumes, the analysis showed no change.

Analysis by GC-MS revealed that and meth-H2Sanethiol were the major odorants in the 3 : 1 and dryfeed slurry headspaces, respectively. Their concentra-tions decayed after sampling at 1 h and were describedby the following equations :

[CH3SH]dry feed\ [0.0078t ] 81 (4)

[H2S]3>1\ [0.02596t ] 294 (5)

for the dry feed (r2\ 0É7466) and 3 : 1 (r2\ 0É6000)slurry headspace samples, respectively, where [CH3SH]and denote the methanethiol and hydrogen sul-[H2S]phide concentrations in mg m~3, respectively. The con-centration decay of the major sulphide in eachheadspace was similar to the decay of the OC. For the3 : 1 diet slurry headspace sample the OC decayed by27% h~1 and the concentration by 32% h~1. Simi-H2Slarly, in the slurry headspace of the dry fed pigs, meth-anethiol dropped in concentration by 35% h~1 and theOC by 24% h~1.

During the OEC run the quantities of methane pro-duced were inversely related to the amount of odorantsemitted from each slurry sample. Slurry from the 3 : 1and dry feed diets produced 70% and 20%, respectively,of the methane produced from the slurry of the 4 : 1 diet(Fig 5).

DISCUSSION

These results show that odours of di†erent composi-tions and behaviour were emitted from slurries ofweaner pigs fed di†erent diets. The odours were com-posed of the odorants identiÐed in previous work(Hobbs et al 1995). Here we have determined the con-centrations in the headspace generated under identicalphysical conditions for emission in the newly developedOEC.

Odorants, with the exception of the sulphides, werequantiÐed in the slurry because the headspace concen-trations were too low to yield accurate results. Further-more, headspace concentration cannot be relied uponfor the determination of the production rates in slurriesbecause of the variation of emission rates for the di†er-ent odorants (for example, 3-methyl indole emits at aslow rate when compared to acetic acid or phenolbecause of kinetic factors). Indeed we noted discrep-ancies in the headspace concentrations which shouldbear a constant ratio to the slurry concentration underthe same physical conditions, according to HenryÏsLaw. For example, butanoic acid gave a higher head-space concentration for the 3 : 1 slurry sample than forthe dry feed slurry sample (Table 2), although the slurryconcentration was greater for the latter (Fig 2). Overallthe headspace analysis was difficult, with standarderrors exceeding 50% because analysis was near to thelimits of detection.

General decreases in the concentrations of odorantsand nitrogen were observed in the slurries as the feedwas diluted ; however greater volumes of slurry andgreater amounts of N were produced from the pigs onincreasingly diluted feed (Table 4). Subtraction of theammonium and nitrate N indicates that considerableamounts of organic N were present. This concentrationdecreased in the slurry as the water content in the dietincreased. Organic N has been shown to be the majororigin of odours. For example, branched-chain volatilefatty acids are known degradation products of proteinsin the human gut (Macfarlane et al 1992) and sulphidesare produced from the sulphur containing proteins(Hammond et al 1989), indole and 3-methyl indole fromtryptophan and phenols from the aromatic amino acids,tyrosine and phenylalanine (Spoelstra 1980). Of theodorant concentrations in the slurry (Figs 2 and 3) onlythe VFA were markedly di†erent even when consideringthe volumes produced, ie 138, 183 and 227 litres for dryfeed, 3 : 1 and 4 : 1 diets, respectively. So while therewere improved digestibility for the diluted diets andhence less substrate to produce odours there was alsothe added possibility of accelerated degradation ofodorants or the precursors that produce odorants. Thiscan be attributed to the reduced slurry viscosity fromthe diets containing more water. Biological activity isusually a positive function of substrate concentration,but high viscosity can restrict access to the nutrients(Faust and Hunter 1976).

