Importância dos arquéios metanogênicos em tratamentos anaeróbicos de esgotos, Notas de estudo de Cultura

Baixe Importância dos arquéios metanogênicos em tratamentos anaeróbicos de esgotos e outras Notas de estudo em PDF para Cultura, somente na Docsity! R I t M Y a 3 b c d e f g H h a A R R A K B B M A W C S 1 d Process Biochemistry 45 (2010) 1214–1225 Contents lists available at ScienceDirect Process Biochemistry journa l homepage: www.e lsev ier .com/ locate /procbio eview mportance of the methanogenic archaea populations in anaerobic wastewater reatments eisam Tabatabaeia,b,∗, Raha Abdul Rahimc, Norhani Abdullahd, André-Denis G. Wrighte, oshihito Shirai f, Kenji Sakaig, Alawi Sulaimanh, Mohd Ali Hassanb,h Microbial Biotechnology and Biosafety Department, Agricultural Biotechnology Research Institute of Iran (ABRII), Seed and Plant Improvement Institute’s Campus, 1535-1897, Mahdasht Road, Karaj, Iran Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Department of Cell and Molecular Biology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Department of Microbiology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Department of Animal Science, University of Vermont, Burlington, VT, USA Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan Laboratory of Soil Microorganisms, Department of Plant Resources, Graduate School of Bioresources and Bioenvironmental Sciences, Kyushu University, 6-10-10 Hakozaki, igashi-ku, Fukuoka 812-8581, Japan Department of Food and Process Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia r t i c l e i n f o rticle history: eceived 18 January 2010 eceived in revised form 1 May 2010 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 summarizesccepted 17 May 2010 eywords: iomethane iomass ethanogens 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.naerobic treatment astewater © 2010 Elsevier Ltd. All rights reserved. ontents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215 2. Types of waste materials and their dominant methanogenic population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215 2.1. Brewery wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215 2.2. Palm oil mill effluent (POME) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1216 2.3. Dairy waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217 2.4. Cheese whey and dairy wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1218 2.5. Pulp and paper wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1218 2.6. Olive oil mill wastewater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1219 3. Anaerobic reactors: designs and operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1220 4. Molecular methods for microbial ecosystem studies during anaerobic di 5. A look to the future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ∗ Corresponding author at: Microbial Biotechnology and Biosafety Department, Agricul eed and Plant Improvement Institute’s Campus, 31535-1897, Mahdasht Road, Karaj, Iran E-mail address: meisam tab@yahoo.com (M. Tabatabaei). 359-5113/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. oi:10.1016/j.procbio.2010.05.017gestion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1221 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1221 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1223 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1223 tural Biotechnology Research Institute of Iran (ABRII), . Tel.: +98261 2703536; fax: +98261 2704539. Bioche 1 f i f i a b w a t o c [ p a t c o d r o o o l a s a a w o b a F c e r r T C M. Tabatabaei et al. / Process . Introduction To date, global energy requirements are heavily dependent on ossil fuels such as oil, coal and natural gas. As the exhaustion of lim- ted fossil fuels is to be anticipated, there is a necessity to search or replacement source of energy [1]. On the other hand, there s a growing amount of organic waste and wastewater produced nnually. Anaerobic digestion technology is an ideal cost-effective iological means for the removal of organic pollutants in waste and astewater which simultaneously produces gaseous methane as n energy resource [2,3]. The many applications of this digestion echnology are the high-rate treatment of high-strength industrial rganic wastewater [1,4], low-strength organic wastewater [5], omplex wastewater containing persistent chemical compounds 4], sulfate-rich wastewaters [6], wastewater discharged at tem- eratures ranging from psychrophilic to thermophilic [2,7] as well s offering potentials for the removal of metals [8], nitrates [9], and oxic substances [10]. The biomethane produced by anaerobic digestion is an odorless, olorless and non-poisonous gas [11]. The process by which anaer- bic bacteria decompose organic matter