Treatment of drinking water residuals: comparing sedimentation and dissolved air flotation performance with optimal cation ratios

Spent filter backwash water (SFBW) and clarifier sludge generally comprise the majority of the waste residual volume generated and in relative terms, these can be collectively referred to as combined filter backwash water (CFBW). CFBW is essentially a low-solids wastewater with metal hydroxide flocs that are typically light and slow to settle. This study evaluates the impact of adding calcium and magnesium carbonates to CFBW in terms of assessing the impacts on the sedimentation and DAF separation processes. Representative CFBW samples were collected from two surface water treatment plants (WTP): Lake Major WTP (Dartmouth, Nova Scotia, Canada) and Victoria Park WTP (Truro, Nova Scotia, Canada). Bench-scale results indicated that improvements in the CFBW settled water quality could be achieved through the addition of the divalent cations, thereby adjusting the monovalent to divalent (M:D) ratios of the wastewater. In general, the DAF process required slightly higher M:D ratios than the sedimentation process. The optimum M:D ratios for DAF and sedimentation were determined to be 1:1 and 0.33:1, respectively. It was concluded that the optimisation of the cation balance between monovalent cations (e.g., Na(+), K(+)) and added divalent cations (i.e., Ca(2+), Mg(2+)) aided in the settling mechanism through charge neutralisation-precipitation. The increase in divalent cation concentrations within the waste residual stream promoted destabilisation of the negatively charged colour molecules within the CFBW, thereby causing the colloidal content to become more hydrophobic.     

Measurement and three-dimensional simulation of flow in a rectangular detention tank

There are two main ways to obtain better knowledge of the hydraulics of ponds, namely measurements and simulations. In this study, the applicability of using three-dimensional simulations as an engineering tool in stormwater pond design was investigated. To do this, three-dimensional simulations were compared with measurements of flow pattern and residence time in a large physical model of a detention tank (13 × 9 × 1 m). The agreement between measurements and simulations concerning both flow pattern and residence time distribution curves was found to be good for high flow rates.     

Investigating dissolved air flotation performance with cyanobacterial cells and filaments

Dissolved air flotation (DAF) performance with two different naturally occurring cyanobacterial morphologies was investigated with respect to the biomass removal efficiency, the toxin release to water and the coagulant demand by different water background natural organic matter (NOM). Coagulation (C)/Flocculation (F)/DAF bench-scale experiments (2 min coagulation at 380 s⁻¹ with polyaluminium chloride (0.5-4 mg/L Al₂O₃, the dose depending on the water NOM content); 8 min flocculation at 70 s⁻¹; 8 min DAF with 5 bar relative pressure and 8% pressurised recycle) were performed with single cells of Microcystis aeruginosa and Planktothrix rubescens filaments spiked in synthetic waters with different NOM contents (hydrophobic vs. hydrophilic NOM; moderate (2-3 mgC/L) vs. moderate-high concentration (ca. 6 mgC/L)). For both morphologies, the results show no apparent cyanobacterial damage (since the water quality did not degrade in dissolved microcystins and the removal of intracellular microcystins matched the removal of chlorophyll a) and high biomass removal efficiencies (93-99% for cells and 92-98% for filaments) provided optimal coagulant dose for chlorophyll a removal was ensured. Charge neutralisation by the polyaluminium chloride was the main coagulation mechanism of the M. aeruginosa cells and most likely also of the P. rubescens filaments. The specific coagulant demand was severely affected by NOM hydrophobicity, hydrophobic NOM (with a specific UV_254nm absorbance, SUVA, above 4 L/(m mgC)) requiring ca. the triple of hydrophilic NOM (SUVA below 3 L/(m mgC)), i.e. 0.7 vs. 0.2-0.3 mg Al₂O₃/mg DOC.     

