Effect of air recirculation and reuse on composting of organic solid waste

Resources, Conservation and Recycling

33 (2001) 93–111

Effect of air recirculation and reuse oncomposting of organic solid waste

Quazi H. Bari a, A. Koenig b,*a Department of Ci�il Engineering, Bangladesh Institute of Technology, Khulna-9203, Bangladesh

b Department of Ci�il Engineering, The Uni�ersity of Hong Kong, Pokfulam Road,Hong Kong, PR China

Received 14 December 1999; received in revised form 2 March 2001; accepted 29 March 2001

Abstract

Composting has become increasingly popular in the past decade as a biological treatmentprocess of organic solid wastes, generated from different sources, with the purpose ofrecovery, stabilization and volume reduction of waste material in the form of compost. Inthis study, the effect of different modes of aeration on composting of solid waste using twoheat insulated closed pilot-scale reactors was investigated. The modes of aeration studiedwere upflow, downflow, alternate upflow/downflow, and internal recirculation in a single-re-actor system as well as reuse of spent air in a two-reactor system in series. Composting testswere performed in two stages of 20–30 days duration each. Temperature at different heightsof the composting mass as well as air velocity were continuously monitored by a datalogger.Air flow was continuous or intermittent depending on temperature. Oxygen content in thespent air was regularly measured. The results show that (i) the application of unidirectionalupflow or downflow aeration creates significant vertical distribution of temperature in thecomposting mass; and (ii) internal recirculation of air in a single-reactor system and reuse ofspent air in a two-reactor system appeared to achieve a more uniform temperature distribu-tion and thereby accelerated degradation of the organic matter. Practical applications incomposting plants are discussed. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Composting; Aeration mode; Degradation; Recirculation; Solid waste; Temperature

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* Corresponding author. Tel.: +852-2859-2655; fax: +852-2559-5337.E-mail address: [email protected] (A. Koenig).

0921-3449/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

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Q.H. Bari, A. Koenig / Resources, Conser�ation and Recycling 33 (2001) 93–11194

1. Introduction

The increasing rate of solid waste generation from different sources, limitedlandfill space and more stringent environmental regulations for new landfill sitesand incinerators have increased the waste disposal fees in many countries of theworld, especially in developed countries. Therefore, municipalities and local govern-ments are under heavy pressure to find sustainable and cost-effective solid wastemanagement policies. Biological treatment offers a cost-effective sustainable solu-tion for urban organic wastes. In practice, the main biological process applied forsolid wastes is composting (Tchobanoglous et al., 1993).

Composting is the biological degradation of highly concentrated biodegradableorganic wastes in the presence of oxygen (aerobically) to carbon dioxide and water,whereby the biologically generated waste heat is sufficient to raise the temperatureof the composting mass to the thermophilic range (50–65°C). The final product ofcomposting is a stable humus-like material known as compost. The popularity ofcomposting has increased in the past decade due to the associated environmentalbenefits such as:� fast conversion of the organic solid waste to a biologically stable end product

(Christensen and Nielsen, 1983);� recovery of waste material in the form of compost for utilization in agriculture,

horticulture or other applications as a soil conditioner, organic fertilizer andlandscaping material;

� effective hygienization of pathogenic bacteria present in the organic waste(Willson, 1983);

� stabilization and volume reduction of the waste materials prior to environmen-tally sound final disposal in landfills; and

� cheap and effective solid waste treatment methods (Stentiford et al., 1985; Sesayet al., 1998).Small-scale composting of nightsoil and other organic wastes has been used

successfully in Chinese agriculture since time immemorial. Currently many largecities in China and south-east Asia are planning to erect or improve existingmunicipal waste composting plants with capacities of up to 1500 t/day (Raninger,1996). In Hong Kong, organic wastes constitute approximately 25% of the wastedisposed of in landfills (Draft Waste Reduction Plan for Hong Kong, 1997) andapproximately 35% of domestic waste (Environment Hong Kong, 1995). To reducethe long-term environmental emission problems of the landfills due to leachate andlandfill gas, it may be necessary for Hong Kong to establish new composting plantsfor separately collected organic waste. A composting plant for livestock wastes with75 t/day capacity is already in operation in Hong Kong.

