Experimental study on the effects of sediment size and porosity on contaminant adsorption/desorption and interfacial diffusion characteristics

The joint effects of the sediment size and porosity on the contaminant adsorption/desorption and interfacial diffusion characteristics were experimentally investigated. The adsorption of Phosphorus (P) on the natural and artificial sediment suspensions was measured with respect to the P adsorption isotherms and kinetics in the experiment. The obtained adsorption isotherms for different grain-sized sediment suspensions fit well with the Langmuir equation, dependent on the initial aqueous concentration and sediment content. The P kinetic adsorption behaviors for cohesive fine-grained and non-cohesive coarse-grained sediment suspensions clearly show the size-dependent feature. On the other hand, the P kinetic release feature of a porous sediment layer is affected by not only the direct desorption of the uppermost sediments, but also the diffusivity in the pore-water within the underlying sediment layer, characterized by the sediment size and porosity, respectively. Furthermore, the temporal contaminant release from the permeable sediment layer into the overlying water column increases with the increasing flow velocity, while this enhancement in mediating the interfacial diffusion flux is somewhat insignificant in an immediate release stage, largely due to the resistance of the diffusive boundary layer on the hydrodynamic disturbance.     

Experimental investigation on diffusive contaminant release from permeable sediment layer under unidirectional unsteady flow

The interfacial diffusive contaminant（phosphorus） release from permeable sediment layer into overlying water column under a unidirectional unsteady（periodic） flow condition was experimentally measured and analyzed. The experimental results indicate that the gross diffusive contaminant release rate is substantially enhanced as compared to that under a steady flow condition, and this enhancement trend is much more pronounced in an immediate release stage. The interfacial diffusive contaminant release rate tends to increase with the increasing flow velocity, decreasing period and augmenting amplitude for the case of the unsteady flow. The additional interfacial diffusive contaminant release under the unsteady flow condition may be related to the hydrodynamic response of the diffusive boundary layer to the flow unsteadiness of the overlying water, depending upon not only the periodic thickness variation of the diffusive boundary layer immediately above the sediment-water interface modulated by the temporal flow velocity of the overlying water column but also the intensified turbulent mixing between the overlying water and the pore-water within the superficial sediment layer induced by an alternate acceleration/deceleration fluctuation during each period.     

Flow structure and phosphorus adsorption in bed sediment at a 90° channel confluence

The distribution of the phosphorus(P) adsorption in a bed sediment at channel confluences is an important issue for understanding the transport of contaminants in channel networks. In this study, the flow structure and its effect on the P distribution in the bed sediment were investigated in a 90° confluence flume. It is shown that the P adsorption amount in the sediment varies significantly in different hydrodynamic zones. The P adsorption amount is high in the flow separation zone where the horizontal velocity of the flow is very small, and it is low in the maximum velocity zone where the flow velocity reaches a maximum. The low P adsorption amount is observed in the downstream portion of the distorted shear layer, while the P enrichment is found in the upstream portion, as is related to the significant downwelling flow in this zone. Thus, the flow structure, especially, the flow velocity, has a significant effect on the distribution of the P adsorption in the bed sediment at the channel confluences.     

EXPERIMENT STUDY ON SEDIMENT INCIPIENCE IN BACKWARDFACING STEP FLOW

Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtained using the technique of Particle Tracking Velocimetry (PTV). The optical amplification technique was employed to measure the instantaneous flow velocities near the bed and the instantaneous bed shear stress was given. The experimental observations revealed a new insight into the oscillation of the large-scale structure and the three-dimensional characteristics of the flow. In particular,very high turbulence intensity,instantaneous horizontal velocity near the bed and the bed shear stress near the reattachment point were observed. The sediment incipient probability obtained from the sequent images of sediment particles near the bed indicates that the critical instantaneous shear stress of the sediment incipience is independent of flow conditions.     

Abrasion test of flexible protective materials on hydraulic structures简

In this study, several kinds of flexible protective materials sprayed with polyurea elastomers （hereinafter referred to as polyurea elastomer protective material） were adopted to meet the abrasion resistance requirement of hydraulic structures, and their abrasion resistances against the water flow with suspended load or bed load were studied systematically through tests. Natural basalt stones were adopted as the abrasive for simulation of the abrasion effect of the water flow with bed load, and test results indicate that the basalt stone is suitable for use in the abrasion resistance test of the flexible protective material. The wear process of the polyurea elastomer protective material is stable, and the wear loss is linear with the time of abrasion. If the wear thickness is regarded as the abrasion resistance evaluation factor, the abrasion resistance of the 351 pure polyurea is about twice those of pure polyurea with a high level of hardness and aliphatic polyurea, and over five times that of high-performance abrasion-resistant concrete under the abrasion of the water flow with suspended load. It is also about 50 times that of high-performance abrasion-resistant concrete under the abrasion of the water flow with bed load. Overall, the abrasion resistance of pure polyurea presented a decreasing trend with increasing hardness. Pure polyurea with a Shore hardness of D30 has the best abrasion resistance, which is 60 to 70 times that of high-performance abrasion-resistant concrete under the abrasion of the water flow with bed load, and has been recommended, among the five kinds of pure polyurea materials with different hardness, in anti-abrasion protection of hydraulic structures.     

