a study of two-stage anaerobic digestion of solid potato waste using reactors under mesophilic and...

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This article was downloaded by: [Nipissing University] On: 08 October 2014, At: 13:51 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20 A Study of Two-Stage Anaerobic Digestion of Solid Potato Waste using Reactors under Mesophilic and Thermophilic Conditions W. Parawira , M. Murto , J. S. Read & B. Mattiasson Published online: 11 May 2010. To cite this article: W. Parawira , M. Murto , J. S. Read & B. Mattiasson (2007) A Study of Two-Stage Anaerobic Digestion of Solid Potato Waste using Reactors under Mesophilic and Thermophilic Conditions, Environmental Technology, 28:11, 1205-1216, DOI: 10.1080/09593332808618881 To link to this article: http://dx.doi.org/10.1080/09593332808618881 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

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Page 1: A Study of Two-Stage Anaerobic Digestion of Solid Potato Waste using Reactors under Mesophilic and Thermophilic Conditions

This article was downloaded by: [Nipissing University]On: 08 October 2014, At: 13:51Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

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

A Study of Two-Stage Anaerobic Digestion of SolidPotato Waste using Reactors under Mesophilic andThermophilic ConditionsW. Parawira , M. Murto , J. S. Read & B. MattiassonPublished online: 11 May 2010.

To cite this article: W. Parawira , M. Murto , J. S. Read & B. Mattiasson (2007) A Study of Two-Stage Anaerobic Digestionof Solid Potato Waste using Reactors under Mesophilic and Thermophilic Conditions, Environmental Technology, 28:11,1205-1216, DOI: 10.1080/09593332808618881

To link to this article: http://dx.doi.org/10.1080/09593332808618881

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources of information. Taylor and Francis shallnot be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and otherliabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.

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

Page 2: A Study of Two-Stage Anaerobic Digestion of Solid Potato Waste using Reactors under Mesophilic and Thermophilic Conditions

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Environmental Technology, Vol. 28. pp 1205-1216© Selper Ltd., 2007

A STUDY OF TWO-STAGE ANAEROBIC DIGESTION OFSOLID POTATO WASTE USING REACTORS UNDERMESOPHILIC AND THERMOPHILIC CONDITIONS

W. PARAWIRA1,2*, M. MURTO1, J. S. READ3 AND B. MATTIASSON1

1Department of Biotechnology, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden2Department of Food Science, University of Zimbabwe, P.O. Box MP167, Mt. Pleasant, Harare, Zimbabwe,

3Department of Applied Biology and Biochemistry, NUST, P.O. Box 939, Ascot, Bulawayo, Zimbabwe

(Received 6 November 2006; Accepted 11 June 2007)

ABSTRACT

A two-stage anaerobic digestion process operated under mesophilic and thermophilic conditions was investigated for thetreatment of solid potato waste to determine optimal methane yield, efficiency of operation and process stability. A solid-bedreactor was used for hydrolysis/acidification stage while an upflow anaerobic sludge blanket (UASB) reactor was used inthe second stage, for methanogenesis. Three sets of conditions were investigated: (I) mesophilic + mesophilic, (II) mesophilic+ thermophilic and (III) thermophilic + thermophilic in the hydrolysis/acidification and methanogenesis reactors,respectively. The methane yield was higher under mesophilic conditions (0.49 l CH4 g COD-1

degraded) than thermophilicconditions (0.41 l CH4 g COD-1

degraded) with reference to the methanogenic reactors. (COD) - chemical oxygen demand.However, the digestion period was shorter in systems II and III than in system I. Also, in system III the UASB reactor(thermophilic conditions) could handle a higher organic loading rate (OLR) (36 g COD l-1d-1) than in system I (11 g COD l-1

d-1) (mesophilic conditions) with stable operation. Higher OLRs in the methanogenic reactors resulted in reactor failure dueto increasing total volatile fatty acid levels. In all systems, the concentration of propionate was one of the highest, higherthan acetic acid, among the volatile fatty acids in the effluent. The results show the feasibility of using a two-stage system totreat solid potato waste under both mesophilic and thermophilic conditions. If the aim is to treat solid potato wastecompletely within a short period of time thermophilic conditions are to be preferred, but to obtain higher methane yieldmesophilic conditions are preferable and therefore there is a need to balance methane yield and complete digestion periodwhen dealing with large quantities of solid potato waste.

