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Influence of initial moisture content on the composting of poultry manure with wheat straw

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Influence of initial moisture content on the composting of poultry manure with wheat straw
  Research Paper: SE d Structures and Environment  Influence of initial moisture content on the compostingof poultry manure with wheat straw Ivan Petric a, *, Almir S ˇ estan b , Indira S ˇ estan c a Department of Process Engineering, Faculty of Technology, University of Tuzla, Univerzitetska 8, 75000 Tuzla, Bosnia and Herzegovina b Department of Chemistry, Faculty of Science, University of Tuzla, Univerzitetska 4, 75000 Tuzla, Bosnia and Herzegovina c Department of Physical Chemistry, Faculty of Technology, University of Tuzla, Univerzitetska 8, 75000 Tuzla, Bosnia and Herzegovina a r t i c l e i n f o Article history: Received 11 October 2008Received in revised form20 May 2009Accepted 10 June 2009Published online 8 July 2009The effect of initial moisture content (MC) on the composting of poultry manure withwheat straw in terms of the temperature of the compost, its emission of carbon dioxideand ammonia, and the rate of conversion of organic matter were investigated. Threeexperiments were carried out in closed laboratory-scale reactors under adiabatic condi-tions. The initial MC of the mixture of poultry manure and wheat straw showeda significant effect on aerobic composting process. The results demonstrated that forcomposting poultry manure with wheat straw, relatively high MCs are better at achieving higher temperatures and retaining them for longer times. However, high MCs can lead toincreased losses of ammonia, which need be controlled by the addition of suitableadditives. The results of this study suggest that an initial MC of around 69% can beconsidered as being suitable for the efficient composting of poultry manure mixed withwheat straw. ª  2009 IAgrE. Published by Elsevier Ltd. All rights reserved. 1. Introduction Poultry and wheat production are two important agriculturalindustries in Bosnia and Herzegovina (Federal Office of Statistics, 2007a, b; Republika Srpska Institute of Statistics,2007a, b) that generate as by products large amounts of manure and straw, respectively. In 2006 the total live poultrystock in Bosnia and Herzegovina was 12731564 birds (FederalOffice of Statistics, 2007a; Republika Srpska Institute of Statistics, 2007a), while total wheat production was 219441 tfrom a harvested area of 67799 ha (Federal Office of Statistics,2007b);RepublikaSrpskaInstituteofStatistics, 2007b).Serious environmental pollution has been caused due to the lack of cost-effectivetechnologiesforprocessingpoultrymanureandthe inappropriate disposal of waste. Composting poultrymanure with wheat straw is an option and that could offermany environmental and economic benefits for countriessuch as Bosnia and Herzegovina. In addition, composting poultry manure with wheat straw is also a good approachfrom the standpoint of process engineering, because poultrymanure has high density and moisture content (MC) but hasa low carbon to nitrogen ratio, whereas wheat straw has theopposite properties. Mixing the two materials should provideimproved MC control and more balanced nutrients formicroorganisms to carry out the composting process.Composting has been widely used for converting organicwastes into relatively stable products for use as fertiliser orsoiladditives(Haug,1993).MChasbeenreferredtoasacriticalfactor in optimising composting systems ( Jeris and Regan,1973), because the decomposition of organic matters depends *  Corresponding author .E-mail address: (I. Petric). Available at www.sciencedirect.comjournal homepage: 1537-5110/$ – see front matter  ª  2009 IAgrE. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.biosystemseng.2009.06.007 biosystems engineering 104 (2009) 125–134  on the presence of water to support microbial activity. Verylow MC can cause early dehydration during composting,which arrests the process, thereby producing a physicallystable but biologically unstable product (De Bertoldi  et al. ,1983), whereas high moisture can produce anaerobic condi-tions as a result of water logging, which prevents the ongoing composting process (Tiquia  et al. , 1996). It has been reportedthat initial moisture optimum contents for composting poultry manure with wheat straw range from 65% to 80% ona wet basis (w.b.) ( Jeris and Regan, 1973; Fernandes  et al. , 1994;Kalyuzhnyi  et al. , 1999). Jeris  et al.  (1973) reported thatoptimum MC range from 75% to 80% for farmyard manure instraw. Fernandes  et al.  (1994) reported that successfulcomposting poultry manure mixed with peat or choppedstraw has been obtained at high initial moisture levels(73–80%). According to the experimental results of  Kalyuzhnyi et al.  (1999), the optimum initial content of the composting mixture (the solid fraction of poultry manure supplementedwithstraw)wasfoundtobe65–70%.Hansen etal. (1989)reportedthatthetypeofaddition,the mixingmethodandtheinitial drysolidfraction(i.e.,initialMC)hadthemostsignificantimpactonthe rate of poultry manure composting out of the sevencontrollable factors they tested (i.e., type of addition, carbon/nitrogenratio,initial mixingmethod,stirring frequency,initialdry solids, particle size, compost temperature).The selection of optimum composting parameters isspecifictothecompostingmaterialsconcernedandneedtobedetermined through experimentation. This study was carriedoutusingaclosedreactortoinvestigatetheeffectofinitialMCon the characteristics of composting poultry manure withwheat strawin termsof composting temperature, emissionof carbon dioxide and ammonia, and the conversion of organicmatter. This information should provide operational param-eters suitable for the effective composting poultry manurewith wheat straw. 2. Materials and methods 2.1. Composting materials Fresh poultry manure and wheat straw were used as experi-mentalmaterialsandwerecollectedinpolyethylenebagsfromfarms near Gracˇanica in the Tuzla Canton of Bosnia andHerzegovina (Table 1). Poultry manure was collected from thesamefarm,directlyfromthetransportationband,whilstwheatstrawwascollectedfromthreedifferentlocations.Wheatstrawwaspurchasedbythebale.Visualevaluationdeemedthestrawtobeofgoodqualitywithnoapparentsignsofmouldordecay.Immediately after transportation to the laboratory, the mate-rials were analysed and prepared for filling thereactors. 2.2. Experimental set-up Laboratory-scaleaerobiccompostingtestswereconductedfor11 days in closed thermally insulated column reactors witheffective volumes of 1 l (with height of 0.20 m and internaldiameter of 0.08 m). The schematic diagram of used experi-mentallaboratory-scalecompostingreactorisshowninFig.1.Thermos bottles (Pengo, Italia) were modified and used assmall reactors. This modification included the rubber stopperwith holes for inlet of air, thermocouples, and outlet of gasmixture (Fig. 1). Reactors were additionally insulated withpolystyrene foam.An aquarium pump CX-0098 (Champion, China) was usedto blow the air with a constant flow (0.9 lmin  1 kg   1 OM) intosmall reactor. Measurement of airflow was carried out using airflow metres (Valved Acrylic Flowmeter, Cole-Parmer, USA).Before introduction to the reactor, the air was bubbledthrough a solution of sodium hydroxide in order to removetraces of carbon dioxide. In order to maintain the humidity atreactor inlet, the air was then passed through a gas-washing bottle filled with distilled water.At outlet, the gas mixture passed through a condenser,a gas-washing bottle with 1 M sodium hydroxide and a gas-washing bottle with 0.65 M boric acid, in order to remove thecondensate, carbon dioxide and ammonia, respectively.Temperature was monitored by type T thermocouples(Digi-Sense, Cole-Parmer, USA), placed in the middle of thesubstrate. Thermocouples were connected through a dataacquisition module (Thermocouple CardAcq, Nomadics, USA)mounted on a laptop computer. Automatic logging of temperature data was performed over the whole period of theexperiment, using special software (Nomadics, USA). Thetemperature in the laboratory was also recorded. 2.3. Experimental design Theinvestigationconsistedofthreeindependentexperiments(three different collections of poultry manure and wheatstraw), with different physico-chemical characteristics of poultrymanureandwheatstraw(Table1).Beforemixingwithmanure, the straw was cut into pieces with 2–5 mm long.Manure and straw were manually mixed (without adding any Table 1 – Physico-chemical characteristics of raw materials in different experiments Experiment Material MC (% w.w.) Organic matter(% d.w.)pH Electrical conductivity(dSm  1 ) 1 Manure 68.31  1.97 71.37  1.35 8.27  0.09 3.94  0.05Straw 10.42  0.83 87.38  1.78 7.87  0.03 1.94  0.042 Manure 72.59  0.97 78.07  1.83 7.71  0.06 3.34  0.10Straw 10.87  0.95 87.91  1.11 7.81  0.05 1.91  0.033 Manure 74.93  1.27 73.20  1.92 8.49  0.06 4.17  0.24Straw 12.55  0.21 90.38  0.74 7.13  0.03 1.24  0.09w.w. – wet weight, d.w. – dry weight biosystems engineering 104 (2009) 125–134 126  water), in plastic boxes for 30 min to achieve homogenisationof materials. The initial MCs in the composting mixtures forthree experiments and two reactors are shown in Table 2.Eachoftheexperimentswasduplicatedtoassesstheabilitytoreplicate process behaviour for given initial MC. 2.4. Analytical methods Volumetric titration was used to determine carbon dioxideandammoniacontents.Forcarbondioxide,analiquotvolumeof sodium hydroxide solution was titrated against a standardsolution of 1 M hydrochloric acid with phenolphthalein