Abstract
Nanofiltration membranes play an important role in the desalination of brackish and seawater as well as membrane mediated waste water reclamation and other industrial separations. Fouling of nanofiltration (NF) membranes is typically caused by inorganic and organic materials present in water that adhere to the surface and pores of the membrane and results in deterioration of performance (reduced membrane flux) with a consequent increase in costs of energy and membrane replacement.
Natural organic matter (NOM) fouling of NF membranes involves interrelationship between physical and chemical interactions and is described in this review. Inorganic fouling due to scale formation of sparingly soluble inorganic salts occurs whenever the ionic salt concentration stream exceeds the equilibrium solubility. Scale formation takes place by homogenous or heterogeneous crystallization mechanisms. Biofilm formation also becomes an issue when its thickness and surface coverage reduces permeability.
There are two strategies that are usually employed to minimize the effect of fouling. The first group includes minimizing of fouling by using adequate feed pretreatment, membrane treatment and membrane modification. The second group involves membrane remediation by chemical cleaning which is carried out to restore membrane fluxes.
A large number of chemical cleaning agents are commercially available, and the commonly used ones fall into six categories: alkalis, acids, metal chelating agents, surfactants, oxidation agents and enzymes. In general, these cleaning agents do improve the membrane flux to certain extent. Combination of these chemical agents has also been tried in order to improve the flux restoration. Even though, many of these cleaning agents can restore the flux over 100% (enhanced flux), they can also impair the selectivity of the membrane reducing of the product water quality.
There are many traditional assessment methods for cleaning and at present these are being supplemented by methods using modern surface analysis techniques. These are being now rapidly developed to give a more precise assessment and a better understanding of cleaning processes. Generally, cleaning is assessed by flux, zeta potential measurement, atomic force microscope (AFM) and Fourier transforms infrared technique (FTIR). Atomic force microscope and related techniques are particularly employed in order to evaluate the cleaning efficiency and other surface phenomena.
There are several factors that can affect the chemical cleaning process which include temperature, pH, concentration of the cleaning chemicals, contact time between the chemical solution and the membrane and the operation conditions such as cross-flow velocity and pressure. The role of temperature and pH in cleaning are membrane dependent. These factors play very important role in flux recovery. A critical review of these factors is also presented.
It appears from the literature that only very few papers on cleaning of NF membrane to regenerate membrane performance have been published up to date, and there is an urgent need for extensive research work to investigate fouling mechanisms in order to obtain fundamental understanding of fouling to provide more feasible, cost-effective cleaning and performance restoration procedures. This also provides further strategies for the avoidance of fouling through better pretreatment and more appropriate membrane fabrication and modification.