Study On Co-digestion And Pretreatment Of Rice Straw For Enhancement Of Methane Yield From Anaerobic Digestion

The utilization of biomass as a renewable source of energy is important from the energetic as well as the environmental viewpoint.It can reduce the rate of fossil fuel depletion caused by the rapid increase in energy consumption.Limited reserves of fossil fuels and greenhouse gas emissions caused by fossil fuel burning,was the reason behind the search for alternative sources of energy,such as biogas from the biomass resources.Rice straw is one of the world's most renewable and available sources of energy,and one of the most abundant lignocellulosic agricultural crop wastes in China,with an annual production ranging between 180 and 270 million tons.Rice straw is used in a number of traditional ways;the biggest part of rice straw is disposed by burning in the field.Burning released substances cause lung and respiratory diseases,which have a bad effect on public health.However,rice straw needs pretreatment or co-digestion with manure to improve the anaerobic digestion efficiency for biogas production.In this research,the effect of co-digestion,fungal,milling and combination pretreatment were studied to maximize the methane yield and the net energy output and to provide a theoretical reference for engineering application.The main results are as follows:1)Co-digestion of rice straw and duck manure was conducted with seven feedstock mixing ratios to evaluate biogas production under different digestion temperature(37? mesophilic and 55? thermophilic)and different initial pH(controlled at 7.0 and the substrate original pH without controlling).Increasing the duck manure in the feedstock resulted in an increased methane yield under 37? and initial pH 7.0,while shorter retention time was achieved by co-digestion of rice straw and duck manure under 55?.Rice straw/duck manure ratio of 40:60 was determined as preferred ratio for optimal biogas,methane content and methane yield.About 90.4%of biogas yield was obtained after 15 days of digestion,with total methane yield of 205 mL/g VS.Based the first set,effect of total solids(TS)content on co-digestion of rice straw and duck manure was evaluated under liquid anaerobic digestion(10%TS)and solid-state anaerobic digestion(20%TS).Considering the volumetric productivity,SS-AD systems(20%TS)showed increases of 155%compared to that of L-AD at 10%TS for(RS:DM,50:50)under 55?.2)Rice straw was subjected to fungal pretreatment using Pleurotus ostreatus and Trichoderma reesei to improve its biodegradability and methane production via solid-state anaerobic digestion(SS-AD).Effects of moisture content(65,75 and 85%),and incubation time(10,20 and 30 d)on lignin,cellulose,and hemicellulose degradation during fungal pretreatment and methane yield during anaerobic digestion were assessed via comparison to untreated rice straw.Pretreatment with P.ostreatus was most effective at 75%moisture content and 20 d incubation resulting in 33.4%lignin removal and a lignin/cellulose removal ratio(selectivity)of 4.2.In comparison Trichoderma reesei was most effective at 75%moisture content and 20 d incubation resulting in 23.6%lignin removal and a lignin/cellulose removal ratio(selectivity)of 2.88.The corresponding methane yields were 263 and 214 L/kg volatile solids(VS),which were 120%and 78.3%higher than for the untreated rice straw,respectively.3)Rice straw was pretreated by different combinations of physical(milling)and biological(incubation with Pleurotus ostreatus fungus)treatment to improve its biodegradability and biogas production during solid-state anaerobic digestion(SS-AD).Effects of milling(?2 mm)and incubation time(10,20 and 30 d),on lignin,cellulose,and hemicellulose degradation during fungal pretreatment and methane yield during digestion were assessed by comparison with untreated rice straw.Both incubation time and milling had significant impacts on both lignin removal during fungal pre-treatment and methane yield during digestion.A combination of fungal pretreatment at 30 days followed by milling prior to anaerobic digestion resulted in 30.4%lignin removal,the highest selectivity value(the ratio between relative lignin removal and relative cellulose removal)of 4.22,and the highest methane yield of 258 L/kg VS.This was equivalent to a 165%increase in methane yield from SS-AD compared to untreated rice straw.4)The continuous anaerobic digestion under liquid AD(10%TS)of pretreated rice straw by fungi and milling was studied.When the organic loading rate increased from 3 to 6 gVS/(L.d),the daily specific biogas yield decreased,and the methane concentration increased and then decreased,and finally stabilized.While,the volumetric biogas yield increased when the organic loading rate increased from 3 to 5 g VS/(L.d).The anaerobic digestion was inhibited,and the daily specific biogas yield was dropped.As a result,the organic loading rate should be controlled below 5 gVS/(L.d).The pretreatment by fungi increased the cumulative biogas yields by 54.6%than control group(autoclaved rice straw).