Study On The Aquacultural Wastewater Purification And The Transformation Of Nitrogen In Aquaculture Pond Ecosystem

As a key freshwater fishery breeding base in our country,the development of the aquaculture surrounded Taihu basin brought huge economic benefits to local residents,as well as brought great pressures to the aquatic environment in Taihu basin.According to the National Water Quality Report,the Taihu Lake was in the mild eutrophication state as a whole.The agricultural diffused pollution is the main source pollution of Taihu.While the pond aquaculture was more developed in Taihu basin,and occupied a high proportion in agriculture diffused pollution.Therefore,studying the method of purify the aquacultural wastewater in the pond aquaculture ecosystem,and the pollutants released law of the sediment in breeding ponds,has certain theoretical and practical significance to improve the breeding environment and relieve the eutrophication of the water in Taihu Lake.This paper took the aquacultural wastewater and sediment which in the pond aquaculture ecosystem as the research objects,and adopted the field monitoring method to study the purification effects on crab breeding water by in-situ purification with different biomasses and the enhanced purification with high density of aquatic plants.This paper also adopted the outdoor simulated method to study the law of the different nitrogen form changes and transition which between the overlying water and the interstitial water of the sediment,and to study the distributions of nitrogen in the pond aquacutural ecosystem which based on the ¹⁵N tracer technique.Through monitoring and analysing the various physical and chemical indexes in the test water and sediments,expected to provide theoretical reference and basis to find a suitable way to control eutrophication in the actual aquaculture production process.（1）This paper comparative studied the purification effects on pond aquaculture water by setting different planting density of waterweed and alternanthera philoxeroides,and studied the strengthen purification effects with high density aquatic plants on pond tail water.The results showed that:the concentrations of suspended solids,COD,NH₄⁺-N,TN and TP had significant negative correlations with the biomasses of the two kinds of aquatic plant（P<0.05）.The average removal rates of the suspended solids,COD,NH₄⁺-N,TN and TP in unit quality of alternanthera philoxeroides was slightly higher than waterweed,but there was no significant difference in the average removal rates of suspended solids,COD,NH₄⁺-N between this two plants（P>0.05）,while the average removal rates of TN and TP had significant difference（P<0.05）.The pond aquacultural tail water couldn’t reach the secondary standard of The Pond Aquaculture Water Discharge Standard Of Taihu Basin during the in-situ purification.But after the strengthen purification by high density of aquatic plants,the concentrations of suspended solids,COD of the tail water in ponds could achieve the secondary emission standard,while the concentrations of NH₄⁺-N,TN and TP could achieve the primary standard.（2）This paper had simulating studied the morphological changes and transition of nitrogen between the interstitial water in the pond sediments and overlying water,the results showed that:The concentrations of NO₃^--N,NH₄⁺-N,TN,TP both from interstitial water and overlying water in reed group were lower than blank group,and they all had significant difference between the two treatments（p<0.05）.With the increase of the sampling depth,in the blank group,the concentration of NO₃^--N from interstitial water decrease,while the concentrations of NH₄⁺-N,TN,TP increase.But there was no obvious regularity in the reed group,the middle layer interstitial water had the minimum concentrations of NO₃^-、N,NH₄⁺-N,TN,TP.Both reed group and blank group,the average concentrations of NO₃^--N,NH₄⁺-N,TN,P from interstitial water had no significant difference between the layers（p<0.05）,but they had very significant positive correlations.In reed group,there were very significant positive correlations between the concentrations of NO₃^--N,NH₄⁺-N,TN from the same layer interstitial water.In blank group,there were very significant negative correlations between the concentrations of NO₃^-、N and NH₄⁺-N,TN from the same layer interstitial water,but significant positive correlations between NH₄⁺-N and TN.In reed group,the concentrations of NO₃^--N,NH₄⁺-N,TN,TP had very significant positive correlations between the overlying water and each layer interstitial water.While the blank group is opposite,the concentrations of NO₃^--N5NH₄⁺-N,TN,TP had very significant negative correlations between the overlying water and each layer interstitial water.In reed group,there were very significant positive correlations between the concentrations of NO₃^--N,NH₄⁺-N,TN from overlying water.In blank group,there were very significant negative correlations between the concentrations of NO₃^--N and NH₄⁺-N,TN from overlying water,but significant positive correlations between NH₄⁺-N and TN.Therefore,the NO₃^--N has the diffusion trend from overlying water to interstitial water,while the NH₄⁺-N,TN and TP has the opposite diffusion trend.The Eh value in reed group was higher than blank group,while the average concentrations of Fe²⁺ of each layer interstitial water were lower than blank group,and both of them had very significant difference between two treatments（P<0.01）.In reed group,the concentrations of N,P from interstitial water had no significant negative correlation with the Eh value,and had no significant positive correlation with the concentrations of Fe²⁺ In blank group,the concentrations of NO₃^--N,TP from interstitial water had significant positive correlation and the concentrations of NH₄⁺-N,TN from interstitial water had significant negative correlation with the Eh value,while there were opposite correlations with the concentrations of Fe²⁺.（3）Based on the stable isotope ¹⁵N tracer technique,this paper simulated studied the retention ratio of ¹⁵N in the pond aquacultural ecosystem.The result showed that:During the test,the contents of total nitrogen in reed group sediment were lower than blank group,but there was no significant difference between two treatments（P>0.05）.At the end of the experiment,the value of delta ¹⁵N in reed was 63161.01‰,in reed group and blank group sediment were 266.53‰ and 7558‰,in reed group and blank group interstitial water were 303.13‰ and 6292.94‰,in reed group and blank group overlying water were 8810.11‰and 63555.44‰.After calculation,in reed group,the reed absorbed 75.97%,sediment stranded 9.83%,interstitial water and overlying water retained 0.01%and 0.22%.In blank group,sediment stranded 81.96%,interstitial water and overlying water retained 0.23%and 6.11%.In conclusion,nutrients which in aquaculture ponds system,was mainly absorbed by plants in environment with aquatic plants.However,nutrients was mainly sediment intercepted with no aquatic plants.Therefore,it has very important practical significance to set a suitable proportion of aquatic plants in aquaculture pond.