Of the numerous aerator designs in use, the simplest and most common type is a vertical riser pipe on a basin inlet. Multiple riser pipes give better aeration by subdividing the flow into thinner sheets. Special caps or spreading aprons are frequently added for the same purpose. Auxilliary pans, shelves or steps add to the efficiency by increasing the spattering effect and the time of contact with the air. The pans may be solid, perforated or slotted. Many aerators instead of getting distribution of water by riser pipes use distributing flumes with the notable effects on one or both sides. The thin sheets may discharge directly into a basin, down a succession of steps, or into successive weir troughs. In one well known type, the water from the distributing flume discharges down an inclined plane studded with iron plates arranged in a herring-bone fashion. Several plants have used aerators of the cascade type for ornamental purposes.