Physical Management and Interpretation of an Environmentally Controlled Composting Ecosystem

8/10/2019 Physical Management and Interpretation of an Environmentally Controlled Composting Ecosystem

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Australian Journal of Experimental Agriculture, 1992,32,657-67

Physical management and interpretation of anenvironmentally controlled composting ecosystem

E. HarperA, F. C .Miller and B . J. M acauley

Department of Microbiology, La Trobe University, Bundoora, Vic. 3083, Australia.B Sylvan Foods Ltd, Worthington, PA 16262, U.S.A.

Summary Compost for mushroom cultivation was 1.23 MJ/kg (range 0.92-1.51 MJ/kg), or 5.11 MJ/kgprepared in an environmentally controlled composting (range 4.04-7.57 MJ/kg) when measured per initi al(ECC) system of 10 t maximum loading. Early in vola til e dry matte r. Hea t e volu tion ave rage dproce ssing , ventilation was manually control led to 18.3 MJ kg decomposed (range 15.4-22.0 MJkg) .provide aerobic conditions. When the desired compost Oxygen usage followed a patte rn similar to thattemperatures were reached, control through temperaturefeedback was used. Physical uniformity of processing

conditions was achieved by recirculating large volumesof air within the reactor.Heat production was found to peak early in the

composting process, reaching a maximum of about8-9 Wlkg initi al wet (67-71%) subs tra te. Whencompost temperatures were allowed to rise to 630C,

of heat reaching a maximum in the55-63OC range. Peak O2 usage was about 9 x 10 kg

O&g compost.s, or in volume terms, 2.9x

10-6m

ai rkg comp0st.s. During temperature feedback control,2 evels were maintained at about 19%.

The enclosed ECC system permitted mass balancedata to be collected for various components. Trialsdemonstrated that temperature and O2 could be closely

maximum heat production occurred at 55-630C. controlled, resulting in-good compost uniformity.Total heat production per initial wet weight averaged

IntroductionPreparation of a nutritional substrate for the cultivation

of the common mushroom (Agaricus brunnescens Peck,synonym Agaricus bisporus) is achieved by compostingvarious agricultural materials. Typical processing includesa largely uncontrolled outdoor Phase I, and anenvironmentally controlled Phase (Fermor et al. 1985).Given a suitable substrate, activity during Phase I isdetermined largely by microbial response to physicalfactors such as temperature and O2 limitations (Milleret al 19 89 ~) . roblems related to Phase I include odours,gaseous NH3 pollution, variations in compost quality,difficult materials handling and inefficient utilisation ofraw materials. Investigation aimed at improvingtraditional composting methods is difficult becauseprecise ecological control is not possible during Phase I,and activity analysis is frustrated by temporal and spatialvariability in composting stacks. Alternatively,development of environmentally controlled composting

(ECC) methods is a means of better understanding theprocess, and of improving commercial practice.Controlling and understanding mushroom composting areinseparably linked.

Various approaches to mushroom compost productionhave been reviewed by Fermor et al. (1985). Recently,various investigators (Smith 1983; Laborde et al. 1986;

Gemts 1987; Pem n and Gaze 1987; Miller et al. 1990)have produced mushroom composts under conditions ofmuch greater environmental control within enclosedsystems. While control strategies have varied, mushroomcropping yields on such composts have beenencouraging. The different approaches have arisen due touncertainty in the defini tion of a good compost formushroom production.

Compost is an ecosystem that responds strongly tochanges in physical factors that select for microbialactivity; different processing scenarios can lead todifferent outcomes. Conversely, composting activity canprofoundly affect physical factors, such as temperature,O2 concentration and moisture content. New processingvariations can be difficult to implement because of anincomplete knowledge of the physical factorsinfluencing compost processing. Rational engineering ofECC systems requires more information on the physicalconsequences of activity, while better engineered

systems can foster a greater scientific understanding ofcomposting ecosystems.

Temperature is a primary factor in controllingcomposting activity (MacGregor et al. 1981; Miller et al.1989~) . Management of metabolically released heat isthe means of controlling temperature. The amount of O2also determines activity, but it can be managed


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conveniently in conjunction with a heat managementstrategy based on ventilation (Finstein et al. 1986).

