This article was downloaded by: [Monash University Library]On: 05 October 2014, At: 16:06Publisher: 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 subscriptioninformation:http://www.tandfonline.com/loi/tent20

The impact of four design parameters onthe performance of a high‐solids anaerobicdigestion of municipal solid waste for fuelgas productionM. Kayhanian a & S. Hardy aa Department of Civil and Environmental Engineering , University ofCalifornia at Davis , Davis, CA, 95616, USAPublished online: 17 Dec 2008.

To cite this article: M. Kayhanian & S. Hardy (1994) The impact of four design parameters on theperformance of a high‐solids anaerobic digestion of municipal solid waste for fuel gas production,Environmental Technology, 15:6, 557-567

To link to this article: http://dx.doi.org/10.1080/09593339409385461

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 ourlicensors 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 thispublication are the opinions and views of the authors, and are not the views of or endorsedby 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 liablefor any losses, actions, claims, proceedings, demands, costs, expenses, damages, and otherliabilities whatsoever or howsoever caused arising directly or indirectly in connection with, inrelation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantialor systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, ordistribution in any form to anyone is expressly forbidden. Terms & Conditions of access and usecan be found at http://www.tandfonline.com/page/terms-and-conditions

Environmental Technology, Vol. 15. pp 557-567© Publications Division Selper Ltd., 1994

THE IMPACT OF FOUR DESIGN PARAMETERS ONTHE PERFORMANCE OF A HIGH-SOLIDS

ANAEROBIC DIGESTION OF MUNICIPAL SOLIDWASTE FOR FUEL GAS PRODUCTION

M. KAYHANIAN* AND S. HARDY

Department of Civil and Environmental Engineering, University of California at Davis,Davis, CA 95616, USA

(Received 24 November 1993; Accepted 14 February 1994)

ABSTRACT

The impact of four design parameters on the performance of a pilot scale high-solids anaerobicdigestion process is evaluated and discussed in this paper. Results obtained from this studyindicate that the rate of methane gas production was inversely proportional to the averagefeedstock particle diameter. Feedstock with various C/N ratios, ranging from 8 to 125, were fedinto the reactor and an optimum C/N ratio, based on biodegradable carbon and total nitrogen, wasfound to be about 25 to 30. Biogas production increased with an increase of organic loading rate upto a rate of 7.5 g BVS kg-1 active reactor mass d. Organic overload occurred at 7.6 g kg-1 andcontinued up to 11.5 g kg-1 active reactor mass at which point the volatile fatty acids accumulationin the reactor reached an inhibitory limit and caused digester failure. Mass retention times of30, 20, and 15 days were investigated, while the organic loading rate was kept constant at about 7 gBVS kg-1 active reactor mass. Shorter MRT's resulted in decreased gas production, presumablydue, at least in part, to the removal of active micro-organisms from the reactor.

Keywords: High-solids anaerobic digestion, performance, design parameter, particle size, C/Nratio, organic loading rate, mass retention time

INTRODUCTION

Municipal solid waste (MSW) disposal is amajor concern in urban areas. Increases inurban waste production have been accompaniedby a decreased availability of landfill sites forwaste disposal. To increase the life of landfills,and to reduce some of the environmentalproblems associated with existing landfills,most communities are forced to divert a largefraction of their MSW from landfills. A majorportion of the organic fraction of the material tobe diverted from landfills can be used for theproduction of compost and methane gas, orconverted to energy in resource recoveryfaculties.

With current environmental regulationsconcerning incineration, and interest inrenewable energy, the use of anaerobic digestionfor energy production from the biodegradableorganic fraction of MSW (BOF/MSW) has been

given serious consideration (1-8). Anaerobicdigestion of BOF/MSW has the advantage ofreducing the fermentable MSW up to 57 percentcompared to the volume that material wouldoccupy in a well compacted landfill (5). Themass lost in this reduction is converted to abiogas with a clean burning quality and mediumthermal energy value, ranging from 19.0 to 24.5MJ m'3. This avoids the air quality problemsassociated with direct MSW combustion. Inmany large cities this can be a major advantage.

