ABSTRACT
To meet stricter environmental regulations, tertiary treatment is generally required to polish the secondary effluent at industry. The polished wastewater, after removing silica and divalent ions such as calcium, magnesium, and sulfate, can be recovered as cooling-tower make-up water. Membrane processes are generally considered as cost-effective for removing these ions. Conventional reverse osmosis (RO) process for desalination is operated in a pressure range of 200-800 psi. Power consumption is a main factor in the operation and maintenance cost. Developments of the membrane technology in the last decade have, resulted in a type of innovated membrane which operated in a typical pressure range of 50-200 psi while still exhibiting ion-rejecting capability. 1 , 2 The type of membrane was developed by FilmTech Co. (USA) and named as “nanofiltration (NF)” membrane owing to its characteristic of rejecting molecules with sizes in the order of one nanometer. The membrane is made of cross-linked aromatic polyamides and has negative surface charges. Thus, the anion repulsion mainly determines the solute rejection. It exhibits typical rejections of 40-70% for monovalent ions and 70-98% for di- or multivalent ions, respectively. Organics with molecular weight above 300 can also be separated from the lower ones. Due to these characteristics, nanofiltration has been applied to partial water desalination, water softening, groundwater treatment, sulfate removal from saline in oil fields, and separating monovalent salts from organics in the MW range of 300-1,000.
