Comparison of Backwash Models for Granular Media

Sholji, Ihsan; Johnson, F.A.

doi:10.1061/(asce)0733-9372(1987)113:3(532)

Cited by 10 publications

(6 citation statements)

References 5 publications

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“…The latter force does not change with backwash intensity, which is possible due to the increase in bed expansion. The amount of pressure loss h min caused by the flow through the fluidized bed can be calculated from Equation (9). It describes the equilibrium of forces at the moment of lifting all grains of the bed for a backwash intensity equal to the minimum fluidization velocity.…”

Section: Calculation Algorithmmentioning

confidence: 99%

“…The differences in energy consumed were mainly due to the different amounts of water required for backwash, and to a lesser extent the different bed resistances during fluidization. Bed resistance to flow during fluidization was determined from Equation (9), and power and energy from Equations ( 10)- (11). The results of calculations for filters washed with the same intensity are summarized in Table 6, with the intensity adjusted monthly to temperature in Table 7, and a comparison of both calculations in Table 8.…”

Section: Analysis Of Annual Energy Savings In a Rapid Filter Plantmentioning

confidence: 99%

“…Since flushing with water alone is not very effective [5][6][7][8], it was necessary to use various procedures to support this effectiveness. However, for a long time in Europe, backwashing started with vigorous mixing of filter bed grains with compressed air, during which sudden changes in pressure gradient occurred and grains rubbed against each other [9][10][11]. First, however, the raw water inflow is closed and the water table in the filter chambers is allowed to drop to a height only a few centimeters above the top surface of the filter bed.…”

Section: Introductionmentioning

confidence: 99%

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Energy and Water Savings during Backwashing of Rapid Filter Plants

Zielina

Dąbrowski

2021

Energies

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This paper describes an analysis of the effects of adjusting the intensity of filter backwash to the water temperature. The consequences of the lack of such adjustment for the life of filter beds, the amount of water used for backwashing, the amount of energy used for backwashing and the quality of the first filtrate are presented. In order to determine the losses and profits resulting from controlling the intensity of backwash water depending on its temperature, an analysis was carried out at a water treatment plant in southern Poland. Laboratory measurements were used to determine the granulation and specific gravity of sand grains filling the filtration beds. On the basis of measurements on a semi-technical scale, the magnitudes of filter bed expansion were determined for average monthly wash water temperatures. They were first calculated from the Richardson–Zaki equation, using different formulae for the value of the exponent of the power in this equation. Due to significant differences in the density and shape of grains covered with a permanent deposit after several years of filter operation, a satisfactory match between the formulae known from the literature and the results of expansion measurements was not obtained. Therefore, an new formula for the bed expansion was developed based on the Richardson–Zaki equation. A good fit of this formula to the experimental results was obtained. Monthly average values of water temperature were compiled, and on this basis the required amount of backwash water and energy was computed. The computations were made for 25% of fluidized bed expansion. Possible energy and water savings were estimated, as well as further gains from keeping the required expansion of the porous bed constant regardless of the wash water temperature.

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Section: Calculation Algorithmmentioning

confidence: 99%

Section: Analysis Of Annual Energy Savings In a Rapid Filter Plantmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Energy and Water Savings during Backwashing of Rapid Filter Plants

Zielina

Dąbrowski

2021

Energies

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“…Filter media fluidization has been intensively investigated and widely accepted formulae exist that relate expanded porosity to the superficial backwashing velocity and the settling velocity of a grain [21,22]. Some of these correlations include Reynolds and Galileo numbers [23,24] or are based on similar approaches [6].…”

Section: Basic Approachmentioning

confidence: 99%

A Model to Predict Granular Activated Carbon Backwash Curves

Dąbrowski

Spaczyńska

Mackie

2008

CLEAN Soil Air Water

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A mathematical model that produces curves for predicting the expansion of granular activated carbon (GAC) filter beds has been developed and applied to several widely used commercial GAC products. Initially, the hydraulic load required to produce a given bed expansion was assumed to be inversely proportional to the viscosity. This assumption was then modified by assuming a power law for the dependence on viscosity. The errors were sufficiently small to allow practical application of the formulae presented in this paper. The potential for application of the method was tested and is illustrated by an example.

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“…Other models by Sholji and Johnson (1987) and Quaye (1987) attempted to introduce a measure of the particle sphericity by using the particle settling velocity as one of the variables, but still required additional variables to obtain a reasonable fit. A recent model by Dabrowski et al (2002) reverts to an empirical approach, which requires a newly calibrated equation for every new media sample.…”

Section: A Review Of Mathematical Bed Expansion Modelsmentioning

confidence: 99%

Filter media expansion during backwash: The effect of biological activity

Clements¹,

Haarhoff²

2007

WSA

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Close observation at a number of South African water treatment plants has shown that media losses during backwashing are excessive-much higher than anticipated. The only likely reasons for this phenomenon are either that insufficient freeboard was provided by the designer or that the mechanical behaviour of the media gradually changes after being placed in the filters. A number of media tests confirmed that the biological fraction of the specific deposit on the filter media (after backwashing) is relatively high-about 50% of the total specific deposit. This led to the hypothesis that the combination of high nutrient concentrations in surface waters, coupled with elevated water temperatures, stimulate biofilm formation on the media grains. These films, in turn, somehow affect the mechanical behaviour of the media bed expansion and backwash. This paper reviews the Dharmarajah bed expansion model (as the most advanced model for media expansion to date) and presents evidence that it predicts the expansion of clean, ovendried media reasonably well. It further shows that media from filters which have been in operation for a while, expand significantly more than predicted by the Dharmarajah model. This finding has major implications for filter design, and suggestions are made on how to adapt design procedures for what is now believed to be the formation of biofilm on media grains.

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Comparison of Backwash Models for Granular Media

Cited by 10 publications

References 5 publications

Energy and Water Savings during Backwashing of Rapid Filter Plants

Energy and Water Savings during Backwashing of Rapid Filter Plants

A Model to Predict Granular Activated Carbon Backwash Curves

Filter media expansion during backwash: The effect of biological activity

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