Emission rates of odorous compounds from pig slurries

J Sci Food Agric 1998, 77, 341È348

Emission Rates of Odorous Compounds fromPig SlurriesPhilip J Hobbs,* Tom H Misselbrook and Brian F PainInstitute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB, UK

(Received 12 August 1996 ; revised version received 7 August 1997 ; accepted 22 October 1997)

Abstract : Techniques to identify odorous compounds and to determine theiremission rates from liquid wastes are described. Odorous compounds, or odor-ants, were analysed by GC-MS and odour concentration was measured by olfac-tometry after being emitted from slurry under controlled environmentalconditions using a specially designed odour emission chamber. The slurries wereobtained from pigs fed commercial and reduced crude protein diets and storedfor 6 weeks, the e†ects of age and sex upon the odours produced in the head-space of the chamber were evaluated. The major odorous compounds were iden-tiÐed as belonging to the sulphide, volatile fatty acid, phenolic and indolicchemical groups. The mean emission rates from 200 litres of stirred slurrysamples with a surface area of 1 m2 using a wind speed of 4 m s~1 were 1É35million Odour Units min~1 for odour and 214, 2É15, 0É21, 0É44, 0É068 and0É02 mg min~1 for hydrogen sulphide, ammonia, phenol, 4-methyl phenol,4-ethyl phenol and indole respectively. Sex and sex/diet interaction e†ects weredemonstrated for emission rates. 1998 SCI.(

J Sci Food Agric 77, 341È348 (1998)

Key words : slurry odours ; emission rates ; pig ; diet ; crude protein

INTRODUCTION

Earlier researchers identiÐed numerous mixtures ofodorants emitted from various farm wastes (OÏNeill andPhillips 1992). However we have identiÐed 15 odorantsas being important (Hobbs et al 1995). Many of theseoriginate from protein sources within the gut and slurry(Hammond et al 1989). These proteins are, by deÐnition,undigested by the pig, they are also synthesised in thehindgut by bacteria and are shed from the walls of thepigs gut. Branched chain volatile fatty acids are degra-dation products from protein (Allison 1978), sulphidesfrom the sulphur containing protein (Hammond et al1989), indole and 3-methyl indole from tryptophan andphenols from the aromatic amino acids, tyrosine andphenylalanine (Spoelstra 1980). Volatile fatty acids(VFAs) are chieÑy produced from lipids, carbohydratesand phenols originating from plant sources.

Emission rates from farm wastes have not received somuch attention as organic compounds of environmentalinterest, such as hydrocarbons, halogenated hydrocar-

* To whom correspondence should be addressed.Contract/grant sponsor : MAFF.

bons and poly aromatic hydrocarbons, although, atconcentrations in the mg litre~1 range and meth-H2Sanethiol have been shown to be fatal in many cases(Donham 1994). The emission rates from a range ofanthropogenic phenols could be considered as a guidefor this experiment. However, they are normally deter-mined when emitted from natural waterways (Thomas1990) and not a livestock slurry which has di†erentphysical, chemical and biological properties.

In this study we investigated the e†ect of diet, age andsex of the pig on the emission rates of odorants andgases from the resulting slurry after 6 weeks storage.This will give a range of the emission rates from slurriesthat would enable estimations of emissions from storagefacilities.

MATERIALS AND METHODS

Livestock

Pigs of commercial stock were obtained from a nationalbreeding company and comprised Ðnishers (65È95 kg)and growers (35È65 kg). Prior to the experiment thepigs were housed in groups and fed ad libitum the

3411998 SCI. J Sci Food Agric 0022È5142/98/$17.50. Printed in Great Britain(

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

respective commercial diets (both containing 13É75 MJdigestible energy and 11 g kg~1 lysine) for their size.

