Research On The Coupling Technology Of Electrochemical,Physical Filtration And Biological Oxidation And Its Influence On "N" Flow In RAS

Physical filtration is the first water treatment unit in traditional RAS,and its treatment efficiency determines the scale and energy consumption of subsequent water treatment units.The treatment efficiency of physical filtration equipment is related to the filtration aperture and the particle size of suspended solids.Restricted by conditions such as process and cost,the filter pore size of physical filter equipment cannot be continuously reduced.Electrocoagulation（EC）can release metal cations into the water,and forming flocculants,thereby adsorbing and flocculating the suspended solids.Using EC technology to increase the particle size of suspended solids in RAS will become one of the ways to improve the efficiency of physical filtration.In this experiment,the application of EC technology in RAS was carried out based on the Litopenaeus vannamei RAS.The research was mainly divided into 4 parts.Part 1,the application of EC-filtration technology in the treatment of aquaculture water was studied,and mainly includes:（1）The effect of EC on the particle size distribution in the aquaculture water,（2）The effect of anode materials,hydraulic retention times and filter pore sizes on EC-filtration water treatment technology,（3）Study on electrode passivation phenomenon and influencing factors of EC-filtration technology in water treatment process,（4）The application of alternating pulse current（APC）in EC-filtration water treatment technology.Part 2,the research on the linkage control of the EC reactor and the microscreen drum fifilter（MDF）was stduied.Part 3,the research on the influence of the EC-MDF coupling technology on the biological filter,studied the effect of EC-MDF coupling technology on water treatment and size of MBBR,and the effect of EC-MDF coupling technology on the MBBR start-up and microorganisms.Part 4,the influence of the coupling technology of EC-MDF-MBBR on the"N"flow in RAS was invesigasted.The results of the study are as follows:1.Application of EC-filtration technology in RAS aquaculture water treatment1.1 The effect of EC on particle size distribution of TSS in aquaculture waterThe effects of anode materials（Al,Fe）and current density（15 A/m²,30 A/m²and 45A/m²）on the size,strength and recovery factor of TSS flocs were investigated.It was found that EC could increase the particle size of TSS,and its effect increased with the increase of current density.Compared with Fe,when Al was used as anode,the flocs produced by EC had higher growth rate,lower intensity factor and higher recovery factor.With the increase of current density,the strength factor of TSS flocs decreased and the recovery factor increased.1.2 Influence of anode materials,hydraulic retention times and filtration pore sizse on EC-filtration water treatment technologyThe effects of anode combination modes,hydraulic retention times（HRTs）and filter pore sizes on the water treatment efficiency of EC-filtration system were investigated.Results showed that EC could significantly enhance the treatment efficiency of the filtration equipment used in subsequent steps.Al-Fe combination electrodes used as anode result in the better processing capacity,and the optimum anode was 3Al+Fe.With the increase of HRT and decrease of filter pore size,the enhanced effect of the EC process on filter was more obvious.When the current density was 19.22 A/m²,the anode was 3Al+Fe,the HRT was 4.5min and the filter pore size was 45μm,the removal efficiency of system for the total number of vibrio,COD_Mn,TAN,NO₂^--N,NO₃^--N,TN,TP,PO₄^3--P was 69.55±0.93%,48.99±1.42%,57.06±1.28%,34.09±2.27%,18.47±1.88%,55.26±1.42%,49.69±1.42%,92.81±1.66%respectively and energy consumption was（26.25±4.95）×10^-3k Wh/m³.1.3 Study on electrode passivation phenomenon and influencing factors of EC-filtration technology in water treatment processUsing response surface methodology（RSM）,we investigated the effect of salinity（10～30）,current density（15 A/m²～75 A/m²）,inlet flow（100 L/h～300 L/h）,and the electrode continuous working time（8 min～40 min）on electrode passivation,treatment efficiency and energy consumption of the EC-MDF system.Results showed that the RSM models obtained could predict the EC behavior for various values.We also found that higher salinity improved current efficiency,alleviated anode passivation,enhanced TSS removal,but aggravated cathode passivation.The current efficiency and TSS removal efficiency increased with the current density,whereas the energy consumption and the cathode passivation were intensified.The current efficiency improved by increasing the inlet flow,while the electrode passivation and energy consumption were alleviated.However,the removal efficiency of TSS was compromised.Under a direct current and continuous increase in electrode continuous working time,the degree of electrode passivation increased,lowering the current efficiency and TSS removal,whereas the energy consumption was upped.1.4 Application of alternating pluse current in EC-filtration water treatment technologyA laboratory-scale continuous flow EC-filtration system was designed for the treatment of marine aquaculture wastewater,and investigated the influence of current density and alternating current on its electrode passivation,water treatment efficiency and energy consumption.By comparing the effects of direct current（DC）and alternating pluse current（APC）on the EC-filtration system under different current densities to understand the