This article was downloaded by: [North Dakota State University]On: 08 December 2014, At: 14:30Publisher: 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

Rheology of sludge from semi‐dry anaerobic digestion ofmunicipal solid wastePaolo Battistoni a , Gabriele Pava a , Franco Cecchi b & Paolo Pavan ba Dipartimento di Scienza del Materiali e della Terra , Universita’ di Ancona , Via BrecceBianche, 1–60100, Ancona, Italyb Dipartimento di Scienze Ambientali , Universita’ di Venezia , Calle Larga S. Marta 2137,I‐30123, Venezia, ItalyPublished online: 17 Dec 2008.

To cite this article: Paolo Battistoni , Gabriele Pava , Franco Cecchi & Paolo Pavan (1991) Rheology of sludge from semi‐dryanaerobic digestion of municipal solid waste, Environmental Technology, 12:10, 897-905, DOI: 10.1080/09593339109385084

To link to this article: http://dx.doi.org/10.1080/09593339109385084

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

Environmental Technology, Vol. 12. pp 837-905© Publication Division Selper Ltd., 1901

RHEOLOGY OF SLUDGE FROM SEMI-DRYANAEROBIC DIGESTION OF MUNICIPAL

SOLID WASTE

PAOLO BATTISTONI1, GABRIELE PAVA1, FRANCO CECCHI2 AND PAOLO PAVAN2

1Dipartimento di Scienza del Materiali e della Terra, Universita' di Ancona, Via Brecce Bianche,1-60100 Ancona, Italy.

2Dipartimento di Scienze Ambientali, Universita' di Venezia, Calle Larga S. Marta 2137, I-30123Venezia, I t a l y . •

(Received 6 December 1990; Accepted 22 July 1991)

ABSTRACT

Rheology of anaerobically digested organic fractions of municipal solid waste was studied. Awide range of total solids content (4-33%) in sludges from a plant operating at mesophilic andthermophilic conditions were examined. The rheological behaviour was described adopting aplastic model. High fluidity and low thixotropy for a content up to 10-12% of total volatile solidsand an exponential correlation between rigidity coefficient or yield stress and total volatilesolids were found. Significant correlations were found between rheological parameters andsludge properties or process performance.

INTRODUCTION

Biological processes offer importantenvironmental advantages with respect to thecurrently available options for material andenergy recovery from waste. The anaerobicdigestion system of organic fractions ofmunicipal solid waste (OPMSW) is one of themost promising of these options (1-4). In the lastfew years the dry fermentation systems,VALORGA and DRANCO (5-7), have receivedwide commercial attention. However, digesterperformances reported that these processes havevarious problems concerning the management of30-35% of total solids (TS) and with the dischargeof wastewater. Recently, an Italian semi-dryprocess, using 18-23% TS in the digester feed,thermophilic conditions and a short retentiontime, has been described (8,9). This process iscomparable to dry systems, and furthermoreoffers a finishing composition step which avoidsany wastewater streams (10). However,rheological investigations of such a system areessential because of its operation close to thelimits of mixing and of heat and mass transfer.

Rheology provides a fluid characterization aswell as useful information concerning thephysical properties of sewage sludge (11,12).Recently rheological parameters have been usedto control filtration, thickening (13,14),dewatering and conditioning (15,16). Theseparameters have been applied to define themaximum permissible solid concentration foranaerobic digestion of industrial waste (17),however, no such data is available for OPMSW.Sludge rheology is complicated by a number ofimportant factors such as: in-line out-linemeasurements, choice of instrumentation, rangeof shear rate and variations in size and nature ofparticulate matter. The literature of the past 30years offers limited information in that it isrestricted in the choice of viscosimeter and datatreatment, and furthermore only describeschemical means of the anaerobic digestion ofsludge (15). More recently, a standardizedmethodology for sewage sludge has beensuggested (18,19). A plastic behaviour withpreliminary correlation between rigiditycoefficient (RC-plastic viscosity) or yield stress(YS) and physico-chemical properties have also

897

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 1

4:30

08

Dec

embe

r 20

14

been presented. A new investigation (20) on awider range of sludges, furnished conclusiveresults on different Theological behaviours.Typical and atypical sludges from wastewatertreatment plants (in terms of total volatile solids(TVS)/TS ratio) were described as Binghamplastic fluids. A general mathematical modelwas adopted to calculate YS and RC in alternativeto the rheological measurement, on the basis ofTVS and capillary suction time (CST).

