The Optimization Study On The Influence Factors Of Duckweed Cultured In Alcohol Wastewater And The Biogas Production Of Duckweed

As we all know,environmental problems and resource shortages?water,food and energy?have affected human health,especially in the developing countries such as China.Among them,a large amount of wastewater was produced in the alcohol production industry,which contains rich nutrients?protein and inorganic salt?.Although the organic pollutants were removed,there were still a certain amount of nutrients?such as N,P?in the wastewater after anaerobic treatment,aerobic treatment or the combination of anaerobic and aerobic treatment.If the wastewater was discharged into natural water bodies without further processing,it would cause serious water pollution?water eutrophication?,thereby affecting the environmental quality and human health.Duckweed,as a kind of aquatic plants,can absorb and utilize the nutrients of the wastewater for biomass accumulation,and purify wastewater.Whereafter,the duckweed biomass could be converted into biogas energy by anaerobic biological technology.So the whole process could achieve environmental and economic benefits.In this paper,select duckweed as the object of study,the influence factors of duckweed growth in alcohol wastewater and its anaerobic digestion were studied.Firstly,the feasibility of duckweed cultured in alcohol wastewater was investigated under the laboratory condition?light incubator?.The appropriate dilution ratio of alcohol wastewater was determined;Then the effects of temperature,pH,light intensity,CO₂ on the duckweed biomass accumulation were studied,in order to determine the optimal conditions for growth.Secondly,the effects of inoculation density and harvesting period on the duckweed growth and composition,and the pre-treatment condition of wastewater were studied under outdoor condition?turnover box?.In addition,the harvested duckweed and excess sludge as the substrates were subjected to anaerobic digestion research,and the optimal conditions for biogas production were determined,in order to provide theoretical guidance for the engineering application.The main research results are as follows:1.The duckweed biomass was increased to 0.60 g when it cultured in the alcohol wastewater diluted 10 times,and the relative growth rate?RGR?was 0.041 g/?g·d?,which was higher than that of the lake group?0.032 g/?g·d??and lower than that of the Hoagland medium group?0.060 g/?g·d??.The result indicated that the alcohol wastewater could be used to culture the duckweed.The optimum conditions for the duckweed biomass accumulation as follows: temperature was 20.0 ?,pH was 7.00,light intensity was 4000 lux,the flux mode of CO₂ was 1 min every two days under the laboratory condition;Regression analysis showed that duckweed could get the maximum growth at the condition of 19.3 ?,6.73 and 4366 lux.2.The maximum relative growth rate?RGR?,C content,N content and protein content?based on dry weight?of duckweed were 0.091 g/?g·d?,417.60 mg/g,45.60 mg/g and 285.00 mg/g respectively when the inoculation density was 149 g/m2;The removal rate of TN,TP,NH₄⁺-N and SCOD of alcohol wastewater were similar under different inoculation density,and its maximum removal rates were 49%,79%,82% and 38% respectively after cultivation 8 days.In addition,the maximum RGR of duckweed were 0.088 g/g/d when the harvestind period was 3 days,and the biomass increased 1 times compared with the initial value,which was higher than the duckweed increment under laboratory conditions.The results indicated that the accumulation of duckweed biomass could be further promoted under the outdoor condition;and duckweed can not only absorb and utilize the N,P and other nutrients of wastewater,but also can adsorb and remove the metal elements to purifty water body further.3.The substrate gas yield was 157 m L/gVS when duckweed was as single substrate to anaerobic digestion,indicating that duckweed has the ability to produce biogas;Duckweed and excess sludge as mixed to anaerobic co-digestion,its cumulative gas yield was 2963 m L,which was 11% higher than the calculated value?2669 m L?,and the methane content was 56.93%,indicating that duckweed mixed with excess sludge could improve the biogas production capacity by the substrates complementary.Furthermore,the methane yield was improved 8% after duckweed pretreated by hot alkali,its value was 1817 m L,and the total gas yield of the pretreatment group was higher than the untreated group during the first 18 days;After 18 days,and the daily gas yield of the pretreatment group was lower than the untreated group,indicating that more available material were used to produce gas in the early stage of the reaction when the duckweed was pretreatmented,to accelerate the gas production rate and increase the methane yield.When the inoculum to substrate ratio was 1:1,the maximum biogas yied,the maximum methane yied and the highest methane ratio were 3309 mL,1883 m L and 56.93%,respectively.At the same tine,the substrate gas yield was 169 mL/gVS,and the removal rates of TS and VS were 26.44% and 36.63%,respectively.4.The analysis results of pH,VFAS,NH₄⁺-N,SCOD,nutrients concentration and enzyme activity of anaerobic system showed that the pH rebound faster after rapid decrease and the time of appropriate pH to produce methane?6.50-7.50?was longer for the anaerobic co-digestion of duckweed and excess sludge;The NH₄⁺-N concentration was maintained within the 400 mg/L,and there was no ammonia inhibition;The amylase activity was stronger during the hydrolysis stage,and the carbohydrate was hydrolyzed to produce sugars,which were utilized during the subsequent fermentation process;Dehydrogenase activity was affected by the change of pH,then its activity was stronger during both of the hydrolysis phase and methanogenic phase due to the stronger microbial activity and abundant nutrients in these stages.5.The duckweed and excess sludge were mixed to anaerobic co-digestion in the 5 L reactor,and the highest daily biogas yield and daily methane yield were 4678 mL and 2569 mL when the running load was 5 gVS/?L·d?.