Textile dyeing industry consumes large quantity of water.
Each kilogram of textile fabric consumes between 60 and
100 kg of water during dyeing and washing operations [1]. Dye
bath effluent, which constitutes approximately 10% of the total
water used, contains up to 75 g/L of inorganic salts such as NaCl
and Na2SO4 along with residual dye molecules [2]. Experiments
have established that up to 71% of the dissolved NaCl can be
recovered along with water as brine solution by commercially
available nanofiltration membranes [3]. The commercial dyeing
units in and around Tiruppur town in Tamil Nadu, India also
employ available nanofiltration membranes for brine recovery
[2]. However, membrane fouling remains a serious challenge
which limits its continuous and long term operation and hence
commercial viability. These dyeing units utilize a variety of dye
molecules based on color and shade requirements. A detailed
study to understand the influence of molecular structure of dye
on NF membrane can throw further light on solving this problem
of membrane fouling. Commercial NF membranes are essentially thin film composites
containing a polyamide layer over a more porous and hydrophobic
polysulfone or poly ether sulfone support layer [4]. The
pore size of the surface layer is in the range of 0.6–1.3 nm which
determines the molecular weight cut off (MWCO) of each NF
membrane [5]. The dye molecules having molecular weights
close to or exceeding the MWCO of individual membranes would
be rejected by these membranes [6]. Nanofiltration membranes
exhibits high and undisturbed flux rates in the presence of
organic acids [7], phenols [8], amines [9], drugs [10], pesticides
[11] and surfactants [12] near the IEP, due to the absence of
strong electrostatic interactions. The present study investigates the effect of molecular charge
of different dyes and the pH of salt solution on water flux and
salt and dye rejections in the context of nanofiltration used in
textile wastewater treatment for the recovery of both water
and salt. Efforts have also been made to measure and interpret
the flux decline due to fouling under each experimental
condition.