Fundamentals of Membrane filtration techniques for dyestuff recovery

Fundamentals of Membrane filtration techniques for dyestuff recovery

Humans have been fascinated by color since prehistoric times. This is justified by the use of colors in various clothing & apparel, furnishings, artifacts, plastics, and paper products. The initial source of colorant was vegetables and other origins. After the discovery of synthetic dyes, the initiative by researchers overall contributed to the development of more than 10000 chemically different dyes that have reached commercial significance today. The synthetic dyestuff industry has undergone significant growth and the highest consumption of these dyes is also in the textile manufacturing segment. So, the textile manufacturing industries offload an enormous amount of color effluent that requires to be looked at with proper dischargeable norms of toxicity and color contaminants. It becomes necessary for maintaining regulations norms for color discharge and treating them for discolorations plus other toxic impurities removal. The approach to solving the wastewater treatment problems will continue as long as the regulations continue to increase and become more restrictive. Membrane filtration techniques used in different modes and forms do remove colorants & its recovery is possible.

The textile industry uses a large volume of water and also discharges a similar quantity as effluent. The wastewater is the principal route by which dyestuffs enter the environment.

Though some basic thoughts for lessening the burden on effluent is also to be reckoned with by following considerations.

1. Adoption in newer & alternative methods of processing
2. Recycling and reuse of industrial water
3. To examine, study, and select low toxic input products as priority
4. Use a lower liquor ratio
5. Reduce rework and practice efficiently right the first time.

The textile effluent consists of mainly :

• Wide range of dyestuffs.
• Dispersing agents & enzymes.
• Dye bath carriers/conditioners/wetting agents.
• Inorganic salts along with reducing & oxidizing agents.
• Emulsifiers.
• Levelling agents.
• Heavy metals.

The presence of such impurities has impacted the majority of quality parameters such as COD, BOD, TDS, SS, TOC, PH, etc. Most of the dyes possess a low BOD value but they add TOC and noticeable color to the effluent.

Color removal methodologies

• Chemical decomposition of the coloring mater.
  • Oxidative degradation (chlorine/ozone, Fenton’s reagent).
  • Reductive degradation (sodium hydrosulphite/formamidine sulfinic acid).
  • Biological degradation.

• Physical removal of coloring mater.
  • Adsorption.
  • (Activated carbon, ion-pair exchange).
  • Filtration (precipitation, flocculation and membrane filtration technologies).

Membrane Filtration technology

Conventional filtration: –

One of the primary stages apart from sedimentation is the conventional filtration process. Most of the wastewater is subjected to filtration due to ease & efficiency in subsequent treatment that helps in lower the cost and quality of discharge or recycling water. The various suspended solids, fibers, and any other matter are removed so that interference with the operation of flow meters, valves, and pumping equipment works uninterruptedly. The characteristics of wastewater like viscosity, ph. temperature is determining factor for the selection of filter elements with porosity. The basic equation used to describe the conventional filter process is,

Flux    =          J_f         =          B X Δ P

                                                µ (L_f + L_c)      

=  Volume
________________________

Unit Area X Unit time

J_f is Flux or rate of filtration per unit area of filter surface.
B is Darcy permeability constant for the filter cake.
Δ P is the pressure drop across both filter material & cake.
µ is the viscosity of the solution in units of mass per distance & time.
L_f is the resistance of the filter material represented as equivalent cake thickness.
L_c is increases in thickness.

