the emission of volatile compounds during the aerobic and the combined anaerobic/aerobic composting...

*Corresponding author. Tel.: 32/9/2645953; Fax: 32/9/2646243; e-mail: [email protected].

Atmospheric Environment 33 (1999) 1295—1303

The emission of volatile compounds during the aerobic and thecombined anaerobic/aerobic composting of biowaste

Erik Smet, Herman Van Langenhove*, Inge De Bo

Department of Organic Chemistry, Faculty of Agricultural and Applied Biological Sciences, University of Ghent, Coupure Links 653,B-9000 Ghent, Belgium

Received 26 February 1998; received in revised form 10 July 1998

Abstract

Two different biowaste composting techniques were compared with regard to their overall emission of volatilecompounds during the active composting period. In the aerobic composting process, the biowaste was aerated duringa 12-week period, while the combined anaerobic/aerobic composting process consisted of a sequence of a 3-weekanaerobic digestion (phase I) and a 2-week aeration period (phase II). While the emission of volatiles during phase I of thecombined anaerobic/aerobic composting process was measured in a full-scale composting plant, the aerobic stages ofboth composting techniques were performed in pilot-scale composting bins. Similar groups of volatile compounds wereanalysed in the biogas and the aerobic composting waste gases, being alcohols, carbonyl compounds, terpenes, esters,sulphur compounds and ethers. Predominance of alcohols (38% wt/wt of the cumulative emission) was observed in theexhaust air of the aerobic composting process, while predominance of terpenes (87%) and ammonia (93%) was observedin phases I and II of the combined anaerobic/aerobic composting process, respectively. In the aerobic compostingprocess, 2-propanol, ethanol, acetone, limonene and ethyl acetate made up about 82% of the total volatile organiccompounds (VOC)-emission. Next to this, the gas analysis during the aerobic composting process revealed a strongdifference in emission profile as a function of time between different groups of volatiles. The total emission of VOC, NH

3and H

2S during the aerobic composting process was 742 g ton~1 biowaste, while the total emission during phases I and

II of the combined anaerobic/aerobic composting process was 236 and 44 g ton~1 biowaste, respectively. Taking intoconsideration the 99% removal efficiency of volatiles upon combustion of the biogas of phase I in the electricitygenerator, the combined anaerobic/aerobic composting process can be considered as an attractive alternative for aerobicbiowaste composting because of its 17 times lower overall emission of the volatiles mentioned. ( 1999 Elsevier ScienceLtd. All rights reserved.

Keywords: Ammonia; Biowaste; Composting; Odour; Terpenes; Volatile organic compounds (VOC)

1. Introduction

In order to reduce the size and volume of solid wasteto be disposed off, composting of the biodegradable frac-

tion of the household waste, being 50—60% of the totalmass, became a widely accepted technique in recent years(Gellens et al., 1995; Epstein, 1997). In all compostingprocesses, the aerobic and/or anaerobic breakdown ofsolid organic matter by micro-organisms is the crucialstep (Derikx et al., 1990a). Mainly aerobic processes areused to convert biowaste into compost. In these plants,the biowaste is aerated during several weeks up to several

1352-2310/99/$— see front matter ( 1999 Elsevier Science Ltd. All rights reserved.PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 2 6 0 - X

Fig. 1. Set-up of the pilot-scale composting experiment: (1) flowcontroller and flow meter, (2) composting bin, (3) compostingmaterial, (4) temperature probe, (5) effluent air, (6) gas samplingunit, (6a) water cooler, (6b) adsorbent sampling tube, (6c) airpump.

months by forced suction or blowing, in order to removemoisture and heat and to create an optimal environmentfor the aerobic mesophilic and thermophilic micro-organisms performing the biodegradation (Haug, 1986).