A newly developed high-solids anaerobicdigestion/aerobic biodrying (anaerobiccomposting) process is under investigation at theUniversity of California at Davis (UC Davis) forthe production of energy and humus fromBOF/MSW. This process has been successfullyoperated for more than three years and itstechnical feasibility has been confirmed (5, 7,8). The high-solids anaerobic digestion processis relatively new and the parameters used to

557

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

design a conventional low-solids anaerobicdigestion process may not be applicable to high-solids system operation design. This study wasundertaken to overcome this problem, and toobtain a range of typical values for four designparameters pertinent to the high-solidsanaerobic digestion of BOF/MSW. Theparameters studied include: (i) feedstockparticle size, (ii) feedstock C/N ratio, (iii)organic loading rate, and (iv) mass retentiontime.

MATERIAL AND METHODS

The materials and methods used to conductthis study are discussed in this section. Thetopics include: (i) description of pilot facility,(ii) analytical and computational methods, (iii)feedstock material and methods of testing, and(iv) experimental methods.

Description of Pilot Facility

The pilot scale high-solids anaerobicdigester used is the first stage of a two stageprocess (see Figure 1). The reactors shown inFigure 1, were designed and constructed byMicrogen Corporation in Ithaca, New York. Thehigh-solids anaerobic reactor is equipped withseveral important features including (i) amechanical agitator to mix the contents of thereactor, (ii) an electric thermal blanket toincrease the temperature of the active reactormass, (iii) a platform scale under the entirereactor to measure the daily mass loss, and (iv)an electrical control panel, which controls thetemperature and mixing period of the reactorsystem. The control panel is equipped with anelectrical system so that the reactor can beoperated in manual or automatic mode. Asummary of the physical and operationalcharacteristics of the pilot-scale high-solids

anaerobic digester is reported in Table 1.Additional information on this pilot facility canbe obtained from reference 5 and 7.

Analytical and Computational Methods

Analytical tests regularly conducted tomonitor the digester include: total solids,volatile solids, ammonia, TKN, volatile fattyacids (VFA), pH, alkalinity, and biogas volumeand composition. All tests were conductedaccording to standard methods for measuringwater and wastewater characteristics (9). A gasChromatograph was used to analyze the biogascomposition. The feedstock's C/N ratio wascomputed based on biodegradable carbon and totalnitrogen, as outlined in Kayhanian andTchobanoglous (10).

Feedstock Material and Methods of Testing

The feedstock for the pilot-scale high-solidsanaerobic digestion process was a blend of clean,source separated wastes mixed to simulate atypical BOF/MSW of the City of Folsom,California in the United States of America. Thephysical and chemical characteristics and thebiodegradability of the BOF/MSW aresummarized in Table 2.

The physical characteristics were measuredto determine the amount of water to be added andto explore the effect of feedstock particle size ongas production r a t e . The chemicalcharacteristics were measured to assess nutrientconcentrations and to determine the feedstocks'C/N ratio. Finally, biodegradability wasmeasured to compute daily organic loading rate,feedstock C/N ratio, substrate removal rate, andbiogas production rate. The physical andchemical characteristics of the feedstock weredetermined according to the test methods outlinedin ASTM for biomass (9, 11). The

Biodegradableorganic fraction

ofMSW

Heat Biogas

A

Complete-mixreactor

Heat Air

Humus

Complete-mixreactor

Anaerobically digested solids

Figure 1. Basic flow diagram of the two stage process.

558

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

Table 1. Summary of the physical and operational characteristics of the pilot scale high-solidsanaerobic digestion process0.

