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The elevation of the clear well is between that of the filter beds and the effluent source of the sedimentation tank. During normal operation, this elevation difference allows effluent from the sedimentation tank to flow through the filter and subsequently to the clear well. Backwash requires approximately ten times the flow rate as normal operation. Filtered water can be sent to the clear by closing the valve leading to the distribution lines whereby water accumulates in the filter beds until the head difference between the filter and the clear well is sufficient and filtered water is diverted to the clear well.
We based our initial design on empirical granular filtration equations from existing literature (reference) on granular filtration. A bench-scale model of our system was built and tested to observe the discrepancy between the actual and the calculated fluidization velocity required to achieve a target bed expansion. We noted an increasing difference between those two sets of values that prompted us to hypothesize that, if we were to base our design on these empirical equations, we need to implement a significant safety factor.
Figure 1: Clear Well Filtration Design
We are discontinuing research of the Clear Well Filtration System because it is not economically feasible due to the large size of the clear well needed to back wash the filters.