The most important problem of operating aerobiccomposting plants is the odour pollution due to theemission of volatile compounds (Krauss et al., 1992).Emission of volatiles already starts upon arrival of thefresh biowaste in the composting plant. According toEitzer (1995), most volatile organic compounds (VOC) inaerobic composting plants are emitted at early stages ofprocessing, i.e. at the tipping floors, at the shredder andat the initial active composting region. Pohle and Kliche(1996) classified the aerobic composting process (ACP) inan acid start stage, a thermophilic stage and a coolingstage, with the production of specific odorants in eachstage. According to Homans and Fischer (1992), mainlyanaerobic conditions in composting piles due to incom-plete or insufficient aeration will produce sulphur com-pounds of intensive smell, while incomplete aerobicdegradation processes result in the emission of alcohols,ketones, esters and organic acids. Van Durme et al. (1992)identified dimethyl sulphide, dimethyl disulphide,limonene and a-pinene as the most significant odorousVOC at a composting facility for wastewater sludge.According to this author, the latter two compounds werereleased from the wood chips used as a bulking agent.During the thermophilic composting stage, pyrolysis,auto-oxidation and Maillard products (e.g. pyridine andpyrazine) can be detected in the composting waste gasesnext to biological metabolites (Homans and Fischer,1992). Krauss et al. (1992) identified 3-hydroxy-4,5-dimethyl-2(5H)-furanone as a typical odorant which isformed at very high temperatures (80°C) and in acid toneutral pH-values of the composting material. Next tothis, an improper nutrient balance (e.g. too much grass)in the biowaste and mixing the compost piles can lead toexcessive VOC and ammonia emission (Williams, 1995;Heining et al., 1995).

A new biowaste composting technology is the com-bined anaerobic/aerobic composting process (CCP) (Sixand De Baere, 1992; Gellens et al., 1995). In Belgium,only one biowaste composting plant is working accord-ing to this technique up to now. In this plant, an intensivethermophilic (50—55°C) solid state fermentation (phase I)takes place in a vertical reactor with a biowaste retentiontime of ca 3 weeks and a biogas production of$100 m3 ton~1 biowaste (Gellens et al., 1995; Sinclairand Kelleher, 1995). As a result of the closed fermentordesign, all volatiles emitted during phase I are collectedin the biogas. On-site, part of this biogas is convertedinto steam for process heating (7%), while the remaininggas is converted into electricity upon burning in an elec-tricity generator. After digestion (phase I), the residue isdewatered in a press and the press cake is aerated duringa 2-week period (phase II) (Riggle, 1996). With regard to

the emission of volatiles, however, no data were found forthese CCP.

The scope of this work was to quantify the emission ofvolatile organic compounds, ammonia and hydrogen sul-phide during the active phase of both the ACP and theCCP. These data can be used to select the optimal odourpollution abatement technique.

2. Material and methods

2.1. Biowaste and press cake

Source separated vegetable, fruit, garden and paperwaste from Brecht (Belgium) and surrounding villages(November 1996) was used, having an average composi-tion of 70% garden waste, 20% kitchen waste and 10%non-recyclable paper (Sinclair and Kelleher, 1995). In thefull-scale combined anaerobic/aerobic composting plantin Brecht, the biowaste was collected, homogenised ina comminuting drum and sieved over a 40 mm sieve.About 120 kg of this material was transported to the labto perform the 12 week pilot-scale ACP. The emission ofvolatiles during phase I of the CCP was monitored on-site in Brecht. After digestion and dewatering of theresidue in a press to a dry matter content of$55%, thematerial was sieved over a 10 mm sieve on-site. About120 kg of this sieved press cake (originating from$250 kg of biowaste) was transported to the lab toperform phase II of the CCP. It should be noticed thatthe same original biowaste was used in both compostingprocesses.

2.2. Pilot-scale composting experiments

Two composting bins with a volume of 224 l wereused to perform both the ACP and phase II of the CCP(Fig. 1). One bin was filled with$120 kg sieved biowasteand the other with$120 kg press cake. Both bins were

1296 E. Smet et al. / Atmospheric Environment 33 (1999) 1295—1303

Fig. 2. (a) Aeration rate (m3 ton~1 biowaste d~1) (£) appliedand temperature (°C) of the composting material (j) during the12 week aerobic composting process. The vertical dotted linesrefer to the mixing and wetting of the composting material.(b) Aeration rate (m3 ton~1 biowaste d~1) (£) applied and tem-perature (°C) of the composting material (j) during phase II ofthe combined anaerobic/aerobic composting process.

covered with insulating material to minimise heat losses.Using a compressor, the composting material in bothbins was aerated with outdoor air and in up-flow regime.The flow rate through the bins was set with a flowcontroller and measured with a flow meter. The temper-ature in the composting material was measured viaa temperature probe. The effluent air of the bins was sentto a hood. For the 12-week ACP, the composting mater-ial was regularly taken out, mixed and wetted in order toprevent channelling and drying-out (Fig. 2a).