Parameters

Physical CharacteristicsReactor typeMixing mechanismTotal reactor volumeTotal active reactor massHeating mechanism

Operational CharacteristicsOperating temperatureMixing rate (intermittent)Reactor TS concentrationTotal wet mass loading ratec

TS loading rateVS loading rateBVS loading rated

Organic loading rateFirst order rate constant, kInfluent substrate cone, Si

Unit

Lkg

Range of values

Complete-mixMechanical

2,2501,800-2,000

Electric thermal blanket

°Cmin/min

% of wet weightkg/dkg/dkg/dkg/d

g BVS kg"1 ARM« de

1/dkg BVS kg"1 feed

53-602/30

20-3058-12618-4215-3410-136-7.2

0.14-0.20.095-0.21

Typical 30db

2,2501,900

552/30

26632117116.5

0.180.19

This table is adapted from reference 8.The values reported are based on a mass retention time of 30 d.Digester was fed 7 days per week.The BVS mass was calculated by using the following expression: BVS mass = (wet mass)(%TS)(%VS of TS)(%BVSofVS)ARM = active reactor mass

biodegradability of the feedstock materials weredetermined by long-term batch digestion studies.

Experimental Procedures

Four experiments were conducted todetermine the operational range and optimumvalue of the selected design parameters. Theprocedures used to conduct these fourexperiments are briefly described below.

Experiment 1 (Feedstock Particle Size):The particle size of the yard and food wastes

used in this study was consistently less than50mm, while the size of the office paper varied.Three different sizes of office paper (a highlybiodegradable component of the organic fractionof MSW) were examined for this study. Becausethe particles of office paper were so irregular, ashape factor, a, was used, with the nominalparticle diameter (Dp), to allow particles ofdifferent sizes and shapes to be compared as ifthey were circular. The values of a used were inthe range of 0.75 to 0.9 for larger and smalleroffice paper particle sizes, respectively. For the

purpose of this study the product aDp will be usedfor particle size. This experiment was conductedunder the operating conditions reported for a 30dmass retention time as specified in Table 1.

Experiment 2 (Feedstock C/N Ratio):A daily feedstock composed of a mixture of

biodegradable organic fractions of MSW withvarious C/N ratios was prepared and fed to thereactor, which was operated under the conditionsreported for a 30d retention time as specified inTable 1. The reactor's performance parametersand stability were monitored and the effect of thefeedstock C/N ratio on digester performance wasnoted. The adjustment of feedstocks to a high orlow C/N ratio was achieved by adding more paperor food and yard waste, respectively. An optimumC/N ratio was identified when the gas productionrate and reactor stability was at its peak.

Experiment 3 (Organic Loading Rate):In this experiment, the process was operated

under the typical conditions for a 30 day massretention time (see Table 1), while the BVSorganic loading rate was gradually increased to

559

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

Table 2. Characteristics of the biodegradable organic fraction of MSW as a feedstock.

Parameter Unit Range of values Typical

Physical Characteristics

Moisture contentBulk densityParticle size

Chemical Characteristics

wet masskg/m3

mm

18-25480-6005-100

2156051

Carbon, CChlorine, ClCobalt, CoHydrogen, HIron, FeMolybdenum, MoNickel, NiNitrogen, NOxygen, 0Phosphorus, PResidueSelenium, SeSulfur, STungsten, W

Biodegradability

Biodegradable fraction15

%%

ppm%

ppmppmppm

%%%%

ppm%

ppm

%VS

43-500.2-0.38<0.1-0.3

5-684-2000.5-2

<0.1-31-3.244-50

0.05-0.15-9

<0.010.08-0.250.05-0.15

64-72

45.660.280.15.99163

11.51.23

40.590.086.00

<0.010.20.1

68

a Feedstock is comprised of 50% paper (12.5% newsprint and 37.5% office paper), 12.5% yard waste and37.5% food waste (% by wet weight) with water added to adjust the total solids. These percentagesrepresent a typical MSW from Folsom, California. This table is adapted from reference 8.

b Determined from a long-term batch study.

determine the digester's operational and critical inert filler, to the feedstock. To evaluate thezones. To evaluate the effects of this change, the effects of MRT, alkalinity, ammoniagas production rate and reactor pH were concentration, gas production rate, massmonitored regularly. removal rate, reactor pH, and volatile fatty acids

concentration were monitored regularly. TheExperiment 4 (Mass Retention Time): digester was operated for a period of two months