Keywords: Anaerobic digestion, mesophilic, thermophilic, performance, solid potato waste

INTRODUCTION

While most anaerobic digestion reactors tend to beoperated under mesophilic conditions (35°C), some industrial

wastewaters are produced at a temperature that makesoperation under thermophilic conditions (55°C) an attractive

alternative. Although single-stage, mesophilic, completely

mixed anaerobic digestion has been widely used for the

treatment of municipal waste (water) and for biogas

production, it requires a long hydraulic retention time (HRT)

and is not efficient in killing pathogenic microorganisms. To

overcome these limitations, thermophilic conditions have

been adopted for anaerobic digestion [1,2]. Thermophilic

regimes have several advantages, such as higher rate of

degradation of organic matter, and hence a lower HRT, higher

biogas production rate, improved solid-liquid separation and

increased destruction of microbial pathogens [3,4]. Reaction

rates under thermophilic conditions may be up to four times

higher than under mesophilic conditions and this affords the

possibility of treating warm, concentrated industrial waste in

small reactors with HRTs measured in hours [5]. Recent

intensive laboratory-scale studies have demonstrated the

potential of the thermophilic anaerobic process for waste

(water) treatment [6-8]. In addition, a few pilot-scale studies

on thermophilic anaerobic treatment of industrial wastewater

(vinasses [9] and brewery waste [10]) have indicated the

feasibility of the process.

Despite these potential advantages of thermophilic

anaerobic digestion, studies of thermophilic and mesophilic

reactors have yielded mixed results [11]. The wide use of

mesophilic over thermophilic digestion is mainly due to the

latter’s reported disadvantages such as poor process stability,

higher energy requirements, a high concentration of volatile

fatty acids (VFAs) in the effluent, high sensitivity to

temperature changes, feed interruption and shock loading

[4,12-14]. However, several laboratory and pilot-scale studies

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on thermophilic anaerobic treatment of industrial wastewater

have shown that the process is stable, can withstand feed

breaks and some temperature fluctuation [7,10]. Some reports

reveal a relatively low sensitivity to temperature changes, and

if a two-stage process is used, thermophilic reactors can be

operated for prolonged periods under extreme loading

conditions (80-100 kg COD m-3d-1) while the concentrations of

VFAs in the effluent remain low [4,13]. In other studies very

low VFA concentrations in the effluent were found in

thermophilic sludge digesters [2,15] and thermophilic

wastewater treatment plants [5,16]. Ahring [6] made a

comparison between similar mesophilic and thermophilic full-

scale biogas reactors in Denmark and concluded that the VFA

concentrations in the effluent were within the same range.

In assessing the potential of thermophilic digestion, the

main technological concern is whether it has advantages over

a mesophilic digestion system for a specific type of waste.

Thermophilic conditions are applied in Europe for the

treatment of manure in one-stage, large-scale biogas plants

and for the treatment of the organic fraction of municipal

solid waste [12,17,18]. A number of medium- and high-

strength liquid wastes, for example, from the canning and

brewing industries, certain waste streams from paper

production, vegetable processing and the potato-processing

industry are, in theory, particularly suitable for thermophilic

anaerobic digestion because they are discharged at high

temperatures [19,20].

Temperature-phased anaerobic digestion (TPAD) is one

of the innovative concepts that combine thermophilic and

mesophilic processes in one treatment system, offering

advantages of the two individual processes [21]. It consists of

a two-stage system, which operates under thermophiliccondition (typically 55 °C) in the first stage and under

mesophilic condition (35-37 °C) in the second stage. It has

been shown to be a reliable and effective means of sludge

stabilization, which achieves bioconversion and methane

production rates higher than those in conventional mesophilic

anaerobic systems [14]. The system has the ability to treat

waste with high solids content and high OLRs with shorter

HRTs, and therefore reactor volumes could be reduced to less

than half the size of conventional systems. The TPAD

arrangement has been modified to include mesophilic +

thermophilic or thermophilic + thermophilic configurations

[18]. The TPAD system has been evaluated with a number of

medium- and high-strength liquid waste streams from, for

example, the canning industry, alcohol distilleries and potato-

processing plants and with model compounds such as VFAs

and carbohydrates [13,22]. Unfortunately, few studies have

been conducted to compare directly the anaerobic digestion

under mesophilic and thermophilic conditions using the same

type of reactor and the same substrate, thus making it difficult

for engineers and policy makers to decide on the best

technology for a given situation.

