I need to design a spiral wound membrane for waste water treatment to bring down the tds from 3000 to below 100. Flow rate is around 2000 lph.
use these equations
π = cRT
π: osmotic pressure (atm.) , c:molar concentration of salt water R: constant (0.082 L .atm/K.mole), T: temperature in Kelvin
Fw=Kw×(∆P-∆π
Fw: water flux rate (L/m2.h), Kw: mass transfer coeff. hr/m, ∆P average imposed pressure gradient (bar), ∆π:osmotic pressure gradient (bar)
· Salt flux (Fs)
Fs=Ks×(Cf – Cp
Fs: mass flux of solute ( g/m2.L) , Ks: mass transfer coeff. m/hr ,Cf:solute concentrate in retentate stream g/m3 ,Cp: solute concentrate in permeate stream g/m3
Hi Bharti,
One approach would be using data available on commercial membranes. You have given a salt range in brackish water RO, with required rejection around 97%. A commercial Dow Filmtec XLE membrane for brackish water RO can give 98.7% rejection with flux of 56-70 L/m2hr at 125 psi (as per the manufacturers). I assume 2000 lph is feed flow rate. Now you need to pick a recovery % and calculate what your permeate flowrate will be. Dividing that number with the commercial flux should give you the area of membrane required for pressure of 125 psi to carry out your filtration.
This is a simple approach for rough estimation only.
The most accurate method is to use the softwares developed by the membrane manufacturers such as ROSA by DOW and TorayDS by Toray industries, as well as you can follow the steps of the membrane assembly design which can be found in the manual books written by the same manufacturers
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