1 month ago


Water treatment

If the distribution of

If the distribution of the specific deposits (σ) is known as function of depth and time, existing headloss models which are functions of σ, can be used to obtain the headloss variation (h) during the filter run. Ohja and Graham (1994) modelled the normalized headloss observations of a SSF unit using the equations 2.2, 2.3, 2.6 and 2.10. They carried out computer-based calculations using a depth step of 1 cm and a time step of 1 day, and assumed that the headloss development in the surface skin of the SSF was not significant. In eq. 2.10 c 1 and c 2 are empirical constants and h 0 is the initial head loss (clean bead). ⎡ σ ⎤ h = ho ⎢1 + ⎥ ⎣ 1− p0 ⎦ c ⎡ p ⎤ 0 ⎢ ⎥ ⎣ p0 −σ ⎦ 1 c 2 (Sembi and Ives, 1982) (2.10) Phenomenological models can be considered empirical models because they are calibrated from actual filter runs and then used to predict filter performance as long as the water to be treated and the operation conditions are properly considered. The trajectory approach focuses on the transport phenomena, by considering granular bed as an assembly of collectors. The particles being transported along streamlines may be captured by interception or transported across streamlines to reach a collector surface. The particle attachment to the collector (removal from the flow) depends on favourable conditions for particle to grain, or particle to particle interaction. The concept was first proposed for air filtration in 1931. It was extended later to water filtration by O'Melia and Stumm (1967), and applied by Yao et al (1971). The single collector efficiency (η) for the three predominant mechanisms in water filtration were presented by Yao et al (1971). The contribution of the diffusion mechanism (η D ) for a single collector can be characterised by the dimensionless Peclet number P e , relating convective and diffusive transport, as follows, 2 3 ⎛ ⎞ 2 3 ⎜ kbT η (2.11) D = 4.04P e = 0.9 ⎜ ⎝ µ d p d V ⎟ c ⎠ in which P e =Vd c /D; D is the Stokes-Einstein diffusion constant, D=k b T/3πµd p ; k b is the Boltzman's constant (1.38x10 -23 J/ºK); T, is the absolute temperature; µ, is the absolute viscosity; d p , is the particle diameter; d c , is the collector (grain) diameter; and V the approach velocity. The contribution of the gravity (sedimentation) mechanism (η G ) for a single collector can be characterised by the dimensionless relationship between the Stoke's settling velocity of the particle (Vp) and the approach velocity (V), as follows, 2 ( ρ p − ρ) gd p ηG = (2.12) 18µ V in which ρ p is the particle mass density; ρ is the water mass density, and g is gravity acceleration constant. The contribution of the interception mechanism (η I ) for a single collector can be characterised by the dimensionless relationship between d p and d c , as follows 2 3 ⎛ d p ⎞ η = 2 ⎜ ⎟ (2.13) I ⎝ dc ⎠ 26

Yao et al (1971) related the removal efficiency in a packed bed filter with the single collector efficiency as shown in the following (2.14) equation, ∂C ∂L 3 (1 − P) ⎡3 (1 − P) ⎤ = − αη C = −⎢ ⎥α ηD G + 2 dc ⎣2 dc ⎦ [ + η η ] I C (2.14) in which α is the collision efficiency factor that accounts for the successful contacts between the particles and the collector, and η is the total collector efficiency, assumed to be the sum of the efficiencies associated to the individual mechanisms of transport. The trajectory analysis does not describe removal efficiencies as particles accumulate within the media, and indicates a sudden decline in η when surface interactions between particles and collectors become unfavourable (Payakates et al, 1974, quoted by Amirtharajah, 1988). However, the actual collection efficiency under a repulsive double layer potential shows a gradual and not a sudden decline, making necessary modifications to the trajectory analysis to account for unfavourable surface interactions (Amirtharajah, 1988). In spite of the limitations of the trajectory analysis to describe the filtering cycles, it has been useful to understand the significance of the removal mechanisms for different particle sizes as shown in fig. 2.6, and to underline the importance of chemical pretreatment in RF units. Figure 2.6 Significant removal mechanisms for a range of particle sizes (Ragapolan and Tien, 1979) The role of the diffusion, sedimentation and interception mechanisms should be even stronger in SFF than in RF because of the lower values associated with V, d c , and considering that a SSF sand bed has approximately twice as many collectors. Haarhoff and Cleasby (1991) show this on the basis of the review of previous work on trajectory analysis. Since the values associated with d c in SSF are about 1/2 of those in RF, then η I would increase by 4 fold, according to eq. 2.13. Considering roughly double number of collectors, the overall theoretical removal efficiencies of particles in SSF can be of the order of 8 times higher than in RF due to interception. Considering sedimentation, V in SSF is of the order of 100 times lower than in RF, so η D would be about 100 times greater. Assuming again double the number of collectors, η D would become 200 fold in favour of SSF (Alternatively, SSF could capture particles that are 1/200 or 5x10 -3 smaller than RF). Similarly, for diffusion, 27

Screen Filtration for Ballast Water Treatment Applications - Cross ...
Pall Aria™ AP-Series Packaged Water Treatment ... - Pall Corporation
Pall Aria™ AP-Series Packaged Water Treatment ... - Pall Corporation
Water Treatment Improvements and Plant Capacity ... -
The Treatment of Scottish Water for Private Communities
Microorganisms (The Coliform Group Bacteria)
Pilot Testing and Evaluation of Three Filtration Technologies - pncwa
Water Treatment Products - Colorfil
Capacity Charts for Water Treatment Systems
Precursor Removal from Ground Water Using GAC ... -
may 4th am water treatment.pdf(18358.4kb) - PNWS-AWWA
Water Quality Report - 2010 - Presidio Trust
Waste water treatment without chemistry Atec Advanced Oxidation ...
The best solutions in water treatment - Istobal
2003 City of Roanoke Water Quality Report - Western Virginia Water ...
Everpure Water Filtration - Lenox-Martell Inc
Council Bluffs Water Works South Water Treatment Plant – Planning ...
Technical Advances in the Treatment of Water and Air for ... - IAAPA
krones Hydronomic Water treatment for all areas of ... - Krones AG
Presentation - National Water Research Institute
Meshing Treatment Objectives, Water Quality Goals ... -
Understanding Emulsified Water Filtration - Donaldson Company, Inc.
City of Roseville - Evaluation of the New Compressible Media Filter
Water Encyclopedia: Domestic, Municipal, and Industrial Water ...
Weatherford in Waste Water Treatment (WWT)
Water Softeners and Wastewater Treatment Systems