Although a number of studies has addressed

thermophilic digestion of liquid waste, few have investigated

the digestion of solid potato waste under mesophilic and

thermophilic conditions. The potato industry generates

considerable quantities of waste, both solid and wastewater.

In the southern part of Sweden alone, about 3,000 tons of

potato solid waste was generated annually. Therefore, the

purpose of this study was to evaluate the performance of a

two-stage anaerobic digestion of solid potato waste under

mesophilic and thermophilic conditions. UASB reactors were

used in the second step of two-stage systems with three

different temperature combinations. The biodegradable solid

waste studied here can be regarded as being representative of

many other kinds of starch-rich biomass suitable for

conversion to biogas.

MATERIALS AND METHODS

Experimental Set-up

The experiments were performed in two-stage systems,

consisting of a hydrolysis/acidification reactor with a

working volume of 2 l and a second-stage methanogenic

UASB reactor with a working volume of 0.84 l, as illustrated

in Figure 1. The reactors were closed at the top with butyl

rubber stoppers. To separate large particles from liquefied

leachate and to prevent clogging of the reactor outlet, a wire

mesh with 2 mm gauge supported by a steel frame was

installed 5 cm from the bottom in the hydrolysis/acidification

acidogenic reactors. Recirculation of the liquid was achieved

by pumping. The leachate in the hydrolysis/acidification

reactor was recirculated at 10 ml min-1 and sprinkled over the

packed bed of potato waste to promote hydrolysis and

solubilisation of the potato solids. The leachate from the

hydrolysis/acidification reactor was then pumped to the

UASB reactor. The UASB reactor contents were recycled at a

constant flow rate of 5 ml min-1 from the top to the bottom of

the reactor in order to provide good contact between the

biomass and wastewater. The outflow from the methanogenic

reactor was recycled back to the hydrolysis/acidification

reactor to replenish the hydrolysis/acidification reactor and

to provide buffering capacity to prevent excessive

acidification. The recirculation of liquid back to the first

reactor was by means of overflow, meaning that the flow rate

was according to what was applied to the methanogenic

reactor.

Substrate Characteristics and Inoculum

Solid potato waste was cut into small pieces using a

kitchen blender. The solid potato waste had a total solids (TS)

content of 19% and a volatile solids (VS) content of 95% of the

TS.

Anaerobic sludge was used as inoculum (0.2 l) in the

hydrolysis/acidification reactor. The anaerobic sludge, which

had a TS content of 1.7% and VS content of 59% of the TS,

came from Ellinge Municipal Wastewater Treatment Plant in

Eslöv, southern Sweden. This plant receives sewage sludge

and industrial effluents mainly from a potato-processing

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System I = Mesophilic reactor + mesophilic UASB reactorSystem II = Mesophilic reactor + thermophilic UASB reactorSystem III= Thermophilic reactor + thermophilic UASB reactor

Figure 1. Experimental set-up for two-stage anaerobic digestion of solid potato waste.

plant. The mesophilic UASB reactor was seeded with granular

sludge from a full-scale UASB reactor processing paper mill

wastewater, (the Netherlands), and the thermophilic UASB

reactors were seeded with granular sludge from a

thermophilic full-scale anaerobic lactate digester in Denmark.

The granular sludge was adapted for a period of one month

by being fed potato leachate once a week.

System Operation

The performance of three temperature-phased systems

were investigated in two replicate experiments: (I) mesophilicfirst stage (37 °C) + mesophilic second stage (37 °C), (II)

mesophilic first stage (37 °C) + thermophilic second stage (55

°C) and (III) thermophilic first stage (55 °C) + thermophilic

second stage (55 °C). The hydrolysis/acidification reactor was

batch fed: 1 kg of potato solids was placed in the reactor and

0.8 l tap water were added to completely saturate the waste as

indicated by the liquid level in the reactor. Feeding of leachate

to the methanogenic reactors was begun after 24 h of

recirculation in the hydrolysis/acidification reactor.