2.3. Analysis of O2

and CO2

Gas sampling of the effluent of the composting bins forO

2and CO

2analysis was performed using a 500 ll gas

syringe, followed by direct injection on the GC. Theanalysis were performed using a GC-8A Shimadzu,equipped with a TCD.

2.4. Analysis of NH3

and H2S

Gastec and Drager detector tubes were used for analy-sis of NH

3(3La: 2.5—200 ppm, 5/A: 5—700 ppm) and H

2S

(4L: 1—240 ppm). The accuracy tolerance of these de-tector tubes is$25%.

2.5. Sampling procedure for VOC

Glass tubes (0.7 cm internal diameter, 19.5 cm length)were filled with 750 mg of Tenax GC (TGC) (60—80 meshsize) or with both 500 mg Tenax TA (TTA) (60—80 meshsize) and 250 mg Carbosieve SIII (CSIII) (60—80 meshsize). Tenax TA is similar to Tenax GC, but produces lessartefacts and is more stable (Matisova and Skrabakova,1995). Because of the reported higher retentivity of CSIIIfor small molecules (Matisova and Skrabakova, 1995),the flow in the TTA/CSIII tubes during sample collectionalways entered at the TTA side. This flow direction wasreversed during desorption of the tubes. The sorbentiawere retained in the glass tubes using glass wool plugs.Prior to use, the tubes were conditioned during 3 h at 220(TGC) or 250°C (TTA/CSIII) under a helium flow(20 mlmin~1). Air was sampled with a membrane pump(ASF Thomas type 5010), while the air sampling flow ratewas adjusted using a rotameter (Matheson 603), pre-viously calibrated with a soap film bubble meter. The airsampling flow rate was set at 0.1 lmin~1 for small samp-ling volumes ((5 l ) and at 0.5 lmin~1 for higher samp-ling volumes (5—10 l ). The air stream was cooled in an icewater cooler prior to adsorption to limit water vapouradsorption (Fig. 1). Sampling was always performed onboth a TGC tube and a TTA/CSIII tube.

2.6. Instrumentation for analysis of VOC

The GC/MS instrument used for quantification andidentification of VOC included a Varian 2700 GC equip-ped with a FID and a Finnigan MAT 112S mass spec-trometer. A desorption oven was constructed in order toanalyse the adsorption tubes. The normal GC injectorwas replaced by a six-way valve provided with a coldtrap. Liquid nitrogen was used to condensate the vol-atiles in the cold trap. During thermal desorption, heliumwas passed through the tubes in order to transport thedesorbed volatiles to the cold trap. Desorption condi-tions were 10 min at 220°C for TGC tubes and 20 min at250°C for TTA/CSIII tubes. The carrier gas was connec-ted directly to the column of the GC. When injecting, thevalve is switched so that the carrier gas first flowsthrough the cold trap before going to the column. Vol-atiles were flash-evaporated by quickly heating the coldtrap by means of a high-intensity 1000 W halogen flood-light. The temperature of injector and FID detector was240°C. A 60 m 100% dimethyl polysiloxane column (filmthickness 1.5 lm, internal diameter 0.53 mm) was tem-perature programmed from 25 to 100°C at 2°Cmin~1

and from 100°C to 220°C at 4°Cmin~1. A splitter wasinstalled between the end of the column and the FIDdetector to divert about 1

4of the effluent via a transfer line

to the mass spectrometer source. Mass spectrometer con-ditions were as follows: temperature of the transfer line:250°C; ionising energy: 70 eV; source pressure: 10~6 torr;

