Aeration Performance of Triangular-Notch Weirs

Hydraulic structures have an impact on the concentration of dissolved oxygen in a river system, even though the water is in contact with the structure for only a short period of time. The same oxygen transfer that would normally occur over several kilometres in a river can occur at a single hydraulic structure, because the flow over a structure is typically highly turbulent, resulting in increased interfacial renewal. Plunging overfall jets from weirs are a good example of this fact, and the aeration properties of such structures have been studied widely in the laboratory and field over a number of years. This technical note (a) describes triangular-notch weirs having a different weir angle α and how they affect aeration performance, and (b) demonstrates that aeration efficiency decreases with increasing weir angle.     

Hydraulic experiments on spillway weir profiles

Spillway weirs are the system for regulating the flow in open channel flow passages. Weirs are commonly used to alter the flow of rivers to prevent floods, measure discharge and also in nuclear reactors for uniform cooling purposes. This paper describes the hydraulic experiments conducted for finding the air entrainment on water under laminar flow conditions in open channel flow passage with various profiles of spillway at flow rates in the range of 100–650?lpm. The concept of the spillway system allows stabilisation of the free level of water and avoids variation in the free level of water along the flow passage as a function of flow rate. The main problem in the spillway is the profiling of weir crest. The criteria that need to be satisfied are: (i) there should be no flow separation from the crest and (ii) there should be a uniform circumferential flow to avoid flow asymmetry in the passage. Separation of flow leads to large impact velocity of the falling water, which would lead to large-scale entrainment of air. When flow adheres to the weir crest and upstream, the impact velocity of water with the free surface would be less and hence entrainment would also be less.     

An experimental investigation of aeration performance in stepped spillways

Dissolved oxygen content is a prime indicator of water quality. The oxygen transfer across the air–water interface at a hydraulic structure, such as a weir or spillway, occurs by self‐aeration along the chute and by flow aeration in the hydraulic jump at the downstream end of the structure. Despite increased research activities in the field of stepped spillways, the aeration efficiency of stepped spillways is not yet known. This paper investigates the aeration efficiency of stepped spillways, in particular the effects of varying chute angle and step height. Empirical correlations predicting length of the nonaerated flow region and aeration efficiency were developed. The results indicate that stepped spillways are effective for oxygen transfer.     

核电站循环水虹吸井出水口改造设计

大亚湾核电站的循环冷却水为海水 ,虹吸井出水口的水流从溢流堰跌落后产生了大量的小水滴和盐雾 ,造成电气设备严重腐蚀 ,同时水流跌落后产生的大量微黄色泡沫严重影响海域景观。为此 ,在模型试验的基础上提出了棱块体挡流堰改造方案 ,并采取了相应的优化措施。模拟试验结果显示 ,除盐雾消沫效果良好     

Influence of mixed liquor recycle ratio and dissolved oxygen on performance of pre-denitrification submerged membrane bioreactors

The conflicting influence of mixed liquor recycle ratio and dissolved oxygen on nitrogen removal and membrane fouling of a pre-denitrification submerged MBR was investigated in this study. It was found that a high aeration rate of 10 L air/min was able to minimize membrane fouling as compared with lower aeration rates of 5 and 2.5L air/min in this study. Faster fouling at lower aeration rate was due to the decrease in cross-flow velocity across the membrane surface. However, high DO concentration (average of 5.1+/-0.5mg O2/L) present in the recycle mixed liquor at an aeration rate of 10 L air/min deteriorated the TN removal efficiency when operating at a recycle ratio of more than 3. A lower aeration rate of 5L air/min, resulting in an average DO concentration of 3.4+/-0.7 mg O2/L in the recycle mixed liquor, led to an improvement in TN removal efficiency: 63%, 80%, 84% and 89% for mixed liquor recycle ratio of 1, 3, 5 and 10, respectively. Further decrease in aeration rate to 2.5L air/min, resulting in an average DO concentration of 1.9+/-0.8 mg O2/L, did not improve the TN removal efficiency. Using a newly developed simplified nitrification-denitrification model, it was calculated that the COD/NO3(-)-N required for denitrification at 10 L air/min aeration rate was higher than those associated with 5 and 2.5L air/min aeration rates. The model also revealed that denitrification at an aeration rate of 10 L air/min was limited by COD concentration present in the wastewater when operating at a mixed liquor recycle ratio of 3 and higher.     

Development of a re‐aeration equation for the simulation of dissolved oxygen in degraded streams

Discharge of domestic and industrial wastewater into the water bodies is the prime source of degradation of the freshwater ecosystem. Re‐aeration of the river allows the rejuvenation of river water quality by absorbing oxygen from the atmosphere at the air‐water interface. An accurate estimation of the re‐aeration coefficient helps to determine the assimilative capacity of streams and wastewater management also. This paper aims to develop the re‐aeration equation and its validation by modelling dissolved oxygen of the Yamuna River. Predictive re‐aeration equations are used to identify their applicability to the study area and the new equation is designed using multivariate statistical regression techniques. QUAL2Kw model is used to validate the equation by modelling dissolved oxygen. Results indicate that the developed equation performs better than the predictive re‐aeration equations for the prediction of water quality.     

