Numerical study of a rational rule for the operation of variable declining rate filters in response to changes in raw water quality

A rule for the operation of Variable Declining Rate (VDR) Filter Plants under conditions of sudden changes in influent water turbidity has been tested using a mathematical model based on the Unit Bed Element (UBE) approach. The rule was postulated previously by Dabrowski and Marzec based on numerical simulation of a filter plant according to the mathematical model of Arboleda et al. Now a UBE model developed by Mackie and Zhao and adapted for Variable Declining Rate operation was used to investigate a plant behaviour in response to rapid changes in raw water quality. Two different cases have been considered. In the first, only the concentration of solid particles was the subject of change, and in the second the particle-size distribution changed as well. Numerical simulations were carried out under various conditions, with backwashing started when the water level above the filters reached the same position. It was found that this resulted in almost identical flow rates through each of the filters, but the length of Miter runs and the effluent quality were significantly different. For waters of stable temperature but different turbidities backwashing should start for the same water level above the filters, even if the raw water quality changes.