Integration of dissolved gas flotation and nanofiltration for M. aeruginosa and associated microcystins removal

The removal of Microcystis aeruginosa and associated microcystins was investigated by a dissolved gas flotation (preceded by coagulation/flocculation)-nanofiltration (NF) sequence. The experiments were conducted with a freshwater spiked with M. aeruginosa cell aggregates to simulate a naturally occurring bloom. Two types of gases were used in the flotation pre-treatment, air (DAF) and a mixture of CO(2)/air. Very good results in terms of NF fluxes, overall removal efficiencies and final water quality were achieved with both sequences. However, the CO(2)/air mixture presented no benefit to the overall sequence, both in terms of toxin release to water during flotation and lower natural organic matter removal by NF, which was due to an overall negative effect of the acid pH. NF was able to completely remove cyanobacteria (100% removal efficiency of chlorophyll a) and microcystins (always under the quantification limit), regardless of the pre-treatment used and the water recovery rate (up to 84%). Therefore, DAF-NF sequence is a safe barrier against M. aeruginosa and microcystins in drinking water. In addition, it ensures an excellent control of particles, disinfection by-products formation, and other micropollutants that may be present in raw water.     

Effects of plume spacing and flowrate on destratification efficiency of air diffusers

This study adopts techniques of computational fluid dynamics (CFD) to analyze the combined effect of adjacent plumes of an air-diffuser system on its destratification efficiency. Lab experiments were carried out to calibrate and verify the CFD models in thermally stratified freshwater. The CFD simulation and lab experiment results were analyzed to relate destratification efficiency with four non-dimensional variables. The results indicate that destratification number, D(N), has the best relationship that includes air flowrate, stratification frequency, water depth, and bubble slip velocity. Since plume spacing and air flowrate are the major control variables of the system, especially in the field, two charts showing the relationships between destratification efficiency, plume spacing, and destratification number are developed for guiding their control in its design and operation.     

Study on inhaled air quality in a personal air-conditioning environment using new scales of ventilation efficiency

A detailed study using computational fluid dynamics (CFD) was conducted on the influence of the difference in the effective diameters of air supply openings (air velocity, assuming the airflow rate to be constant) when using personal air-conditioning (PAC) with isothermal air currents. A new method to analyze the age of air (SVE3*) for individual supplies and the residual lifetime of air (SVE6*) for individual exhausts was developed and proposed. The study focuses on the individual supply openings and exhaust openings in a room with multiple supply openings and exhaust openings when using PAC. PAC with the larger supply opening resulted in less mixture with the surrounding air and a lower age of air than the smaller diameter, which therefore indicated better ventilation characteristics.     

The influence of contact zone configuration on the flow structure in a dissolved air flotation pilot plant

The dissolved air flotation process is used in water and wastewater treatment. Among many parameters the fluid dynamics determine the capacity of the process. The contact zone is assumed to be important for the removal function, as it is believed to be the location for the aggregation of bubbles and flocs. This paper presents an experimental study on the flow structure in a contact zone of a DAF pilot tank and the influence of contact zone configuration. The flow structure in the contact zone was examined for different horizontal lengths of the zone and for different heights and inclinations of the shaft wall. The hydraulic surface loading was 11 m/h over the total tank surface area and the recycle rate was constant at 10% of the main flow. The examined hydraulic surface loading over the contact zone ranged from 40 to 98 m/h. Water velocities in the longitudinal, central section of the tank were measured with an acoustical Doppler velocimeter in a grid net for the different contact zone configurations, giving an insight into the flow structure. The result showed that the flow structure in the contact zone was characterised by a turbulent lower region and a plug-flow higher region. The hydraulic surface loading, a function of the length of the contact zone, seemingly determined the extension of the turbulent region. A higher hydraulic surface loading decreased the turbulent region while the lower loading increased it. A hydraulic surface loading of 65 m/h was suggested for design. It was not possible to determine the turbulent intensity quantitatively due to the measurement method. The height and inclination of the shaft wall did not seem to have a significant influence on the turbulent region. However, an increased height of the contact zone enhanced the higher, plug flow region and a recommended height of 0.81 m or higher for the recommended hydraulic surface loading was suggested when both mixing and plug-flow are desired. The separation zone was characterised by a stratified flow structure, mainly influenced by the cross-flow velocity that is a function of the distance between the shaft wall top and the water surface. A cross-flow velocity of 37m/h or higher resulted in a clearly defined stratification, believed to be crucial for an efficient separation of flocs. Finally, the extension of the lower, denser plug-flow region in the separation zone increased when the shaft wall height increased.     

