composting reactors the return of the jedi · oxygen supply throughout the composting media. ......

Composting Bioreactors DesignIIIReportWinter2011Jamaleddine,Eyad[260282587]Rainville,Cloé[260282662]

DEPARTMENTOFBIORESOURCEENGINEERING

JAMALEDDINE&RAINVILLE2011

2

ABSTRACT

Considering the present push towards greener industrial and residential activities,

composting is once again a hot topic amongst Ecological Engineers. Uniform composting

conditions are necessary to ensure the destruction of pathogens andmaintain thewhole

system at the same composting stage, so it is essential to maintain a homogeneous

temperaturethroughoutthecompost.Inthequesttoaccomplishthelatter,anin‐vesselheat

redistributionsystemwasconstructedandtested.Thesystemrequiresnoexternal inputs

of energy, but exploits the principles of conductive and convective heat exchange. Once

composting gets underway and temperature differentials arise within the compost bed,

changes in buoyancy cause water to flow through a closed coil of copper tubing,

redistributing the core heat throughout the medium. Heat is also conducted along the

copper tubing. In the past, a controlled experiment was conducted to test the design. A

statisticalanalysisoftheexperimentalresultsdemonstratesthatthevesselsfittedwiththe

heat redistribution system exhibit lower temperature gradients within the compost bed

thanincontrolvesselswithoutthesystem.Thepresentwilldealwithanairredistribution

systemtobefittedtotheaforementioneddesign.Thelatterwouldpermitwarmairexiting

fromthetopfourinchwholetobecooledandre‐circulatedtothebottomfour‐inchwholeof

the barrel. The overall objective is essential to reduce heat losseswhilemaintained the

replenishingoftheoxygensupplythroughoutthecompostingmedia.


3

Introduction

Compostisessentiallythedecayingoforganicmatter.Primarily,amesophilicphase

occurs, followedbyathermophilicphase.Forcenturiesmanhasbeenutilizingthe

latter process to increase soil fertility, reduce organic ordure volumes and treat

contaminated soils. Composting is a practice gaining popularity amongst the

agricultural community, theengineering realmand,onamoregeneral scale, even

with theaverage individual.Essentially, asmoreapplications involveutilizing this

ancient technique, one must account for the numerous limitations that can be

encountered when composting. Of the latter, the inability to ensure that the

compostingmediaisfullycuredafteracertainperiodorensuringthattheentirety

of the media has attained the crucial thermophilic phase, where pathogenic

organisms are destroyed, are limiting factors when considering composting as a

means to an end. More so, the production of volatile fatty acids from

microorganisms is the sourceofunpleasantodors that candeter individuals from

settingupacompostingbin.Theaforementionedlimitingfactorsareduetothenon‐

homogenous nature of compost and the presence of anaerobic digestion within

pockets of the compost media. In the past, a heat redistribution system was

designedinthequesttoredistributethecoretemperatureofthecompostingmedia

uniformlythroughoutthecompostingvessel,withoutanyexternalinputsofenergy.

To do so, the heat produced by the activated microorganisms was uniformly

distributedbyawatertightsystemconsistingofcopperandplastictubingconnected

toaheatercoreplacedatthecenterofthecompostingmixture.Nextwedesigned

theabovementionedheatredistributionsystemandtesteditusingsixtwohundred

literpolyethylenebarrels.Thelatterwasdonebyfittingthebarrelswithafour‐inch

holeatthetopandbottomandameshgridat8inchesfromthebottomtoholdthe

0.15m3ofcompost.Amixtureofdogfoodandwoodchipswasutilizedduetotheir

low cost and availability. To insure the statistical validity of the results, six

compostingvesselswerebuilt,threecontrolsandthreebarrelsfittedwiththeheat

redistributionsystem;astatisticalanalysiswasthenconductedtodetermineifwe


4

had obtained valid results. The objective was to test the effectiveness of the

designedsystemtotransferheatthroughoutthecompostmediaandpermituniform

composting throughout the latter, therefore a fully cured final product. Themain

constraintwithinthedesignof theheatredistributionsystemwasthefact thatno

external inputs of energywere to be added; the systemwas to be self‐sufficient.

Results obtained in the past suggested to a 99.5% confidence coefficient that the

heat redistribution system was meeting it’s objective of distributing the heat

uniformlythroughoutthebarrel.Wewerealsoabletosuggestthatthesystemhad

thepotentialofaccelerating thecompostingphaseandproducingacuredproduct

quicker than the barrels not fitted with the heat redistribution system. We did

howevernoticethatasignificantamountofheatwaslostfromthefourinchwhole

at the topof thepolyethylenecontainer.Thesewholes,oneat the topandbottom

wereputinplacetoinsurethattheairwouldcirculatethroughoutthecomposting

bed, favoring aerobic bacterial growth and reducing the production of odorous

gasses.Therefore,inaquesttoreduceheatlossandfavorhighertemperaturesand

a prolonged thermophilic phase we have gone about designing an air exchange

system. This system (AES) should be able to permit oxygen to be replenished

throughout the composting media while minimizing losses through the four‐inch

wholes.Sketchesoftheheatredistributionsystem(HRS)andtheairredistribution

systemcanbefoundinAppendixB,Figure12,andAppendixC,Figure13.


5

DesignoftheHRSSystem

FUNCTION

Theheatredistributionsystemutilizestheheatproduced,undertheformof

energy frommicroorganisms, anddistributes it throughout the compostingmedia

permittingtemperaturegradientstobeloweredandthecomposttobeatthesame

compostingphasethroughouttheprocess.Essentially,astheheatercoreplacedat

thecenterofthebarrelisheatedbythemicrobialactivity,thedensityofthewater

withinthelatterdrops.Thedensitygradientofthewaterwithintheheatercoreand

coppertubingcausesthewatertoflowfromtheheatercoretotheplasticpipingand

intothecoppertubingthroughoutthecompostheap.Thisprocessiscalledthermal

driving. Thedifferencebetween the forcesofgravityexertedon the twovolumes

will cause the warmer fluid to rise and the colder fluid to sink. The continuous

warmingofthebarrelisbasedonthisprinciple.Asthemicroorganismswithinthe

compost bin begin to digest the nutrients, heatwill be dissipated and once it has

elevated thewater’s temperature to the proper level, thewarmwaterwill slowly

rise as the coldwater spirals down the copper tubing towards the bottom of the

barrel. Whentheaforementionedoccurs, theheat fromthecenterof thebarrel is

evenlydistributedthroughoutthecompostingmixtureduetothehighconductivity

of the copper tubing, without external inputs of energy. The latter permits the

compost to be at the same microbial phase, whether that be mesophilic or

thermophilic, essentially eliminating pockets of undigested organic mater and

ensuringthatthefinalproductiscompletelycured.Thefour‐inchholes,madeatthe

top and bottom of the barrel and the clearance produced by the grill and V‐bent

steel supports also permit air to flow, through convection, throughout themedia,

permittingaerobicconditions.ThelatterreducestheamountofVOA(volatilefatty

acids) emitted by the compost, therefore reducing the unpleasant ammonia smell

causedbythedecompositionprocess.