Observations on heat and mass changes occurringduring composting have been made for straw (Carlyleand Norman 1941), refuse (Wiley 1957), sewage sludge(Miller 1984), leaves (Finstein et al. 1986), and ricehulls and flour (Hogan et al. 1989), but only preliminarywork has been reported for mushroom compostingsubstrates (Miller et al. 1989b). In practice, maximalrates of heat evolution define the heat removal capacityneeded to control temperature. Similarly, peak O2demand defines the minimum ventilation required tomaintain aerobic conditions. Other physical factorsrelated to heat, mass and gas transfer are needed torefine system control.

In a previous paper, Miller et al. (1990) described aseries of mushroom composting trials in which an ECCsystem was used to gain uniform control overtemperature and O2 concentration. The rationale for, andresults of, processing compost under moderatelythermophilic and aerobic conditions, including cropyield and other cultural data from growing trials, werereported. Continuing the analysis of these previousinvestigations , this paper examines the results of severalECC trials with reference to heat, mass change and otherphysical factors.

Materials and methodsComposting reactor

The trial ECC system used here is based on astandard (26.4 m3) refrigerated container (often referredto as a ' tunnel'), with a forced ventilation system (Fig. 1).Previous work (Miller et. al . 1989b) showed that a

recirculation fan with a nominal capacity of 0.7 m3/s wassufficient to control temperatures to within the upperlimits of mesophillic activity (40-55OC) for a vessel thissize. An axial fan (Woods, 0.37-m-diameter, 2500 rpm)delivered approximately this amount with a maximumstatic head of about 390 Pa (40 mm water). This highrate of recirculation was used to minimise thetemperature gradient across the compost and to createuniform conditions within the compost mass.

The exhaust/recirculation proportion was controlledby a balancing damper located midway between theexhaust and inlet ports. Fresh air intake was controlledby a servo damper system. Control was based on airtemperature on the intake side of the system after freshair had been drawn in through a filter. The damper wasactivated by a temperature controller, although manualoffsetting of this damper was used in the early part oftrials to ensure aerobic conditions. Air exhaust wasdetermined by the rate of fresh air induction. With thefresh air damper fully closed, leakage in this system wasaround 3 of the recirculation rate. Air from the fan wasdistributed along open-topped, longitudinally orientated,

Fig. 1 Main elements of the composting tunnel construction. Stippledarea represents compost in the tunnel during routine operation; m o w srepresent the direction of airflow. 1, insulated structure of the modifiedcontainer; 2, tunnel free headspace above compost; 3 air-permeablefalse floor for ventilation; 4, ventilative underduct; 5, inlet airtemperature control sensor; 6, in duct fan; 7 automatically controlledfresh air inlet damper; 8, fresh air inlet with air filter; 9, exhaust port;10, manually set exhaust flow control balancing damper; 11, exhaustair temperature sensor; 1 2, recirculated air; 13, mixed fresh and

recirculated air; 14, double doors with air seals for loading andunloading.

5-cm-deep, aluminium channels in the tunnel floor, andinto the compost. The floor consisted of shade mesh,underlain with 4-cm steel mesh, which was in turnsupported by hardwood planks, resulting in a total airporosity of about 60 .

MeasurementsWherever possible a datalogger (Datataker DT100F)

was used to collect information. Time interval ofscanning was normally 5 min. Datalogging was used forthe following parameters.

Compost temperature. Type T thermocouples werefixed to wooden dowels and inserted in the compost atloading. Both horizontal and vertical orientations wereused to monitor variations in temperature within thecompost.

Air intake and exhaust conditions. A combination ofplatinum resistance and accurate solid state temperaturethermometers were used to monitor inflow and exhausttemperatures. Wet and dry bulb temperatures of theintake air were measured by a forced aspiration systemin a standard Stevenson screen, placed as close aspractical to the fresh air intake.

Mass loss. Electronic scales (Barlo Titan 10) with amaximum capacity of 10 t were placed under the

container for later trials (trial 6 on), and the totalcontainer and compost mass was measured. In addition,compost volume was assessed by visual estimation ofcompost height through an access port.

