Effects Of Adding Different Bioflocs To Constructed Wetland On Its Ability To Treat Phosphate In Aquaculture Wastewater

With the rapid development of aquaculture,the environmental problems caused by the random discharge of aquaculture wastewater are becoming more and more obvious.In the treatment of aquaculture water,there are few phosphorus recovery processes,which are directly discharged into the environment,resulting in water eutrophication and other problems.At the same time,there are few studies on how to deal with the waste flocs in BFT.Based on these,as an integrated water treatment system,constructed wetland may be able to solve these two problems together.On the one hand,constructed wetland has the advantages of low cost,easy management and landscape benefits.The substrate,microorganisms and plants in the wetland have a certain phosphorus removal effect.On the other hand,constructed wetlands use a variety of substrates,including gravel,ceramsite,steel slag,soil or sludge,so bioflocs can be added to constructed wetlands as substrates.In this experiment,three groups of constructed wetlands with different bioflocs and one group of constructed wetlands without bioflocs were constructed.They are recorded as groups A,B,C and D.Group A was added with sugar added bioflocs,group B was added with non sugar added bioflocs,group C was added with SBR cultured mature granular flocs,and group D was not added with bioflocs.After the constructed wetland is mature,the purification ability of the constructed wetland with different bioflocs for aquaculture wastewater is studied,especially for the removal of phosphorus;At the same time,the changes of phosphorus form and content in each constructed wetland with different bioflocs before and after the experiment were studied;In addition,the plant growth and total phosphorus content of constructed wetland with different bioflocs were studied.This experiment is mainly divided into the following three parts:1.Study on the purification capacity of aquaculture wastewater of constructed wetlands with different bioflocsIn order to study the effects of adding different bioflocs on the purification capacity of the constructed wetland in the treatment of aquaculture tail water,the water quality at the outlet of each constructed wetland was continuously monitored for up to 8 days after feeding the aquaculture tail water,and the difference between nitrification intensity and denitrification intensity was measured in connection with the difference of nutritional composition of the added flocs,combined with the push flow first-order kinetic model,the pollutant removal rate is analyzed.The results showed that group A>group B>group C（P<0.05）,which had a direct impact on the phosphorus removal rate in the subsequent water quality indexes;Except for the increase of ammonia nitrogen in group A（P<0.05）,all indexes in other groups decreased significantly.It was found that the fitting effect of quasi second-order kinetic equation was the best,mainly chemical adsorption;In terms of phosphorus removal,group D without bioflocs showed better removal effect（P<0.05）,and the removal rate was 97%,in which matrix adsorption and precipitation was the main way;The removal efficiency of COD and DOC is good,and the average removal rate is more than 60%;Within 8 days,the p H increased from 8.03 to 7.20,and the alkalinity increased more from 310 mg/L Ca CO₃,especially 630 mg/L Ca CO₃ in group A;The first-order push flow kinetic model is used to fit the relationship between pollutant concentration and HRT.The results show that the fitting effect of nitrite nitrogen,phosphate and COD is good;Group A had the lowest nitrification intensity but the highest denitrification intensity,and group B had the highest nitrification intensity but the lowest denitrification ability;In addition,this chapter also compares and analyzes the experimental results with the continuous monitoring in the film hanging stage.The results show that due to the high temperature in the film hanging stage and the low temperature in the experimental stage,only the removal rates of some indicators such as nitrate,total nitrogen and phosphate have been improved.Affected by microbial activities,the nitrification intensity and denitrification intensity decreased,and the ammonia nitrogen removal rate and COD_Mnremoval rate decreased.2.Effect of adding different bioflocs on phosphorus form and content in constructed wetlands substratesIn the experiment,the matrix phosphorus content of each constructed wetland was measured by SMT protocol.Langmuir and Freundlich equation models were used to fit and analyze the isothermal adsorption process of phosphorus on the three substrates,and the quasi first-order and second-order kinetic equation models and intra particle diffusion equation models were used to fit and analyze the adsorption kinetic process of phosphorus on the three substrates.In addition,the unit mass matrix microbial biomass of each wetland was measured.The results showed that the total phosphorus content of various substrates of each constructed wetland increased