Root stimulated nitrogen removal: only a local effect or important for water treatment?

Plants in constructed wetlands serve as carriers for attached microbial growth. They mainly transfer oxygen and release exsudates to the root zone. In consequence of this an area around the roots, called the rhizosphere exists, in which bacteria are stimulated by root growth. The goals were to ascertain whether stimulating the microbial cenosis only has a local effect on the rhizoplane, and to establish the importance of this stimulation for the water purification process in the root zone. Observations were carried out in a laboratory batch reactor filled with sand and planted with reeds (Phragmites australis). A small section was separated with gauze to avoid root growth outside this zone. The reactor was incubated with an artificial waste water containing a high concentration of ammonium. Samples were taken at intervals of 10 mm away from the gauze. The chemical and physical conditions and enzyme activities in soil sections at different distances from the roots affecting the efficiency of nitrogen removal were characterized. An influence was detectable by several parameters up to a specific root distance. Indirect parameters such as the total bacterial number and the DNA amount seem to be affected up to a distance of 50 mm from the root whereas the oxygen amount and DOC are unaffected at a distance exceeding 20-30 mm. This is an initial indication that improved nitrogen removal is also possible in the wider root surroundings. In view of the average root-to-root distance of 35 mm, the root-influenced area could therefore be expanded to the whole rooted zone in a constructed wetland. The influence on bacteria by roots is not just a local effect but may also play an important role in the whole purification process.     

The coupling between hydrodynamic and purification efficiencies of ecological porous spur-dike in field drainage ditch

In this study, a series of ecological porous spur-dikes are arranged in an experiment channel to simulate a real field drainage ditch. The inside and outside flow fields of spur-dikes are determined by numerical simulations and experimental methods. An Ammonia-Nitrogen(NH_3-N) degradation evaluation model is built to calculate the pollution removal rate by coupling with the inner flow field of the porous spur-dikes. The variations of the total pollutant removal rate in the channel are discussed in terms of different porosities and gap distances between spur-dikes and inlet flow velocities. It is indicated that a reasonable parameter matching of the porosity and the gap distance with the flow velocity of the ditch can bring about a satisfactory purification efficiency with a small delivery quantity of ecological porous materials.     

Mean flow,secondary currents and bed shear stress at a 180-degree laboratory bend with and without enhanced permeable groins as an Eco-friendly river structure

River restoration aims to apply environmentally-friendly structures for bank protection in meandering rivers to restore their natural habitat. Enhanced Permeable Groin (EPG) is a novel river restoration technique that can improve the fish habitat environment in a river system by creating a series of eco-friendly scour pools. This study reports the results of two groups of 3D velocity measurements in a 180-degree channel bend in cases with and without an EPG for clear water conditions to characterize the mechanisms leading to the primary stages of the scouring phenomenon. The analysis revealed that the presence of an EPG amplified the velocity magnitude in the regions near the tip of the vane and increased its value in the middle of the channel 1.13 times the bend without the structure. In addition, the comparison showed that the EPG reduced the velocity magnitude in the recirculation zone by an average of 38%. Secondary currents including main and outer bank cells were observed in the case without the structure. The presence of the EPG in the flow field effectively increased the outer-bank cell strength by 11 times compared to that without the structure. The low-value contours of the bed shear stresses were observed in the zone downstream of the structure for a distance of 6 times the effective length of the structure. Based on the results of this study, the generation of a recirculation zone with low-velocity and shear stress values can provide suitable conditions for aquatic habitats, deep-bodied fish assemblages, aquatic vegetation, shrub roots, and tree roots along the outer bank.     

A PHYSICAL MODEL FOR PREDICTING THE PRESSURE DROP OF GAS-LIQUID SLUG FLOW IN HORIZONTAL PIPES

A comprehensive treatment of all sources of pressure drop within intermittent gas-liquid flow is presented. A slug unit is divided into three parts and the pressure gradient of each part is calculated separately. In the mixing zone the momentum theory is employed and the mixing process between the film and slug is simulated by a two-dimensional wall jet entering a large reservoir to calculate the mixing length. The boundary layer theory is utilized to calculate the pressure drop for the slug body and the momentum equation of the film zone is integrated to calculate the pressure drop for the film zone. The pressure drop predicted in present model is in good agreement with all the measurements.     

