co‐composting for managing effluent from thermophilic anaerobic digestion of municipal solid waste

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This article was downloaded by: [Linnaeus University] On: 11 October 2014, At: 03:53 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20 Cocomposting for managing effluent from thermophilic anaerobic digestion of municipal solid waste A. Pera a , G. Vallini a , Stefania Frassinetti a & F. Cecchi b a Soil Microbiology Center , CNR , Via del Borghetto 80, 56124, Pisa, Italy b Department of Environmental Sciences , University of Venice , Calle Larga S. Marta, 2137, 30123, Venice, Italy Published online: 17 Dec 2008. To cite this article: A. Pera , G. Vallini , Stefania Frassinetti & F. Cecchi (1991) Cocomposting for managing effluent from thermophilic anaerobic digestion of municipal solid waste, Environmental Technology, 12:12, 1137-1145, DOI: 10.1080/09593339109385114 To link to this article: http://dx.doi.org/10.1080/09593339109385114 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

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Page 1: Co‐composting for managing effluent from thermophilic anaerobic digestion of municipal solid waste

This article was downloaded by: [Linnaeus University]On: 11 October 2014, At: 03:53Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Environmental TechnologyPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tent20

Co‐composting for managing effluent from thermophilicanaerobic digestion of municipal solid wasteA. Pera a , G. Vallini a , Stefania Frassinetti a & F. Cecchi ba Soil Microbiology Center , CNR , Via del Borghetto 80, 56124, Pisa, Italyb Department of Environmental Sciences , University of Venice , Calle Larga S. Marta, 2137,30123, Venice, ItalyPublished online: 17 Dec 2008.

To cite this article: A. Pera , G. Vallini , Stefania Frassinetti & F. Cecchi (1991) Co‐composting for managing effluentfrom thermophilic anaerobic digestion of municipal solid waste, Environmental Technology, 12:12, 1137-1145, DOI:10.1080/09593339109385114

To link to this article: http://dx.doi.org/10.1080/09593339109385114

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in thepublications on our platform. However, Taylor & Francis, our agents, and our licensors make no representationsor warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Anyopinions and views expressed in this publication are the opinions and views of the authors, and are not theviews of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should beindependently verified with primary sources of information. Taylor and Francis shall not be liable for any losses,actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoevercaused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Co‐composting for managing effluent from thermophilic anaerobic digestion of municipal solid waste

Environmental Technology, Vot 12 ppU37-114SO Publications Division Seiner Ltd, 1991

CO-COMPOSTING FOR MANAGING EFFLUENTFROM THERMOPHILIC ANAEROBIC DIGESTION

OF MUNICIPAL SOLID WASTE

A. PERA1, G. VALLINI1*, STEFANIA FRASSINETTI1 AND P. CECCHI2

1 CNR, Soil Microbiology Center, Via del Borghetto 80, 56124 Pisa, Italy2 University of Venice, Department of Environmental Sciences,

Calle Larga S. Marta, 2137, 30123 Venice, Italy

*To whom correspondence should be addressed

(Received 25 April 1991; Accepted 21 September 1991)

ABSTRACT

Anaerobic effluent from thermophilic digestion (55°C) of organic fraction of municipal solidwaste (OFMSW) was ultimately treated through a co-composting process. OFMSW sorted by anindustrial plant was used as bulking agent mixed to the digested effluent in the co-compostingexperiment. Compost detoxification of the anaerobic effluent was carried out in a static windrowaerated by blowing air in it. The bio-oxidative post-treatment technology adopted allowed theanaerobic effluent mixed with the OFMSW to overcome phytotoxicity and to reach maturity for apossible use as organic amendant in agriculture. Short term composting (5 weeks) proved also toact efficiently in drying and hygienizing the initial waste biomass.

INTRODUCTION

Anaerobic digestion of complex organicfeedstocks gives rise in any case to a low-putrescibility effluent that is normally rich inreduced metabolites. These compounds areresponsible for phytotoxicity phenomena whenthe anaerobic effluent is directly used asfertiliser in agriculture [1].

The number of microorganisms indicativeof faecal contamination, if not even the presenceof pathogens, may also represent a furthernegative aspect of this effluent. Onlythermophilic anaerobic digestion has shownto be satisfactorily destructive to pathogenicorganisms [2,3].

