repeat the backwash process if after cleansing the desired number of cells cell target the headloss has not been restored the plc will increase the dwell time value for cleansing each cell thereby attempting to minimize the number of cells above the cell target that must be cleansed in a backwash cycle to achieve the desired permeability of the filter bed if headloss is restored after cleansing less than the cell target the plc will decrease the dwell time for cleansing each cell so that during the next subsequent backwash cycle the actual number of cells cleansed should increase to approach the cell target the cell target value is set to achieve the goal of maintaining a percentage of the filter bed in a ripened state the degree of ripening effects both filtrate quality and the headloss the percentage of the ripened bed to be maintained depends on specific filtrate quality objectives that are established for each individual application when headloss is restored to the desired and expected level the plc will cause the backwash cycle to terminate by suspending operation of the pumps and drive unit on the bridge the backwash mechanism will remain stationary in position over but not sealed with the last cell that was backwashed until a next subsequent backwash is initiated the electronic output from at least one turbidimeter immersed in the backwash flow from the cell being backwashed is monitored by the plc if during the backwash cycle the dwell time on a cell is exceeded or if all the cells in the filter bed are backwashed indicating the cell.