Thereafter, increasing the flow rate from the

hydrolysis/acidification reactor to the methanogenic reactor

increased the OLR, to assess the maximum OLR sustainable

by the methanogenic reactor. The changes in OLR were

undertaken after about four days of operation at around the

same OLR, and when the methanogenic reactor performance

was thus stable (in terms of chemical oxygen demand (COD)

removal). The OLRs applied to the UASB reactor ranged from

2.2 to 11.0 g COD l-1d-1 in System I, from 4.5 to 22.3 g COD l-1d-

1 in System II, and from 1.3 to 36.0 g COD l-1d-1 in System III

during stable operation. Refer to Figure 1 for definition of

systems I, II and III. In this study, the OLR rate increase was

not the same for the different systems because the hydrolysis

/solubilisation in the first-stage reactors fed (batchwise) with

solid potatoes was different resulting in varying soluble COD

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1208

concentrations. Under such operation the systems would

never reach stationary or steady state conditions.

On the 17th day in System I and the 14th day in System

III the UASB reactors showed signs of overloading (a dramatic

decrease in the partial alkalinity and increased amounts of

VFAs) at OLRs of 13.2 g COD l-1d-1 and 45.6 g COD l-1d-1,

respectively. The feeding to the UASB reactor was therefore

stopped until the systems recovered and the experiments

were then continued. In System II no overload of the UASB

reactor was observed because the solid potatoes load was

quickly degraded completely before the OLR was at a level

high enough to cause system failure. The experiments were

run for a period of up to 40 days at which time biogas

production in the methanogenic reactors was insignificant,

and the potato waste had been completely degraded in the

hydrolysis/acidification reactors.

Analyses

During operation, VFAs concentrations, pH, soluble

COD, gas composition and production rate were measured

regularly to monitor the progression of the acidogenic and

methanogenic fermentation. Samples (10 ml per day) were

collected from the lines out of the reactors, see Figure 1.

Partial alkalinity was only measured from samples taken from

the effluent from the methanogenic reactors.

The TS, VS and COD were determined according to

standard methods [23]. The alkalinity of the sample was

evaluated as partial alkalinity (PA) by titration to pH 5.75, and

total alkalinity (TA) by titration to pH 4.3 with 0.1 M HCl

using a TitraLabTM 80 titrator (Radiometer, Copenhagen,

Denmark) and expressed as mg CaCO3 l-1. Samples for

alkalinity and VFA measurements were centrifuged (WIFUG

Lab. Centrifuges STUDIE-M, England) at 3 000xg for 3 min

and the supernatant was collected for analysis. VFAs were

analysed with HPLC according to Parawira et al. [24].

The biogas produced in the reactors was collected in

gas-tight bags. The volume of biogas was measured using a

wet-type precision gas meter (Schlumberger, Karlsruhe,

Germany). The biogas composition was measured from gas

samples taken from the gas collection line every two days and

analysed using a gas chromatograph, Varian 3350 (Walnut

Creek, CA, USA), fitted with a HaySep Q 80/100 mesh

column, a molecular sieve column and a thermal conductivity

detector. Helium was used as the carrier gas at a flow rate of12 ml min-1. The column temperature was 70 °C and the

injector and detector temperatures were 110 °C and 150 °C,

respectively. The compounds detected were methane, carbon

dioxide, nitrogen and oxygen.

RESULTS

The results presented in this study are an average from

two repeat experiments. Figure 2 shows the performance of

the three temperature-phased, two-stage anaerobic digestion

systems in terms of total methane yield and the OLR applied

to the second-stage UASB reactors. High methane content and

good biogas production were achieved in all the systems,

although it varied from each system. The highest methane

yield, 0.49 l CH4 g COD-1degraded was reached in System I, in

System II, the highest value was 0.41 l CH4 g COD-1degraded and

in System III, 0.31 l CH4 g COD-1degraded. The methane yield in

Systems I and III decreased to below 0.1 l CH4 g COD-1degraded

when overload was being approached. However, the methane

yield then returned to 0.49 l CH4 g COD-1degraded in System I

and increased to 0.28 l CH4 g COD-1degraded in System III after

feeding was resumed, indicating that the microorganisms had

not lost their methanogenic activity.

The maximum cumulative biogas production was 113

litres under system III, compared with 108 litres under

Systems I and II from digestion of 1 kg potato solid waste

(Figure 3). The composition of methane ranged from 50 to

76% in System I, from 60 to 70% in System II, and from 54 to

74% in System III (data not shown).

