Membrane softening The use of membranes for removing hardness from water most often accomplished using NF membranes Net driving pressure The hydraulic pressure differential across the membrane minus the osmotic pressure differential across the membrane that is available to force water through the membrane Permeate,product(filtrate) The membrane output stream that contains lower TDS (lower TSS)than the feed stream Pressure vessel A single tube with several membrane elements in series Raw Input stream to the membrane process Recovery The ratio of the permeate flow to the feed flow expressed as a percentage Rejection Percentage solute concentration reduction of the permeate stream relative to the feed stream Scaling Precipitation of solids in an element due to solute concentration in the feed stream Solute Dissolved solids in raw,feed,permeate,and concentrate streams Solvent Liquid,usually water,containing dissolved solids Stage or bank Parallel pressure vessels System arrays Several arrays that produce the required plant flow Transmembrane pressure Pressure drop across the membrane barrier Abbreviation key:TDS,total dissolved solids;TSS,total suspended solids;MF,microfiltration;UF,ultrafiltra- tion:NF,nanofiltration;RO,reverse osmosis.Tongji University