passivation of the Al electrode and rummage approaches to alleviate the electrode passivation,which is expected to promote the application of the EC-filter system in actual production.The study found that with the increase of current density and system running time,electrode passivation became more serious.And using direct current,when the current density was 45A/m²,the weight of the electrode plate increased by about 1546.67±54.50 mg after the system running for 90 min.In addition,using APC can significantly alleviate electrode passivation,reduce energy consumptionand improve the removal effect of TSS,COD_Mnand TN by EC filtration system.When using APC with current conversion period（CCP）was 1min,current density was 45 A/m²,and the system ran for 10 min,the removal efficiency of TSS,COD_Mnand TN by EC-filtration system was the best,which was 53.55±1.66%,47.76±0.18%and 15.55±0.31%,respectively,the energy consumption was 11.88×10^-3k Wh/m^-3.2.The linkage control of EC reactor and microscreen drum filterIn this experiment,the TSS concentration of RAS was used as an intermediate variable to construct the relationship model between the output current(I_EC)of the EC reactor and the backwash cycle(T_MDF)of the microscreen drum filter（MDF）:I_EC=(3217.69T_MDF^-1.262+1.66)Q/Φ（Q was the flow rate andΦwas the current efficiency of EC）.And the linkage control system of EC reactor and MDF was established in Litopenaeus vannamei RAS.It was found that the EC reactor can improve the removal efficiency of TSS and COD_Mnby MDF and foam separator（FS）.Under the action of EC reactor-MDF linkage control system,the average removal efficiency of TSS and COD_Mnby RAS increased by 24.07%and 24.30%respectively,and the additional energy consumption of RAS increased by 3.25 k Wh/d,and the loss of Fe anode was about 350 g/d.3.The effect of EC-MDF coupling technology on MBBR3.1 The effect of EC-MDF coupling technology on water treatment and size of MBBRA continuous flow EC-MDF-MBBR water treatment system was designed.And the effects of three current densities（5 A/m²,15 A/m²,30 A/m²）and two anode materials（Al,Fe）on EC-MDF-MBBR system were investigated.The results show that EC reactor can remove COD_Mn,TN and TAN in aquaculture water,and the higher the current density is,the better the removal effect is.EC can shorten the backwashing cycle of MDF and improve the filtration efficiency.After pretreatment by EC-MDF system,the influent load of MBBR was significantly reduced,which reduced the requirement of MBBR size.The concentrations of TSS and TN in the influent of MBBR were relatively lower when Al was used as anode in EC reactor,while the concentrations of COD_Mnand TAN in the influent of MBBR were relatively lower when Fe was used as anode.From the perspective of TAN removal efficiency of 100%,the size of MBBR could be reduced by more than 25%when the current density of EC reactor was 30 A/m²and Fe was used as anode.3.2 The effect of EC-MDF coupling technology on MBBR start-up and microorganismThe effect of EC-MDF coupling technology on MBBR start-up and microorganism was studied.In the first 31 days of the experiment,the EC reactor was opened to study the effect of EC-MDF coupling technology on the water treatment efficiency in the start-up stage of MBBR.After the 31st day,the EC reactor was shut down,and the change of MBBR water treatment efficiency was investigated.It was found that the MBBR start-up time in the EC treatment group was shorter,and it was the shortest in the Fe anode treatment group.Fe anode and Al anode treatment groups can respectively remove 41.02%and 28.86%of COD_Mn,19.86%and 10.29%of TAN in influent of MBBR,which reducing the processing load of MBBR and shortening the start-up time of MBBR.When the EC reactor was closed,the Fe anode treatment group has the largest change in the treatment efficiency of COD_Mnand TAN.Moreover,the accumulation of NO₂^--N existed during the start-up of MBBR,and the control group was relatively serious,followed by Al treatment group and Fe treatment group.In addition,EC could change the color and morphology of biofilm in MBBR,thus changing the structure of biofilm and microbial community structure.4.The effect of EC-MDF-MBBR coupling water treatment technology on the flow of"N"in RASIn this experiment,the effect of EC-MDF-MBBR coupling water treatment technology on the flow of"N"in RAS was studied.It was found that 34.22%of the nitrogen input was converted into harvested nitrogen,27.67%was removed by MDF,13.35%was removed from RAS by water exchange,10.89%was removed by FS,3.83%was removed from RAS in the form of shrimp shells,1.43%was removed by MBBR,1.32%was removed by EC reactor,0.71%was removed from RAS in the form of dead shrimp,and 6.57%was accumulated in RAS.The EC reactor not only removed nitrogen from the aquaculture water through electro-oxidation reduction,but also increased the removal of nitrogen by MDF and FS.MBBR was the water treatment unit with the highest energy consumption and cost in RAS.It could remove 26.30%of DN,but part of the absorbed DN would be converted into SN and returned to aquaculture water,resulting in the increase of SN in MBBR effluent.FS could not only remove SN,but also removed DN,and had a certain removal effect on TAN,NO₂^--N and NO₃^--N.In addition,the study found that the DN removal efficiency in RAS was only28.85%.And the water exchange of RAS would cause 13.35%of nitrogen to enter the surrounding environment.Purification and effective use of RAS tail water is an important guarantee for RAS as a green aquaculture method.