6

A Solid Works model was designed to better understand the layout of the heat

redistributionsystemanditsimplementationbeforetheconstructionandtestingof

thebarrelsthatwasdone.FromtheSolidWorksdrawingsandpreviouswork,the

construction of the vessels was much facilitated; see figures in Appendix B. The

latteralsoprobablycontributedtothefinalresultsnotbeinghinderedbytechnical

errors. Furthermore another simulation model was designed including the ARS

systemtoestablishabetterreferenceintermsofspaceandsizing.Thelatterwould

beusedtosimplyhaveabetterunderstandingof thewaythesystemwouldcome

togetherwhentheHRSandtheARSsystemareimplemented.NotethatinAppendix

BDrawings1 to6, the lidof the200LiterPolyethylenebarrelswasnot included.

Thelatteristoensurethatthesystemcanbeproperlyseen.

CONSTRUCTION

Asmentionedprevious,thematerialsusedwithintheHRSdesigninvolveda

heatercore,fivefeet(1.50meters)ofcoppertubingandabout2feet(0.6meters)of

braidedplasticpiping.Therefore,forthebarrelsfittedwiththeheatredistribution

system, three five‐foot coils of copper tubing,with an inner diameter of 4/8 inch

(0.01meters)wereusedalongsidethree2‐feetsegmentsof5/8in.(0.0158meters)

inner diameter plastic piping and a 3‐way control valve. A zinc coatedmesh grid

wouldbefittedintoeachofthesixcompostingvessels.V‐bentsteelbarstoholdthe

totalweightofcompostwouldsupportthelatter.Afour‐inch(0.1m)holewouldbe

madeatthecenterofthetoplidofthebarrelandanotherfour‐inchholewouldbe

made 8 inches (0.2m) from ground height, to insure there would be airflow

throughoutthecompostingmedia. Theheatredistributionsystemwasassembled

and tested by inserting the heater core into a water bath and increasing the

temperatureofthewaterbathuntilwatermotioncouldbeobservedthroughoutthe


7

clearplastic,carewastakentoensureflowwasoccurringthroughoutthepipingand

the water motion did not only consist of localized turbulence. It had been

determinedthataround35°Cwaterwouldstartflowing.Aftertestingallthreeofthe

heatredistributionsystem,framesweredesignedtoholdthe latterandinsurethe

heatercorewouldbeatthecenterofthetwohundred‐literpolyethylenebarrels,as

shown in Figure 2. The heat redistribution systems were then fitted to their

respectivebarrels.The200Lplasticvesselsweretheninsulatedwithmineralwool

and bubble rap tominimize heat losses from the sides of the barrels. All vessels

were then transported to the Bioresource Engineering Laboratory for further

testing.

Figure1:Picture of the heat redistributionsystem, before being placed into theinsulatedcompostbarrel.


8

HRSCALCULATIONSThermalDriving

Thermaldrivingheadistheforcethatcausesnaturalcirculationtotakeplace.Itis

causedbythedifferenceindensitybetweentwobodiesorareasoffluid.Whenwe

have two volumes that are at different temperatures, then the volume with the

highertemperaturewillhavea lowerdensityandhence lessmass. The inverse is

also true, which is why the volumewith a lower temperaturewill have a higher

density and a greatermass. The higher temperaturewill not only bring about a

lower mass, it will also lower the force exerted on the fluid by gravity. The

differencebetweenthe forceofgravityexertedon the twovolumeswill cause the

warmerfluidtoriseandthecolderfluidtosink(Munsonetal.,2005).

The continuous warming of the barrel is based on this principle. As the

microorganismswithin thecompostbinbegin todigest thenutrients,heatwillbe

dissipatedandonceithaselevatedthewaterstemperaturetotheproperlevel,the

warm water will slowly rise as the cold water spirals down the copper tubing

towardsthebottomofthebarrel.

Frictioninthepipes

Twomainfactorswereconsidered:theReynoldsnumberandtheheadlossdueto

friction. The former is necessary to determine whether the flow is laminar or

turbulent and the latter to establish the losses in the systemdue to the choice of

material.Itisimportanttomaintainalaminarflowbecauseitismorestableandit

willlowerthepressuredropinthepipes.Turbulentflowisamuchmorecomplex

process although it should theoretically enhance the heat and mass transfer

processes.


11

Materials

HOMOGENEOUSCOMPOST

Twooptionswereavailablefortheorganicwastematerial:dogfoodandchickenor

cowmanure.Dogfoodwaschosenovermanurechieflyforitsconformity.Sincethe

experimentwillberepeatedinthefuture,amoreconformmaterialwasfavoredto

avoiddiscrepanciesbetweenexperimentsandbetweenthe6barrelsthatwereset

up for this design. Additional features include the greatly reduced amount of

pathogenicorganismsanditsFDAapproval(FDA2010).Anotherbeneficialaspect

isthedogfood’swaterabsorptioncapacity.Severalmaterialswerealsoconsidered

for the bulking agent: sawdust, shreddedpaper, straw, andwood chips. Sawdust

wasrejectedsinceweneededamaterialthatcouldprovidestructuretothemixture.

