| Bioretention Cell?BRC?have many advantages such as low cost,good synergy with landscape and simple maintenance.Bioretention cells also play an important role in decentralized and small scale wastewater treatment.At present,the research on BRC is limited to the macro-analysis of hydrological/water quality effect,and there is a lack of in-depth evaluation of long-term operation performance,especially nitrogen removal effect.Analyzing the seepage process in each structural layer of BRC and carrying out the research on long-term operation efficiency has an important practical significance to improve its benefit of entire life cycle.In current study,the seepage characteristics and denitrification performance of the BRC system were tested and analyzed for treatment of sewage wastewater.Two methods were evaluated to enhance the comprehensive efficiency of the BRC system.Both methods are based on the experimental and simulation analysis of the seepage characteristics of BRC under the clean water condition.The main results of the current study are as follows:1)The simulation and theoretical analysis of BRC seepage characteristics under the clean water condition were carried out.Computational fluid dynamics?CFD?simulation shows that the flow distribution was uniform when hydraulic load was 1.0-1.5m3/?m2·d?.The tracer experiment results show that under this hydraulic load,the flow field distribution of the BRC filter structure was the most uniform,the hydraulic efficiency was the highest,and the seepage process was close to the push flow pattern.Results revealed that when the hydraulic load was 1.0m3/?m2/d?,the stability of the system takes 13 days,and the permeability coefficient decreases from 1.31×10-4m/s to 1.07×10-4m/s.On the basis of CFD simulation and experimental verification,the four-stage theory of"decrease-increase-then decline-stability"of BRC seepage coefficient was put forward.This phenomenon reveals the relaxation process of pressure expansion when the packing particles are disturbed by hydraulic load.2)Experiments on seepage characteristics and denitrification performance of BRC under sewage conditions were carried out.The results show that the performance of the system was the best when the hydraulic load was 1.0m3/?m2/d?.Under the short-term continuous operation condition,the hydraulic performance of the system was stable for 15 days,the total amount of overcurrent was0.075m3,and the permeability coefficient was decreased from 3.89×10-4m/s to 3.20×10-4m/s.Compared with the empty bed,the stability and performance were slightly lower,indicating that the microorganisms on the surface of the packing material have a certain influence on the seepage process.Under the long-term intermittent operation condition,the hydraulic performance was stable for 123 days,the total amount of overcurrent was 0.077m3,and the permeability coefficient was reduced to 0.89×10-4m/s.The time required for the hydraulic performance of the system to achieve stability was only related to the accumulated overflow,while the permeability coefficient of the hydraulic performance was related to the cumulative number of pressurized expansion of the system.3)The seepage characteristics of microbial fuel cell?MFC?coupled BRC system?BRC-MFC?and catalytic iron coupled BRC-MFC system?BRC-MFC-CI?were analyzed.Experimental conditions were as follows:MFC electrode spacing 30cm,hydraulic load of 1.0m3/?m2/d?,1 hour of influent and 7 hours of intermittent operation.It takes about 113 days and 95 days respectively for the hydraulic performance of BRC-MFC and BRC-MFC-CI to be stable.The results revealed that permeability coefficient was decreased from 3.89×10-4m/s to 1.86×10-4m/s and 2.15×10-4m/s,respectively.Results of Midi LOGGER GL820 electrode voltage,Miseq high throughput sequencing and confocal laser scanning microscopy?CLSM?show that compared with the BRC system,the additional electron transfer in the BRC-MFC system accelerates the growth of microorganisms.At the same time,the microbial community was increased from 14 to 19 and compared with the BRC-MFC system,the catalytic iron in the BRC-MFC-CI system increases the additional electron transfer and the microbial membrane grows faster.The biofilm on the surface of packing particles has the function of stabilizing the spatial structure of the system,which is beneficial to the earlier stability of the hydraulic performance of the system.The skeleton effect of biofilm slows down the influence of pressure expansion which is beneficial to increase the permeability coefficient of the system when the system is stable.4)The nitrogen removal performances of BRC,BRC-MFC and BRC-MFC-CI systems were also analyzed.The initial removal rates of ammonia nitrogen?NH3-N?and total nitrogen?TN?were 55.2%and 22.2%,respectively.Under the intermittent operation condition,the time required for the nitrogen removal performance of BRC,BRC-MFC and BRC-MFC-CI systems to achieve the stable removal efficiency was 130d,120d and 120d,respectively.At the same time,the removal rates of NH3-N and TN were 51.6%/15.9%,62.0%/52.2%and 64.8%/66.0%,respectively.The average voltage of BRC-MFC and BRC-MFC-CI systems reaches to 35.5mV and 120mV respectively,and the maximum power density was 103.83mW/m3 and 290.21 mW/m3.Under the two kinds of electric enhancement conditions,the electron migration in the system is beneficial to denitrification and improves the overall nitrogen removal performance.At the same time,there is a limit of dynamic equilibrium for microbial proliferation,and the time to reach the stability in the system was almost similar to the mass transfer.The research results provide a technical guideline to enhance the long term operation reliability and denitrification performance of BRC systems for the intermittent domestic sewage treatment. |
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