Ultrafiltration: –

The term ultrafiltration is generally used for filtration of soluble molecules or polymers having nominal diameters /sizes 10-1000 A0. The pressure used for this type of filtration is generally low. If there is a significant difference in the osmotic pressure of the solution being filtered and that of the filtrate, filtration will occur at a very slow rate or not at all.  The filtration rate decrease is caused by the gel layer which is commonly called gel polarization that depends on chemical/polymer material under filtration. As the material being filtered increasingly concentrates at the surface and again filtration rate is decreased and it does not have any impact on pressure. This second effect is called concentration polarization. Most of the P.V.A. size recovery cycle and indigo dye recovery system is using ultrafiltration process from stage one onwards. Both R.O. & U.F. are pressure-driven membrane processes that remove solutes from solution based on particle or molecular size differentials. In all pressure-driven membrane filtration techniques, the liquid to be treated flows in a turbulent manner over the surface by means of a cross-flow technique. The separation mechanism for U.F. is based on membrane pore size. Though membrane pore size in R.O. is too small.

Crossflow filtration: –

In the conventional filtration system, the filter surface will get fast choked up due to accumulated impurities of suspended solids. The cake thickness also increases obstructing the easy way out for the solution to penetrate. The surface area plays an important role in efficiency, but its limitation is the surface size. Frequent backwash/cleaning and replacements are practiced. As a result, the design is modified so that suspended solids don’t accumulate as a filter cake. The solution being filtered is directed parallel rather than perpendicular, to the filter surface. The formation of the filter cake is minimized as the solution being filtered sweeps the membrane or filter surface thus gel polarization is restricted to quite an extent. This type of filtration is called cross filtration. The momentum of the flow of solution at parallel to the membrane reduces the likelihood of gel polarization, which ultimately helps in membrane fouling.

Cross filtration is used to separate or recover suspended solids, soluble polymers, and inorganic salts from the waste stream at ambient conditions. Commercial filters are fabricated from organic, ceramic, metallic materials and may have tubular or plate configurations.

The average efficiency of different filtration methods for treating wastewater stream in a typical dye house (PUBLISHED BY ENVIRONMENT CHEMISTRY OF DYED & PIGMENTS –JHON WILEY & SONS, INC.)

Parameters	Biological %	Conventional filtration %	Ultrafiltration %	Reverse osmosis %
BOD	50	30	50	95
COD	30	30	50	95
TDS TOTAL (INCLUDING FIBERS)	25	20	50	95
S.S.	50	30	98	98
TDS	40	0.0	20	95
COLOUR	30	10	50	98

Reverse Osmosis: –

Osmosis is a natural process involving the diffusion of a fluid through a semipermeable membrane separating two solutions of different concentrations and tending to equalize the concentration on both sides of the membrane. Semipermeable membranes have the ability to prevent the passage of dissolved molecules while allowing the passage of a solvent (usually water) through the membrane under a pressure gradient. If the pure water is in contact with both sides of a semipermeable membrane at equal pressure and temperature, then there is no net flow across the membrane, because the chemical potential is equal on both sides. If a soluble salt is added to one side of the membrane, the chemical potential of the water on that side is reduced. Osmosis occurs when the pure water passes through the membrane to the salt solution side until equilibrium of the solvent chemical potential is restored. True reverse osmosis has operational pressure is always high in the range of 100-1000 psi & whereas UF system operates at usual pressure 10-100 psi.

The Equilibrium occurs when the pressure differential from the volume change on the two sides is equal to the osmotic pressure, a solution property that is independent of the membrane.

If the pressure higher than the osmotic pressure is applied to the salt–solution side, it will cause a reversal of the osmotic flow and drive the water back through the membrane to the pure waterside. This phenomenon is called reverse osmosis. (Hyperfilteration)

The membrane filtration technique is playing an important role in the areas of removal of coloring contaminants for both dyestuffs manufacturing and dyeing houses. Dye-containing wastewater disposal unit is obliged to use membrane technologies in a selective way. The color reduction by chemical reaction also has its disadvantage as the use of oxidizing/reducing agents adds into the additional reaction products and increases the load of treatment. Though filtration methods have definite advantages in discoloration and recycling of water cannot be limited by cost. The average return takes 3-4 years in breakeven calculations.

Range of application of membrane technology – (Courtesy ATCC)

For further information Contact   CTD@atira.in