Recent applications of large-scale composting have often been plagued bytechnological problems, lack of understanding of the biological fundamentals, andunsuitable waste materials resulting in unacceptable product quality as well asenvironmental nuisances such as odors, leachates, and contaminated aerosols. Theformer Chai Wan composting facility, Hong Kong, represents such a negativeexample. The forced aeration composting process has been proposed to overcome

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the above problems (ORCA, 1991). However, few scientific results or evaluationshave been published on full-scale forced aeration composting plants (Baumann etal., 1995) and more work needs to be done to determine the best control methodsand design/operation specifications (Hyatt, 1995). In recent years, several pilot-scalestudies have been conducted, emphasizing comparison of forced and naturalaeration methods or control of oxygen concentration in the composting mass(Leton and Stentiford, 1990; Sartaj et al., 1997; Fernandes and Sartaj, 1997; Vander Gheynst et al., 1997). All pilot-studies reported vertically non-homogenoustemperature and moisture distribution in the composting mass, but no specificattempts were made to evaluate or control this process-limiting condition. Thepurpose of this pilot-scale study was to systematically investigate various forcedaeration modes to improve the vertical distribution of temperature and moisturethroughout the height of the composting mass, thereby increasing the rate andextent of organic waste degradation. Special emphasis was given to evaluate theaeration modes employing recirculation of spent air in a single-reactor system andreuse of spent air in a two-reactor system. The pilot-scale experiments weredesigned to obtain a better understanding of the effect of aeration methods onprocess efficiency and to extend the findings for improved design of future compost-ing plants.

2. Materials and methods

2.1. Source and type of wastes

For the composting tests, organic solid wastes from the University of HongKong were used. These consisted of two types of waste, namely food waste (fromthe student canteen) and mixed office waste paper. In addition, saw dust from theUniversity Industrial Center was used as a bulking agent. The average initialmoisture content of the raw, presorted food waste, wastepaper and saw dust was71.7, 8.5 and 8.3%. The initial waste mixtures of all tests contained approximately75.0, 12.5, and 12.5% by wet weight of food waste, wastepaper, and sawdust,respectively. The mixing procedure for the preparation of the initial waste mixturesfor composting and their composition has been reported elsewhere (Koenig andBari, 1998).

2.2. Pilot-scale reactors

Two specially designed heat insulated closed batch reactors were used in thisresearch. Each reactor had a total volume of approximately 200 l, with a height of2.2 m. The reactors were insulated with a 10-cm-thick layer of polyurethane on allsides. The waste mixture was put into the reactors from the top, which could beeasily opened. The thermocouples for measurement of temperature during thecomposting tests were placed inside the reactor at different depths of compostingmass: at 10, 30, 50, 70, 90, and 110 cm, respectively, starting from the bottom.


Forced aeration was provided by an air blower, with an option for continuous orintermittent operation. After leaving the reactor, the hot spent air cooled downnaturally by passing through a large diameter vertical aluminum pipe (approx. 2 mlong). The lower end of the pipe was connected to a collection chamber for thecondensate. Finally, prior to discharge to the atmosphere, the spent air was filteredthrough a biofilter (cross-sectional area 0.09 m2), which was made up of a lower layer(20 cm) of gravel and an upper layer (90 cm) of mature compost with a moisturecontent of 55–65%. The pipes for air recirculation or reuse were insulated with a5-cm-thick layer of Fanyalon to prevent heat loss and minimize condensation. Internalair recirculation could be varied according to the air recirculation ratio r which isdefined as the ratio of recirculated air over fresh air input, in percent. The flow ofreused air was equal to the flow of spent air from first stage composting, i.e.approximately equal to fresh air input. Leachate was collected at the bottom of thereactor column in an intermediate storage tank and intermittently recycled to the topof the column. A schematic diagram of the experimental pilot scale two-reactorcomposting system in the upflow mode is shown in Fig. 1. These two reactors couldbe used either as single reactors or as a two-reactor system in series by modifyingthe valves and pipe connections.