E. Smet et al. / Atmospheric Environment 33 (1999) 1295—1303 1297

Table 1Concentration of volatile compounds (mg m~3) in the biogas before and after the gas bags and after the electricity generator(EG)

Compound Before gas bags(mg m~3)

After gas bags(mg m~3)

After EG(mg m~3)

Alcohols 44 26 (0.1Carbonyl compounds 28 44 0.6Terpenes 2060 1950 0.3Esters 3.1 1.9 0Org Sulphur compounds 17 7.9 3.0Ethers 3.0 4.2 (0.1NH

318 0 0

H2S 170 270 0

Others 12 8.9 0.3

Total 2360 2320 4.3

scan range: 30—250 me~1; scan speed: 2.5 s scan~1 (VanLangenhove et al., 1982). Except for CS

2, quantification

was performed by integration of the FID-signal witha Nelson Analytical Chromatography Software (4.1) sys-tem. Since the response of the FID was found to be verysimilar within one chemical group of volatiles (except fororganic sulphur compounds), the response factor for onlyone representative compound of a group was determinedas the average value of three 1 ll injections of a 500—1000 ng ll~1 solution. For the organic sulphur com-pounds, standards for each individual molecule weredetermined. Because of the low response of the FID forCS

2, quantification of this compound was performed by

integration of the Total Ion Count Chromatogram of themass spectrometer.

2.7. Data processing

For every volatile organic compound analysed duringthe whole test period, plots of concentrations on bothadsorption tubes were made. Both tubes were consideredto be interchangeable since the slope of the plots equalled1.0$0.2. The only exception was isoprene, which yieldedhigher concentrations on the TGC tube. Except for iso-prene, the average value of the analysis on both the TGCand TTA/SIII tubes was used for processing data.

2.8. Analysis on the biowaste and the compost

The dry matter content was calculated by the weightdifference before and after drying at 105°C to constantweight. NH`

4—N and NO~

x—N contents, pH and electri-

cal conductivity (EC) were determined according to thestandard methods (APHA, 1980). The terpene contentof the biowaste and compost was determined using acombined steam distillation-solvent extraction by meansof a modified Likens—Nickerson extraction apparatus

(Godefroot et al., 1981). About 50 ml dichloromethanewas used to extract 500 g material. In a preliminaryexperiment, an extraction efficiency of 97% was obtainedafter a 3 h extraction period for a limonene standardadded to a mature compost sample. Consequently, thisextraction time was applied for further terpene analysis.

3. Results

3.1. Emission of volatiles during phase I of the CCP

During phase I of the CCP, biogas is extracted fromthe top of the fermentor and is sent to gas bags witha storage capacity equivalent to a 5-day biogas produc-tion. The biogas was analysed before the gas bags, afterthe gas bags (resulting in a time-averaged concentration)and after passing the electricity generator. Next to thepresence of the characteristic biogas compounds CH

4($49% v/v) and CO

2($51% v/v), the total concentra-

tion of inorganic and organic volatile compounds in thebiogas was 2.36 g m~3 with a predominance of terpenes(87%) (Table 1). Taking into consideration the averagebiogas production of 100 m3 ton~1 biowaste, the totalemission of volatiles (not including CO

2and CH

4) during

phase I is 236 g ton~1 biowaste. The cumulative terpeneemission during phase I (206 g ton~1 biowaste) corre-sponded to 104% of the experimentally determined ori-ginal terpene content in the fresh biowaste (199 g ton~1)(Table 2). However, p-cymene was found to be the pre-dominant (60%) terpene molecule in the biogas, whilelimonene made up about 91% of the terpene content inthe fresh biowaste.