Because of the usual percentages of totalsolid (TS) (5-10% in conventional processes andgreater than 15% in semi-dry conditions), adigested effluent has a diluted fertilising valueon a NPK basis, with obvious negativeimplications in terms of economical costs forstorage, t ransport and land application.Moreover, raw (i.e. undewatered) anaerobiceffluents at high application rates can promote todifferent extent the collapse of pedological

structure that will result in a poor soil aeration[4]. Another problem associated with spreading ofhigh-water-content digested effluent might bealso the risk of groundwater contaminationdue to leaching of chemical or biologicalcontaminants. However, drying of a digestereffluent, unless carried out on thickening beds,might sometimes result costly and energy-consuming. Such considerations make theabatement of effluent phytotoxicity and moisturethe main goal to be achieved possibly by means ofsimple and reliable technology.

Co-composting seems to be a feasible post-treatment technology in the realistic scheme ofFigure 1 where anaerobic digestion and aerobicbiostabilisation could be efficiently integratedfor the exhaustive processing of wastewater andOFMSW. In a previous study the authorspresented the application of the co-compostingtechnique to an effluent coming from themesophilic digestion (35°C) of OFMSW [5,6]. Thepresent paper describes the ultimate bio-treatment of effluent from the anaerobicdigestion of organic fraction of MSW accordingto exactly the technological diagram proposed inFigure 1.

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WHITEWATERSEPARATELY

COLLECTED OFMSK

WASTEWATERTREATMENT PLANT

SEWAGESLUDGE

UNSORTED OFMSW

IHAZARDOUS WASTE

SORTWG PLANT

ORGANICFRACTION C0MBUSTI8LE

DIGESTER

RDFINERTS

AND _OTHERä

HAZARDOUSLANDFILL

INCINERATOR

LANDUTILIZATION

ASH LANDFILL

ENER6YSAVING

IMATERIALS'RECYCLE

ENVIRONMENTALNEGATIVE IMPACT

: MAIN FLOWS : ALTERNATIVE FLOW : THIS PAPER STRATEGY FOR OFMSW

Figure 1. Flow diagram for the exhaustive treatment of the organic fraction of MSW through acombination of high soilds anaerobic digestion and co-composting process.

EXPERIMENTAL

Anaerobic effluent

Anaerobically digested outlet sludge wasobtained from ä 3 cubic meter working volume,mechanically stirred, pilot digester [7] runningunder thermophilic conditions (55±1°C). Thereactor was fed with OFMSW from the full-scalesorting facilities of S. Giorgio di Nogaro (Udine,North-East Italy) [8] after dilution in a storagetank, in order to get a TS concentration in therange 20-25%. Some characteristics of thisanaerobic effluent are reported in Table 1.Further information dealing with anaerobicdigestion trials of OFMSW is reported elsewhere[9].

Organic fraction of MSW

Due to its water content, the anaerobiceffluent could not be composted before beingmixed with a bulking material which allowed it tobe arranged in a stable windrow. Following thescheme proposed in Figure 1 the biodegradableorganic fraction of MSW was chosen as co-

substrate in our composting experiments. Originand characteristics of this material were thesame of the organic fraction fed to the anaerobicdigester (Table 1). The OFMSW used in thisexperiment underwent a transient self-heatingstage during the truck loading and transportationfrom the MSW sorting plant to the experimentalcomposting yard in Pisa (Central Italy).

Composting system

Once mixed together with the anaerobiceffluent the organic fraction of MSW was formedin a pile of about 3.5 metric tonnes. The effluentand the OFMSW have been mixed in the ratio of35% to 65%, approximately 15% and 85% drymatter contributions respectively, in order toachieve a moisture content (Table 2, time 0)compatible with an optimal biological evolutionof the process without adding any more water [10].The pile was arranged on a composting yardequipped for the control of the bio-oxidativeprocess according to the Rutgers strategy [11,12]in which microbial heat generation, temperature,aeration and vaporization of water areinteracting factors. A temperature controller

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Table 1. Main physico-chemical characteristics of the anaerobic effluent and the organic fraction ofMSW used for the co-composting experiment.

Anaerobic effluent Organic fraction of MSW

MoisturepHAsh (%)*Organic carbon (%)*Total nitrogen (%)*C/N ratioPhytotoxity (%)**

83.087.2

54.7817.322.138.10

48.20

49.037.8

56.5023.07

0.6535.5030.50

on a dry matter basis germination index (%) (see caption of figure 3)

Table 2. Dynamics- of physico-chemical parameters during composting of organic fraction of MSWmixed with anaerobic effluent from thermophilic digester.

