The flow patterns of bubble plume in an MBBR

The flow patterns of the gas-liquid two-phase flow in a Moving-Bed Biofilm Reactor (MBBR) have a critical effect upon the mass transfer by the convection.Bubble plumes promote unsteadily fluctuating two-phase flows during the aeration.This article studies the unsteady structure of bubble plumes through experiments.The time-serial bubble plume images in various cases of the tank are analyzed.The Recursive Cross Correlation-Particle Image Velocimetry (RCC-PIV) is used to calculate the velocities in those cases, and then the time-serial vortex, the total turbulence intensity, the time-serial streamline are obtained.It is shown that the aspect ratio and the void fraction are the dominant factors influencing the unsteady structure of bubble plumes.When the aspect ratio is unity and the void fraction is high, the bubble plumes see a symmetrical vortex structure with a long residence time, which is beneficial for optimizing the aeration system and enhancing the applied range of bubble plumes.     

水流作用下气泡帘的形成及防悬浮物扩散特性

气泡上升形成的气泡帘能较好地防止耙吸挖泥船作业引起的悬浮物扩散。采用CFD仿真的方法研究了在水流作用下气泡帘的形成过程及防悬浮物扩散机理。研究结果表明：气泡帘的形成过程受气泡孔直径、通气速度和水流速度的影响;增加通气量并选取合适的气泡孔直径可以增强气泡帘防止污染物扩散的能力,而过大的水流速度会导致气泡帘的失效;对于给定的工况,存在一个疏浚悬浮物的临界阻隔粒径。研究成果可为合理确定气泡防污帘的结构参数和操作参数提供理论依据。     

不同曝气量和纵横比下针孔喷射气泡羽流摆动特性的数值模拟研究

该文采用三维欧拉-欧拉法结合标准k-ε紊流模型模拟了矩形鼓泡塔内针孔喷射的气泡群的瞬时流动特性,并开展了纵横比和曝气量对气泡羽流摆动特性的影响分析。数值模拟结果很好地复现了低空隙率气泡羽流的周期性运动特性。当纵横比H/W=1.0时,气泡羽流的流动形态和结构稳定;当纵横比H/W=2.25和H/W=3.0时,气泡羽流的流动形态不稳定,呈现周期性摆动规律。在模拟计算中曝气量Q_0的范围为13.3 mL/s–170.4 mL/s。随着Q_0的增加,气泡羽流摆动周期减小并且趋于稳定,液体的含气率在增加并呈线性趋势;两种纵横比对气泡羽流的摆动周期和气体总空隙率的影响较小。     

生物倍增反应器气泡流态特性分析

利用激光粒子图像测速技术(PIV)对生物倍增反应器在7种不同进水流量下的气泡流态特性进行了测量和分析,利用特征正交分解方法对所测2个截面的气泡流场进行分解,提取流场中含能大尺度结构。结果表明, 进水流量的增加导致了气泡轴向速度减小,径向速度增大,气泡附壁效应增强;不同进水流量对气泡的扰流影响程度不同,气泡流态的速度场和涡量面积分布存在差异,在特定工况下气泡流场产生了涡旋。上部区域进水流量为0时,1阶模态占能比例最高,约为7.2%;下部区域进水流量为0.18 m3/h时,1阶模态占能比例最高,约为4.5%。低阶特征模态中存在明显的大尺度结构,且大尺度的流动结构与液相混合行为间接相关。当进水流量为0.90 m3/h时,2个截面的涡量面积分布最均匀,气泡死区最少,此时气泡流态最佳。在本试验条件下,进水流量为0.90 m3/h时,反应器整体气泡流态最佳。     

基于图像二值化处理的气泡羽流不稳定结构分析

气泡羽流是在曝气过程中产生的一种复杂的两相流动。对气泡羽流中气泡速度信息的提取是粒子图像测速算法(PIV)的核心部分。速度信息的提取即数字图像的预处理过程。本研究主要是在读取灰度值后,通过大津法得到相应的最佳阈值分割点,再采用该灰度阈值对图像进行二值化,以此来减少噪声对流场的影响,降低图像后处理的难度,这是气泡羽流的运动规律研究的一个重要过程。图像的二值化处理能有效地提高PIV技术计算流场的精度,为研究气泡羽流在曝气过程中产生的不稳定结构奠定了基础。     

Membrane bioreactor aeration: investigation of the velocity flow pattern.