Air flow recirculation and fresh air intake rates.These were measured by pressure differentials acrossrounded and sharp-edged orifices, respectively, and theuse of standard American Society of Heating


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Refrigerating and Air-conditioning Engineers(ASHRAE) orifice equations (ASHRAE 1981), or byderivation from mass loss rates as explained below. Adifferential pressure detector was installed across theintake orifice and continuously logged. The pressureprofile within the compost was measured by placingrigid plastic tubing taped to dowels in the compost.Permanent fixtures under the floor, above the compost,and in pressure taps in the ventilation ductwork werealso used. Simple flu id manometers were used tomeasure pressures.

Data collected manually at 4-8 h intervals includedgas concentrations and compost sampling. Ammonia andC 02 were measured with Draeger or Gastec tubes. Sideby side comparison of these 2 tubes gave very goodagreement. Oxygen was measured with either a Teledyne3 2 0 P P or a Draeger Oxywarn 100 O2 analyser. Gassamples were taken from sample ports on the exhaustside of the recirculation system before fresh air addition.Occasionally, 0 samples were withdrawn from the

pressure profile installation, to monitor the 0 usageprofile within the compost. At the beginning and end ofeach trial, compost was subsampled throughout loadingand unloading. Samples were analysed for moisturecontent, Kjeldahl N, NH3, pH, ash and total carbon(see Miller et al. 1990). At the completion of some trials,detailed moisture and density samples were taken atknown locations.

Data analysisDue to the enclosed nature of the ECC system, with

point source exhaust and entry ports, the overall massbalance can be expressed in finite difference form, overshort time periods, as

dMldt o

where M is mass; t is time; and q and q re the massflow rates at the exhaust and inlet ports, respectively. Inaddition, mass loss can be partitioned to dry matter andwater losses:

M = M d W r

where Md is the compost dry matter, and Wr is the waterremaining. Correction can be made for the metabolicproduction of water, assuming 0.55 kg of water isproduced under aerobic conditions (Griffin 1977; Hoganet al. 1989) for each kg of dry matter lost. Alternatively,metabolic water production can be estimated from thedifference between the calculated and the measured finalmoisture content. The partitioning of mass losses tothese components requires individual gas components tobe evaluated.

Gases of importance are NH3, CO,, 02, H20, N2 and ArSummation of the components per unit mass of ry air isconducted at both the inlet and exhaust ports, andincludes the change in the mass leaving the container as

a result of biological activity and increasing moistureload. Nitrogen and Ar are assumed to be inert withrespect to the composting process. Other gas masses arecalculated from measured concentrations NH3, C0 2 andO2 at the exhaust port), or from standard atmosphericvalues (0 2, 20.9 ; C02 , 0.03 ; and NH3, 0 byvolume at the inlet port) by the use of the gas laws, withcorrections for non-ideal behaviour (Barrow 1973) beingapplied. Moisture loads are calculated using ASHRAE(ASHRAE 198 1) and ASAE (Young and Day 1988)psychometric equations. Measured wet and dry bulbtemperatures are used for the inlet air, and the exhaust airwas assumed to be at 100 relative humidity (RH)(Hogan et al. 1989).

Air intake can also be expressed in terms of volumeper unit time, using the moist air density evaluatedthrough the ASHRAE equations (ASHRAE 1981). Aftercalibrat ion of the airflow pressure transducer, air volumeflow rates can be used to calculate mass flow rates, andhence dM/dt.

Following this mass balance, a heat balance equationcan be constructed:

where H is the heat production rate (W); Eo and Ei rethe enthalpy out and enthalpy in (W); is the summationof all heat storage components (J), the importantcomponents being compost moisture and dry matter, thecontainer, and the internal air volume; K is the apparentthermal conductivity (WI0C) of the container andventilation system as a whole; dT is the thermal gradient(OC) from the compost to ambient conditions; and P isthe mechanical gain in energy (W) from the fan.