After passing the gas bags, NH3

was no longer detect-able in the biogas (Table 1), most probably due to dis-solution of this molecule in the water of condensationthat accumulated on the surface of the gas bags. For


Table 2Terpene content (g ton~1) in the biowaste and cumulative ter-pene emission (g ton~1) during the aerobic composting process(ACP) and during phases I and II of the combined anaer-obic/aerobic composting process (CCP). The terpene content inboth compost materials was not detectable

Compound Biowaste ACP CCP(g ton~1) (g ton~1) Phase I Phase II

(g ton~1) (g ton~1)

Limonene 180 56 68 0.7a-Pinene 6.1 8.0 7.7 0.1Thujone 5.9 6.8 2.4 (0.1p-Cymene 2.1 2.9 123 0.8b-Pinene 3.9 5.1 2.6 (0.1Other 0.6 3.8 2.0 0.5

Total 199 82 206 2.1

Table 3Characteristics of the biowaste and the compost from the aero-bic composting process (ACP) and from the combined anaer-obic/aerobic composting process (CCP)

Biowaste CompostACP

CompostCCP

Dry matter (%) 39 46 52EC (mS cm~1) 2.7 1.8 1.8pH 4.6 8.2 8.6NH`

4—N (g ton~1) 570 0 440

(NO~2#NO~

3)

—N (g ton~1)20 240 60

Fig. 3. CO2

concentration (%) (h, j) in the exhaust air andcumulative CO

2-emission (kg ton~1) (£, m) during the aerobic

composting process (h, n) and phase II of the combined an-aerobic/aerobic composting process (j, m).

other compounds, time-averaged concentrations afterthe gas bags were in the same order of magnitude asconcentrations before the gas bags. Upon passing theelectricity generator, on the other hand, methane (datanot shown) and the other volatiles were efficiently('99%) removed from the waste gas by incineration(Table 1).

After phase I, the residue of the fermentation is dew-atered in a press and the press cake is sent to phase II.The emission of volatiles during dewatering and thepresence of volatiles in the press water were notmonitored in this study. Based on the observed effect ofNH

3dissolution in the water of condensation in the gas

bags, however, significant losses of volatiles during thedewatering are only to be expected for NH

3.

3.2. Operation of the pilot-scale composting bins

Both bins were aerated in order to prevent the temper-ature of the composting material to exceed 70°C. Thetemperature of the composting material shifts to 60—70°Cin 1 week during the ACP (Fig. 2a), while this takes only2 days in phase II of the CCP (Fig. 2b). The temperatureincrease during week 3 in the ACP is due to a decrease inaeration rate (from 135 to 75 m3 ton~1d~1) and also dueto the mixing of the composting material during week 2.The cumulative aeration rate was 5234 m3 ton~1 bio-waste and 1165 m3 ton~1 press cake (or 555 m3 ton~1

biowaste) in the ACP and phase II of the CCP, respect-ively. Both composting processes resulted in a final com-post product with a higher dry matter content anda higher pH-value in comparison with the fresh biowaste(Table 3). The 12-week ACP of the 120 kg biowasteyielded 50 kg compost, while the the aeration during

phase II of the CCP of the 120 kg press cake, originatingfrom$250 kg biowaste, resulted in 105 kg compost.

3.3. Emission of inorganic compounds during the ACP andphase II of the CCP

In Fig. 3, the gradual decrease in microbial respirationrate during both composting tests is illustrated. Linearregression between oxygen (%O

2) and carbon dioxide

(%CO2) concentration measurements during both pilot-

scale tests resulted in a good correlation:

(%O2)"21.94—0.98 (%CO

2) (n"16; r2"0.98).

The cumulative CO2-respiration was 291 kg ton~1 bio-

waste and 37 kg ton~1 press cake (equivalent to17 kg ton~1 biowaste) during the ACP and phase II ofthe CCP, respectively, illustrating the significantly highertotal metabolic activity during the ACP. Apparently,phase I of the CCP already results in a strong reductionof the biodegradable fraction of the composting material.Up to 90% of the cumulative CO

2-respiration during the


Fig. 4. (a) Ammonia concentration (mg m~3) (s) in the exhaustair and temperature of the composting material (j) during theaerobic composting process. (b) Ammonia concentration(mg m~3) (s) in the exhaust air and temperature of the compost-ing material (j) during phase II of the combined anaer-obic/aerobic composting process.

