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Process Biochemistry 45 (2010) 1214–1225
Contents lists available at ScienceDirect
Process Biochemistry
journal homepage: www.elsevier.com/locate/procbio
Review
Importance of the methanogenic archaea populations in anaerobic wastewater
treatments
Meisam Tabatabaei a,b,∗ , Raha Abdul Rahim c , Norhani Abdullah d , André-Denis G. Wright e ,
Yoshihito Shirai f , Kenji Sakai g , Alawi Sulaiman h , Mohd Ali Hassan b,h
a
Microbial Biotechnology and Biosafety Department, Agricultural Biotechnology Research Institute of Iran (ABRII), Seed and Plant Improvement Institute’s Campus,
31535-1897, Mahdasht Road, Karaj, Iran
b
Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
c
Department of Cell and Molecular Biology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
d
Department of Microbiology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
e
Department of Animal Science, University of Vermont, Burlington, VT, USA
f
Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan
g
Laboratory of Soil Microorganisms, Department of Plant Resources, Graduate School of Bioresources and Bioenvironmental Sciences, Kyushu University, 6-10-10 Hakozaki,
Higashi-ku, Fukuoka 812-8581, Japan
h
Department of Food and Process Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
a r t i c l e
i n f o
Article history:
Received 18 January 2010
Received in revised form 1 May 2010
Accepted 17 May 2010
Keywords:
Biomethane
Biomass
Methanogens
Anaerobic treatment
Wastewater
a b s t r a c t
Methane derived from anaerobic treatment of organic wastes has a great potential to be an alternative
fuel. Abundant biomass from various industries could be a source for biomethane production where
combination of waste treatment and energy production would be an advantage. This article summarizes
the importance of the microbial population, with a focus on the methanogenic archaea, on the anaerobic
fermentative biomethane production from biomass. Types of major wastewaters that could be the source
for biomethane generation such as brewery wastewater, palm oil mill effluent, dairy wastes, cheese whey
and dairy wastewater, pulp and paper wastewaters and olive oil mill wastewaters in relevance to their
dominant methanogenic population are fully discussed in this article.
© 2010 Elsevier Ltd. All rights reserved.
Contents
1.
2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Types of waste materials and their dominant methanogenic population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.
Brewery wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.
Palm oil mill effluent (POME) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.
Dairy waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.
Cheese whey and dairy wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.
Pulp and paper wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.
Olive oil mill wastewater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anaerobic reactors: designs and operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Molecular methods for microbial ecosystem studies during anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A look to the future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗ Corresponding author at: Microbial Biotechnology and Biosafety Department, Agricultural Biotechnology Research Institute of Iran (ABRII),
Seed and Plant Improvement Institute’s Campus, 31535-1897, Mahdasht Road, Karaj, Iran. Tel.: +98261 2703536; fax: +98261 2704539.
E-mail address: meisam tab@yahoo.com (M. Tabatabaei).
1359-5113/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.procbio.2010.05.017
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1. Introduction
To date, global energy requirements are heavily dependent on
fossil fuels such as oil, coal and natural gas. As the exhaustion of limited fossil fuels is to be anticipated, there is a necessity to search
for replacement source of energy [1]. On the other hand, there
is a growing amount of organic waste and wastewater produced
annually. Anaerobic digestion technology is an ideal cost-effective
biological means for the removal of organic pollutants in waste and
wastewater which simultaneously produces gaseous methane as
an energy resource [2,3]. The many applications of this digestion
technology are the high-rate treatment of high-strength industrial
organic wastewater [1,4], low-strength organic wastewater [5],
complex wastewater containing persistent chemical compounds
[4], sulfate-rich wastewaters [6], wastewater discharged at temperatures ranging from psychrophilic to thermophilic [2,7] as well
as offering potentials for the removal of metals [8], nitrates [9], and
toxic substances [10].
The biomethane produced by anaerobic digestion is an odorless,
colorless and non-poisonous gas [11]. The process by which anaerobic bacteria decompose organic matter into biomethane, carbon
dioxide, and a nutrient-rich sludge involves a step-wise series of
reactions requiring the cooperative action of several organisms. It
occurs in three basic stages as the result of the activity of a variety
of microorganisms. Initially, a group of microorganisms converts
organic material to a form that a second group of organisms utilizes to form organic acids. Methane-generating (methanogenic)
anaerobic archaea utilize these acids and complete the decomposition process. Table 1 presents the classification of methanogenic
archaea as outlined by Demirel and Scherer [12]. In the first stage,
a variety of primary producers (acidogens) break down the raw
wastes into simpler fatty acids. In the second stage, a different group
of organisms (methanogens) consumes the organic acids produced
by the acidogens, generating biogas as a metabolic byproduct. On
average, acidogens grow much more quickly than methanogens.
Finally, the organic acids are converted to biogas [13]. Moreover,
compared with ethanol or other liquid biofuels, biomethane is
easily separated from liquid phase, which can contribute to the
reduction of the process costs [14].
Renewable biomass is the most versatile non-petroleum based
resource that is generated from various industries as waste mate-
rials. Animal manure, agricultural waste, municipal solid waste,
sewerage, food industry waste, and forest industry residues—all
of these sources can be used for production of biogas especially
biomethane [15] and it would be estimated that at least 25% of
all bioenergy can in the future originate from biogas produced
from waste [16]. The conversion of the waste to biomethane is
not only an alternative cost-effective way of energy production,
but it also contributes to very large overall reductions of greenhouse gas emissions as leakages of methane into the atmosphere
are avoided. Although biomass energy is more costly than fossilfuel-derived energy, trends to minimize carbon dioxide and other
emissions through emission regulations, carbon taxes, and subsidies of biomass energy would make it cost competitive [17].
In order to take full advantage of renewable biomass through
anaerobic digestion technology, one the most advanced fields associated with the technology which is the microbiology of anaerobic
digestion processes should be fully understood. The knowledge
of the ecology and function of the microbial community in these
processes is required to better control the biological processes
as the process is ultimately dependent on an active biomass
for operational efficiency. Therefore considerable attempts have
been made to understand the microbial community structure
by using culture-dependent and culture-independent molecular
approaches [2,18,19]. Through these analyses, particularly those
targeting the 16S rRNA gene, comprehensive pictures of the community compositions have been documented.
