| With the gradual acceleration of country’s urbanization process and the increasing scale of the aquaculture industry,the treatment of aquaculture wastewater has gradually become an important task of water pollution control.Constructed wetlands are widely used to treat various types of wastewater due to their economic,high efficiency,and easy management.Based on the low C/N characteristics of aquaculture wastewater,this study constructed two sets of tidal flow-underflow constructed wetland pilot plants,namely,a pure ceramsite filler constructed wetland(PCW)system and a ceramsite+iron-carbon composite filler constructed wetland(CCW).system.The focus is on the sewage treatment effect of the system under different inundation times and different influent C/N ratios,and the removal of pollutants during the operation cycle.The main results obtained are as follows:(1)When COD=300 mg/L and NH4+-N=100 mg/L,the first-level vacancy time was kept for 4 h to investigate the effects of inundation time on nitrogen and phosphorus removal in the two groups of constructed wetlands.The removal of pollutants mainly occurs in the first-level tidal unit.The removal effect of COD in the two groups of constructed wetlands is basically not affected by the submergence time,the removal effect of TP becomes worse with the decrease of submergence time,and the accumulation amount of NO3--N and NO2--N gradually increases with the decrease of submergence time.In the CCW system,the removal rates of NH4+-N and TN decreased with the decrease of submergence time,while in the PCW system,the removal rates of NH4+-N and TN increased first and then decreased with the decrease of submergence time.The optimal inundation time of PCW system is 8 h and 12 h,and that of CCW system is 8 h and 24 h.The effects of different influent C/N ratios on the denitrification and phosphorus removal efficiency of the two groups of constructed wetlands were investigated under the optimal inungation time respectively.It was found that the TP removal efficiency of the two groups of constructed wetlands was basically not affected by C/N.The TP effluent concentration of CCW system was within the range of 0.11-0.13 mg/L,and the TP removal efficiency of CCW system was about 4.86%higher(0.35-0.36 mg/L)than that of PCW system.With the decrease of C/N,the effluent concentrations of COD,NH4+-N and TN in the two groups of constructed wetlands gradually increase.When C/N>2,there is little difference in the removal efficiency of COD,TN and NH4+-N in the two groups of constructed wetlands.However,when C/N=1,compared with PCW system,the removal rate of NH4+-N and TN in CCW system increased by 7.2%and 6.37%,and the effluent concentration of NO3--N was lower than that in PCW system.(2)By analyzing the variation trend of pollutants in the inundation cycle,it was found that the removal of COD,NH4+-N,TN and TP in the alternated dry/wet tidal flow constructed wetland was basically completed within 2 h after the inflow of water,so O2 and ORP in the first-level system have a significant decrease after the inflow of water for 2 h.Two undercurrent in the artificial wetland COD,NH4+-N,TN and TP removal mainly occurs within 4 h after the water in the water 6 h after the removal rate of the secondary PCW system basically stable.However,the removal of NH4+-N,TN and TP in the second-stage CCW system was still proceeding slowly,indicating that the addition of iron-carbon filler can prolong the reaction time of pollutants in the CCW system and improve the removal effect of pollutants.(3)The removal effect of COD and TN at different heights along the process in the first-stage tidal flow constructed wetland system and the second-stage PCW system was the upper layer(medium particle size)>middle layer(small size)>Lower layer(large size).This indicates that the spatial location of the bed was more important than the matrix particle size for the removal of COD and TN.The removal effect of TP in primary and second-stage PCW was middle level>upper layer>Lower layer,the substrate with small particle size was more favorable for TP removal.The removal effect of COD and TN in the second-stage CCW was still shown as upper layer>middle layer>lower layer,and the removal effect of each height was better than that of the second-stage PCW.The removal effect of TP was upper layer(iron-carbon layer)>middle layer(iron-carbon layer)>lower layer,the addition of iron-carbon micro-electrolytic fillers improves the removal rate of TP in the second-stage CCW.(4)During the investigation of microbial community diversity,a total of 9 phyla were detected from 9 samples at different heights in the three reactors,and the main phyla were Proteobacteria,Bacteroidetes and Actinobacteria.Among which the proportion of Proteobacteria in different regions along the three reactors was the highest.The addition of Fe-C microelectrolytic filler promoted the growth of Proteobacteria and inhibited the growth of Bacteroidetes.In the first unit,the dominant genera at 20 cm was not obvious,while the two genera with high relative abundance were NS9_marine_group(9.12%)and Rhodanobacter(9.11%).The only dominant genus at 40 cm was Plasticcumulans(76.42%),and Alicycliphilus at 60 cm was dominant(21.91%).In the second-stage CCW system,Acinetobacter was the dominant genus at 20 cm(23.8%),and OPB56 and Rhizobacter were the top two genera in abundance at 40 cm and 60 cm.Acinetobacter were the dominant bacteria in different positions of second-stage CCW,and the proportion was respectively 15.96%17%and 38.82%.Plasticicumulans bacteria accumulated in primary tidal flow constructed wetlands are important bacteria for the synthesis of poly(P-hydroxybutyric acid)(PHB)and poly(hydroxy fatty acid ester)(PHA),which may be the reason for the efficient denitrification at low C/N in primary tidal flow constructed wetlands.The addition of iron carbon filler can promote the growth of Acinetobacter,which is a heterotrophic nitrification-aerobic denitrification bacterium,and can effectively realize the nitrogen removal treatment in the nutrient-poor state. |
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