ACP took place during the first 4 weeks (Fig. 3). Therespiration rate at the end of both composting processeswas comparable, being 0.4 kg CO

2ton~1 biowaste d~1 and

1.4 kg ton~1 press cake d~1 (0.7 kg ton~1 biowaste d~1)for the ACP and phase II of the CCP, respectively. While noH

2S (concentration (0.35 mg m~3) was detected in the

composting waste gases, very high NH3

concentrationswere measured during both aerobic processes (Fig. 4aand b). The emission of NH

3strongly correlated with the

temperature of the composting material. The cumulativeNH

3-emission was 152 g ton~1 biowaste and 87 g ton~1

press cake (equivalent to 41 g ton~1 biowaste) during theACP and phase II of the CCP, respectively. The NH

3-

emission data for phase II of the CCP are, however, anunderestimation since the observed NH

3-concentrations

exceeded the maximum detectable concentration of500 mg m~3 during the period from day 1 to 3. Next tothis, the NH

3-emission during phase II of the CCP was

not completely finished after the 14 day period (Fig. 4b).In accordance with this, the compost of the CCP stillcontained 440 g NH`

4—N ton~1 biowaste after phase II

of the CCP, while ammonium was no longer detectablein the aerobic compost due to complete nitrification(Table 3).

3.4. Emission of volatile organic compounds (VOC) duringthe ACP and phase II of the CCP

Alcohols, carbonyl compounds, terpenes, esters, or-ganic sulphur compounds and ethers were analysed inthe waste gases of both aerobic processes (Table 4). Fattyacids were not detected with the technique used. Thetotal cumulative VOC-emission during the ACP wasabout 200 times higher than during phase II of the CCP.During the ACP, the total weight loss due to VOC-emission corresponded to 0.059% of the original bio-waste. In both processes, the emission of aliphatic andaromatic compounds was very low ((0.003 g ton~1d~1)and mainly due to the presence of these compounds inthe outdoor air used to aerate the bins (data not shown).In the ACP, 2-propanol, ethanol, acetone, limonene andethyl acetate made up about 82% of the total VOC-emission. Maximum observed VOC-concentrations inthe waste gases were 194 mg m~3 ethanol and 19 mgm~3 acetone at the start of the ACP and phase II of theCCP, respectively. Similar terpene molecules were foundin the ACP waste gases as in the original biowaste.However, only 41% of the original terpene content of thebiowaste was recovered in the ACP waste gas (Table 2).For the different groups of VOC, different emission pro-files during the ACP were observed (Fig. 5): alcohols,carbonyl compounds, esters and ethers were mainly emit-ted during the initial composting stage, while the volatileorganic sulphur compounds were mainly emitted duringthe thermophilic stage. For all terpenes except p-cymene,on the other hand, a zero-order decrease in emission rateversus time was observed.

4. Discussion

Both composting processes resulted in a 60% weightreduction of the biowaste and yielded a compost materialwith a higher dry matter content and higher pH-value incomparison with the original biowaste (Table 3). Al-though the final respiration rate of both compost mater-ials was comparable, the compost of the CCP can beconsidered as less mature, since only 11% of the am-monium—nitrogen was converted by the nitrifyingmicro-organisms at the end of phase II. Apparently, thestart-up period for the nitrification process exceeds a2-week aeration period.

In the biogas of phase I of the CCP, the total concen-tration of volatiles (not including CO

2and CH

4) was

2.36 g m~3 with a predominance of terpenes (87%).However, while NH

3was removed from the biogas upon

transit through the gas bags, the remaining volatiles wereefficiently ('99%) removed in the electricity generator,yielding a very low overall emission of 0.4 g vol-atiles ton~1 biowaste during phase I of the CCP.