It is a common practice now to use mass stable operation at each MRT and an optimumretention time (MRT) in a high-solids anaerobic MRT based on the above parameters wasdigestion process rather than the conventional identified,hydraulic retention time (5, 12). The massretention time refers to the inflow stream and is RESULTS AND DISCUSSIONdefined as the ratio of the total reactor wet massto the total influent wet mass flow rate, which The operating ranges and typical values ofincludes substrate, dilution water, and other the tested design parameters, based on methanecomponents. Mass retention times of 30, 20, and gas production and reactor stability, of a high-15 days were investigated, while the organic solids anaerobic digestion process are reportedloading rate was kept constant at about 7g BVS kg"1 below. In addition, the effect of each designactive reactor mass'd. This was accomplished parameter on process stability and fuel gasby the addition of the dry humus end product, as an production is discussed.

560

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

Feedstock Particle Size

Feedstock particle size reductioninfluences biological processes in two ways: (i)natural barriers to bacterial attack aredisrupted-these barriers may include films,waxy coatings, and other surface protectants thatimpede the microbe's access to the components tobe decomposed, and (ii) the ratio of surface areato mass is increased. The larger the surface areaexposed to bacterial attack or bio-chemicalactivity, the more rapid may be the bio-conversion (13).

The effect of office paper particle size, inthe feedstock mixture, on methane gas productionfor a constant organic loading rate is shown inFigure 2. As shown in Figure 2, a 25 percentincrease in gas production was achieved when theoffice paper particle size diameter was reducedfrom 215 mm to about 41 mm. Similar resultswere achieved by other researchers when theeffect of particle size on the anaerobic digestionof solid waste residue in a landfill and tomatowastes was considered (14, 15). It can be shownthat the rate of methane gas production is

inversely proportional to the average substrateparticle diameter. In theory, this statementimplies that the more closely the dimensions of aparticle approach the molecular, the more rapidand thorough will be the bio-conversion. Whilesmaller particle size will reduce materialhandling difficulties and help to elevate the gasproduction rate, the reduction of feedstockparticles below a certain size is costly andeconomically unjustified (16).

Feedstock C/N Ratio

To develop anaerobic digestion at thecommercial level, attention must be focused onprocess stability. Nutritional deficiencies mayresult in reactor instability and incompletebioconversion of the organic substrates. Whenthe anaerobic digestion process is applied to thebiodegradable organic fraction of MSW,bacterial nutritional requirements must beaddressed, and nutrient supplementation may berequired (17).

Methanogenic bacteria have a variety ofmineral nutrient requirements for robust growth(18, 19). The roles of organic carbon and total

05

I

TO03

ra

3.0

2.8 -

2.1 2.4TS%2

2.6 —

2.2 —

a 2.0 —

1.8

1 1 1 1

, , , ,

1 1 1 1

<

\ *

\

1 1 1 1

, , , ,

1 1 1 1

•

. , , ,

1 1 1 1

_

;

_

-

, . , ,

50 100 150 200

Particle size diameter, aDp (mm)

250

Figure 2. Effect of particle size on methane gas production rate Feedstock C/N Ratio.

561

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

Table 3. Performance of the high-solids anaerobic digestion process under four feedstock C/Nratios.

Parameter Unit 10

Average valueAverage feedstock's C/N ratio

25 40 82

AlkalinityAmmonia nitrogenBiogas production ratea

Biogas production rateMass removal rateMass removal efficiencyPHVolatile fatty acids

mg I/1 as CaCO3

mg I/1

V/active reactor Vm3 kg"1 BVS added'dg BVS kg"1 ARM» db

% BVS mass fed

mg L"1 as acetic acid

80001,600

2.50.452.6

406.65

1,200

12,000750

5.40.855.5

847.25

750

10,000450

3.20.554

707

900

9,500300

30.513.5

606.9

850

a The biogas production rate reported is based on dry and standard conditions (0 °C and 1 atm.).b ARM = active reactor mass.

available nitrogen in cell synthesis, growth, andmetabolism of anaerobic bacteria are veryimportant. For proper nutrition, these elementsmust be present in the substrate in the correctratio. An exact optimum ratio for the high-solidsanaerobic digestion of the organic fraction ofMSW for methane gas production has not beenestablished.

