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Comparison of Thermophilic and Mesophilic One-Stage, Batch, High-Solids Anaerobic DigestionG. Hegde & P. PullammanappallilPublished online: 11 May 2010.

To cite this article: G. Hegde & P. Pullammanappallil (2007) Comparison of Thermophilic and Mesophilic One-Stage,Batch, High-Solids Anaerobic Digestion, Environmental Technology, 28:4, 361-369, DOI: 10.1080/09593332808618797

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361

Environmental Technology, Vol. 28. pp 361-369© Selper Ltd., 2007

COMPARISON OF THERMOPHILIC AND MESOPHILICONE-STAGE, BATCH, HIGH-SOLIDS ANAEROBIC

DIGESTION

G. HEGDE1 AND P. PULLAMMANAPPALLIL1,2*

School of Environmental Science, Murdoch University, Perth, WA 6150, AustraliaDepartment of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611-0570, USA

(Received 1 May 2006; Accepted 14 November 2006)

ABSTRACT

The concept of starting up a batch, high-solids anaerobic digester by simply flooding the bed with a pH-buffer solution wastested using a mixture of vegetable waste and wood chips as feedstock at mesophilic (38 oC) and thermophilic (55 oC)conditions. At both temperatures stable and balanced methanogenesis was rapidly established within four days and wassustained until substrate was exhausted. Methanogenesis was more rapidly initiated in the thermophilic digester than in themesophilic digester. Acetic, propionic and butyric acids accumulated in the leachate of both digesters during the start-up ofdigestion of uninoculated batch of waste. Thereafter all acids were degraded; which was quicker in the thermophilicdigester. The accumulation and degradation of these acids was slower in the mesophilic digester. These studies showedthat inoculum for carrying out thermophilic and mesophilic anaerobic digestion is readily available within the waste and itsactivity for complete mineralization of organic matter can be enhanced and sustained by providing adequate alkalinity. Byemploying a process in which anaerobic digestion of subsequent batches of waste was carried out by flooding with leachatedrained from the digestion of a previous batch of waste, the volatile organic acid accumulation was maintained low and 95%of the methane yield potential of the waste was produced in 11 days under thermophilic conditions as opposed to 27 daysunder mesophilic conditions.

Keywords: Anaerobic digestion, high-solids, leach-bed, thermophilic, mesophilic

INTRODUCTION

Anaerobic digestion is a biochemical process that

mineralizes solid organic matter like carbohydrates, proteins

and fats to methane and carbon dioxide through a concerted

action involving several populations of microorganisms. The

process can be carried out at mesophilic (27oC - 38 oC) or

thermophilic (50oC - 58oC) temperatures. Among various

technologies that are available for anaerobic digestion of solid

organic waste streams, the batch, high-solids process has

several advantages. It is a one-stage process, and does not

require fine shredding of feedstock, does not require mixing,

agitation or movement of digester contents, requires only

minimal water addition and does not require bulky,

expensive, high-pressure vessels as it can be operated at low

(ambient) pressures. Moreover, this design is modular, can be

scaled up easily and implemented in bioreactor landfills.

However, due to the nature of the process, it is subjected to a

high organic loading initially which makes its start-up a

challenge. Usually, the first batch of waste that is processed is

mixed with anaerobically digested sewage sludge and then

digested [1]. Anaerobically digested sewage sludge serves as

an inoculum to seed the waste with methanogenic organisms.

Anaerobic digestion of subsequent batches of waste can be

initiated by employing a leachate management strategy that

recirculates leachate between digested and fresh waste. This

strategy is successfully employed in the SEBAC (Sequential

Batch Anaerobic Composting) process [2,3].