Results of investigations concerning membrane bioreactor aeration are presented which were carried out at the Institute of Environmental Engineering of RWTH Aachen University (ISA) in cooperation with the Department of Chemical Engineering of the Technische Universit?t Berlin. In the field of industrial and municipal wastewater treatment the use of membrane bioreactors (MBR) is of increasing interest especially due to the high requirements on effluent quality nowadays. The design of aeration systems is a very important aspect of MBR development because it influences both cost of operation and filtration flux. The ISA has carried out tests concerning the velocity flow pattern in flat sheet membrane modules (developed by the A3 Water Solutions GmbH) to identify the effects of different aeration systems, aeration intensities and module constructions. The Department of Chemical Engineering is currently using the results obtained from the ADV to calibrate a numerical model which simulates two phase water and gas flow within an aerated membrane module. Optical investigations concerning the bubble distribution give a better understanding of the flow conditions in MBR. Developing a numerical tool for membrane module optimization concerning the hydrodynamics is the aim of the investigation of membrane bioreactor aeration.     

高速掺气水流的气泡级配

用针式掺气流速仪观测高速掺气水流中气泡的数量和直径。将小浪底工程泄洪洞中闸室原型与模型中相应测点的气泡资料对比后表明，原型中d＞2mm的气泡基本符合重力相似准则，可以在模型中模拟。     

Bubble dynamics and oxygen transfer in a hypolimnetic aerator

A hypolimnetic aerator operating in one of the City of Norfolk's water supply reservoirs was tested. Dissolved oxygen (DO) profiles, water flow rate, and gas-phase holdup were measured over a wide range of applied air flow rates. A model that was developed to predict oxygen transfer in a Speece Cone was modified to conform to the conditions of the hypolimnetic aerator. By varying a single parameter (the initial bubble size) the model was found to provide a close fit to the experimental DO profiles as well as the observed gasphase holdup. The model was used to show that a doubling in oxygen transfer may be achieved if initial bubble size is reduced from 5 mm to 2.5 mm. Knowing the initial bubble size, it should be possible to predict water velocity by incorporating the effect of momentum. Further work is now underway to test this approach and to examine the possibility of extending this generalized model to cover the range of hypolimnetic aeration and oxygenation devices.     

Evaluating a strategy for maintaining nitrifier activity during long-term starvation in a moving bed biofilm reactor (MBBR) treating reverse osmosis concentrate

A two-stage moving bed biofilm reactor (MBBR) was applied at the Bundamba advanced water treatment plant (AWTP) (Queensland, Australia) to treat the reverse osmosis concentrate (ROC) for inorganic nutrient removal. One of the operational challenges for the system was to cope with the large fluctuations of the ROC flow. This study investigated the decay rates of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) and biofilm detachment in MBBR during starvation for up to one month. An intermittent aeration strategy of 15 min aeration every 6 h was applied. This study also evaluated the activity recovery of both AOB and NOB after normal operation was resumed. The results showed that the activity loss of AOB and NOB was relatively minor (<20%) within 10 days of starvation, which ensured relatively quick recovery of ammonium removal when normal operation resumed. In contrast, the AOB and NOB activity loss reached 60-80% when the starvation time was longer than 20 days, resulting in slower recovery of ammonium removal after starvation. Starvation for less than 20 days didn't result in an apparent biomass detachment from carriers.     

Innovations in wastewater treatment: the moving bed biofilm process.

