Dynamics of variable declining rate filters during backwash

Variable declining rate (VDR) filters are usually controlled by the interaction of laminar head losses in the filter beds and turbulent head losses in the drainage systems and orifices installed at the outflow of each filter. The non-linear turbulent head losses are much higher for clean filters, but as the filter media becomes clogged the flow rate is reduced and the turbulent head losses decline. The disconnection of one filter during backwash causes a sudden increase in the water table level above theremaining filters, and so produces a rapid increase in the flow rates through them. This change in flow rate can have a significant impact on effluent quality. A simple and efficient method of calculating acceleration at the beginning of a backwash has been developed and verified by numerical solutions of the equations governing the hydraulics of VDR filters. The results show that the flow rate increases are approximately proportional to the flow rates during normal plant operation. A practical method of calculating the highest possible water table fluctuation during backwash has been developed.