Diets

In this comparative test, a standard commercial dietand a reduced crude protein (RCP) diet were fed to eachof the age groups. The diets were appropriate to the ageof the pig. As a consequence, there were two diets forthe growers (G) ; a commercial (a) and a RCP diet (b)and also a commercial (c) and RCP diet (d) for the Ðn-ishers (F). Analysis of the RCP and commercial dietsare shown in Table 1. The least cost RCP diets wereformulated to contain approximately same protein asthe commercial diet and ratios of lysine tomethionine] cystine, threonine and tryptophan at 1 to0É6, 0É65 and 0É20, respectively. The ideal lysine todigestible energy (DE) ratios were close to the ideal of0É9 and 0É8 for the growers and Ðnishers respectively

(Wang and Fuller 1989). In addition, the diets containedcopper sulphate at approximately 175 and 100 lg g~1for the growers and Ðnishers respectively as a growthpromoter.

Experimental design

Experiments were performed with pens containing pigsof di†erent sex and age, each age was fed their respec-tive RCP and commercial diet. There were twelve pigsin each of the eight pens, four pens contained males (m)(all of which were entire) and four pens containedfemales (f). The amount of feed o†ered to the pigs was0É95 of the calculated ad libitum DE intake based on thepig liveweight at the start of the acclimatisation period.The ad libitum DE intake, measured in MJ per day wascalculated using the equation 4É1 ] W 0Õ5 (where W isthe liveweight of the pig in kg). Slurry was accumulatedseparately under each pen for 1 month. During month

TABLE 1Formulations and results of analysis showing nutrient composition (g kg~1 as

fed) of diets with total available amino acid contents showna

Grower diets Finisher diets

RCP(b) Commercial(a)b RCP(d) Commercial(c)c

Food ingredientBarley 689É0 È 323É5 ÈWheat 42É5 È 410É5 ÈBiscuit È È 100É0 ÈSoya 86É5 È 10É0 ÈPeas 75É0 È 80É1 ÈBeans È È È ÈFishmeal 20É0 È 19É1 ÈOil/Fat 44É0 È 5É5 ÈMajor minerals 15É9 È 32É3 ÈMin/Vit mix 4É0 È 4É0 ÈLysine-HCI 5É705 È 6É590 ÈMethionine 2É024 È 1É931 ÈThreonine 2É357 È 2É769 ÈTryptophan 0É408 È 0É468 È

NutrientsDE MJ kg~1 14É7 14É3 14É5 14É3Dry matter 881 872 866 864Crude Protein 161É2 208É4 139É9 189É2Lysine 12É5 13É0 12É0 12É0Lysine : DE 0É85 0É91 0É83 0É84Lysine : CP 0É078 0É062 0É086 0É063Oil 69 60 54 55NDF 97 110 87 115Ash 52 57 45 48Threonine 8É1 9É6 7É6 8É0Methionine 3É8 3É4 3É8 3É0Met ] Cys 6É4 6É7 6É3 6É3

a Nutrients analysed as per standard methods used by ADAS, Wolverhampton,UK.b Dalgety Ultra Gain.c Dalgety Ultra Lean.

Emission rates of odorous compounds from pig slurries 343

one mG were fed either diet a or b ; month two mF werefed either diet c or d ; month three fG were fed eitherdiet a or b and month four fG were fed either diet c or dto acquire slurry from each diet and age group. A slurryvolume of 200 litres was taken from each slurry storeafter thorough mixing to determine emission rates inthe odours emission chamber (OEC). Emission rates forthe eight slurry samples were determined to after 6weeks storage under anaerobic conditions at 15¡C after4 weeks accumulation beneath the pen.

