A pattern of water flow in which a solution (water or regenerant usually) enters at the bottom of the vessel or column and flows out at the top of the vessel or column during any phase of the treatment unit’s operating cycle. The term is used to describe ion exchange system flow patterns or water flow through filter media. A system can have up-flow during the treatment cycle and down-flow during regeneration. Up-flow is also called counter current flow. Countercurrent flow means regeneration flows and service flows are in the opposite directions.