The enthalpy components are calculated using theASHRAE equations (ASHRAE 1981) and the mass f lowrates calculated above. Corrections are made for-theincreased mass and altered specific heat capacity of theexhaust air (Smith and Van Ness 1975). The thermalproperties of the compost dry matter are assumed to besimilar to those of 'haylage' (Jiang e t a l . 1986).Calibration of the tunnel heat loss and fan energy inputwas undertaken. Container heat loss is taken as 64 WI0C,and the fan energy input as 1.44 kW. Most results arepresented, for convenience, in units of MJkg of moistcompost, or a rate derived from these units.

basic sensitivitv analysis of the computer programsshows P and K towhave negligible effects on heatproduction calculations. Error analysis (Fritschen andGay 1979) shows the component with the largestuncertainty to be the saturated vapour pressure at theexhaust port. Calibration shows the error in temperaturemeasurement at the exhaust port to be 0.2OC. The othermajor source of error is the uncertainty in themeasurement of the 0 concentration, which, in theabsence of C 0 2 measurements, was used to calculate the


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mass of exhaust air and, hence, to partition moisture anddry matter losses. These errors, and numerical errorswhen running the model, accumulate to


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20 4 6 8 1 12 14 16 18

Time h)2 4 6 8 1 12 14 16 18

Time h)

Fig 2. Trial 15. Relatio nship of various parameters with time. Cooldown events occur at 1 0 and 8 0 h, and a small adjustment is made at 8 5 h tostabilise the compost temperature at47OC Composting is completed at 180 h, when the cooldown for unloading begins.a) Mean compostemperature -), and ir inlet - -) and exhaust - - -) temperatures; b) resh ir intake rate m3/s.kg compost initially loaded);c) recirculationate m31s.kg compo st initially loaded); 4 oxygen concentration in the recirculationir in the exhaust side @), and 0 2 usage -); e) proportion

of 0 2 n the fresh air intake quantity utilised in the compost;f) ammonia concentration *), nd calculated losses in kg/kg compost.s - - -) from theexhaust port; g) heat output s a function of time; h ) components of heat removed, s percentage of total latent heat - ), and sensible heatosses in exhaust air - -), and con ductive heat losses from the container and ventilation system-).


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Physical management and interpretation of cornposting 663

heat production was markedly reduced, and in someinstances, temperature decreases were recorded beforemore oxygenated conditions could be re-introduced.

Trial 15 demonstrates the main pattern of O 2 usageduring the composting process. Oxygen usage increasedduring the temperature build-up to pasteurisation andpeaked in the 55-63OC range (Fig. 2 4 . Cooldown toconstant temperature then began, so data for highertemperatures were unavailable. Oxygen usage declinedafter the cooldown, typically to around 5 0 4 0 of thepeak value. Frequently, a smaller increase in O2 usagewas found, as seen during the 54OC phase at 40 h,presumably due to th e new populations utilisingremaining substrate and/or unstable products of the hightemperature stage. Similar, but smaller, rises were notedoccasionally after establishment of the 470C phase.

Peak O2 usage for trial 15 was about 9 x 10-7 kg02/kg .s (moist compost) or, in terms of fresh air at 20C,about 0.025 m3/s. Figure 2e shows the proportion of thefresh air intake required to maintain aerobic conditions

in the compost. During peak O2 demand, this proportionreached 40 of the manually set intake rate. At thistime, the measured O2 concentration in the exhaust airdropped to around 12 . Previous experience showedthat O2 concentrations lower than this could result intemperature declines, as mentioned above. After thecooldown, 2 rose to about 19 , and slowly increasedwith time throughout the tr ial. A decline in theproportion of the fresh air intake required to oxygenatethe compost was evident. From the cooldown to 54OCuntil the end of the trial, the majority of fresh air wasrequired to remove excess heat rather than to supply O2to the compost. Trials conducted at 47OC were moremoderate in O2 demand, exhibiting a demand similar tothat seen in the 54OC constant temperature period fortrial 15.

Amm onia production and releaseTypically, NH3 concentrations in the exhaust and

recirculation air rose to their peak values within 20 h.Peak NH3 values coincided with peak temperatures intrials where temperatures were allowed to rise to 63OC.As the fresh air intake was at a minimum untilpasteurisation was completed, the location of this peakcan be explained by rapid volatilisation of NH3,combined with a low exhaust rate. Losses andconcentrations of NH3 for trial 15 are shown in Fig. 2f.