This research extends the study to OFMSWanaerobically digested, with the specific aim toexamine the rheplogy in a TS content higher thanthat previously utilized, and to define parametersnecessary to prevent fluid-dynamic problems insemi-dry reactors.

MATERIALS AND METHODS

Pilot Plant

The pilot plant used was a 3m3 mechanicallystirred digester. The working temperatures werecontrolled in the mesophilic and thermophilicranges (37 and 55 ± 1°C respectively). The gaspressure inside the digester was in the 200-300mm w.c. range. The reactor feeding was semi-continuous (twice a day) with total solid substrateconcentration in the 16-22% range.

The OFMSW came from the full-scalesorting plant of S.Giorgio di Nogaro (UD-Italy)where it was pre-composted. A supply of OFMSWwas guarantied twice a month. Analyses for

substrate characterization were carried out overthe whole experimental period and distribution ofrefuse fractions is reported (Table. 1). Themonitoring of the process was performed tocontrol mass balance and stability processparameters. The list of the parameters followedare: TS, TVS, total chemical oxygen demand(TCOD), total organic carbon (TOC), soluble totalsolids (STS), soluble volatile solids (SVS),soluble COD (SCOD) of the influent and digester,biogas and feed flow rates, pH, total alkalinity(TA), volatile fatty acids (VFA) of the reactor,content and percentage of CO2 in the biogas. Thegas production pattern between two subsequentdigester feedings was recorded, and the valuebefore feeding was used for this study (GP)according to a new approach on the anaerobicdigestion process modelling (21). Details on theexperimental devices, substrate origin, itscharacterization and analytical methods adoptedare reported elsewhere (22).

Rheology measurement

Sludge sampled at steady state was firstsieved with a N° 20 (0.841 mm) U.S. sieve toeliminate plastic, wood and coarse particleaggregates; then TS, TVS were determined, andacidic surface groups per unit of TS (GAT) weretitrated (19). From each sludge, a set of samplesat several solid concentrations (TS from 4.0 to33%) was obtained by thickening and/or low speed

Table 1 Chemical characteristics and fractions of the pre-composted OFMSW used in theexperiments. This was sorted in the industrial plant of S. Giorgio di Nogaro (Ud - Italy).

OFMSWCHARACTERISTICSTS,gKg-iTVS, %TSTCOD, % TSTC, % TSTKN, %TSP, %TS .STS, %TSSVS, % TVSSCOD,%TS

OFMSWFRACTIONS:PutresciblePaperWoodPlasticInert

Range

513.129.123.37,8-1.2

0.05-3.2-3.2-3.1 -

- 952.0-57.4-90.4•37.1-3.4

0.2221.625.428.8

%TS

58.95.11.11.8

33.1

Average

763.043.958.820.62.20.118.19.6

10.3

S.D.

81.35.4

17.45.80.5

No.Samples

21021041

18720

0.03 593.94.36.7

818127

&TVS

77.97.22.23.49.3

898

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 1

4:30

08

Dec

embe

r 20

14

centrifugation. For each sample CST wasdetermined before rheological measurement.Rheological measurements were carried out by acommercial rotational viscosimeter (HAAKERV 12) using cells with a profiled sensor system(MVIIP and SVIIP) to prevent sludge slippage oninner rotor surface and outer cylinder;temperature was maintained at 20 ± 0.5 °C. Thedetails of the procedure adopted are describedelsewhere (19). The measurement conditionswere:- sample stirred directly inside the rheometer

for 4 min at shear rate of 100 sec*1

- increase in shear rate from 0 to 100 sec"1 in120 seconds

- stirring kept at 100 sec"1 for 60 seconds- decrease in shear rate from 100 to 0 sec"1 in

120 seconds.The plot of shear stress versus shear rate was

obtained directly from 100 pairs of data, thentransmitted to an interfaced computer tocalculate the best fitting equation (rising shearrate of the flow curve). Results are expressed asan average value of three measurements.