Theuseofshreddedpaperwasaninterestingoptionsinceitpermittedtherecycling

ofoldmaterial,butaswithsawdust,itwouldn’tprovideadequatestructure.When

comparingtheremainingtwomaterials,aswithdogfoodandmanure,theissueof

availabilitywasnoted. Sinceanotherexperiment run is scheduled for this spring,

similar materials must be available at that time. Straw would have been more

difficulttoobtainthanwoodchipsatthatpoint,andifobtainedwouldhavebeenofa

different quality than the fresh straw collected in the fall. Wood chipswere also

favoredfortheirlargersize,providingsuitablestructuretothecompost,aswellas

theiravailabilityandconsistency.

Inordertodeterminethetotalmassofcompostmaterials,avolumeanddensityhad

tobeestablished.Theheightofcompostwaschosentobe26inches(0.6604m)and

thediameterof the200‐litrepolyethylenebarrelwas21 inches (0.5334m).From

this information, the volume of compostmaterial was found to be 0.15m3. The

density of themixturewas assumed to be 550 kg/m3, after consultationwith an

expertonthematter(Dr.S.Barrington, PhD,Agr.Eng.,McGillUniversity),yieldinga


13

woodchipsaccountedfor4.9litersofwater(basedoninformationinTable1)and

so44.6litersoftapwatertheoreticallyhadtobeaddedtothecompostmixture.To

ensurethatsucha largequantityofwaterwouldremainwithinoursystemrather

than leak out through the bottom 4‐inch hole, thewood chipswere soaked for 3

daysinawhiteplasticbinwithadepth,width,andheightof0.57,0.86and0.58m.

Oncethematerialswerepurchased,theywereanalyzedinalaboratoryformoisture

content,density,percent total solidsandpercent ash content. Todetermine total

solids,threesamplesofeachmaterialwereweighed,placedinanovenat103°Cfor

24 hours andweighed oncemore (see Sample Calculations, Eq. 5). The remains

were then placed in a furnace, set to a temperature of 550°C for 5 hours, to

determine theash contentofbothmaterials (seeSampleCalculationsEq.6). Ash

contentisexpressedasapercentofthetotalsolids.Thedensitywasmeasuredby

weighing the samples in a crucible of known volume. Characteristics of the final

compostmixturewereanalyzedinthesamemanorasthedogfoodandwoodchips,

however6samplesweretestedinsteadof3.Resultsfromthelaboratoryanalysisof

thecompostmaterialswillbefurtherdiscussedintheAnalysissection.Withthese

results,calculationswereverifiedanditerationswereconductedoncemoretoyield

moreaccuratemassesofeachingredient,basedonmeasuredparameters.

Oncethematerialswerepurchased,threesamplesofeachdogfoodandwoodchips

were analyzed according to the aforementioned methods. The data obtained is

presented in Appendix C: Tables 5, 6 and 7. In tables 2 and 3, results formean

moisturecontent,meantotalsolidscontentandmeanashcontentarepresentedfor

bothdogfoodandwoodchips.Themoisturecontentandtotalsolidscontentwere

close to the values that had been initially assumed. This indicates that the

calculationsmadetoarriveatdesiredmassesdogfoodandwoodchipsbasedona

theoretical C/N ratio respected the characteristics of the chosen composting

materials.However,thedensitieswerequitedifferentwith341.3and162.0kg/m3

for dog food andwood chips respectively. Also, themoisture content of thewet


17

A Hewitt Packer Data logger (Model #: 34970A) was used to acquire the

temperaturereadingsfromthethreepre‐determinedheightspreviouslymentioned.

Thelatterwassettotaketemperaturereadingsatfifteen‐minuteintervalsandthe

datawasextractedfromthedataloggereveryday.

The datawas collected for a period of thirty days, running three control barrels,

labeled CX‐# and three barrels fittedwith the heat redistribution system, labeled

HR‐X#(XvariesfromAtoCand#varyfrom1to3)asshowninfigure3.

Figure3:Labelingschematicofthethermocouples.


19

Three things are apparentwhen evaluating the graphs above (Fig. 4). One of the

latterwouldbethefactthatthegreenlinesonallofthecharts,representingthetop

thermocouples,seemtohavemoreaggressiveandunpredictablevariationsthanthe

otherlines.Thisisduetotheunforeseeneffectsofcompaction.Itwasnottakeninto

accountthatcompostvolumewouldbereducedtothatextent(70mmdecreasein

height), exposing the top thermocouples. The aforementioned lead the top

thermocouplestomeasureambientairwithinthecompostingvesselsinsteadofthe

actualtemperatureatthetopofthecompostmedia.Anotheraspectworthnotingis

thehigher temperature that thevessels fittedwith theheat redistribution system

(HRS) attain. The preceding is assumed to be due to the heat being uniformly

distributed throughout the composting vessels fitted with the HRS, favoring the

microorganisms of thermophilic nature, permitting the latter to attain full

maturationandintheprocesspermittingthevesselstoattainhighertemperatures.

The third phenomenon that can be observed involves the smaller temperature

differences noticed between the center andbottom thermocouples (Red andBlue

lines respectively) in the vessels fittedwith theHRS, notably between 35 and 50

degrees Celsius. More so, after 400 data acquisitions, it can be noticed that the

temperature starts decreasing in the control barrels, whereas the HR vessels

temperaturescontinuetorise.Thisalsocanbeattributedtotheheatredistribution

systemandwillbediscussedfurtherinthediscussionsection.Abetterdepictionof

the latter can be observed in Figure 5 and Figure 6, where the difference of

temperaturesbetweenthecenterandbottomthermocoupleswereaveragedoutfor

thecontrolandHRSvessels.ItcanbeobservedthattemperatureoftheHRSvessels

donotattainaslargedifferencesasthecontrol,demonstratingthatovertheperiod

ofninedaysand861dataacquisitions,thevesselsfittedwiththeheatredistribution

systemseemtohaveamoreuniformtemperaturegradient.