Numerical investigation of the swirling air diffuser: Parametric study and optimization

During the recent decade, high induction diffusers have become more appealing in applications which require relatively high ventilation airflow rates, such as clean rooms. In this research, the effect of geometric parameters on the performance of a specific type of swirling air diffuser is investigated numerically. The results show that although the diffuser slots geometry, namely their angle and aspect ratio, is impressive on the diffuser performance, it is not as important as the swirling blade angle and the performance is almost constant in a wide range of slots specifications. The results also demonstrate that the diffuser performance and the resultant indoor airflow distribution highly depends on the blades angle and their sensitivity is surprisingly significant when the angle is between 30° and 35°. It is also worth mentioning that the optimum angle of 32° is almost independent of the diffuser airflow rate. Finally, the diffuser jet indexes, namely jet decay coefficient and entrainment ratio, are evaluated for the optimum diffuser and are compared with other common types of diffusers. This research and its results are useful in gaining a better understanding of the swirling diffuser airflow and the effect of diffuser geometry features on its performance.     

An approach to improve the separation of solid-liquid suspensions in inclined plate settlers: CFD simulation and experimental validation

The most important requirements for achieving effective separation conditions in inclined plate settler (IPS) are its hydraulic performance and the equal distribution of suspensions between settler channels, both of which depend on the inlet configuration. In this study, three different inlet structures were used to explore the effect of feeding a bench scale IPS via a nozzle distributor on its hydraulic performance and separation efficiency. Experimental and Computational Fluid Dynamic (CFD) analyses were carried out to evaluate the hydraulic characteristics of the IPS. Comparing the experimental results with the predicted results by CFD simulation implies that the CFD software can play a useful role in studying the hydraulic performance of the IPS by employing residence time distribution (RTD) curves. The results also show that the use of a nozzle distributor can significantly enhance the hydraulic performance of the IPS, which contributes to the improvement of its separation efficiency.     

Numerical simulation of wind flow over transverse and pyramid dunes

Wind tunnel experiment and computational fluid dynamics (CFD) simulation of the flow past a scaled transverse dune model were performed, after which a validated numerical method was used to evaluate the flow characteristics over a full-scale pyramid dune. The dimensional effect of the CFD simulation was analyzed by comparing the results of two- and three-dimensional (2D and 3D) models with the same mesh method and flow condition. There was excellent agreement between the wind tunnel measurements and the 2D and 3D sectional CFD results, proving that the simplified numerical model could simulate the sectional flow pattern of dunes. However, the lateral inhomogeneity of the flow field is significant although with simple transverse dunes, and 3D simulation is required to capture the lateral inhomogeneity of flow patterns after the dune crest line. Survey data of the topography of a pyramid dune, located at the eastern foot of the Mingsha Megadune, Dunhuang, China, were used to build numerical geometry. The flow fields over this pyramid dune under three inlet flow directions are significantly different from each other, and the lateral variances of flow patterns at different vertical planes are profound. The location, shape, and magnitude of the reverse flow zone change corresponding to the dune topography.     

Performance analysis of a ground-assisted direct evaporative cooling air conditioner

In this paper, the results of performance analysis of a ground-assisted hybrid evaporative cooling system in Tehran have been discussed. A Ground Coupled Circuit (GCC) provides the necessary pre-cooling effects, enabling a Direct Evaporative Cooler (DEC) that cools the air even below its wet-bulb temperature. The GCC includes four vertical ground heat exchangers (GHE) which were arrayed in series configuration. In order to have an accurate prediction of the optimum performance of a GCC, a computational fluid dynamic simulation was performed. Simulation results revealed that the combination of GCC and DEC system could provide comfort condition whereas DEC alone did not. Based on the simulation results the cooling effectiveness of a hybrid system is more than 100%. Thus, this novel hybrid system could decrease the air temperature below the ambient wet-bulb temperature. This environmentally clean and energy efficient system can be considered as an alternative to the mechanical vapor compression systems.     

