Nutrient Budget And Optimization Of Vertical Flow Wetland Of Grass Carp And Largemouth Bass

Grass carp（Ctenopharyngodon idella）and largemouth bass（Micropterus salmoides）are the most representative freshwater aquaculture species in China,and their production has increased rapidly in the past decades.Grass carp is a traditional freshwater fish in China and the production has reached 5.5 million tons,accounting for 18%of freshwater aquaculture production.Largemouth bass has become the main carnivorous fish for freshwater aquaculture in China due to its rapid growth rate and delicious meat quality.In 2019,the production of largemouth bass has reached400,000 tons（Ministry of Agriculture and Rural Affairs of the People’s Republic of China,2020）.At present,the intensive aquaculture models of grass carp and largemouth bass has a negative impact on the aquaculture system and surrounding environment.Due to the different feeding habits and physiological characteristics of grass carp and largemouth bass,their feed nutrients are also quite different.In this study,constructed wetlands suitable for grass carp and largemouth bass were constructed by studying the characteristics of the waste water of these two fish and their optimal substrate proportion and plant density.In order to explore the ability of eco-substrate to remove nutrients in ponds,the in-situ water restoration experiment was carried out.The main conclusions are as follows:（1）Study on nutrient fate of grass carp and largemouth bass aquaculture systems.At present,waste water treatment has become the"bottleneck"problem for the sustainable development of aquaculture.Study on the nutrient fate in different aquaculture modes is the premise of treating waste water.In this study,the fate of carbon（C）,nitrogen（N）and phosphorus（P）in aquaculture systems of typical herbivorous fish（grass carp）and typical carnivorous fish（largemouth bass）was analyzed by laboratory experiments.Results showed that grass carp could utilize22.8%of C,50.9%of N and 14.34%of P in the feed,and largemouth bass could utilize 17.85%of C,30.4%of N and 4.87%of P.The other nutrient were discharged into the aquaculture environment.It revealed that the environmental loadings of grass carp was lower than that of largemouth bass.These results could supply basis for the design and optimization of waste water treatment systems of these two fish.（2）Optimization of substrate and plant for grass carp and largemouth bass waste water treatment.Results showed that the combination of zeolite,ceramsite and coconut activated carbon could effectively remove nitrogen and phosphorus in the waste water of grass carp and largemouth bass aquaculture systems.The waste water of grass carp,and the removal rates of NO₃^--N、NO₂^--N、NH₄⁺-N、TN、TP and PO₄^3--P in the waste water reached 78.86%、90.35%、81.23%、68.72%、85.93%and 88.56%,and in the waste water of largemouth bass,and the removal rates of NO₃^--N、NO₂^--N、NH₄⁺-N、TN、TP and PO₄^3--P in the waste water reached 69.88%、34.98%、63.12%、41.77%、90.23%and 85.13%,respectively.The combination of hornwort（Ceratophyllum demersum）,iris（Iris tectorum）,and Cyperus alternifolius could also effectively remove nitrogen and phosphorus in the waste water of grass carp and largemouth bass aquaculture systems.The waste water of grass carp,and the removal rates of NO₃^--N、NO₂^--N、NH₄⁺-N、TN、TP and PO₄^3--P in the waste water reached 75.81%、89.12%、90.11%、66.75%、82.33%and 70.97%,and in the waste water of largemouth bass,and the removal rates of NO₃^--N、NO₂^--N、NH₄⁺-N、TN、TP and PO₄^3--P in the waste water reached 68.33%、34.59%、63.91%、42.15%、85.39%and 89.18%,respectively.In view of the economic cost,the optimal ratio of the purification substrate of grass carp waste water was zeolite:ceramsite:coconut activated carbon=1:1:2,and the optimal ratio of largemouth bass tail water was zeolite:ceramsite:coconut activated carbon=2:1:1.The optimal density of the purification plant of grass carp waste water was hornwort 5.2 kg/m²,iris 9.75 kg/m²and C.alternifolius 8 kg/m².The density of largemouth bass waste water was ornwort 5.2 kg/m²,iris 9.75 kg/m²and C.alternifolius 4 kg/m².（3）Effect of Hydraulic retention time（HRT）on the vertical flow constructed wetlands for grass carp and largemouth bass.Results showed that when the hydraulic retention time was 16 h,the constructed wetland had the highest removal rate of nitrogen and phosphorus in the waste water of grass carp,and the removal rates of NO₃^--N、NO₂^--N、NH₄⁺-N、TN、TP and PO₄^3--P in the waste water reached 49.38%、35.12%、28.33%、36.19%、25.67%and 29.18%,respectively.When the hydraulic retention time was 24 h,the constructed wetland had the highest removal rate of nitrogen and phosphorus in the waste water of largemouth bass,and the removal rates of NO₃^--N、NO₂^--N、NH₄⁺-N、TN、TP and PO₄^3--P in the waste water reached 31.56%、45.17%、43.91%、33.95%、37.65%and 43.28%,respectively.When the hydraulic retention time was 32 h,the removal rate of heavy metals in the waste water of grass carp was the highest and the removal rates of Cr,As,Cd,Hg,Pb,Cu and Zn were 63.42%、40.14%、79.56%、62.33%、85.57%、72.36%and 90.92%,respectively.When the hydraulic retention time was 16 h,the removal rate of heavy metals in the waste water of largemouth bass was the highest and the removal rates of Cr,As,Cd,Hg,Pb,Cu and Zn were 53.46%、45.93%、66.73%、68.21%、82.36%、71.43%and 77.86%,respectively.（4）The effect of periphyton substrate on accumulation of nitrogen and phosphorus in the largemouth bass（Micropterus salmoides）aquaculture ponds.In order to study the effect of periphyton substrate on the accumulation of nitrogen and phosphorus in largemouth bass farming ponds,a 6-month experiment was carried out on largemouth bass aquaculture pond.During the study period,the Nitrite nitrogen(NO^2--N),nitrate nitrogen(NO^3--N),ammonium nitrogen（NH₄⁺-N）,phosphate(PO₄^3--P),total nitrogen（TN）,total phosphorus（TP）,total organic carbon（TOC）content and the total nitrogen（TN）,total phosphorus（TP）,and total organic carbon（TOC）content of the aquaculture pond sediment were determined.The results showed that TN,NH₄⁺-N,NO₃^--N in the treatment group was significantly lower than those of the control group,and the accumulation of N was significantly lower than that of the control group;the TP content of the treatment group was significantly lower than that of the control group,and the accumulation of P in the water was significantly lower than that of the control group.TOC,TN,and TP content in the sediment of the treatment group and the control group showed no significant difference.Results indicated that the periphyton substrate has an effect on reducing nitrogen and phosphorus content in the largemouth bass pond.