The constantly growing use of enzymes is leading to a great interest in biocatalysts that demonstrate improved or new properties. Biological (enzymatic) procedures offer advantages like simplicity of activity and control, flexibility to change in temperature and they are quick.
Enzymes are exceptionally explicit and proficient catalysts; they can corrupt a specific pollutant without influencing different parts in effluent. Hence they beat common industrial processes.
Breakdown of Organic Compounds
Organic compounds make up a small fraction of the earth’s crust but are central to all known life. They form through the conversion of inorganic carbon dioxide and hydrogen from water by autotrophic organisms, which use sunlight or other sources as energy. The breakdown of these organic molecules then forms complex organic materials like trees, soil and plants and, in turn, provide raw material for other biodegradable substances such as proteins and fats.
Wastewater treatment bacteria generate enzymes such as ligninases, lipases, cellulose-degrading cellulases, urease and xylanases which are specific to removing certain organic pollutants. These enzymes work to break down these pollutants into smaller parts which can be more easily managed and recycled.
Oxidative enzymes such as peroxidases and polyphenol oxidases are also used to remove phenolic, aromatic and inorganic pollutants through oxidation reactions. These enzymes utilise similar redox mechanisms as chemical oxidants but provide the additional benefit of a high level of regio- and stereoselectivity.
These enzymatic reactions occur under mild reaction conditions (ambient/physiological temperatures, atmospheric pressure and neutral pH) with a catalytic efficiency many times greater than inorganic chemical catalysts. The reactions also induce fewer side-reactions which may result in toxic byproducts. Compared to different chemical and physical strategies, enzymatic methods are viewed as clean/green technologies that are less prone to introducing extra contaminants into the environment.
Hydrolysis
Water (H2O) is an exceptionally useful solvent, owing to its polar structure. This makes it act as a nucleophile, which is a chemical substance that attacks and cleaves other chemicals to produce a new compound. This process is called hydrolysis.
The microorganisms that degrade non-settable organic substances in biological wastewater treatment generate a wide range of highly specific hydrolytic enzyme products. These include protease, lipase, a-glucosidase and cellulase. These enzymes cleave the sensitive linkage/bonding of large organic polymer molecules to simpler monomer units that can transfer across the membrane and be metabolised by bacteria.
As a result, these enzymes improve sludge settleability, dewaterability and anaerobic digestion of sludge to increase biogas production and energy recovery. These enzymes also reduce the need for conventional inorganic chemical catalysts.
Enzyme extraction from the waste activated sludge (AS) generated by biological wastewater treatment processes is an important technology to improve the economic and environmental sustainability of the water industry. However, the complexity of AS sludge composition and enzyme distribution has limited commercial enzyme extraction from it to date. To overcome this barrier, researchers are developing novel methods to stimulate the growth of microorganisms and enhance sludge treatment performance. This includes the addition of high-value chemical substrates and secondary materials such as industrial organic wastes. This approach has the potential to provide a practical method to upscaling the enzyme recovery process for full-scale industrial application.
Catalytic Reactions
Catalysts increase the reaction rate by lowering the activation energy needed to get the reaction started. Unlike reactants, catalysts are not consumed during the reaction. They can be recycled for continued use. There are two types of catalysts – homogeneous and heterogeneous. Homogeneous catalysts are in the same phase as the reactants, while heterogeneous catalysts are in a different phase from the reactants. Enzymes are examples of biological catalysts.
Heterogeneous catalysis uses a solid material to promote reactions that occur in the gas or liquid phase. In heterogeneous catalysis, the reaction molecules bind to the catalyst surface in a process called adsorption. The size of the solid, its pore dimension and its surface area determine its ability to promote the reaction. Porous materials like zeolites, silica gels and gamma alumina have large surface areas for a given volume.
The Advanced Oxidation Process (AOP) is a highly efficient chemical reaction that is non-toxic, environmentally friendly and able to remove organics from water at a molecular level. This method cleans and disinfects wastewater by oxidizing aqueous organic matter to form short-lived hydroxyl radicals. This results in a mineralized, sterilized effluent that contains no harmful chemicals and has a neutral pH. It can be used to treat a wide variety of contaminants in the water, including invasive blue-green algae, organic chemicals, odors and pathogenic microorganisms.
Biodegradation
The biodegradation process transforms organic waste into simple, benign forms that can be easily absorbed and used by microorganisms as a source of energy. This is achieved by the metabolic and enzymatic actions of microorganisms in soil. The organisms break down matter into its constituent parts and produce carbon dioxide, water and other byproducts for respiration. The two main modes of biodegradation are mineralization and cometabolism. The former involves the breaking down of matter into mineral form, while the latter requires a primary carbon source for the breakdown of organic matter.
There are many kinds of bacteria involved in the biodegradation of organic waste. These include anaerobic, aerobic and facultative bacteria. Aerobic bacteria thrive in the presence of oxygen and can quickly convert to anaerobic digestion when oxygen levels decrease. Facultative bacteria are capable of living both under aerobic and anaerobic conditions, but they prefer to breathe aerobically if oxygen is present.
The enzymes that are generated by these bacteria can be used in the treatment of sewage and other organic wastes. These enzymes are very specific in the type of matter they break down. For example, ligninase helps in the biodegradation of plant material, amylase breaks down starches and xylanase breaks down cellulose. Enzymes are biological in origin and their biodegradability diminishes their unfavorable impact on the environment, making enzymatic wastewater treatment an ecologically sustainable technique.