In this review, we focus on microbiological aspects of anaerobic digestion of various renewable biomasses with a focus on their
dominant methanogenic population, the leading factor of their successful anaerobic treatment, and update the recent findings in this
field. In addition, we highlight the importance of molecular techniques in moving from the conventional monitoring systems of
anaerobic digesters to biomonitoring procedures.
2. Types of waste materials and their dominant
methanogenic population
2.1. Brewery wastewater
The brewing process generates a unique, high-strength wastewater as a byproduct. Even though substantial technological
Table 1
Classification of methanogenic archaea as outlined by Demirel and Scherer [12].
Class I. Methanobacteria
Order I. Methanobacteriales
Family II. Methanothermaceae
Family II. Methanothermaceae
Family I. Methanococcaceae
Class II. Methanococci
Order I. Methanococcales
Family II. Methanocaldococcaceae
Family I. Methanomicrobiaceae
Order I. Methanomicrobiales
Family II. Methanocorpusculaceae
Family III. Methanospirillaceae
Class III. Methanomicrobia
Order II. Methanosarcinales
Family I. Methansarcinaceae
Family II. Methanosaetaceae
Genus I. Methanobacterium
Genus II. Methanobrevibacter
Genus III. Methanosphaera
Genus IV. Methanothermobacter
Genus I. Methanothermus
Genus I. Methanococcus
Genus II. Methanothermococcus
Genus I. Methanocaldococcus
Genus II. Methanotorris
Genus I. Methanomicrobium
Genus II. Methanoculleus
Genus III. Methanofollis
Genus IV. Methanogenium
Genus V. Methanolacinia
Genus VI. Methanoplanus
Genus I. Methanocorpusculum
Genus I. Methanospirillum
Genus I. Methanosarcina
Genus II. Methanococcoides
Genus III. Methanohalobium
Genus IV. Methanohalophilus
Genus V. Methanolobus
Genus VI. Methanomethylovorans
Genus VII. Methanimicrococcus
Genus VIII. Methanosalsum
Genus I. Methanosaeta
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Table 2
List of the abbreviations and definitions.
Abbreviation
Definition
Abbreviation
Definition
ABR
AFB
BOD
CDT
COD
CSTR
DGGE
EGSB
FBL
FELDA
FISH
GAC
GRABBR
HHW
HRT
JSPS
LCFA
Anaerobic baffled reactor
Anaerobic fluidized bed
Biological oxygen demand
Closed digester tank
Chemical oxygen demand
Completely stirred tank reactor
Denaturing gradient gel electrophoresis
Expanded granular sludge blanket
Fixed-bed loop
Federal Land Development Authority
Fluorescent in situ hybridization
Granular activated carbon
Granular bed baffled reactor
Household waste
Hydraulic retention time
Japan Society for the Promotion of Science
Long chain fatty acids
LH-PCR
M. concilii
MAS
MOSTI
Length heterogeneity PCR
Methanosaeta concilii
Membrane anaerobic system
Ministry of Science, Technology and Innovation, Malaysia
OMW
POME
RISA
SEC
SRT
SS
SSCP
T-RFLP
UASB
UASFF
VFA
VOL
Olive mill wastewater
Palm oil mill effluent
Ribosomal intergenic spacer analysis
Sulfite evaporator condensate
Sludge retention time
Suspended solids
Single-strand conformation polymorphism
Terminal restriction fragment length polymorphism
Upflow anaerobic blanket reactor
Upflow anaerobic sludge fixed film reactor
Volatile fatty acids
Volumetric organic loading
improvements have been made in the past, it has been estimated
that for each liter of beer produced in breweries approximately
3–10 l of waste effluent is generated [20]. The high level of soluble biological oxygen demand (BOD) (Table 2) and the warm
temperature (>37 ◦ C) make brewery wastewater an ideal substrate
for anaerobic treatment. Brewery wastewater is characterized by
high-strength soluble organic pollutants and suspended solids (SS)
[21]. Therefore, aerobic treatment due to the need for an intensive amount of energy for aeration and a large amount of wasted
sludge produced is not a favorable choice [22]. Hence, anaerobic digestion using high-rate anaerobic reactors such as upflow
anaerobic blanket reactor (UASB) [23], anaerobic granular bed baffled reactor (GRABBR) [24] and anaerobic fluidized bed (AFB)[25]
have been reported to treat brewery wastewater with a satisfactory chemical oxygen demand (COD) reduction. Díaz et al. found
Methanosaeta the dominant genus (between 75 and 95% of total
archaeal cells) in a UASB reactor treating brewery wastewater, with
Methanosaeta concilii accounting for 70% of the archaeal clones
[26] (Fig. 1A and B). Methanosarcina mazei and Methanospirillum hungatei were also present. This could be explained by the
favorable concentration of acetate in brewery wastewater [27], as
Methanosarcina has a higher maximum growth rate, but a lower
affinity for acetate (max , 0.21 day−1 ; Ks , 4 mM) than Methanosaeta
(max , 0.11 day−1 ; Ks , 0.44 mM) [28,29]. In addition, the majority
(87%) of the total bacterial clones obtained in his study belonged
to the phyla Deferribacteres, Nitrospira, and Chloroflexi [26]. Uncultured clades belonging to the phylum Deferribacteres represented
34% of the bacterial population. This high occurrence in such
methanogenic ecosystems indicates that they might play a role in
part of the food web for the methanogenic degradation of organic
compounds [22].
In a similar study, all archaeal clones were affiliated
with Methanosaeta concillii, and no clones were related to
hydrogenotrophic methanogens. However, electron microscopic
examination detected hydrogen-consuming Methanosarcina-like
cells and Methanobrevibacter-like cells [30]. This difference was
due to the bias of the rRNA approach when there are significant differences in the number of studied microorganisms [31].