The performance of the high-solidsanaerobic digester, based various parameters, atfour feedstock C/N ratios is reported in Table 3.

In addition, the reactor stability, based onpH, at various feedtosck C/N ratio is shown inFigure 3. As shown in Figure 3, the reactor wasoperated at feedstock C/N ratios ranging from 8to 125. Long-term operation of the digester at a

200

160

g2 120

XLÜ

2</)•oa>

80

40

50 200100 150

Operational period (d)Figure 3. Effect of feedstock's C/N ratio on digester pH.

250

562

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

high or low C/N ratio was normally associatedwith unstable reactor performance (i.e., pHdecreased to 6.5 or below). The principalproblem associated with a high C/N ratio isnutrient deficiency, resulting in lower gasproduction and lower substrate removalefficiency, which can be alleviated by removing aportion of the carbonaceous material or by addingmanure or sewage sludge. Removal ofcarbonaceous material can be accomplishedthrough the reclamation of the paper fraction forother uses. The principal problem associatedwith low C/N ratio, in the high-solids anaerobicdigestion process, is ammonia toxicity. Based onthe work at UC Davis, it can be said that a C/Nratio below 20 will probably result in ammoniatoxicity (10, 20). Ammonia toxicity problems canbe corrected by adjusting the C/N ratio of thefeedstock or by diluting with water (20).

Long-term operation of the pilot facility atUC Davis revealed that when the BOP/MSW wasused as a feedstock and when the feedstock C/Nratio was adjusted to between 25 to 30, the pHnormalized at 7.2 and operation was steady.

Organic Loading Rate

Organic loading rate (OLR) is a major

design factor in an anaerobic digestion system,affecting reactor size, total waste massstabilization, and methane gas production. It canbe the determining factor which economicallyjustifies the application of an anaerobicdigestion design.

The rate at which organic matter isintroduced into the digester has a stronginfluence on the performance of the process. Anincreased feeding rate causes a rapid increase inthe population of the acidogenic bacteria relativeto the slower growing methanogenic bacteria. Ifcontinued, overloading occurs, causing unstableconditions which may eventually lead to processfailure. Consequently, the organic loading ratehas a strong influence on gas production. Also,sudden changes in the organic loading rate canstress operation. Therefore changes in loadingrate were made gradually to allow the microbialpopulation time to adapt to the new rate. Onlyafter operation had stabilized were resultscollected.

The influence of BVS organic loading rateon gas production and reactor stability, asindicated by pH, is shown in Figure 4. As can beseen, biogas production increased with anincrease of BVS/OLR up to 7.5 g kg'1 activereactor massed. Organic overload occurred at

<x>• sco

co

4.0

3.5

3.0

2.5

2.0

1.5

T32Q.

W

ca

cl 1.0

0.5

0.0

Figure 4.

o.

cu

7.5

- 7.0

- 6.5

- 6.0

- 5.5

5.02 4 6 8 10 12

BVS OLR (g BVS/kg active mass/d)

Effect of BVS organic loading rate on the reactor pH and methane gas production rate.

•

-

I I

, ,

, I

I ,

L

IM

ll,

i

/ / / ^

/C normal' J operating/ ^ range

/ /

Á

'- Dry

, , , i , ,

I

S

0 O

methane

, i

production

i i i

>o » Y

o\o \

o \

o

/

rate at STP

i

i '

Reactor

/

\

\ V

A"° ° \

i . *

pH ;

-

-

\A« —

563

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

7.6 g BVS kg"1 active reactor massed, at whichpoint volatile fatty acids were produced at a rapidrate. The methanogen population could notconsume the VFA's as fast as they were producedand reactor pH fell. Organic overload continuedup to 11.5 g BVS kg"1 active reactor mass at whichpoint the volatile fatty acids accumulation in thereactor reached an inhibitory limit, pH droopedto about 5.5 and the digester failed. Although it ispossible to operate the digester in the range of 7.6to 11.5 g BVS kg"1 acitve reactor mass, operationis less stable, less efficient, and more prone tofailure due to other stresses.