The maximum OLR for the thermophilic UASB reactor

in System III was 36 g COD l-1d-1 compared with 11 g COD

l-1d-1 for the mesophilic UASB reactor while maintaining

stable operation. This corresponds to an HRT of 16 h in the

thermophilic UASB reactor in System III and 48 h in the

mesophilic UASB reactor. In System II no overload was

observed and the thermophilic UASB reactor was stable up to

an OLR of 22 g COD l-1d-1 and HRT of 25 h. The total digestion

period was 38 days for System I, 29 days for System II and 25

days for System III.

The concentration of soluble COD from the

hydrolysis/acidification in the first-stage reactors and the

soluble COD removal in the second-stage reactors is shown in

Figure 4. In all the three systems, the soluble COD

concentration in hydrolysis/acidification reactors initially

increased and then stabilized before declining. The soluble

COD concentration in the initial 5 days was different in all the

systems (System I: 22-32 g COD l-1, System II: 18-25 g COD l-1,

System III: 15-28 g COD l-1). In Systems I and II the soluble

COD concentration stayed fairly constant from day 1 to day

15, even though OLR was increased probably because

hydrolysis and solubilisation was faster, resulting in

accumulation of soluble COD in the first-stage reactors. The

removal of soluble COD in the methanogenic reactors

decreased to 31% on day 17 in System I, when the UASB

reactor had become overloaded, due to OLRs above 11 g COD

l-1d-1 (Figure 4a). The removal of soluble COD decreased to

20% on day 14 in System III, when the UASB reactor had

become overloaded at OLRs above 36 g COD l-1d-1 (Figure 4c).

Feeding was therefore stopped in these systems for two to

three days and the overloaded UASB reactors were allowed to

recover. The soluble COD removal was 54% at an OLR of 36 g

COD l-1d-1 and HRT of 16 h in the thermophilic UASB reactor

(System III), the maximum for stable operation.

Under both mesophilic and thermophilic conditions,

the dominant VFAs in the hydrolysis/acidification stage were

acetic acid, iso-butyric acid and propionic acid, whereas

there were low amounts of normal butyric acid, and iso- and

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Figure 2. Methane yield obtained and organic loading rate applied to the methanogenic reactors in the two-stage,

temperature-phased systems: (a) System I, (b) System II, (c) System III.

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Figure 3. Accumulated biogas productions in System I, System II, System III during two-stage anaerobic digestion of solid

potato waste.

Figure 4. Soluble COD reduction in the methanogenic reactors and concentration of soluble COD in outflow from the

hydrolysis/acidification reactors entering the methanogenic reactor: (a) System I, (b) System II, (c) System III.

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normal-valeric acids in the hydrolysis/acidification reactors

(data not shown). This trend is in agreement with production

of VFAs during anaerobic mesophilic digestion of solid potato

waste reported by Parawira et al. [24]).

The concentration of the total VFAs (TVFAs) and

individual VFAs in the effluent from the methanogenic UASB

reactors of the different systems are shown in Figure 5. The

concentrations of acetic acid and propionic acid were higher

than those of other VFAs in both the mesophilic and

thermophilic UASB reactors. The concentrations of the TVFAs

Figure 5. Volatile fatty acid concentration in effluent from the methanogenic UASB reactors of the anaerobic digestion

systems: (a) System I, (b) System II, (c) System III. (Note the different vertical scales in the figures).

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and individual VFAs increased during overload periods in

Systems I and III, but decreased when feeding was stopped.

Eventually, the concentrations of TVFAs decreased to below

0.2 g l-1 at the end of the digestion period.

The partial alkalinity (PA) in the methanogenic reactors

and pH profiles in the hydrolysis/acidification and

methanogenic reactors are shown in Figure 6. The pH of the

methanogenic reactors ranged between 7.0 and 7.9 but a

decrease in both the pH and PA was observed during the

overloading periods in Systems I and III. The PA increased to

Figure 6. Partial alkalinity in the methanogenic reactors and pH profiles in the acidogenic and methanogenic reactors for the

two-stage anaerobic digestion systems: (a) System I, (b) System II, (c) System III.

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above 4.0 g CaCO3 l-1 in both the mesophilic and thermophilic

methanogenic reactors until overloading was experienced,

when it fell to zero and feeding to the UASB reactors was

therefore stopped. When the systems had recovered, the PA

increased again, this time to around 3 g CaCO3 l-1, until

digestion was complete. The pH of the leachate in the

hydrolysis/acidification reactors decreased from 6.8 to about

4.5 after 24 h and remained in the range 4.2-4.5 during the first

20 days of operation. The pH then increased gradually to

above 7.0 when digestion had been completed. A summary of

the results from the three systems is shown in Table 1.

