An apparatus for use in a filter system for filtering water or wastewater. The apparatus preferably includes an underdrain lateral having a plurality of chambers. Each of the plurality of chambers is preferably configured to direct a combined washing liquid and washing gas into a filter bed during a cleaning or washing cycle. The apparatus further includes one or more flow control members operably associated with the underdrain lateral for improving distribution of the combined washing liquid and washing gas through the filter bed. Each of the one or more flow control members is preferably a baffle assembly having a plate and a plurality of baffles operably associated with the plate. Preferably, one or more baffle assemblies are configured to be inserted into one or more chambers of the underdrain lateral.