Numerical research for contaminant release from un-suspended bottom sediment under different hydrodynamic conditions

Contaminant release from bottom sediments in rivers is one of the main problems to study the environmental hydrodynamics. Contaminant will release into the overlying water column under different hydrodynamic conditions through pore-water in sediment, the release mechanism can be roughly divided into convection diffusion, molecular diffusion and adsorption/desorption. In this article, phosphorus was as a typical contaminant with sorption. Through theoretical analysis of the contaminant release, according to different particle-sized and hydraulic conditions, the mathematics model of contaminant release can be established by the coupled Navier-Stokes equation, Darcy equation, solute transport equation and adsorption/desorption equation. Then that model was verified by flume experiment. Numerical studies show that, under different velocity, the instantaneous concentration of convection diffusion is about 6 times to 50 times larger than that of molecular diffusion during initial stages. The concentration of molecular diffusion is about 1 times to 4 times larger than to that of convection diffusion during late stages. Through analysis, the diffusive boundary layer near the interface can be obtained. In addition, the release will increase with particle size decreasing, and the release will be influenced much more by the size change when the particle size is relatively big under different velocity.     

Sediment rarefaction resuspension and contaminant release under tidal curren- ts

Based on experiment in tidal flume, this paper analyzes the sediment rarefactive phenomenon and hydraulic characteristics of sediment resuspension with different physical properties under the effect of tidal current. According to this experiment, sediment resuspension is related to the hydraulic characteristics of overlying water and its own dry density, namely the moisture content of sediment and deposition time. Generally, river sediment can be classified into the upper layer of floating sludge and lower layer of deposit sediment. Incipient velocity goes higher as the sediment layer goes thicker. Based on the experiment, incipient velocity formula of sediment can be obtained. There is a cohesive force among natural fine sediment whose resuspension is almost irrelevant to their diameters. Therefore, the critical incipient velocity is determined by the cohesive force instead of particle diameter. The lower layer of deposit sediment is generally not so easy to start up. And it will be rarified and release into the overlying water when contacting with overlying water. However, this rarefaction release velocity is gentle and slow. Under the same flow condition, annual loss amount of lower layer deposited sediment is about one fifth of upper layer of floating sediment. Flow velocity of tidal river and variation of the water level are asymmetrical, both of which vary under different tidal cycles. During long tidal cycle, flow velocity and water level change in the same phase and amplitude with tide. During the whole ebb and flow, flow direction does not change as the water level goes under the influence of acceleration and deceleration. As the tide cycle increases, the incipient velocity of sediment goes higher. This means that the long period tide cycle plays buffer effect on the resuspension of sediment, which makes the sediment not so easy both to start up and to suspend.     

2-D NUMERICAL SIMULATION OF CRUSH BEDROCK RIVER

In this paper, the erosion-resisting coefficient was introduced to compute bed deformation in a crush bedrock river. In the case of crush bedrock, there has been no proper control equation to describe bed stability, which leads to difficulty in calculation of the bed deformation with conventional methods. The data from field survey were used to give the erosion-resisting capability with an appropriate coefficient. After the determination of longitudinal distribution expressed by polynomial regression and transversal distribution expressed by normal distribution function, the plane distribution of erosion-resisting coefficient in a crush bedrock river was obtained. With the computational results from a 2-D horizontal flow mathematical model, the erosion-resisting coefficient and controlling condition of local stability were employed to compute the values of bed deformation when riverbed is stable.The above method was applied in a case study, and the computational results of flow and bed deformations are in good agreement with physical model test data.     

