# Rationale for the Combined Use of Biological Processes and AOPs in Wastewater Treatment Tasks
## 1 Introduction
Considering the distribution of the proportion of publications on wastewater treatment topics (Figure 1), there has been an increase in publication activity in this area since 1970, with an intensive build-up of research on various areas of wastewater treatment beginning in 2005. The rapid growth of publication activity began in 2016, and peaked in 2020. In recent decades, there has been a clear trend towards the mainstreaming of water conservation issues in different regions of the world, with the formation of a number of integrated solutions for treatment of wastewater of different origins [10,11]. Thus, Figure 2 shows the thematic area of research according to different sources of publications, showing that there is an interdisciplinary approach to the topic of wastewater treatment. Publications in engineering, chemical sciences, and ecology, as well as materials science and energy, predominate. At the interface of the sciences, there is also interaction with the trend of engineering implementation of solutions for wastewater management and treatment of sewage sludge, development of sustainable water management and its energy potential, and environmental and economic justification of wastewater treatment processes. At the same time, biological processes for wastewater and sludge treatment have been widely used [12,13]. Special attention is given to the processes of studying the inhibitory effects of various toxic substances on the bioprocess of wastewater treatment [14]. For example, [PERSON] et al. [14] investigated the modes of operation of a hybrid membrane bioreactor with directed osmosis microfiltration (MF-FOMBR) in the treatment of municipal wastewater. The effects of 20 commonly used antibiotics were investigated. At concentrations of 4.1-716.9 ng/L (regraffloxacin, sulfamethazine, and cephalxin were the most common antibiotics, followed by amoxicillin, lomefloxacin, and ampicillin), the system showed total removal of 58.9-100% of all antibiotics [14]. However, it is worth noting the wide range in the removal efficiency of these antibiotics, which indicates the different levels of resistance to them of the activated sludge's biocenosis. A study by [PERSON] et al. [15] operated aerobic granular sludge (AGS) at high ammonium concentration to remove three fluoroquinolones (FQs), i.e., ciprofloxacin, ofloxacin, and nonfloxacin at 3, 300, and 900 ug/L, respectively. The main objectives were to study the differential distribution of antibiotic resistance genes and mobile genetic elements in the sludge fractions in the nitrification and denitrification processes. AGS demonstrated excellent stability under the influence of FQs, with nitrite-oxidizing bacteria being more sensitive to FQs than ammonium-oxidizing bacteria [15]. Thus, it was determined that different ecological and trophic levels of the biocenosis have their own threshold of adaptation to increasing concentrations of antibiotics, which is also important to consider when optimizing the wastewater treatment process. Figure 1: Publication activity curve by year based on the Scopus database using the keyword combination “wastewater” and “treatment”. A study by [PERSON] et al. [16] described the effects of antibiotics and antibiotic-resistance genes in biological treatment technologies and combined processes in wastewater treatment plants. Adsorption is the main antibiotic removal mechanism for tetracyclines, fluorquinolones, and macrolides, while biodegradation is the pathway for antibiotic removal for sulfonamides and \(\beta\)-lactams. In addition, the process of microbial co-metabolism contributes to the biodegradation of antibiotics. Process engineering and continuous monitoring and control of bioremediation parameters, the flexibility of the system to adapt to fluctuations, and capital and operating costs are all important [16]. Microbial community biodiversity is shaped by many coexisting factors, including seasonality, the material of water system fixtures and biofilm carriers, and the location of sampling sites.