21

ANALYSISOFRESULTS

Havingplottedtheaveragedifferencebetweenthemiddleandbottomtemperature

readings of both the control barrels and the heat redistribution barrels, it is

essentialtoestablishwhetherthesamplemeansfortemperaturedifferencesofthe

control and heat redistribution systems are significantly different. Using a right‐

handone‐tailedtestabouttheequalityoftwopopulationmeans, itwasfoundthat

the average temperature difference in the control barrelswas significantly larger

fromtheaveragetemperaturedifferenceintheheatredistributionsystem. Itmay

be affirmed that the average temperature difference in the heat redistribution

system is in fact smaller than the averagedifference in the control barrelswith a

confidence intervalof99.5%. Thisconfirmsthat thedesign is functional in that it

succeededinwarmingthecompostmixtureinamoreuniformmannerandattaining

highertemperaturesthanthecontrols,withoutanyexternal inputsofenergy.This

testwas basedon the samplemeans: 5.493°C for the control and4.742°C for the

heat redistribution system, and the samplevariances:9.652 for the control, 5.019

for the heat redistribution system. Detailed calculations are presented in Sample

Calculationssetb,AppendixG.

Fromtheaboveanalysis, theHRShasdemonstrateditseffectiveness inpermitting

the barrels to attain higher temperatures than the controls and to have a more

uniform temperature gradient throughout the compostmedia. Thismore uniform

temperature gradient could be effective in destroying pathogenic organisms,

increasing the quality of the final cured product and potentially reducing the

compostingtime. Itshouldbementionedthatevenwithout thetopthermocouple,

the results are valid and statistically sound. The comparisonbetween center and

bottom thermocouple readings to determine temperature distribution and

uniformity is statistically sound. More so, in the controls, midway through the

experiment it seems that the temperature stabilizes and starts to decrease. The

latterismostlikelyduetothecompostnotbeingabletoattainthesecondtierofthe


22

thermophilicphasedue topocketsat lower temperatures inhibiting thegrowthof

the required thermophilic bacteria and slowing down the composting process.

Whereas the barrels fitted with the HRS are able to continue their gains in

temperature and alsomaintain, on a general basis, a smaller overall temperature

gap. When evaluating Figure 5 and Figure 6, it was observed that overall the

temperature differencewithin the control vessels is higher than the temperature

difference within the barrels fitted with the HRS. One also has to take into

consideration that, asmentioned previously temperature starts decreasing in the

control vessels around the four hundredth scan,whereas the temperaturewithin

the HRS barrels continues to increase even after the last data scan that was

recorded. ThelattercouldexplainthesecondhalfofFigure5,wheretemperature

differences within the controls seems to drop, but this may be explained by the

generaldecreasewithinthosebarrelsasmicrobialactivitydiminishes.

Relying on the statistical analysis procured in the Sample calculations set b, it is

possible to declare that the heat redistribution system has attained the set out

objective of homogenizing the temperature throughout the composting media,

increasing the speed of composting and permitting the barrels to attain higher

temperatures possibly destroying pathogenic organisms throughout the compost.

The speedof the compostingprocesswill be furtherdiscussed in thenext report,

since data gathering is still underway. The latter gives a strong argument to

continuethedevelopmentandameliorationsprocuredintheimprovementssection

and the possible implementation of the HRS on a larger scale, increasing the

efficiency of composting and it’s applications in a residential, industrial and

commercialbasis.


23

RiskAssessment&FailureMode

A limitationof thedesignwouldbe thepotential failureof theheat redistribution

system. Failure could occur during the filling phase, where the dumping of the

compostonto theHRScouldcauseawater leakandhencedisrupt the flow in the

coppertubing.However,carewastakenwhenthelatterwasdone.Moreso,another

factorthatcannotbefullyremediatedfororobservedduringtherunningphaseof

the compostwouldbe the reductionor stoppageof flow throughout thepipingof

theHRS.The lattercouldoccur if anairbubblewere toenter thesystem,causing

blockage,orifakinkwascausedbytheweightofthecompostitself.However,one

must take into consideration that even if there were to be some sort of limiting

factor thatwould cause the stoppage of flow, the high conductivity of the copper

tubing could still permit the heat to be transferred from the center of the barrel,

throughtheheatercore,intothewaterwithinthelatterandthroughouttherestof

thepiping and fluidby conduction.The latter shouldbe taken into consideration,

because uniformity in the temperature gradient does not necessarily imply flow

withinthepiping.ItisimportanttomentionthattheHRSwastestedbyinsertingit

inaheatedwaterbathsetat20°Candincreasingthetemperatureinincrementsof

fivedegreesuntilflowcouldbeobservedthroughoutthetransparentplastictubing,

therefore demonstrating that the design was sound and able, under ideal

circumstance to transmit warmed water throughout the piping. Flow occurred

around35°CinallthreeoftheHRSsystems.Amethodofreducingkinksandleakage

would involve reducing the lengthof theplasticbraidedpiping toavoidexcessive

twistingmotions,andevaluatingthecoppertubing,beforefilling,ensuringthereare

noapparentorhiddenkinksandavoidinganyabruptchangesinthedirectionofthe

copper tubing. Another important aspect tomention is the environment inwhich

thebarrelsarerun.Thelocationwheretheunitswereplacedwasmaintainedata

temperatureof20degreesforthefirsttwodays(192scans)andthenat25degrees

Celsius for the rest of the experiment. The latter should be taken into account in


24

future runs. To avoid additional discrepancies in the future, a logbook should be

keptwiththedateandtimeatwhichtheBioresourceEngineeringLaboratorywas

accessedsinceitwasbroughttoourattentionthatfellowstudentswhohadworkto

completeintheEngineeringLabwouldleavethedooropeninanattempttoreduce

thefowlsmellsemanatingfromthecompostbarrels.Thebreezecreatedinthebarn

could be responsible for the erratic behavior of the top (exposed) thermocouple

readings. Finally, in terms of risk assessment it is important to have respiratory

protection when dealing with any types of large volumes of compost. For the

purposesofthisexperimentdatawasuploadedonceadayandarespiratorysafety

devicewasused.Anotherimportantsafetyaspectwouldinvolvecontactavoidance

withcompostthatisleftasresidueonthesidesofthebarrels.

ISSUES

DuringthetestingoftheHRS,amultitudeofissueswereencountered.Ofthelatter,

heatlossfromthetopfour‐inchhole,madeforaerationwasprobablyofthehighest

significance. Other issues include the large amount of leachate produced by the

compostandthefactthatithadtobemanuallycollectedandresuppliedtothetop

of thecompostpile.The lossofnitrogenouscompoundsduringthedecomposition

processoccursmainlythroughemissionofgasessuchasNH3andNOx,aspreviously

mentioned. This loss of nutrients may have a significant impact on the nutrient

balanceofoursystem.Sincethecompostvesseliswellisolated,itisassumedthat

themajorityofnitrogenousemissionsareexitingthroughthe4‐inchholeatthetop

of the barrel. Heat is also lost through the same opening. These issues will be

addressedbyrecirculationthewarmairproducedbythecompostbymeansofan

air‐to‐air heat exchanger. Also, the reduction in compost volume was not

anticipatedtobesolarge.


