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: 13:51Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK
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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
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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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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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