Recovering & Reusing Caustic using Membrane Filtration
-Shanti Bhushan, Department - GEA Filtration, GEA Process Engineering A/S

Recovering and reusing of caustic in process industries is a crucial process which can not only help organisations reduce process cost, but also allow them 'to go clean and green' This article explains how Membrane Filtration Technology can be used to attain the objective.

Caustic is one of the most common cleaning agents used across various industries. It is also used as an integral process chemical in many industries. Often, it has been found that contaminated caustic effluent is discharged after use. Reuse of the caustic after purification not only minimise the cleaning and process cost but also show that there is increasing effort by the industries to go clean and green. Tightening regulation for effluent discharge has also led to the growing consciousness for purification before discharge or for reuse.

Membrane filtration is a unit operation that is being increasingly used for numerous applications in different industries. Amongst the four categories into which the membrane filtration has been divided into, the microfiltration (MF) process separates using the most open membranes followed by ultrafiltration (UF), then nanofiltration (NF) and reverse osmosis (RO) operates using the tightest membrane. MF and UF are primarily size based separation processes such that the solutes bigger in size are retained by these membranes and the smaller solutes permeate through these membranes. NF and RO also separate largely on the principle of size but these membranes also possess charge. NF and RO membranes used by water industries are mainly made up of cellulose acetate materials and therefore possess neutral charge. However, the NF and RO membranes used by the process industries are primarily called "Thin Film Composite" and these membranes possess negative charge and therefore, affect the separation of charged and ionic components.

Spiral-wound, flat sheet, hollow fiber and tubular are the most common configurations in which different membranes are available. Tubular membrane because of the wide channels finds its use for the separation of streams containing suspended and precipitated solids. Spiral-wound membrane is made by folding few layers of flat sheet membranes and therefore, possesses a high density of membrane area making it the most economical. Spiral membranes have narrow channel spacer and can be used only for streams free of suspended solids.

Tubular membranes can be made of inorganic or organic materials. Inorganic tubular membranes made up of titania, zirconia or alpha-alumina are robust membranes capable of handling extremes of pH and temperature. These properties of the inorganic tubular membranes make them useful for purification of variety of effluent streams. The cleaning of these membranes is also eased because of the high temperature and pH that can be used during CIP (clean-in-place). However, while several MF and UF membranes of different molecular weight cut-offs have been successfully cast into the inorganic tubular modules, the same is not true for the NF and RO membranes. As a result, the use of the inorganic tubular membranes is still primarily restricted to the MF and UF operations. A picture of a ceramic inorganic tubular plant is shown in Figure 1a and that of a ceramic module and membranes are displayed in Figure 1b. (on previous page) Spiral membranes used in the process industries are normally available as either normal membrane or specialised membrane. The former work in the pH range of 3-10 during production and 2-11 during CIP. 50 degree°C is the maximum temperature that can be handled by the normal membranes. On the other hand, the specialised membranes can work in extremes of pH (0-14) and also at temperature as high as 70 degree C. Because of specialised chemistry , the latter is very expensive as compared to the normal membranes. Nevertheless, the specialised membranes are still very cheap as compared to the tubular inorganic membranes. Picture of a spiral filtration system can be viewed in Figure 2a and that of a spiral membrane in Figure 2b.

We know that caustic permeates through all the MF, UF, NF and RO membranes. If the effluent caustic stream contains suspended solids, the first step often employs using the inorganic tubular MF or UF membranes for the rejection of those materials. Sometimes, the MF/UF permeate is clean enough to be reused as process caustic. However, a number of times this permeate requires further cleaning using NF spiral membranes. The caustic present in the NF permeate is normally pure enough to be reused as process and primary cleaning caustic for a number of applications.

Spiral NF step can be directly inserted for the concentration of the caustic effluent if the stream is free of suspended solids. Irrespective of whether the NF is used alone or after MF/UF, the NF spiral membranes that are used for caustic applications are specialized NF membranes. For some applications , caustic purified by membrane filtration is concentrated by evaporators to generate concentrated caustic for proper reuse.