Odour emissions chamber

The OEC which initially encloses 40 m3 of air wasdesigned by Cumby et al (1995) and has a slurry con-tainer with a capacity of 200 litres which was stirred toexpose a new surface to the air. The stirring rate wassuch that minimal splashing and particulate emissionoccurred. The slurry container (2 m] 0É5 m ] 0É25 m)has a water pressure seal to allow uninhibited gas pro-duction during storage. The container was placed intothe OEC prior to the experimental determination ofemission rates. The temperature of the slurry and the airwere controlled at 15 and 20¡C, respectively, during theemission rate determination, primarily so that thehigher air temperatures would minimise condensationonto the internal OEC surfaces. The OEC consists ofstainless steel U-shaped ducting (0É5 m ] 0É5 m internalsection) whose ends were connected into a Tedlar bag(Fig 1). Minimal pressure was applied to the air in theOEC by the bag wrapping around a roller movingdown rails under gravity. This prevented external airbeing drawn through any leaks to prevent dilution ofthe OEC volume. The pressured OEC leaks 28 m3 ofair over a three hour period. Air was circulated at1É0 m3 s~1. Before a determination of emission rates theOEC was Ñushed with ozone for 2 h and then withambient air to e†ectively eliminate any contamination.

Analysis of odorants

Volatile compounds were preconcentrated from a600 ml sample of the headspace volume above the pigslurry by adsorption onto silica (Orbo 52, Supelco Inc.Supelco Park, Bellefonte, PA, USA) and carbon (Orbo32) based adsorbents. The concentrated odorants were

Fig 1. A schematic of the odour emission chamber.

then thermally desorbed from the adsorbents into theGC-MS system for identiÐcation and quantiÐcation.

Liquid or slurry samples were stored at 3¡C andanalysed within 3 days of arriving at the laboratory tominimise the likelihood of further or di†erent microbio-logical processes occurring. An accurate standard ofeach odorant in a mixture was prepared in distilledwater and used for calibration (also stored at 3¡C).Samples of between 50 and a 100 ml of slurry were cen-trifuged at 5000] g for 30 min. The concentration ofselected odorants was determined by the direct injectionof 0É5 ll of supernatant liquid from the centrifugedslurry sample into the GC-MS system.

A Hewlett-Packard (hp) 5890 II Series gas chromato-graph and a 5972A mass selective detector (MSD II)were used to analyse all the samples. A 25 m fused silica(cross-linked methyl siloxane) hp-1 column with an id of0É2 mm and a 1É00 lm Ðlm with a 1 m deactivatedfused silica guard column (0É25 mm id) was employed toanalyse the sulphide component from the OEC and acolumn with an increased Ðlm thickness of 0É34 lm wasused to analyse the headspace and slurry samples. Thecolumn Ñow rate was 0É75 ml min~1. The Optic tem-perature programmable injector (Ai Cambridge Ltd,Pampisford, Cambridge, CB2 4EF) was used to ther-mally desorb headspace samples and was initially at30¡C and heated to 250¡C at 16¡C s~1 for 1 min forheadspace samples. The programming was the same forliquid samples, except the initial temperature was at60¡C. An electronic pressure controller was used too†set peak pressure broadening with increasing GCcolumn temperature. The GC oven temperature was ini-tially 27¡C and was increased at 15¡C min~1 to 220¡Cand maintained for 1 min. The GC-MS interface was at280¡C. The mass spectrometer scanned from 32 to 250mass units every 0É2 s to give sensitivities down to50 pg. Retention time was used to identify odorants andthis was conÐrmed by matching the spectra with that inthe mass spectral library.

Non-odorous methane and carbon dioxide emissionrates were determined by GC-FID and infrared spec-troscopy, respectively.

Odorant concentrations in the headspace of slurrysamples were subject to regression analysis where thee†ects of diet on odorants in the slurry in terms of sexof pig for the growers and Ðnishers were determinedand expressed as a di†erence in the concentration,usually as a quadratic equation.

Olfactometry

The olfactometric response of samples was conductedusing dynamic dilution olfactometers (Project ResearchAmsterdam bv). Each olfactometer had two sniffingports and was of the forced choice type, with odourlessair being presented to the panellist through one port


and diluted odorous air through the other as describedin Hobbs et al (1995).