Entrainment and its implications on microclimate ventilation systems: Scaling the velocity and temperature field of a round free jet

Research on microclimate ventilation systems, which mostly involve free jets, points to delivery of better ventilation in breathing zones. While the literature is comprehensive, the influence of contaminant entrainment in jet flows and its implications on the delivery of supplied air is not fully addressed. This paper presents and discusses entrainment characteristics of a jet issued from a round nozzle (0.05 m diameter), in relation to ventilation, by exploring the velocity and temperature fields of the jet flow. The results show a trend suggesting that increasing the Reynold number (Re) reduces ambient entrainment. As shown herein, about 30% concentration of ambient air entrained into the bulk jet flow at Re 2541 while Re 9233 had about 13% and 19% for Re = 6537/12 026 at downstream distance of 8 diameters (40 cm). The study discusses that “moderate to high” Re may be ideal to reduce contaminant entrainment, but this is limited by delivery distance and possibly the risk of occupant discomfort. Incorporating the entrainment mixing factor (the ratio of room contaminants entrained into a jet flow) in performance measurements is proposed, and further studies are recommended to verify results herein and test whether this is general to other nozzle configurations.     

针阀在城市给水厂和污水厂中的应用

从避免气蚀和调节水、气流量等方面介绍了电动针阀在城市给水厂和污水处理厂中的应用，并结合杭州西区水厂和七格污水处理厂的实例阐述了电动针阀结合流量计在水厂输水管线流量调节中的控制方法，以及溶氧仪和针阀联动控制曝气池DO的应用。     

Predication of discharge coefficient of cylindrical weir-gate using adaptive neuro fuzzy inference systems (ANFIS)

Settlement of sediments behind weirs and accumulation of materials floating on water behind gates decreases the performance of these structures. Weir-gate is a combination of weir and gate structures which solves them Infirmities. Proposing a circular shape for crest of weirs to improve their performance, investigators have proposed cylindrical shape to improve the performance of weir-gate structure and call it cylindrical weir-gate. In this research, discharge coefficient of weir-gate was predicated using adaptive neuro fuzzy inference systems (ANFIS). To compare the performance of ANFIS with other types of soft computing techniques, multilayer perceptron neural network (MLP) was prepared as well. Results of MLP and ANFIS showed that both models have high ability for modeling and predicting discharge coefficient; however, ANFIS is a bit more accurate. The sensitivity analysis of MLP and ANFIS showed that Froude number of flow at upstream of weir and ratio of gate opening height to the diameter of weir are the most effective parameters on discharge coefficient.     

Particle Image Velocimetry (PIV) application in the measurement of indoor air distribution by an active chilled beam

We study the turbulent air flow behaviours of the attached plane jet discharged from an active chilled beam in a room using Particle Image Velocimetry (PIV). PIV is an innovative technology to study indoor air flow which began in the eighties of the last century for the measurement of whole air flow fields in fractions of a second. Here an experimental PIV system was built to reveal the structure of a turbulent attached plane jet in the entrainment process of the ambient air downstream from the jet slot. For the particle seeding in the PIV experiments, a few different particles were tested with the attached jet PIV application in a room. The results presented in this paper show the clear structure of the turbulent attached plane jet in the entrainment process after issuing from the chilled beam slot. The PIV visualisation results proved that the jet will attach to the ceiling and become fully turbulent a short distance downstream from the slot. The jet velocity vector fields show that the volume flow rate of the attached plane jet increases because of the large vortex mixing mechanism in the outer region of the jet. In three measurement cases, the air jet grows faster at a Reynolds number of 960 than at Reynolds numbers of 1320 and 1680. The calculated spreading angles in the cases with lower Reynolds numbers have similar values compared with the visualisation results.     

纯氧曝气用于污泥高温好氧消化的研究

采用纯氧曝气高温好氧消化技术(TAD)处理污水厂污泥,并与传统空气曝气进行了对比,系统考察了纯氧曝气高温好氧消化反应器的运行性能.研究发现,通过合理调节纯氧曝气量来控制反应器内的溶解氧浓度,可以实现污泥的高温好氧消化,7 d后对VSS的去除率可达40%以上,达到了我国城镇污水处理厂污泥的排放标准.纯氧曝气用于污泥高温好氧消化的氧气利用效率明显高于空气(<25%),最高可达80%.在达到同等污泥稳定化效率的情况下,纯氧曝气量远小于空气曝气量,由于尾气排放造成的能量损失较少,有利于反应器的保温.纯氧曝气速率过高会导致反应器内溶解氧的积累,而在高温条件下过高的溶解氧浓度(>15 mg/L)会对微生物产生一定的毒害作用.     