The bacterial clones in this study were mostly affiliated with a
not-yet-cultured Clostridium cluster (>50%) [30]. In general, acetoclastic methanogens in particular Methanosaeta concillii are more
abundant than hydrogenotrophic ones in methanogenic consortia
during anaerobic digestion of brewery wastewater (Table 3).
2.2. Palm oil mill effluent (POME)
Palm oil mill effluent is unquestionably the largest waste generated from the oil extraction process [32]. For every tonne of oil palm
fresh fruit bunch, it is estimated that 0.5–0.75 t of POME will be discharged from the mill [33]. This wastewater is a viscous, brownish
liquid containing about 95–96% water, 0.6–0.7% oil and 4–5% total
solids (including 2–4% suspended solids). It is acidic (pH 4–5), with
a high temperature (80–90 ◦ C), high organic COD, (50,000 mg l−1 ),
and high BOD, (25,000 mg l−1 ) [34]. It is 100 times more recalcitrant than domestic wastewater [35]. Therefore, due to its highly
polluting properties (high BOD and COD), the most cost-effective
technology is anaerobic treatment [36].
Over the past decade, several cost-effective anaerobic treatment
technologies have been developed for the treatment of POME such
as closed digester tank (CDT) [32], completely stirred tank reactor
(CSTR) [37], the modified anaerobic baffled reactor [38], anaerobic filter and anaerobic fluidized bed reactor [39], thermophilic
Fig. 1. Typical fluorescent in situ-hybridized cells of dominant methanogens in anaerobic treatment of the majority of various wastewaters: (A) Methanosaeta concilii; (B) a
cluster of Methanosaeta concilii and (C) Methanosarcina sp. hybridized with FITC-labeled methanogens probe (MSMX860) (provided by the authors).
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Table 3
A summary of kinetic data, main characteristics, and methanogenic population of major wastewaters.
Type of wastewater
Main characteristics
Dominant
Maethanogens
max (day−1 )
Ks (mM)
Other Methanogens
Reference
Brewery wastewater
Favorable
concentration of
acetate for
Methanosaeta
Methanosaeta concilii
0.11a
0.44
Methanosarcina mazei,
Methanospirillum hungatei
[26,27,28,29]
Palm oil mill effluent
Dairy wastes
Cheese whey and
dairy wastewater
Pulp and paper
wastewaters
Olive oil mill
wastewaters
Highly favorable
concentration of
acetate for
Methanosaeta
High levels of free
ammonia and VFAs
Presence of LCFA in
particular oleic acid
Toxic and resistant to
biodegradation
compounds (i.e.
lignins, resins, tannins
and highly chlorinated
organics such as
chlorophenolic
compounds)
Acidic pH, presence of
inhibitory/toxic
compounds such as
high sodium
conce-ntration, high
content of polyphenols,
tannins, and lipids
Methanosaeta concilii
0.11
0.44
Hydrogen-consuming
Methanosarcina-like cells and
Methanobrevibacter-like cells
Methanosarcina sp.
Methanosarcinaceae
0.21
4
Methanomicrobiales
[28,54,58,61]
Methanobacterium
thermoautotrophicum
Methanosaeta spp.
–
–
0.11
0.44
Methanobrevibacter sp.
[78,79,80,81]
Methanococcus spp.
(towards the end of
operation)
Methanosarcina sp.
(Methanosarcina
barkeri)
–
–
Methanosarcina sp.
0.023 (h−1 )
320 (as mgCOD/l−1 )
Methanobacterium sp.
Methanosaeta sp.
(Methanosaeta concilii)
Methanobacteriaceae
(Methanobacterium
formicicum)
0.11
0.44
0.053 (h−1 )
–
Methanobrevibacter
arboriphilus
Methanosaeta sp.
[29,31]
[12,94,95,100,101,
110,111,112,113]
[12,118,131,133,134]
Methanomicrobiaceae
a
Growth on acetate.
upflow anaerobic filter [40], membrane anaerobic system (MAS)
[41], UASB reactor [42,43], and rotating biological contactors [44].
To date, only a few studies have been conducted on the microbial aspects of POME anaerobic treatment [31,35,45,46]. Tabatabaei
et al. conducted a comprehensive study on the methanogenic
diversity during the anaerobic treatment of POME in a CDT. The
majority (>99%) of the total methanogens counted by using fluorescent in situ hybridization (FISH) in their study belonged to
the genus Methanosaeta (Table 3). However, 16S rRNA cloning
along with denaturing gradient gel electrophoresis (DGGE) analysis
showed that M. concilii was the only member of the genus present.
Methanosarcina accounted for <1% of the whole methanogenic population [31] (Fig. 1). The high number of M. concilii was attributed
to the highly favorable concentration of acetate in POME.
2.3. Dairy waste
An average dairy cow (450 kg) produces approximately 37 kg
of waste (manure and urine) d−1 ; thus, a 1000-cow dairy produces approximately 13,500,000 kg of waste annually [47]. This
waste is usually stored in lagoons until it can be applied to agricultural fields as a soil fertilizer for crops. However, there are serious
drawbacks for the current procedure. First, cow manure may contain pathogenic bacteria to both humans and animals, such as
Escherichia coli O157:H7 [48], Campylobacter spp. [49], Salmonella
spp. [50], and Mycobacterium spp. [51,52,53]. Therefore, crops fertilized with dairy waste may transmit these pathogens to livestock
or humans who consume them. Second, the release of odorous com-
pounds into the air severely affects the air quality [47]. Veterinary
studies indicate that anaerobic treatment at 60 ◦ C with a guaranteed holding period of 4–6 h before waste is pumped out of the
reactor is acceptable in order to treat potentially dangerous wastes
[54]. Hence, the use of high-rate anaerobic treatment technologies
for dairy waste before it enters the lagoons is an advantageous solution in order to reduce organic matters [55], and pathogens [56] as
well as methane production.