This paper describes the moving bed biofilm reactor (MBBR) and presents applications of wastewater treatment processes in which this reactor is used. The MBBR processes have been extensively used for BOD/COD-removal, as well as for nitrification and denitrification in municipal and industrial wastewater treatment. This paper focuses on the municipal applications. The most frequent process combinations are presented and discussed. Basic design data obtained through research, as well as data from practical operation of various plants, are presented. It is demonstrated that the MBBR may be used in an extremely compact high-rate process (<1 h total HRT) for secondary treatment. Most European plants require P-removal and performance data from plants combining MBBR and chemical precipitation is presented. Likewise, data from plants in Italy and Switzerland that are implementing nitrification in addition to secondary treatment are presented. The results from three Norwegian plants that are using the so-called combined denitrification MBBR process are discussed. Nitrification rates as high as 1.2 g NH4-N/m2 d at complete nitrification were demonstrated in practical operation at low temperatures (11 degrees C), while denitrification rates were as high as 3.5g NO3-Nequiv./m2.d. Depending on the extent of pretreatment, the total HRT of the MBBR for N-removal will be in the range of 3 to 5 h.     

掺气水流不同粒径气泡运动特性研究

采用三维数值模拟方法和离散相模型(DPM模型)模拟了不同比尺高水头直槽式泄洪洞和带反弧泄洪洞掺气水流中不同粒径气泡的运动特性,分析了不同粒径气泡对近壁掺气浓度的影响范围,研究成果可为掺气浓度的缩尺效应提供依据。分析表明,气泡的影响范围与流速成正比,与气泡的粒径成反比;泄洪洞底坡越小,气泡的影响范围越大;泄洪洞反弧段半径越大,气泡的影响范围越大。     

改进型移动床生物膜反应器处理有机废水的试验

改进型移动床生物膜反应器(CMCBR)是在普通移动床生物膜反应器中引入导流板,使填料在全池循环移动,消除了普通移动床生物膜反应器的死角。在CMCBR处理模拟生活污水的试验中,研究了有机物的去除效果,考察了容积负荷、水力停留时间、冲击负荷等参数对处理效果的影响。试验发现,在填料填充比例为50%(体积比),进水COD质量浓度为320~550mg/L,水力停留时间为3 h的条件下,出水COD质量浓度小于100 mg/L,达到国家污水综合排放标准的一级标准。反应器具有较强的抗冲击负荷能力,出水水质稳定。     

MBBR法处理桃浦工业区废水的中试研究

采用MBBR法处理桃浦工业区废水 ,同原有的SBR法相比无论在有机物的去除效果上还是对有机物的耐冲击负荷上都更优越。试验结果表明 ,当HRT为 6～ 15h时 ,MBBR法对COD的平均去除率大于 75%。     

Multivariate statistical analysis of a high rate biofilm process treating kraft mill bleach plant effluent.

This study reports on a multivariate analysis of the moving bed biofilm reactor (MBBR) wastewater treatment system at a Canadian pulp mill. The modelling approach involved a data overview by principal component analysis (PCA) followed by partial least squares (PLS) modelling with the objective of explaining and predicting changes in the BOD output of the reactor. Over two years of data with 87 process measurements were used to build the models. Variables were collected from the MBBR control scheme as well as upstream in the bleach plant and in digestion. To account for process dynamics, a variable lagging approach was used for variables with significant temporal correlations. It was found that wood type pulped at the mill was a significant variable governing reactor performance. Other important variables included flow parameters, faults in the temperature or pH control of the reactor, and some potential indirect indicators of biomass activity (residual nitrogen and pH out). The most predictive model was found to have an RMSEP value of 606 kgBOD/d, representing a 14.5% average error. This was a good fit, given the measurement error of the BOD test. Overall, the statistical approach was effective in describing and predicting MBBR treatment performance.     

Effect of organic loading rate on a wastewater treatment process combining moving bed biofilm and membrane reactors.

