Distribution and activity of ammonia oxidizing bacteria in a large full-scale trickling filter

The biofilm in a full-scale nitrifying trickling filter (NTF) treating municipal wastewater has been investigated with microbiological methods using fluorescence in situ hybridization (FISH) with 16S rRNA oligonucleotide probes in combination with confocal laser scanning microscopy (CLSM) and mathematical modeling using a dynamic multi-species biofilm reactor model. Ammonia oxidizing bacteria (AOB) were found to belong to the genus Nitrosomonas at different depths in the NTF at every sampling occasion, corresponding to different long-term operational conditions for the NTF. Both the measurements and the corresponding simulated predictions showed the same general trend of a decrease with filter depth of the amount of biofilm, the proportion of AOB to all bacteria and the total amount of AOB. The latter decreased by several times from top to bottom of the NTF. Measurements and simulations of potential ammonium oxidizing activity in the biofilm also showed a decreasing activity with depth in the NTF, which generally was operating at close to complete nitrification. However, no difference was observed when the activity was normalized to the amount of biofilm, despite decreasing proportions of AOB to all bacteria with depth in the NTF. This could be explained by diffusion limitations in the biofilm from the upper parts of the NTF according to the biofilm reactor model. The relatively good agreement between the simulations and the measurements shows that the kind of biofilm reactor model used can qualitatively describe an averaged behavior and averaged composition of the biofilm in the reactor.     

Approximate algebraic solution for a biofilm model with the monod kinetic expression

A steady-state biofilm model, which describes attached-biofilm reactors, is often derived by simultaneously considering microbial kinetics and diffusion through a biofilm. When Monod kinetics is used to describe the microbial kinetics in the biofilm, the resulting equation becomes analytically insolvable. In this paper, an approximate solution, having an explicit algebraic form, was developed using the orthogonal collocation method to relate the steady-state substrate flux into the biofilm to the bulk substrate concentration, and to predict the effluent substrate concentration from a continuous stirred-tank reactor. The accuracy of the approximate solution was evaluated by comparing the solution to an accurate numerical solution. The comparison indicated that the approximate solution is very accurate for the case in which the substrate fully penetrates the biofilm (i.e. a shallow biofilm), and is inaccurate for the case in which the substrate partially penetrates the biofilm (i.e. a deep biofilm). The accuracy was found to depend on biofilm thickness and substrate concentration in the bulk liquid. In addition, the region in which the approximate solution is accurate was graphically developed along with the regions of three limiting cases (i.e. zero-order, first-order and deep biofilm). These regions graphically illustrate where the approximate solution and each limiting case are valid in a two-dimensional space of substrate concentration and biofilm thickness.     

A compartmental model to describe hydraulics in a full-scale waste stabilization pond

The advancement of experimental and computational resources has facilitated the use of computational fluid dynamics (CFD) models as a predictive tool for mixing behaviour in full-scale waste stabilization pond systems. However, in view of combining hydraulic behaviour with a biokinetic process model, the computational load is still too high for practical use. This contribution presents a method that uses a validated CFD model with tracer experiments as a platform for the development of a simpler compartmental model (CM) to describe the hydraulics in a full-scale maturation pond (7 ha) of a waste stabilization ponds complex in Cuenca (Ecuador). 3D CFD models were validated with experimental data from pulse tracer experiments, showing a sufficient agreement. Based on the CFD model results, a number of compartments were selected considering the turbulence characteristics of the flow, the residence time distribution (RTD) curves and the dominant velocity component at different pond locations. The arrangement of compartments based on the introduction of recirculation flow rate between adjacent compartments, which in turn is dependent on the turbulence diffusion coefficient, is illustrated. Simulated RTD’s from a systemic tanks-in-series (TIS) model and the developed CM were compared. The TIS was unable to capture the measured RTD, whereas the CM predicted convincingly the peaks and lags of the tracer experiment using only a minimal fraction of the computational demand of the CFD model. Finally, a biokinetic model was coupled to both approaches demonstrating the impact an insufficient hydraulic model can have on the outcome of a modelling exercise. TIS and CM showed drastic differences in the output loads implying that the CM approach is to be used when modelling the biological performance of the full-scale system.     