Recently, it was shown that rather than inoculation,

provision of pH buffer and alkalinity was the critical factor

for the initiation of methanogenesis in leach-bed digesters

treating organic fraction of municipal solid waste, OFMSW [4,

5]. In these experiments which were carried out at mesophilic

temperatures methanogenesis was initiated by simply

flooding a bed of OFMSW with a solution of sodium

bicarbonate. Could a similar start-up be achieved at

thermophilic temperatures? Despite enhanced kinetics, there

are contradictory reports in the literature on the stability of

thermophilic anaerobic digestion [6-9]. Typically

thermophilic digesters are started up using mesophilic

inoculum employing elaborate start-up and acclimatization

procedures whilst a start-up procedure using pH-buffer

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solution would rely on the activity and growth of native

microorganisms within the waste to initiate and sustain

anaerobic digestion. Moreover, the question remains whether

native organisms exist in more homogeneous feedstocks like

vegetable wastes that were used for the present study. A

process in which subsequent batches of waste are inoculated

with leachate from the previously stabilized residue would

introduce robust cultures of native microorganisms that have

been acclimatized to the waste and operating conditions.

These adapted microbial populations may guarantee better

process stability and improved kinetics of waste

mineralization. In this paper the effect of such an operational

strategy on the methane production rate, methane yield,

volatile organic acid accumulation during thermophilic and

mesophilic anaerobic digestion was investigated and

compared.

MATERIALS AND METHODS

Anaerobic Digesters

Two, 200 liter, insulated, 316 l-stainless steel vessels that

were designed and fabricated to Australian Standards AS

1210 were used in this study. The waste loaded into these

vessels was supported by a stainless steel screen at the

bottom. The temperature of the waste in each reactor was

controlled by two Helios Electroheat heating tapes helically

mounted on the outside of the reactor. Electrical power input

to the heating tapes was controlled by a microcomputer based

controller (JCS 33 A from Shinko Technos Co., Ltd, Japan).

Externally mounted interconnected centrifugal pumps were

used to pump or recirculate the leachate. On-line pH

measurement was done using electrodes with replaceable

sensing cartridges (Amalgamated Instruments Co Pty Ltd).

Gas production from the reactors was measured using a

positive displacement gas meter. The device consisted of a

perspex (acrylic) U tube filled with Silicone Fluid (200 Fluid,

50 centistokes, manufactured by Dow Corning), a relay, a float

switch, a timer, a counter and a solenoid valve. Datataker DT

50 data logger was used for data acquisition and online

monitoring of parameters. T type thermocouples sheathed in a

stainless steel tube were used for temperature sensing.

Besides temperature, pH and gas output were also logged.

Feedstock

Feedstock comprised of vegetable waste mixed with

wood chips, the latter improved structural strength and

imparted bulking properties. To eliminate the effects of

variation in the composition, feedstock for all the digestion

runs was prepared simultaneously in the run up to the study

and stored in deep freezers at -250C. Vegetable waste was

sourced from the Canning Vale Markets, Perth and shredded

to approximately 20 mm x 10 mm x 20 mm size. The feed

mixture was prepared in batches of 30 kg and loaded into 100

liter polypropylene boxes with tight lids. Each batch of 30 kg

of feed mixture composed of 21 kg or 70% of vegetable

wastes, and 9 kg or 30% wood chips. The feed mixture was

filled into onion bags (nylon thread, 0.5 mm thickness with

6mm x 6 mm openings) before placing in the boxes to

facilitate loading and unloading into the reactors. The

contents of two boxes were used to load each digester. A total

of 14 boxes of feed mixture (approximately 420 kg) were

prepared. The feedstock was stored in commercial deep

freezing facilities of Fremantle Cold Stores, Perth.

Anaerobic Digestion Protocol

The boxes were removed from the deep freezer

approximately 14 hours prior to the loading of the digester to

allow sufficient time for thawing the waste. Approximately 60

kg of defrosted feed mix was loaded from the top into the

digester for each run. Two runs under each category i.e.,

thermophilic and mesophilic temperatures, were conducted.

The first run under each category was done with no addition

of inoculum while leachate from the end of the first run was

used as an inoculum for the second run. For the first of the

two runs in each category 125 liters of buffer solution was

pumped into the digester from one of the bottom inlets.

