Laboratory and numerical experiments into efficient management of VDR filter plants

Abstract

One of the operation rules of variable declining rate (VDR) filters states that the longest filtration runs are achieved when the highest flow through the most recently backwashed filter and the highest available head loss coincide. This rule has previously been published by Dąbrowski, who used a simplified mathematical model of VDR filters developed by Di Bernardo. However, until now no experimental verification of this optimisation approach has been presented. A series of tests were carried out on a laboratory VDR filter plant, using a suspension collected from an industrial sedimentation tank and using alum as a coagulant. The results of the experiments were compared with computations carried out according to a combined unit bed element (UBE)-phenomenological model of deep bed filtration adapted to VDR operation by Mackie and Zhao. It was confirmed that the longest filtration runs did indeed correspond to the pairs of the highest H and q1/qavr. The UBE-phenomenological model of deep bed filtration gave results of computations quite close to experimental results and improved the accuracy of calculations based on the theoretical model by Di Bernardo.

Energy and Water Savings during Backwashing of Rapid Filter Plants

“…However, according to the calculations, a slightly better filtrate was obtained in the system with a constant filtration rate, when the filter cycles were of the same length [43]. First, theoretically [47], and then experimentally [48], it has been shown that in the case of filter plants controlled by variable declining rate operation systems, in comparison with constant rate systems, longer filter cycles, and thus saving water for backwashing, are achieved in the VDRF operation systems if the ratio of the hydraulic load of the most recently backwashed filter to the average hydraulic load of the all filters in the plant is higher than 1.2. This VDR system can be used for the time-varying turbidity of raw water [49,50], but it requires adjustment when changing the water temperature and it is troublesome to use this system in large stations due to small fluctuations in the water table above the filters, which makes it difficult to determine the appropriate time of subsequent backwashing in stations [51].…”