This option consists of installing a weir or weir gate in a concrete weir box on
the outlet piping of each filter. The weir provides a positive method of
maintaining the tailwater elevation above the sand level, with the weir
elevation typically set at 6” – 12” above the sand level. An adjustable weir
can allow the operator to recover the 6” – 12” of driving head pressure
initially lost from forcing the water level to be above the sand level. Once the
headloss across the filter reaches a minimum of one foot, the operator can
recover this driving head by lowering the weir to the sand level. However,
there is an increased risk of air binding if the weir is lowered to far.