Buffer solution was prepared by dissolving 5 g l-1 sodium

bicarbonate in tap water. Nearly uniform temperature was

maintained by continuously mixing the leachate. Once the gas

production from the first set of runs (uninoculated) tapered

down suggesting the major portion of the gas potential has

been realized, the leachate was pumped out and stored in

containers with lids. The remaining residue was removed

from the top and the finely broken portion of the feedstock

that passed through the screen/grate was removed from the

hand hole at the bottom. The second run in each category

was initiated by adding the stored leachate to the fresh stock

of feed mix loaded into the reactor. No further quantity of

sodium bicarbonate was added for this run. Thermophilic

digestion was carried out at 55 ± 2 oC and mesophilic

digestion at 38 ± 2 oC.

Analysis

Moisture content was determined by drying in a 105 0C

oven until constant weight. Volatile solids content was

determined by burning dried samples in a muffle furnace at

560 0C until constant weight. Acetic, propionic and butyric

acids were analyzed using a Varian Star 3400 Model Gas

Chromatograph. Gas chromatograms were recorded and

processed by using the Varian Star System Software, version

4.02. GA 2000 gas analyzer from Geotechnical Instruments

(UK) Ltd. was used to measure the CH4 and CO2 composition

of gas.

The performance of the four runs was evaluated by

fitting the cumulative methane production data to the

modified Gompertz equation [10]. The Gompertz equation

describes cumulative methane production from batch

digesters assuming that methane production is a function of

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bacterial growth, i.e.

M = P�exp �expRm �e

P�� t( )+1

�

���

�

��

�

��

�

where M is the cumulative methane production (l kg VS-1) at

any time t, P is the methane yield potential (l kg VS-1), Rm isthe maximum methane production rate (l d-1), � is the

duration of lag phase (d) and t is the duration of the digestion

process at which cumulative methane production M iscalculated (d). The parameters P, � and Rm were estimated for

each of the 4 data sets by using the ‘Solver’ feature in MS-

Excel. The value of parameters which minimized the sum of

the square of errors between fit and experimental data were

determined.

RESULTS AND DISCUSSION

Results

Moisture and volatile solids content of individual

components of feedstock and the mixture are listed in Table 1.

Volatile solids loaded into the digesters in each run were

approximately 15 kg. Figure 1 shows methane composition,

daily methane and cumulative methane production from the

four runs. Data from all four runs are plotted together so as to

be able to compare the trends between the runs. Irrespective

of the temperature of operation, methanogenesis was initiated

in Run 1 even though the digesters were not inoculated. By

day 2, methane content was 0.1% and 0.2% in the gas phase of

the thermophilic and mesophilic digesters respectively. The

methane content continued to rise reaching 75% in both

digesters. Daily methane production also increased peaking

at 70 l d-1 in the thermophilic digester on day 14 and 45 l d-1 in

the mesophilic digester on day 18. The methane yield

approached 50 l (kg VS)-1 in both the digesters.

Run 1 (both thermophilic and mesophilic) relied on the

native microbial populations existing within the waste bed to

initiate fermentation and mineralization of the organic matter

to methane and carbon dioxide. This was successfully

accomplished by only providing adequate alkalinity (2,500 mg

CaCO3 l-1 from 5 g l-1 sodium bicarbonate solution). The pH of

leachate was 6.6 initially and rose to 7.6 by the end of the

experiment. Concentrations of volatile organic acids namely

acetic, propionic and butyric are plotted in Figure 2. Volatile

organic acids first accumulated in both the thermophilic and

mesophilic digesters, but these were subsequently degraded.

