Influence of Media Characteristics on Energy Dissipation in Filter Backwashing

Abstract

Effective cleaning of granular filters during backwashing processes needs maximum turbulence and maximum shear in the fluid particle field. The energy dissipation in a backwashed filter as a particulate fluidized bed arises due to the suspending and random motions of particles and turbulent fluctuations in the bed. Size, density, and sphericity of the filter materials greatly influence the fluidization behavior of the media. In this study, a new model is proposed for predicting the energy dissipation parameters namely the hydrodynamic shear stress (tau(a)), the velocity gradient (G(a)), the turbulence dissipation coefficient (C(a)), and the turbulence parameter (C(a)0.5/Re) in backwashing of filters for different types of filter materials (sand, anthracite, and glass ball). The hydrodynamic shear stress is the dominant mechanism of filter cleaning and appears to increase with increasing the density and size of the filter media particles. Using the basic set of data, a step by step procedure is developed to compute the velocity gradient G(a), the turbulence dissipation coefficient C(a), the hydrodynamic shear stress tau(a), and the turbulent parameter (C(a)0.5/ Re).

Characterization of polystyrene granules as granular media filters

“…In the equations presented by Turan et al [8] and Naseer et al [9] (presented below) for the simulation of the physical parameters of the backwash in conventional media, sand and anthracite were used. Through this parameter, it is possible to identify the influence of density during the backwash and other physical parameters of PS granules.…”

“…The models presented by Turan et al [8] and Naseer et al [9], describe the behavior of the media filter during backwashing. In fact, the efficiency of backwashing depends on other aspects: the physicalchemical parameters, the number of active sites, and the specific surface of the grain, but the suggested models only consider the physical parameters.…”

“…Subsequently, there were related variables of porosity and solid fractions, obtained through of the superficial velocities assigned among 0-83.3 mm/s (0-300 m/h). Thus, it was necessary to follow the reasoning suggested by the authors [8][9][10] to get the answers of the physical parameters of backwashing. For all simulations, a temperature of 25˚C was considered.…”

“…Energy dissipation parameters: The model presented by Turan [8] was developed to describe the influence among different types of filter materials with regard to the energy dissipation parameters such as hydrodynamic stress, velocity gradient, turbulence dissipation coefficient (Ca), and turbulence parameter ðCa 0:5 =ReÞ in backwashing filters. Thus, the velocity gradient was developed as Eq.…”

Assessment methodology of backwash in pressurized sand filters

“…To perform a backwash, it is necessary to conduct previously filtered water through one or more filters of the filtration set, in the opposite direction of the filtration process (usually upward) with enough velocity to promote an adequate level of expansion and disturbance of the filter bed (Kau & Lawler, 1995;Turan et al, 2003;Ramirez et al, 2011). In filter bed expansion during the backwash, the required value of superficial velocity increases with the increment in the desired expansion level and with the filter media granulometry, varying its magnitude between the different construction models of sand filter drains (Ramirez et al, 2011;Mesquita et al, 2012).…”