2.3. Experimental program and mode of aeration

A number of extensive pilot-scale composting tests were performed in two or threestages of 20–30 days duration each. The modes of aeration studied were upflow,downflow, up/down flow, and internal recirculation in a single-reactor system as wellas reuse of spent air in the two-reactor system in series. The first stage compostingtests were conducted using fresh organic solid waste mixtures. For the second stagecomposting tests, the fresh compost produced in the first reactor was removed, re-mixedand adjusted for moisture content prior to transfer into the same or the second reactor.The same procedure was used for the third stage composting tests. The aeration modesand duration of the different composting tests are summarized in Table 1.

Based on previous composting tests (Koenig and Tao, 1996) the range of air flowrate was set at 17–20 l/min (equivalent to approx. 0.67 l/min per kg initial VS) duringthe first 2 days. This was followed by an air flow rate of 8–10 l/min according tothe following rule: (i) if the temperature at any measuring point in the compostingmass exceeded 60°C, continuous aeration; (ii) between 55 and 60°C, intermittentaeration according to a pre-set cycle of 5-min aeration and 5-min pause; and (iii) fortemperatures in the compost mass below 55°C, intermittent aeration according to apre-set cycle of 5-min aeration followed by 10-min pause.

No replicate tests were performed because of (i) the large scale at which theexperiments were carried out, with full practical height of the composting mass asin actual composting plants, rendering replication impractical; (ii) the time limit dueto the long duration of the experiments; and (iii) the need to test a wide range ofdifferent aeration modes. Besides, it has already been demonstrated that replicate testswith pilot-scale reactors are capable of providing reproducible results (Van der Gheynstet al., 1997).


2.4. Physico-chemical monitoring and analyses

Temperatures at different heights were continuously monitored by means of thethermocouples which were connected to a datalogger. In addition, for the determi-nation of air flow rates, air velocity was continuously monitored by a dataloggerthrough an air velocity transmitter. Oxygen content in the spent air was regularlymeasured by means of an oxygen meter (MSI Elektronik GmbH type MSI 150). Atthe beginning and end of each composting test, the initial waste mixture (foodwaste and waste paper) and/or compost was analyzed with regard to the mostrelevant physico-chemical parameters. All analyses were carried out according tothe Austrian standard methods for analysis of compost, which are widely used inEurope (OENORM S 2023, 1986).

Fig. 1. Two-reactor composting system with reuse of spent air in the upflow aeration mode.


Table 1Experimental program and setup of pilot-scale composting tests

Reactor systemStage Aeration mode Duration (days)Test

Upflow1 28SR1aUpflow/downflow for 10 days, reuse of air21b 28 (56)*TRfrom stage 1Downflow1 24SR2a

2b SR Upflow 32 (56)23a SR Upflow, internal recirculation (r=100–200%) 291

Upflow/downflow for 17 days, reuse of air2 TR 39 (68)3bfrom stage 1Upflow, internal recirculation (r=20–80%) 21SR14a

TR2 Upflow/down flow for 9 days, reuse of air4b 20from stage 1

SR3 Upflow/downflow alternatively every 3–4 days 11 (52)4c

a=first stage; b=second stage; c= third stage; SR=single-reactor system; TR= two-reactor system;r=recirculation ratio; *total duration including first, second and third stages.

2.5. Self-heating test

The self-heating test was applied to determine the degree of biological stability ofthe waste after composting (Koenig and Bari, 1998; LAGA, 1985). In the self-heat-ing test, suitably prepared waste samples of optimally adjusted moisture content areloosely filled into Dewar bottles (volume=1.5 l, inner diameter=100 mm) open tothe atmosphere. A temperature sensor is inserted for monitoring. The bottles arethen kept at room temperature of approximately 23°C. If the waste is not yetbiologically stable, it will further degrade aerobically, generating heat which willcause a temperature rise. Usually, the maximum temperature Tmax is reached after2–5 days. The test ends after Tmax is culminated and rapidly declining temperaturesare observed, at the latest after 10 days. Tmax attained is used as an indicator ofbiological stability to define the stability index SI. The SI of waste is classified indegrees from I to V, in ascending order of biological stability, and ranges from raw,unstabilized waste (SI=I) to completely stabilized waste (SI=V). Tmax alsocorresponds approximately to the area under the temperature curve A72 (after 72 h)as shown in Table 2 (LAGA, 1985).