into biomethane, carbon ioxide, and a nutrient-rich sludge involves a step-wise series of eactions requiring the cooperative action of several organisms. It ccurs in three basic stages as the result of the activity of a variety f microorganisms. Initially, a group of microorganisms converts rganic material to a form that a second group of organisms uti- izes to form organic acids. Methane-generating (methanogenic) naerobic archaea utilize these acids and complete the decompo- ition process. Table 1 presents the classification of methanogenic rchaea as outlined by Demirel and Scherer [12]. In the first stage, variety of primary producers (acidogens) break down the raw astes into simpler fatty acids. In the second stage, a different group f organisms (methanogens) consumes the organic acids produced y the acidogens, generating biogas as a metabolic byproduct. On verage, acidogens grow much more quickly than methanogens. inally, the organic acids are converted to biogas [13]. Moreover, ompared with ethanol or other liquid biofuels, biomethane is asily separated from liquid phase, which can contribute to the eduction of the process costs [14]. Renewable biomass is the most versatile non-petroleum based esource that is generated from various industries as waste mate- able 1 lassification of methanogenic archaea as outlined by Demirel and Scherer [12]. Class I. Methanobacteria Order I. Methanobacteriales F F Class II. Methanococci Order I. Methanococcales F F Class III. Methanomicrobia Order I. Methanomicrobiales F F F Order II. Methanosarcinales F F mistry 45 (2010) 1214–1225 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 green- house gas emissions as leakages of methane into the atmosphere are avoided. Although biomass energy is more costly than fossil- fuel-derived energy, trends to minimize carbon dioxide and other emissions through emission regulations, carbon taxes, and subsi- dies 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 asso- ciated 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 com- munity compositions have been documented. In this review, we focus on microbiological aspects of anaero- bic digestion of various renewable biomasses with a focus on their dominant methanogenic population, the leading factor of their suc- cessful anaerobic treatment, and update the recent findings in this field. In addition, we highlight the importance of molecular tech- niques in moving from the conventional monitoring systems of anaerobic digesters to biomonitoring procedures. 2. Types of waste materials and their dominant methanogenic population2.1. Brewery wastewater The brewing process generates a unique, high-strength wastew- ater as a byproduct. Even though substantial technological amily II. Methanothermaceae Genus I. Methanobacterium Genus II. Methanobrevibacter Genus III. Methanosphaera Genus IV. Methanothermobacter amily II. Methanothermaceae Genus I. Methanothermus amily I. Methanococcaceae Genus I. Methanococcus Genus II. Methanothermococcus amily II. Methanocaldococcaceae Genus I. Methanocaldococcus Genus II. Methanotorris amily I. Methanomicrobiaceae Genus I. Methanomicrobium Genus II. Methanoculleus Genus III. Methanofollis Genus IV. Methanogenium Genus V. Methanolacinia Genus VI. Methanoplanus amily II. Methanocorpusculaceae Genus I. Methanocorpusculum amily III. Methanospirillaceae Genus I. Methanospirillum amily I. Methansarcinaceae Genus I. Methanosarcina Genus II. Methanococcoides Genus III. Methanohalobium Genus IV. Methanohalophilus Genus V. Methanolobus Genus VI. Methanomethylovorans Genus VII. Methanimicrococcus Genus VIII. Methanosalsum amily II. Methanosaetaceae Genus I. Methanosaeta 1 Bioche a a h i a t h t a M s g t r t c m g i a M i o t M a T a M a i l t w a i [ t o 2 c 1 r o i a l g s m m m i o T t s d r i m c 218 M. Tabatabaei et al. / Process nd VFA levels [58,59,60]. In contrast, using 16S rRNA sequence nalysis, low levels of members of the Methanosarcinaceae and igh levels of members of the Methanomicrobiales were observed n a full-scale manure-fed reactor [61]. Ahring counted acetate- nd hydrogen-utilizing methanogens in thermophilic biogas reac- ors treating a mixture of cow and pig manure and found the ydrogen-utilizing methanogens in particular Methanobacterium hermoautotrophicum absolutely dominant [54]. In his study, all cetate-utilizing methanogens identified belonged to the genus ethanosarcina and the majority were in the form of individual ingle cells in the reactor. Hence, it could be concluded that the enus Methanosaeta plays no or little role in acetate conversion in he therrnophilic biogas reactors [54,62,63]. An experiment with adio-labeled acetate (14CH3COOH) [54] showed that acetate in he thermophilic anaerobic reactor was