Composting time, days

Moisture (%)pHAsh (%)*Organic carbon (%)*Total nitrogen (%)*C/N ratio

0

62.17.0

63.220.2

0.825.2

3

56.86.3

64.220.0

0.825.0

7

53.27.4

64.818.3

0.726.1

14

48.58.3

65.115.7

0.722.4

21

46.38.0

66.013.8

0.719.7

28

44.38.2

65.413.4

0.719.1

35

43.18.6

67.413.3

0.719.0

50

38.78.6

69.113.2

0.816.5

* on a dry weight basis

unit set on 55°C was connected with athermocouple placed in the pile. Whentemperature at the thermocouple reached 55°C thecontroller governed operation of a blower untilthe temperature in the pile went down again below55°C. This temperature feedback was to restrictthe temperature ascent in the pile around 60°C. Attemperatures lower than 55°C, pile ventilationwas guaranteed by blower operation throughtimer-schedule. During the experimental trialsthe ambient air temperature ranged from 5 to14°C with a mean of 11°C.

Analytical methods

Physico-chemical analyses:Physico-chemical, microbiological and

phytotoxicity analyses were performed onrepresentative samples of the substrates used forthe experiment; 9 separate 0.5 kg samples weretaken from three different layers in the pile(three respectively from different locations at10, 50, 90 cm in the pile) and mixed together toobtain a single sample. A multi-channelpyrometer connected with three temperaturesensors placed in the composting pile was used

for temperature measuring at 10, 50 and 90 cmfrom the windrow surface. Moisture content wasdetermined by drying 100g samples in a oven at105°C until constant weight was reached. pHmeasurement was performed by electrometricdetermination. Ash content resulted by ignitingdried samples at 550°C to constant weight in amuffle. The potassium dichromate oxidationmethod was used for COD determination and thevalues were empirically related to the organiccarbon content of the samples [13]. Total nitrogencontent was determined by the Kjeldahlprocedure.

Microbiological determinations:The methods and media used for the culture

and counts of the total populations of aerobicbacteria, actinomycetes and fungi were thosedescribed by Pochon & Tardieux [14].Cellulolytic fungi were counted following amodification of the procedure suggested byHudson [15]. Actually rifampicin at the finalconcentration of 50 ug/ml was added to theagarized medium. The same culturing mediumwithout antibiotic was used for the determinationof cellulolytic bacteria.

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Enumeration of autotrophic ammonia-oxidizing and nitrite-oxidizing bacteria by mostprobable number (MPN) was made according tothe methods reported by Schmidt & Belser [16].Total and faecal coliforms, faecal streptococciand Salmonella sp. were isolated as previouslysuggested by de Bertoldi et al. [17],

Detection of enteroviruses was performedthrough the classic multistep procedure based ona first observation of the cytopathic effect of theviral agent isolated, followed by an eventualimmunological reaction for identification.Samples of anaerobic effluent, OFMSW orcomposting mixture of both were homogenized inRPMI 1640 Medium (Sigma Chemical Company)at the ratio of 1 to 9, by stirring in flask with amagnetic stirrer for lh at room temperature.Stirred samples were then clarified bycentrifugation at 3,000 rpm for 15min. 10ml ofthe supernatant from each sample weredisinfected by adding 250 units of penicillin,200ug streptomycin and 50ug neomycin. Onceantibiotised the supernatants were centrifugedagain (at 4°C for 1 h at 4,000 rpm) and then storedat -30°C until they were inoculated into cellcultures. Hep-2, Vero and LLC-MK cell lines(Collection of the Department of Biomedicine,University of Pisa) were cultivated andpropagated by s tandard techniques [18].Identification of enteroviruses was carried outaccording to the procedures suggested byKapsenberg [19].