A STUDY ON NONEQUILIBRIUM TRANSPORT OF NONUNIFORM BEDLOAD IN STEADY FLOW

In the case of steady flow, an equilibrium state for an alluvial reach referring tothe input of sediment over a sufficiently long time (time interval in the order of months) is equalto the output of sediment. A nonequilibrium state exists under steady flow when the sedimenttransport rate changes with time and space, so that there is no balance between input and outputof sediment. The experiments were performed in an open circuit tilting flume with the bed slope of 0.001-0. 009, the flume length of 30m and flume width of 0. 5m. The boundary condition atupstream and in all experiments, the rate of sedimant supply was zero. The coal or sand was se-lected as the Wed material. The grain diameter of the bed material varied in the range of 0.05-20. 00ram. The equation of nonequilibrium transport of nonuniform bedload is derived. Thegrain distribution of the size distribution of bedload is calculated by modified Gessler's formula.The grain size distribution of bed material is calculated by CARICHAR mixed layer model. These equations are applied to numerical simulation of armoring of riverbeds. The results ofthe bedload transport rate and the grain size distribution of the bedload at the end of the down-stream in the process of time, as well as the grain size distribution of the bed material in the pro-cess of time and distance are all in close agreement with the measurements.     

RAYLEIGH-TAYLOR INSTABILITIES BY OVERTURNING EXPERIMENTS IN TANK

A laboratory study of the turbulence front initiated by the Rayleigh-Taylor instabilities was conducted by overturning a two-layer stratified flow in a tank. Dye was introduced to the upper layer and the concentration of the dye was determined using a video imaging method. The mass center of the heavier upper layer moved continuously to a lower elevation because of the intrusion of the heavier fluid into the lower layer and the lighter fluid into the upper layer. The downward movement of the mass center was accurately determined from the dye concentration profile. It is concluded that the initial advancement of the mass center follows a quadratic relation with time and the final advancement obeys a linear relation with time.     

曝气生物滤池降解规律试验研究

采用实际分散型生活污水作为试验原水,对曝气生物滤池的降解规律进行了试验研究。结果表明,与传统的观点不同,曝气生物滤池在处理低浓度有机废水时并不发生严格的有机物与氨氮的分段去除现象,有机废水氨氮与有机质基本在同一区域内去除,滤料在10～40cm段的去除率增长速度最快,在70cm之后去除率的增长开始逐渐变缓。在进口段,由于有机质浓度高,供氧充足,有机质的去除率较高;由于异养菌和氨化菌的作用,氨氮浓度在进口段不但不减少,反有微小升高。     

4种生物滞留填料对径流污染净化效果对比

为提高生物滞留系统净化效果,探讨了绿沸石、蛭石、活性炭、珍珠岩等材料用作特殊填料时对道路径流雨水中NH_4~+-N、TN、TP、COD等污染物的吸附性能。静态摇床实验研究了填料对各污染物的吸附能力,并建立了各填料等温吸附模型。结果表明沸石的综合吸附性能较好,活性炭次之,4种填料的等温吸附过程拟合结果基本符合朗格缪尔(Langmuir)吸附模型。以沸石作为特殊填料的小试结果表明,生物滞留系统对NH_4~+-N、TN、TP、COD的负荷削减率可分别达到70.59%、85.83%、79.98%、66.02%。从净化性能及经济性方面考虑,沸石作为生物滞留系统特殊填料是一种较好的选择。     

Experimental study on nitrification in a submerged aerated biofilter.

The aim of the present work was to evaluate the performance of a semi-pilot scale BAF in order to obtain a highly polished effluent in terms of removal of organic matter, suspended solids and ammonia and to observe the influence of temperature, pH and nitrite accumulation on the nitrification process. The ammonia removal efficiency during summer and winter and the nitrite accumulation in presence of free ammonia were observed. The biomass density was measured at different filter bed heights and the sludge production from the effluent of the backwashing water was evaluated. The results obtained were used to calibrate a mathematical model for the prediction of the ammonia removal profile in the filter bed and of biomass thickness.     

曝气生物滤地在污水回用中去除氨氮的试验研究

采用曝气生物滤地(BAF)工艺处理某污水处理厂模拟二级出水,研究了水力负荷及滤料层高度对NH_3-N去除效果的影响.试验结果表明,当进水NH_3-N≤25 mg/L时,平均NH_3-N去除率达到93.12%,出水NH_3-N可降至3 mg/L以下.当水力负荷为1.44 m~3/(m~2·h),HRT为1.8 h时,氨氮的去除效果最好,去除率达到95.32%,出水NH_3-N在1.5 mg/L以下.BAF对NH_3-N的去除作用主要发生在滤料的中上层.     

