## 6 Types and Mechanisms of Removing Xenobiotics from Water
### Challenges of Enzymatic Bioremediation
Higher mixing speeds result in increased biodegradation efficiency due to better oxygen availability in the aerobic processes [280]. Processes carried out in bioreactors are more environmentally friendly, as well as cost- and time-efficient [273]. There are two groups of bioreactors: (i) enzyme bioreactors (EBR) and (ii) enzyme membrane reactors (EMR) [311]. In membrane bioreactors, enzyme molecules can be immobilized in or on the membrane primarily by covalent bonds to prevent elution and loss of the catalytic properties of the enzyme during successive reaction cycles. The enzyme, as well as the structure, physicochemical properties and size of the expected biodegradation products affect the choice of the bioreactor membrane. Therefore, it is important to know the membrane characteristics, such as pore size, material, properties, presence of functional groups, hydrophilicity and surface charge [273; 312; 313]. A large surface area for enzyme attachment and good porosity and spatial structure are sought-after properties of good membrane support. All these factors facilitate the access of the reaction mixture (polluted sewage) to the active sites of enzymes. The membrane should be made of a material that is inert to the reaction components and mechanically resistant [159]. Based on the pore size of the membrane used, wastewater treatment processes can be divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). MF, UF and NF membranes are used to remove contaminants of 100-1000, 5-100 and 1-5 nm, respectively [269]. Membrane fouling is a key factor in EMR efficiency, so it is important to use pretreated wastewater without solids and operate the bioreactor in a cross-flow filtration mode [261; 160; 273]. Another significant factor in the course of bioremediation in a membrane bioreactor, especially in a batch bioreactor, is the enzyme dose. In general, the efficiency of degradation increases with increasing enzyme concentration until a certain optimum is reached. Degradation efficiency at a specific concentration of the enzyme maintains the appropriate reaction parameters, e.g., contaminant concentrations in the feed material in the batch bioreactor system remain constant [280]. When designing a bioreactor, a thorough study of the biological system used should be carried out, and capital costs, plant construction, maintenance costs, stability and scaling should be considered [159]. In EMR, the membrane should act as both a carrier of enzymes and a separator of products and substrates, which will simplify the process and reduce its costs. The choice of reactor type and operating mode (batch or continuous) is made depending on the process conditions, enzymatic activity, use of a free or immobilized enzyme, the form of the carrier if the enzyme is immobilized and the required purity of the product [273]. Enzyme catalytic activity should be at the highest possible level in terms of enzyme units per gram of support [159]. Currently, bioremediation allows the transformation of a significant part of difficult-to-degrade substances into simpler compounds, but it is not able to completely remove pollutants. Therefore, it is necessary to combine enzymatic degradation with other wastewater treatment techniques, such as photocatalysis, activated sludge or anaerobic digestion to achieve the full removal, detoxification and mineralization of pollutants [154; 160; 263].