860 K.Li et al.Renewable and Sustainable Energy Reviews 54(2016)857-865 feed in the reactor fermented at 38C 76.Moreover,a solar- straw [57].The theoretical methane yield of crops straw is greatly powered water-heating system is also proved feasible for tem- higher than its practical methane yield.Previous studies in China perature keeping (77].Another case is concerned the Minghe showed that methane yield of cotton stalk was about 240 mL/g-VS Animal Husbandry Company's biogas plant with continuous stir- while its theoretical yield was 356.7 mL/g-VS [92].Similarly,the red tank reactor(CSTR)and CHP technology which disposes 500 t practical and theoretical yield of wheat straw were 297 mL/g-VS chicken manure and daily generates electricity 60,000 kW h [78]. and 426 mL/g-VS.respectively [93].This may be largely due to the The biogas power-generation project of the Hainan Luoniushan Pig fact that crop straw contains high content of lignocelluloses Farm generates 420,000 kW h per year,with an annual biogas compounds and tight structure between cellulose,hemicellulose output of 600,000 m79.This high efficiency of electricity gen- and lignin,which makes it difficult to degrade and results in low eration may mainly results from its biogas plus electricity pro- methane yield.Pretreatment of lignocellulosic materials can duction system. decompose hemicellulose and cellulose,and break down the linkage between polysaccharides and lignin,thus improving digestibility 94.Therefore,pretreatment is normally a necessary 5.Methanogenic fermentation of fruit and vegetable waste step in the anaerobic digestion processes for straw. 5.1.Anaerobic digestion characteristics of fruit and vegetable waste 6.1.Pretreatment of crop residue for enhanced methane yield The C/N ratios of majority FVW are ranged from 20:1 to 35:1 At present,the pretreatment methods are generally classified [6,80-82]and then FVW can be digested without additiona into chemical,physical,biological pretreatment methods or their nitrogen source [83].For those FVW with unsuitable C/N ratio. combination (95.Among them,pretreatment with alkali,steam mixing two or more sorts of them together to improve nutrients explosion and microbial agents are the most broadly implemented balance is a practical method. methods to treat lignocellulosic materials like straw in China FVW contains 75%easy biodegradable organic matter and are Each method is surely efective despite their limitations.But low in nitrogen concentration 6.When digested alone,redun- chemicals especially alkaline are superior to fungus96,bacter- dant volatile fatty acids will be produced in the process [84].But ium agents 97.enzyme 98.Alkaline can lead to the saponifi- when the FVW is co-digested with swine feces and urine which cation of uronic bonds between hemicelluloses and lignin,facil- contain high level of nitrogen,the amount of inhibit acids can be itating biological hydrolytic breakdowns 99.Study shows that decreased (85. rice straw pretreated by 4-10%NaOH can obtain 3.2-58.1%more In fact,the VFA from vegetable wastes acidification can loose biogas yield than untreated rice straw,corresponding to the lignocellulose structure in flower stems and helps its degradation reduction of 8.0-44.5%,35.2-54.2%and 14.2-16.4%in total lignin. 86.Also,adding corn straw to algae with a C/N ratio of 20/1 can hemicellulose,and cellulose contents,respectively 100.Hydro- obviously enhance the methane yield of algae from 201 mL/g VS to gen peroxide is also an excellent choice for pretreatment,since it 325mL/gVs[871. has strong oxidizability and hardly leaves residues as secondary For continuous stirred-tank test,the best ratio for the co- products in biomass.Improved biodegradability of lignin,cellu- digestion of fruit and vegetable waste with food waste was 1:1. lose,and hemicellulose has been reported when rice straw treated At this ratio and organic loading rate of 3 kg VS/(m.day).the with hydrogen peroxide,and the optimal conditions result in methane yield was 0.49 m2 CH4/kg VS compared with 0.3 m3 CH4 88.0%higher methane yield(290 mLg-1 VS)than untreated rice kg VS of FVW as single substrate.In addition,increasing the pro- straw 101.Approximately,biogas yield of 327.5 and 319.7 mL g- portion of FVW can lead to a higher degree of acidification in two- VS could be obtained from 3%and 4%H2O2 pretreated rice straw. phase AD system88.It seems that FVW can be well co-digested respectively.And pretreatment with H2O2 got higher performance with substrates rich in nitrogen.For carbohydrate-rich wastes. than pretreatment using ammonium hydroxide [102. alkali addition is another suitable measures in addition to mixing Physical pretreatment means using machine or heat etc.to substrates to maintain stable digestion. reduce particle size and crystallinity of fiber,thus increases the available surface for enzymatic attack [103,104].This method is 5.2.Development of biogas plant for fruit and vegetable waste energy consuming but quite efficient.Under high temperature, treatment in China high pressure and saturated steam conditions,steam explosion pretreatment can break down the links between amorphous cel- 130 million tons FVW arc annually produccd in China.Unfor- lulosc and crystallinc ccllulosc by abruptly releasing pressurc tunately,these wastes were usually discarded [88,89].Due to the [105].Guo et al.reported that the highest amount of organic acid high moisture and biodegradability of FVW.anaerobic digestion is (2.99 g/l)was obtained from corn stalk pretreated by steam more suitable for treating FVW in industrial scale rather than explosion after 3 days fermentation at 50C,and the amount of combustions or landfill deposition.Limited by shortages,biogas 2.74.1.96.1.45.and 2.21 g/l organic acid were respectively plants seldom use FVW as single substrate,but mix FVW with obtained from other pretreatments,including sodium hydroxide. manure,food waste and straw for co-digestion.In this way,not sulfuric acid,acetic acid,and aqueous ammonia 106. only nutritional balance can be maintained in large-scale biogas Biological methods use lignocellulose-degrading microorgan- plant,but also reduce the effect of acidification caused by high isms or enzymes to decompose substrate into simpler and more sugar content in FVW [90].In these biogas technologies,CSTR are biodegradable molecules like sugar,starch and pectin.Thus widely applied in China [70].In regard of FVW's easily acidifica- shorter digestion time,improved total solid reduction and biogas tion.anaerobic digesters for FVW had better adopt two-phase conversion rate could be achieved during this process 107,108]. digestion technology [91]. The key step of this technology is to look for the high lignocellulose-degrading strains.For example,Zhong et al.treated corn straw with 0.01%dose of a new complex microbial agents 6.Methanogenic fermentation of lignocellulosic biomass resulting in 75.57%increased CHa yield and 34.6%digestion time reduction for 15 days at 20C 109. The biogas yield of wheat straw is the highest among agri Additionally,there are also some other effective pretreatment cultural residue straws,followed by rice,corn,peanut and soybean methods.For example,biogas yield of rice straw pretreated by