Development Of Ecological Purification System For Low C/N River And Lake Water And Its Purification Efficiency

Shahe reservoir is the critical node of the upper North Canal.According to the surface water function area divisions in Beijing,the nature of the upper north canal is landscape water,and the water quality should reach the Class IV standard of Surface Water Environmental Quality Standards in China（GB3838-2002）.However,the present water environment quality of Shahe Reservoir is not satisfied.There is an urgent need to improve the water quality and the water ecology of the Shahe reservoir,which is of considerable significance to the construction of the North Canal ecological corridor.This paper investigates the water quality of shahe reservoir,establishes an ecological treatment system based on the characteristics of the water quality,studies its water purification efficiency and water purification mechanism,and conducts field pilot tests.The water quality of shahe reservoir was firstly monitored from March 2018 to November 2019.Results showed that the COD,BOD₅,TN and TP in Shahe reservoir were 32.53 mg/L,8.71 mg/L,6.97mg/L and 0.25 mg/L,respectively,which did not meet the Class IV standard of Surface Water Environmental Quality Standards.Due to the sediment pollutant release and overflow pollution,the pollution indexes show seasonal variation.The organic pollutants were high in spring and summer and low in autumn.Except for June,the TP concentration showed an obvious"inverted V-shaped"change trend on the whole,while TN showed a significant"V-shaped"change trend.In summer（flood season）,nitrogen pollution in the reservoir area is mainly NH₃-N,while in other seasons（spring and autumn）,nitrogen pollution is mainly NO₃-N,which is a typical low C/N sluice dam type water body.Shahe reservoir is a typical low C/N sluice dam type reservoir supplied with recycled water,and TN and TP were the main pollution factors.Based on the water quality characteristics of Shahe reservoir,we developed a combined ecosystem,carbon release pond--horizontal subsurface flow constructed wetland--submerged plant pond,to treat low C/N surface water streams.There was a carbon release bed in the carbon release pond,which was a sandwich structure with wood,gravel and corn cob.The corn cod can release carbon,and the COD release rate was 0.75 mg/g·d.The ecological purification system is used to improve the water quality in the reservoir area,and the water purification efficiency of the system in different operation stages is studied.The results showed that the effluent COD,TN and TP were 18.46 mg/L,1.41 mg/L and 0.04 mg/L.The removal rates were 2%,81%and 92%,and the compliance rates were 99%,73%and 100%,respectively.Entering the late autumn,the effluent TN was beyond the Class IV limit,mainly because the denitrification bacteria activity and plant growth were affected by the temperature drop.The horizontal subsurface flow constructed wetland was the core unit of the combined ecosystem,and its contribution to the TN and TP removal was higher than that of carbon release pond and submerged plant pond,which was 43%and 40%,respectively.The microbial community structure in the ecological purification system and the carbon release bed were analyzed through High-throughput sequencing,respectively.The results showed that the bacterial community structure in the ecological purification system was abundant.The dominant bacterial phylum included Proteobacteria,Actinobacteria and Chloroflexi,etc.,and the dominant bacteria genera included Arthrobacter,Bacillus and Pseudomonas,etc.They were all related to nitrogen and phosphorus removal.Among them,the relative abundance of microorganisms in constructed wetland units in the middle stage of the experiment was the highest,and the relative abundance of genus was 27.75%.While in the carbon release bed,the dominant bacterial phylum was Proteobacteria,Bacteroidetes and Firmicutes,etc.,and the dominant bacteria genera was Hydrogenophaga,Azospira and Clostridium,etc.They gradually changed from denitrification and dephosphorization related phylum（genus）to cellulose degradation related phylum（genus）as the running time elapsed.Based on the laboratory scale study results,we designed and constructed a pilot-scale project in the north shore of shahe reservoir.The treatment capacity was 1.5 m³/d.The periodic operation（10/09/2019-11/12/2019）results showed that the effluent COD,TN and TP in water were 42.27 mg/L,6.76 mg/L,and0.21 mg/L,with the removal efficiency of 36%,23%and 57%,respectively.The effluent water quality index cannot reach the Class IV standard.That was because the wetland plants littered and microbial activity reduced in winter.The further investigation related to the pilot projects should be carried out.