Trials at constant temperature (47OC) demonstratedsignificantly lower peak NH values but similar overall

NH3 losses from the system. High temperatures werefound to affect NH3 concentrations. In trial 14, whichinadverte ntly heated to 70-720C during the initialcomposting phase, the highest peak value of 1500 pL/Lwas recorded. This resulted in the highest NH3 exhaustrat e of 2.5 10-8 kg/s.kg compost. It also provedimpossible to clear NH3 in the 47OC phase of this trial,

even by extending the processing time to 250 h.Ammonia remained at 200 pL/L, which implies avolatilisation rate of about 8.3 10-9 kg/s.kg compostthroughout this period. In a second trial (trial 13), wherethe straw was heated to 80C before mixing andcomposting, total NH3 losses again were high. Wherepasteurisation was completed at more moderatetemperatures, the volatilisation rate had fallen to zero bythe end of the trial.

Calculation of total N at the beginning and end ofeach trial indicated large inaccuracies in either thesampling scheme or the chemical analysis technique, asa net gain in total N was recorded in 2 trials. The massflow model was therefore the preferred method ofcalculation for NH3 losses.

Trials gave varying losses, ranging from 3.6 x 10-5 kgNH3/kg compost for trial 7 to 1.1 1 0 4 kg NH3/kgcompost for trial 14. Trial 7 was a 47OC constanttemperature trial, with low initial N (1.44 ).

No odiferous compounds other than NH3 were

detected while composting was aerobic. Anaerobicconditions resulted in unidentified, but pungent, odours.

Comp ost density mass loss and moistureMass loss during cooldown events was almost totally

due to loss of water vapour, as indicated above. Veryearly in processing, when the fresh air intake was small,gravitational drainage of water from the compost, andsubsequent leakage from the tunnel, was sometimesresponsible for very high mass loss rates. When thisoccurred, these time periods were excluded from modelcalculations.

Both the wet and dry compost densities showed aninitial rise. This initial increase was the result of thedecrease in compost height due to self loading and,

hence, compression of the underlying compost in thefirst 24 h. The decrease in compost height then slowed.After this period, wet density demonstrated anexponential decline. Dry matter loss was a much smallercomponent of mass loss and remained relatively constantthroughout the remainder of the trial. The final wet anddry densities were similar through all trials, having meanvalues of 298 and 101 kg/m3, respectively. However, thefinal density at unloading had little effect on thecropping density, as the compost was pressed into thetrays by a hydraulic ram.

Sampling at the completion of trials showed littlehorizontal variation, but a large vertical change indensity. Dry matter density increased with depth, from a

surface value of 95 kg/m3 to a maximum of 170 kg/m3about 0.3 m from the floor, and appeared to be a result ofself loading and compression of the base compost. Theconstant density below 0.3 m suggests that the composthad reached maximum compressibility and that higherinitial loadings are unlikely to increase greatly the basedensity of the compost. This increase in density, and


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therefore reduction in the pore space available for airflow, contributed to the large pressure gradien t measuredacross the compost.

A pronounced moisture gradient from the base to thesurface was noted in all trials, averaging 4 in trial 15.Evaporative drying, particularly in a 0.15-m-high regionnear the floor, was extensive. gradient in moisture wasalso present lengthways down the tunnel, the compostbeing 4 wetter on average at the end furthest from thefan. This effect was presumed to be due to uneven airflow distributions, the increased path length down thecontainer increasing the resistance to air flow. Localisedareas of very high moisture content (in excess of 80 )also occurred at sites where condensation formed. The2 major causes of condensation were air leakage fromseals (near examination ports and doors), and theformation of condensation on the exposed walls and roofvia conductive heat losses.

The final moisture content predicted by the model fortrial 15 was 64.6 , which compared favourably with themeasured value of 65.7 . This discrepancy may beexplained by the uncertainty in O2 concentration and,hence, in the partitioning of losses to water and drymatter losses, but more likely, it was due to variability inthe compost moisture content. Calculated metabolicwater production ranged from 0.29 to 0.71 kgkg drymatter lost, with a mean value of 0.48 kg/kg. While thismean is similar to that found by Griffin (1977) andHogan et al. (1989), the large range for similar substratesindicates sampling or analysis error.