converted by the aceto- lastic reaction at high concentrations of acetate and by a two-step echanism involving the microbial oxidization of acetate to hydro- en and carbon dioxide and the transformation of these products nto methane by hydrogen-utilizing methanogens [64] when the cetate concentration was lower than the threshold level for the ethanosarcina species and in the absence of Methanosaeta species n the reactor. In addition, a mixture of both types of metabolism ccurred close to the threshold level. In general, in contrast to sludge digesters where, Methanosae- aceae are the main acetoclastic methanogens (Table 3) [26,30,31], ethanosarcinaceae are either the only or the most abundant cetate-utilizing methanogens in manure digesters [543,58,61]. he predominance of Methanosarcinaceae could be indirectly ttributed to the high free ammonia levels of manure which restrict ethanosaetaceae [58]. Methanosarcinaceae particularly M. concilii re the most ammonia-sensitive methanogen, and it is completely nhibited at a concentration of 560 mg (total) NH4-N l−1 at a pH evel of 7.0 [65,66]. Therefore, high free ammonia levels cause he accumulation of VFA, which then allow Methanosarcinaceae hich have a higher threshold for acetate [28,29,67] to outcompete nd restrict Methanosaetaceae. Finally, reducing ammonia levels or ts inhibitory effect such as by addition of lipid-containing waste 68] in manure digesters should change the equation in favor of he members of the Methanosaetaceae and consequently reduce rganic acid levels considerably [58]. .4. Cheese whey and dairy wastewater The liquid waste in a dairy originates from manufacturing pro- ess, utilities and service sections with a high COD ranging from 1 to 0 g l−1 and a high BOD5 ranging from 0.3 to 5.9 g l−1 [69–71] rep- esenting its high organic content. Moreover, the dairy industry is ne of the largest sources of industrial effluents for instance, a typ- cal European dairy generates about 180,000 m3 of waste effluent nnually [72]. However, there are high seasonal variations corre- ated with the volume of milk received for processing; which is in eneral high in summer and low in winter months [73]. The various ources of waste generation from a dairy are spilled milk, spoiled ilk, skimmed milk, whey, wash water from milk cans, equip- ent, bottles and floor washing [69]. Among those, whey is the ost difficult high-strength waste product of cheese manufactur- ng (COD of more than 60 g l−1) [71,74] which contains a proportion f the milk proteins, water-soluble vitamins and mineral salts. herefore, high COD concentration of dairy effluents, their high emperature, no requirement for aeration, low amount of excess ludge production and low area demand make them ideal candi- ates for anaerobic treatment [71] using various types of anaerobic eactors [75–77]. The acetoclastic methanogenic activity measured n anaerobic treatment of dairy wastewater was found to be due ostly to Methanosaeta species whilst Methanosarcina-like species ontributed insignificantly [78]. However, Methanococcus speciesmistry 45 (2010) 1214–1225 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 anaero- bic bacteria appeared to have taken place with the predominance of acidogenic bacteria in the initial compartments and the pre- dominance of methanogenic bacteria in the final compartments. It also showed that the dominant methanogens in the initial com- partments 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 autofluo- rescent 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 particu- lar 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 concentra- tion 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 methanogen- esis 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 con- sequently 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 concen- tration below 100 mg l−1, and after careful acclimatization of the digester culture to casein in order to develop proteolytic enzy- matic 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 pro- cess or hardly affected the overall performance [87,89]. It was also reported that the anaerobic biodegradation rate of fat-rich wastew- aters is slower than that of fat-poor wastewaters, due to the slower rate of the fat hydrolysis step [89]. Having considered various fac- tors, 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 after- wards. 