Sampling

Slurry samples from the growers, fed diets a and b, andthe Ðnishing pigs fed diets c and d were stored for 6weeks at 15¡C prior to quantifying the emission rates inthe OEC. Headspace samples were taken at seven presettime periods during the 225 min experiment to measureodorous and non-odorous emissions by passive sam-pling using a TeÑon FEP gas sampling bags (AdtechPolymer Engineering, Gloucestershire, UK). Headspacesamples from the OEC were taken through a port andanalysis of odour concentration (OC), odorous com-pounds, methane and carbon dioxide was performed.

Slurry samples of 500 ml were taken before and afterthe OEC experiment for every sample. As well asanalysis of the usual parameters, such as pH,ammonium-N, nitrate-N, total N and solids, odorantswere quantiÐed by GC-MS analysis.

Operational and computational parameters

The emission rates were calculated at zero time of theOEC run. The reasons for this are three-fold. First, thesuppression of emission by the mass present in theheadspace would be minimised as inferred by HenryÏsLaw, because an increasing headspace concentrationwould reduce the chemical potential or energy for emis-sion to occur and would not reÑect the real situationwhere odorants are rapidly removed. Second, physicaland chemical interactions with other odorants whichare polar and reactive will unnecessarily complicate theresults. Third, oxidative processes that occur in theslurry because of stirring may introduce another factorinto the calculation that is difficult to evaluate. Themost appropriate solution was, therefore, to di†eren-

tiate the equation of the total mass emitted and deter-mine the slope. The equation of the total mass emittedwas the sum of the integrated equations for the massleaked and that present in the chamber and this wasbest expressed in a quadratic form. The emission ratesare expressed as mass emitted per unit area.

RESULTS

The OEC was found to function e†ectively when quan-tifying major gases to determine the emission rates.However, VFAs and skatole gave variable concentra-tions because they were at or below the limit of detec-tion even with preconcentration onto adsorbents.Hydrogen sulphide was analysed on the day of theOEC experiment as it was found to decay at a rate ofabout 20% per day when present in an odour sample.Other odorants were analysed within 24 h. Generallysecond order polynomial equations could be Ðtted tothe concentration proÐle in the OEC.

Emission rates are presented in Tables 2 and 3 forgrowing and Ðnishing pigs respectively. The mean andstandard deviation (SD) of the emissions for all samplesare presented in Table 4 to give a range of expectedemissions from 200 litres of slurry with a 1 m2 surfacearea that was replenished by stirring. The mean emis-sion rate was 1É35 million OU min~1 for odour and214, 2É15, 0É21, 0É44, 0É068 and 0É02 mg min~1 forhydrogen sulphide, ammonia, phenol, 4-methyl phenol,4-ethyl phenol and indole respectively.

To establish if HenryÏs Law was obeyed, we lookedfor a relationship between the emission rate and theconcentration in the slurry. Expectations that these con-centrations would be reÑected in the emission rateswere unfounded. For the odorants that were quantiÐedin both air and slurry, ammonia, the VFAs, the phenolsand indole showed no relationship (P[ 0É1). Physicaland chemical parameters may a†ect the emission rate,

TABLE 2Emission rates of odorants and gases from slurries of the growing pigs fed commercial (a) and RCP (b) diets (Units are

mg m~2 min~1 unless otherwise stated)

Male Female Probability

Diet a Diet b Diet a Diet b SED Diet Sex Cross-over

OC (OU m~2 min~1) 6É17E] 05 3É23E] 05 5É70E] 05 6É53E] 05 1É77E] 05 NSa 0É097 NSCO2 1017 1362 1224 879 57É9 0É088 NS 0É046Methane 6É67 12É92 5É68 4É82 1É07 NS NS NSH2S 110 105 198 232 14É1 0É016 0É026 0É016NH3 11É8 2É5 20É7 6É5 Èb NS NS NSPhenol 0É011 0É012 0É007 0É576 0É024 0É001 NS 0É0014-Methyl phenol 0É012 0É042 1É065 0É941 0É058 NS 0É003 NS4-Ethyl phenol 1É23E[ 04 0É0048 n.d. 0É1817 0É0078 0É002 NS 0É021Indole 0É0016 1É24E[ 05 0É3790 0É0919 0É0291 0É003 NS 0É024

a NS: not signiÐcant, P\ 0É2.b È Not determined as only two measurements were taken at the end of a run.