25

AirRedistributionSystem

ARSINITIALDESIGN

Todealwiththeheatlossfromthetopfourinchwhole,wehadtofindamethodto

keeptheprocessaerobicwhilereducingheatflowfromthetopofthecontainer.To

achievetheaforementionedanAirRedistributionSystem(ARS)wasdesigned.The

latterwouldsimplyconsistofachimneythatwouldbeable tore‐circulate theair

into the bottom four‐inch hole while maintaining the oxygen supply. The initial

designisdepictedinFigure7.

Figure 7: Sketch of the design III concept.


27

inflow of ambient, quiescent air due to kinks that might occur during the

constructionorassemblyprocesses.

Since the designed system has no external input of energy, it relies on free

convectiontodrivetheheatexchanger.Thisfreeconvectionoriginateswhenabody

forceactsonafluidinwhichtherearedensitygradients.Theneteffectisknownas

buoyancyforceanditinducesfreeconvectioncurrents(Incroperaetal.,2007). In

this case, the body force is gravity and the density gradient is temperature. The

processbeginsasthewarmair,resultingfromthemicrobialactivity,risesthrough

theinnercylinderduetobuoyancysincethedensityofthewarmairislowerthan

thatoftheambientair.Atthispoint,wehaveassumedthatthewarmairisevenly

distributed throughout the entire cylinder, up until the point where it leaves the

innercylinder throughasimilar4 inchdiameter(101.6mm). The innercylinder

shouldbecomposedofahighlyconductivematerial.Below,inTable5,arealistof

metallicandnon‐metallicmaterialswithrelativelyhighconductiveproperties.

Metal

Conductivity, k (W/m*K) at 330 K

Silicon Carbide 490 Silver 428 Copper (pure) 399 Beryllium Oxide 247 Aluminum (pure) 238 Magnesium 155 Tungsten 141 Zinc 114.3 Iron 77 Tin 65 Commercial Bronze (09% Cu, 10% Al)

52

Chromium steels 48.2 Diamond 2047

Table5:Conductivepropertiesofmetalsandnon‐metalmaterials. Source:Incroperaetal.,2007.


28

Obviously, diamondandpure silver arenot inourbudget. Silicon carbide canbe

purchased in Canada at a price of 277.00$ for a 50mm by 50mm sheet ():

110’800$/m2.This ismuchmore expensive than copper,which canbepurchased

for26$,fora1ftx2ftsheet(www.whimsie.com/coppersheetwire),around140$/m2.

Thus, copper ismoreaffordableandstillhasveryhighconductivity,k. As for the

outercylinder,thesamepolyethylenematerialthatcompostvessel ismadeofwill

be used, alongwith the same insulatingmaterials, whichweremineralwool and

bubblefoilinsulationwiththermalconductivitiesof0.042W/mKand0.034W/mK

respectively(Incroperaetal,2007,AppendixA).

DETERMININGTHEBOUNDARYLAYER

As the ambient air, approximated at 20 °C, comes into contact with the warm

surfaceoftheinnercylinder,athermalboundarywilldevelopduetothedifference

in temperatures. The fluidparticles coming intocontact themetallic surfacewill

achievethermalequilibriumatthesolidssurfacetemperature.Theseparticleswill

then exchange energy with those adjacent to them in the fluid, creating a

temperature gradient in the fluid. The region in which this gradient occurs is

definedasthethermalboundarylayer,attheleadingedgeofwhichthetemperature

profilewillbeuniform,withT(y)equatingT∞(definedastheambienttemperature).

However,thestandardequationsdonotapplyinthiscase,sincethereisnoforced

convection and the plate, or rather cylindrical surface, is vertical. As a result, the

governingequationwillinvolvethedimensionlessparameterGr(Grashoffnumber),

whose functionmaybe compared to thatof theReynoldsnumber in situationsof

forced convection, and thatmeasures the ratio of buoyancy forces to the viscous

forcesactingonthefluid(Incroperaetal.,2007).


33

The5mmdistancewillbethelengthbetweentheinnercylinder’ssurfaceandthe

perimeteroftheouterinsulatedcylinder.Itisalsoimportanttonotethestateofthe

fluid.FlowisconsideredlaminariftheproductoftheGrashoff(Gr)andPrandtl(Pr)

numbersarebelow1x109(Incroperaetal.,2007).Inourcase,Gr0.8mandPrwere

1’970’259’723 and 0.706 respectively, which yields a value of 1.3 x109. Since the

valueobtainedisveryclosetothe109 limit, this indicatesthatat theheightof0.8

meters, the flow is beginning to transition from laminar to turbulent. However,

throughout most of its length (0m to 0.7m), the air redistribution system

demonstrates laminar flow patterns, according to the equations previously

mentioned.

Additionally, in order to increase the surface area of the inner, heat conducting

cylinder, twooptionswereavailable:verticallyaligned fins (straight, triangularor

parabolic)orfoldingthecoppersheettocreateripplesalongthesurface.Although

thefinsmightgenerateaslightlylargersurfacearea,itwasmorerealistictocreate

thefoldsonthesurfaceofthecoppersheetthantofirmlyattach60individualfins.

To determine the amount of folds required, the perimeter of the inner cylinder

(319.9mm)was divided by the number of spacings. In the table below, several

spacingswerecalculated.


34

Number of Spacings

Width of spacing

(mm)

Length of Fold (mm)

New Perimeter

(mm)

Perimeter with Fins

(mm) 10 31.92 5.00 99.92 346.19 15 21.28 4.43 132.95 361.19 20 15.96 4.12 164.83 376.19 25 12.77 3.92 196.11 391.19 30 10.64 3.78 227.05 406.19 35 9.12 3.68 257.77 421.19 40 7.98 3.60 288.33 436.19 45 7.09 3.54 318.79 451.19 50 6.38 3.49 349.17 466.19 55 5.80 3.45 379.49 481.19 60 5.32 3.41 409.76 496.19 65 4.91 3.38 439.99 511.19 70 4.56 3.36 470.20 526.19 75 4.26 3.34 500.38 541.19

Table 8: Values for the perimeter of the inner cylinder, dependant on the number ofspacingsassigned.