Heat production and removalTotal heat production was insensitive to the

temperature of composting. The 47OC constanttemperature trials, with the exception of trial 11, releasedamounts of heat similar to the higher temperature regime(Table 2). Multiple regression of the initial startingmixture components against total heat production

Table 2 Heat output MJ/k g compo st), dry matter, initial volatilematter or decomp osed material, and heat output rate kJ1kg.h) for

trials 6 15

Trial Compost ry Initial Decomposed Heat outputmatter volatile matter material (kJ/kg.h)

6 1.42 5.70 6.47 18.17 1.01 4.35 5.56 16.68 1.14 4.96 5.78 19.2

9 1.05 4.70 5.57 15.410 1.36 5.58 6.46 16.511 0.92 3.06 4.0412 1.51 6.70 7.57 20.613 1.34 5.61 6.84 22.014 1.34 5.45 6.47 19.915 1.21 5.00 5.74 16.7Mean 1.23 5.11 6.01 18.3

1 2 3

Time h)

Fig. 3 Cumulative heat output as a function of time for all trials- - 470C constant temperature trials; -, pasteurised trials).rial 15 is indicated.

showed the only significant factor (at a 15 level) to bethe quantity of deep litter DL) used.

Total heat production ranged from 0.92 to 1.5 1 MJ/kgmoist compost loaded. The low value for trial 11 is theresult of pre-composting outside the tunnel for 4 days,when any heat released is unaccounted for; however,trials 7 and 9 also received pre-composting, but thisresulted in higher total heat production. Trial 9 did notinclude a DL component, cotton seed meal being the onlyN source (straw/cotton seed meal, 0.43). As DL is astrong microbial inoculum, the high heat yield of trial 9may have been due to a slower biomass build-up in thepre-composting period. Trial 7 had a low DL component(s trawPL, 0.24) and only 2 days pre-composting.

There was little difference in heat output between the47OC constant temperature trials and trials whichreceived pasteurising temperatures (Fig. 3). Heatproduction in the pasteurising trials was higher duringthe temperature build-up stage than in the constanttemperature trials, but this period of high heat output waslimited. Heat production for trial 15 is shown in Fig. 2g.In common with the other pasteurised trials, heatproduction reaches a maximum in the 55-63OC range(see Fig. 2a). The maximum value recorded in all trialswas 9.0 W/kg of initial compost (or 38 W/kg initialvolatile matter) in trial 14.

Of note was the extremely rapid increase in heatoutput (Fig. 2 g , indicating a rapid biomass increase.When fresh air was added there was a decline in heatproduction. Heat production did not reach the same levelafter pasteurisation, but in common with 2 usage, itshowed a second peak around 40 h. The establishment ofa new, stable population structure, and the utilisation ofunstable products from the pasteurisation stage, mayhave been the cause. Heat production then gradually fellas substrate nutrition became limiting. The constant


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relationship indicated that heat was produced by asimilar mechanism throughout the temperature range

7 employed.

Discussion6

w The ECC system used in these trials performedadequately under the designed compost loadings and

5 gave good temperature and O2 control. Temperaturec variations vertically through the compost were generally

. 4


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within the temperature range used here could bepredicted reliably from this relationship.

Most work on microbial heat production has beencarried out in small scale, pure culture systems. Incontrast, the ECC system is a-large scale, mixed culturesystem which may lead to differences in total substrateutilisation. Previous measures of heat output inmushroom composts have only been made in small scaleeactors (Miller e t al 1989). In this work, heat outputanged from 8.7 W/kg at 550C to 10.5 W/kg at 45OC on

a volatile matter basis, with rates averaged over 12-hperiods. A similar analysis for trial 15 results in a heatoutput of 18 W/kg volatile matter at 550C. Peak heatoutput over short periods, although known lessaccurately than the above averages, is about 30 W/kgvolatile matter in the 55-630C range. This increased heatoutput is due to the ECC system being operated underconditions for optimal activity early in the processing,which, in turn, builds up a large biomass. Total heatproduction (Table 2) is similar to that of otherenergy-dense composts. Using the small scale reactormentioned above, Miller (1984) obtained a range of15.2-21.8 MJ/kg decomposed dry matter for seweragesludge .and wood chip mixtures. Hogan t al (1989),using the same system with a rice hulls and rice floursubs trate , obtai ned a range of 14.2-16.7 MJ/kgdecomposed dry matter. The large range in total heatoutput encountered in these trials may be due to widelydiffering initial components and to the effect of heat lostduring pre-composting. Attempts to predict the heatoutput from the initial components were unsuccessful.While deep litter is a major determining factor in heatoutput from the compost mixtures used here, thevariability in moisture-and composition precludes the

