Reverse osmosis (RO) and nanofiltration (NF) membranes are typically used in the form of elements, also called modules, such as spiral wound elements, hollow fiber elements or tubular elements. A number of elements, typically between 1 and 8, are mounted in series in a pressure vessel, alternatively called a housing, with a feed inlet, concentrate outlet, and permeate outlet. Multiple pressure vessels may be connected together in parallel to form a bank, alternatively called a stage, in a filtration system. The stages of a filtration system are collectively referred to as a membrane block.
A filtration system may have multiple stages connected together in various configurations. In concentrate staging, alternatively called a multi-stage array, feed water is first pumped into a first stage of elements. Concentrate from each upstream stage is fed to each downstream stage. The concentrate port of the last stage is fitted with a concentrate valve. The flow and pressure through the membrane block are controlled by the feed pump and concentrate valve. Permeate flows from each stage to a common permeate header. The concentrate staging increases permeate recovery. Filtration systems with high recovery rates, for example 80% or more, typically have at least two stages.