Preparation of Thin Film Composite (TFC) Charged Nanofiltration (NF) Membranes

Introduction

This technology refers to a novel process for preparation of Thin Film Composite (TFC) Charged Nanofiltration (NF) Membranes containing surface negative charge in the form of fixed sulfonic acid ( -SO₃^-H⁺) groups. This has tremendous potentials for applications in aqueous stream separations such as in production of potable water from partially brackish hard water, removal of heavy metal contaminants, removal of microbial (bacteria/virus) contaminations, pretreatment of saline water for desalination processes like RO and MSF, and a host of other areas like pharmaceuticals and bio technological industries, downstream processing, food and beverage industries, dairy industry, waste water treatment.

Salient Features

• The flat sheet membranes give a unique performance in terms of solute rejections. The rejections of salts containing bivalent anions are very high and that for salts containing monovalent anions are very low.
• An interesting feature of these membrane is the huge difference between the solute rejections for high rejecting solutes (Na₂SO₄ : 96 %) and low rejecting solutes (NaCl : 25%) which enables the membranes fractionally separate them when they are present together in a mixture.
• These membranes are particularly suitable for management of hardness (Ca and Mg containing solutes) and partial salinity in water for production of potable water as well as water for industrial purposes.
• Membranes of a range of performances, according to the need of any application, can be prepared by variation of chemical compositions.
• The membranes can be used in any suitable configurations such as spiral module, plate and frame module, etc. Further, the modules can be used in isolation or in combination with other types of modules such as RO modules.

Working Principle

The membranes work under the principle similar to reverse osmosis where pressure is applied on the feed side of the membrane resulting in the desired operation. However, the performance of these charged nanofiltration membranes can be tailored by incorporation of appropriate charge in them which results in the unique property of the membranes enabling them for fractional separation of various solutes. The membranes operate under lower pressure than the reverse osmosis but still produce higher flux. Generally 10 bar applied pressure is suitable for these operations with a feed concentration of about 2000 ppm.

Applications

• Water purification, particularly for production of drinking water by removal of hardness and partial removal of salinity from moderately brackish water.
• Removal of hardness for water use in industrial purpose.
• Pretreatment of saline water for desalination processes like multistage flash (MSF) and reverse osmosis (RO).
• Removal of heavy metal impurities.
• Fractional separation of mixed systems containing monovalent and multivalent ions where the membranes preferentially retain multivalent anions such as sulfates.
• Applications in aqueous stream separations in pharmaceutical, food and beverage industries, dairy industry, waste water treatment, etc.
• The membranes can be applied in a wide range of scales starting from domestic to industrial level.
  

Space required

The space requirement will depend on the size and capacity of the membrane casting facility. For domestic applications for production of 1012 (1 inch dia x 12 inch length) and 2512 (2.5 inch dia x 12 inch length) type spiral modules 125 m² space may be adequate to accommodate the casting facilities of polysulfone as well as TFC coatings and the spiral module rolling facility.

Manpower Requirement

6-8 persons on a regular basis for a facility to produce membranes and modules of about 10,000 units of 1012 spiral modules per annum for domestic applications.