The effect of moving bed biofilm reactor (MBBR) loading rate on membrane fouling rate was studied in two parallel units combining MBBR and membrane reactor. Hollow fiber membranes with molecular weight cut-off of 30 kD were used. The HRTs of the MBBRs varied from 45 min to 4 h and the COD loading rates ranged from 4.1 to 26.6 g COD m(-2) d(-1). The trans-membrane pressure (TMP) was very sensitive to fluxes for the used membranes and the experiments were carried out at relatively low fluxes (3.3-5.6 l m(-2) h(-1)). Beside the test with the highest flux, there were no consistent differences in fouling rate between the low- and high-rate reactors. Also, the removal efficiencies were quite similar in both systems. The average COD removal efficiencies in the total process were 87% at 3-4 h HRT and 83% at 0.75-1 h HRT. At high loading rates, there was a shift in particle size distribution towards smaller particles in the MBBR effluents. However, 79-81% of the COD was in particles that were separated by membranes, explaining the relatively small differences in the removal efficiencies at different loading rates. The COD fractionation also indicated that the choice of membrane pore size within the range of 30 kD to 0.1 microm has very small effect on the COD removal in the MBBR/membrane process, especially with low-rate MBBRs.     

曝气池中气液两相流数值模拟

应用数值模拟的方法,对曝气池气液两相流运动规律进行了探讨。研究表明:当孔径相同,空气流量越大,紊动强度越大,气液混合越充分,曝气效果越好。对于相同的水深和相同的孔径,流量是影响气泡大小和速度的主要因素。当气体流量或速度增加时,气泡体积增大,气相速度也明显增大。     

Prediction of alpha factor values for fine pore aeration systems.

The objective of this work was to analyse the impact of different geometric and operating parameters on the alpha factor value for fine bubble aeration systems equipped with EPDM membrane diffusers. Measurements have been performed on nitrifying plants operating under extended aeration and treating mainly domestic wastewater. Measurements performed on 14 nitrifying plants showed that, for domestic wastewater treatment under very low F/M ratios, the alpha factor is comprised between 0.44 and 0.98. A new composite variable (the Equivalent Contact Time, ECT) has been defined and makes it possible for a given aeration tank, knowing the MCRT, the clean water oxygen transfer coefficient and the supplied air flow rate, to predict the alpha factor value. ECT combines the effect on mass transfer of all generally accepted factors affecting oxygen transfer performances (air flow rate, diffuser submergence, horizontal flow).     

THE VOID FRACTION DISTRIBUTION IN TWO-DIMENSIONAL GAS-LIQUID TWO PHASE FLOW USING IMAGE PROCESS METHOD

1. INTRODUCTION Bubble-driven plumes have been widely used in industrial plants or environmental applications. Including aeration in wastewater treatment, gas stirring of liquid metal, desertification of reservoirs,ice prevention in lakes, etc.. Generally…     

挑坎掺气槽过流空腔区掺气特性的研究

本文探讨了挑坎掺气槽过游空腔区的掺气特性。对掺气机理、临界掺气所条件、所泡上浮特性、紊动扩散系数、断面含气液度和通气量进行了较详细的理论分析。主要结果得到实测资料的验证。     

基于PIV测量技术的变曝气量下气液两相流速度场研究

粒子图像测速技术(PIV)作为一种无扰的全流场速度测量手段,在多相流研究领域具有较高的学术意义和实用价值。本研究针对曝气过程中的复杂气液两相流动,使用高速摄影机获取气液两相流流场的高帧图像,再通过图像预处理手段及PIV测速算法得到气液两相流速度场信息,并对不同工况下气液两相流的流态流速规律进行了研究分析。本文提出的图像预处理方案,通过采用图像的运动目标分割、降噪与反相等手段进行图像预处理,可以有效地减少后续速度场计算中的信息量及误差。研究结果为工业生产过程中有效提高曝气效率提供了重要的参考依据。