The 24-h unsteady analysis of air flow and temperature in a real city by high-speed radiation calculation method

Heat island phenomenon is an important issue in environmental studies. Many studies involving observations and simulations have been performed. Computational Fluid Dynamics (CFD) analysis including the effects of solar radiation and longwave radiation heating/cooling are limited in the extreme conditions at midday, when solar radiation intensity are at maximum; and the 24-h unsteady analyses are not done due to the difficulties of the boundary conditions. Authors developed Computer Graphics (CG) method for calculating solar radiation and longwave radiation with high speed, and developed the 24-h unsteady analytical method from the data calculated by Weather Research and Forecasting (WRF). The integrated CFD was applied to the real city. The results showed that the integrated CFD was the useful tool to analyze the heat island phenomena.     

One-dimensional modelling of the secondary clarifier-factors affecting simulation in the clarification zone and the assessment of the thickening flow dependence

This paper presents a one-dimensional (1-D) model of the secondary settling tank (SST), and the drive for model development is discussed using steady-state simulation results generated with a 2-D computational fluid dynamics (CFD) model. Concentration profiles measured in a flat-bottom circular (fbSST) that is equipped with suction-lift sludge removal, served for CFD model validation. We investigate the factors that may deteriorate the 1-D simulation performance of clarifier models, which incorporate dispersion both in terms of the effluent sludge concentration and in terms of the sludge blanket height. Furthermore, dispersion-models can account for the effect of clarifier flow conditions on thickening performance. However, correlations, found in the literature, are shown to have limited efficiency under the wide flow boundary conditions examined in this study. Optimisation of the 1-D clarifier model structure is proposed by using an overflow dependent dispersion coefficient and including a feed flow dependent reduction factor in the downward convective velocity term. Results obtained show improved simulation performance in the clarification zone and allow for an efficient flow-dependency formulation of the thickening performance.     

CFD simulation on the air flow in a sauna

Sauna is the Finnish word for a wood-lined and insulated room, heated by a special stove containing stones, and erected specifically to create the right environment for a certain kind of dry bath. Its fundamental purpose is to induce perspiration and thus to cleanse the skin and body. The authors have now applied scientific data to the mystique and culture of the often misunderstood Sauna which has been used by the Finns for some two thousand years.     

暖通空调气流组织数值模拟的特殊性

从计算流体力学CFD应用于暖通空调工程的角度，阐述了通风空调房间内气流组织数值计算的特殊性和难点，对风口模型、湍流模型、辐射模型、热源模型、计算收敛速度以及复杂物理条件的描述等作了分析和介绍，并提出了相应的对策或解决思路。     

Particle dispersion and deposition in ventilated rooms: Testing and evaluation of different Eulerian and Lagrangian models

Indoor particle dispersion in a three-dimensional ventilated room is simulated by a Lagrangian discrete random walk (DRW) model and two Eulerian models: drift flux model and mixture model. The simulated results are compared with the published measured data to check the performance of the three models for indoor particle dispersion simulation. The deposition velocity of the particles is also computed and compared with published data. The turbulent airflow is modeled with the renormalization group (RNG) k−ε and a zero equation turbulence model. Comparison of the calculated air velocities with measurement shows that both the two turbulence models can simulate the airflow well for the presented case. For the Lagrangian DRW model, a post-process program is used to state the particle trajectories and transfer the results to particle concentration distribution. For Eulerian models, the effect of particle deposition towards wall surfaces is incorporated with a semi-empirical particle deposition model. The comparison shows that both the Lagrangian DRW model and drift flux model yield satisfactory predictions, while the predicted results by the mixture model are not satisfied. The deposition velocity obtained by the three models match the experimental data well.     