3. Results and discussion

3.1. Changes in physico-chemical parameters of compost

Changes in total mass, moisture content (MC) and volatile solids (VS) duringpilot-scale composting tests are presented in Table 3, with MC reported on a wetbasis and VS on a dry basis. The initial weight of waste put inside the reactor forthe first stage tests varied from 55 to 77 kg and for the second stage tests from 41


to 52 kg. Initial moisture content of the waste mixtures varied between 55 and 65%with an average of 58.7%, thus staying within the recommended range (Koenig andBari, 1998). The final average moisture content of first stage tests ranged from 31%to 47% and that of second stage tests ranged from 51 to 66%. In all initial wastemixtures, volatile solids amounted to more than 94% of the total solids; however,the volatile solids originating from paper and sawdust were considered less readilybiodegradable due to their high cellulosic content. Depending on biodegradablevolatile solids content in the initial waste mixture, the total VS degradation indifferent tests after second or third stage varied between 36 and 41%, which isconsidered quite high in view of the high cellulose content (mixed waste paper,sawdust) of the waste. Generally, first stage composting of approximately 26 daysduration comprised approximately 65–70% of the overall volatile solids removal,while approximately 30 days duration of second stage composting removed theother 30–35%.

3.2. Effect of aeration modes on temperature

The temperature patterns in different compost layers of tests 1a, 4a and 4b arepresented in Figs. 2–4. During the first 2 days of most first stage tests, evaporativecooling reduced the temperature in the layers near air inlet by 5–8°C from theambient temperature (23–25°C). Evaporative cooling also happened successively inthe top or bottom layers of test 4b, where alternate upflow/downflow aeration wasapplied before series connection. In all first stage tests, 1–1.5 days of lag periodwere observed in the temperature curves of all layers whereas the temperatures inmost second stage tests rose quickly within the first 6–12 h to thermophilic range(above 50°C).

In test 4a upflow aeration was employed with internal recirculation ratios of20–80% whereas in test 1a only upflow aeration was applied with no internal airrecirculation. The average temperature in test 4a remained above 50°C for morethan 16 days, while in test 1a the average temperature was above 50°C for 6 daysonly. The narrow band width between minimum and maximum temperature curves

Table 2Classification of waste according to degree of biological stability

Degree of IIIIIIIIVVUnstable�Stablebiological stability

(stability index SI)

20–30 30–40 40–50 50–60 60–70 �70Tmax (°C)0.3–0.45Imax (°C/h) 0.45–0.8�0.3 0.8–1.4 1.4–2.0 �2.0

Area A72 (°C×h) 1700–2000�1700 2000–2500 2500–3000 3000–3500 �3500

Tmax=maximum temperature; Imax=maximum temperature increase; A72=area under temperaturecurve after 72 h.

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Table 3Changes in total mass, moisture content (MC) and volatile solids (VS) during pilot-scale composting tests

Final VS �VSTest �VSInitial mass Totala �VSFinal mass Initial MC Final MC Initial VS(%) (kg) (%) (%)(%)(kg) (%)(%)(kg)

47.05 94.26 91.81 7.49 29.291a 37.1763.66 57.3989.77 1.79 11.17 37.1890.6941.801b 54.6857.7236.15

96.0960.33 95.30 6.33 24.7233.12 55.83 38.952a94.0441.28 92.15 2.88 18.43 38.5930.30 59.77 51.562b

94.75 6.34 25.3595.173a 31.0158.9332.2863.7491.54 2.63 21.31 41.263b 52.55 45.02 65.29 66.05 93.3793.03 5.68 23.8095.014a 37.2962.6834.9067.21

50.70 92.99 91.77 2.41 13.914b 33.9041.84 55.4691.90 0.34 02.67 36.1691.7730.89 55.9955.434c 31.05

�VS=VS degraded.a Total % �VS is determined on the basis of initial VS of the first stage. For calculation purposes, the final VS after the first stage should be equal to

the initial VS of the second stage. However, since the initial VS of the second stage was mostly less due to sample taking for physico-chemical analysis, theactual values of the initial VS and final VS for the second stage were proportionally adjusted for the calculation.


Fig. 2. Temperature variation at different heights of composting mass during first stage test 1a, upflowaeration.

in test 4a demonstrates clearly that the internal recirculation of air led to a morehomogeneous temperature distribution throughout the composting mass as com-pared to no recirculation.