To bring a digester back to normal

operation after organic overload has occurred,the following actions are recommended: (i) stopfeeding the digester for a few days, (ii) add astrong base, such as sodium bicarbonate, asrecommended in Ten Brummuler and Koster(21), to neutralize the acids, and (iii) resumefeeding at a lower organic loading rate, when thereactor pH reaches 6.8 to 7, gradually increasingto a normal level.

Mass Retention Time

The process performance for each MRTperiod is reported in Table 4. As indicated, the

Table 4. Summary of the effect of mass retention time on the performance of the pilot scale high-solids anaerobic digestion process.

Parameter

MRT=30 dAlkalinityAmmonia nitrogenBiogas production ratea

Biogas production rateMethane concentrationMass removal rateMass removal efficiencyPHVolatile fatty acids

MRT =20 dAlkalinityAmmonia nitrogenBiogas production rateBiogas production rateMethane concentrationMass removal rateMass removal efficiencyPHVolatile fatty acids

MRT=15dc

AlkalinityAmmonia nitrogenBiogas production rateBiogas production rateMethane concentrationMass removal rateMass removal efficiencypHVolatile fatty acids

Unit

mg I/1 as CaCO3

mgLr1

V/active reactor Vm3 kg-1 BVS added»d

% biogasg BVS kg"1 ARM»db

% BVS mass fed

mg Lr1 as acetic acid

mg I/1 as CaCO3

mg I/1

V/active reactor Vm3 kg"1 BVS added'd

% biogasg BVS kg"1 ARM»db

% BVS mass fed

mg I/1 as acetic acid

mg I/1 as CaCO3

mgl/ 1

V/active reactor Vm3 kg"1 BVS added'd

% biogasg BVS kg"1 ARM»db

% BVS mass fed

mg I/1 as acetic acid

ValueRange

10,000-13,000250-1,600

4-60.75-150-545-6.570-937-7.5

600-1000

9,000-13,000350-1700

4-5.50.75-0.9

49-524-6.565-90

6.9-7.3600-1100

8,500-10,000500-1900

3-50.6-0.847-50

3.5-5.660-806.6-7

700-1,200

Typical

12,000750

5.40.85

525.5

837.25

750

10,000800

4.80.8

505.2

797

850

9,000900

40.7

494.8

736.8

1,000a The biogas production rate reported is based on dry and standard conditions (0 °C and 1 atm.).b ARM = active reactor mass.c These data were obtained when dairy manure was added as a nutrient suppliment.

564

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

performance was evaluated using variousparameters. In general, the process performedbetter when a mass retention time of 20 days ormore was used. At a MRT of 15 days, the reactorwas less stable and the addition of nutrient wasrequired to ensure continuous operation. Similarresults have been obtained when MSW andwastewater sludge have been anaerobically co-digested at high solids concentrations (22).

Preliminary research data indicate thatadding dairy manure at a rate of 2 to 2.5 g dry kg"1

active reactor massed, as a nutrient supplement,may allow the high-solids anaerobic digester tooperate at a MRT of 15 days.

In an anaerobic digestion process, methaneproduction correlates directly with the reductionof the biodegradable fraction of organicsubstrates. The reduction of the biodegradablefraction of an organic substrate and the massretention time are mutually dependent variables.They are interrelated in that the organic loading

rate and the feedstock solids concentrationdefine the retention time for a given activereactor mass. Therefore, for a given organicloading rate, mass retention time can onlydecrease by increasing the feedstock solidsconcentration. At long mass retention times, theconversion of biodegradable organic substratewill be essentially complete, as has been shownby Richards et al. (12) using a kinetic analysis.