Demirer and Chen reported that two-phase anaerobic digestion
for unscreened dairy manure at a ratio of sludge retention time
(SRT) to hydraulic retention time (HRT) of 10 days (2 days acidogenic and 8 days methanogenic) resulted in 50 and 67% higher
biogas production at OLRs of 5 and 6 g VS l−1 d−1 , respectively, relative to a conventional one-phase configuration with SRT/HRT of
20 days. Moreover, it made an elevated OLR of 12.6 g VS l−1 d−1
possible which was not achievable for conventional one-phase configuration and therefore, was made significant cost savings due to
both superior performance and reduced volume requirements [57].
The dominant methanogens in manure digesters have never
been well documented. Karakashev et al. reported that organisms
assumed to be acetoclastic (Methanosarcinaceae and Methanosaetaceae) are more abundant than organisms assumed to be
hydrogenotrophic (Methanobacteriales, Methanomicrobiales, and
Methanococcales) in anaerobic treatment of dairy waste. However,
as manure contains high levels of ammonia and of volatile fatty
acids (VFA), its anaerobic digestion leads to the domination of
members of the Methanosarcinaceae (Fig. 1C) [58] due to the intolerance of members of the Methanosaetaceae for high ammonia
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and VFA levels [58,59,60]. In contrast, using 16S rRNA sequence
analysis, low levels of members of the Methanosarcinaceae and
high levels of members of the Methanomicrobiales were observed
in a full-scale manure-fed reactor [61]. Ahring counted acetateand hydrogen-utilizing methanogens in thermophilic biogas reactors treating a mixture of cow and pig manure and found the
hydrogen-utilizing methanogens in particular Methanobacterium
thermoautotrophicum absolutely dominant [54]. In his study, all
acetate-utilizing methanogens identified belonged to the genus
Methanosarcina and the majority were in the form of individual
single cells in the reactor. Hence, it could be concluded that the
genus Methanosaeta plays no or little role in acetate conversion in
the therrnophilic biogas reactors [54,62,63]. An experiment with
radio-labeled acetate (14 CH3 COOH) [54] showed that acetate in
the thermophilic anaerobic reactor was converted by the acetoclastic reaction at high concentrations of acetate and by a two-step
mechanism involving the microbial oxidization of acetate to hydrogen and carbon dioxide and the transformation of these products
into methane by hydrogen-utilizing methanogens [64] when the
acetate concentration was lower than the threshold level for the
Methanosarcina species and in the absence of Methanosaeta species
in the reactor. In addition, a mixture of both types of metabolism
occurred close to the threshold level.
In general, in contrast to sludge digesters where, Methanosaetaceae are the main acetoclastic methanogens (Table 3) [26,30,31],
Methanosarcinaceae are either the only or the most abundant
acetate-utilizing methanogens in manure digesters [543,58,61].
The predominance of Methanosarcinaceae could be indirectly
attributed to the high free ammonia levels of manure which restrict
Methanosaetaceae [58]. Methanosarcinaceae particularly M. concilii
are the most ammonia-sensitive methanogen, and it is completely
inhibited at a concentration of 560 mg (total) NH4 -N l−1 at a pH
level of 7.0 [65,66]. Therefore, high free ammonia levels cause
the accumulation of VFA, which then allow Methanosarcinaceae
which have a higher threshold for acetate [28,29,67] to outcompete
and restrict Methanosaetaceae. Finally, reducing ammonia levels or
its inhibitory effect such as by addition of lipid-containing waste
[68] in manure digesters should change the equation in favor of
the members of the Methanosaetaceae and consequently reduce
organic acid levels considerably [58].
2.4. Cheese whey and dairy wastewater
The liquid waste in a dairy originates from manufacturing process, utilities and service sections with a high COD ranging from 1 to
10 g l−1 and a high BOD5 ranging from 0.3 to 5.9 g l−1 [69–71] representing its high organic content. Moreover, the dairy industry is
one of the largest sources of industrial effluents for instance, a typical European dairy generates about 180,000 m3 of waste effluent
annually [72]. However, there are high seasonal variations correlated with the volume of milk received for processing; which is in
general high in summer and low in winter months [73]. The various
sources of waste generation from a dairy are spilled milk, spoiled
milk, skimmed milk, whey, wash water from milk cans, equipment, bottles and floor washing [69]. Among those, whey is the
most difficult high-strength waste product of cheese manufacturing (COD of more than 60 g l−1 ) [71,74] which contains a proportion
of the milk proteins, water-soluble vitamins and mineral salts.
Therefore, high COD concentration of dairy effluents, their high
temperature, no requirement for aeration, low amount of excess
sludge production and low area demand make them ideal candidates for anaerobic treatment [71] using various types of anaerobic
reactors [75–77]. The acetoclastic methanogenic activity measured
in anaerobic treatment of dairy wastewater was found to be due
mostly to Methanosaeta species whilst Methanosarcina-like species
contributed insignificantly [78]. However, Methanococcus species
seemed to become the most dominant group towards the end of
the operation [78,79].
A study where a polymer-amended anaerobic baffled reactor
(ABR) was used revealed that partial spatial separation of anaerobic bacteria appeared to have taken place with the predominance
of acidogenic bacteria in the initial compartments and the predominance of methanogenic bacteria in the final compartments.
It also showed that the dominant methanogens in the initial compartments of the ABR were those which could consume H2 /CO2
and formate as substrate, i.e. Methanobrevibacter, Methanococcus,
with populations shifting to acetate utilizers (i.e. Methanosaeta,
Methanosarcina) in the final compartments [79].
Milk fat was found to have an immediate influence on reducing
methane gas production rate in reactors to which it was added [80].
Similar observations were reported by Uyanik et al and Demirel and
Yenigun indicating that the densities of total bacteria and autofluorescent methanogens both decreased during start-up operation of
dairy wastewater anaerobic treatment [79,81]. This was explained
due to the presence of long chain fatty acids and in particular oleic acid, which is a major derivative of milk fat hydrolysis
[80]. Oleic acid was found to have inhibitory effects on methane
production and on ATP concentration in the sludge which is an
indicator of sludge’s total physiological activity [80] particularly
through acetoclastic methanogenesis. Oleic acid at a concentration of 4.4 mM (300–1500 mg l−1 ) resulted in 50% inhibition in
methanogenesis from acetate at 30 ◦ C [82]. Under thermophilic
conditions (55 ◦ C), 100–1000 mg l−1 oleic acid inhibited acetic acid
removal [83]. Lalman and Bagley also reported that oleic acid at
concentration above 30 mg l−1 inhibited acetoclastic methanogenesis at 20 ◦ C [84]. They also pointed out that slight inhibition of
hydrogenotrophic methanogenesis occurred. Furthermore, milk fat
also contributes to the sludge flotation problems [80,85] which consequently plays a role in biomass wash-out from the reactor [48].