Fig. 3. Temperature variation at different heights of composting mass during first stage test 4a, upflowaeration with internal air recirculation, r=20–80%.


Fig. 4. Temperature variation at different heights of composting mass during second stage test 4b,upflow/downflow aeration for 9 days and then reuse of spent air from stage 1. U=upflow aeration andD=downflow aeration.

In tests 1b, 3b and 4b reuse of spent air from first stage tests was applied in atwo-reactor system after 10, 17 and 9 days, respectively. In all these cases, anarrower band width between minimum and maximum temperature curves wasobserved after series connection thus creating a most homogeneous temperaturedistribution throughout the composting mass as shown in Fig. 4 for test 4b. Beforeand after series connection in test 4b, the maximum variation between minimumand maximum temperature was 43°C and 7°C, respectively. In this test, the seriesconnection was disconnected 1 day before termination of the test which againcaused a wider band width between minimum and maximum temperature. There-fore, reuse of spent air in the second stage appeared to achieve a more uniformtemperature distribution and thereby accelerating degradation of the remainingorganic matter.

3.3. Air supply and oxygen consumption

The pattern of daily air supply, oxygen consumption rate and cumulative oxygenconsumption is shown in Fig. 5 using test 1a as an example. In the initialdecomposition phase of all first and second stage tests, the oxygen consumptionrate as well as the daily air supply was higher due to the higher degradation andheat removal demand. After 15–20 days, the O2 consumption rate had decreased toa very low, relatively stable value in all tests, indicating the completion of the firststage of composting.

Total air supply and oxygen consumption in each pilot-scale composting test arepresented in Table 4. The O2 concentration in the waste air fluctuated between 10


and 20%, with an average of approximately 18%. The minimum O2 concentration(9–14% for first stage tests) in the waste air was observed at the beginning of eachtest, when the degradation of VS and the resulting O2 demand were highest,whereas the maximum O2 concentrations occurred towards the end of each test.The range of total O2 consumption in different first stage tests was 7.2–10.2 kg fordifferent aeration modes, amounts of waste mixture, and duration of each test. Thetotal O2 consumption in the different second stage tests ranged only from 3.6 to 4.1kg. Therefore the O2 consumption in second stage tests was approximately 40–50%of that in the first stage tests.

3.4. Biological stability

A series of self-heating tests were performed for the end products of eachcomposting stage. Table 5 shows the maximum temperature, ambient temperature,other parameters and the stability index calculated from six self-heating tests aftereach pilot-scale test. In the self-heating test of samples from test 1a (upflowaeration), a maximum temperature of 60.28°C was reached with A72 (A72=areaunder temperature curve after 72 h) of 3302°C×h and Imax (Imax=maximumtemperature increase) of 0.9°C/h, which indicates a biological stability index of Iaccording to Table 2. The self-heating tests of samples from tests 1b (reuse of spentair) attained maximum temperatures of 39.26°C (with A72 of 1670°C×h andImax=0.26), respectively, resulting in stability indices of IV. For samples of test2a+2b (downflow and upflow aeration was applied in first and second stage,respectively, with no modification) only stability index III was achieved, althoughthe total composting time of 56 days was the same as for test 1a+1b.

Fig. 5. Daily air supply, oxygen consumption rate and cumulative oxygen consumption during first stagetest 1a.

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Table 4Air supply and oxygen consumption in pilot-scale composting tests

Total O2 O2 usedTotal air suppliedTest % O2 in the spent air Total O2

(Ave.) supplied used per �VS(Min)m3/kg VSbm3/kg VSaSpentFresh(kg) (kg) (kg/kg)(m3)(m3)

17.30 51.65 10.28 1.351a 13.97.43–19057 11.7 17.87 44.78 3.76 2.06128 3.551b 11.36

17.23 48.91 8.512a 1.33180 – 7.02 14.719.32 35.06 4.11 1.4113.82b 17.198.25–12917.45 52.44 8.31 1.313a 195 – 7.80 9.319.91 73.69 3.82 1.0516.23b 16.297.7517513216.694a 48.18179 7.23 1.28– 7.50 13.818.82 44.41 3.61 1.4813.44b 70 109 4.0319.704c 6.8525 0.47 1.38– 2.02 14.10 17.1

�VS=VS degraded.a Initial VS at the beginning of each stage.b Initial VS at the beginning of first stage.

