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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 [email protected] (M. Tabatabaei). 1359-5113/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2010.05.017 1215 1215 1215 1216 1217 1218 1218 1219 1220 1221 1221 1223 1223 M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225 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- 1215 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 1216 M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225 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). M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225 1217 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 1218 M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225 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 M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225 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 1219 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, 1220 M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225 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). 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. Type of substrate applied 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 M. Tabatabaei et al. / Process Biochemistry 45 (2010) 1214–1225 Class 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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