Evidently, the new perimeter has to be larger than the perimeter of the 4 inch

diameter,discardingallspacingsunder46.Onceagain,feasibilityofconstructionis

key;weneedthehighestnumberofspacingspossiblewithoutitbeingtoosmallfor

ustoactuallybuild.Wedecidedon60spacings,yieldinganewperimeterof409.76

mm, a 29% increase compared to the initial 4 inch diameter. From Table 8, the

columnonthecompleterightindicateswhattheperimetercouldhavebeen,hadwe

chosen the impractical fins. It is higher than the folds, however if both are

compared at 60 spacings, the difference is less noticeable than at 15 spacings. A

graphical depiction facilitates the comparison: the perimeterwith folds increases

morerapidlythattheperimeterwithfins(Figure11).


36

VELOCITYANDMASSFLOWRATEOFWARMEDAMBIENTAIR

The mass flow rate of the warmed air was also determined. Mass flow rate is a

function of the air density, velocity and cross sectional area of flow (Incropera,

2007).

ρ=1.0682(at328K)A=(πD2)/4=(π*(0.1016m)2)/4=0.008107m2

where:

ν:velocity,m/s g:gravitationalacceleration,9.81m/s2

L:verticaldistancefrombottomofthesurface,m.

ΔT:Temperaturedifference, Ts‐T∞=35KT∞:Ambient“room”temperature,293K

ν=0.6m/s

Hence,themassflowrateofthefluidatroomtemperature(293K)hasavelocityof

0.6m/sandamassflowrateof0.005196kg/sasitcomesintocontactwiththehot

metallicsurfacethattheinnercylinderconsistsof.


37

ARSSUMMARY

Theairredistributionsystemconsistsof twoverticalconcentriccylinders,bothof

0.8minheight.Theinnercylinderwillbeconstructedusingathincoppersheetto

enhancetheconductionofheatfromtheexhaustairofthecompostandwillbeleft

openattheend. Theinnercylinderwillhaveastarformationwith60spacingsof

5.32mmeach,providingaperimeterof409.76mmandasurfaceareaofcontactof

327’808mm2(0.328m2).Additionally,thelengthofeachofthe120foldswillbeof

3.41mm.

The outer cylinder will be made of polyethylene, covered in the appropriate

insulation as described above, and the top surfacewill not be left open since the

freshincomingairwillbedirectedtowardsthebottomofthecompostvessel,rather

thanlosttoanopeningatthetop.Toensureinflowoffreshair,theoutercylinder

will be punctured 5 cm intervals from the bottom, 2 cm intervals along the

horizontal, and a well insulated 2 inch diameter piping system will connect the

bottomsectionoftheoutercylindertotwothe2inchopeningsthatwillbepresent

oneitherside,atthebottomofthevessels.SeeAppendixC.

TESTING&SIMULATION

Constructionandtestingoftheairredistributionsystemshouldbeginthissummer.

Theseresultsshouldhelpbetterdeterminetheprecisionofourcalculationsandthe

overallefficiencyofthesystemitself.Duringtheexperiment,weintendoninserting

3thermocoupleswithintheinnercopper,star‐shapedcylinder:thefirstjustabove

thecompostvessel’s4inchopening,thesecondat0.4minheightandthethirdat

theexit(0.8m). Thermocoupleswillalsobeplacedinthesectionbetweenbothof

thevertical cylinders, at thesameheights. The lastpairof thermocoupleswillbe


38

measuringthetemperatureoftheairasitleavestheinsulatedpipingandentersthe

compostvesselfromtheopeningatthebottom.

Essentially the testing is a process where calculation methodologies will be

compared to theirempirical standingsandadjustmentswillbemade tomend the

discrepancies.


39

EconomicOverview

The fact that the HRS and ARS system can be implemented on a small scale, for

exampleonamunicipallevel,onamediumorlargeindustriallevelforanumberof

reasonsincreasetheprobabilitiesthatsuchasystemmightbecommercialized.The

purposeofourdesignbeingaproofofconceptweconcentratedondemonstrating

thatthesystemwasfunctional.However,consideringtheincreasingpricesof land

filling and the popularity of bioremediation the system could be a cost‐effective

solutionforthelatter.

Note that the approximate cost of the HRS & ARS systems, including the two

hundred litter vessels hovers around 90$, that said one could cut costs and used

recycledmaterial tobuildand implement theCompostingBioreactors.Land filling

in Canada has an average cost of around 85$ per ton, however the total

environmentalcostofthelatterincludingtheeco‐systembenefitsthatalandfillwill

destroy or hinder does not have a set value. Note also that land‐filling will be

subjected to higher taxes in the upcoming years and that the Composting

BioreactorsbearingHRSandARSsystemcouldbeusedforthousandsofcomposting

cycleswithvery littlemaintenanceconsidering therearenomechanicalpartsand

thatitutilizesnoexternalinputsofenergy.Overallthecompostingreactors,onthe

short run,mightbemoreexpensive,however the futurebenefitsaremuchhigher

than the initial cost considering the final product could be used or sold as soil

fertilizerandthefactthatland‐fillingisseenasanoutdatedmethodology.

Assuming that a composting bioreactor can hold 90 kg of compost per run and

requiresthirtydaystocompostthelatterwithaninitialcostof90$andalifecycleof

athirtyrunsthetotalcostfor2700kgoforganicmatteris90$.Thelatterdoesno

includethe labor that it requires.Assumeaperson ispaid12$perhour to fill the

barrelasasecondarytasktotheirmainemploymentandthatittakefifteenminutes

tofill/emptyabarrelthatis180$laborexpensefor30runssumminguptoatotalof


40

270$for2700kgtherefore,about100$perton.Nowconsiderthatthelandnotused

by the organic waste is kept as a natural habitat and a part is utilized for the

buildingofahousingprojectorapark.ThelatterwouldofferEco‐Systembenefits

thatthelandfillwouldontheotherhandtakeaway.Thebioremediationcapacities

ofcompostingcouldalsobeutilizedonasmallorlarge‐scaleoperation.