气流床反应器液体停留时间研究

文章介绍在D100×1500的有机玻璃气流床反应器中,采用脉冲示踪法测定了气流床反应器液体停留时间分布,分析了喷嘴个数、液体流量和气体流量对液体停留时间的影响。基于实验观察和分析,建立了液体停留时间数学模型,模型计算结果与实验吻合良好。     

Residence time distribution of a model hydrodynamic vortex separator

This study investigates the macromixing within a hydrodynamic vortex separator (HDVS). The device is a scale model of a prototype unit and is operated with zero baseflow. The device under investigation is typically used for the removal of settleable and colloidal solids. The macromixing is investigated by conducting tracer experiments from which the residence time distribution (RTD) is obtained and interpreted to characterise the mixing regime within the HDVS. The method of moments and non-linear regression are used to obtain various RTD functions and flow-model parameters to aid in the characterisation of the device's mixing regime and the degree of any non-ideal flow behaviour. The axial dispersion model (ADM) and tanks-in-series model (TISM) are used in this study. The RTD imperfectly approximates a plug-flow distribution but, the device has some amount of dispersion and is equal to approximately 2–3 perfectly stirred tanks in series. The ADM seems to give a closer representation of the experimental curves compared to the TISM. The sludge hopper appears to be acting as a stagnant zone.     

Design optimization of a self-cleaning moving-bed bioreactor for seawater denitrification

The hydrodynamic behavior and denitrification capacity of a down flow self-cleaning moving bed bioreactor (MBBR) using floating packed-bed carriers were investigated. Water mixing in the MBBR was determined by tracer studies, which showed a completely stirred tank reactor. Eleven different types of patented plastic packed-bed carriers were selected based on their density, shape and specific surface area. Video studies of the carriers were used to determine operating conditions and select the Cascade 1A and Tri-Pack 1 carriers for denitrification experiments. Both carriers showed a similar performance level in terms of the denitrification capacity of the resulting MBBR. The improvement in liquid circulation is thought to explain the high denitrification rates that reached 27 gNm(-2)d(-1) and the maintenance of a thin biofilm. Increasing the overall liquid velocity profile led to an increase of up to 30% in the denitrification rate in conditions with a 1-month-old biofilm. This MBBR design seemed to control biofilm development and could easily be scaled up to denitrify seawater or freshwater systems.     

Residence time distribution in submerged biofilters

A mathematical model is presented for the residence time distribution in a submerged denitrification filter, in which pronounced tailing is experienced due to hold-up in the biofilm. Experiments with a dye tracer, Eosine-Y, and with Tritium show that adsorption of the dye makes it impossible to determine biofilm characteristics. The same is true for any tracer if nitrogen bubbles are present in the filter. An experiment with Tritium in the filter without bubbles indicates that the surface of the media is only partly covered with biofilm. Biofilm characteristics can be determined only with great uncertainty, due to uneven distribution.     

Experimental and modeling approach to study sorption of dissolved hydrophobic organic contaminants to microbial biofilms

A biofilm reactor was developed to investigate the sorption of polycyclic aromatic hydrocarbons (PAH) as model compounds for hydrophobic organic contaminants (HOC) to intact microbial biofilms at environmentally realistic concentrations. When operated as a differential column batch reactor, the system can be used to study the thermodynamics as well as the kinetics of the exchange of HOC between an aqueous phase and microbial biofilms. Organic carbon normalized partition coefficients (K(oc)) for phenanthrene, fluoranthene and pyrene were at the lower end of those known for other organic sorbents. Intra-biofilm diffusion coefficients (D) were calculated from decrease in solute concentration over time using a model for diffusion through a plane sheet and ranged from 0.23 to 0.45x10(-9)cm(2)s(-1) for the three PAH. These diffusion coefficients are about four orders of magnitude lower than those reported in literature for free aqueous solution. These data and the experimental approach presented here are useful to assess the importance of microbial biofilms for exchange processes of HOC in heterogeneous systems such as water distribution systems, membranes and aquifers, sewer systems or surface soils.     