Volatile organic acids accumulated rapidly in the

thermophilic digester. Acetic acid, propionic acid and butyric

acid accumulated to 3,750 mg l-1 and 2,600 mg l-1 within 4

days of start up, but were rapidly degraded to concentrations

less than 500 mg l-1 by day 12. By day 18 these acids had

dropped below 50 mg l-1. Butyric acid accumulated to 1,800

mg l-1 immediately upon start up. But by day 11 the

concentration had dropped below 50 mg l-1. In the mesophilic

digester, volatile organic acids accumulated much slower

than in the thermophilic digester. It also took a longer

duration for the acids to degrade. Acetic acid increased to

3,750 mg l-1 by day 10 and dropped to less than 500 mg l-1 by

day 30. Propionic acid increased to 1,200 mg l-1 by day 6 and

remained at about this concentration until day 26, after which

it began to drop. Butyric acid accumulated to above 1,200 mg

l-1 by day 14, thereafter it was degraded more rapidly than

acetic or propionic acid to below 75 mg l-1 by day 20.

The next set of digestion runs (Run 2) at thermophilic

and mesophilic temperatures were started up by flooding

with leachate that was drained from the thermophilic and

mesophilic digesters respectively at the end of Run 1. No

further alkalinity was added to the leachate. It can be seen in

Figure 1 that methanogenesis was initiated quicker in both

digesters when compared to start-up in Run 1. One day after

start-up methane content was 22.8% in the thermophilic

digester and 31.4% in the mesophilic digester. Methane

content reached a maximum value of around 60% within 6

days in thermophilic digester and 12 days in mesophilic

digester. Within 6 days of start up daily methane production

rate rapidly peaked to 120 l d-1 in the thermophilic digester.

Methane production peaked to 55 l d-1 within 12 days in the

mesophilic digester. Methane yield approached 46 l (kg VS)-1

in the thermophilic digester. The mesophilic digestion

experiment was not taken to completion as it was expected

that methane production would be sustained in this digester

like in the others; hence experimental data on methane yield

was not available.

Table 1. Moisture and volatile solids content of individual constituents and feedstock mixture.

Moisture

Content (%)

Volatile solids

(%)

Vegetable wastes 91 80

Wood chips 22 97

Feedstock mixture 72 87

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Figure 1. Methane production and composition from digesters.

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Figure 2. Volatile organic acid concentrations in digesters.

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Volatile organic acid concentrations in leachate from

Run 2 are shown in Figure 2. Levels of volatile organic acid

were lower in this experiment. Acetic acid accumulated to

1,000 mg l-1 and propionic acid concentration to 400 mg l-1 in

both digesters. Acetic and propionic acids were degraded

quickly to less than 50 mg l-1 by day 10 in the thermophilic

digester. These acids persisted longer at about 400 – 500 mg l-1

in the mesophilic digester. Propionic acid dropped below 50

mg l-1 by day 18. Butyric acid accumulated to 1,050 mg l-1

immediately on start-up in the thermophilic digester, but

quickly dropped below detection limits in a couple of days.

Butyric acid concentrations were between 75 and 100 mg l-1 in

the mesophilic digester. During the course of digestion, pH of

leachate in both digesters was higher in Run 2. Initially it was

6.8 and increased to 7.6 by the end of the experiment.

The Gompertz equation was fit to all four sets of

cumulative methane production data. The Gompertz fit is

shown as lines in the cumulative methane production plot ofFigure 1. Values of parameters �, Rm and P are listed in Table

2. For these values of parameters, the Gompertz equation

yielded an excellent fit to all sets of cumulative methane data.