usedas an indicator. To determine ammonia content, an aliquotvolume of boric acid solution was titrated against a standardsolution of 1 M hydrochloric acid using bromocresol green-methyl indicator.The gas-washing bottles were changed daily for determi-nation of produced gases.MC was determined by dry weight in oven at 105  C for 24 h(APHA, 1995). Organic matter was evaluated on the residuefrom moisture analysis using incineration at 550   C for 6 h(APHA, 1995).The loss (or conversion) of organic matter  k  was calculatedfromtheinitialandfinalorganicmattercontents,accordingtothe following equation (Haug, 1993; Diaz  et al. , 2002; Ku ¨ lcu &Yaldiz, 2007): k  ¼  OM m ð % Þ  OM p ð % Þ  100OM m ð % Þ  100  OM p ð % Þ   (1)where OM m  is the organic matter content at the beginning of the process (%); and OM p  is the organic matter content at theend of the process (%).Electrical conductivity and pH were measured in aqueousextract. This aqueous extract was obtained by mechanicallyshaking the samples with distilled water at a solid:water ratioof 1:10 (w/v) for 1 h. The suspension was centrifuged andfiltered through a Whatman no. 42 filter paper. The pH andelectrical conductivity measurements were carried out using a PC 510 Bench pH/Conductivity meter (Oakton, Singapore)with two separate electrodes.Each analysis was triplicated with the mean valuecalculated. 2.5. Statistical analysis Statistical analysis (analysis of variance, and the least signif-icant difference for mean at 95%) was performed using a Statgraphic statistical package (STATGRAPHICS, 1996) ondata obtained at the different composting times. 3. Results and discussion 3.1. Temperature profiles After the initial filling of the reactors, a rapid increase intemperaturewasproducedinallreactors,indicatingamarkedmicrobial activity. The changes in compost temperature fol-lowed a pattern similar to a typical composting process.Initially, the temperature of composting mixtures rose asa consequence of the rapid breakdown of the readily availableorganic matter and nitrogenous compounds by microorgan-isms (thermophilic phase). As the organic matter becamemore stabilised, microbial activity, the organic matterdecomposition rate, and the temperature all decreasedgradually to ambient levels. The temperature regimes of the Fig. 1 – Schematic diagram of laboratory reactor system (1 – aquarium pump, 2 – airflow metre, 3 – gas-washing bottle withsolutionofsodiumhydroxide,4– gas-washingbottlewithdistilledwater,5– smallreactor,6– thermocouple,7–condenser,8 – graduated cylinder, 9 – gas-washing bottle with solution of sodium hydroxide, 10 – gas-washing bottle with solution of  boric acid, 11 – laptop).Table 2 – Mixing ratios and initial MCs in the mixtures Experiment Reactor Ratio of manureto straw (d.w.)Initial MC(% w.w.) 1 1 9.68:1 67.652 4.30:1 62.852 1 4.86:1 69.112 1.31:1 60.553 1 10.10:1 69.432 2.10:1 66.19w.w. – wet weight, d.w. – dry weight biosystems engineering 104 (2009) 125–134  127  composting reactors containing different mixtures in threeexperiments are illustrated in Fig. 2. The term optimum MCrepresents a trade-off between moisture requirements of microorganisms and their simultaneous need for adequateoxygen supply (Haug, 1993). Too high a MC may inhibit thestartofthecompostingprocess.Inthisstudy,eventhehighestinitial MC used (69.43%) did not inhibit the start of the com-posting process. However, the highest MC inhibited the com-posting process immediately after reaching very shortthermophilic phase. After several hours the temperature inthe composting mass started to rise due to intense biodegra-dation. Several hours after start of the process, there weresmall differences in temperature (below 40   C) between thereactors in all three experiments. Above 40   C, the MC of thecompost material had a significant effect on the microbialactivity and temperature. The initial MC of 69.43% in experi-ment 3 (Fig. 2c) resulted in a cooling effect. Such high watercontent could influence gaseous exchange by limiting diffu-siontherebyrestrictingoxygenutilisationbymicroorganisms,resultingindecreasedmicrobialactivity.Therefore,thisinitialMCwouldnotbesuitableforcompostingpoultrymanurewithwheat straw. In all three experiments the higher temperatureprofiles were achieved in the reactor 1 than in the reactor 2. Itshould be noted that reactor 1 had higher initial MC in allexperiments (Table 2). In the experiment 1 (Fig. 1a), the com- posting process reached the maximumtemperature of 56.5   Cafter 1.2 days in reactor 1, whereas reactor 2 reached lowermaximum temperatures (52.5   C after 1.2 days). In the exper-iment 2 (Fig. 2b), the composting process reached a maximumtemperature of 64.6   C after 1.3 days in reactor 1, whereasreactor 2 reached lower maximum temperatures (51.0   C after1 day). In the experiment 3 (Fig. 2c), the