ENZYMATIC REMEDIATION OF INDUSTRIAL WASTES

Enzymes are increasingly being used in process industries to create a “cleaner” technology and decrease the use of harsh chemicals that are not environment friendly.⁵⁶ Enzymes are powerful tools for sustaining a clean environment in several ways. They are effectually utilized in a number of industries like agro-food, oil, animal feed, detergent, pulp and paper, textile, leather, petroleum, chemical, and biochemical.

5.4.1 FOOD INDUSTRY

Enzymes have diverse potentials in the industries like manufacturing of cheese, vinegar and wine, leavening of bread, brewing of beer, and so on.⁵⁶ Enzymes support food processes by saving energy and resources, and hence improving the efficiency of the overall process. The use of enzymes decreases the number, variety, and toxicity of by-products and effluents, hi the food industry, amylases are an effective alternative to strong acids and high temperatures required for the breakdown of starch. The industrial xylanase is a good enzyme for wheat separation because it shows a high level of activity toward soluble arabinoxylans and a low level of activity toward insoluble arabinoxylans and thus releasing previously bound starch molecules and proteins. Only a minimum amount of water is needed to wash out the starch. Hence, apart from the higher yield, water saving is one of the most important advantages of using xylanase. Besides, less water is used in the process, and hence, less wastewater is disposed of into the environment and leads to a substantial economic benefit. Moreover, the reduced viscosity of the soluble fraction makes the removal of liquid easier in the subsequent starch concentration step. Thus, less energy is required for the final evaporation stages.

5.4.2 EDIBLE OIL INDUSTRY

Even though enzymes are extensively used in carbohydrate processing, they play little role in edible oil and fat processing because of the high expense of the necessary enzymes like phospholipases for the degumming of oils.⁵⁷ The introduction of enzymes has opened up a new avenue for vegetable oil extraction. Hexane was the traditionally used solvent to dissolve oil from crushed seeds. It is not only dangerous to breathe in but also highly explosive. Enzymatic extraction process in aqueous medium replaces hexane-based technology.⁵⁶

5.4.3 ANIMAL FEED INDUSTRY

It is estimated that billion tons of feces and mine are produced each year by livestock. The way of treatment of these hazardous materials is one of the serious pollution threats in the United States. The animal feed industry has gone through many changes in the past few years. The consumers and the industry itself have looked more closely than before into the production of animal feeds, rearing of animals, and the impact of animal husbandly systems on the environment. In the animal feed industry, enzymes offer a relatively new development.⁵⁸ The enzymatic treatment of catfish and the inclusion of fiber degrading enzymes in the diets of grower pigs result in the reduction of polluted wastes. Enzymes in the feed industry are mostly used to either enhance the feed digestibility or improve the utilization of raw material. They increase the amounts of the diy feed matter, particularly nitrogen and soluble carbohydrates, which can be retained by the animal, thus reducing excretions and hence reducing the pollution load. Commonly used enzymes for this purpose are amylases, pectinases, proteases, p-glucanases, xylanases, and hemicellulases.⁵⁶-⁵⁹

5.4.4 DETERGENT INDUSTRY

Eutrophication of water bodies caused by the presence of phosphorous in detergents paved the way for the introduction of enzymes in detergent formulations. Enzymes have been used in laundry detergents since the 1960s.^{56 60} Proteases were the first enzymes to appear, followed by amylases, lipases, and cellulases. The detergent industry has remained the largest market for industrial enzymes, and new enzyme products are constantly being developed for such use. The efficiency of the enzymes is reflected in the necessity of trace amounts, typically less than 1% by weight, for remediation. Washing at lower temperatures in comparison with conventional high-temperature washing cycles is one of the mam environmental benefits of using enzymes. Moreover, enzymes are consistent with the principal necessities in the detergent industry like cost-effectiveness, acceptable environmental profile, biodegradability, no negative impact on sewage treatment processes, and saving of energy by lower temperature washing. They are also multifunctional, providing an advantage in stain removal, anti-redeposition, whiteness/ brightness retention, color maintenance, and fabric softening. Furthermore, the addition of enzymes to detergent formulations reduces the amounts of bleach, caustic, and phosphate.

5.4.5 PULP AND PAPER INDUSTRY

The paper-pulp industry uses harsh chemicals and bleaching compounds at many stages of the processes and discharges considerable amounts of deleterious by-products like lignin into the environment. Hence, paper-pulp industry is one of the most eminent sectors for environmental pollution.⁶¹ Even though the introduction of enzymes has not fully substituted any manufacturing task to date, they are veiy helpful in either improving the processes or in reducing pollution. During the manufacture of paper, in the pulping process, lignocellulosic raw material is digested with calcium sulfite, and the digested raw material is then dewatered and refined. Subsequently, the refined pulp is bleached with Cl, to make the pulp completely white. In this bleaching process, the residual lignin is virtually removed completely, and enzymes like xylanases and ligninases are introduced for effective bleaching of the pulp reducing the use of oxidizing chemicals to about 15-20%. Cellulases are beneficial in deinking, an important part of secondary fiber processing.⁵⁶ Cellulases release ink particles, thereby enhancing the removal of ink by flotation and consequently substantial decrease in the usage of deinking chemicals.⁵⁹-⁶¹

