858 K.Li et al.Renewable and Sustainable Energy Reviews 54(2016)857-865 livestock,fruits and vegetable wastes produced during store, are the same with the ones of Methanosaeta except enzymes in the transportation and handling of vegetables,and lignocellulosic process of acetyl-CoA synthese.As a result,Methanosaeta has biomass such as rice straw,wheat straw,corn Stover,etc.[3-6]. lower minimum threshold acetate concentration than Methano- They are biodegradable,rich in organic matter and can be used for sarcina.While Methanosarcina species grow faster than Methano- methane production by anaerobic digestion 7.In 2012,846 saeta species when acetate concentrations are beyond 1 mM[19]. million tons crops residues and 3.21 billion tons livestock manure The methanogens population are not only affected by acetate were produced in China.If these wastes are utilized for anaerobic concentration,but also by temperature 20]or substrate 21. fermentation,4.23x 101 m3 biogas could be produced ]How- When temperature is increased within a certain range,methano- ever,these resources have not been used efficiently.72.5%of crop gens can grow and multiply.Generally.mesophilic anaerobic straw is combusted,lost or discarded,left only 0.5%for biogas digestion is more stable than thermophilic fermentation for lower production 9].Besides,the uncontrolled decomposition of animal ratio of free ammonia to total ammonium ion,as well as greater manures have caused serious environmental problems.Obviously. diversity and evenness of bacterial communities [22].Methano- technologies for biomass utilization are urgently needed in China sarcinales is a common major methanogenic order in mesophilic either for energy or for environmental purpose. conditions 23.but in thermophilic reactor hydrgenotrophic Biomass can be converted to gaseous.liquid and solid biofuels methanogens dominated [241.The order of Methanosarcinales only by technologies like anaerobic digestion and gasification,pyrolysis occupied a percentage of 10%in methanogenic population at and carbonization [10,11.Compared with other bioenergy tech- temperatures of 55C,in which Methanosaeta was almost absent nologies,biogas technology is quite mature and has already been Additionally,reducing digester feeding frequency can increase at its industrialization stage.Furthermore,biogas project combines Methanosarcina predominance,and Methanosarcina-enriched together the ideas of ecological agriculture and recycle agriculture. reactor has better performance than Methanosaeta-enriched and develops several practical agricultural production patterns like reactor [25.In the industry application,Methanosaeta is important the "Pig-Biogas-Fruit"model in south China and the "Four in One for the upflow anaerobic sludge blanket (UASB)reactor,due to its model in north China [12,13].Extensive fundamental researches enhancing granulation [26]and attachment ability [27].Hydro- about biogas have also been conducted.For example,microbial genotrophic methanogens can tolerate the ammonium con- community shifts during thermophilic methane fermentation centration of 6000 mg/L which is sixfold higher than the threshold fermentation characteristics of crop straw pretreated by microbial ammonium concentration of Methanosaeta and twofold higher community or steam explosion [14J,and trace elements'stabiliz- than that of Methanosarcina (24,28.Therefore,the stable methane ing effects on long-term anaerobic digestion have been studied production when ammonia level increases may be explained by [15.It has also been found that digested liquid swine manure can the increasing activity of hydrogen-utilizing methanogens. control soybean cyst nematode 16,and biogas fertilizer can Overall,several species of methanogens participate in the improve the yield of cherry tomato 17. terminal steps of methane generation.They are high susceptible to However,understanding the process of methane fermentation the environmental conditions.As a result,anaerobic digestion is difficult because of various influence factors.The broad appli- process is fragile and highly sensitive to external influences.The cation of biogas technology is greatly limited by the disconnection prevalence of certain members of archaeal methanogens depends of practical operation from its mature research status.So,in order on physical and chemical conditions like temperature,pH,NH3-N to improve the practical efficiency of anaerobic digestion,to grasp and VFA etc.The shifting of this anaerobic microbe community the current study of anaerobic digestion for biogas production in might influence the biogas yield as well as the stability of China is necessary. biomethanation. This article reviews the metabolism of methanogens and the present situation of biogas technology dealing with three main agricultural residues including livestock manure.fruit and vege- 3.The adaptability of substrate materials for methanogenic table waste (FVW)and lignocellulosic biomass in China.The cur- fermentation rent status of large,medium biogas plant built in China are also summarized.In addition,problems and recommendations for The feedstock of anaerobic digestion is widely tolerant,but if future development are put forward based on current situation of the maximum biogas yield is expected to obtain,several factors biogas technology,with the hope of improving anaerobic digestion should be concerned,including C/N ratio,substrate concentration, efficiency and promoting the application of biogas technology physical and chemical property of feedstock. in China. In terms of C/N ratio,low C/N ratio may Icad to ammonia releasing [29].while methanogenesis inhibition also occurs when the ratio is high either due to nitrogen shortage or to organic acid 2.Methanogens in anaerobic digestion accumulation caused by excess carbon 30,31.As a result,various feedstock with various C/N should be matched properly to produce Methanogens are strictly anaerobic archaea and have limited a proper ratio range,generally proposed 20/1 to 30/1 [32,33. substrate range.Thus the bioconversion of organic waste to Substrate concentration refers to the proportion of total solids methane generally needs the consortia of interacting micro- accounting for the reactor medium [34].Proper solid content is organisms including fermentative bacteria,H2-producing aceto- important for biogas production.Too much water in the fluid can genic bacteria and methanogens.and the final generation of cause low organic loading rate and too little can lead to organic methane by methanogens is the rate-limiting step. matter accumulation resulting in fermentation inhibition,both of Currently,two acetate-utilizing methanogens,Methanosarcina which will decrease biogas yield.The substrate concentration and Methanosaeta,are most explored among methanogens,partly capable of producing biogas is quite wide.TS content ranging from because 70%methane in nature derives from methyl group of 1%to 40%[35].even to 55%is feasible [36].In rural area of China. acetate 18.Methanosarcina have one times larger genomes than substrate concentration of 6-12%is generally adopted during obligate hydrogenotrophic methanogens,which reflects powerful anaerobic digestion [37]. metabolic capabilities.For example,it has the broadest substrate Physically,anaerobic digestion can be divided into liquid anae spectrum,acetate,methylated Ci compounds and H2/CO2.The key robic digestion (L-AD)and solid-state anaerobic digestion (SS-AD). enzymes for methane formation from acetate in Methanosarcina with solid concentrations between 0.5%and 15%,and higher than