It should be noted that even though mesophilic digestion in

Experiment 2 was not taken to completion, the cumulative

methane yield data could be extrapolated by using the

Gompertz equation. In addition, Table 2 also lists the time

taken to produce 95% of methane yield potential (P) as

calculated using the Gompertz equation. Methane yield

potential varied little and was between 45 and 50 l (kg VS)-1 in

all experiments, indicating uniformity in the feedstock. The

lag time prior to exponential growth in uninoculated

thermophilic digester was shorter than in the mesophilic

digester. Further, the lag time was considerably shortened to

1.6 days in the thermophilic digester of Run 2 by inoculating

with leachate from the previous thermophilic digestion

experiment. Maximum daily methane production was higher

in the thermophilic digester and it increased by 64% and 22%

in Run 2 of thermophilic and mesophilic digesters

respectively. Time taken to produce 95% of methane yield

potential can be used as a parameter to compare rates of

digestion. Since the cumulative methane production curve

only asymptotically approaches the methane yield, a digester

would take infinite time to produce 100% of methane

potential. Therefore, the 95% value was arbitrarily chosen. In

Run 2, it took 11 days to produce 95% of methane potential in

the thermophilic digester compared to 27 days in the

mesophilic digester. Maximum methane production rate in

Run 2 from thermophilic digester was 106 l d-1 whereas that

from mesophilic digester was 45.8 l d-1, i.e. for the same

amount of feedstock loaded into the digesters, the rate of

methane production from thermophilic digester was more

than double that from mesophilic digester.

Percentage solids reduction was calculated following

draining of leachate and determination of the moisture

content of solids that remained in the digester. Any solids

removed with the leachate were also included in the

calculation. The thermophilic experiments yielded a solids

reduction of 32% in Run 1 and 29% in Run 2, whereas the

mesophilic experiments yielded a solids reduction of 27% in

Run 1 and 13% in Run 2. Solids reduction in mesophilic

digester of Experiment 2 was low because it was operated for

only 16 days and not taken to completion. Discounting solids

reduction in this experiment, the average solids reduction was

around 29%. It should be noted that among the constituents

of feedstock primarily vegetable wastes would undergo

degradation. Biodegradable organic carbon content of the

solids in the feedmix was determined to be 45% through a test

for lignin content. When this aspect was taken into account,

the degradation of biodegradable volatile solids in the

thermophilic experiments was 80% and 75% respectively in

Run 1 and Run 2, whereas in the mesophilic experiment it

was 68% in Run 1.

Even though the volatile organic acid concentration in

the leachate at the end of each experiment was very low, the

leachate still contained some soluble COD. For example, the

soluble COD of leachate at the end of Thermophilic Run 1 was

2,330 mg l-1. This indicated that some of the solubilized

Table 2. Comparison of performance of thermophilic and mesophilic anaerobic digestion experiments based on a Gompertz fit

to cumulative methane data.

Lag time,�

(d)

Maximum methane

production rate,

Rm

(l d-1)

Methane yield

potential, P

(l kg VS-1)

Approximate

duration to produce

95% methane yield

potential

(d)

Thermophilic Run 1 8.6 65.6 48.4 25

Thermophilic Run 2 1.6 106 44.6 11

Mesophilic Run 1 11 37.6 45.1 37

Mesophilic Run 2 5.1 45.8 46 27

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organic matter from the feedstock persisted in the leachate

and this could not be anaerobically digested over the duration

of experiment. Depending on the discharge requirments, the

leachate from the digester may require further treatment to

reduce COD.

DISCUSSION

The methane yield from the feedstock varied between

45 l (kg VS)-1 and 50 l (kg VS) -1. This yield is low compared

to literature values of 160 l (kg VS)-1 to 420 l (kg VS)-1 for fruit

and vegetable wastes [11]. Even though the volatile solids

loaded into the digesters was 15 kg the actual degradable

volatile solids would only be contributed from vegetable

wastes. It should be noted that wood chips contributed

significantly to the volatile solids content loaded into the

digester but these are not readily degradable over the

duration of the experiments due to high lignin content. Lignin

content of wood chips was found to be 42% of VS. Retardation

of anaerobic biodegradation due to lignin is thought to be

primarily due to physical inhibition related to sheathing of

cellulose by lignin rather than due to a chemical inhibition.

Volatile solids from vegetable wastes loaded into each

digester were approximately 3 kg VS. A corrected methane

yield that only accounts for volatile solids from vegetable

wastes would be 225 l (kg VS)-1 to 250 l (kg VS)-1 which is

more consistent with methane yields from the literature.