黄河源草甸型弯曲河流的悬臂式崩岸机制

弯曲河流的滨河草甸对抑制河岸崩塌和维持河湾蜿蜒具有重要作用。2011—2014年,在黄河源区兰木错曲的弯曲河段对河岸植被根系、崩塌块几何尺寸、根土复合体物理和力学特性等开展野外调查,并通过原位拉拔试验测定河岸崩塌面植被根系抗拉强度,进而分析草甸型弯曲河道的崩岸机制。草甸型弯曲河流的滨河植被根系网络具有较强缠绕和固结土体作用,河岸根系的力学特性是抑制凹岸崩岸的关键因子。近岸水流淘刷作用集中于根系作用范围以下的砂卵石层,当砂卵石层被淘刷至上部悬空,在自重作用下达到临界状况,河岸沿纵深方向出现贯穿性裂缝,最后崩塌块垮塌贴住河岸。崩块的破坏形式以悬臂式张拉破坏为主。崩塌块的植被根系长度与崩塌块厚度呈明显对数函数关系,其体积也随根系长度增大而增加。分析草甸型弯曲河流的新崩岸模式,建立了近岸根土复合体崩岸的临界力矩平衡方程,并简化得到崩塌块的临界宽度表达式。崩塌块体的临界宽度是崩塌块根系长度和根系产生的抗拉强度增量的函数,临界宽度的计算值与实测值较为符合。     

干旱区不同灌溉方式下枣树根系分布特性研究

采用土钻取样和扫描分析法,对干旱区两年的滴灌和沟灌两种灌溉方式下的枣树根系分布进行了调查研究。结果表明:滴灌处理的总根长、吸收根根长、根尖数、分叉数以及根表面积和体积分别比沟灌提高了15.4%、13.9%、24.8%、39.7%、29.9%和47%,说明滴灌使根系增多,从而提高了枣树根系吸收水分和养分的能力。并用指数函数建立了枣树有效根长密度分布的二维模型,为干旱区枣树的滴灌技术改进提供依据。     

水库运行对下游河岸潜流带水位-温度影响研究

选取浙江省新安江大坝下游尾水至富春江水库某一河段作为试验场地,实时监测同一河流剖面的河道水位、温度以及河岸水位与温度分布。通过分析侧向潜流交换流量,定量描述侧向潜流交换的空间非均质性和动态特征,刻画河岸潜流带与河道水体水位、温度之间的响应关系。结果表明:本次试验剖面上的侧向潜流交换流量变化范围为-8.36~3.87 m2/s,监测时间段内单位长度上的潜流交换量可达2.49 m2;河岸带侧向潜流区同一剖面不同观测井处的潜流流量呈线性相关,且离河道越远,潜流区水位波动振幅消减越多,响应越滞后,潜流交换流量越小;受水库下泄水流影响,河道水温日波幅很小,河岸含水层具有显著的垂向温度分层。在垂直方向上,冬季浅层河岸温度较低,深层河岸温度较高,夏季则相反,同时浅层温度梯度较大,尤以地面以下0.8~1.7 m范围内温度梯度最大,深层温度梯度小;在水平方向上,离河道越远,受河道入渗水影响越小,温度越高。揭示了水库运行对下游河岸潜流带的影响规律,为河流潜流带生态影响评估提供参考。     

干旱区滴灌核桃树根系空间分布特性研究

为了研究核桃树根系空间分布特性,本文使用分层分段挖掘法对干旱区滴灌方式下核桃树根系总根长、根重、有效吸收根根长的空间分布规律进行了研究。研究表明：核桃树根系总根长水平方向主要分布在0～120cm处,占其总分布的90．84％;垂直方向主要分布在0～90cm处,占其总分布的78．75％。根重水平方向主要分布在0～30cm处,占其总分布的52．49％;垂直方向主要分布在15～75em处,占其总分布的61．12％。有效吸水根根长在水平距离60cm和垂直深度60cm处为分布密集区。在距离树干水平距离60cm和垂直深度60cm附近的根区,应作为核桃树水肥管理的重点区域。     

