Water. desalination + reuse

RESEARCH | 44 | Desalination & Water Reuse | August-September 2014 The UDV method requires additional pipe arrangements which can cause changes in flows that introduce errors. LDV is a highly accurate flow measurement method but still not suited to the analysis of three-dimensional flows. Moreover, the performance of both UDV and LDV is often limited in conditions where there are large volumes of micro-bubbles in water as encountered typically in seawater DAF pre-treatment for desalination. A third method using acoustic doppler velocimetry (ADV ) can overcome the drawbacks seen with UDV and LDV. Researcher Mans Lundh and coworkers 2,3 have suggested that an ADV based technique can be used to measure actual flow characteristics on different operation parameters. And there are ways to further enhance ADV applications for the analysis of DAF. OpERAtiOnAl SEtup The DAF system used in this study (figure 1) was designed for seawater application at a loading rate of 10-50 m/h. For verification of the ADV flow measurement method, bench-scale DAF was operated at 10 m/h with raw seawater obtained from the Tampa bay area in Florida, USA. The flow measurement in this study was then tested at a loading of 30 m/h on the DAF pilot with the same raw seawater. The recycling rate was controlled in the range of 10-20% at 75 psi, and then it was fixed to an optimized rate at each loading rate. Coagulants and flocculants were not used in this study so the evaluation of performance in solids removal was confined to flow patterns. REliAbility tESt To evaluate the reliability of the ADV, averaged flow velocity in certain planes was investigated in a bench-scale DAF. The calculated velocity indicated the plug flow velocity at each channel. The ideal velocities and measured velocities in different planes showed more than 95% accuracy. Measured velocity data was then also compared with CFD simulation (figure 3). The flow measurement using ADV was accurate and could be useful in making sure that CFD models are well simulated. tHREE-dimEnSiOnAl viSuAlizAtiOn The two shapes of the bench- and pilot- DAF tanks were plotted by computer-aided design (CAD) on an AutoCAD platform. Figure 1. DAF tanks: bench-scale (left) and pilot-scale (right). Editor's note: dissolved air flotation and velocimetric analysis DAF removes suspended solids from a wastewater or seawater stream. Contaminants are removed when air dissolved in the water stream is released from solution in the form of micron-sized bubbles that attach to the contaminants and carry them to the surface. Once at the surface the bubbles and contaminants form a floating bed of sludge that is removed by a surface skimmer. This reduces the total suspended solids, oils and greases and biochemical oxygen demand in the influent. The dissolved air is produced by injecting air under pressure into a stream of recycled clarified effluent from the DAF tank. This recycle stream is then combined and mixed with the wastewater or seawater influent in an internal contact chamber where the bubbles are released. laser doppler velocimetry LDV measures the velocity of microscopic particles in a fluid or solid surfaces. A laser beam is split and the two arms are made to cross in a space in line with a sensor. This is called the probe volume. Where the arms cross, an interference pattern is generated, seen as light and dark stripes inside the probe volume. These stripes are called fringes. As a microscopic particle passes across the fringes they reflect light from the light stripes which is picked up by the sensor. The distance between the fringes is a factor of the wavelength of the laser so the speed of the particle can be calculated from the fringe width and the time between each reflection. The direction of movement and therefore the velocity can be determined from the Doppler shift in the reflection created by the directional components of each particle's movement relative to the line of the laser arm. ultrasound doppler velocimetry In UDV a transducer sends regular short ultrasonic bursts and receives the echoes reflected by micro particles contained in a flowing liquid. By sampling the incoming echoes at the same time relative to the emission of the pulses, the variation in the positions of the echoes and the time lapse between the emission and the reception are measured to give the velocity of the particle. The term ultrasound doppler velocimetry implies that the velocity is measured by finding the Doppler frequency in the received signal, as it is the case in laser doppler velocimetry. In fact, in UDV, this is never the case. Velocities are derived from shifts in positions between pulses, and the Doppler effect plays a minor role. Accoustic doppler velocimetry In ADV a transmitter sends out a sound pulse at a fixed frequency which bounces off moving particles in the water. Three receiving probes aligned along three-dimensional axes pick up the change in frequency of the returned waves caused by the Doppler effect. The ADV then calculates the velocity of the flow in the x, y, and z directions.

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