Table 4Cumulative emission (g ton~1) of volatile organic compounds, ammonia (NH

3) and hydrogen sulphide (H

2S) during the ACP and

phases I and II of the CCP, together with the maximum observed concentration (mg m~3) during the ACP and phase II of the CCP andthe concentration in the biogas of phase II of the CCP

ACP CCP

Emission Max conc Phase I Phase II(g ton~1) (mg m~3) Emission Biogas conc Emission Max conc

(g ton~1) (mg m~3) (g ton~1) (mg m~3)

Alcohols2-Propanol 134 95 1.4 14 n.d. n.d.Ethanol 133 194 2.2 22 (0.1 0.2Isobutanol 5.8 15 0.4 3.8 (0.1 0.42-Butanol 3.7 15 0.4 3.7 n.d. n.d.Other 8.5 0 (0.1Sum 285 4.4 (0.1

Carbonyl CompoundsAcetone 125 114 0.7 7.4 0.2 19Butanone 22 61 0.8 7.8 0.1 8.32-Heptanone 1.4 2.4 0.4 3.6 (0.1 0.73-Methylbutanal 4.0 4.0 0.1 1.0 0.1 5.5Other 5.6 0.8 0.2Sum 158 2.8 0.6

¹erpenesLimonene 56 57 68 679 0.7 3.4a-Pinene 8.0 6.9 7.7 77 0.1 2.7Thujone 6.8 4.9 2.4 23 (0.1 2.0p-Cymene 2.9 3.4 123 1233 0.8 5.1Other 8.9 4.6 0.5Sum 82 206 2.1

EstersEthyl acetate 35 66 0.3 3.1 (0.1 0.4Methyl acetate 9.6 24 n.d. n.d. n.d. n.d.Methyl propionate 2.1 5.9 n.d. n.d. n.d. n.d.Propyl propionate 1.0 2.7 n.d. n.d. n.d. n.d.Other 5.3 0Sum 53 0.3 (0.1

Sulphur CompoundsDimethyl sulphide 8.2 8.2 0.3 3.3 0.1 0.6Dimethyl disulphide 0.4 0.8 0.5 5.4 (0.1 1.3Carbon disulphide 0.4 0.4 n.d. n.d. (0.1 0.1Methyl propyl disulphide 0.2 0.1 0.4 4.2 (0.1 0.1Other 0 0.4 0Sum 9.2 1.7 0.2

Ethers2-Ethyl furane 1.6 4.0 n.d. n.d. (0.1 0.12-Methyl furane 0.9 0.2 0.2 1.6 (0.1 0.2Diethyl ether 0.2 0.5 0.1 1.4 n.d. n.d.Sum 2.7 0.3 (0.1

Other VOC 0 1.2 0

Total VOC 590 217 3.0

NH3

152 227 1.8 18 41 '500H

2S n.d. n.d. 17 170 n.d. n.d.

Total Volatiles 742 236 44

n.d."not detectable.


Fig. 5. Emission (g ton~1 d~1) of 2-propanol (n), limonene (m),dimethyl sulphide (s) and ammonia (h) versus time during theaerobic composting process, together with the temperature (j)of the composting material.

Due to the performant fermentation during phase I ofthe CCP, an 8 times lower cumulative CO

2-respiration

was observed in phase II of the CCP in comparison withthe ACP, while the cumulative VOC emission duringphase II of the CCP was about 200 times lower incomparison with the ACP (Table 4). During phase II ofthe CCP, cumulative emission values of all compoundsanalysed were lower than 1 g ton~1 biowaste. In theACP, alcohols and carbonyl compounds made up 75%of the total VOC-emission and were mainly emittedduring week 1 (Fig. 5), being the period with reducedoxygen content (12—17%) in the waste gas. The cumulat-ive emission data for 2-propanol, ethanol and acetoneduring the ACP exceeded a value of 100 g ton~1 bio-waste. Together with limonene and ethyl acetate, thesemolecules made up about 82% of the total VOC-emis-sion during the ACP. A surprising similarity in VOC-composition was observed between the ACP waste gasand the biogas of phase I of the CCP. Indeed, volatilesidentified in both the ACP waste gas and in the biogasmade up 98% of the total VOC-content of the biogas(Table 4). Together with the low oxygen contents mea-sured in the ACP waste gas, these observations furtherindicate that production in anaerobic microsites of thebiowaste piles is a major formation process for VOCduring the ACP.