About 70% of milk lipids are adsorbed by the granular sludge [86]
which reduced the adhered fraction of biomass [87]. Rinzema et
al. reported sludge flotation and a total sludge wash-out in a UASB
reactor with a lipid loading rate more than 2–3 gCOD l−1 d−1 [88].
Taking all into account, Perle et al advised to treat dairy effluents
by anaerobic digestion only after reduction of the milk fat concentration below 100 mg l−1 , and after careful acclimatization of the
digester culture to casein in order to develop proteolytic enzymatic system [80]. To the contrary, some studies reported that
the intermediates of fat degradation (mainly oleic acid) seem not
to reach concentrations high enough to affect the anaerobic process or hardly affected the overall performance [87,89]. It was also
reported that the anaerobic biodegradation rate of fat-rich wastewaters is slower than that of fat-poor wastewaters, due to the slower
rate of the fat hydrolysis step [89]. Having considered various factors, Vidal et al. recommended reactor operation for anaerobic
treatment of dairy wastewater at COD concentrations between 3
and 5 kg COD m−3 to ensure the highest levels of biodegradability
and biomethanation of both wastewaters and eliminate flotation
problems [89]. It was documented that anaerobic treatment of a
fat-rich dairy wastewater is enhanced when repeated pulse feeding
is applied by promoting the accumulation of long chain fatty acids
(LCFA) into the biomass and allowing them to be biodegraded afterwards. This is attributed to the fact that LCFA degradation process
increased the tolerance of the acetoclastic methanogens to LCFA
effect, by significantly dropping the lag phases observed before the
beginning of methane production [90].
2.5. Pulp and paper wastewater
The pulp and paper industry is a very water-demanding industry and can consume as high as 35 m3 of freshwater t−1 of paper
produced [91]. This results in the generation of various types of
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wastewater such as papermaking effluent, de-inking process effluent and pulping process effluent with an average COD value as high
as 11,000 mg l−1 [89]. For each tonne of manufactured pulp, the
wastewater discharge volume will be a minimum of 30 m3 [92].
The characteristics of the pulp and paper-effluent are highly dependent on raw materials and manufacturing process adopted [92,93].
Moreover, these effluents are strongly polluting and toxic owing
to the presence of lignins, resins, tannins and chlorophenolic compounds that are resistant to biodegradation [94,95].
Application of a “zero liquid effluent” process was reported as
a feasible option for the paper mills and found to be profitable
[96,97]. The wastewater is generated at a temperature ranging from
50 to 60 ◦ C and therefore, thermophilic anaerobic treatment complemented with appropriate post-treatment is considered as the
most cost-effective solution to meet re-use criteria of the process
water as well as maintaining its temperature [98]. Anaerobic treatment is well feasible for effluents generated by recycle paper mills,
mechanical pulping (peroxide bleached), semi-chemical pulping
and sulfite and kraft evaporator condensates. [99]. This is due to
the tolerance to toxicity of anaerobic microorganisms [100]. In
the proposed closed-cycle, the anaerobic treatment step removes
the largest fraction of the biodegradable COD and sulfur as H2 S
from the effluent, without the use of additional chemicals, and is
regarded as the only possible location to eliminate sulfur from the
process water cycle [98]. Buzzini and Pires studied the treatment
of diluted black liquor from a kraft pulp plant by using a UASB reactor and obtained a COD removal efficiency of 80% [101]. The black
liquor comprises only 10–15% of the total wastewater, however,
contributes approximately 95% of the total pollution load of pulp
and paper mill effluents [102]. Therefore, due to its higher contents
of chemicals and organic substances and consequent high pollution strength, low influent concentration was found essential for
granulation when UASB reactors are applied [103]. Van Lier et al.
compared H2 S stripping efficiency of UASB and gas-supplied UASB
reactors treating paper mill effluent and showed 3–4 times higher
values in the gas-supplied UASB [98]. In a study where an anaerobic
baffled reactor was used for continuous anaerobic digestion of pulp
and paper mill black liquors, OLRs higher than 5 kg COD m−3 d−1
resulted in loss of reactor’s stability which was apparent by the
decrease in biogas production rate and its methane content [104].
This was attributed to the toxic effect of the high concentration
of tannin and lignin present in black liquor on methanogens [105].
However, in a similar study in an anaerobic baffled reactor by using
an immobilized cell system, the reactor maintained its stability
with higher OLRs (7 kg COD m−3 d−1 )[106]. This was due to the
advantages of immobilization technologies such as the retention
of catalytic activity, a high ratio of sludge retention time (SRT) to
hydraulic retention time (HRT) and in particular, the protection of
cells from the effects of inhibitory/toxic substances [107,108].
