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 Dabrowski, 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 sedimen-tation tank and using alum as a coagulant. The results of the experiments were compared with com-putations 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-pheno-menological 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.