A study of substrate removal in a microbial film reactor

A mathematical model is formulated describing the mechanism of substrate removal by a microbial slime over which a film of liquid, containing the substrate as dissolved biodegradable material, is flowing. It is assumed that a lack of either organic carbon, oxygen, or both simultaneously, can limit the overall rate of the process. Basic chemical engineering principles of interfacial mass transfer, diffusion and biochemical reaction are used in the formulation of the model and the resulting set of equations is solved by digital computer using typical kinetic parameters taken from the literature. Predictions of whether organic carbon, oxygen, or both simultaneously, limit the process, the substrate removal rate, and the active depth of the biofilm are made.Data were obtained in support of the model by measuring substrate removals on a vertically mounted experimental biofilm reactor over a range of hydraulic and organic loadings typical of industrial-scale operation. Good agreement between the experimental results and the model predictions was obtained with the exception of the data pertaining to hydraulic loadings approaching the minimum wetting rate. These data deviated from the predicted values at high substrate concentrations indicating that under conditions of low hydraulic load the model is less satisfactory for describing the system.Conversely it may be that at low hydraulic loads and high applied substrate concentrations experimental accuracy is poor.     

裂隙中滞水区对溶质运移影响的模拟分析

 无论是野外还是室内试验所涉及到的均是变隙宽的粗糙裂隙。重点探讨带滞水区的裂隙形态对其中溶质式示踪剂运移的影响及导致的运移特征变化。采用的模拟方法是考虑对流与分子扩散步骤的随机粒子追踪方法。数值模型区域选择均质导水介质。不同程度的分子扩散模式用于比较其对溶质运移结果——穿透曲线特征的影响。尝试从不同穿透曲线的特征分析来解析带滞水区裂隙的几何形态及相对分子扩散程度对溶质运移的影响。研究结果表明，在不带滞水区的单裂隙中，溶质几乎同时流出系统。对于相同分子扩散尺度下的带较宽或较深滞水区的裂隙，其对应的穿透曲线的高峰值明显降低并伴随着明显的“拖尾”现象。模拟结果还表明在较高尺度分子扩散条件下，其穿透曲线具有相类似的特征。     

曝气量对SBBR生物除磷的影响研究

为了考察曝气量对序批式生物膜反应器（SBBR）除磷效果的影响,采用厌氧/好氧交替运行的方式,通过控制好氧反应过程中的曝气量,研究了不同曝气量时SBBR的好氧吸磷效果,以及不同曝气量对生物膜脱落量的影响,并推导了生物除磷过程中生物膜内溶解氧的扩散模型。结果表明,曝气量是影响生物除磷效果的一个重要因素,为了满足生物膜内聚磷菌对氧的需求量,加快氧的传递速率,增加活性生物膜的厚度,加快聚磷菌的好氧吸磷速率,必须提高液相主体中溶解氧的含量。选择适宜的曝气量能够促进生物膜的脱落与更新,起到调控污泥龄的作用,从而增强生物除磷的稳定性。     

Epuration hydrocarbonne en couche fluidisee triphasique etude experimentale et modelisation

