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Nitrogen Removal of an Integrated A/O Membrane Bioreactor and Its Application on Shipboard Wastewater Treatment

Author: LiJianFeng
Tutor: YangFengLin
School: Dalian University of Technology
Course: Environmental Engineering
Keywords: Membrane bioreactor Nitrogen removal Membrane fouling Activated sludge Shipboard wastewater treatment
CLC: X703
Type: PhD thesis
Year: 2008
Downloads: 620
Quote: 5
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Nitrogen is one of the key nutrients that result in water eutrophication. Simultaneous removal of organic compounds and nitrogen in a single MBR has many advantages, such as savings of organics, decrease in greenhouse gas emission and less sludge production, in according with the concept of sustainable development, which has broad development prospects. However, membrane fouling reduces productivity and increases maintenance and operating costs, and is commonly considered one of the most relevant factors affecting the wide spreading of the MBR technology. It is crucial to study membrane fouling behaviors and fouling mechanism for membrane fouling control.By combining nitrification, denitrification and membrane filtration in a single reactor, an integrated A/O-MBR is estabilished. The reactor has the ability of simultaneous removal of organic matter and nitrogen. The nitrogen removal efficiency, mechanism, factors affecting nitrogen removal as well as many aspects of membrane fouling were systematically investigated in this study. Moreover, the performance of integrated shipboard wastewater treatment by Integrated A/O-MBR was also studied. This study was performed as follows:(1) COD and TN removal efficiency under steady state was 98% and 78% respectively. Due to the DO gradient in the reactor, bacteria community structure in aerobic zone and anoxic zone was different. The microorganisms from the aerobic zone had a higher level of nitrifying capability, while the microbes from the anoxic zone had a higher level of denitrifying capability. The aerobic zone and anoxic zone in this reactor acted as the active centers for nitrification and denitrification respectively, in which ammonia nitrogen was converted to nitrate, and nitrate was then converted to nitrogen simultaneously. As internal recycle rate increased, TN removal was increased, but the effect can be ignored when internal recycle rate was too high. It was found that HRT of minimum 8h was needed for complete nitrification and denitrification. However, TN removal efficiency was unchangeable regardless of the variation of HRTs between 8 and 12h. As COD/TN ratio increased from 4 to 10, the nitrogen removal efficiency increased. Thus higher COD/TN ratios benefit the nitrogen removal.(2) Membrane fouling rate of start up period was obviously higher than that of steady state. Bacteria community structure has changed as time elapsed, in which some bacteria lost their predominated situation while some others became dominated bacteria. Because of the substrate change from real wastewater to simple compounds, the bacteria diversity of the reactor decreased until the reactor went to steady state. As the reactor gradually stabilized, the MLSS concentration increased, EPS and SMP concentration decreased, flocs became bigger and looser. It is believed that the variations of microbiologically produced substances (EPS and SMP), morphological and rheological properties of activated sludge, which caused by variation of bacteria community structure induced to the variations of the membrane fouling rate in this period.(3) The impact of filamentous bacteria on membrane fouling was through their effects on activated sludge morphology. Filament Index (FI) has no statistical correlation with EPS and SMP. But as the backbone of floc, filamentous bacteria have an important impact on floc morphology. Higher FI value was introduced to the formation of bigger floc size with looser structure, the same trends vice versa. The filamentous bacteria have no significant impact on membrane-fouling rate in this case mainly due to the specific experimental condition.(4) The TMP profiles were different under super-critical flux and sub-critical flux conditions. In super-critical condition, the TMP increased linearly and the membrane fouling rate was relatively high. The major contributions of membrane fouling resistance came from specific cake resistance, and floc size and fractal dimension were the key factors affecting membrane fouling rate in this period. While in sub-critical operation, there was a point to discriminate the fouling behaviors into two-stage, firstly the TMP was increased gradually, after some critical times, TMP started to "jump". During sub-critical flux operation, slow accumulation of trace foulant increased the membrane resistance, the fouling of these substances induced to the heterogeneous of the membrane surface and lead to a distribution in local fluxes. The sudden rise in TMP is believed to be caused by local flux in some regions of the membrane increasing and exceeding the critical flux of the dominant foulant, which transit to super-critical flux condition. The sub-critical flux period has a relatively long period of TMP gradual increase, the frequency of membrane washing, membrane renewing, as well as the costs of maintenance was low under this condition, consequently it is better to operate MBR under sub-critical condition. Moreover, inhibiting EPS and SMP excretions by optimizing operational parameters is a practical tool to prolong the operation of MBR.(5) Most of the hydrocarbons in the fuel oil were biodegraded by activated sludge after 4h. Orthogonal tests showed that surfactant concentration and salinity have no obvious effects on oil removal efficiency. However, oil concentration and bacteria culture had statistical effects on oil removal. Oil removal was increased as oil concentration increased. The commercial culture had its priority over common activated sludge on oil removal. The main pollutants removal efficiency by Integrated A/O-MBR was COD: 94.5%, TN: 63.7%, TP: 33%, TSS: 99.9%, respectively. GC-MS results showed that the hydrocarbons in the influent were mostly removed by Integrated A/O-MBR. As Integrated A/O-MBR has the ability to treat integrated wastewater from ships, application of Integrated A/O-MBR could simplify the treatment process and reduce the cost of maintenance. Moreover, the water quality of effluent was quite better than the discharge standards of MARPOL and it can meet all the other strict requirements of the world.

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CLC: > Environmental science, safety science > Processing and comprehensive utilization of waste > General issues > Wastewater treatment and utilization
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