The substrate mass removal efficiency andmethane production rates for the three MRTsstudied are shown in Figure 5. As expected, atlong MRTs a higher mass removal efficiencyand gas production rate were achieved since theconversion of biodegradable organic substratewas more complete. The slight decrease in gasproduction during MRTs of 20 days or less is due,at least in part, to the removal of a higherpercentage of the active microorganisms fromthe reactor. At MRTs of 15 days or less, substrateis apparently removed from the reactor before

. . . .I I Subslrale Removal ••— -MelhaneGas

Efficiency, % Production

90 -

80 -

¿ 70 -oI 6 0 -

iS en .

s1 40 -CECD o n

£ 30 -tó"§ 20 -co

1 0 -

with nutrient supplement

I / 3- without nutrisnt /: supplement /

: /

\

• 1

m

m

-

m

-

•

• 3.0

• 2 . 5 S0)

• 2 . o |CU

2• 1.5 c

en

o

leth

ane

pro

duct

io

15 15 20 30

Mass Retention Time (d)

Figure 5. Efect of MRT on the reactor mass removal efficiency and methane gas production rate.

565

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

Table 5. Summary of the ranges, typical and optimum values for the four design parametersstudied.

Average valueDesign parameter

Particle sizeFeedstock C/N ratiob

Organic loading rate (OLR)Mass retention time (MRT)

Unit

mm

gBVSkg"1 ARM«dc

d

Range

1-2508-1254-1115-30

Typical

1-5525-306-8

20-30

Optimum

<50a

277.230

a The optimum particle size for methane gas production may be less than 50 mm, but the cost associated with asmaller particle size may not be economically justified.

b The reported feedstock C/N ratio is based on biodegradable carbon and total nitrogen.c ARM = active reactor mass.

sufficient nutrient has been solubilized from thesubstrate mass. This may cause an additionaldecrease in gas production unless nutrient isadded.

To ensure the complete removal of inputsubstrate BVS mass, it is essential to extend theMRT to 60 days or more. However, mostdigesters are designed to operate at shorterMRT's, to maximize system cost efficiency.

SUMMARY AND CONCLUSIONS

A summary of the ranges, typical andoptimum values for the four design parametersstudied is reported in Table 5.

The conclusions that can be drawn from thisstudy are as follows:( i ) The particle size of the feed material has a

direct effect on the performance of thedigester - a smaller particle sizeincreases the rate of substrate utilizationand hence increases the gas productionrate. Reducing the particle size may alsoreduce material handling difficulties.

( i i ) A C/N ratio in the range of 25 to 30, basedon biodegradable carbon, is recommendedfor the high-solids anaerobic digestionprocess for robust microbial growth andactivity and for optimum digester

performance.( i i i ) The high-solids anaerobic reactor can be

operated an organic loading rate of up to7.5 g BVS kg"1 active reactor mass 'd withminimal overloading effects. Organicloading rates higher than 7.6 g BVS kg"1

active reactor massed gradually reducedthe digester's performance, as measuredby gas production, up to an OLR of 11.5 atwhich point digester failure occurred.Normal OLR for a high-solids digester wasdetermined to be in the range of 2.5 to 7.5with an optimum value of 7.2 g BVS kg"1

active reactor mass 'd .(iv) While it was possible to operate the high-

solids anaerobic digestion process at a 15day retention time, the reactor was lessstable at this shorter retention time. For amore complete waste stabilization, highergas production rate and stable digesteroperation, a MRT of 20 or higher isrecommended.

ACKNOWLEDGEMENTS

The work reported upon in this paper wassupported by a grant from the California EnergyCommission (CEC) and Prison IndustryAuthority (PIA) of the State of California.

REFERENCES

1. Rivard J.C., Himmel M.E., Vinzant T.B., Adney W.S., Wyman C.E. and Krohmann K.,Anaerobic High Solids Fermentation of Processed Municipal Solid Wastes for the Production ofMethane. Biotechnology Research Branch, Solar Energy Research Institute, Golden, Colorado80401 (1987).

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

566

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4

3. Ten Brummuler E., Koster I.W. and Zeevalkink J.A., Dry anaerobic digestion of the organicfraction of municipal solid waste. In: Advances in Water Pollution Control, Hall E.R. andHabson P.N. (eds.), Pergamon Press, Oxford, pp. 335-344 (1988).

4. Kayhanian M., Lindenauer K.G., Hardy S. and Tchobanoglous G., Two stage process combinesanaerobic and aerobic methods. BioCycle, 32, 48-52 (1991).