Phytotoxicity testing:Phytotoxicity analyses were performed on

germinating cress (Lepidium satiuum) asproposed by Zucconi et al. [20,21]. Standardconditions for the bioassay were 24 h incubationin the dark at 27°C. Seeds were placed in 5cmdiameter Petri dishes lined with filter paperwetted with lml of aqueous extract from thematerial tested. Ten replications of seven seedswere performed for the control (germination onsterile distilled water) and each dilution (3, 10and 30%) of the original extract. Extraction wasby compression for 15min at 250atm aftercorrecting moisture content of the materialinvestigated to 60%. Extracts were then sterilizedby filtration through a 0.2um disposable filterunit. Germination index was obtained byapplication of the following equation:

where Rc and Rt are respectively the number of

seeds germinated on control Petri dishes and ontreated plates, while Lc and L t are root lenghts ofgerminated seeds in control and extract-addeddishes respectively.

RESULTS AND DISCUSSION

Stabilisation parameters

Temperature:As illustrated in Figure 2 the adoption of a

system designed to prevent excessive rising oftemperature in the self-heating organic biomasscaused the temperature to peak only at themaximum value of 64°C during the period offeedback control. This operating ceiling ofapproximately 60°C sustained a high rate ofmicrobial metabolic activity by avoiding thecollapse of microbial populations which wouldhave happened in composting piles without atemperature control unit, where 70°C and more,lethal to most microorganisms, could last a fewdays. Blowing air through the composting.mixture"by demand" via temperature feedback controlensured heat removal and oxygen supplyto the biomass. A consequence of this wasmaximization of decomposition rate with asubstantial reduction of the stabilisation time.The intensive oxidative metabolism caused therapid depletion of putrescible fractions in thecomposting mixture. Therefore heat generationprogressively ceased and temperature came downto near ambient air temperature value in about 5weeks (Figure 2).

Moisture:During biostabilisation the moisture content

of the composting mixture decreased from 62.1 to38.7% within fifty days (Table 2). The mainmechanism of water removal in the strategyadopted was vaporization as consequence of themicrobial heat generation. Water vaporizationcaused a continuous heat removal so that thetemperature did not become inhibitively highfor the microorganisms. On the other hand,ventilative heat removal in metabolically activecomposting pile allowed the initial substratebiomass to dry progressively. Therefore thecourse of drying during composting was anindicator of organic matter decomposition andstabilisation [22].

Carbon/Nitogen ratio:The progressive decrease in the C/N ratio

shown in Table 2 indicated maturation (i.e.stabilisation) of the composting mixture (23).

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Page 6: Co‐composting for managing effluent from thermophilic anaerobic digestion of municipal solid waste

80

70-

c? 60-

LüÙL31-

ÙLLüQ.

LJJ

50-

40-

30-

20-

10-

00

DEPTHS FROM THE WINDROW TOP: 0-10 cm50-60 cm90-100 cm

10 20 30 40COMPOSTING TIME (DAYS)

50 60

Figure 2. Profile of temperature variations in the composting windrow during the aerobic ultimatestabilisation of fresh OPMSW mixed with anaerobic effluent from thermophilic digestionof garbage putrescibles.

During humification of organic substratesthroughout a controlled composting processsimilar to thst adopted in the present research,the C/N ratio usually declines for up to the first 4-5 weeks, the decrease then becomes slow as timeproceeds (24). The same trend was observed inour composting windrow in which the C/N ratioreached a practically constant value as the heatgeneration (i.e. intensive oxidative metabolism)ceased.

Phytotoxicity:A further parameter used as indicator of the

degree of maturity of the composting mixture wasthe phytotoxicity. A correlation between presenceof phytotoxic compounds in the compost and itsimmaturity has been well pointed out in theliterature [25-28]. According to the cressbioassay, the most probative 30% dilution of theaqueous extract from the composting mixture wasconsidered in order to determine plant toxicity.At this concentration of the extract correspondeda germination index higher than 60% (thresholdof phytotoxicity) in between the second and thethird week of composting (Figure 3) when the

thermophilic step of the process was practicallyover.