Methane production was initiated in Run 1 at both

thermophilic and mesophilic temperatures by simply flooding

the bed with water containing only buffer. A balanced

anaerobic digestion was established in the mesophilic digester

within 12 days. Similar start-up was observed with OFMSW, a

heterogeneous feedstock at mesophilic temperature [4].

Experiments here showed that such a start-up protocol was

also applicable to a more uniform/homogeneous feedstock

like vegetable wastes and that too at both mesophilic and

thermophilic temperatures. Investigations here revealed that

appropriate microbial populations required for anaerobic

digestion of the feedstock was naturally present within the

feedstock and that there is no need to seed with methanogenic

organisms. On the other hand, a start-up strategy wherein

anaerobically digested sewage sludge was mixed with

OFMSW at a 1:1 (by wet weight) ratio accompanied by

maintaining pH at 7.0, took over 100 days to establish

sustained and balanced anaerobic digestion at mesophilic

conditions [1]. Mineralization of organic matter to methane

and carbon dioxide in an anaerobic digester requires the

concerted action of several populations of microorganisms.

Of these populations, those involved in acidogenesis

(conversion to volatile organic acids), acetogenesis

(conversion of higher chain volatile organic acids like

propionic and butyric acids to acetic acid) and

methanogenesis (formation of methane from acetic acid and

methane from hydrogen and carbon dioxide) are of primary

importance. It is well known that acidogenic organisms

readily establish within anaerobic environments wherein they

cause rapid fermentation of organic matter due to their high

growth rates. This was also confirmed here as addition of

water and establishment of anaerobic environment caused

volatile organic acids to build up. On the contrary, acetogenic

and methanogenic populations like aceticlastic methane

populations are slow growing organisms. Highly imbalanced

anaerobic digestion process, like sludge digesters with high

concentrations of propionic and butyric acids, have been

known to take long periods of time to recover requiring

elaborate operational protocols with close monitoring to nurse

them back to balanced conditions. However, it was seen here

that even though higher chain volatile organic acids like

propionic and butyric acid accumulated, these were degraded

at a later stage. This degradation was mediated by microbial

populations naturally occurring within the waste bed as an

external inoculum was not added to the digester in Run 1.

Initiation and sustenance of methane production also

indicated the presence of adequate number of methanogenic

populations.

Both accumulation and degradation of volatile organic

acids was more rapid in the thermophilic digester than in the

mesophilic one. As a rule of thumb it is generally considered

that bacterial growth rates double for every ten degree rise in

temperature. At thermophilic temperature activity of

acidifying organisms would be higher causing volatile

organic acids to build up quickly in the thermophilic digester.

The activity of acetic, propionic and butyric acid degrading

bacteria are threefold higher at 55 oC than at 38 oC [12]. In line

with these findings the accumulated acids were degraded

within ten days in the thermophilic digester whereas it took

longer up to 30 days for acids to drop below 200 mg l-1 in the

mesophilic digester.

When commissioning a thermophilic digestion process

the start-up of digestion of the first batch of feedstock needs

to be addressed. This is usually done by using a mesophilic

inoculum like anaerobically digested sewage sludge. An

elaborate start-up procedure for converting a mesophilic

anaerobic sewage sludge digester to a thermophilic one was

described in Ahring et al [8]. It has been shown that during

high-solids digestion of organic matter typically two to three

weeks are required for starting up thermophilic digestion

using a mesophilic inoculum [13, 14]. However, the start-up

procedure evaluated here initiated stable and sustained

methanogenesis within four to six days, by which time

methane content in gas phase had climbed to 30% and daily

methane production was 10 l d-1 and volatile organic acids

began to drop in the leachate.

An improved process for high-solids leach-bed

digestion was developed here. Unlike the SEBACTM

(sequential batch anaerobic composting) process which

requires two vessels [2,3,15], this process required only one

vessel and moreover it did not involve leachate exchange

between beds of stabilized residue and fresh waste for

inoculation. Flooding a fresh waste bed with leachate drained

at the end of a previous digestion was sufficient to start up

and sustain methanogenesis.