5.4.6 TEXTILE INDUSTRY

Enzymes can replace harsh organic or inorganic chemicals currently used in the textile industry.⁵⁶ Consequently, the utilization of highly specific enzymes for various textile processing applications is popular. Chemicals such as acids widely used in desizing of cloths, bleaching chemicals, and the production of coloring agents and dyes are responsible for the pollution associated with the textile industry.⁶² In denim manufacturing, even though the wastewater streams can be readily disposed of by maximum dilution with cleaner waters, the stone washing is associated with a problem of disposal of the sand produced by eroding of stones. The pumice grit may clog up drains, and hence, then exclusion from the wastewater is essential. An enzyme substitute for pumice stones can alleviate this environmental problem. With the appropriate enzymes, stones are no longer needed. The cost of enzymes is comparable to the cost of stones yet when other savings are taken into account; the stone-free process turns out as being more affordable. The major advantages of a stone-free process include (1) some people rejected jeans as the stones damage the denim, (2) less wear on laundry machines, (3) no storage and handling of bulky stones, (4) no destoning and no need to wash off pumice grit, (5) more room for garments in the wash loads, (6) no disposal problems due to solid pumice waste, and (7) less variation in wash results. In the textile industry, desizing is essential for the removal of all the starch paste fr om the fabric. The starch strengthens the waip thread without breaking during the weaving process. Up to that time, textiles soaked in water for several days or treated with acid, alkali, or oxidizing agents to break down the starch. However, both of these methods are difficult to control and sometimes may damage or decolorize the material. Therefore, crude enzyme extracts in the form of malt extract or pancreas extract are used to cany out desizing.

Fabric softening is another area in the textile industry where enzymes are replacing the traditional practices. For years, chemicals are used by the manufacturers in order to make fabrics softer. However, the softness is removed after continuous washing, and the chemicals used are deleterious to the environment. Biopolishing or enzymatic softening offers long-lasting softness to the fabric. Moreover, it offers a green softening procedure. Recent enzymatic development in the textile industry moved away from the use of chemical bleaches, particularly chlorine- and hydrogen peroxide-based bleaches. In the degradation of hydrogen peroxide by the enzyme catalase, there is a significant reduction in the amount of water consumed in the overall process, since the series of rinses between primary bleaching and dyeing processes can be excluded. Enzymes can also be utilized in the dyeing step by lowering their amounts in the effluents. Cellulases, amylases, and proteases are useful in the prewashing step of dyeing to improve the dyestuff uptake. Enzymes can improve the degr ee of whiteness prior to dyeing, color shade, and felting behavior. Further, LiP, MnP, and laccase have the potential to degrade common dyes found in the textile effluents.

5.4.7 LEATHER INDUSTRY

Animal hides and skins are excessively used in the leather industry. Leather manufacturing is a multistep process including curing, soaking, dehairing, dewooling, bating, and tanning.⁵⁶ Traditionally, dehairing and dewooling requires an extremely alkaline condition in the soaked and swollen epidermis and corium of the skin, followed by the reaction of hair proteins with sulfides to break the bonding of hair protein fibrils and dissolve the proteins of hair root. The presence of these chemicals in the effluents poses a severe environmental problem. The incorporation of enzymes, especially proteases, is beneficial for improving the leather quality and reducing the pollution. In the presence of enzymes, the hair is not dissolved and can be filtered out from the liming float. In this manner, chemical oxygen demand (COD) and biological oxygen demand (BOD) of the waste discharges can be reduced to a considerable extent. Bating makes the leather soft, supple, and able to accept an even dye and demonstrate the grain in an adequate fashion, and encompasses deliming and deswelling and degrading of the collagen of the skins and the protein fibers. Bating is helpful for the control of the quality of leather: Stiff leather is used for soles and is only lightly bated, while the soft leather used in gloves results from intense bating. The introduction of trypsin leads to a reliable process reducing the BOD content in the effluent. Degreasing or the removal of fat from the leather before tanning is significant since once most of the natural fat has been removed, and subsequent chemical treatments such as tanning, retanning, and dyeing have a better effect. In addition to the final quality of the product, lipases offer an environmentally better way of removing the fat. The practice of lipases in the degreasing step permits the partial or full replacement of the solvents or surfactants that may be harmful to the environment.

5.4.8 PETROLEUM INDUSTRY

Enzymes are progressively piercing the sectors of petroleum industry, where their use was once considered not viable because of either technical or economic reasons.⁵⁶-⁶³ Enzymes can remove sulfur from petroleum at mild operating conditions like temperature and pressure. In addition to the cost advantages compared to the existing methods, biodesulfurization technology offers the benefit of operating under milder conditions, consuming less energy, and emitting fewer greenhouse gases. The future may realize the practice of enzymes to catalyze cracking, viscosity reduction, and demetal- ization in a petroleum refinery.⁶³