Nonlinear eddy viscosity modeling and experimental study of jet spreading rates

Indoor airflow pattern is strongly influenced by turbulent shear and turbulent normal stresses that are responsible for entrainment effects and turbulence‐driven secondary motion. Therefore, an accurate prediction of room airflows requires reliable modeling of these turbulent quantities. The most widely used turbulence models include RANS‐based models that provide quick solutions but are known to fail in turbulent free shear and wall‐affected flows. In order to cope with this deficiency, this study presents a nonlinear k‐ε turbulence model and evaluates it along with linear k‐ε models for an indoor isothermal linear diffuser jet flow measured in two model rooms using PIV. The results show that the flow contains a free jet near the inlet region and a wall‐affected region downstream where the jet is pushed toward the ceiling by entrainment through the well‐known Coanda effect. The CFD results show that an accurate prediction of the entrainment process is very important and that the nonlinear eddy viscosity model is able to predict the turbulence‐driven secondary motions. Furthermore, turbulence models that are calibrated for high Reynolds free shear layer flows were not able to reproduce the measured velocity distributions, and it is suggested that the model constants of turbulence models should be adjusted before they are used for room airflow simulations.     

On the reoxygenation efficiency of diffused air aeration

One technique used to increase the dissolved oxygen concentration of polluted waters is the bubbling of air through a diffuser pipe located at depth, thereby producing a bubble curtain from which oxygen transfer to the water occurs.

The results of laboratory studies on the aeration efficiency of a diffuser placed along the entire width of a flume, perpendicular to a cross flow are presented (two dimensional aeration). Parameters investigated include (1) diffuser type-porous materials with mean pore sizes of 40, 90 and 180 μm and perforated pipes with 0.4, 0.6 and 1.0 mm diameter ports (2) air flow rate per unit width 3–53 m³ (m h)⁻¹ and (3) cross flow velocity (2.5–15 cm s⁻¹. The effect of variation from the two dimensional situation is also discussed as well as the consequence of using oxygen instead of air, and the sensitivity to discharge angle, port spacing and the free surface. The measured efficiencies are compared with theory as well as available laboratory and field data.

The major conclusions are (1) aeration efficiencies using diffused air aeration are on the order of 2–13%m⁻¹ (2) the aeration efficiency increases with increasing cross flow velocity and decreasing air flow rate per unit width (3) aeration efficiencies using porous filters, for air flow rates less than 40 m³ (m h)⁻¹, are significantly higher (a factor of 2–3) than those achieved using perforated pipes (4) changing the pore size from 40 to 180 μm, the port size from 0.4 to 1.0 mm or the port spacing does not significantly effect the aeration efficiency (5) aeration using oxygen is somewhat less efficient than that using air. However, since equivalent oxygen bubbles contain approximately five times more oxygen than air bubbles, more oxygen is transferred on an absolute basis at the same gas flow rate using compressed oxygen as opposed to air (6) aeration efficiency resulting from aeration over a portion of the entire width can be reasonably predicted using the results of the two dimensional studies and (7) the available laboratory and field data compare well with the results of these laboratory studies.     

Dauerhaftigkeit der Verschlüsse von Schleusen und Wehranlagen

Durability of the gates of locks and weir plants. Hydraulic steel constructions are special steel structures. They have a high relevance for the operability and bearing capacity of a water‐structural plant and are subject to special requirements and operating conditions. Therefore high demands are made on their durability. Due to the immersion medium “Water” the steel structures of the gates of shiplocks and movable weirs are exposed to corrosion hazards exceedingly. Gates of ship locks are stresses by fatigue additionally. Concerning the steel structures of weirs corrosive, abrasive and/or hydrodynamic effects dominate. An overview will be given which methods and possibilities are used to achieve an appropriate durability during the intended service life under these circumstances. In this context the importance of a regular structure‐maintenance will be shown too. Furthermore new innovative gate structures may offer advantages with regard to fatigue strength, simplified maintenance and inspection possibilities and favorable corrosion protection characteristics.     