development of any general relationship. Pre-compostingmay lower total heat output if significant activity isallowed to occur before loading. Measures ofcomposting activity through heat evolution in the ECCsystem may provide a means of improved processmanagement.

Loss of mass in ECC is mostly through loss of water,which is used to cool the compost evaporatively. Drymatter losses are restricted to around 30 . This is asignificant saving in raw materials compared withconventional composting, which can result in ry matterlosses of up to 60 . However, dry matter losses inconventional composting are strongly affected by thelength of the Phase I stage (Flegg and Randle 1981), anddry matter losses similar to those of ECC can beobtained by shortening Phase I to 3 days.

Air flow is the most important design factor in ECCsystems. Sufficient air capacity should be provided tocarry away the maximum heat load. The quantity offresh air required for this is about 90 m3lt.h (assumingthat fresh air RH is 80 at 250C). More energy-rich or

energy-dense composts may have even higher heatoutputs. Heat production at the 54OC constanttemperature stage may provide a guide: using the aboveassumptions, this period would require around 45 m3lt.h.Ventilation capacity should obviously be based on theheat removal requirement rather than the O2 demand ofcompost.

The ECC system can produce information for therational design of other large scale composting systemswith substrates other than mushroom compost. Further,basic research is made possible in a realistic system thatpermits execution of controlled environmentinvestigations into composting ecology. Microbialactivity can be monitored as it occurs by measuringmetabolic heat evolution. The effects of various physicalfactors on composting activity can be determinedthrough direct experimentation. Control of physicalvariables can also permit investigations into substratenutrition and subsequent mushroom nutrition. Spatialuniformity and controlled environment allow meaningful

population studies in a system of special interest for itsrapid population changes.

cknowledgmentsThis research was supported by grants from the

Australian Mushroom Growers Association and theAustralian Horticultural Research and DevelopmentCorporation. The authors would like to acknowledge theextensive assistance given by the staff of MelbourneMushrooms Pty Ltd, David Blunt and Frank Bizzotto(Bulla Mushrooms Ltd), and Kevin Gorman (KevinGorman and Associates).

ReferencesAmerican Society of Heating Refrigerating and Air-

Conditioning Engineers. (1981). ASHRAE Handbook 1981Fundamentals.' (American Society of Heating Refrigeratingand Air-conditioning Engineers Inc.: Altanta, GA, U.S.A.)

Barrow, G. M. (1973). 'Physical Chemistry.' (McGraw-HillKogakusha: Tokyo.)

Carlyle, R. E., and Norman, A. G. (1941). Microbialthermogenesis in the decomposition of plantmaterials-Part 11. Factors involved. Journal ofBacteriology 41 699-724.

Cooper, S. C., and Sumner, H. R. (1985). Airflow resistance ofselected biomass materials. Transactions of the AmericanSociety o f Agricultural Engineers28 1309-1 2.

Fermor, T. R., Randle, P. E., and Smith, J. F. (1985). Compostas a substrate and its preparation. In 'The Biology andTechnology of the Cultivated Mushroom.' (Eds P. B. Flegg,D M. Spencer and D A. Wood.) pp. 81-110. (John Wileyand Sons: Chichester, UK.

Finstein, M. S., Miller, F. C., and Strom, P. F. (1986). Wastetreatment composting as a controlled system.In 'Biotechnology, Vol. 8.' (Eds H. J Rehm and G. Reed.)pp. 363-98. (VCH Verlagsgesellschaft: Weinheim,Germany.)

Flegg, P. B., and Randle, P. E. (1981). Relation between theinitial nitrogen content of mushroom compost and theduration of composting. Scientia Horticulturae 15,9-15.


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Physical Management and Interpretation of an Environmentally Controlled Composting Ecosystem