en Water quality and wastewater treatment and reuse Original articles <div style="text-align: justify;"><strong><em>Introduction:</em></strong> One of biological wastewater treatment methods that utilizes to both digesting waste activated sludge and methane production is anaerobic digestion (AD). It is believed to be most effective solution in terms of energy crisis and environmental pollution issues.<br> <strong><em>Materials and Methods:</em></strong> In this study the sludge was digested anaerobically sampled from a full-scale WWTP, located at south of Tehran, Iran for evaluation. To study the microbial community within the sludge the MiSeq Sequencing method utilized. Based on our field data and microbial community data, a schematic diagram of probable leading pathways was made in the studied digester.<br> <strong><em>Results:</em></strong> At first, the community variety in the bulk sludge and richness were enhanced followed by loading increasing. Meanwhile, the loading change enhanced the community richness and variety of the sludge. By comparing the rank-abundance distributions, a shallow gradient would show high evenness since the abundances of diverse species are alike. The results showed all the communities were extremely diverse and 15 phyla were distinguished in the sludge sample. The dominant phyla of the community were <em>Bacteroidetes</em> and <em>Firmicutes</em> and quantity of the two phyla were 21% and 11%, respectively. <em>Anaerobaculum</em>, <em>Acinetobacter</em>, <em>Syntrophomonas,</em> and <em>Coprothermobacter</em> were the chief genera for the microbial communities and the sum of four genera were 7%, 3%, 3%, and 2%, respectively.<br> <strong><em>Conclusion:</em></strong> It was shown that syntrophic acetate oxidizing bacterias (SAOBs) metabolized acetate through hydrogen trophic methanogenesis in the digester. Generally, the findings may be useful to help the wastewater operators to utilize an effective method that able to treat waste sludge plus methane production, simultaneously.</div> Microbial Community, Methane Production Pathway, Biogas, Anaerobic Digester. 1091 1102 http://jehsd.ssu.ac.ir/browse.php?a_code=A-10-212-2&slc_lang=en&sid=1 Reza Barati Rashvanlou rezabarati8770@gmail.com 0000-0002-7086-5830 No Research Center for Environmental Health Technology, Iran University of Medical Sciences, Tehran, Iran. Mahdi Farzadkia Mahdi.farzadkia@gmail.com 0000-0001-7795-9461 No Research Center for Environmental Health Technology, Iran University of Medical Sciences, Tehran, Iran. Abbas Ali Moserzadeh A.moserzadeh@tpww.ir 0000-0003-3908-3360 No Civil Engineering, Department of Civil Engineering, Amirkabir University of Technology, Tehran Asghar Riazati Riazati158@gmail.com 0000-0002-4739-062X No Managing director of Tehran Water and Wastewater Co., Tehran, Iran. Chiang Wei d87622005@ntu.edu.tw 0000-0003-4983-4594 No Research Fellow/Division Chief Planning Division, The Experimental Forest, College of Bio-Resources and Agriculture, National Taiwan University, Nantou 55750, Taiwan Edris Hoseinzadeh edris-2009@live.com 0000-0002-0674-281X Yes Student Research Committee, Saveh University of Medical Sciences, Saveh, Iran.