Degradation of odours in the slurry is an importantand a preferable means of reducing odours because its isperformed at source. VFA are degraded by severalmeans, with methanogenesis being the principlepathway. There are various causes for the inhibition ofmethanogenesis which are common and relevant to thisstudy, including overloading with organic matter andhigh concentrations of ammonia, hydrogen sulphide orVFA in the slurry or the presence of heavy metals

444 P J Hobbs, T H Misselbrook, B F Pain

(Spoelstra 1980). Our results suggest that the totalsolids and/or ammonia have an inhibiting e†ect, butonly when considered as mass per unit volume. Theincreasing total solids concentrations of 0É021, 0É033and 0É043 kg m~3 corresponded to a noticeabledecrease in methane production of [170, 120 and40 mg m~3 for the 4 : 1, 3 : 1 and dry feed diets, respec-tively (Fig 5). It can be argued that inhibition of meth-anogenesis results in increased odorant production dueto reduced degradation rates of VFA. Thus, any factorsthat enhance methane production will reduce odorantconcentration in the slurry and should also be of beneÐtto those pig producers who use methane as a source ofenergy. Whether more methane would have been pro-duced during the total degradation of the slurry wasoutside the scope of this work, but is of concern becauseit is a powerful greenhouse gas (Bouwman 1990).

Characteristic curves for methane concentration thatincreased to a maximum value were observed for emis-sion for each slurry. The plateau may indicate that equi-librium had been reached between the concentrations inthe headspace and the slurry. These characteristiccurves were obtained using a chamber of the samedesign to investigate headspace emissions of ammoniaand methane (Cumby et al 1995). QuantiÐcation of theheavier and more water-soluble components in theheadspace proved difficult with a wide degree of varia-bility of response. This was in part due to the very lowconcentrations in the TeÑon bags and determinationwas made possible only by using adsorption tubes topreconcentrate the odorants for quantiÐcation.

Olfactory data from equal volumes of all the slurriesdid not demonstrate equilibrium behaviour as a plateauon the concentration curve. Odour concentrations fromthe headspace of the slurries from the 3 : 1 feed and thedry feed diet peaked after about 60 min and thendeclined, which can be attributed to the reduction of theamounts of sulphides present. The OC from the slurryof the 4 : 1 feed, which had a negligible amount of sul-phide in the headspace, gave an increasing responsewhich would conÐrm that sulphide degradation wasassociated with the decay of OC for the other samples.The increasing OC from the slurry of the 4 : 1 dietwould also indicate that the odorants had not reachedequilibrium after three hours as would be necessary toallow thermodynamic determination of their emissionrates. This also conÐrms the sulphides as the major con-tributors to the OC from the dry feed and 3 : 1 slurries.The decay of the OC for the 3 : 1 and dry feed slurriescan be explained by the decay of the more unstablemethanethiol and hydrogen sulphide in the respectivesamples. The di†erent rates of decay of the OC for thetwo samples can be explained because the methanethioldid not contribute so much to the odour from the slurryof the dry feed diet as the hydrogen sulphide did to theodour from the slurry of the 3 : 1 diet ; hence the relativechange was greater for the latter. After conducting

further studies (unpublished work) we found that hydro-gen sulphide decayed at a rate of about 20% per daywhen samples were stored in TeÑon bags under thesame physical conditions as we have used here, so addi-tional factors were contributing to increase decay of thesamples in this study. Other chemicals present in theheadspace samples could increase the decompositionrate by, for example, catalytic oxidation (Boon 1992).

A general picture of the role of the odorous unstablesulphides appears to be emerging. However comparisonof OC and odorant concentrations requires investiga-tion because we have to consider interactive or syn-ergistic e†ects between odorants which have not beenevaluated, although odour detection threshold valuesare known (Table 3). Knowledge of the chemical com-positions of odours is important if we are to reduceodours from livestock sources e†ectively.

Despite these difficulties the OEC has proved ane†ective research tool. Its size improves uponlaboratory-scale experiments of atmospheric events,giving a greater degree of control over wind velocityand slurry stirring rate.

ACKNOWLEDGEMENTS

The authors would like to thank Dr Trevor Cumby ofSilsoe Research Institute for manufacturing the OEC,University of Plymouth for supplying slurry samples,Dr David Chadwick for methane analysis and MAFFfor directly funding this research.

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