TABLE 3Emission rates from Ðnishing pigs fed commercial (c) and RCP (d) diets (Units of mg m~2 min~1 unless otherwise stated.)

Male Female Probabilities

Diet c Diet d Diet c Diet d SED Diet Sex Cross-over

OC (OU m~2 min~1) 2É50E] 06 1É04E] 06 2É49E] 06 2É65E] 06 1É87E] 05 0É001 0É011 0É001CO2 1003 1660 757 549 49É4 0É01 0É001 NSaMethane 6É43 6É08 17É9 13É2 1É08 0É075 NS NSH2S 173 337 274 289 12É1 0É026 0É09 0É001NH3 16É6 8É7 42É1 7É6 Èb NS NS NSPhenol 0É48 0É17 0É01 0É45 0É058 NS 0É003 0É0754-Methyl Phenol 0É49 0É33 0É54 0É13 0É0341 0É159 0É095 NS4-Ethyl phenol 0É11 0É04 0É07 n.d. 0É0466 0É001 0É001 NSIndole 0É48 0É01 0É21 0É40 0É0498 0É012 0É001 0É001

a NS: not signiÐcant.b È Not determined as only two measurements were taken at the end of a run.

with, for example, dry matter (DM) (Fig 2) ranging from8É4 to 4% by weight. The chemical composition of theslurries also varied, but the VFAs, total N andammonia were generally less for the RCP diets than forthe corresponding commercial diets for age and sex ofthe pigs (Figs 2, 3 and 4).

The identiÐcation of a relationship of an individualodorant in the slurry with the olfactory response wasmade difficult because of the variability of the OC fromsamples taken during the OEC run. However, there wasone relationship concerning the olfactory emission ratethat was signiÐcant (r2\ 0É6509, P\ 0É10) which waslinear with the concentration of 4-methyl phenol [4-mp], described by eqn (1)

[4-mp]\ 3.3698e~5x ] 10.521 (1)

where x is the odour emission rate (OU m~2 min~1).Other relationships to the odour emission rate were

sought concerning hydrogen sulphide, methane andcarbon dioxide emission rates to simplify odour moni-toring, but no signiÐcant relationships were found.

Although the four diets were di†erent in order toaccommodate the dietary requirements of the two age

groups some common factors were noted. First, themagnitude of the di†erences between odour emissionrates between the pig sexes suggest no e†ect for theRCP diet for the female pigs of both age groups (Tables

Fig 2. Analysis of slurry samples prior to the determination ofemission rates. Nitrogen content components are expressed as

g litre~1.

TABLE 4Range of emission rates of all pigs and diets (Units of mg m~2 min~1 unless otherwise

stated)

Emission Standard Min Maxrate Deviation

OC (OU m~3 min~1) 1É36E] 06 1É01E] 06 3É23E] 05 2É65E] 06Carbon dioxide 1056 352 548É7 1660Methane 9É22 4É80 4É8 17É9Hydrogen sulphide 214É7 83É9 105 337Ammonia 2É15 1É75 0É35 5É85Phenol 0É21 0É247 0É0068 0É584-Methyl phenol 0É44 0É397 0É0125 1É064-Ethyl phenol 0É07 0É069 0É0001 0É182Indole 0É20 0É199 0É000 01 0É475


Fig 3. Analysis of VFAs in the slurry samples prior to thedetermination of emission rates.

2 and 3). This was reÑected by a higher hydrogen sul-phide emission rate for the female pigs. Second theslurry from the Ðnishing pigs emitted odours at agreater rate than that from the growers per unit volumeof slurry.

