Combining Algal Pond And Constructed Wetland Systems For The Efficient Treatment Of Low Carbon Nitrogen Ration Wastewater:Performance And Mechanisms

With the rapid and healthy development of economy in China,the discharge of domestic sewage in villages and towns is increasing,and shows a trend of low carbon/nitrogen ratio（C/N）.Such sewage has high nitrogen content,if it is directly discharged into rivers and lakes,it will cause eutrophication of water body,reduce aquatic biodiversity and destroy water ecological balance.Constructed wetlands（CWs）technology is widely used in all kinds of sewage treatment because of its high treatment efficiency,low operation and maintenance cost,environmental friendliness and good ecological value.However,there are still problems such as poor reoxygenation capacity and insufficient carbon source,resulting in poor removal effect of organics and nitrogen,which affect the promotion and application of CWs technology.Microalgae purification technology can not only absorb nitrogen,phosphorus and other nutrients in water for their own growth and reproduction,but also generate oxygen,organic carbon and other substances through photosynthesis.Therefore,the coupled wastewater treatment process based on algal ponds combined with CWs is developed to realize efficient wastewater purification with low C/N.In this study,the optimal operating conditions of algal pond system were determined by simulating the cultivation of Scenedesmus obliquus（S.obliquus）in low C/N wastewater.The combining system of algal pond and CW was constructed to treat low C/N domestic sewage.And the long-term purification effect,the N2O release,and the algal cells physiological characteristics were systematically investigated.In addition,the migration and transformation characteristics of pollutants in the coupling system were mastered.At the same time,three-dimensional fluorescence excitation and emission matrix spectroscopy（EEM）and parallel factor analysis（PARAFAC）was used to study the content regulation and component distribution characteristics of dissolved organic matter（DOM）in the algal pond and CWs coupling system.The microbial community diversity and characteristics in CWs were analyzed by using Illumina High-throughput Sequencing technique to initially define the removal mechanism of pollutants.The main research conclusions are as follows:（1）The algal density,chlorophyll a,polysaccharide and protein contents reached the maximum,which were 4.45×10⁶ cells m L^-1,1.59 mg L^-1,724.92μg and 94.83μg,respectively when cultured S.obliquus in simulated low C/N wastewater for 2 days.At the same time,algae cells had a better removal effect on pollutants,with the removal efficiencies of DOC,NH4⁺-N and TP being 84.43%,36.23%and 50.70%,respectively.In addition,the DOM produced by S.obliquus was more on the second day in cultivation cycles which was mainly humic substances.（2）The algal pond and CW coupling system had a better removal effect on pollutants,with the removal efficiencies of COD,NH4⁺-N,TN and TP being 85.04～87.26%,97.79～98.38%,33.31～57.74%and 39.60～58.57%,respectively.The N2O emission flux(13.91～35.21μg m^-2 h^-1)of the coupling system was significantly lower than that of the CW system(69.55μg m^-2 h^-1).Moreover,the pollutants removal efficiency in the coupling system increased gradually with the increase of sewage C/N.And the biomass,chlorophyll a,polysaccharide and protein contents of S.obliquus all increased.During the typical cycle,the alternating aerobic-anaerobic environment provided a favorable environment for the pollutants removal in the algal ponds and CWs coupling system.Under light condition,the biomass,chlorophyll a and COD concentrations in the algal ponds increased gradually,but decreased gradually under dark condition.The accumulation of NO3^--N and NO2^--N was less in algal ponds.The COD concentration in the CW showed a trend of increase-decrease continuously,providing organics for denitrification.The NH4⁺-N concentration decreased and then kept in a stable value,while the NO3^--N concentration increased firstly and then stabilized and little NO2^--N accumulation.（3）The coupling system has a good effect on DOM assimilation,absorption and utilization,and the effluent DOC content in the coupling system(4.02～5.28 mg L^-1)was lower than that of the CW(5.36 mg L^-1).EEM-PARAFAC analysis showed that the effluent DOM of the coupling system was dominated by humus substances and the effluent DOM had higher aromaticity,humification degree and large molecular weight which were not easily degraded by microorganism.In addition,the microbial diversity in the CWs was higher and the microbial richness in the upper layer was higher than that in the lower layer.The results indicated that the prominent bacteria in the CW system were Proteobacteria phylum.The upper and lower of the filler layers relative abundance was 45.24%and39.79%,respectively.In the genus level,the Nitrospira genus related to nitrogen removal was enriched in the CW system,and the relative abundance in the upper filler（10.10%）was higher than that in the lower filler（5.51%）.Most of the microflora in the CW are related to denitrification microorganisms,which is beneficial to improve denitrification efficiency,while the lack of available carbon source limits the denitrification.