DISCUSSION

The performance of anaerobic digestion in our reactors

was quantified in terms of methane yield, maximum OLR and

COD removal, three of the most important economic factors

when considering the feasibility of an anaerobic digestion

process. The fact that the methane yield was highest in System

I showed that a higher temperature had no positive effect on

methane yield. The lower methane yield from the

thermophilic UASB reactors could also be due to high loss of

methane potential through production of CO2 and H2 in the

acidogenic reactor. Another reason may be the lower OLRs

(and therefore higher HRT) applied to the mesophilic UASB

reactor in System I than to the thermophilic UASB reactors in

Systems II and III. Sung and Santha [14] also reported higher

methane yields from a mesophilic reactor than from a

thermophilic reactor for TPAD systems treating dairy cattle

manure. However, Ahn and Forster [3] obtained a higher

methane yield in a thermophilic reactor than in a mesophilic

reactor treating paper-pulp liquors. At similar OLRs, the

methane yields in Systems I and II were comparable and that

in System III was much lower. This could be due to the

difference in temperature of the hydrolysis reactor. However

the volatile fatty acids composition and concentrations

coming out of the mesophilic and thermophilic hydrolysis

reactors were significantly different.

In our study, the methane yield increased after Systems

I and III had recovered from overload which demonstrates

that some of the methanogenic microorganisms did not lose

their methanogenic activity after being exposed to

overloading conditions. In an earlier study a methane yield of

0.39 l CH4 g VSadded was obtain in a two-stage system with a

UASB as the methanogenic reactor [25]. Furthermore, in batch

digestion of solid potato waste a yield of 0.32 l CH4 g VSdegraded

was achieved [26]. Our results on biogas production (Figure

3) are also in agreement with those of Dugba and Zhang [18],

who reported that thermophilic + mesophilic systems

performed better in terms of biogas production rates than did

mesophilic + mesophilic systems while treating dairy

wastewater.

Mackie and Bryant [27] stated that in order to take

advantage of anaerobic digestion at thermophilic

temperatures, the reactors must be operated at high OLRs and

short HRTs. In our study, however, the thermophilic

methanogenic UASB reactor in System III could handle a

maximum OLR three times higher than the mesophilic UASB

reactor. The maximum OLR in the mesophilic UASB reactor

in System I (11 g COD l-1d-1) was twice that obtained

previously when the performance of a UASB reactor was

compared with that of an anaerobic packed-bed reactor

treating potato waste leachate [28]. In that study we found

that at OLRs above 6 g COD l-1d-1, stable mesophilic digestion

was not possible. The higher OLR in this study was probably

due to the fact that the overflow from the methanogenic

reactors was recycled back to the hydrolysis/acidification

reactors to replenish them and to provide buffering capacity,

which might have improved the stability of the system.

Demirer and Chen [28] reported that treating unscreened

dairy waste in a two-stage system could tolerate a higher OLR

compared to a conventional one-stage reactor. All

methanogenic reactors in this study could handle a higher

OLR than that reported by Linke [30] where solid waste from

solid waste processing was treated in a CSTR at thermophilic

conditions. In that study the reactor was stable up to an OLR

of 3.4 g VS l-1d-1 [30].

To provide good contact between the biomass and

wastewater, furthermore, UASB reactor liquid was recycled at

a constant flow rate of 5 ml min-1 from the top to the bottom

Table 1. Summary of results from the three systems studied.

Parameter Mesophilic-Mesophilic

(System I)

Mesophilic-Thermophilic

(System II)

Thermophilic-Thermophilic

(System III)

Methane yield (l CH4 g COD-ldegraded) 0.49 0.41 0.31

Accumulated biogas volume (l)

from 1 kg solid potato waste

108 108 113

Maximum OLR (g COD l-1d-1)

for stable operation

11 22 36

Complete digestion period (d) 36 29 25

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of the reactor. Our findings indicate that such recirculation

tends to balance the effective COD and VFAs concentrations

variations in UASB reactors and reduce the influent alkalinity

requirements per influent COD [30].