composting processreachedthe maximumtemperatureof 59.5   C after1.2 daysinreactor 1, whereas reactor 2 reached lower maximumtemperatures (50.0   C after 1.2 days). There were statisticallysignificant differences in the temperature regime betweenreactors 1 and 2 in all three experiments (experiment 1 – fromthe first to the tenth day; experiment 2 – from the first to thefourth day; experiment 3 – from the first to the fourth day)( P < 0.05). The temperature in reactor 1 in the experiment 2(Fig. 2b) was maintained above 55   C for 2 days, which shouldbe sufficient to maximise sanitation (Stentiford, 1996) and todestroy pathogens (Strauch & Ballarini, 1994). Reactor 1 inexperiment 1 and 3, and reactor 2 in all three experiments didnotprovidesuitableconditionsforfullsanitationordestroying pathogens.Addingawaterabsorbent,suchasthewheatstrawin this study, is a good method to adjust the initial MC of compost,becauseitproduceslongperiodsforthethermophilicand inhibitive temperature stages (Luo  et al. , 2008). Vander-gheynst  et al.  (1997) found that maximum rates of oxygenconsumption and maximum temperatures increase withincreasing MC. Our results confirm these findings up to MC of 69.11%. It should be noted that MC of 69.43% in experiment3(Fig.2c)didnotprovidethelowerrateofoxygenconsumption (greater emission of carbon dioxide) and lower temperaturesthanMCof69.11%inexperiment2(Fig.2b).ThissuggestedthattheinitialMCof69.11%wasthemostsuitablefortheevolutionthe temperature especially in the thermophilic range. Resultsshowed that relatively high MCs were better for reaching higher temperatures and retaining them for longer periods.The initial MC of 69.11% provided the longest thermophilicphase during the composting process, indicating the highestrates of microbial activity and organic matter stabilisation.Further,themesophilic, thermophilic andcoolingstageswereclearly shown.Secondary temperature peaks occurred in all reactorsexcept in reactor 2 in experiment 3. Shin and Jeong (1996)explained secondary temperature peaks as indicating thedegradation of cellulose after all readily degradable matter isconsumed. Secondary temperature peaks could also be theresult of recovered thermophilic microbial population. At theend of the process, when there is a smaller amount of easilydegradable organic matter, the composting mixture cooleddown and the temperature of the decomposing materialapproached the ambient temperature.Temperature profile of compost mixture in reactor 2 inexperiment3 (Fig. 2c) showeda veryshort thermophilic phasewithout secondary temperature peaks. This is probablybecause of higher aeration effect. Addition of more wheatstraw provided the smaller initial MC and the greater free airspace (FAS). Because of greater pores inside the compostmixture, the constant airflow in the reactor enhanced cooling  Fig. 2 – Temperature changes in the substrate during thecomposting process (reactor 1 (  C  ), reactor 2 (  :  ), ambient (-)): (a) experiment 1; (b) experiment 2; (c) experiment 3. biosystems engineering 104 (2009) 125–134 128  of the composting mixture. Therefore, readily degradablematter was not consumed completely by thermophilicmicroorganisms. Liang   et al.  (2003) showed that MC hasa greater influence on microbial activities than temperature.For this study, thermos bottles (vacuum flasks) were usedas small reactors. Using vacuum as an insulator reduces heattransfer by conduction and convection. Thermal radiativelosses were minimised by applying a reflective coating tosurfaces (silver reflected any radiation that falls on it fromthe inner bottle). The contents of the reactor reachedthermal equilibrium with the inner wall. The wall is thin,with low thermal capacity, so it did not exchange much heatwith the contents of the reactors, affecting their temperaturelittle. At the temperatures for which reactors were used(maximum temperature was 64.6   C in three experiments),and with the use of reflective coatings, there was littleradiative transfer. In addition, the reactors in this study wereinsulated with polystyrene foam. By increasing thermalresistance by use of insulating materials, it can be expectedthat the overall heat transfer coefficient for the reactorsdecreased significantly, thus minimising conductive heatlosses. For a given volume and height, the cylindricalshape of the reactors minimised the surface-area-to-volumeratio thereby providing the minimum surface heat losses(Petiot and de Guardia, 2004).Aeration rates are a key factor in the control of ventilationheat losses. At laboratory-scale, ventilation heat losses areusually considerably lower than those expected for full-scalesystems (Mason and Milke, 2005b).Differences observed in the composting process betweenfull-scale, pilot-scale and laboratory-scale operations inreactors arise due to different