RESULTS

The pilot plant operated for more than twoyears at various steady state operativeconditions. This allowed the study andoptimization of the semi-dry mesophilic andthermophilic anaerobic digestion processesapplied to the mechanically sorted OPMSW (8-10,22). During this investigation, rheologicalmeasurements relative to eight processconditions were carried out. Reactorperformances, summarized in Table 2, grouptogether conventional and unconventionalparameters; GPbf and SGPbf represent theinstantaneous and specific gas productionrespectively in terms of total volatile solidcontent inside the digester before a new feed (thereactor was fed twice a day in a semi-continuousway). Those parameters also reflect thebiodégradation obtained. The gas productionpattern between consecutive feedings ischaracteristic of the available substrateaccording to the step-diffusional model (21). Afurther parameter not utilized usually is Fb,which refers to gas or methane production and isdefined as the ratio between specific gas ormethane produced and the ultimate gas ormethane production (23, 24).

The properties of sludges used aresummarized on Table 3. GAT are present atvalues less than 0.16 meq.g-1 TS in agreement

with the typical values of the anaerobic digestionprocess. Rheological properties are obtainedthrough a plastic model, since both plastic (eq.I)and pseudoplastic models (eq.II) give a standarderror (SE) less then 5% in all cases. Actually, thebest model varies with TS content. A trend isobserved between plastic to pseudoplastic as TSincreases. Those results are inverse withrespect to those previously reported for sewagesludges (18). The high TS content allowsunusually high CST values and an unexpected lowrange of RC and YS (Table 3). Thixotropypresents very low values ranging from 12 to 161Pa s"1.

YS + RCD (I)

(II)

where D = shear rateX = shear stressK = semiempirical coefficientn < l

DISCUSSION

The comparison of rheological data for thedifferent sludges is normally carried outconsidering the TS content, however in this wayany observation can be incorrect because sludgeswith different TVS to TS ratios are compared. Abetter analysis can be drawn representingrheological data versus TVS (Figs. 1 a-b) sincethe mineral fraction of TS has a low to null effecton rheology (20). At least three basicconsiderations are evident:1) OFMSW sludges up to 10-12% TVS have RC's

an order lower than those from wastewaterplants (20) (aerobically and anaerobicallydigested). Although no experimental data areavailable for sewage sludge at higher TVScontent (they do not represent attainableprocess conditions), data extrapolation onthat region shows a flattening trend leading tothe same plastic viscosity for both sludges.On the contrary, yield stress is always lowerindependently from the TVS content (Fig.lb).

2) The patterns of rheological data versus TVSindicate an exponential relation in OFMSWsludges compared to the power typecorrelation observed in sewage sludges (20)(compare eqs. Ill and IV, VI and VII onTables 4-5).

3) The exponential equations for YS (IV) and RC(VII) consent the rheologicalcharacterization of dry and semi-dry

899

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 1

4:30

08

Dec

embe

r 20

14

processes. Supposing an equal TVS to TS Pa were excluded for the YS). Despite theratio (50%), results shown in Table 6 stress different types of sludges, the sustained numberthe management problems in dry digestion of data (42 experimental points) leads towith a TS increase from 20.5 to 32.5%, YS and acceptable standard errors for RC and YSRC rise from a half to one order, correlations (eqs. IV and VII). However betterrespectively. results have been obtained by introducing CST, a

quick and easy measurable parameter (Tables 4Coefficients, exponents and constants of and 5). In this case, a very low SE (5%) for RC was

Theological properties correlated with TVS, are obtained (eq. X); while for YS the improvementreported in Tables 4 and 5 (values lower than 1.5 on the SE is reduced from 28 to 25% (eq.V).

Table 2 Peed and digester characteristics, operative conditions, digester performance andparameters used for the rheology behaviour modeling at steady-state mesophilic (M) andthermophilic (T) conditions.