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 indus- try 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 Bioche w e a w T d M t p a [ 5 p m w m m a t t t f r p o t l c a o t g c r v b a r d T o H a w a o h c b t [ t s p r e a i d m w [ r a M. Tabatabaei et al. / Process astewater such as papermaking effluent, de-inking process efflu- nt and pulping process effluent with an average COD value as high s 11,000 mg l−1 [89]. For each tonne of manufactured pulp, the astewater discharge volume will be a minimum of 30 m3 [92]. he characteristics of the pulp and paper-effluent are highly depen- ent on raw materials and manufacturing process adopted [92,93]. oreover, these effluents are strongly polluting and toxic owing o the presence of lignins, resins, tannins and chlorophenolic com- ounds that are resistant to biodegradation [94,95]. Application of a “zero liquid effluent” process was reported as feasible option for the paper mills and found to be profitable 96,97]. The wastewater is generated at a temperature ranging from 0 to 60 ◦C and therefore, thermophilic anaerobic treatment com- lemented with appropriate post-treatment is considered as the ost cost-effective solution to meet re-use criteria of the process ater as well as maintaining its temperature [98]. Anaerobic treat- ent is well feasible for effluents generated by recycle paper mills, echanical pulping (peroxide bleached), semi-chemical pulping nd sulfite and kraft evaporator condensates. [99]. This is due to he tolerance to toxicity of anaerobic microorganisms [100]. In he proposed closed-cycle, the anaerobic treatment step removes he largest fraction of the biodegradable COD and sulfur as H2S rom the effluent, without the use of additional chemicals, and is egarded as the only possible location to eliminate sulfur from the rocess water cycle [98]. Buzzini and Pires studied the treatment f diluted black liquor from a kraft pulp plant by using a UASB reac- or and obtained a COD removal efficiency of 80% [101]. The black iquor comprises only 10–15% of the total wastewater, however, ontributes approximately 95% of the total pollution load of pulp nd paper mill effluents [102]. Therefore, due to its higher contents f chemicals and organic substances and consequent high pollu- ion strength, low influent concentration was found essential for ranulation when UASB reactors are applied [103]. Van Lier et al. ompared H2S stripping efficiency of UASB and gas-supplied UASB eactors treating paper mill effluent and showed 3–4 times higher alues in the gas-supplied UASB [98]. In a study where an anaerobic affled reactor was used for continuous anaerobic digestion of pulp nd paper mill black liquors, OLRs higher than 5 kg COD m−3 d−1 esulted in loss of reactor’s stability which was apparent by the ecrease in biogas production rate and its methane content [104]. his was attributed to the toxic effect of the high concentration f tannin and lignin present in black liquor on methanogens [105]. owever, in a similar study in an anaerobic baffled reactor by using n immobilized cell system, the reactor maintained its stability ith higher OLRs (7 kg COD m−3 d−1)[106]. This was due to the dvantages of immobilization technologies such as the retention f catalytic activity, a high ratio of sludge retention time (SRT) to ydraulic retention time (HRT) and in particular, the protection of ells from the effects of inhibitory/toxic substances [107,108]. Several studies have demonstrated the capacity of the micro- ial consortium e.g. methanogenic archaea to adapt to potentially oxic effluents present in the effluents of pulp and paper mills 101,109]. The adaptation depends on the concentration of the oxicants and the operating conditions and the acclimation of the ludge substantially reduces the degree of inhibition [109]. The redomination of Methanosarcina spp. and Methanosaeta spp. was eported during the anaerobic treatment of paper and pulp mill ffluent using USAB reactors [100,101,110]. In a study, Roest et l. monitored microbial populations in a UASB reactor for treat- ng paper mill wastewater over 3 years with a combination of ifferent molecular techniques and conventional microbiological ethodology. The authors were able to confirm that Methanosaeta as the most abundant archaeal genus throughout the operation 111]. They also reported the domination of sulfate-reducing bacte- ia and syntrophic fatty acid-oxidizing microorganisms during the naerobic treatment of paper mill wastewater [111]. Methanogenicmistry 45 (2010) 1214–1225 1219 consortia (Methanosaeta sp., Methanosarcina sp., and Methanobac- terium 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 anaer- obic 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, Methanobre- vibacter 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 effi- ciency of almost 90% [113]. 