The Effect Of The Combination Of Hydroponic Plants And Microorganisms In Cold Water Aquaculture Wastewater Treatment

In recent years,more and more cold flowing aquaculture have been applied,and the environmental pressure brought by wastewater discharge has also increased.However,the treatment of wastewater has been neglected.Cold flowing aquaculture is characterized by low water temperature and high water exchange frequency.Therefore,the cost of constructing cold flowing aquaculture wastewater treatment facilities according to the ex-situ treatment mode is relatively high,so in-situ treatment may be a more suitable choice.There have been many reports on the combined application of hydroponic plants and biofilm in the treatment of aquaculture wastewater,but the effect and mechanism of the interaction between hydroponic plants and microorganisms on the effect of wastewater treatment are not clear.Therefore,the purpose of this study is to construct a composite in-situ treatment system combined with hydroponic plants and microorganisms,evaluate the treatment efficiency of this system on the wastewater of sturgeon cold flowing aquaculture,clarify the nutrient removal pathway in this system,and clarify the influence and mechanism of the interaction between hydroponic plants and rhizosphere microorganisms on the effect of wastewater treatment,provide a reference for the treatment of wastewater of cold-flowing water aquaculture.The results obtained are as follows:Experiment 1:A composite in-situ treatment system combined with hydroponic plants and microorganisms was constructed,and its treatment efficiency on wastewater of sturgeon cold flowing aquaculture was clarified,low-temperature wastewater treatment plants were screened,and the microbial composition of water and biofilm were detected.The correlation between the changes of water quality indicators and bacterial abundance was analyzed,and the absolute abundance of nitrogen and phosphorus cycling genes was determined.The results showed that the removal efficiency of NH₄⁺-N,NO₃^--N,TN,TP and COD produced by the experimental cultured sturgeon were 90%,100%,100%,100%and 48%,respectively.Chinese cabbage（Brassica rapa var.chinensis）and Water dropwort（Oenanthe javanica）showed significant growth in the four plants,which contribute to the removal of nutrients from wastewater.In addition,in the biofilm,Proteobacteria,Bacteroidetes,and Verrucomicrobia became the top three dominant flora at the phylum level,and Flavobacterium,Rhodoferax,Sphaerotilus and Chitinomonas were dominant at the genus level.The dominant bacteria were beneficial to denitrification and phosphorus uptake,and promoted the removal of nitrogen and phosphorus in wastewater.The results of Functional Annotation of Prokaryotic Taxa（FAPROTAX）analysis showed that the highest functions within the carbon and nitrogen metabolisms were significantly identified in the biofilm,such as chemoheterotrophy,aerobic chemoheterotrophy and nitrate reduction.Further,the absolute abundance of denitrification genes membrane-bound dissimilatory nitrate reductases G（nar G）and periplasmic dissimilatory nitrate reductases（nap A）was significantly higher than that of nitrification-associated genes nitrite oxidoreductase B （nxr B）and ammonia monooxygenase A（amo A）,indicating the more active denitrifying process.In summary,the combined in-situ treatment system of hydroponic plants and microorganisms can effectively remove nutrients in cold flowing aquaculture wastewater,and hydroponic plants and biofilm rich in denitrifying bacteria play an important role in this process.Experiment 2:The effect of the interaction between hydroponic plants and rhizosphere microorganisms on the removal of nitrogen nutrients was investigated.On the basis of Experiment 1,the screening of plants for low-temperature wastewater treatment was continued,and it was found that watercress is also a suitable plant for low-temperature wastewater treatment.Therefore,watercress was used in this experiment to conduct subsequent research on the interaction between hydroponic plants and rhizosphere microorganisms.The denitrifying bacteria in the watercress rhizosphere were isolated and identified,and their denitrification efficiency was detected;the difference in the root exudates of watercress with and without rhizosphere microorganisms was analyzed by gas chromatography-mass spectrometry（GC-MS）,and the identification root exudates with relatively high abundance were detected;the effects of different concentrations（20,40,60,80 and 100μM）of exudates on the growth,biofilm formation and nitrogen removal efficiency of denitrifying bacteria were further tested.The results showed that among the 10strains of rhizosphere denitrifying bacteria,except strain 1-1,the remaining 9 strains all had good denitrification and denitrification ability,and could completely remove 100 mg/L nitrate nitrogen of simulated wastewater within 48 h,and there is no accumulation of nitrite nitrogen and ammonia nitrogen.The relative abundance of watercress root exudates was palmitic acid.80 and 100μM palmitic acid treatment of strain 1-1,which has weak denitrification and denitrification ability,can promote the biofilm formation of strain 1-1 and improve the denitrification ability of strain 1-1.Especially at 24 h,the removal efficiency of TN in the 80μM and 100μM treatment groups was about 1.5 times that of the control and the 20μM treatment group.In summary,watercress can affect the denitrification ability of rhizosphere microorganisms through root exudates.In summary,this study constructed a composite in-situ treatment system of combined application of hydroponic plants and microorganisms,and the system can effectively remove nutrients in the cold flowing aquaculture water of sturgeon.In addition,we also found that watercress roots can secrete palmitic acid,thereby promoting the biofilm formation and nitrogen removal efficiency of denitrifying microorganisms in the rhizosphere.