Run IM

PEED CHARACTERISTICS:

TS.gKg"1 .TVS, gKg"1

STS,%TSSVS, % TVSTCOD, g Kg-1

SCOD.gKg-1

TOC,% TSTVFA, g CH3COOH I"1

pHTA6, g CaCC-3 HTA4, g CaCOa I"1

222.8110.5

4.23.3

105.319.314.52.767.061.14.5

REACTOR CHARACTERISTICS:TS.gKg-lTVS, g Kg"1

STS, % TSSVS, % TVSTCOD, g Kg"1

SCOD.gKg-1

TVFA, g CHaCOOHl"1

pHTA6, g CaCO3HTA4, g CaCO3H

163.573.6

5.75.9

94.46.30.277.134.88.8

OPERATIVE CONDITIONS:T, °CHRT, daysOLR, KgTVS/m3d

37.414.7

7.5

DIGESTER PERFORMANCE:GPR, m3/m3dSGP, m3/KgTVSfGPbf, m3!!-1

SGPbf, mSh-^KgTVSrFb, %CH4, %TVS removal, %

1.40.200.140.65

87.152 '23

IT

161.186.312.313.9

102.719.016.67.286.480.54.9

141.666.4

6.17.2

69.05.90.397.442.96.0

55.014.65.9

2.50.430.211.05

966148

3T

230.5145.4

7.89.8

130.518.422.54.876.690.85.3

194.777.3

3.14.5

75.05.20.597.363.26.8

54.911.39.2

2.70.290.210.92

645734

5T

224.6106.0

6.99.0

183.014.723.10.787.170.83.8

173.376.1

6.17.6

86.05.50.697.243.06.9

54.67.8

13.5

4.10.300.451.97

605337

6T

210.6121.5

6.96.2

159.110.928.82.617.420.85.3

179.186.1

7.07.6

93.38.72.007.172.47.6

51.56.1

19.9

4.70.230.582.22

495727

7T

205.1120.9

8.39.1

145.115.328.5

3.67.161.15.3

182.390.3

8.77.7

104.45.90.457.293.79.2

47.76.1

18.8

4.30.240.431.43

546127

8T

216.9123.7

8.79.4

134.513.026.53.137.280.94.7

186.890.3

6.47.3

117.25.00.427.254.36.7

48.17.8

13.9

3.50.290.341.29

5862

3

9T*

195.6105.810.810.6

128.722.0. .

11.46.30.69.7

143.274.8

6.85.6

99.14.30.337.64.89.2

55.27.4

12.9

5.20.370.291.24

505840

This run was carried out using fresh mechanically sorted OFMSW (uncomposted).

900

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 1

4:30

08

Dec

embe

r 20

14

1E2-

1E 1-

8

en

toQ.

IE 0-

TVSX

Figure la RC vs total volatile solids TVS%

1E2-

S.

tocuc_-uVi•ai—icu

2 4 B B ^ l o 12 \4 16 18 2TYS%

TA = Typical and Atypical sewage sludges

Figure lb YS vs total volatile solids TVS%

901

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 1

4:30

08

Dec

embe

r 20

14

Table 3 Physico-chemical characteristics of sludges. Analysis carried out for the rheologicalmeasurement.

SLUDGE

IMIT3T5T6T7T8T9T

TS%

4.4+185.0+32.77.1+27.94.8+10.09.8+27.28.6+20.97.7+18.47.5+25.2

TVS/TS%

51.547.549.045.259.345.346.849.7

GATmeqg-1 TS

0.110.130.070.140.100.030.160.13

CSTs-

1688-30623200+41001700+34002700+43002060+32002400-31001646+2921

RCmPas8+54

10+42013+1319+32

23+84016+30017+7317+710

YSPa

0.25+1.20.26+37.71.4+13.51.2+3.12.1+63.01.4+36.71.7+5.8

0.04+5.06

TxPa/s

12+12536+16117+5022+3819+100

.18+6013+48

n.d.