5. Kayhanian M., Lindenauer K.G., Hardy S. and Tchobanoglous G., The Recovery of Energy andProduction of Compost from the Biodegradable Organic Fraction of MSW using the High-SolidsAnaerobic Digestion/Aerobic Biodrying Process, A report prepared for the Prison IndustryAuthority, State of California. Department of Civil and Environmental Engineering, Univ. ofCalif., Davis (1991).

6. Cecchi F., Mata-Alvarez J. and Pohland F. (eds.), Proceeding of the International Symposium onAnaerobic Digestion of Solid Waste, Venice, Italy, April 15-17 (1992).

7. Kayhanian M. and Tchobanoglous G., Innovative two-stage process for the recovery of energy andcompost from the organic fraction of municipal solid waste (MSW). Water Sci. Technol., 27(2),133-143 (1993).

8. Kayhanian M. and Tchobanoglous G., Characteristics of humus produced from the anaerobiccomposting of the biodegradable organic fraction of municipal solid waste. Environ. Technol.,14, 815-829 (1993).

9. ASTM, Standard Methods for the Examination of Water and Wastewater, APHA-AWWA-WPCFPublications, John D. Lucas Co., Baltimore, 17th edn. (1991).

10. Kayhanian M. and Tchobanoglous G., Computation of C/N Ratios for various organic fractions.BioCycle, 33(5), 58-60 (1992).

11. ASTM, Annual Book of ASTM Standards. American Society for Testing and Materials,Philadelphia, PA (1983).

12. Richards K.B., Cummings R.J., White E.T. and Jewell W.J., Methods for kinetic analysis ofmethane fermentation in high solids biomass digesters. Biomass Bioenergy, 1(2), 65-73 (1991).

13. Trezek G.J. and Savage G., MSW Component Size Distributions Obtained from the Cal ResourceRecovery System. Resource Recovery and Conservation, 1(2), 67-77 (1976).

14. Chain E.S., Dewalle F.B. and Hannerberg E., Effect of Moisture Regime and other Factors onMSW Stabilization in Management of Leachate in Landfills. Banerji S.K. (ed.), USEPA 600/9-77-026, Washington D.C. (1977).

15. Hills D.J. and Nakano K., Effects of particle size on anaerobic digestion of tomato solids wastes.Agricultural Wastes, 10, 285-295 (1984).

16. Diaz F.L., Three key factors in refuse size reduction. Resource Recovery and Conservation, 1(2),111-113 (1975).

17. Rivard C.J., Vinzant T.B., Adney W.S. and Grohmann K., Nutrient requirements for aerobic andanaerobic digestion of processed municipal solid waste. J. Environ. Health, 52, 96-99 (1987).

18. Speece R.E. and McCarty P.L., Nutrient requirements and biological solids accumulation inanaerobic digestion. In: Advance in Water Pollution Research, Vol. 2, Eckenfelder W.W. (ed.),Pergamon Press Ltd, Oxford, England, pp. 305-322 (1964).

19. Speece R.E. and Parkin G.F., Nutrient requirements for anaerobic digestion. In:Biotechnological Advances in Processing Municipal Waste for Fuels and Chemicals,Antonopoulos A.A. (ed.), ANL/CNSV-TM-167, Argonne National Laboratory, Argonne, Illinois,pp. 195-221 (1985).

20. Kayhanian M., Performance of a high-solids anaerobic digestion process under various ammoniaconcentrations. J. Chem. Technol. Biotechnol., in press (1993).

21. Ten Brummeler E. and Koster I.W., The effect of several pH control chemicals on the dry batchdigestion of the organic fraction of municipal solid waste. Resources, Conservation andRecycling, 3, 19-32 (1989).

22. Poggi-Varaldo H.M. and Oleszkiewicz J.A., Anaerobic co-composting of municipal solid wasteand waste sludge at high total solids levels. Environ. Technol., 13, 409-421 (1992).

567

Dow

nloa

ded

by [

Mon

ash

Uni

vers

ity L

ibra

ry]

at 1

6:06

05

Oct

ober

201

4