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Table 5Maximum temperature, calculated temperature parameters and stability index (SI) of composts according to self-heating test

Atotal tmaxTest Duration of pilot-Ta SITmax Tmax−Ta Imax A72

(°C×h) (days) scale test(°C×h)(°C/h)(°C)(°C)(°C)(days)

21056 3.74 28 I0.901a 330239.0460.2821.2422.20 1670 20647 10.21 28 (56) IV39.26 17.061b 0.26

30635 10.34 242a II22.19 54.09 31.90 0.76 267018737 2.84 32 (56) III27192b 0.4920.5444.1623.6217733 4.92 29 I3a 19.95 60.29 40.34 0.94 3063

7445 6.95 39 (68) V16023b 0.052.5121.5819.07216204a 4.5726.62 20 I64.25 37.63 1.06 359412855 6.19 21 III25060.394b 24.95 46.17 21.22151614c 11.5521.81 11 (52) V36.84 15.03 0.19 1652

Ta=ambient temperature; Atotal=area under temperature curve after test completion; tmax= time needed to reach Tmax, in days; (), total duration of first,second and third stages, in days.


Fig. 6. Estimated mass and energy balances for the degradation of 1000-g volatile solids (VS) for tests3a and 3b.

According to the self-heating tests, the stability index of the end products was I,III and V, respectively, after pilot-scale tests 4a (upflow aeration with internal airrecirculation), 4b (reuse of spent air) and 4c (alternate upflow downflow aeration).It is of interest to note that the only other completely mature end product withstability index V was obtained from the two stage tests 3a and 3b after 68 days,where internal air recirculation and reuse of spent air was also applied in the firstand second stage, respectively. Composting in three stages with internal air recircu-lation and reuse of spent air produced a completely stable end product in evenshorter time (approx. 50 days) as demonstrated by tests 4a, 4b and 4c. It appears,therefore, that even with the best possible aeration practices of internal airrecirculation and reuse of spent air a minimum of 7 weeks (approx. 50 days)composting time is necessary to achieve a completely stable end product.

3.5. Material and energy balance

Fig. 6 shows two typical examples of an estimated mass and energy balance fortests 3a and 3b. Mass and energy balances were estimated for the completedegradation of 1000-g volatile solids, whereas the water balances were estimated ona total mass basis. According to the mass balance of test 3a, 1311 g O2 was usedto degrade 1000 g VS which produced 1802 g CO2, 14.8 g NH3-N and 493.6 g H2Owhile releasing 15 890 kJ heat energy. Based on the water balances of tests 3a and3b, approximately 53% of the heat energy released during degradation were usedfor evaporation of water (sum of condensate and water lost in the spent air), whichresults in 3.0–3.5 kg of water evaporated per kg volatile solids degraded. Theremaining energy was used to heat the composting mass and spent air, or was lostthrough conduction and radiation to the environment. If all of the leachate had


also evaporated, the percentage of evaporative heat loss would have been up to70%. Although evaporative heat loss is related to size, shape and insulation of thereactor, it is difficult to predict whether it would be significantly higher in afull-scale plant, because no reliable energy balances are available for comparison.

3.6. Water balance and quality of leachates and condensates

According to the water balance of test 3a as shown in Fig. 7, 37.56 kg of waterwas present in the initial mix. Metabolism of the VS produced another 3.13 kgwater. During the test, 5.20 kg water escaped through the outlet air, 16.17 kg wascollected as condensate and 6.75 kg was leachate (leachate plus condensate ofrecycled air). The final compost of test 3a held 12.57 kg of water. Approximately55% of the total initial water was removed by the air in test 3a, whereas in thesecond stage test 3b the percentage of water removed by the air was less than half(24%) of the first stage test. Therefore, water removal in the first stage test wasalmost double than that of the second stage test, roughly corresponding to theamounts of volatile solids degraded and/or heat energy generated.

