| China is one of the largest agricultural countries in the world with large amounts of crop straws being abandoned annually, which always causes environmental pollution due to the unreasoned utilization. Anaerobic digestion of crop straws is a feasibility technology to utilize straws, as it can not only supply renewable bioenergy but also reduce the agricultural pollution. However, the high content of lignocellulose in straws limit the attack by anaerobic microorganisms, therefore lead to lower digestion rate and biogas production, which prevent them from large-scale utilizating. Meanwhile, the characters such as low density, bulky, expanding and floating after water absorption, poor fluidity, etc., made crop straws hard to be processed by the existing anaerobic digestion craft nowadays. To address the problems in anaerobic digesting of the crop straws, we need to determine a suitable pretreatment method with low cost and high efficiency and develop a specific anaerobic digestion craft for efficient biogasification of crop straws. Thus, in order to provide technological support for highly efficient of biogasification banana straw, the effect of chemical pretreatment on banana straw anaerobic digestion efficiency was explored by the automatic methane potential test system (AMPTS), the optimized pretreatment parameters were gotten as well; The newly designed integrated two-phase anaerobic reactor (ITPAR) was used to investigate its stability and systematic efficiency during processing banana straw in this study. The main results are as follows:(1) Banana straw had a high content of cellulose and hemicellulose, low content of lignin, whose monosaccharide components were mainy consisted by glucose and xylose; The 5 mm particle size could obviously shorten the technical digestion time of banana straw; It was difficult to ensure a stable anaerobic digestion of banana straw in the continuous stirred tank reactor at high organic loading rate. The easy winding and bunching banana straw was easy to present a heterogeneity state in reactor and resulted in a loss of homogeneous anaerobic sludge along with feeding, which caused the accumulation of volatile fatty acids and even a high concentration of propionic acid in the reactor with the unstable operation and low system efficiency.(2) Two acid and two alkali with 4% mass percent were chosen to pretreat banana straw at three different pretreatment tempreture and pretreatment time. The chemical composition of banana straw and its anaerobic digestion efficiency showed that the degradation rate of hemicellulose was directly related to the pretreatment time. The degradation rate of cellulose and lignin remained stable due to the pretreatment time. The extended pretreatment time and raised pretreatment tempreture would increase the loss rate of dry matter in straw. The 4% NaOH-pretreatment at 30 ℃ for 7 days resulted in a highest methane production rate of 343.8 NmL CH4/gVS. As a result of this, NaOH was chosen as the best pretreating agent for banana straw.(3) The anaerobic digestion performances of banana straw by different mass percent of NaOH-pretreatment and at different TS feeding concentration showed that the optimal pretreatment conditions and anaerobic digestion parameters were as follow:moisture content of banana straw was 90%, NaOH mass percentage of straw dry weight for 4%, treating at 30 ℃ for 7 days, the TS feeding concentration of 6% could get a highest cumulative methane production and methane production rate. The methane production rate was 81.1% higher than that of untreated banana straw. The technical digestion time was shortened by nearly 25%. The continuous anaerobic digestion showed that the continuous stirred tank reactor could reached a stable state when processing NaOH-pretreated banana straw at the hydraulic retention time of 36 d and the organic loading rate of 1.8 kgVS/(m3·d), with a VS removal rate of 55.4%, gas production rate of 0.64 Nm3/(m3 · d), and the highest methane production yield of 0.31 Nm3CH4/kgVS.(4) The newly integrated two-phase anaerobic digestion (ITPAR) has operated stably for 220 days with a semi-continuous feed batch mode by a cycling time of 10 days. Under the maximum organic loading rate of 2.2 kgVS/(m3 · d), the fresh banana straw had an average gas production rate of 1.27 Nm3/(m3 · d), a maximum methane production yield of 0.325 Nm3CH4/kgVS, a VS removal rate of 62.5%; The NaOH-treated banana straw boosted the cumulative gas production by 11% in an operation stage. The gas production rate reached 1.41 Nm3/(m3· d), the methane production yield got to 0.37 Nm3CH4/kgVS, the VS removal rate came to 68.38%. The anaerobic biodegradation rate of NaOH-treated banana straw reached to more than 90% in ITPAR.(5) There was an uneven distribution of microorganisms in ITPAR. In general, the diversity of bacterial and archaea were higher in acidogenesis unit than in methanogenesis units. The acidogenesis unit of ITPAR had a large number of cellulolytic bacteria, and the relative abundance of the acetoclastic methanogens balanced the hydrogenotrophic methanogens; The acetoclastic methanogens overwhelmed in methanogenesis unit. That indicated a major methanogenesis pathway by acetoclastic methanogens in methanogenesis unit, the acetoclastic and hydrogenotrophic methanogenesis pathway co-existence in acidogenesis unit of ITPAR. The ITPAR achieved the separation of solid and liquid as well as the solid retention time-hydraulic retention time-mean cell retention time in one reactor, with high-efficient biogasification and non-scuming in treating banana straw. |
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