曝气生物滤池(BAF)用于建筑中水处理试验研究

曝气生物滤池是一种介于生物接触氧化法与生物流化床之间的生物膜污水处理工艺,其紧凑性、能耗及运行维护亦介于两者之间,非常适合用于建筑中水处理。为检验曝气生物滤池处理建筑中水的性能,分别采用人工配水和洗浴废水进行了试验测定和实际验证。验证试验表明,经曝气生物滤池处理后的洗浴废水几乎不需任何后续处理(混凝、沉淀、过滤)便可满足中水回用标准要求。     

Volatile organic compound adsorption in a gas-solid fluidized bed.

Fluidization finds many process applications in the areas of catalytic reactions, drying, coating, combustion, gasification and microbial culturing. This work aims to compare the dynamic adsorption characteristics and adsorption rates in a bubbling fluidized bed and a fixed bed at the same gas flow-rate, gas residence time and bed height. Adsorption with 520 ppm methanol and 489 ppm isobutane by the ZSM-5 zeolite of different particle size in the two beds enabled the differentiation of the adsorption characteristics and rates due to bed type, intraparticle mass transfer and adsorbate-adsorbent interaction. Adsorption of isobutane by the more commonly used activated carbon provided the comparison of adsorption between the two adsorbent types. With the same gas residence time of 0.79 seconds in both the bubbling bed and fixed bed of the same bed size of 40 mm diameter and 48 mm height, the experimental results showed a higher rate of adsorption in the bubbling bed as compared to the fixed bed. Intraparticle mass transfer and adsorbent-adsorbate interaction played significant roles in affecting the rate of adsorption, with intraparticle mass transfer being more dominant. The bubbling bed was observed to have a steeper decline in adsorption rate with respect to increasing outlet concentration compared to the fixed bed. The adsorption capacities of zeolite for the adsorbates studied were comparatively similar in both beds; fluidizing, and using smaller particles in the bubbling bed did not increase the adsorption capacity of the ZSM-5 zeolite. The adsorption capacity of activated carbon for isobutane was much higher than the ZSM-5 zeolite for isobutane, although at a lower adsorption rate. Fourier transform infra-red (FTIR) spectroscopy was used as an analytical tool for the quantification of gas concentration. Calibration was done using a series of standards prepared by in situ dilution with nitrogen gas, based on the ideal gas law and relating partial pressure to gas concentration. Concentrations up to 220 ppm for methanol and 75 ppm for isobutane were prepared using this method.     

空气曝气与纯氧曝气生物滤池应用于污水深度处理的研究

采用两种不同气源的曝气生物滤池对污水处理厂二级出水进行深度处理中试研究.结果表明,空气曝气和纯氧曝气生物滤池对COD_(Mn)的去除率相近,分别为30.5%和30.9%;以纯氧为气源的生物滤池硝化能力远强于空气曝气生物滤池,氨氮总去除率77.4%,出水氨氮为2.1～6.3 mg/L;纯氧曝气提高了滤池上部氨氮去除效果,维持滤池内溶解氧在6 mg/L以上,沿水流方向使pH逐渐下降.纯氧曝气生物滤池是污水深度处理中去除氨氮的有效工艺.     

三种改性吸附剂对氨氮去除的试验研究

以氨氮为处理对象,通过杯罐试验考察了分别用盐酸、氢氧化钠、氯化钠改性的膨润土、沸石、粉煤灰对氨氮的去除效果,初步遴选较优的改性吸附剂。试验结果表明:在水温25℃,振荡频率100 r/min,吸附时间2 h的条件下,改性后的吸附剂对氨氮去除率都有一定提高,氯化钠改性沸石对氨氮的去除效果最佳。     

北方城市库水季节性氨氮增高原因分析与对策

对北方城市水库水的特征,水库水季节性氨氮增高现象的原因进行了分析,并对常用的氯化法、沸石吸附法、生物陶粒接触氧化法、生物沸石法和扬水曝气法等工程措施进行了讨论.认为生物沸石过滤法是一种优势明显、应用前景广阔的微污染水处理方法.     

基于藻类连续生长的生物慢滤池深度净化污水的研究

利用藻类连续生长的生物慢滤池开展污水深度净化实验,通过分析生物慢滤池不同深度区域污染物浓度,考察其对氮磷等污染物质的降解规律。研究结果表明:在生物慢滤池中,总氮的去除主要依靠填料表层中微生物的降解和藻类吸收作用,其降解幅度可达31.5%;总磷的去除主要依靠滤柱深层的滤料的吸附作用以及上覆水中藻类的吸收,其降解幅度可达47.0%;氨氮浓度的降低主要依靠硝化转化作用和滤池上覆水表面的挥发作用,其降解幅度为21.8%。可见,生物慢滤池的上覆水和填料表面存在有大量的微生物和藻类,其对水质的净化起到了关键作用。