Several studies have demonstrated the capacity of the microbial consortium e.g. methanogenic archaea to adapt to potentially
toxic effluents present in the effluents of pulp and paper mills
[101,109]. The adaptation depends on the concentration of the
toxicants and the operating conditions and the acclimation of the
sludge substantially reduces the degree of inhibition [109]. The
predomination of Methanosarcina spp. and Methanosaeta spp. was
reported during the anaerobic treatment of paper and pulp mill
effluent using USAB reactors [100,101,110]. In a study, Roest et
al. monitored microbial populations in a UASB reactor for treating paper mill wastewater over 3 years with a combination of
different molecular techniques and conventional microbiological
methodology. The authors were able to confirm that Methanosaeta
was the most abundant archaeal genus throughout the operation
[111]. They also reported the domination of sulfate-reducing bacteria and syntrophic fatty acid-oxidizing microorganisms during the
anaerobic treatment of paper mill wastewater [111]. Methanogenic
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consortia (Methanosaeta sp., Methanosarcina sp., and Methanobacterium sp.) were found to have an important role in the degradation
of highly chlorinated compounds such as chlorophenols present
in paper mill wastewaters (Table 3) [112]. This was supported by
the findings of Buzzini et al. who reported the capability of anaerobic treatment using UASB reactors dominated by Methanosaeta
sp. and Methanosarcina sp. to treat this kind of wastewater with
chlorinated organics removal efficiency ranging from 71 to 99.7%
[110]. Using a high-rate fixed-bed loop (FBL) reactor, Ney et al.
successfully treated sulfite evaporator condensate (SEC) which is
a wastewater from pulp and paper [113]. He showed that with
a consortium consisting of Methanosarcina barkeri, Methanobrevibacter arboriphilus, M. concilii and Desulfovibrio furfuralis, all the
constituents of a synthetic SEC including furfural, which is a toxic
compound to anaerobic bacteria [114], were degraded at an efficiency of almost 90% [113].
2.6. Olive oil mill wastewater
Olive mill wastewater (OMW) generated by the olive oil extraction process is the main waste product of this industry. The world
annual production of olives is approximately 10 million tonnes
where the majority of olives is produced in the Mediterranean
countries and 90% is processed for oil production [115]. The average amount of olive mill wastewater produced during the milling
process is 1.2–1.8 m3 t−1 of olives [116]. Therefore, the OMW resulting from the production process exceeds 13.5 million m3 annually.
Treatment of OMW is becoming a serious environmental problem,
due to its high BOD and COD concentration (100–220 g l−1 ; which
is on average 100 times greater than those of common municipal wastewater [114,117]), high sodium concentration [118] as
concentrations exceeding 10 g l−1 strongly inhibits methanogenesis [119], low pH (∼5), low alkalinity (∼0.6 g CaCO3 l−1 ) [120]
and finally because of its resistance to biodegradation due to its
high content of polyphenols, tannins, and lipids and consequent
negatively impacts on methanogenic cells. Despite the presence
of inhibitory/toxic compounds, the high organic content of OMW,
makes anaerobic treatment processes with biogas production a
considerable option. Besides the previously mentioned advantages
of anaerobic treatment, easy restart after several months of shutdown before seasonal production campaigns as it is the case for
OMW anaerobic treatment should be stressed.
To date due to the characteristics of OMW, various anaerobic
treatment approaches have been applied such as high dilution of
OMW with tap water [121,122], combined treatment (co-digestion)
of OMW together with other waste such as manure, household
waste (HHW) or sewage sludge to compensate for its low alkalinity and nitrogen [123,124], the use of pretreatment systems
before anaerobic treatment such as sand filtration and subsequent
treatment with powdered activated carbon [116], using biological agents such as Aspergillus strains, Azotobacter chroococcum,
Geotrichum candidum [125,126] and pretreatments with Ca(OH)2
and bentonite [127].
Dalis et al. found the employment of the upflow type digester
such as UASB as an economical and effective treatment for significantly reducing the organic load of total raw olive oil wastewater
(83% COD removal and 75% reduction of phenolic compounds).
More satisfactory results were obtained when a fixed-bed-type
digester was connected in series with a previous one as a second
treatment stage [128]. COD reductions of 70–80% using laboratoryscale UASB reactors were reported by other researcher [122,129].
Anaerobic biofilm reactors packed with granular activated carbon (GAC) and ‘Manville’ silica beads showed approximately 60,
250, and 100% improvement in COD removal, phenol reduction
and methane yield, respectively, when compared with treatment in
conventional anaerobic contact bioreactors [130]. In a similar study,
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Table 4
The classification of anaerobic reactors and typical examples by Fannin and Biljetina [137].
Category
Retention characteristics
Examples
A
B
Microorganisms retention time is equal to that of the solid and liquid (RTm = RTs = RTl )
Microorganisms and solid retention time is higher than that of the liquid (RTm and RTs > RTl )
C
Microorganisms retention time is higher than that of the solid and liquid (RTm > RTs and RTl )
CSTR, CDT
CSTR or CDT with solid recycle,
UASB, Baffled flow reactor
Membrane bioreactor, UASFF
RTm = Retention time of microorganism; RTs = Retention time of solid; RTl = Retention time of liquid; CDT = Closed digester tank; CSTR = Continuously stirred tank reactor;
UASB = Upflow anaerobic sludge blanket; UASFF = Upflow anaerobic sludge fixed film reactor.
Bertin et al. [131] used a GAC-bioreactor to treat OMW and reported
about 100 and 300% improved efficiency in terms of removal of COD
and phenolic compounds, respectively, and by 70% in terms of CH4
production [131]. GAC provides the microorganisms with a place
to grow and allow them to live stably in the reactor by minimizing
the inhibitory/toxic effect of the present compounds. Hence, it provides the bioreactor with increased tolerance to high and variable
organic loads along with a volumetric productivity in terms of COD
and phenolic compound removal. Taking all things into account,
results of single anaerobic treatment are not always satisfactory
and some form of pretreatment, apart from simple dilution and
nutrient/alkalinity adjustment, is usually necessary [132].
Bertin et al. (2006) analyzed the microbial diversity of a GAC
reactor during anaerobic digestion of OMW and found a member of Methanobacteriaceae as the sole dominant species, i.e.,
hydrogenotrophic Methanobacterium formicicum representing the
whole archaeal community [131]. This methanogen was also dominant and persistent in a UASB pilot plant treating OMW [133]. The
absence of acetoclastic methanogens which are highly pH-sensitive
as well [134] was due to the acidic pH environments occurred in
the reactors. In contrast to these studies, Methanobacteriaceae and
Methanosaeta were both the main methanogens in a laboratoryscale upflow anaerobic digester treating olive mill effluent [134].
