Effects of Flow Structure on Particle Separation in Dissolved Air Flotation

Abstract

Dissolved Air Flotation (DAF) is a separation process in water and wastewater treatment. The removal function is based on reducing the density of the particle by producing aggregates of the particles and added micro-bubbles of air, thus lifting the aggregates to the water surface, where they can be removed as sludge. One parameter that is believed to be of crucial importance is fluid dynamics. The hypothesis is that complex hydraulic effects occur inside a DAF process for clarification, and that these effects have consequences for the separation of particles. Hydraulic loading, air content and internal geometry are believed to control the hydraulic behaviour inside a specific DAF tank. By mapping the flow structure and defining the relationship to these factors, it should be possible to partially predict the separation performance of the unit. The measurements were performed in a rectangular pilot tank. The hydraulic loading over the separation zone was varied in nine cases, from 11.3 m/h to 24.7 m/h, and the recycle rate varied from 5% to 15% of the head flow, generating air contents of 3-12 ml air/l water. Changes in internal geometry were achieved by modifications of the contact zone shaft wall. The flow structure was defined through measurements with an Acoustical Doppler Velocimeter and the separation efficiency was determined through measurements of suspended solids. Numerical and analytical analyses of the flow structure were performed with tracer measurements. A conceptual model was defined based on the cascade box model and the dispersion plug-flow model. The results show that a stratification of the water body exists inside the separation zone. The structure is generated by density gradients caused by spatial differences in air content. The stratification is achieved for the low to average hydraulic surface loading and for the average to high air-content. For the low air-content, a short-circuit flow structure was developed. There appears to be a relationship between the flow structure and the removal efficiency of particles. A high concentration of suspended solids seem to influence the short-circuit flow structure by a transformation towards a stratified flow structure. This, in turn, was observed to improve the separation of particles as well, further strengthening the hypothesis that the flow structure is important for the separation function and that a stratified structure should be maintained. Numerically, there seem to be a relationship between the estimated number of boxes in a cascade box model, calculated from the measured time-concentration curve (retention time distribution) from a tracer test. Visually, the time-concentration curve displays a high, narrow peak for the stratified flow structure and a low, more spread-out distribution for the short-circuit flow structure. Analytical analysis of the stratified flow structure seems feasible with the cascade box model or the dispersion plug-flow model in the upper layer of the stratification. However, the model can only be used for defining the stratified flow structure.

• author: Lundh, Måns
• supervisor: [unknown] [unknown]
• opponent: Prof Haarhoff, Johannes
• organization: Departments at LTH
• publishing date: 2002
• type: Thesis
• publication status: published
• subject: Water Engineering; Environmental Engineering
• keywords: plasma, Environmental technology, pollution control, Miljöteknik, kontroll av utsläpp, Gases, fluid dynamics, plasmas, Gaser, fluiddynamik, Teknik, Technological sciences, modelling, retention time distribution, acoustic doppler velocimeter, flow structure, separation, water treatment, wastewater treatment
• pages: 198 pages
• defense location: 2002-05-28, LTH, John Erikssonsväg 1, Lund, V-Building, V:A,
• defense date: 2002-05-28 10:15:00
• external identifiers: other:ISRN:LUTVDG (TVVA-1007) / 001-198 / (2002)
• ISBN: 91-628-5220-5
• language: English
• LU publication?: yes
• additional info: Article: Måns Lundh, Lennart Jönsson and Jan Dahlquist: Evaluation of an ejector-based air saturation system for dissolved air flotation. Submitted to Vatten. Article: Måns Lundh and Lennart Jönsson: Performance evaluation of the Acoustical Doppler Velocimeter in water with high contents of micro-bubbles. Submitted to Journal of Hydraulic Research. Article: Måns Lundh, Lennart Jönsson and Jan Dahlquist: The influence of contact zone configuration on the flow structure in a dissolved air flotation pilot plant. Water Research 36(6), pp.1585-1595 (2002). Article: Måns Lundh, Lennart Jönsson and Jan Dahlquist: Experimental studies of the fluid dynamics in the separation zone in dissolved air flotation. Water Research 34(1), pp.21-30. (2000). Article: Måns Lundh, Lennart Jönsson and Jan Dahlquist: The flow Structure in the Separation Zone of a DAF Pilot Plant and the Relation with Bubble Concentration. Water Science & Technology 43(8), pp.185-194. (2001). Article: Måns Lundh, Lennart Jönsson and Jan Dahlquist: Flow structures in a dissolved air flotation pilot tank and the influence on the separation of MBBR floc. In press for Water Science & Technology: Water Supply. Article: Måns Lundh, Lennart Jönsson and Jan Dahlquist: Relating the separation of biological floc from the Kaldnes moving bio bed process to the flow structure in a dissolved air flotation pilot tank. Submitted to Water Environment Research. Article: Måns Lundh and Lennart Jönsson. RTD characterization of the flow structure in dissolved air flotation. Submitted to Journal of Hydraulic Engineering.
• id: d932e0b5-869a-404f-8b7f-a435840f6aaa (old id 464631)
• date added to LUP: 2016-04-01 15:31:42
• date last changed: 2018-11-21 20:34:54