Numerical modelling of flow and transport in rough fractures

Simulation of flow and transport through rough walled rock fractures is investigated using the latticeBoltzmann method （LBM） and random walk （RW）, respectively. The numerical implementation isdeveloped and validated on general purpose graphic processing units （GPGPUs）. Both the LBM and RWmethod are well suited to parallel implementation on GPGPUs because they require only next-neighbourcommunication and thus can reduce expenses. The LBM model is an order of magnitude faster onGPGPUs than published results for LBM simulations run on modern CPUs. The fluid model is verified forparallel plate flow, backward facing step and single fracture flow; and the RWmodel is verified for pointsourcediffusion, Taylor-Aris dispersion and breakthrough behaviour in a single fracture. Both algorithmsplace limitations on the discrete displacement of fluid or particle transport per time step to minimise thenumerical error that must be considered during implementation.
2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.     

The effect of influent temperature variations in a sedimentation tank for potable water treatment--a computational fluid dynamics study

A computational fluid dynamics (CFD) model is used to assess the effect of influent temperature variation on solids settling in a sedimentation tank for potable water treatment. The model is based on the CFD code Fluent and exploits several specific aspects of the potable water application to derive a computational tool much more efficient than the corresponding tools employed to simulate primary and secondary wastewater settling tanks. The linearity of the particle conservation equations allows separate calculations for each particle size class, leading to the uncoupling of the CFD problem from a particular inlet particle size distribution. The usually unknown and difficult to be measured particle density is determined by matching the theoretical to the easily measured experimental total settling efficiency. The present model is adjusted against data from a real sedimentation tank and then it is used to assess the significance of influent temperature variation. It is found that a temperature difference of only 1 degrees C between influent and tank content is enough to induce a density current. When the influent temperature rises, the tank exhibits a rising buoyant plume that changes the direction of the main circular current. This process keeps the particles in suspension and leads to a higher effluent suspended solids concentration, thus, worse settling. As the warmer water keeps coming in, the temperature differential decreases, the current starts going back to its original position, and, thus, the suspended solids concentration decreases.     

Characterizing transportation of indoor gaseous contaminant using the state space method

This paper employs the state space method to characterize transportation of indoor gaseous pollutant in steady airflow field. From the differential equations governing contaminant transportation in space, the state space equation for transportation is proposed and the analytical solution is obtained. In the method, the matrix covering hologram of the transportation is derived. The state space equation is validated with the analytic solution for the case of the simultaneous transportation of the pollution for piston flow. Similarly, the concentration from the proposed method for a 2-D case also agrees well with the result from CFD method based on the experimentally validated flow field. Based upon the analytic solution of the state equation, it is easily known that the influence of the initial concentration distribution and the pollution source on the concentration at the specific point. In addition, assisted by Chen’s zero equation turbulence model [1], the concentration field for a 3-D case is simulated by the presented method. It is found that there exists a regular stage at which the relative effect of the initial concentration distribution and the source on the concentration field will not change with time.     

Numerical simulation of inter-flat air cross-contamination under the condition of single-sided natural ventilation

The objective of this study is to investigate the mechanism of inter-flat airborne disease transmission under the condition of single-sided natural ventilation. The focus is on one of the typical designs in residential buildings with a rectangular plan layout and having a common corridor separating the two sides, each of which has a flat façade with openable windows. When the wind speed is extremely low, with doors closed and windows opened, the flats become single-sided naturally ventilated driven by buoyancy effects. The air pollutants can travel from a lower flat to a vertically adjacent upper flat through open windows, caused by indoor/outdoor temperature-difference induced buoyancy. Computational fluid dynamics is employed to explore the characteristics of this process. Based on the comparison with experimental data about the air flow distribution in and around a single-sided naturally ventilated room, the renormalization group based k?ε model, together with carbon dioxide used as a tracer, is chosen to reveal this air cross-contamination. The simulation results are in agreement with our prior on-site tracer-gas measurements, revealing that the windows flush with a flat façade can be a major route of the air cross-contamination in high-rise residential buildings. Finally, an assessment index is proposed to evaluate the potential infection risks associated with this inter-flat air flow occurring in high-rise residential buildings.