In the latter study, at a volumetric organic loading (VOL) of 6 g
COD l−1 day−1 , the hydrogenotrophic Methanobacterium predominated in the reactor but decreased from 1011 to 108 cells g−1 sludge
when the VOL was increased to 10 g COD l−1 day−1 . By increasing
the VOL, the non-dominant methanogenic family i.e. Methanomicrobiaceae increased from 104 to 106 cells g−1 sludge. On the
other hand, hylotypes belonging to the acetoclastic Methanosaeta
were stable throughout VOL variation and at 10 g COD l−1 day−1
dominated in the biofilm (109 cells g−1 sludge) [135]. With the
above results, we may suggest, Methanosaeta as the most tolerant
methanogen to the inhibitory/toxic substances present in wastewaters such as OMW. This could be attributed to its high affinity for
acetate enabling it to occupy the deepest or in other words, more
protected niches in the granule or biofilm with low concentration
of substrate [136].
3. Anaerobic reactors: designs and operation
Various types of anaerobic reactors have been successfully
designed, studied and applied to a wide range of organic rich
wastewaters such as UASB [23,42,43], GRABBR [24], AFB [25],
CDT [32], CSTR [37], the modified anaerobic baffled reactor [38],
anaerobic filter and anaerobic fluidized bed reactor [39], thermophilic upflow anaerobic filter [40], MAS [41], rotating biological
contactors [44], polymer-amended ABR [79], anaerobic biofilm
reactors packed with GAC and ‘Manville’ silica beads [130,131].
This section shall briefly review the common anaerobic reactor
designs used in the treatment of organic rich wastewaters and further discuss their classification and operation. The classification
of anaerobic reactors was best described by Fannin and Biljetina
according to the retention time characteristics of microorganisms,
solid and liquid in the reactor system and is simplified in Table 4
[137].
The simplest reactor design is class A, such as CDT and CSTR,
where the retention time of microorganisms, solid and liquid is
equal. This type of digesters is characterized by lowest construction cost and simplest operation among all types of reactors. The
schematic diagrams of various anaerobic reactors are presented in
Figs. 2 and 3. In the CDT reactor [32], the substrate is fed through
the bottom inlet and displaces the treated effluent inside the tank
out through the top outlet of the CDT (Fig. 2A). The mixing is
achieved using a centrifugal pump which circulates the effluent
intermittently inside the digester. Alternatively, an agitator could
also be used for the mixing purpose. The mixing will release the
entrapped biogas at the bottom of the tank and provides a good
contact between microorganisms and substrate inside the digester.
The biogas produced will flow out of the digester through the top
outlet for further processing. In the CSTR reactor [138], the influent
is pumped into the CSTR through the bottom inlet and the effluent will flow out from the top outlet (Fig. 2B). Mixing is achieved
using an agitator mounted at the top of the CSTR. The biogas is
produced inside the CSTR and released through the top outlet. The
mixing could be continuous or intermittent but must be achieved
completely in the digester.
Fig. 2. Schematic diagrams of closed digester tank (CDT) (A), continuous stirred tank reactor (CSTR) (B), and CDT with solid recycle system (C).
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M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225
1221
Fig. 3. Schematic diagrams of upflow anaerobic sludge blanket bioreactor (UASB) (A), membrane filter bioreactor (B) and upflow anaerobic sludge fixed film bioreactor
(UASFF) (C).
For class B anaerobic reactors such as CSTR or CDT with solid
recycling system, UASB and baffled flow reactor, the retention time
of microorganisms and solid is higher than that of the liquid in
order to accomplish higher process efficiency. The basic schematic
diagram of a CDT reactor with solid recycle system is shown in
Fig. 2C [45]. The basic operation of such reactor is identical to that
of the normal CDT, however, a settling tank is added to the system to recycle the biomass and consequently increase the retention
time of the reactor. The recycling of biomass would also help to
supplement nutrients and alkalinity to the reactor. Recently, Busu
et al. reported that sludge recycling improved the overall process
performance of the reactor [139]. The schematic diagram of the
UASB reactor is shown in Fig. 3A [140]. In this reactor, the substrate is fed through the bottom inlet and flows upward through
the sludge blanket phase. The treated effluent will overflow at the
top of the reactor. The most important feature of this reactor is
the sludge blanket situated at the bottom part of the reactor which
aids to maintain a high amount of microorganisms including both
acidogens and methanogens in the system.
To achieve a higher biomass concentration in the reactor for
higher reactor efficiency and biogas performance, Fannin and Biljetina suggested class C of the reactor designs [137]. The typical
examples of such design are MAS and upflow anaerobic sludge
fixed film reactor (UASFF)[141]. In the membrane anaerobic reactor, the influent is pumped through the top inlet to be utilized
by the microorganisms inside the reactor (Fig. 3B). The mixture
of substrate and microorganisms is then pumped into the membrane filtration unit for separation. The treated effluent is allowed
to leave the reactor while the microorganism fraction is returned
to the reactor. The key and the most important part of this system
is the membrane unit designed to efficiently capture the microbial
mass responsible for acidogenesis and methanogenesis processes.
The schematic diagram of the UASFF reactor is presented in Fig. 3C
[34]. The reactor column is divided into three different compartments. The bottom part is designed like a UASB section while the
middle and top portions are designed similar to a fixed film reactor
and a gas–liquid separator, respectively. The influent is fed to the
reactor from the base using a pump and flows through the reactor. The incoming influent will displace an equal volume of the
treated effluent flowing out through the top outlet of the reactor.
The combination of sludge blanket at the bottom and fixed film at
the middle ensures high biomass retention in the system for high
organic removal efficiency and biogas production. The comparison
of performance of various reactor designs utilized for the anaerobic
treatment of different types of wastewaters and biogas generation
is summarized in Table 5.
4. Molecular methods for microbial ecosystem studies
during anaerobic digestion
Although anaerobic digestion has been applied in wastewater
treatment successfully over the last 100 years, however, molecular
methods have only been applied to the analysis of communities
in anaerobic digesters since the late 1990s [142,143]. As previously mentioned, methanogenesis from complex organic matter
is achieved by the microbial consortia comprising members of
both the bacteria and the archaea. At the moment, optimization
of methane production is carried out empirically and the process
is generally monitored by the determination of VFA concentrations
in the digester as described by Ahring et al. [144]. Biomonitoring
digesters using molecular methods would not only be useful to
avoid failures and optimize the production of methane, but could
also lead to the identification of new species.