一年野生草本植物根系力学性能动态变化特征

土壤-根系复合体力学性能与根系力学性能密切相关。通过野外小区随机取样,测定了一年野生草本植物—狗尾巴草和稗草的根系参数和力学性能。结果显示:研究期间,狗尾巴草根系直径和根系密度参数随生长历时先增大后减小,稗草根系直径和根重密度随生长历时持续减小,而其含根量则随之先增大后减小;狗尾巴草根系直径小于稗草,但其含根量、根重密度、根长密度以及草根抗拉力和抗拉强度均大于稗草;两种植物单根抗拉力和抗拉强度分别与根系直径呈极显著幂函数正相关和对数负相关(P≤0.001),除较粗根系(0.6 mm)外,单根抗拉力和抗拉强度均随生长历时先增大后减小。试验结果有助于进一步了解土壤-根系复合体力学性能的动态变化特性征,以为固土护坡野生草本植物的选择提供一定理论依据。     

Analysis of flow around impermeable groynes on one side of symmetrical compound channel:An experimental study

This paper presents the results of an experimental study on the influences of floodplain impermeable groynes on flow structure,velocity,and water depth around the groyne(s).A wooden symmetrical compound channel was used.Groyne models with three different groyne relative lengths,0.5,0.75,and 1.0,were used on one floodplain with single and series arrangements.Analysis of the experimental results using the measured flow velocity and water depth values showed that flow structure,velocity,and water depth mainly depend on groyne relative length and the relative distance between series groynes.The flow velocity at the main channel eenterline increased by about 40%,60%,and 85%,and in other parts on the horizontal plane at the floodplain mid-water depth by about 75%,125%,and 175% of its original value in cases of one-side floodplain groyne(s)with relative lengths of 0.5,0.75,and 1.0,respectively.The effective distance between two groynes in series arrangement ranges from 3 to 4 times the groyne length.Using an impermeable groyne with a large relative length in river floodplains increases the generation of eddy and roller zones downstream of the groyne,leading to more scouring and deposition.To avoid that,the groyne relative length must be kept below half the floodplain width.     

交汇角对明渠交汇口污染物输运特性影响的数值模拟分析

针对等宽矩形断面明渠交汇口建立三维两相流水动力-污染物耦合数学模型,将污染物视作保守物质,不考虑其降解作用。该耦合模型以雷诺应力模型为基础,定量研究不同交汇角工况下交汇口的分离区几何特性及污染物输运规律。计算结果表明：交汇口分离区几何形状、污染物分布特性均存在明显的三维特征,且受交汇角影响;交汇角较大时分离区的几何对称性更强,不同交汇角下断面环流的位置不同,污染物的分布也不同,污染带最大宽度、混合界面宽度均随交汇角的增大而增大;分离区及其上游污染物混合速率随交汇角的增大而加快,在流速恢复区及其下游,混合速率基本不受交汇角影响,其沿程变化符合指数函数关系。     

华北山前平原灌溉农田深层土壤水分动态特征及渗漏量估算

以中国科学院栾城农业生态系统试验站的大埋深土壤水分剖面观测设施为依托,利用中子水分仪对15.4m深度的土壤水分进行连续1年的定位观测,同时通过定期采集深层土壤和水分样品,对该区土壤水分变动特征和深层入渗量进行了定量研究。结果表明:土壤水分在垂直剖面上的分布受土壤质地组成控制,同时在年内受降水事件和灌溉的影响而波动,随着土壤深度的增加,土壤水分的变异性呈减弱趋势;根区土壤水分受降水(灌溉)和作物耗水的共同影响而变化剧烈,根区以下土壤水分对降水的响应有一定延迟;在观测期间,整个15.5m深度的土壤剖面上有水分的盈余,土壤水贮量增加了216mm,但在小麦生育期土壤水分表现出耗损过程(土壤水贮量减少了208mm)。最后利用氯质量平衡法估算560cm深度处渗漏量约为65mm。     

明渠混合污染物侧向和垂向扩散系数的计算方法及其应用

明渠的侧向和垂向扩散系数是衡量岸边水流对污染物质混合输移能力的重要水质参数之一, 其值的准确与否直接关系到明渠水质预测预报成果的可靠性。基于污染混合区的理论计算方法,推导了污染混合区外边界标准曲线和曲面的统一方程,包含最大长度Ls、最大宽度bs和最大深度ds等特征尺度。该曲线形状近似于半椭圆,曲面形状为近似椭球体的一部分,表明污染混合区具有相似性。给出了由岸边污染混合区外边界最大长度、最大宽度或最大深度和平均流速确定侧向或垂向扩散系数的计算公式,提出了采用污染混合区面积或体积进行总体控制的侧向或垂向扩散系数计算方法和采用水面横向积分浓度确定垂向扩散系数的实用方法。通过现场观测结果分析,给出了洸府河下游河段枯水期的侧向扩散系数0.27 m2/s。     

