Research On Biogas Anaerobic Fermentation Coupled With Microalgae Culture In Membrane Photobioreactor

Large amounts of biogas slurry containing organic matter,nitrogen,phosphorus and other pollutants are being produced from biogas anaerobic fermentation.If untreated biogas slurry was discharged at will,it would not only pollute the environment,but also lead to resources waste.On the other hand,microalgae can be cultivated with biogas slurry,which could make the biogass slurry harmless and obtain some high-value microalgae products at the same time.This research designed a system of biogas anaerobic fermentation coupled with microalgae cultured in membrane photobioreactor(Membrane photobioreactor,MPBR).The coupled system combined biogas anaerobic fermentation with microalgae culture,by which the biogas slurry could be treated and recycled with microalgae products output,and could also improve methane concentration of biogas.Firstly,the microalgae cultivation strategy was enhanced in the system of membrane photobioreactor coupled with biogas anaerobic fermentation(Coupled system)to improve treatment efficiency of biogas slurry by microalgae and compatibility between MPBR and anaerobic fermentation tank.Then,the operation of the coupled system was investigated under the condition of biogas slurry reflux,and the effects on improving methane purity improvement was also studied.Mass balance and‘energy cost-environment-economic’evaluation of coupled system was done finally.Some of the main research conclusions were listed as follows:(1)The system of semi-continuous biogas anaerobic fermentation coupled with MPBR was constructed,and the grading lighting strategy for microalgae culture in MPBR was established:400 mol/m2·s from 0 to 24 hours,500 mol/m2·s from 24 to 72 hours,and 600 mol/m2·s from 72 to 96 hours.Under this illumination strategy,C.sorokiniana had optimum biomass(0.73 g/L)and ammonia removal efficiency of biogas slurry(90.1%).Besides,it was determined that treatment efficiency of biogas slurry by MPBR could be preferably compatible with production rate of biogas slurry from anaerobic fermentation tank when Hydraulic retention time(HRT)of anaerobic fermentation was 40 d,and C.sorokiniana also had optimum biomass(1.05 g/L)and ammonia removal rate of biogas slurry(21.3 mg/L·d)in MPBR.(2)The bio gas slurry treated by microalgae was refluxed in coupled system,that was,treated biogas slurry all was refluxed into the anaerobic fermentation tank to reduce biogas slurry discharge.The results showed that,the ammonia nitrogen concentration(～1200 mg/L),volatile fatty acid(～2600 mg/L),viscosity(～40 mPa·s)and daily biogas production(～1100 mL/L·d)in coupled system had no significant differences with that of control group.The removal efficiencies of ammonia nitrogen and phosphate respectively reached 99.80%and 99.50%,with the final ammonia concentration lower than 5 mg/L and phosphate concentration lower than 0.5 mg/L.High biomass(1.05 g/L),production rate(160.2 mg/L·d)and protein content(55.0%)of C.sorokiniana were also obtained in coupled system.(3)With microbial community analysis,it was found that the relative abundance of nitrophilous genus in Firmicutes(Peptostreptococcus,Peptoniphilus、Clostridium,Sporanaerobacter and Tepidimicrobium)increased with the increase of ammonia concentration in the anaerobic reactor,resulting in an excessive acetic acid and then inhibited biogas generation.Besides,compared with the circulation reflux group of treated biogas slurry(0.64%and 0.02%),the relative abundance of Methanosarcina(0.41%)with poor ammonia nitrogen tolerance in the circulation reflux group of untreated biogas slurry decreased,while the relative abundance of Methanobrevibacter(0.07%)with strong ammonia nitrogen tolerance increased on account of the influence of accumulated high ammonia nitrogen concentration on methanoarchaea.The total relative abundance of methanoarchaea in the reflux group of untreated biogas slurry showed a decreasing trend,leading to a decrease in methane production.(4)The performance of the ultrafiltration membrane of MPBR in coupled system was investigated.After 80-days’ operation,the average growth rate of the transmembrane pressure(0.26 kPa/d)of MPBR was lower than that(0.41 kPa/d)of normal membrane bioreactor(MBR).The total resistance of MPBR(6.8×1012 m-1)was lower than that of MBR(9.4×1012 m-1)as well.The filtration flux of MBR was only 66.1%of that of MPBR,which indicated that MPBR could better alleviate membrane pollution compared with MBR.(5)The effect of coupled system on improving CH4 purity was investigated.The results showed that the highest CH4 purity(84.3%)with a volume ratio of biogas/biogas slurry(7.5 L/2 L),which had sufficient biogas slurry medium and gas carbon source(CO2).The optimum microalgae biomass(1.27 g/L)and CH4 purity improvement(24.9%)were obtained with sufficient CO2 provided by crude biogas containing 40%(v/v)CH4 for microalgae growth,.By comparison of different processes of biogas purification by microalgae,current data have shown that the coupling system had excellent biogas purification efficiency(7.49%/d),removal rate of ammonia nitrogen(63.4 mg/L·d)and phosphate(5.52 mg/L·d),and microalgae production rate(292.5 mg/L·d).(6)Mass balance and economic analysis of coupled system were finally conducted.A benefits of 241.07 yuan with 1 m3/d biogas slurry treatment could be made with high removal efficiencies of both ammonia nitrogen(99.6%)and phosphate(99.8%).With a comprehensive analysis of "energy-environmental-economic" indicators of different biogas slurry treatment process,the economy(38.99 yuan/ton)and green degree(0.440 gd/ton)of microalgae method were quite outstanding except slightly higher energy consumption(139.31 MJ/ton),which was an excellent biogas slurry treatment method under comprehensive consideration.