Pumped from the bottom. The basin sits on a raised box that holds the internals, including a pump for draining and a water valve for filling. Basically it uses a timer that commands the pump and a sump circuit for the fill valve detecting high, low, and overflow conditions.

The pump runs for three minutes every hour for sixteen hours between 0600 and 2200. The three minutes is enough to drain the entire thing with enough time for the clean fill water to rinse the basin and be drained away before stopping the pump and letting the fill top it off with clean water.