In the paper industry, caustic is used in a number of steps during the manufacturing of the sheets of Kraft pulp. One of the most important steps is the bleaching process in which the cellulose fibers are washed and bleached using caustic, chlorine dioxide, hydrogen peroxide and oxygen to produce a white product which can be used to make end products such as towels, tissue, high quality printing, writing and book papers. After the end of the bleaching process, the waste stream which contains about 8-10 percent caustic is sent to the waste treatment facility for further treatment before discharge. With the introduction of special NF (in some cases special spiral UF) membranes, the caustic in the waste stream can be purified and then concentrated by evaporators before being reused again in the bleaching process.

During the manufacturing of the rayon using the rayon-viscose process, cellulose pulp is immersed in 17-20 per cent aqueous caustic in order to swell the cellulosic fibers and to convert cellulose to alkali cellulose. This step is called steeping. The swollen alkali cellulose is then pressed to a wet weight and the caustic is discharged. As stated in the above paragraph, we can use specialised NF membrane to purify the caustic and then recycle it for the steeping step.

Caustic also finds its use in the mercerisation of textiles, particularly cotton to improve fiber strength, shrinkage resistance, luster and dye affinity resulting into a high quality fiber. Caustic rearranges the cellulose molecules in the fiber to effect these changes. A 25 per cent caustic strength is used for the mercerisation bath of the cotton fibers. After the reaction is over, the cotton is washed with warm water thereby discharging the caustic. The contaminated stream containing about 6 percent caustic can be purified using the specialised NF membranes and then evaporated to 25 per cent caustic before reused for the mercerisation process.

One of the steps in the process of purification of cane sugar is the decolorisation of the sugar using the anion exchange resin. The decolorisation happens due to the adsorption of the high molecular weight colorants on the resin. Alkaline sodium chloride is then used to elute the color molecules. The spent brine is a highly saline waste stream full of colored organic matter and consequently possessing high COD value. Specialised NF membranes have been used to purify this stream by retaining the color molecules and permeating the sodium chloride and caustic . The purified stream is then recycled for further decolorisation of cane sugar (Figure 3).

In the citrus juice industry, caustic is used for peeling of the fruits and also for cleaning of the several process equipments such as evaporators. By using tubular inorganic MF membranes, the caustic can be purified to the extent that it can be reused for primary cleaning of the equipment. The concentrate of the MF can be used for the peeling process. Thus MF process leads to zero discharge of the caustic effluent.

If the MF permeate does not generate pure enough caustic for reuse, then specialised NF is used on MF permeate to produce caustic that is acceptable for reuse in the process. For example, in the tomato processing industries the caustic is first purified by MF and then treated by special NF to generate caustic which is then concentrated by the evaporators and sent for reuse. Alternatively, the effluent is also treated directly by the specialised NF membranes if it is devoid of suspended solids.

Last but not the least, caustic is simply used as primary cleaning chemical in a number of industries for the cleaning of equipments and process lines and piping. One such example is use of caustic for cleaning of evaporators across various industries. Another example is using caustic for bottle washing in the beverage industry. Depending upon the presence of suspended solids or not, such cleaning caustic effluent can also be recovered and reused using MF, NF or a combination of both.

In conclusion, membrane filtration has a substantial role to play in the purification of caustic used in the process industries. Recovery and reuse of caustic by purification can not only reduce the cost of buying fresh caustic but it can also minimise the cost of further treatment before discharge. Presently only a small percentage of the used caustic is purified and reused. Mostly, the used caustic is discharged and drained causing environmental problems. Ignorance on the part of the industries about purification is one of the primary reasons for discharge. Another reason is relaxed regulation in many parts of the world which ultimately does not help in enforcing a sense of green technology. However, we have seen that in the recent years, there has been a growing effort by people and industries to keep their environment clean and with this hope we sense that it is the right time to increase further awareness in whatever way we all can do. The present article has been written with this outlook.