Biological treatment by immobilized cells on a submerged support allows important volume reduction of units owing to the high micro-organism concentration in the process. The efficiency of classical reactors, i.e. preoxygenated fixed bed, is, nevertheless, limited by oxygen supply to the reactive medium and the unit must be worked discontinuously because of the bed clogging.A 9.4 cm i.d. three-phase fluidized reactor has been tested in an attempt to solve these problems; the turbulence induced by air injection should remove the excess biomass. Step injections of synthetic sewage have been made for different values of the working parameters. Interesting efficiency results have been observed: elimination efficiency of 10 kg BOD₅ m^?3 day^?1 without recycle, is independent of space-time over a wide range.Limitations of a three-phase fluidized reactor have also been found: insufficient oxygen transfer to the liquid phase due to rapid bubble coalescence is a consequence of fluidizing small particles and inefficient sloughing which allows the reactor to be segregated depending on biofilm thickness and, sometimes, clogged in the upper part of the column.A model based on previous work in fixed bed has been proposed. Interphase mass transfer and zero order reaction within the biofilm are taken into account and the liquid phase is assumed to be perfectly mixed. The two parameters of the model are a combination of the numbers of transfer units within the biofilm and through the boundary layer and of the number of reaction units in the biofilm. The limiting substrate may be oxygen, organic carbon or any other nutrient.Experimental efficiencies can be predicted by the model with 15% accuracy. Particles size distribution must be corrected with regard to segregation.Physicobiochemical assumptions can be used in the modelling of a reactor which would not be perfectly macromixed and could be basically used in a unified theory.     

Epuration hydrocarbonne en couche fluidisee triphasique etude experimentale et modelisationHydrocarbon removal in a three-phase fluidized experimental bed and modelling of the process

Biological treatment by immobilized cells on a submerged support allows important volume reduction of units owing to the high micro-organism concentration in the process. The efficiency of classical reactors, i.e. preoxygenated fixed bed, is, nevertheless, limited by oxygen supply to the reactive medium and the unit must be worked discontinuously because of the bed clogging.A 9.4 cm i.d. three-phase fluidized reactor has been tested in an attempt to solve these problems; the turbulence induced by air injection should remove the excess biomass. Step injections of synthetic sewage have been made for different values of the working parameters. Interesting efficiency results have been observed: elimination efficiency of 10 kg BOD₅ m⁻³ day⁻¹ without recycle, is independent of space-time over a wide range.Limitations of a three-phase fluidized reactor have also been found: insufficient oxygen transfer to the liquid phase due to rapid bubble coalescence is a consequence of fluidizing small particles and inefficient sloughing which allows the reactor to be segregated depending on biofilm thickness and, sometimes, clogged in the upper part of the column.A model based on previous work in fixed bed has been proposed. Interphase mass transfer and zero order reaction within the biofilm are taken into account and the liquid phase is assumed to be perfectly mixed. The two parameters of the model are a combination of the numbers of transfer units within the biofilm and through the boundary layer and of the number of reaction units in the biofilm. The limiting substrate may be oxygen, organic carbon or any other nutrient.Experimental efficiencies can be predicted by the model with 15% accuracy. Particles size distribution must be corrected with regard to segregation.Physicobiochemical assumptions can be used in the modelling of a reactor which would not be perfectly macromixed and could be basically used in a unified theory.     

Mass transfer in a membrane aerated biofilm

We present experimental results of mass transfer of a non reactive tracer gas (neon) measured in aerobic heterotrophic biofilm developed from activated sludge. Biofilms are grown in various hydrodynamic conditions and the effective diffusivity is used to quantify the mass transfer through the biofilm. Beyond some cross-flow conditions, the effective diffusivity through the biofilm seems larger than in the bulk. This can be explained by a dispersion generated by convection inside the biofilm, as supported by an analytical flow model and in accordance to the numerical simulation proposed by Aspa et al. (2011).     

A simple biofilm model of bacterial competition for attached surface

A simple biofilm model of competition in bacterial growth for an attached surface is developed. Competition for the attached surface is expressed with the crowded and detachment effects. The developed model is verified by comparing simulated results with data obtained in the experiments of batch culture of nitrifier and continuous treatment of actual sewage with biofilm reactor. This model can favorably simulate the growth competition between autotrophic and heterotrophic bacteria for the attached surface. Then some parameters for nitrification process are discussed with this model. It is clarified that the effective removal of organic matter before nitrification tank is required for effective nitrification in the biofilm reactor.     