TVS/TS - total volatile to total solids ratio; RC - rigidity coefficient, GAT - surface acidic groups;YS - yield stress;Tx - thixotropy; n.d. - not detected

Table 4 Correlation between yield stress (measurements > 1.5 Pa) and total volatile solids(TVS), operative plant parameters and capillary suction tim (CST).

model(III) YS

(IV) YS

(V) YS

= K TVSa

= Ke*TVS

= K ea TVS * CSTb

statistical andn = 21

R2 = 0.56SE = 40

n = 21R2 = 0.78SE = 28

n = 21R2 = 0.81SE = 25

correlationK = 0.33a = 1.42

K = 0.70a = 0.28

k = 143a = 0.24b = -0.65

parametersP < 0.05P < 0.001

P < 0.14P < 0.001

P < 0.09P < 0.001P < 0.07

Table 5 Correlation between rigidity coefficient (mPas), total volatile solids, (%TVS), operativeplant parameters and capillary suction time (CST)

model statistical and correlation parameters(VI) RC = KTVS a

(VII) RC = K e a T V S

(VIII) RC = K

(IX) RC = K

(X) RC = K e a T V S *

n = 42R2 = 0.77SE = 14

n = 42R2 = 0.85SE = 11

n = 42R2

s 0.88SE = 9n = 42

R2= 0.88SE = 9

n = 42R2 = 0.97SE = 5

K=1.3a

Ea

kbc

kab

kab

= 1.93

: = 4.2= 0.36

= 50.4= 0.35= -0.57= 3.6= 0.37= 0.46

= 223= 0.36= -0.49

P < 0.32P < 0.001

P < 0.001P < 0.001

P < 0.001P < 0.001P < 0.009P < 0.001P < 0.001P < 0.006

P < 0.002P < 0.001P < 0.003

n - number of data; RC - rigidity coefficient (mPas); R2 - Pearson coefficient;TVS - total volatile solids (%); SE - standard error (%); CST - capillary suction time (s);SGP - SGPbf (Table 2); YS - yield stress (Pa)

902

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 1

4:30

08

Dec

embe

r 20

14

Both equations show a negative CST exponent; thiscircumstance appears unexpected and as yet hasnever been reported for sewage sludges (18),nevertheless it can be explained by consideringhow CST varies with TS. Different patterns canbe clearly noted for OFMSW sludge, CSTdecreases with TS following a parabolic modelXI. On the contrary, in sewage sludge, CSTincreases with TS exponentially (XII) or in apower mode (XIII).

The different trends in the same TS region(5-15 %) suggest different sludge characteristicsand particle size distribution.

CST=a + bTS + cTS2 k O . a a n d o O (XI)

CST = k e a T S 0<a< l (XII)

CST = k T S b b>2 (XIII)

Although the introduction of CST optimizedthe equations, the fact remains that asemiempirical parameter is used to calculaterheology and it is useful only for already existingsystems. For this reason it was attempted toutilize the operative digester parameters, inorder to find a mathematical model able to:

a - predict fluid dynamic characteristics withoutany specific measurement;

b - describe the behaviour of the digester underdifferent operative conditions.

The inherited difficulties in obtaining all theprocess parameters were surmounted becausethis work was included in a wider researchprogramme where the process performance wasalso studied at different operative conditions. Amultivariate statistical procedure conducted overall the available parameters (Table 2) allows usto obtain satisfactory results only for RC assummarized by eqs. VIII and IX (Table 5).Among all the parameters tested, only Fb and/orSGPbf contribute effectively to sludge viscosity.In spite of the different physical meaning, Fb andSGPbf result with the same power in the viscositydefinition as illustrated by the associated SE.SGPbf gives information on the instantaneouslevel of the substrate utilization inside thedigester; but under the steady conditions used, itremains constant and connected with Fb.Clearly a micro-organism content inside thedigester and with a degree of feed biodégradationconstant, must give the same information onrheology, since sludge sampled is constituted of

anaerobic biomass and residual substrate. Theseconsiderations suggest the possibility of usingSGPbf to model a non-steady state.