Onalargerscaleassuming10.8tonesoforganicmatter,therefore10800kgneeded

tobelandfilledonaspanof1yearandthatthecompostingbioreactorwouldcost

70$ to mass produce (recycled material). If one were to land‐fill the 10 tones it

wouldcostatotalof920$,whereas10bioreactorscouldhaveanoutputof900kg

per cycle and 10 800kg output per year. The total cost of the reactorswould be

(10*70)+(10*0.25*12*12*2)= 1420$. However, cured compost can be resold at a

priceofapproximately25$/ton.Therefore(10800kg/1000)*25=270$.Thatsaid,

thetotalcostofcompostingthesoilandsellingitwouldcomebacktoalossof230$

includingthehassleofhavingsomeonefillandemptythevessels.

Fromtheeconomicpointofview,keepinginmindthatthereisnodiscounting,the

overallcostofoperatingatwohundredlitervesselseemstobehigherthansimply

sending the contaminated soil to a land fill. However, as taxation increases for

landfilling and as public opinion turns against the latter practice, it will not be a

viable option. Also, the reactors could be built on a larger scale to save time and

money. Also, some items like theheatexchangerwithin theHRSsystemcouldbe

recycled from a junkyard. Amore in debt analysis utilizing discount rates, Initial

Costanalysisandexact itempriceswouldyieldmoreconcurrentdataonwhether

suchaprojectcouldhaveanindustrialormunicipalapplication.


41

Conclusion

Havingdemonstratedtoa99.5%confidencelevelthattheHRSsystemisfunctional

andisdistributingtheheatuniformlythroughoutourcompostingmediawecansay

thattheinitialobjectiveoftestingandassessingtheeffectivenessoftheHRSsystem

has been attained. The secondpart being the design of theARS systemwas also

completed. Although the feat of designing and testing theHRS system is a design

project in itself theengineeringprocesswasalsoutilized in thesecondhalfof the

project, as the ARS system required a very rugged mathematical and simulation

intensive design process. One should note that the Composting Bioreactors have

receivedasignificantamountofattention in thepast fewmonths, investmentand

funding for the latter project, including an application for a patent is now a

possibility. Future considerations should involve the testing of theARS system as

done for the HRS system, meaning a three standard to three ARS fitted 200L

polyethylenevesselsshouldberunalongsidetodeterminetheeffectivenessofthe

latter. The ARS system should be able to maintain higher temperatures than the

standardbarrelsforaprolongedperiodoftime.Thereforethedataanalysiswould

compare maximal temperatures attained and the length of the time these

temperatures can be maintained in the standards and in the ARS fitted systems.

Another testcould involve fitting threevesselswith theHRSandARSdesignsand

threeonlywiththeHRS.Eitherofthelattertestswouldrequirearuggedstatistical

analysistovalidatethedataonceitiscompiled.

Inconclusion,onecanwithgreatcertaintydeclarethat fromanacademicpointof

viewtheCompostingBioreactorsareasuccessinthattheproofofconcepthasbeen

attained. From an economical or industrial point of view much testing and

manipulations should be done to better the reactors for larger scale use and to

lowerthecostofthefinalproduct.Inessence,theCompostingReactorsareawork

in progress as any other feasible engineering design; there is much room for

improvement.


42

REFERENCES

Barrington,S.,D.Choiniere,M.TriguiandW.Knight.2002.Effectofcarbonsourceoncompostnitrogenandcarbonlosses.BioresourceTechnology83:189‐194.CCME. 2005. Guidelines for compost quality. Canadian Council ofMinisters of theEnvironment.PublicationNo.CCME1340.Ottawa,ON.ISBN1‐896997‐60‐0Duteilleul, P., D.E. Mather and B. Pelletier. 2009. Lecture Notes of the StatiscticalMehtods 1 Course (AEMA 310), Fall 2009 edition, McGill University, MacdonaldCampus.FoodandDrugAdministration(FDA).2010. Animal&Veterinary:FDARegulationofPetFood.http://www.fda.gov/animalveterinary/products/animalfoodfeeds/petfood/ucm2006475(2010/11/28).Golueke,C.1992.Bacteriologyofcomposting.Biocycle33:55‐55.Goodfellow. “Materials for Scientific and Industrial Research and Manufacturing,”retrieved on 04/01/2001, from: http://www.goodfellow.com/E/Silicon‐Carbide%27‐Sheet.html

Herrmann, R.F. and J.F. Shann. 1997. Microbial Community Changes During theCompostingofMunicipalSolidWaste.MicrobialEcology33:78‐85.Incropera,Dewitt,etal.2007.FundamentalsofHeatandMassTransfer,6thedition,JohnWiley&Sons,Inc,NJ,US.Kakaç, S. and H. Liu. 2002.Heat exchangers: selection, rating, and thermal design:CRCPress.Kawano, S., C. R. Kleijn, et al. (2004). Computational technologies forfluid/thermal/structural/chemical systems with industrial applications‐‐2004:presented at the2004ASMEPressureVessels andPipingConference : SanDiego,California,USA,AmericanSocietyofMechanicalEngineers.Kostolomov, I. and A. Kutushev. 2006. "Numerical investigation of air freeconvectioninaroomwithheatsource."ThermophysicsandAeromechanics13(3):393‐401.Kutz,M.2009.EnvironmentallyConsciousMaterialsHandling:JohnWiley&Sons.


43

Martins,O.andT.Dewes.1992.Lossofnitrogenouscompoundsduringcompostingofanimalwastes.BioresourceTechnology42:103‐111.MetcalfandEddy,Inc.2003.WastewaterEngineering,Treatment,Disposal,Reuse,4thEdition,McGraw‐Hill,NewYork.Munson, B., Young, D. and Okiishi, H. 2005. Fundamentals of Fluid Mechanics, 5thedition,WileyPublisher,800pages.Napolitano, L. G. 1982. "Surface and buoyancy driven free convection." ActaAstronautica9(4):199‐215.Nakasaki,K.andOhtaki,A.2002.“Asimplenumericalmodelforpredictingorganicmatter decomposition on a fed‐batch composting operation,” Journal ofEnvironmentalQuality31:997‐1003.Ostrach,S.1953.“AnAnalysisofLaminerFreeConvectionFlowandHeatTransferAboutaFlatPlateParalleltotheDirectionoftheGeneratingBodyForce,”NationalAdvisoryCommitteeforAeronautics,Report1111.