Dynamics of microbial groups:As it can be seen in Table 3, the microbial

populations of total bacteria, actinomycetes andfungi remained fairly stable during thethermophilic step of the process with a moreevident drop as the highest temperature valueswere reached in the pile. This massivepresence of microorganisms in the compostingmixture was made possible by controlling aprolongated overheating of the windrow with thetemperature-feedback control embodied in thestrategy adopted. Cellulose degrading aerobicmicroorganisms were detectable throughout thewhole composting. In the later phases of theprocess an important increase of the number ofcellulolytic eumycetes occurred in agreementwith previous findings [25,29]. Autotrophicnitrifying bacteria (both ammonia-oxidizers andnitrite-oxidizers) were monitored on purpose ofdetermining the degree of compost maturityin terms of nitrification as suggested byFinstein & Miller [30]. As this kind of bacteria

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Page 7: Co‐composting for managing effluent from thermophilic anaerobic digestion of municipal solid waste

100

^ 9 0

S 802 701 60§ 505 401 30S 20

10

• GERMINATION INDEX+ NH4-OXIDIZING (•)« NOL, -OXIDIZING (*)

Threshold of phytotox[city__

10

-9

"8 o-7 S-6

-4

-10

CO

" 0 5 10 15 20 25 30 35 40 45 50 55 60COMPOSTING TIME (DAYS)

Figure 3. Dynamics of phytotoxicity and counts of autotrophic nitrifying bacteria in the compostingmixture during the aerobic stabilisation (germination indices of anaerobic effluent andOFMSW are in table 1). Germination tests were carried out on cress seeds treated with the30% dilution of the aqueous extract from the composting substrate.

Table 3. Total counts of different groups of microorganisms during composting of the mixture oforganic fraction of MSW and anaerobic effluent from thermophilic digester (cells g"1 d.w.)

Composting time :Windrow temp CO*

Total aerobic bacteria (x 108)

Actinomycetes (x 103)

Total fungi (x 104)

Cellulolytic bacteria (x 103)

Cellulolytic fungi (x 104)

018

20.0

4.1

1.4

1.9

0.4

351

0.46

0.13

0.15

1.10

0.38

760

0.04

0.57

0.04

0.98

0.09

1464

0.73

0.26

0.48

0.78

0.17

2149

0.19

0.341.40

3.80

0.79

2841

9.3

20.0

16.0

12.0

13.0

3544

11.0

220.0

110.0

42.0

72.0

5019

12.0

390.0

260.0

46.0

210.0

*measured at 50-60cm from the top of the windrow

Table 4. Monitoring of some pathogenic microbes during composting of organic fraction of MSWmixed with anaerobic effluent (cells g"1 d.w.)

Stabilisation time (days)

Total colliforms (x 104)

Faecal colliforms (x 105)

Faecal streptococci (x 104)

Salmonella sp.

0

460

2.7

4000

n.f.

7

60

n.f.

76

n.f.

21

4.3

n.f.

48

n.f.

35

4 .1

n.f.

0.43

n.f.

50

0.62

n.f.

0.089

n.f.

n.f. not found

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become detectable in a composting biomass andnitrification takes place in the organic substrate,the compost can be considered stabilised. Duringthe composting trial here discussed the activity ofammonia and nitrite-oxidizers was revealedafter 4 weeks since the process had begun (Figure3). This is in accordance with the otherstabilisation parameters discussed above.

Deactivation of pathogens:The composting mixture of organic fraction

of MSW and anaerobic effluent underwent adrastic reduction in the number of cells of totalcoliforms, faecal coliforms and streptococci(Table 4). Although Salmonella sp. was not foundeither in digester -effluent or in fresh organicfraction of MSW (probably because of theuncontrolled self-heating stage the OFMSWsuffered a t the sorting plant), previousinvestigations [17] reported the effectiveness ofthe composting process as regards the destructionof this pathogen. The inoculated primary cellcultures for viral identification showed cytotoxicphenomena (i.e. induction of cell roundness)probably provoked by the presence of toxicmolecules in all the samples investigated.Therefore subcultures were prepared in order toverify the actual presence of viruses.

Nevertheless diagnosis of cytopathic effectswas negative, testifying for the absence of viralagents both in the starting substrates (anaerobiceffluent and OFMSW) and in the end product(compost). It is interesting to point out that freshOFMSW resulted free from enteroviruses inspite of the ascertained occurrence of viralagents in garbage putrescibles [31]. The transientascent of temperature (several hours over 60°C)with rising of ammonia concentration and pH inthe OFMSW mass, during transportation to thecomposting yard, may have acted as prominent

factors in a substantial reduction of recoverableviruses, likely to non-detectable levels. On theother hand analysis of the anaerobic effluent tendto confirm the virucidal efficacy of thermophilicdigestion.

CONCLUSIONS

The research performed has shown thatco-composting effluent from thermophilicanaerobic digestion of OFMSW mixed with freshOFMSW may be a reliable alternative in thecontext of an exhaustive management of the urbansolid waste and wastewater.

