64    NANOFILTRATION MOTIVATES FUTURE WASTEWATER
RECLAMATION
Mao-Yuan Tur, Group Leader
Ming-Shean Chou, Group Leader
Hung-Yuan Fang, Department Manager
Environmental Engineering Department
Refining & Manufacturing Research Center
Chinese Petroleum Corporation
Taiwan 60036, R.O.C.
INTRODUCTION
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.
This article will be focused on the applicability of this nanofiltration technique to wastewater
reclamation in petroleum refining industry. Demineralization efficiency of nanofiltration membrane
NF-70 (FilmTech Co.) was tested. Operating and maintenance costs for this process were then estimated and compared with those of reverse osmosis.
EXPERIMENTAL
Feed
Feed water was prepared by filtrating secondary effluent from CPC's refinery through a mixed
media followed by activated carbon adsorption to remove suspended solids, colloids, and trace
organics contained therein. Characteristics of the feed water are shown in Table I. For preventing the
membrane from scaling upon a recovery of 80%, the feed was brought to a Langelier Saturation
Index (LSI) of -1.70 by adding 85 mg/L hydrochloric acid and 4 mg/L scale inhibitor (Flocon-100,
Pfizer Co.) following the suggestions of ASTM: D-3739-83.3
Test Unit and Test Method
Schematic diagram of the nanofiltration test unit is shown in Figure 1. The whole NF system is
equipped with a 1.5 gpm procon pump capable of maintaining a liquid system pressure of up to 150
psig. The system contains a chamber which holds an element of spiral-wound membrane (NF 70-2514,
FilmTec) with an effective area of 5 sq. ft. System pressure and concentrate flowrate were regulated
with a needle valve. Cooling system is not required for the concentrated flow because of neglecting
heat dissipation at a system pressure of less than 130 psig and a total operation time of less than 90
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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