Molecular methods can be generally classified into (i) analysis of small subunit ribosomal RNA (SSU rRNA) clone libraries,
(ii) community fingerprinting techniques using SSU rRNA gene
such as denaturing gradient gel electrophoresis (DGGE), terminal restriction fragment length polymorphism (T-RFLP), ribosomal
intergenic spacer analysis (RISA), single-strand conformation polymorphism (SSCP), and length heterogeneity PCR (LH-PCR), and
(iii) dot blotting, FISH, and stable isotope probing. Most of the
molecular approaches used so far are based on the analysis of SSU
rRNA but recent studies also use quantification of functional gene
expressions. Furthermore, several quantitative methods have been
developed such as quantitative real-time PCR assay [145,146] and
quantitative FISH to investigate methanogens [31]. A recent review
published by Talbot et al. highlighted the principles of cultureindependent nucleic-acid-based methods for analyzing microbial
communities in anaerobic digesters [147]. Tabatabaei et al. also
demonstrated that by employing FISH combined with community
fingerprinting DGGE and cloning/sequencing analyses can successfully study the methanogenic population dominating a particular
substrate [31]. Therefore, a combination of molecular techniques
seems to be an ultimate tool in microbial ecosystem studies during
anaerobic digestion.
5. A look to the future
A look to the future of environmental biotechnology, microbial ecology and anaerobic digestion should focus on scientific
advances that open new possibilities that pull them towards practical goals. Looking first at the science side, the capabilities of
molecular methods to shed light on how microbial communities
1222
Table 5
Different classes of anaerobic reactors used for different wastewaters, maximum performances, advantages, and drawbacks of each design.
Reactor
Ref.
HRT (d)
COD inlet
level
(kg m−3 )
OLR
(kg COD m−3 d)
COD Removal
(%)
Advantages
Drawbacks
A
Palm oil mil effluent
CDT
[32]
10
56.45
5.55
>90
Low capital, operating and
maintenance costs, adaptable to high
OLR range, simple design, construction
and operation, less technical skills
requirement, uniform distribution of
nutrients, pH, substrate and
temperature, no scum layer formation,
plugging, gas entrapment and
channeling, easily modeled
[137,152,153].
Less stable system, less biomass
retention, more suitable for particulate,
colloidal and soluble wastes substrates,
larger digester volume requirement
and problem of microorganisms
wash-out, longer retention time
requirement, complete mixing
problem at large scale. [137,152,153].
Swine waste
Manure slurry
CSTR
CSTR
[148]
[149]
1
16.2
NA
NA
37.4%
32.8%
Municipal sludge
Brewery slurry
CSTR
Anaerobic
sequencing
batch reactor
Stirred tank
Modified ABR
[150]
–
NA
13.5
21.2
61 at 10%
slurry
58.1
0.18–80
3.9
8.57
42%
88.9
[151]
[38]
5.6
3
70
16
12.60
5.33
97
77.3
Simple construction except gas-solid
separator, high loading rate, low
suspended solid influent and effluent,
no mechanical mixing and costly
support media, higher biomass
retention inside the digester, higher
quality effluent [137,153].
Requires effective gas-liquid separator,
need efficient distribution of feed, may
lose microorganisms and foaming at
high loading rate, may lose a portion of
sludge bed during hydraulic surge of
toxic effect, requires effluent recycling
for bed expansion, longer start-up
period for granulation [137,153].
No mixing system, smaller tank
volume, more stable system, adaptable
to load variation, longer retention of
microorganism, more rapid restart
after shutdowns., short retention time,
higher biomass retention, tolerable to
shock loading, suitable for diluted
wastewater [137,153].
Higher filter materials cost, pressure
drop problem, longer start-up period,
higher energy and maintenance cost,
not suitable for high solid and grease
content effluent, support media
wash-out problem, lower OLR if
influent contains high suspended solid
[137,153].
B
Palm oil mil effluent
Palm oil mil effluent
UASB
[154]
1
2
2
26
UASB
[154]
1
3
3
50
UASB
[155]
11 h
3
6.6
90–99
C
Pharmaceutical
wastewaters
containing N-propanol
Pharmaceutical
wastewaters
containing
dimethylformamide
Wastewater containing
VFA and nitrate
Piggery waste
Palm oil mil effluent
Palm oil mil effluent
Distillery wastewater
UASB
EGSB
UASB
UASFF
[156]
[157]
[43]
[158]
2
2
4
8
10.1
80
42.5
110–190
4.1
17.5
10.6
23.25
39.1
91%
96
64
Palm oil mil effluent
Distillery wastewater
Distillery wastewater
Food wastewater
UASFF
UASFF
AFB
Membrane
bioreactor
Anaerobic
filter
AFB
[34]
[159]
[159]
[160]
1.5
2.5
2.5
60 h
26.21
15
15
2–15
17.47
20
5.88
4.5
90.2
76
96
81–94
[161]
1.0
10.0
10.0
>90
[161]
0.25
2.5
10.0
>90
Palm oil mil effluent
Palm oil mil effluent
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applied
M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225
Class
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M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225
function will continue to expand and generate much larger quantities of throughput. A big challenge is knowing what to do with
all the data. On the practical side, a chemical engineer must understand that we have already moved from the old-way monitoring
techniques to biomonitoring procedures of anaerobic digestion
process using molecular techniques. For those applications, the
challenge will be to improve reliability, particularly for use at a
large scale or even any scale. This will include using a cost-effective
monitoring of anaerobic digestion using a combination of molecular methods. Ultimately, if optimum process efficiency is to be
fully realized, then future developments in anaerobic treatment
processes will still require a much greater understanding of the fundamental relationships between archeal and bacterial populations
within the biomass.
Acknowledgements
The authors would like to thank Federal Land Development
Authority (FELDA), Ministry of Science, Technology and Innovation,
Malaysia (MOSTI) and Japan Society for the Promotion of Science
(JSPS).
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