Emission rates from growing pigs

The OC, which could be interpreted as integrating thetotal odorant content, as well as including the e†ects ofnon-odorous emissions on the odorous components,was shown to di†er between the sexes (P\ 0É097).Carbon dioxide, hydrogen sulphide, phenol, 4-ethylphenol and indole showed a strong sex/diet interaction(Table 2). Reduction in concentration of nitrogen com-ponents in slurry from pigs fed the RCP diet wasgreater for male than female pigs (Fig 2). Ammoniaemission rates from the slurry were lower for the RCPdiet fed to pigs of both sexes (Table 2).

Emission rates from Ðnishing pigs

Higher OC emission rates were observed from slurriesfrom Ðnishers than from growers (Table 3). E†ect of dietwas only evident for slurry from male pigs with adecrease in odour emission rate for pigs fed RCP diet.Generally the emission rates of OC, hydrogen sulphide,phenol and indole all showed a sex/diet interaction.Although there was a higher OC emission rate for theslurries from the male pigs fed the commercial diet, themajor odorant hydrogen sulphide was emitted at alower rate than that from the slurry of the pigs fed theRCP diet. The other odorants demonstrated a higheremission rate from the slurry of the male pigs fed thecommercial diet than the slurry of the female pigs fedthe RCP diet.

DISCUSSION

Quantifying odorants to determine realistic emissionrates can be difficult as some odorants have low odour

Fig 4. Analysis of phenols and indoles in the slurry samples prior to the determination of emission rates.


thresholds of about 1 ppb(v). In order to determinetheir emission rates we require concentrations near1 ppm(v) which are higher than those found in normallivestock practices. We attained these concentrations inthe OEC for most odorants for the slurry samples underconstant conditions of temperature, air Ñow and slurrystirring rate. The primary aim was to determine therange of emission rates (Table 4) using slurries of di†er-ent composition. They relate to limiting conditions witha slurry volume of 200 litres at 15¡C, a stirring rate thatdoes not cause splashing and a wind speed of 4 m s~1.

A comparison of the emission rates was also madewith urine and faeces from pigs accumulated over fourweeks and then stored for 6 weeks as slurries. Therefore,di†erences in the emission rates due to diet, age and sexof pigs as measured in the OEC may not represent dif-ferences in the total emission over the lifetime of theslurry, but do give us a range of emission rates thatmight be expected from a storage facility during stirring.

Emission rates from slurries may be a†ected by windspeed (Liu et al 1995), stirring and temperature. In otherexperiments, increasing stirring rate increased emissionof odorants (unpublished data) although Cumby et al(1995) found that ammonia emission rates decreasedwith stirring. Increasing slurry temperature physicallyincreases emissions and bacterial biogenesis of odor-ants. Indirectly odour emissions may be increased asmore carbon dioxide and methane are produced withincreasing temperature (Husted 1994) to strip the odor-ants from the waste.

Emission rates also depend on the chemical behav-iour of the gas or odorant, for example the biogenesis ofmethane will generally be greater in a larger store perunit volume because the greater volume to area ratiocreates a more stable and necessary anaerobic environ-ment. Methane also has a low solubility and should beemitted quickly after biogenesis however there is evi-dence of physical inhibition of emission (Hobbs et al1997). In contrast phenol, present at about 50 mglitre~1 in slurry was emitted at 0É21 mg m~2 min~1and would take over 200 min to deplete assuming noregeneration. VFAs demonstrated ambiguous emissionbehaviour giving a variable concentration in the head-space with no discernable pattern. Phenols generallyhad a higher concentration than the VFAs in the head-space although phenol concentration in the slurry wasless by an order of magnitude than that of the VFAs.HenryÏs Law states that a thermodynamic equilibriaexists between the concentration in the headspace andthe slurry and that the chemical energy driving emis-sions is proportional to the concentration in the slurry.Phenols and indoles (the latter when measurable)demonstrated characteristic concentration curvesincreasing and commensurate with HenryÏs Law.However, HenryÏs Law applies to pure solutions ratherthan the complex mixture present in slurries. Electro-lytes and surfactants in the form of polymeric proteins,

saccharides and lipids all contribute to deviations fromthis law (Thomas 1990). A ratio of the concentration inthe slurry to the emission rate did not reveal any consis-tent results, with the exception of 4-methyl phenol, dueto the large variation of concentration in the headspaceof several orders of magnitude.