Combined aerobic heterotrophic oxidation, nitrification and denitrification in a permeable-support biofilm

A biofilm reactor, termed the permeable-support biofilm (PSB), was developed in which oxygen was supplied to the interior of the biofilm through a permeable membrane. The reactor was tested on filtered sewage supplemented with nutrient broth; the bulk solution was anoxic and the interior of the biofilm was supplied with pure oxygen. All tests were performed on a non-steady state biofilm with a depth of 1 mm. Mass balances on total organic carbon, ammonia, organic nitrogen and nitrate showed that combined heterotrophic oxidation of organics, denitrification and nitrification occurred simultaneously within the biofilm. The advantages of such a reactor are discussed.     

Multidimensional modeling of biofilm development and fluid dynamics in a hydrogen-based,membrane biofilm reactor (MBfR)

A two-dimensional, particle-based biofilm model coupled with mass transport and computational fluid dynamics was developed to simulate autotrophic denitrification in a spiral-wound membrane biofilm reactor (MBfR), where hydrogen is supplied via hollow-fiber membrane fabric. The spiral-wound configuration consists of alternating layers of plastic spacer net and membrane fabric that create rows of flow channels, with the top and bottom walls comprised of membranes. The transversal filaments of the spacer partially obstruct the channel flow, producing complex mixing and shear patterns that require multidimensional representation. This study investigated the effect of hydrogen and nitrate concentrations, as well as spacer configuration, on biofilm development and denitrification fluxes. The model results indicate that the cavity spacer filaments, which rest on the bottom membranes, cause uneven biofilm growth. Most biofilm resided on the bottom membranes, only in the wake of the filaments where low shear zones formed. In this way, filament configuration may help achieve a desired biofilm thickness. For the conditions tested in this study, the highest nitrate fluxes were attained by minimizing the filament diameter and maximizing the filament spacing. This lowered the shear stress at the top membranes, allowing for more biofilm growth. For the scenarios studied, biomass limitation at the top membranes hindered performance more significantly than diffusion limitation in the thick biofilms at the bottom membranes. The results also highlighted the importance of two-dimensional modeling to capture uneven biofilm growth on a substratum with geometrical complexity.     

驯化污泥固定化快速启动BAC反应器的研究

采用驯化污泥以动态挂膜和静态膜固定的方法进行了生物活性炭(BAC)反应器的快速挂膜和启动研究。将驯化污泥配制成500 mg/L的污泥菌液,控制挂膜污泥量为2~4 g/L、挂膜液循环流量为400~560 mL/min并以下向流循环方式运行,在1~2 d内就能实现BAC的稳定挂膜。启动后应用其处理某垃圾焚烧厂渗滤液处理站的生化处理出水,对COD的去除率70%,显著优于SBR法的处理效果。     

低溶解氧对生物膜特性的影响研究

通过考察生物膜的生长特性、除污效率、脱氢酶活性以及微生物群落组成,分析了低溶解氧对生物膜特性的影响,验证了在低溶解氧条件下采用生物膜反应器处理低浓度生活污水的可行性.试验发现,在低溶解氧条件下生物膜具有特殊的生长特性与外观结构,对COD、SS的去除率分别达到60%和90%以上;由于液相中的低浓度有机物向生物膜内的扩散通量较小,使其成为生物膜降解活性的主要限制因素;长期在低溶解氧下运行会使生物膜内环境更适合兼性异养菌的生长,而总的细菌含量变化不大,保证了生物膜具有较高的处理效率.这为降低污水处理能耗,拓宽生物膜反应器的应用范围提供了新的思路和理论基础.     

膨胀颗粒污泥床厌氧反应器的快速启动研究

以膨胀颗粒污泥床（EGSB）反应器生产实践为基础进行的处理模拟生活污水的试验发现：在启动过程中，接种污泥含有厌氧颗粒污泥菌种和适量的絮状厌氧污泥可以加快反应器启动；采用添加微量金属营养元素和进水COD负荷基本不变及稳定后增加负荷的三段式进水操作方式，有助于颗粒污泥的形成和加快反应器的启动；在培养过程中出现的反应器壁膜对颗粒污泥的生成可起到促进和辅助作用，并能提高反应器的抗冲击负荷能力。研究结果对EGSB反应器的生产实践具有重要的指导作用，可作为反应器实际运行的参考依据。