Water. desalination + reuse

RESEARCH | 42 | Desalination & Water Reuse | August-September 2014 _________ Jun Kim, Lan Huong Doan, Hyung Keun Roh Doosan Hydro Technology ___ Editor's note: dissolved air flotation (DAF) is a well established method for removing suspended solids from wastewater that is gaining ground in the pretreatment of seawater for desalination. The authors describe a method for analyzing the flow of water in DAF to inform better design of DAF tanks. THE REMOVAL of suspended solids in seawater pre-treatment for desalination can be particularly challenging for operations in climes where difficult- to-remove microorganisms, present in the seawater, can prove damaging to membrane and thermal desalination. This is a particularly severe problem in the Middle East and North Africa (MENA) region where at certain times of the year the proliferation of harmful algal blooms is a feature. Dissolved air flotation (DAF) systems for the removal of suspended solids have gained a significant profile in wastewater and surface water treatment and are currently growing in importance in seawater desalination pre-treatment particularly in the MENA region. DAF removes suspended solids by lifting them on microscopic air bubbles to create a blanket of sludge at the surface Innovative Flow Analysis Method for High Loading Rate DAF Doosan's dissolved air flotation system is used in pre treatment at Ras Al Khair seawater desalination plant in Saudi Arabia. of a flotation tank (see box - DAF and volumetric analysis). The technique's growth in profile has arisen because of its competitive performance in removing particles and microorganisms to supply stable feed water for membrane processes even where harmful microorganisms are present. To improve further the performance of DAF treatment in desalination requires an improvement from the typical loading rate seen in wastewater treatment of 4-12 m/h to one of more than 30 m/h. To achieve high loading rates requires high accuracy in analysis and optimization of flow patterns in DAF tanks. To that end, a new profiling and three-dimensional visualization method for water flows using acoustic doppler velocimetry (ADV ) and Matlab computing language has been developed for application with any type of DAF or open channel system. The performance of DAF is usually facilitated by optimizing the hydrodynamic properties of the influent including velocity, flow direction, recycling rate and micro- bubble generation. Several researchers have, however, suggested different methods to evaluate design parameters to enhance its performance. Researcher Nick Fawcett 1 in 1997 described a computational fluid dynamics (CFD) modelling in the simulation of flow patterns and optimization of DAF design parameters. While computer simulation can provide fundamental guidelines for DAF geometry, it is still a theoretical estimation and usually limited to two-dimensional systems. FLow mEASuREmEnt mEtHoDS Other researchers have adopted ultrasonic doppler velocimetry (UDV ) or laser doppler velocimetry (LDV ) to measure flow velocity in DAF tanks.

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