Table 6 Dry and semi-dry process fluid-dynamiccharacterization on the basis ofexponential equations of RC (VID and YS(IV)

Process TS%

TVS RCmPa*s

YSPa

DrySemi-dry

32.520.5

16.310.3

1485171

6712

OFMSW sludges reveal a low thixotropycompared to sewage sludge (19). A tentativecorrelation between thixotropy and processperformance shows a power type equation withTVS and Fb (XIV). The TVS exponent is 1.01against 3.0 for sewage sludge (19) (It remainsunchanged with or without Fb). This demonstratesthe effect of the different nature of volatile solidson thixotropy. Naturally Fb, representing thedegree of feed transformation from OFMSW toanaerobically digested sludge, is relatedpositively to thixotropy.

Tx = 0.13 TVS101 * Fb0-95 R2 0.77, SE 9% (XIV)

CONCLUSIONS

Rheology of anaerobically digested OFMSWwas studied. Measurements in a wide range of TScontents (4-33%) of sludges sampled in eightmesophilic and thermophilic reactors at steadystate, consent to evaluate the fluid dynamic of asemi-dry process. The main results can besummarized in the following:- Both Bingham plastic and Ostwald pseudo-plastic models can well describe rheologicalbehaviour of OFMSW sludge; plastic model waspreferred as it allows a better representation ofthe physical characterization of viscosity(rigidity coefficient).

A higher fluidity and lower thixotropy ofOFMSW, containing TVS up to 10-12%, withrespect to wastewater treatment sludges arenoted; at the same time an exponential relationbetween RC or YS and TVS is observed.

A mathematical model based on theexperimental data is formulated to calculaterheological parameters by means of a simpledetermination of TVS and CST.

Fb or SGP digester performance parametersconsent to predict sludge rheology even if with a

903

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 1

4:30

08

Dec

embe

r 20

14

lower precision/Furthermore, the physical state - SGPbf could be a useful parameter forof the sludge examined cannot be represented modeling rheological behaviouT of sludges insimply by any classical feed description digesters operating under non-steady conditions,parameter, reactor characteristics or operative This is a topic of research in progress,conditions.

REFERENCES

1. K.M. Richards, The U.K. landfill gas and MSW digestion industry, in Landfill Gas andAnaerobic Digestion of Solid Waste, Y.R. Alston and G.E. Richards, eds., Harwell LaboratoryPubl., 1989, 12-36 .

2 . A. Brown, Refuse the energy option, in Landfill Gas and Anaerobic Digestion of Solid Waste,Y.R. Alston and G.E. Richards, eds., Harwell Laboratory Publ., 1989, 3-11.

3. D.P. Chynoweth and R. Legrand, Anaerobic digestion an integrate part of MSW management, inLandfill Gas and Anaerobic Digestion of Solid Waste, Y.R. Alston and G.E. Richards, eds.,Harwell Laboratory Publ., 1989, 467-480.

4. J. Coombs and Y. Alston, The potential of recovery of energy from solid waste using fabricateddigester system, in Landfill Gas and Anaerobic Digestion of Solid Waste, Y.R. Alston and G.E.Richards, eds., Harwell Laboratory Publ., 1989, 454-466.

5. B. De Wild, W. Six and L. De Baere, Recycling of solid organic wastes throw dry anaerobicconversion, in Recycling International, K.J.Thomè-Kozmiensky, ed, EF. Verlag GmbH, Berlin,1989, 248-255.

6. O. Begoven, E. Thiebaut, A. Pavia and J.P. Peillex, Thermophilic anaerobic digestion of theMSW by Valorga process, in 5th Int. Symp. on Anaerobic Digestion - Poster Papers, A. Tilcheand A. Rozzi, eds., Monduzzi Ed. Bologna ITALY, 1987, 789-792.

7. F. Cecchi, P.G. Traverso, J. Mata-Alvarez, J. Clancy and C. Zaror, State of the art of R & D in theanaerobic digestion process of MSW in Europe, Biomass, 16, 257-284 (1988).

