Study On Characteristic Of Anaerobic Co-digestion Of Straw Depolymerization Wastewater And Corn Stovers

The process of stovers depolymerization with dilute sulfuric acid,usually encountered in the production liquid fuel,generates large amounts of sulfate-containing straw depolymerization wastewater.According to production practice,a ton of liquid fuel produces approximately 20-30 tonnes of straw depolymerization wastewater.When discharged untreated or disposed improperly,it might cause and its direct discharge or improper treatment severe environmental pollution.Due to the high temperature,suitable carbon/nitrogen ratio?about 28/1?and the abundance in volatile fatty acids?VFAs?of straw depolymerization wastewater,it is very suitable as raw material for anaerobic digestion to produce biogas.However,high concentration of sulfate may inhibit the anaerobic digestion microorganisms,when it is directly used for biogas materials for methane production.In addition,the preparation of biogas by anaerobic co-digestion of corn stovers requires hydration,nitrogen and heat supplementation.The straw depolymerization wastewater has the above functions.Therefore,anaerobic co-digestion of straw depolymerization wastewater and corn stovers is expected to improve biogas production performance,meanwhile,reducing the operating cost of anaerobic digestion system.However,the characteristics of the straw depolymerization wastewater and the reaction conditions and digestion performance of the anaerobic co-digestion straw depolymerization wastewater and corn stovers were not clear,and there is no relevant literature that has been reported.In order to unravel characteristics of anaerobic co-digestion of straw depolymerization wastewater and corn stovers,the following four aspects of researches were carried out.?1?Anaerobic digestion of corn stovers under different sulfate concentrations were preformed,the mechanism of the influence of sulfate on anaerobic digestion process were studied by analyzing VFAs,sulfate removal rate,organic matter degradation and mass balance of C and S elements,et al.?2?Under the different substrate concentrations,anaerobic co-digestion of straw depolymerization wastewater and corn stovers were carried out.The effects of straw depolymerization wastewater on anaerobic digestion performance of corn stovers were studied by comparing the methane production potential,VFAs concentrations,and microbial community of co-digestion of straw depolymerization wastewater and corn stovers as well as mono-digestion of corn stovers.?3?Anaerobic co-digestion of straw depolymerization wastewater and corn stovers under different sulfate concentration and substrate concentration were carried out.The variance analysis was used to study the significance of the influence of sulfate concentration and substrate concentration on methane production.The best digestion condition was obtained by analyzing methane production potential,methane production rate and microbial community analysis.?4?Straw depolymerization wastewater with high sulfate concentration was pretreated by calcium hydroxide precipitation,then co-digestion of straw depolymerization wastewater with different sulfate concentration after pretreatment and corn stovers were performed.The effect of the ratio of total carbon to sulfate(TC/SO₄^2-)on anaerobic digestion performance was assessed and the tolerance of anaerobic digestion system for sulfate loading was measured.The main findings and conclusions are as follows.?1?Mechanism of the influence of sulfate on anaerobic digestion processIn the presence of sulfate,the sulfate reducing bacteria can promote the oxidation of propionic acid and butyric acid to acetic acid,thereby promoting the methanogenesis.In the anaerobic digestion reactor,the sulfate oxidation process and the sulfate reduction process co-existence with the methanogenic process,in which the sulfate oxidation process occurs mainly in the early stage of the methanogenesis process,prior to the sulfate reduction process.?2?Effect of straw depolymerization wastewater on anaerobic digestion performanceAnaerobic co-digestion of straw depolymerization wastewater and corn stovers can significantly increase methane production and buffering capacity of anaerobic digestion system.Maximum methane production from anaerobic co-digestion of straw depolymerization wastewater and corn stovers of 214.81 m L/g VS was about 13.54% higher than for mono-digestion of corn stovers of 189.20 m L/g VS.The lag time for methane generation from the anaerobic co-digestion of straw depolymerization wastewater and corn stovers was 10 days,while the lag time for mono-digestion of corn stovers was 23 days for a substrate concentration of 36.2 g/L.The removal rate for sulfate increased from 10.43% to 58.40% when the substrate concentration was increased from 25.9 to 45.3 g/L for the anaerobic co-digestion of straw depolymerization wastewater and corn stovers;Microbial communities were analyzed using 16 S r DNA sequencing technology which showed that anaerobic co-digestion of straw depolymerization wastewater and corn stovers promotes the growth of methanogens.The relative abundance of these methanogens?Euryarchaeota?for the anaerobic co-digestion of straw depolymerization wastewater and corn stovers was increased significantly,being approximately 9.84% higher than that of mono-digestion of corn stovers,which was at a substrate concentration of 36.2 g/L.?3?The effects of concentration of sulfate and substrate on anaerobic digestion performanceThe results showed that sulfate concentration and substrate concentration have significant effects on methane production from co-digestion of straw depolymerization wastewater and corn stovers,and the effect of substrate concentration on methane production is greater than the sulfate concentration.When the sulfate concentration is 0.4 g/L and the substrate concentration is 32.3 g/L,the hightest methane production potential of 249.93 m L/g VS was obtained.The maximum relative abundance of methanogens?Methanoculleus and Methanosarcina?in the digestion broth was achieved at this condition.?4?Optimum range of TC/SO₄^2-from anaerobic digestion of sulfate containing substrateStraw depolymerization wastewater with high sulfate concentration was pretreated by calcium hydroxide precipitation,then co-digestion of straw depolymerization wastewater with different sulfate concentration after pretreatment and corn stovers were performed.The results showed that calcium hydroxide pretreatment can change the characteristics of straw depolymerization wastewater and improve the biodegradability of straw depolymerization wastewater.When amount of addition of calcium hydroxide increased from Ca²⁺/SO₄^2-of 0.50 to 1.75,the concentrations of sulfate and Fe³⁺ in the straw depolymerization wastewater decreased from 16.70 g/L and 7.52 g/L to 0.35 g/L and 1.45 g/L,respectively,while Ca²⁺ concentrations increaseed from 0.34 g/L and 1.01 g/L.The concentrations of Ca²⁺ and Fe³⁺ were positively and negatively correlated with methanogens.The ratio of total carbon to sulfate(TC/SO₄^2-)in the digestion liquor was was established as a parameter to measure the stability of anaerobic co-digestion system,which could directly reflected the tolerance of anaerobic digestion system for sulfate loading.A maximum methane production of 260.14 m L/g VS was obtained at TC/SO₄^2-of 35.However,anaerobic digestion process is inhibited at TC/SO₄^2-of 6 for anaerobic co-digestion of straw depolymerization wastewater and corn stovers.Anaerobic co-digestion of straw depolymerization wastewater and corn stovers can effectively remove sulfate from straw depolymerization wastewater.When TC/SO₄^2-is 16,the maximum sulfate removal rate was 93.43%,and the concentration of sulfate in the biogas slurry after anaerobic co-digestion is less than 0.1 g/L regardless of initial TC/SO₄^2-in this study,which can meet the discharge standard.Anaerobic co-digestion of straw depolymerization wastewater and corn stovers can promote the degradation of lignocellulose.Co-digestion improved the destruction of cellulose,and the lowest crystallinity of 15.08 was obtained at TC/sulfate of 35,which was 49.70% less than undigested corn stovers of 29.98.When TC/SO₄^2-decreased from 110 to 6,the removal rate of lignin increased from 5.39% to 29.61%;...