The maximum OLR in System I was lower than the 16 g

COD l-1d-1 previously achieved by Shin et al. [32] in a

mesophilic UASB reactor treating leachate from

hydrolysis/acidification reactor in two-stage anaerobic

digestion of food waste. The maximum OLR for the

mesophilic UASB in the current study is also comparable to

the OLR of 10 g COD l-1d-1 reported by Dinsdale et al. [33] in a

study of comparing mesophilic and thermophilic UASB

reactors treating instant coffee production wastewater. The

maximum OLR obtained in System III with a thermophilic

UASB reactor (36 g COD l-1d-1) was significantly higher than

the 11.4 g COD l-1d-1 reported by Dinsdale et al. [33] using a

thermophilic UASB reactor. The solid potatoes load was

quickly degraded completely in System II before the OLR was

increased to a level high enough to cause system failure.

Therefore, the OLR of 22 g COD l-1d-1 in that system should

not be taken as a maximum sustainable OLR. It follows that

an increase in OLR would have led to process failure.

What is also important is the fact that the digestion

period of the solid potato waste was shorter (by 24%) in

System II and in System III (by 34%), than in the System I,

indicating the advantage of digestion at thermophilic

conditions. This can be explained by the faster degradation at

thermophilic conditions and the higher OLRs possible.

Comparison between the solubilisation of solid waste in

the mesophilic and thermophilic hydrolysis/acidification

reactors showed that soluble COD was higher in the

thermophilic hydrolysis reactor than in the mesophilic

hydrolysis reactors, suggesting that it might be advantageous

to use thermophilic conditions for the hydrolysis/acidification

step during two-stage anaerobic digestion of solid potato

waste. The reduction in soluble COD in Systems II and III was

comparable to that of System I, despite the fact that the former

was operated at a higher OLR. However, Ahn and Forster [11]

reported better soluble COD removal by a thermophilic

upflow filter than with a mesophilic upflow filter at OLRs of

12-17 kg COD m-3d-1. The better performance of the

thermophilic process was attributable to the fact that

thermophilic microorganisms have higher growth rates than

mesophilic ones [5].

One of the major criticisms of the use of thermophilic

conditions in a process is that the final effluent has higher

concentrations of VFAs than those from a process conducted

under mesophilic conditions [4,20,34]. The thermophilic

methanogenic reactors were just as stable as the mesophilic

reactor. Precise comparisons between the effluent of the

mesophilic and thermophilic reactors could not be made

because of the different OLRs applied to the systems in this

study. However, the concentration of the TVFAs was below 5

g l-1 in the effluent from UASB reactors at both mesophilic and

thermophilic conditions before the overload period. The

accumulation of residual acetate was due to the fact that the

conversion of acetic acid to methane was the rate-limiting step

or was inhibited at overload. These results are in agreement

with previously reported results where VFA concentrations

were in the same range in the effluent from mesophilic and

thermophilic full-scale reactors digesting manure and

industrial organic waste [6]. Ahn and Forster [10] also

reported the same range of effluent VFA concentration in

mesophilic and thermophilic laboratory-scale upflow

anaerobic filters at lower OLRs (1-8 kg COD m-3d-1). However,

at higher OLRs (12 -17 kg COD m-3d-1) the thermophilic

reactor performed much better than the mesophilic one in

terms of reduction of the concentration of VFAs.

The pH of the effluent from the methanogenic reactors

was maintained above neutral, showing that the alkalinity

present in the reactors was able to neutralise the excess VFAs

to maintain the pH in the optimum range for methanogenesis.

However, at overload the alkalinity of the sludge was no

longer sufficient to neutralise the high VFA concentrations

prevalent in the reactors and there was a drop in pH.

CONCLUSIONS

Solid potato waste can be considered to represent any

carbohydrate-rich organic waste and thus this research can be

applied to household, farm and industrial biodegradable

waste to produce biogas. This research also contributes to our

knowledge regarding the choice of either mesophilic or

thermophilic conditions and the associated advantages for

biogas production. The conclusion is that the anaerobic

digestion of solid potato waste can be achieved using either

mesophilic or thermophilic conditions depending on the

circumstances. The feasibility of using a two-stage system has

also been demonstrated since potatoes are highly degradable,

producing a high concentration of volatile fatty acids in the

early stage of the anaerobic digestion process, and would be

difficult to digest using a one-stage system.

ACKNOWLEDGEMENTS

Funding for this research was provided by the Swedish

Agency for Research Cooperation (SAREC) and the Swedish

National Energy Administration (STEM).

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