working conditions (tempera-ture, oxygen and moisture). The quality of oxygen supply willvary as well as the rate of heat transfer and water loss leading to specific values for mean temperature and moisture. Thus,the main problem is to control or minimise the differencesbetween different scale operations in reactors.Diffusion of air into the heart of the substrate is easierwhen the thickness of product is less than a metre, such as inlaboratory-scale reactors. When it reaches several metres,such as in full-scale reactors, it is more difficult. Thus, inlaboratory experiments the porosity of the substrate will behigher than in full-scale, leading to potentially greaterexchanges.Thiscanincreasethedegradationrate,inducedbya better supply of oxygen and enhanced water exportationexchange. This can lead to excessive heat losses and prema-ture drying of the substrate. To compensate phenomenonwith enhanced water exportation, saturated air was used inthis study.Air pathway management in laboratory-scale reactors ismore likely to provide conditions (oxygen, moisture andtemperature) that suitaerobic treatment than thoseoccurring at full-scale (Petiot and de Guardia, 2004).Laboratory- and pilot-scale reactor procedures typically donotincludemixing.In thiscase theysimulatetheoperationof static bed reactors, such as aerated static piles, passivelyaerated piles or non-agitated tower reactors (Mason andMilke,2005a).Simulatingacompostingprocessinalaboratoryreactor requires the conditions of the substrate to be adjustedto, those that occur at full-scale.Givenidenticalrawmaterialandoperatingconditions(e.g.,moistureaddition,mixing),full-scaleparametersmaybeusedincombinationwiththegeneralshapecharacteristics,maybeused to evaluate the extent to which laboratory- and pilot-scale temperature data provides a good simulation of full-scale profiles (Mason and Milke, 2005b). Laboratory-scalereactor temperatures typically return to under 40   C withinrelatively short time periods, with lower temperature–timeprofile parameters than those measured for full-scalesystems. However, temperature shape characteristics aregenerally similar to full-scale profiles.The experiments in this study were undertaken in labo-ratory reactors in order to simulate conditions in forced-aeration static-pile composting system. Although it seemsthat 11 days is too short a period for composting, forced-aeration static-pile composting usually takes longer, thetemperature profiles of the reactors (Fig. 2) has demonstrateda highly accelerated process, especially in the first few hoursof composting, indicating a marked microbial activity. Fig. 2showsthreeofthefourphasesinaerobiccompostingprocess:mesophilic (initial) phase, thermophilic phase and cooling phase. The maturation phase was not shown because it wasnottheaimofthisstudywhichwastoinvestigateactive,high-rate composting in a reactor. Compost maturity was notexamined in this study since the experiments were termi-nated after the compost temperature reached the ambientlevel. A minimum curing period of one month after activecomposting is usually recommended to complete the process(Rynk, 1992). 3.2. Evolution of carbon dioxide and ammonia The results of the carbon dioxide and ammonia changesinside the reactors 1 and 2 in all three experiments are shownin Fig. 3. The masses of carbon dioxide and ammoniaproduced in all the reactors increased in all reactors propor-tionally to activity of the microorganisms during the processand these values were higher in the reactor 1 than in thereactor 2 in all experiments. The highest masses of carbondioxide and ammonia were produced in experiment 2, Fig. 3c,d, and were related to temperature changes (Fig. 2b). Theinitial MC of 69.43% (in reactor 1, experiment 3) producedlower masses of carbon dioxide and ammonia (Fig. 3e, f) thanthe initial MC of 69.11% (reactor 1, experiment 2, Fig. 3c, d).This confirmed the previous conclusion that the initial MC of 69.11% was prefered for composting poultry manure withwheat straw. It was noticed that the profile of generatedcarbon dioxide was similar to that of the temperature. Thisindicated that the change of temperature was closely relatedto the change of carbon dioxide mass, suggesting that carbondioxide mass might be used as another indicator formeasuring the composting process besides temperature. Thisdid not occur with ammonia. There were statistically signifi-cant differences in carbon dioxide and ammonia evolutionbetween reactor 1 and reactor 2 in all experiments ( P < 0.05).Thelowest emission of carbondioxide was observed in thereactor2 in experiment3 (Fig. 3e) whichcouldbe explained byits temperature profile. Immediately after reaching themaximum temperature (50   C) the compost mixture was biosystems engineering 104 (2009) 125–134  129
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