An integrated treatment such as thatdescribed here appears to be advantageous on thebasis of the work done for the following reasons:i ) the abatement of the phytotoxic effects

exerted by the digested effluent,i i ) a greater hygienization of the initial organic

biomasses,i i i ) the possibility to close the digester water

balance.The overall waste management plan

proposed will result in energy saving (bio-gas)and production of a stabilised residue (compost)for land application as organic amendment.Undoubtedly this residue can be more easilystored, transported and disposed of than thestarting materials.

ACKNOWLEDGEMENTS

The authors thank Mr Sandro Scatena of theInst i tu te of Agricultural Microbiology,University of Pisa, for technical assistance insetting up the composting experiments. Thesupport of Mrs Lucia Barontini (Dpt. ofBiomedicine, University of Pisa) for virologicalanalyses was also highly appreciated.

REFERENCES

1. Zucconi F., Monaco A., Forte M. V. and de Bertoldi M. (1985). Phytotoxins during thestabilisation of organic matter. In: Composting of Agricultural and Other Wastes (J. K. R.Gasser ed.), pp. 73-85, Elsevier Appl. Sci. Publ., London.

2. Lund E., Lydholm B. and Nielsen A. L. (1983). The fate of viruses during sludge stabilisation,especially during thermophilic digestion. In: Disinfection of Sewage Sludge: Technico-economical and Microbiological Aspects. Proc. CEC Workshop held in Zurich, May 11-13, 1982(A. M. Bruce, A. H. Havelaar & P. L'Hermite eds.), pp. 114-124, D. Reidel Publ. Company,Dordrecht.

3 . Demuynck M., Nyns E. J. and Naveau H. P. (1985). A review of the effects of anaerobicdigestion on odour and disease survival. In: Composting of Agricultural and Other Wastes (J.K. R. Gasser ed.), pp. 257-269. Elsevier Appl. Sci. Publ., London.

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4. Epstein E., Taylor J. M. and Chaney R. L. (1976). Effects of sewage sludge and sludge compostapplied to soil on some soil physical and chemical properties. J. Environ. Qual., 5(4), 422-426.

5. Vallini G., Pera A., Cecchi F. and Mata-Alvarez J. (1989). Co-composting as ultimate step forreclaiming digested organic fraction of municipal solid waste as agricultural fertilizer. Proc.Int. Symp. on Compost Recycling of Wastes, Athens, Oct. 4-7, (in press).

6. Cecchi P., Vallini G. and Mata-Alvarez J. (1990). Anaerobic digestion and composting in anintegrated strategy for managing vegetable residues from agro-industries or sorted organicfraction of municipal solid waste. Water Sci. and Technology, 22(9), 33-41.

7. Cecchi F., Traverso P. G. and Cescon P. (1986). Anaerobic digestion of organic fraction ofmunicipal solid waste: digester performance. The Science of Total Environ., 56, 183-197.

8. Cecchi F., Marcomini A., Pavan P., Fazzini G. and Mata-Alvarez, J. (1990). Mesophilicdigestion of the refuse organic fraction sorted by plant. Performance and kinetic study. WasteManag. & Res., 8, 33-44.

9. Cecchi F., Pavan P., Mata-Alvarez J., Bassetti A. and Cozzolino C. (1991). Anaerobic digestionof municipal solid waste. Thermophilic versus mesophilic performance at high solids. WasteManag. & Res., 9, 305-315.

10. de Bertoldi M., Vallini G., Pera A. and Zucconi F (1985). Technological aspects of compostingincluding modelling and microbiology. In: Composting of Agricultural and Other Wastes (J. K.R. Gasser ed.), pp. 27-41, Elsevier Appl. Sci. Publ., London.

11. MacGregor S. T., Miller F. C, Psarianos K. M. and Finstein M. S. (1981). Composting processcontrol based on interaction between microbial heat output and temperature. Appl. Environ.Microbiol, 41, 1321-1330.

12. Finstein M. S., Miller F. C , Strom P. F., MacGregor S. T. and Psarianos K. M. (1983).Composting ecosystem management for waste treatment. Bio / Technol., 1(14), 347-353.

13. APHA, AWWA, WPCF (1985). Standard Methods for The Examination of Water andWastewater. 16th Edition, Washington, DC.

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