数学模型在污水处理厂优化控制过程中的应用

通过小试结合模拟计算,分析了某污水处理厂现有运行存在的不足.试验表明,内回流比对提高脱氮率的作用是有限制条件的;对于溶解氧的控制,要根据系统硝化反应的需要来确定;缺氧区/总体积的比例是影响脱氮效果的一个关键因素,在保证硝化不受影响的前提下,增大缺氧区使反硝化进行得更充分,有助于提高系统的脱氮能力.通过模拟计算,对其工艺参数进行了优化组合,在不改变池容、不增加其他处理设施的情况下提出了优化方案,确定其最佳工艺控制参数为:缺氧区/总体积的比例为0.45,混合液内回流比为200%,好氧区溶解氧2 mg/L.调整后的出水总氮为21.77 mg/L,去除率较改造前提高了4%,出水氨氮浓度为1.16 mg/L,去除率提高了7.2 %.     

Removal of total suspended solids and turbidity within experimental vegetated channel: optimization through response surface methodology

Vegetation plays an important role in transport and settling of suspended sediment in natural waterways. In this study, effectiveness of emergent vegetation in an open channel for the removal of Total Suspended Solids (TSS) and turbidity is empirically investigated. To this end, response surface methodology (RSM) was applied to optimize three variables of vegetated channel including flow velocity at the entrance, density of vegetation, and the length of vegetation zone. Analysis of Variance (ANOVA) was employed to verify the precision of the mathematical models and their relative parameters. Optimum values generated by RSM were compared with experimental results of percentage TSS and turbidity removal and good agreement was observed.     

污染混合区的二维解析计算及移流扩散方程的分类简化条件

在恒定时间连续点源条件下，从移流扩散（离散）方程的完全二维解析解出发，给出了污染混合区的二维解析计算方法和等浓度曲线方程，分析了污染混合区的形状变化规律；以简化二维移流扩散条件下的污染混合区长度L为特征长度，定义了佩克莱特数Pe ，给出了污染混合区无量纲上、下游长度、最大宽度及相应纵坐标和面积的试算公式及诺莫图。表明污染混合区的无量纲尺度主要取决于佩克莱特数，其次无量纲最大宽度和面积还与 有关。给出了非保守物质污染混合区的修正计算方法以及保守与非保守物质的计算分区图；完整系统地提出了二维移流扩散方程定量化的分类简化条件，具有很好的可操作性和实用性。     

Coupling of the flow field and the purification efficiency in root system region of ecological floating bed under different hydrodynamic conditions

The artificial ecological floating bed is a commonly adopted in situ treatment technique for repairing and purifying polluted water. The plant root system of the floating bed is the primary region to absorb and degrade the pollutant of water. Its inner flow field characteristics and the interactive water quantity with the surrounding water greatly impact the purification efficiency of the floating bed. In this paper, the particle image velocimetry(PIV) technology and the boundary velocity direct extraction method are used to study the velocity distribution of the root system region by numerical simulations and experiments in an experimental water channel. A pollution removal rate(PRR) evaluation model is built to calculate the PRR by coupling with the flow velocity field of the root system region. The variations of the total pollutant removal rate(TPRR) are discussed for different center distances( L= 0.30 m, 0.45 m, 0.60 m), flow velocities( v= 0.007 m/s, 0.015 m/s, 0.025 m/s, 0.040 m/s, 0.055 m/s, 0.070 m/s) and root system porosities( P= 54.73%, 68.33%, 79.17%). The results indicate that the position arrangement of the floating beds influences the TPRR significantly, and the distance should be limited in a reasonable range for a high purification efficiency. Moreover, the root systems with higher porosity( P= 68.33%, 79.17%) have higher TPRR value than a lower porosity root system(P= 54.73%) within a certain flow velocity range, and the higher porosity root system has less fluctuation of the TPRR value than a lower porosity situation within a wide flow velocity range. Furthermore, under the same center distance condition, the lower flow velocity condition brings about a significantly higher TPRR value than the higher flow velocity situation.     