The emission profile of NH3

as a function of timeduring both the ACP and phase II of the CCP wasstrongly related to the temperature of the compostingmaterial. The observed NH

3-concentrations exceeded

the MAK-value (maximum concentration value in theworkplace, defined by the Deutsche Forschungsgemein-schaft) of 35 mg~3 (Fig. 4a and b) (ACGIH, 1991). Ac-cording to Verschueren (1983), severe toxic effects ofNH

3for man appear at 350 mg m~3 after a 1 min expo-

sure period. For the ACP, NH3-volatilisation correspon-

ded to 27% of the initial NH`4—N in the biowaste, while

42% of the initial NH`4—N was converted into nitrite and

nitrate by the nitrifying micro-organisms (Table 3). Asa possible explanation for the 31% gap in this NH`

4—

balance, N-immobilisation in the biomass and N2- and

N2O-emission can be mentioned (Hellman, 1995; Czepiel

et al., 1996; Morgenroth et al., 1996). During phase II ofthe CCP, only 7% of the initial NH`

4—N in the biowaste

was lost by volatilisation. This amount of ammonia,however, made up 93% of the total emission of volatiles(VOC#NH

3) during phase II of the CCP (Table 4).

The cumulative emission of organic sulphur com-pounds during the ACP (9.2 g ton~1"4.9 g S ton~1)was in the same range as was found during the produc-tion of mushroom cultivation compost (8.3 g S ton~1)(Derikx et al., 1990b) and occurred mainly during thethermophilic composting stage. Epstein (1997) and Pohleand Kliche (1996) reported good correlations betweendimethyl disulphide emission concentrations and odouremission concentrations during aerobic composting pro-cesses. In this work, however, dimethyl sulphide wasfound to be the dominant sulphur compound, witha maximum concentration during the ACP exceedingabout 1000 times the odour treshold value of 6 lg m~3

(Devos et al., 1990).Both in the ACP waste gas and the waste gas of phases

I and II of the CCP, similar terpenes were detected(Table 2). Moreover, these terpenes were found to beextracted from the biowaste material during the initialstages of both composting processes. During the CCP,$104% in mass of the original terpene content in thebiowaste was collected in the biogas, while only $1%was collected in the waste gas of phase II. Apparently,the thermophilic conditions in the anaerobic fermentorpromote complete extraction of terpenes out of the bio-waste and the conversion of limonene into p-cymene. Inthe ACP, only 41% of the original terpene content in thebiowaste was collected in the waste gas. As a possibleexplanation for this gap, chemical oxidation and aerobicbiodegradation of these terpenes in the ACP can bementioned (Kutty et al., 1994). Indeed, limonene removalrates of 500 g m~3 d~1 were reported for a compostbiofilter without the need for a long start-up period (Smetet al., 1997). The contribution of this biowaste strippingprocess for the emission of other volatiles analysed needsfurther research.

As a conclusion, it is stated that the collection ofvolatiles in the biogas and the subsequent combustion inthe electricity generator result in a very low overall emis-sion of volatiles during phase I of the CCP. Next to this,the total air flow requirement (m3 air ton~1 biowaste) forperforming phase II of the CCP is about 10 times lowerin comparison with the ACP. In composting plants, bio-filters are most often used to control the odour andVOC-emission. Since these biotechniques usually are di-mensioned on the basis of the total air flow, a signifi-cantly smaller biofilter surface area will be required in the


CCP (Leson and Winer, 1991). However, the phase IIwaste gas is predominated by NH

3which makes up

about 93% in mass of the total emission of volatiles andis emitted at rather high concentrations (more than500 mg m~3). Since NH

3is a toxic compound for biofil-

ters at concentrations exceeding 45 mg m~3 (Hartikainenet al. 1996), a chemical NH

3-scrubber preceding the bio-

filter is recommended to control the odour pollution inthese CCP plants. For the ACP plants, a large biofiltersurface area will be required.

Acknowledgements

This work was financed by a scholarship of theFlemish Institute for support of Scientific/TechnologicalResearch in the Industry (IWT) (OZM/960008). Theauthors thank O.W.S. nv for helping with the experi-mental part.

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the emission of volatile compounds during the aerobic and the combined anaerobic/aerobic composting...

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