The kinetics of anaerobic digestion improved

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considerably from digestion of first batch of feedstock to the

second batch. Based on the time needed to produce 95% of

methane potential (Table 2), under thermophilic conditions

the rate doubled whereas under mesophilic conditions the

rate improved by 1.5 times. This showed that introducing

leachate from a previous digested material inoculated the

fresh bed of feedstock with appropriate microorganisms to

carry on the digestion process. Typically commercial designs

like Dranco and Kompogas processes mix part of the digested

residue with the incoming wastes for inoculation [16].

However, it was shown here that there is no need to mix the

solid residue itself to inoculate. It is far easier to introduce

leachate than mix solid residue.

A comparison of the thermophilic and mesophilic

digestion here showed that even though methane yields are

similar, the rate at which methane is produced, consequently

the rate at which organic matter is degraded is 2.5 times faster

under thermophilic conditions (Table 2). In agreement with

findings here, high solids anaerobic digestion of OFMSW at

thermophilic temperatures yielded two to three times more

gas production rates than at mesophilic temperatures [17].

Despite enhanced kinetics, there are contradicting reports on

the stability of thermophilic digestion process. Volatile fatty

acids accumulated when market wastes were anaerobically

digested in one stage reactors [6]. When assessing anaerobic

digestion of kitchen wastes, mesophilic digestion medium

was reported to have a greater buffering capacity and to be

more robust to changes or accumulation of inhibitory

chemicals and hence more stable compared to thermophilic

process [9]. It has been reported that thermophilic anaerobic

digestion often manifests chronically higher volatile fatty

acids (particularly propionate) and hence propionate

degradation is generally the rate limiting factor at

thermophilic conditions [7]. Our findings are contrary to

these but are in agreement with Ahring et al. [8] where it is

stressed that thermophilic digesters can be operated stably

provided that proper start-up is ensured by employing an

appropriate strategy that allows for optimal growth of the

necessary thermophilic minority populations without over-

growth of ubiquitous and innumerable fermentative

microbes. The successful strategy here was to provide

adequate alkalinity allowing naturally occurring

microorganisms in the waste to activate. In the leach-bed

design, as solid organic matter is usually introduced as

received or after only coarse shredding, hydrolysis or

solubilization is a rate limiting step. Therefore, excessive

fermentation of this organic matter cannot happen rapidly as

with liquid waste streams or finely ground organic matter.

This allows for minority populations to develop in phase with

fermentative populations preventing huge accumulation of

volatile organic acids. In addition, the provision of adequate

buffer ensured that even if volatile organic acids accumulated

to some extent the pH did not drop to acidic conditions which

would have inhibited the minority populations, namely

acetogenic and methanogenic microorganisms. After the

start-up of the first batch of waste with buffer, subsequent

digestion by flooding the feedstock bed with leachate drained

from the previous digestion ensured that a new batch of

feedstock was inoculated with all required microorganisms.

This was proven by the lower accumulation of volatile

organic acids in Run 2. Similar start-up and inoculation

procedure was also valid for mesophilic digestion.

CONCLUSIONS

Both mesophilic and thermophilic, high-solids leach-

bed anaerobic digestion of vegetable wastes was rapidly

started up by simply flooding a bed of waste with a pH

buffer. A process was developed in which anaerobic

digestion in a batch of waste was started up by flooding with

leachate drained at the end of a previous digestion process.

The performance of the thermophilic digestion was superior

to that of mesophilic digestion. During start-up, degradation

of volatile organic acids including propionic and butyric

occurred more rapidly in the thermophilic digester. The rate

of digestion was 2.5 times faster in the thermophilic digester

than in the mesophilic digester.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the Australian

Research Council and Western Australia State Government

for funding and Dr. Suresh Nair, Research Fellow, Murdoch

University for carrying out the volatile organic acids analysis.

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