Polpraset, C. 2007. Organic Waste Recycling; Technology and Management, 3rdedition.London,UK:IWAPublishing.Suleyman Yigit, K. “Applied Thermal Engineering,” Volume 25, Issues 17‐18,December2005,pages2790‐2799.To,W.M.andJ.A.C.Humphrey.1986."Numericalsimulationofbuoyant,turbulentflow‐‐I.Freeconvectionalongaheated,vertical,flatplate."InternationalJournalofHeatandMassTransfer29(4):573‐592.Whimsie Studio. “Copper Sheet Metal,” retrieved on 04/01/2001, from:http://www.whimsie.com/coppersheetwire.html


53

APPENDIXD

SKETCHESOFTHECOMPOSTBIOREACTORS

Drawing1:HRS&ARSFull‐SizeRepresentation


54

Drawing2:HRS&ARSFull‐SizeRepresentation

Drawing3:HRS&ARSfull‐sizerepresentationwithinnercontentsvisible.


59

APPENDIXGSAMPLECALCULATIONS

Calculatingthecarboncontentindogfood(sameprocedureappliestowoodchips):

(Eq.2)

Meanpercentashobtainedfromlaboratory:6.824(Std.deviation:0.298).

Calculating the percentage of nitrogen in dog (same procedure applies to woodchips):C/Nratiois18,therefore:

Thereis2.83%Ninthedogfoodsamplesanalyzed.

Calculatingthecarboncontentperkgofwetdogfood:

(Eq. 3)

Calculatingthenitrogencontentperkgofwetdogfood:

(Eq. 4)

=0.0258kgC/kgwetdogfood


60

Calculatingtotalsolids:

(Eq.5)

Calculatingashcontent:

(Eq.6)

Calculatingmoisturecontent,wetbasis.Theexamplebelowusesexperimentalvaluesfordogfood,sample1:

(Eq.7)

Where:Mwb:isthemoisturecontentonawetbasis(%)Si:istheinitialmassofthesample(g)Sdry:isthesamplemassafter24‐hourdrying(g)

Calculatingthemassofwaterabsorbedbywetwoodchips:Newmoisturecontent(MC)–initialMC=(53.21–16.57)%=36.64%MC

Thetotalmassofwoodchipstobeaddedperbarrelis19kg,therefore:

Andso,6.96litersofwaterwereabsorbedbythewoodchips.


61

STATISTICSTheexampleusesmoisturecontents(%)calculatedfor3dogfoodsamples:Sample1:9.158Sample2:8.686Sample3:8.606Determiningsamplemean.

Determiningsamplevariance:

Determiningsamplestandarddeviation:

S=+√(S2)

S=√(0.089)

S=0.298


62

SamplecalculationssetbCalculations for the statistical analysis on the equality of means are based onprocedurefromP.Dutilleuletal.(2009).1) Assumptions:

a. Populationsarenormallydistributed

2) Samplemeanforaveragedifferenceincontrolbarrels, =5.493Samplemeanforaveragedifferenceinheatredistributionbarrels, =4.742Samplevarianceforcontrols,S2C=9.652SamplevarianceforHRsystems,S2HR=5.019Samplesizesareidenticalforbothsamples:nc=nHR=825

3) Thenullhypothesis,H0:μc=μHR,statesthatthereisnosignificantdifferencebetweenbothpopulationmeans.The alternative hypothesis, H1 : μc > μHR , states that there is a statisticallysignificantdifferencebetweenbothpopulationmeans,theaveragetemperaturedifferencebeinggreaterinthecontrolsthanintheHRS.

4) Since the twopopulationvariances(σ2Candσ2HR)arenotknown,a two‐tailedtestabouttheequalityofthetwopopulationvariancesiscarriedout.Thenullandalternativehypothesesareasfollows:H0:σ2C=σ2HRH1:σ2C/σ2HR>1orσ2C/σ2HR<1The Fisher‐Snedecor’s F sampling distribution is used along with the teststatistic:F=S2C/S2HR~F(nC‐1,nHR‐1)Choosingasignificancelevelof5%,α=0.05,H0willberejectedifeitherofthefollowingconditionsaremet: S2C/S2HR>F1‐α/2(nC‐1,nHR‐1)or S2C/S2HR<Fα/2(nC‐1,nHR‐1) S2C/S2HR=9.652/5.019=1.923


63

F0.975(824,824)=1.13(TableFfromDutilleuiletal.2009)Since1.923>1.13,thenullhypothesisisrejectedandthepopulationvariancesmaynotbeassumedtobeequal.

5) Since the hypothesis for homogeneity of population varianceswas rejected, atest statistic that follows a t distribution is then used to show whether thesamplemeansaresignificantlydifferent.

Teststatistic:

==5.632

6) Next,thenullhypothesisisrejectediftheobservedvalueoftheteststatistic

(5.632)isgreaterthanthecriticalvalue.Criticalvalue:t1‐α(effectivedegreesoffreedom)Effectivedegreesoffreedomdefinedby:

Effectivedegreesoffreedom=1499.35Significancelevel:α=0.05t1‐α(1499.35)=doesnotappearonthetablefortdistribution;thesecond

highestvaluebeing120.Hence,thevalueof∞willbeusedasdegreeoffreedom.


64

T0.95(∞)=1.645;thenullhypothesisisrejectedsincetheobservedvalue

(5.632)isgreaterthan1.645.Evenwithasmallersignificancelevel,thenullhypothesisisrejected:T0.995(∞)=2.58;thenullhypothesisisrejectedsincetheobservedvalue

(5.632)isgreaterthan2.58.

7) Therejectionofthenullhypothesisindicatesthatthepopulationmeansarenotequalandthatindeedtheaveragetemperaturedifferenceinthecontrolbarrelsis larger than the average temperature difference in the heat redistributionsystems,andthismaybesaidwitha99.5%confidenceinterval.

composting reactors the return of the jedi · oxygen supply throughout the composting media. ......

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