DESIGN OF IMPLANTABLE AXIAL-FLOW BLOOD PUMP AND NUMERICAL STUDIES ON ITS PERFORMANCE

This article presents the design of a new implantable axial-flow blood pump. The special feature of the flow channel inside the blood pump is that the blood is driven by a big-small tandem impeller installed in the inner hole of the cylinder magnet of a brushless direct current motor. The inner hole makes the main flow channel possible, while the gap between the inner end of the stator and the outer end of the cylinder magnet gives the shape of the tributary flow channel. There is no motor magnet inside the main flow channel, therefore, more blood can pass through it. The gap of the tributary flow channel is very small, but the blood flow in it is not blocked. Thus, the efficiency is increased and the volume and weight of blood pump can be reduced greatly. The outer diameter, length and weight of the manufactured implantable axial-flow blood pump are 29.6 mm, 76 mm and 158 g, respectively. The impeller spins at the speed of 9000 rpm and can generate a pressure head of 100 mmHg and a flow rate of 8 L/min. In an animal experiment, the blood pump has been successfully applied as a Ventricular Assist Device (VAD) in the chest of a small cow. Besides, a mathematical model is established to simulate the flow inside an axial-flow blood pump of implantable VAD. The numerical studies on the performance of the implantable axial-flow blood pump are carried out by combining this mathematical model and the Fluent software. The numerical results agree well with those of experiments, with the maximum error less than 10%.     

The association of E. coli and soil particles in overland flow.

The removal of E. coli from overland flow under saturation-excess runoff conditions was investigated in experimental field plots that were 1 m wide and 5 m long. Variation in the attenuation of bacteria and distance transported was quantified under contrasting flow conditions. In addition, the impact of soil tillage upon microbial attenuation was examined by comparing results derived from grassed plots (intact) with those subject to tillage with the soil left bare (cultivated). For intact plots subjected to a flow of 2 L/min, 27% of the E. coli in the flow was removed after 5 m with removal following a logarithmic function with respect to distance. For the higher flow rates of 6 L/min and 20 L/min, no attenuation trend was observed over this distance. E. coli removal during flow across the cultivated plots was significantly greater compared to the intact plots. This was attributed to a greater infiltration rate in the cultivated plots (due to the tillage) which promoted a greater volume of flow to pass through the soil matrix, providing the opportunity for filtration and adsorption of microbes. Logarithmic trends with respect to distance were observed for all flow rates tested on the cultivated plots (2, 6 and 20 L/min). Total removal after 5 m at a flow rate of 2 L/min was 41% and again removal efficiency decreased as the flow rate increased. Analysis of the transported state of the E. coli revealed that the bacteria were being transported predominantly in particles less than 20 microm in diameter and were not attached to large (dense) soil particles. The limited removal (< 50%) of bacteria from overland flow under saturation-excess runoff conditions in these experiments appeared, therefore, to be primarily due to a lack of settling or deposition. Instead, most bacteria remained entrained within the overland flow down the length of the plots.     

Velocity and concentration measurements in initial region of submerged round jets in stagnant environment and in coflow

Velocity and concentration fields are measured in submerged round jets in a stagnant environment and in coflow using laser Doppler anemometry and laser-induced fluorescence. Measurements are made in the initial region within distances of 40 jet exit diameter at jet Reynolds number between 1000 and 5000 and coflow-to-jet velocity ratio from 0 to 0.43. Different behaviors of jet spreading and dilution are found in jets at three different ranges of Reynolds number in which the jets are classified as initially laminar, transitional or turbulent. In the zone of established flow, the jet centerline velocity and concentration decay with downstream distance at different rates in the three groups of jets. For jets in coflow, axial development of normalized forms of centerline mean excess velocity and mean concentration at different velocity ratios can be reasonably well collapsed onto universal trends through the use of momentum length scale. Turbulence properties inside a jet are increased by the presence of a strong coflow. Inside the zone of flow establishment, some strange features are observed on jet turbulence properties. The length of zone of flow establishment increases from the turbulent jets, to the transition jets and to the laminar jets. The zone lengths for concentration are shorter than those for velocity by one to two jet exit diameters. Both lengths are shortened further in the presence of a coflow. For jets a stagnant environment and in the strong jet flow region of jets in coflow, jet widths increase linearly with downstream distance in transitional and turbulent jets. Self-similarity of radial profiles of mean velocity or excess velocity, mean concentration, turbulence intensities and concentration fluctuation level is explored in the zone of established flow.