Previous workers have attempted to Ðnd a relation-ship between OC and the concentration of a single com-ponent in the headspace to simplify odourmeasurements (Dorling 1977 ; Schaefer 1977). Here weidentiÐed a relationship between OC and the concentra-tion of 4-methyl phenol in the slurry despite hydrogensulphide being the major odorant in the headspace.Establishing an indicative relationship between OC andany component can be difficult because of the variationof odour measurements during an OEC run. However,emission rates for methane were lower than expected,possibly because the methanogens that producemethane and are strict anaerobes were inhibited by theoxygen infusing into the slurry during stirring. Theemission rates of hydrogen sulphide were high whencompared to those for ammonia which were lower forreasons previously stated. Hydrogen sulphide is nothighly soluble in water and additional stirring may haveinduced rapid biogenesis of this gas. Rapid emissions ofhydrogen sulphide have been shown to cause fatalities(Donham 1994) and these set of criteria may be repro-duced in the OEC.

The e†ects of age, sex and diet of the pig on the emis-sion rates were varied, of the eight emission rates for thegrowers Ðve showed a sex/diet interaction, in additionthere was a sex e†ect for the OC. The Ðnishers had foursex/diet interactions, three sex e†ects and a diet e†ectwhere methane biogenesis was reduced for the RCPdiet. A common factor was that the RCP diets reducedthe ammonia emission rates (Tables 2 and 3) and con-centrations in the slurries (Fig 2) for pigs of the samesex and age, this was also true of the total nitrogencontent in the slurry (Fig 2).

Analysis of the slurry (Fig 2) also demonstrated alower dry matter and pH for the RCP diets for pigs ofthe same sex and age, such factors may be beneÐcial inthe management of slurry, and may involve lessresources for say slurry stirring prior to land spreading.Odorant concentrations in the slurry (Figs 2 and 3) ofmale pigs were generally lower for both ages when fedthe RCP diets than for slurries from male pigs fed com-mercial diets. These results were slightly di†erent fromthose for fresh slurries produced by mixed sex pigs fedthe same diets (Hobbs et al 1996) possibly due to bac-terial action during storage mobilising the undigestedcrude, dead bacterial and endogenous protein from thepig to form a new pattern of microbial degradation, anda di†erent odour proÐle. The rate of microbial activitymay di†er between slurry samples so that each samplemay be at a di†erent stage of decay at the time of sam-pling. Generally the female pigs did not show a dietary


e†ect on the slurry odorant concentration. It could beconcluded that microbial fauna from the female pig guthas a di†erent e†ect upon the storage of waste than themale pig and that RCP diets are not so e†ective inreducing odorant concentrations in slurry from thefemale pigs.

In conclusion emission rates from stored pig slurrieshave been determined. Should they be used to calculateemissions from storage facilities then considerationshould be given to the limiting conditions previouslydiscussed. Further studies would enhance our under-standing of the variables of wind speed, slurry stirringrate and temperature. However variable microbialactivity appears to contribute greater changes, espe-cially through the ageing of wastes which alter the emis-sion rates of individual odorants.

ACKNOWLEDGEMENTS

The authors are grateful to Dr Andrew Rook for thestatistical analysis ; Mr Roger Kay, ADAS, Terrington,for supplying samples ; and MAFF for their Ðnancialsupport to perform this work. IGER is supported by theBiology and Biotechnology Science Research Council(BBSRC).

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Emission rates of odorous compounds from pig slurries