Nanofiltration

Overview

Position of nanofiltration

Properties of nanofiltration membranes

Transport and separation mechanisms

Challenges of nanofiltration

Recent Research interests

Applications

Robert Petersen, in his Foreword to Elsevier's Nanofiltration principles and applications, describes

reverse osmosis (especially in the water treatment business) as the main course, the steak perhaps, of a

meal, whereas nanofiltration is like the wine menu an opportunity for creativity and exploration

Position of nanofiltration (NF) membranes

Source: ACS medio ambiente

The fundamental principle of NF : use of pressure to separate soluble contaminants from water using a semi-permeable membrane.

Nanofiltration properties Pore size distribution (0.5 to 2 nm) based on successful atomic force microscopy (AFM) characterization by Bowen et al.

Tight

Very Loose

MWCO is between 150 and 1000 Da

Typically: NF operating pressure range is from 5 to 40 bar

Compared to 60 to 70 bar for RO

Lower operating pressures mean lower energy requirements and costs

Also NF have higher fluxes

Additional information

Presently most NF membranes are made predominantly in spiral-wound configurations using thin-film flat sheet composite membranes

NF is very effective in reducing hardness, organics and particulate contaminants

The current trend is incorporating NF as pretreatment before RO and multistage flash (MSF) allowing the RO and MSF to achieve higher recoveries

Retentions higher than 90% for multivalent ions and about 60-70% for monovalent ions

Selectivity of NF membranes

Transport and separation mechanisms

Major separation mechanisms involve steric hindrance (size exclusion for uncharged solutes) and electrostatic partitioning interaction (Donnan and dielectric exclusion)

For nonporous structures the separation between solute and solvent may occur due to difference in diffusion rates

The charge effects play a big role in NF

More about role of charge on NF

Membranes in contact with aqueous solutions develop charges by

Dissociation of surface functional groups

Adsorption of ions from the solutions

Adsorption of ionic surfactants and macromolecules

This charging can occur both on external surface as well as internal pores because of ion distribution to maintain electro neutrality In this mechanism ions with same charge as membrane surface are repulsed serving as an additional rejection mechanism

Source: M.R.Teixeira et al. 2005 Role of membrane charge on nanofiltration performance

Challenges of NF Flux decline due to fouling and concentration polarization

are major areas of challenges

Several sources of fouling: inorganic (carbonates, sulphates etc.) and organic (humic acids, colloids, bacteria etc.)

Leading to higher energy penalty, higher maintenance costs, and reduced lifetime of membranes

To consider suitable pretreatment, proper understanding of foulant materials is important

Recent Research Interests - 1

R. Lamsal et al., 2012 made use of surface enhanced spectroscopy combined with scanning electron microscopy (SEM) technique to study fouling

SEM was used to examine the morphology and nature of foulants while the surface enhanced Raman spectroscopy allowed identification of the foulant materials

Virgin DK-NF membrane

Natural organic matter Fouled membrane

Recent Research interests - 2 membranes are key to NF

performance

Recently, there is a growing interest to develop novel NF hollow fiber membranes

because hollow fiber module may provide a much larger membrane area per unit membrane module volume, self-mechanical support and ease of handling during module fabrication and system operation

S.P. Sun et al. ,2012 Novel thin-film composite nanofiltration hollow fiber membranes with double repulsion for effective removal of emerging organic matters from water

Recent Research interests - 3 The majority of commercially available nanofiltration

membranes are negatively charged

Positively charged NF membranes hold potentials for high rejections for multivalent cations with applications to pharmaceutical industries, textile industry wastewater treatment etc.

Cheng et al.,2012 demonstrated and characterized a novel self assembled positively charged membrane and tested it in the lab for removal of dye from a dye-house wastewater

Some key models and Applications

Brief look at some key models used to characterize and predict NF membrane processes:

Models based on extended Nernst-Planck equation

Models based on Spiegler-Kedem model

Many applications beginning from water softening, pretreatment of RO feed water, wastewater treatment in paper, textile, leather etc. industry, recovery of products in food and pharmaceuticals industry etc.

Thank you