As examples of varying quality of condensate, pH, COD and NH3-N (in g) in thecondensate produced during tests 4a and 4b are shown in Figs. 8 and 9, respec-tively. During the peak degradation period, the maximum condensate productionrate was 3–4 l/day. After that condensate was not collected frequently, as the dailyproduction rate greatly decreased to a lower quantity. The total condensateproduced in first stage tests 1a, 2a, 3a and 4a were 10.95, 7.01, 16.16 and 20.52 l,respectively, or 1.46, 1.11, 2.55 and 3.61 l/kg VS degraded. The correspondingvalues for second stage tests 1b, 2b, 3b and 4b were 4.45, 5.60, 8.05 and 13.49 l,respectively, or 2.49, 1.94, 3.06 and 5.60 l/kg VS degraded. As expected, the highestamounts of condensate correlated well with the highest temperatures in the com-posting mass. Approximately 1.5–2.5 times more condensate was produced in firststage tests than in second stage tests. It should be pointed out that the amount of

Fig. 7. Estimated total water balance for tests 3a and 3b. All the numbers are in kg.


Fig. 8. pH and total amount of COD and NH3-N in the produced condensate during pilot-scale test 4a.Total amount of COD was calculated as concentration of COD× total volume of condensate collectedon that day (as shown in figure). The same procedure was applied for NH3-N.

leachate produced per kg VS degraded depends not only on the percentage ofevaporative heat loss, but also on the moisture content of the composting mass, theairflow applied, and the temperature decrease of the spent air in the air conveyingsystem prior to discharge to the atmosphere. The leachate production during the

Fig. 9. pH and total amount of COD and NH3-N in the produced condensate during pilot-scale test 4b.Total amount of COD was calculated as concentration of COD× total volume of condensate collectedon that day (as shown in figure). The same procedure was applied for NH3-N.


pilot-scale tests was very low, approximately 0.0–0.9 l over the whole duration ofa test. During first stage tests, the initial total COD of the condensate variedbetween 6 and 12 g/day and after 5–10 days it sharply decreased to a value nearzero, while at the same time the pH increased to its maximum value as shown inFig. 8. At the beginning of all the first stage tests, the NH3-N content in thecondensate was very low, but after that the NH3-N content gradually increasedtowards a maximum value. Therefore, attention should be paid to the large amountof highly concentrated condensate produced in a large scale composting plant forproper collection, recirculation or treatment.

4. Practical application in composting plants

The results of this study can be used for a better understanding of the effect ofdifferent modes of forced aeration on the composting process and may help in thedesign and operation of future composting plants. The application of unidirectionalupflow or downflow aeration creates significant vertical distribution of temperaturein the composting mass, which does not subject all waste particles to the sametreatment conditions. Periodical mixing of the composting mass is therefore neces-sary to produce a more uniform product quality when applying unidirectionalaeration systems. Similar to the single reactor system with internal air recirculation(tests 1a, 3a and 4a) of this study, the increasingly popular small scale tunnelcomposters or container composters (approx. 1000 t/year capacity) for acceleratedcomposting apply a single stage, static composting process with internal recircula-tion of air. In a few cases, reuse of spent air is applied. An almost perfect exampleof the two-reactor composting system is the huge composting plant of Bangkok(approx. 400 000 t/year input capacity) which applies a two-stage compostingprocess, whereby the spent air of the first stage (21 days of downflow) is used foraeration of the second stage (21 days of upflow), achieving a very stable endproduct. Controlled air flow in an enclosed system and application of a biofilterresulted in complete elimination of odor problems. Enclosed composting systemswith upflow aeration and internal air recirculation or reuse of spent air may besuitable for many applications in densely populated urban areas where stringentenvironmental emission standards have to be met or where availability of land islimited. Therefore, modern design has to find the optimum combination of fre-quency of material turnover as well as intensity, duration, and mode of aeration.

5. Conclusions

The application of unidirectional upflow or downflow aeration creates significantvertical distribution of temperature in the composting mass.

The two-reactor system produced a more stable compost than the single reactorsystem in the same composting time.


Internal recirculation of air in the first stage leads to a more homogeneoustemperature distribution throughout the composting mass as compared to norecirculation, resulting in improved organic matter degradation.

Reuse of spent air in the second stage appeared to achieve a more uniformtemperature distribution thereby accelerating degradation of the remaining organicmatter while lowering energy requirements.

The findings of this study are applicable for design of modern composting plantsindicating the importance of intensity, duration, and mode of aeration as well asfrequency of material turnover.

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Effect of air recirculation and reuse on composting of organic solid waste