K.Li et al.Renewable and Sustainable Energy Reviews 54(2016)857-865 859 15%,respectively [31.L-AD is a widely used technology to produce Spartina alterniflora.Similarly,co-digestion of goat manure with biogas and most commercial digesters in China are L-AD systems three crop residues,wheat straw,corn stalks and rice straw,obtain which can process municipal organic waste for energy production. 62.1%,83.02%and 111.28%higher biogas yield than corresponding SS-AD is hindered from wide application due to its low operational single crop residues,and 23.04%,54.44%and 51.31%higher than stability,although it is superior to L-AD for less heating energy,less single goat manure [61.Moreover,the co-digestion of dairy and leachate,higher volumetric loading capacity and less floating pro- chicken manure mixture with corn stalk was compared with single blems [35,38.Moreover,the particle size of straw or fruit vege- manure co-digestion with corn stalks,the former got better results table waste(FVW)like substrate also have certain requirements.For under both temperature of 15C and 35C [62].The apparent example,corn stover had better be cut into range of 0.25-1.0 mm synergistic effect in co-digestion can be attributed to a better buffer [39.When particle size is bigger than 1 mm,the cumulative yield capacity,balance nutrients,as well as less risk for the occurrence of of hydrogen,acetate and butyrate is decreased [40. ammonia/ammonium inhibition [63.Trace elements in manure When it comes to the chemical feature,it is generally required may be another reason [64].Yue et al.also considered that micro to be easily available for microbe.Straw,mainly composed of lig- bial community composition shifted with different feed composi- nin,cellulose and hemicelluloses,is difficult to be broken down. tion which has an effect on the hydrolysis of fiber [65]. Therefore.for the purpose of improving degradation efficiency. Still.the species.growth stages of animal as well as the fraction preprocessing straw firstly is necessary because straw composi- of manure can exert direct effects on methane production from tions can be partly degraded into easily utilized organics in this livestock manures [66].For example.the solid fraction of pig process. manure contains higher organic matter content than liquid frac- Clearly,different substrates have different features such as tion and produces more biogas than raw pig manure [36];the nitrogen-rich or carbon-rich which directly influence anaerobic manure excreted by gestating sow,post-weaned piglet and digestion.Thus,the optimal process methods for different feed- growing fattening pig have different components.This may be stock should be explored based on the characteristics of each kind caused by their different nutrient metabolic capacity and feed of biomass materials. formulas,additive at different growth stages (67]. In conclusion,manures characterized by rich in nitrogen likely lead to ammonia accumulation in reactor.Simultaneously treating 4.Methanogenic fermentation of livestock manures manures with other nutrient complementary feedstocks can improve the anaerobic process yields.Additionally,higher 4.1.Anaerobic digestion characteristics of livestock manures methane yield can also be obtained by manure pretreatment for accelerating hydrolysis. Animal manure is a kind of low C/N ratio feedstock and it is characterized by high organic matter and high moisture [41.as 4.2.Development of biogas plant for manure treatment in China well as fast conversion rate (42,43.Manure ferments well and can be used as mono-substrate if proper temperature,substrate con- In the past years,the promotion of household biogas project centration as well as inoculum [44-46]are given.For example. fed with manure was strategically coupled with the idea of ecology human excreta and chicken manure with a C/N ratio of 2.88:1 47] in China.During the development of rural household biogas, and 8.84:1 [48]respectively can digested alone [49]. several ecological models have been evolved.Two successful As sole substrate for methane fermentation,fresh pig manure models among them are the "Pig-Biogas-Fruit"model in south can rapidly generate biogas without adjusting C/N ratio 50.The China and"Four in One"model in north China.In "Pig-Biogas- CH4 content of biogas from pig manure anaerobic digestion is Fruit"model,pigpen and fruit trees are combined with biogas quite high,typically in the rang of 70-80%51.yielding biogas digester through manure and biogas residue 68.The "Four in 495 ml/g VS.while the CH4 content of biogas from cattle manure One"model is popular in cold area for it solves the problem of has a slightly lower CH4 content,in the rang of 55-75%,potentially biogas digester that can not operate under cold weather.In this giving 398 mL /g VS.Methane yield of 324 mL/g VS for chicken model,pigpen,toilet and a biogas digester are built in a green- manure and 294 mL/g VS for human feces were obtained [52]. house which can provide suitable temperature for the growth of Rabbit manure was degraded faster than duck and sheep manure, pig,vegetable and the production of biogas.In return,burning for 87.8%,67.6%and 65.0%of total biogas yield could respectively biogas increases the temperature of greenhouse and provide CO be obtained after 20 days of digestion at 35+1C [53].Great and light for vegetable.Meanwhile,biogas digestate can be used as differenccs of biogas yicld and biogas production ratc among organic fertilizer for vegctablc production [69]. various livestock manures can be explained by the different con- The biogas-plant industry has grown quickly in China.By 2010 tent of protein,carbohydrate,fat and lignin in substrate.Manures more than 40 million household biogas project have been built. with higher liquid content have high methane potential due to the and there were 27,410 medium-and large-scaled biogas plants highest energy density in fat.but longest retention time is dealing with manure [70.The fermentation material is mainly required for the poor bioavailability of fat.Conversely,carbohy- composed of swine manure,cattle manure and chicken manure drates and proteins can degraded rapidly 42,541. 71.In reality,the anaerobic digestion of single manure has low Currently,the study of manure digestion in China mainly con- performance caused by ammonia inhibition and nutrient imbal- cerned with the co-digestion of animal manure with other biosolid ance 72.In some biogas plants which were built in well devel- wastes.The advantages of co-digestion are summarized as: oped country like German,livestock manure was seldom used as adjusting the C/N ratio of low nitrogen material with manure to single substrate but mixed with organic residues 73.Presently. generate biogas stably:increasing methane yield and proportion in the development of biogas plants are far from maturity and still biogas [55].According to comparative studies in China.when low exist deficiencies in respects of electricity production [74].hea nitrogen agricultural residues like dried grass silage [56].maize preservation in winter [75].etc.However,some biogas plants have straw 57].vegetable stalks [58]were digested for methane pro- obtained great achievements.For example,a large-scale biogas duction.it is feasible to improve feedstock adaptability by adjusting plant located in Tongzhou District of Beijing treats 30t dairy C/N ratio with high nitrogen substrates like pig manure and urea manure and 30 t wastewater per day,and particularly adopts 59].For example,Chen et al.[60]reported that 7.09-44.26%higher combined heat and power(CHP)systems which has an installed methane yield was obtained by co-digestion of cow feces and electricity capacity of 160 kw.Waste heat from generator keeps