8. F. Cecchi, G. Fazzini, P. Pavan, A. Bassetti, A. Farneti and U. Barbaresi, Dry thermophilicanaerobic fermentation of the organic fraction of M.S.W. in conventional reactor, in 5th EECConf. Biomass for Energy and Industry, Abstracts, Lisbon, Portugal, 9-13 October, 1989, 15-17 .

9. F. Cecchi, P. Pavan, J. Mata-Alvarez, A. Bassetti and A. Farneti, Analysis of the thermophilicsemi-dry anaerobic fermentation of the organic fraction of the Municipal Solid Waste sorted byplan mechanisms, in Recycling International, K. J. Thomè-Kozmiensky, ed., EF.- VerlagGmbH, Berlin, 1989, 241-247.

10. F. Cecchi, P. Pavan, A. Bassetti, A. Farneti, U. Barbaresi and A. Frigerio, Semi-dry fermentationdella sostanza organica dei R.S.U. selezionata in implanto, in Trattamento e Smaltimento deiRifiuti Solidi Urbani e Industriali, CI.ESSE.I Ed., Milano, 1990, 116-123.

11. G.A. Carthew, CA. Goehring and J.E. Van Teylingen, Development of dynamic head losscriteria for raw sludge pumping, J Water Pollut. Control Fed., 55, 472-483 (1983).

12. M.C. Mulbarger, S.R. Copas, J.R. Kordic and F.M. Cash,_Pipeline friction losses for wastewatersludges, J Water Pollut. Control Fed, 53, 1303-1313 (1981).

13. R.C. Frost and J.A. Ovens, A method of estimating viscosity and designing pumping systems forthickened heterogeneous sludges, in 8th Int. Conf. "Hydr. Trans. Solids in pipes", Johannesburg,South Africa, 1982, August 25, 485-501.

14. A. Valioulis, Inter-relation between sludge characteristics, Master Thesis, Cornell Univ.,Ithaca (N.Y., U.SA.) (1985).

15 H.W. Campbell and P.J. Crescuolo, The use of rheology for sludge characterization, Water Sci.Technol., 14, 475-489 (1982).

16. H.W. Campbell and P.J. Crescuolo, Assessment of sludge conditionability using rheologicalproperties, in Proc. E.E.D. Work, Dublin, 6 July 1983, D. Reidel, Dordrecht, The Netherlands,(1983), 31-46.

17. F. Colin, Characterization of the physical state of sludges, in Seminaire Methanisation du

904

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 1

4:30

08

Dec

embe

r 20

14

COMES, Sofia, Antinopolis, June 10-11, 1982.18. P. Battistoni and G. Fava, Fanghi di depurazione: comportamento reologico e correlazioni con

alcune propriété chimiche e fisiche, in Congresso Biennale A.N.D.I.S., Rome, Italy, 16-17December, 1988, 295-309.

19. P. Battistoni, G. Fava and L. Ruello, Rheology of sewage sludge (I): development of a standardizedmethodology, Ing. Sanit., 2, 10-19 (1990).

20. P. Battistoni, G. Fava and L. Ruello, Rheology of sewage sludge. (III) Rheological parameters andsludge characteristics, Environ. Prot, Eng., in press (1991).

21 . F. Cecchi, J. Mata-Alvarez, P.G. Traverso, F. Medici, G. Fazzini, A new approach to the kineticstudies of anaerobic degradation of the organic fraction of MSW, Biomass, 23, 79-102 (1990).

22. F. Cecchi, A. Marcomini, P. Pavan, G. Fazzini and J. Mata-Alvarez, Mesophilic digestion of theorganic fraction of refuse: performance and kinetic study, Waste Man. Res., 8, 33-44 (1990).

23. J. Mata-Alvarez, F. Cecchi, P. Pavan and P. Llabres, The performances of digester treating theorganic fraction of MSW defferently sorted, Biological Wastes, 33, 181-199 (1990).

24. Y. Chen and A. Hashimoto, Kinetics of methane fermentation, Biotech. Bioeng. Symp., 8, 269-282(1978).

90S

Dow

nloa

ded

by [

Nor

th D

akot

a St

ate

Uni

vers

ity]

at 1

4:30

08

Dec

embe

r 20

14