Study On Wastewater Treatment Performance Of Microalgae-Dynamic Membrane Composite Process

Microalgae has a good environmental adaptability,which could be applied to sewage treatment and produce useful algal biomass.It is one of the important ways to achieve simultaneous wastewater treatment and resource utilization.However,under the natural conditions of microalgae cultivation have some problem,such as low biomass and difficult to enrich and separation,which would be became an important bottleneck restricting the promotion and application of microalgae wastewater resource treatment technology.Based on the optimization of wastewater matrix composition of Chlorella vulgaris in mixotrophic cultivation,this study constructed a new wastewater treatment composite process by combining photobioreactor with dynamic membrane reactor.Simultaneously,the performance of composite process was researched for the cultivation and harvesting of microalgae and the treatment of wastwater.The main research contents and results are as follow:（1）To enhance the overall performance of the mixotrophic cultivation of Chlorella vulgaris,single factor test and response surface methodology（RSM）was applied to optimize the concentration of wastewater composition.The results showed that under the optimal condition of wastewater composition（COD=1271 mg/L,NH₄⁺-N=20 mg/L and TP=18 mg/L）,the maximum biomass concentration of Chlorella vulgaris reached0.52 g/L,which was enhanced by more than three times as compared to that of control substrate（0.15 g/L）.Moreover,when the growth cycle of Chlorella vulgaris was in the range of 3-5 d,the removal efficiencies of COD,NH₄⁺-N and TP were in the ranges of70%-83%,51%-91%and 30%-94%,respectively.And during the growth cycle of Chlorella vulgaris,good positive correlations among the biomass and removed pollutants were observed,which facilitates the removal of contaminants and the accumulation of biomass.（2）Basd on the pre-test,the microalgae-dynamic membrane composite process had been successful constructed,including photobioreactor（PBR）and diatomaceous dynamic membrane reactor（DDMBR）.The composite process not only efficiently produced,separated and enriched microalgal biomass,but also had a good efficiency for the wastewater treatment.During the stable operation of the PBR,it was found that the growth of Chlorella vulgaris was very well,and the average biomass productivity of Chlorella vulgaris achieved 0.50 g/L·d.Turbidity and algae density of the DDMBR enffluent were less than 5 NTU and 10⁵ cells/mL,and the algae concentration increasing from 0.50 g/L to 8.00 g/L.After treating by the composite process,the removal efficiencies of COD,TN,NH₄⁺-N and TP achieved>80%,>95%,>97%,60%-80%,respectively.Especially,the effluent concentration of TN and NH₄⁺-N were1-2 mg/L,much lower than the pollutant discharge standard of urban sewage treatment plants（first level A）.（3）To analyze possible membrane fouling mechanism,a series of instrumental analysis methods were applied to systematically investigate the properties of dissolved organic matter（DOM）and the morphological structure of membrane fouling layer.The sample of DOM was taken from influent,microalgal mixture,effluent and backwash mixture,respectively.The results revealed that DOM was the main reason of membrane fouling,in which polysaccharides and protein substances were easy to accumulate on the membrane surface and contribute more to membrane fouling.The GFC analysis found that DOM had a broad molecular weight distribution,and the medium molecular weight organics dominanting the substance of membrane fouling.At the same time,SEM-EDX analysis found that the surface of the membrane was covered with the filter cake layer,including the diatomite layer and the algae layer.It could effectively prevent DOM contaminating from membrane material.And the filtration performance of the dynamic membrane was effectively restored after physical cleaning.In addition,the molecular biology method was used to investigate the changes of microbial population structure in the PBR-DDMBR process.It was found that bacteria also promoted the removal of pollutants.