2.6. Olive oil mill wastewater Olive mill wastewater (OMW) generated by the olive oil extrac- tion 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 aver- age amount of olive mill wastewater produced during the milling process is 1.2–1.8 m3 t−1 of olives [116]. Therefore, the OMW result- ing 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 munic- ipal wastewater [114,117]), high sodium concentration [118] as concentrations exceeding 10 g l−1 strongly inhibits methanogen- esis [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 shut- down 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 alka- linity 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 biolog- ical 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 signif- icantly 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 laboratory- scale UASB reactors were reported by other researcher [122,129].Anaerobic biofilm reactors packed with granular activated car- bon (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 Microorganisms retention time is equal to that of the solid and liquid (RTm = RTs = RTl) CSTR, CDT B Microorganisms and solid retention time is higher than that of the liquid (RTm and RTs > RTl) CSTR or CDT with solid recycle, UASB, Baffled flow reactor C Microorganisms retention time is higher than that of the solid and liquid (RTm > RTs and RTl) Membrane bioreactor, UASFF R ion tim U m reac B a a p t t v o a r a n r b h w i a a t M s I C n w t c o w d a m a a p o Tm = Retention time of microorganism; RTs = Retention time of solid; RTl = Retent ASB = Upflow anaerobic sludge blanket; UASFF = Upflow anaerobic sludge fixed fil ertin et al. [131] used a GAC-bioreactor to treat OMW and reported bout 100 and 300% improved efficiency in terms of removal of COD nd phenolic compounds, respectively, and by 70% in terms of CH4 roduction [131]. GAC provides the microorganisms with a place o grow and allow them to live stably in the reactor by minimizing he inhibitory/toxic effect of the present compounds. Hence, it pro- ides the bioreactor with increased tolerance to high and variable rganic loads along with a volumetric productivity in terms of COD nd phenolic compound removal. Taking all things into account, esults of single anaerobic treatment are not always satisfactory nd some form of pretreatment, apart from simple dilution and utrient/alkalinity adjustment, is usually necessary [132]. Bertin et al. (2006) analyzed the microbial diversity of a GAC eactor during anaerobic digestion of OMW and found a mem- er of Methanobacteriaceae as the sole dominant species, i.e., ydrogenotrophic Methanobacterium formicicum representing the hole archaeal community [131]. This methanogen was also dom- nant and persistent in a UASB pilot plant treating OMW [133]. The bsence of acetoclastic methanogens which are highly pH-sensitive s well [134] was due to the acidic pH environments occurred in he reactors. In contrast to these studies, Methanobacteriaceae and ethanosaeta were both the main methanogens in a laboratory- cale upflow anaerobic digester treating olive mill effluent [134]. n the latter study, at a volumetric organic loading (VOL) of 6 g OD l−1 day−1, the hydrogenotrophic Methanobacterium predomi- ated in the reactor but decreased from 1011 to 108 cells g−1 sludge hen the VOL was increased to 10 g COD l−1 day−1. By increasing he VOL, the non-dominant methanogenic family i.e. Methanomi- robiaceae increased from 104 to 106 cells g−1 sludge. On the ther hand, hylotypes belonging to the acetoclastic Methanosaeta ere stable throughout VOL variation and at 10 g COD l−1 day−1 ominated in the biofilm (109 cells g−1 sludge) [135]. With the bove results, we may suggest, Methanosaeta as the most tolerant ethanogen to the inhibitory/toxic substances present in wastew-ters such as OMW. This could be attributed to its high affinity for cetate enabling it to occupy the deepest or in other words, more rotected niches in the granule or biofilm with low concentration f substrate [136]. Fig. 2. Schematic diagrams of closed digester tank (CDT) (A), continuous ste of liquid; CDT = Closed digester tank; CSTR = Continuously stirred tank reactor; tor. 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], ther- mophilic 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 fur- ther 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 construc- tion 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 efflu- ent 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. irred tank reactor (CSTR) (B), and CDT with solid recycle system (C). Bioche f t a s t p c l m u f p d w A A M ( R M. Tabatabaei et al. / Process unction will continue to expand and generate much larger quan- ities of throughput. A big challenge is knowing what to do with ll the data. On the practical side, a chemical engineer must under- tand that we have already moved from the old-way monitoring echniques to biomonitoring procedures of anaerobic digestion rocess using molecular techniques. 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