Oxygen transfer and uptake, nutrient removal, and energy footprint of parallel full-scale IFAS and activated sludge processes

Integrated fixed-film activated sludge (IFAS) processes are becoming more popular for both secondary and sidestream treatment in wastewater facilities. These processes are a combination of biofilm reactors and activated sludge processes, achieved by introducing and retaining biofilm carrier media in activated sludge reactors. A full-scale train of three IFAS reactors equipped with AnoxKaldnes media and coarse-bubble aeration was tested using off-gas analysis. This was operated independently in parallel to an existing full-scale activated sludge process. Both processes achieved the same percent removal of COD and ammonia, despite the double oxygen demand on the IFAS reactors. In order to prevent kinetic limitations associated with DO diffusional gradients through the IFAS biofilm, this systems was operated at an elevated dissolved oxygen concentration, in line with the manufacturer’s recommendation. Also, to avoid media coalescence on the reactor surface and promote biofilm contact with the substrate, high mixing requirements are specified. Therefore, the air flux in the IFAS reactors was much higher than that of the parallel activated sludge reactors. However, the standardized oxygen transfer efficiency in process water was almost same for both processes. In theory, when the oxygen transfer efficiency is the same, the air used per unit load removed should be the same. However, due to the high DO and mixing requirements, the IFAS reactors were characterized by elevated air flux and air use per unit load treated. This directly reflected in the relative energy footprint for aeration, which in this case was much higher for the IFAS system than activated sludge.     

Hydrodynamic parameters of diffused air systems

The influence of hydrodynamic parameters of diffused air systems on the oxygen transfer efficiency and the longitudinal dispersion of liquid in diffused air aeration wastewater treatment tanks have been evaluated using dimensional analysis, which involved the identification of significant parameters, development of equations to relate the dimensionless mass transfer and dispersion criteria with dimensionless hydrodynamic and geometric variables, and experimentation to determine the necessary numerical values for these equations. The developed relationships can be used as a guide for the design and optimization of aeration tanks.     

Jet entrainment effect on fire-induced flow in isothermal model applied to reduced-scale tunnel

Based on the entrainment rate of isothermal jet, an application method of the isothermal model in tunnel fire was presented through the more reasonable estimation of total smoke flow rate and its density at jet exit. Acetone LIF measurement was performed to identify the entrainment of ambient air qualitatively, and the entrainment constants were provided with the results of a previous study of which jet velocity corresponded to that of this study. The entrainment effect was more remarkable for the smaller fire, because the normalized axial distance to ceiling was increased with decreasing the fire size. These results suggested that the present model considering the jet entrainment might enhance the previous isothermal model in tunnel fire.     

A study of optimum aeration efficiency of a lab‐scale air‐diffused system

Up to now, tremendous efforts have been devoted to modelling the oxygen transfer coefficient (k_La) for diffused aeration systems, while not considering the corresponding energy consumption. Enhancing k_La is favorable for an exemplary oxygenation process, but may come at the cost of greater energy withdrawal, an unwelcome tradeoff. Assessing the aeration efficiency (the rate of oxygen delivered per unit energy) reflects the overall effectiveness of an aeration process and guarantees a superior system performance. Presented here is a lab‐scale study that investigates the effect of the orifice diameter, the airflow rate and the water column on the aeration efficiency. Various combinations of the studied parameters were tested using a cylindrical tank with a single orifice for air injection. An optimal performance of the aeration efficiency was observed at an orifice diameter of 0.3 mm when tested under 0.91 m water column and an airflow rate of 0.05 SLPM. Furthermore, a new empirical formula of aeration efficiency was established with a high correlation index (R² = 0.97) to allow preliminary prediction of aeration efficiency.     

Studies on dissolved oxygen concentration and sludge retention time affecting the full-scale activated sludge process

We have made a series of experiments on dissolved oxygen (DO) concentration and sludge retention time (SRT) to affect the full-scale activated sludge process.

For DO control, a better effluent water quality and a reduced current consumption have been achieved as compared with those in a constant air flow operation. The best control point for the DO control operation is at the outlet side of the aeration tank.

For SRT control, effluent COD seems to fluctuate slightly, and is favorably stable when SRT is between 10 and 20 days. It takes about twice as many days as SRT value set up for the total mass of sludge in the process (M) to be stabilized in case of making set value change as a result of simulation using mathematical models.     

复合生物反应器的同步硝化反硝化研究

以实际生活污水为处理对象,利用有效容积为12 L的间歇式复合生物反应器（填料填充率：30%,运行方式：瞬间进水—曝气660 min—沉淀40 min—排水20 min）,研究了DO、COD/TN值、MLSS对同步硝化反硝化的影响。结果表明：当溶解氧浓度从4 mg/L降到0.5 mg/L时,对总氮的去除率从48.9%升至74.2%;当污泥浓度从1 000 mg/L提高至6 000 mg/L时,对总氮的去除率从63.4%升至81.6%;当COD/TN值从3升至15.6时,对总氮的去除率从59%提高至82.5%,但当COD/TN值〉8后,对总氮的去除率提高得并不明显。整个试验过程中SVI〈105mL/g,污泥的沉降性能良好。复合生物反应器易于实现稳定的同步硝化反硝化,并可通过控制DO、MLSS等参数来有效提高对总氮的去除率。