in the film hanging stage and experimental stage compared with the initial value,and the content ratio of various phosphorus forms was as follows:TP（Total phosphorus）>IP（Inorganic phosphorus）>HCl-P（Extraction of phosphorus with hydrochloric acid）>OP（Organophosphorus）>Na OH-P（Extraction of phosphorus with sodium hydroxide）.When comparing the overall effect of the wetland,compared with that before the experiment,the increase of total phosphorus per unit mass matrix of A,B,C and D is about 0.57 g/kg,0.66 g/kg,0.56 g/kg and 0.85 g/kg.It can be seen that the matrix adsorption phosphorus of constructed wetland D is higher（P<0.05）,the adsorption capacity of constructed wetland B is the second,and the adsorption capacity of A and C is the lowest（P<0.05）.It shows that adding flocs in constructed wetland can not significantly increase the adsorption capacity of substrate for phosphorus;Limestone has higher fitting degree and better adsorption effect under Langmuir equation model.The fitting degree of gravel and ceramsite is good,but the adsorption capacity is poor;From the perspective of phosphorus adsorption kinetics,the gravel fits well under the quasi first-order kinetic equation model,and the maximum adsorption capacity is close to the experimental value;The experimental value of ceramsite is relatively close to the fitting value of quasi first-order kinetic equation,and the diffusion constant in the particle is low,indicating that the adsorption effect is not strong and the internal diffusion ability is poor;Limestone fits well under the quasi first-order kinetic equation model,the maximum adsorption capacity is close to the actual value,the reaction rate constant is higher than the quasi second-order kinetic equation,and the fitting effect of limestone’s intra particle diffusion equation is better,the diffusion constant is 0.136,which is much larger than that of ceramsite and gravel,indicating that limestone has strong adsorption of phosphorus and obvious internal diffusion;The results of microbial biomass per unit mass matrix of constructed wetlands showed that the microbial biomass per unit mass matrix of groups A and B was significantly higher than that of groups C and D（P<0.05）,and there was no significant difference between groups A and B,and between groups C and D（P>0.05）.3.Effect of adding different bioflocs on phosphorus removal capacity of constructed wetland plantsIn order to further illustrate the effect of adding different bioflocs on the phosphorus removal capacity of constructed wetlands,the plant treatment capacity of each group of constructed wetlands was studied in this chapter.The plant height and dry weight of plants were measured to analyze the growth status of plants.The changes of total phosphorus content of plants in different periods were also measured.The phosphorus absorption of plants in constructed wetland was analyzed by calculating the changes of phosphorus content in aboveground parts of plants.The results show that in terms of plant growth,the growth of Acorus calamus and windmill grass in constructed wetlands A and B is general,and the plant growth in wetland C is better.The possible reason is that the added floc has a high content of organic matter,which leads to the increase of humus,which is conducive to the absorption of phosphorus by plants.Windmill grass in wetland D grew well and its dry weight increased rapidly,indicating that adding flocs with high organic matter content was beneficial to the growth of Acorus calamus and windmill grass;In terms of plant phosphorus content,the phosphorus content of windmill grass in wetland a changed little,and the total phosphorus content of Acorus calamus increased by 73%（P<0.05）.Windmill grass in wetland B increased by 30%and calamus increased by 70%（P<0.05）.The total phosphorus content of windmill grass in wetland C changed little,and that of Acorus calamus increased by 41%;Windmill grass in wetland D increased by 46%.The results of calculating the phosphorus content of the aboveground part of wetland plants show that the plants in wetland B absorbed more phosphorus during the whole experiment,followed by wetland a,and wetland C absorbed the least phosphorus,indicating that the bioflocs added in wetland B are the most conducive to plant phosphorus removal,which may be due to the high total phosphorus content of flocs A and B（P<0.05）.To sum up,on the one hand,the addition of biological flocs affects the phosphorus removal capacity of constructed wetlands.The constructed wetland without biological flocs had better phosphorus removal ability（P<0.05）.On the other hand,the microorganisms introduced by biological flocs will be adsorbed by the matrix.The higher the total phosphorus of flocs,the earlier the matrix reaches saturation,and the worse the purification effect of phosphorus in aquaculture tail water（P<0.05）.In addition,the amount of phosphorus absorbed by plants may be positively correlated with the total phosphorus content of flocs（P<0.05）,and the quality of plant growth may be positively correlated with the organic matter content of flocs（P<0.05）.