Study On COD And Nitrogen Removal And Optimization Of Oxic-Anoxic Treatment System For Ship Sewage

Ship domestic sewage directly discharged into the sea will cause marine environmental problems,like eutrophication and algal bloom.The International Maritime Organization（IMO）has set increasingly stringent discharge standards for ship sewage treatment,which restrict the ship domestic sewage can not be discharged directly without treatment,leading to an emergent need to develop advanced ship sewage treatment methods.However,recently developed treatment systems are too large to fit into the available space in a ship and unable to achieve the desired continuous stable nitrogen removal.Additionally,ship domestic sewage treatment system may encounter several unexpected situations,including change of influent wastewater composition,temperature and salinty variation,hydraulic load shock,etc.However,existing studies solely focus on reactor design and performance optimization,and it is difficult to take above sudden situations into consideration,which makes the sewage treatment system unable to deal with the possible faults in complex working conditions.In order to solve above problems,we develop a new ship domestic sewage treatment system,Oxic-Anoxic Micro-Sludge MBR（O-AMSMBR）,which incorporating multiple biological treatment technologies.Based on dynamic modelling and molecular biological tools,we investigate the mechanism of the synergistic removal of carbon and nitrogen in the ship domestic sewage treatment technology.Firstly,the removal efficiency of COD,NH₄⁺-N,NO₃^--N and NO₂^--N was investigated by orthogonal experiment under different COD and TN loads,C/N,p H and temperature.In addition,high-throughput sequencing was used to analyze the synergistic mechanism of different functional microbial groups in this novel marine domestic sewage treatment system.The results show that the O-AMSMBR can start-up quickly,which is suitable for ships.In common condition,the aerobic/anaerobic microenvironment is constructed due to the built-in settler and the existence of the anoxic zone in the aerobic tank.Therefore,the simultaneous nitrification and denitrification（SND）became the main nitrogen removal method in the O-AMSMBR.When the system operates in extreme conditions,such as low C/N,low p H and low temperature,the biofilm removal efficiencies of TN at anaerobic tank increases by 22%,although the TN removal efficiency of SND achieved by activated sludge drops,which proves the potential of"micro-anaerobic sludge effect"in actual ship domestic wastewater.Microbial sequencing data showed that the synergistic effect between nitrifying and denitrifying bacteria communities in aerobic zone and anaerobic biofilm are the key factors to affect the SND and"anaerobic micro-sludge effect",which ensures stable nitrogen removal efficiency under different influent conditions and environmental factors by O-AMSMBR system.Secondly,aiming at the disconnection between existing research experiment research and engineering operation in ship domestic sewage treatment,the dynamic kinetic model was established to explore the limiting factors of carbon and nitrogen degradation of ship sewage under different operating conditions.The potential solutions are also proposed in this study.Firstly,the feasibility of the model was tested by dynamic simulation,and the dynamics and stoichiometric parameters were determined by sensitivity analysis of the model,and then these parameters were validated by pilot experiment.We found that the high salinity and low C/N of marine domestic sewage had negative effect on the microbial activity of biological system,but the nitrifying bacteria were more sensitive to salinity.In addition,the denitrifying bacteria were more inhibited due to the low carbon content available for denitrification and the increasing dissolved oxygen in the anaerobic zone.In addition,NH₄⁺-N is accumulated at low temperature due to the inhibition of ammonia oxidizing bacteria（AOB）.It is suggested to install heating devices in the aerobic zone to improve the removal of NH₄⁺-N.However,the research of temperature sensitivity coefficient indicated that O-AMSMBR technology is more resistant to low temperature conditions.The experimental results of dynamic simulation show that the difference of key parameters between the pilot experiment setup and the actual wastewater treatment system mainly because some denitrifying strains can use refractory COD as carbon source for denitrification.Finally,new ship omestic sewage treatment technology may has a unsatisfactory COD and nitrogen degradation under sudden temperature changes and hydraulic load shocks,we used the the wavelet neural network（WNN）and dynamic model to optimize the condition.The simulation outcome of WNN shows that temperature and HRT have a major impact on the removal efficiency of COD and TN.Therefore,we can install a buffer pool and heating system to mitigate hydraulic load shocks and temperature variations.Different optimization approaches,like improving DO concentrations,sludge recycle rate,MLSS concentrations and adding carbon sources,were adopted to improve the TN removal efficiency of O-AMSMBR prototype.For low C/N marine domestic sewage,the optimal solution is to maintain the MLSS at around 5000-6000 mg/L in the reactor.For normal C/N ship domestic sewage,the optimization approach is to maintain the MLSS at around 6000-7000 mg/L in the reactor.Alternatively,increasing the sludge reflux ratio to 70%can also improve TN removal efficiency.The optimization strategy can be built into the control system by combining with hardware and software coordination.When the sensor detects the change of influent and effluent parameters,the control system can automatically adjust the operation parameters of the engineering prototype according to the encoded optimization strategy,which allows an intelligent operation of the ship sewage treatment system.