Sources of Antibiotic Contamination in Wastewater and Approaches to Their Removal—An Overview

: In the practice of medicine, antibiotics are extremely important and are employed in the treatment of infections. A lot of antibiotics are consumed by humans and excreted via urine and feces into sewage systems and treatment plants. These are considered to be non-biodegradable, and over the years they accumulate in the aquatic environment. The presence of antibiotics in water resources causes the emergence of antibiotic-resistant bacteria, posing a serious threat to the health of human beings. Water bodies must be adequately treated before being discharged to prevent the spread of antibiotic resistance. In the present article, the sources of antibiotics and strategies used for their effective removal, such as ultraﬁltration, microﬁltration, nanoﬁltration, membranous biological reactor treatment, Advanced Oxidation Process (AOP), Reverse Osmosis (RO) and Nano sorbents, are discussed. Conventional wastewater treatment plants are not able to eliminate antibiotics deposition/resistance genes effectively and efﬁciently. In this regard, the adsorption method is the most effective way of removing antibiotics from wastewater from various sources.


Introduction
Water, a renewable but limited resource available in nature, is essential to human existence as well as environmental sustainability.In certain developing nations, chemical pollution of surface waterways, mostly owing to industrial and agricultural emissions, poses a serious threat [1].Most developing nations respond to public health protection needs through pathogen management and water management [2][3][4].Antibiotics are considered the most essential drugs in hospitals to treat/prevent diseases.The improper disposal of these antibiotics in various ways causes them to reach water bodies [5].Continual exposure to and sub-inhibitory concentrations of antibiotics pave way to bacterial resistance to numerous antibiotics, and form multidrug-resistant bacteria that not only cause resistance but interfere in the bio-geo-chemical cycle [6,7].Besides hospital waste, municipal wastewater is considered as one of the biggest "hotspots" for antibiotics.[8].Feces and carcasses of human and animal sources are the major sources of the introduction of antibiotics, as well as antibiotic-resistant microbes, into water ecosystems.These bacteria can transmit their genes to waterborne pathogenic microorganisms too, thus inducing resistance [9,10].Thus, antibiotic resistance could become a critical issue in modern medicine, as it poses the threat of catastrophic epidemics [11].It becomes highly essential to ensure adequate processing before release into the environment, or else these substances can cause severe pollution and disturb the natural balance.Nevertheless, eliminating antibiotics from wastewater is a major challenge [12].The most common conventional methods used in removing these antibiotics are chemical precipitation, ion exchange, biosorption, reverse osmosis, nanofiltration, etc. [13].Each process's characteristics, as well as biological and chemical oxygen demand, water quality conditions, and environmental factors, influence the antibiotic elimination efficiency [14].Irrespective of all of this, antibiotics are not entirely eliminated in wastewater treatment facilities, as they have been found in stagnant wastewater ponds, municipal sewage, hospital sewage, surface water, and groundwater [15][16][17].At present, membrane technology is considered a promising method for removing antibiotics from effluents [12].An efficient treatment or conservation method would have a positive impact, playing a key role in delivering noteworthy benefits in terms of economic development, either directly or indirectly [18].In this paper, various sources of antibiotics, and the different strategies for their removal, are outlined in order to help prevent the emergence of antibiotic-resistant genes and bacteria that give rise to new diseases, and to help combat the adverse effects on humans and the environment.

Antibiotics in Water
Antibiotics are categorized into 11 types based on their chemical structure, including aminoglycosides, lactams, glycopeptides, macrolides, oxazolidinones, polymyxins, quinolones, streptogramins, sulfonamides and tetracyclines [19].Tetracyclines, sulfonamides, and macrolides are common antibiotics used in human and veterinary medicine.These antibiotics are used to ensure and protect the health of animals, as well as to diagnose and treat infectious disorders in humans [20].The most popular and oldest antibiotic is Penicillin (a category of lactams), which contains naturally occurring penicillin G, extended spectrum penicillin-piperacillin and aminopenicillins-ampicillin [21].The most commonly found antibiotics in effluent, sewage sludge, sediments, soils, and foods are sulfonamides, macrolides, and quinolones.Thus, antibiotic pollution has become a severe matter of concern in many rivers and seas, as it can bioaccumulate in aquatic creatures, leading to a loss of biodiversity [22,23].The presence of antibiotics could pose threats to the food chain, limiting soil decomposition, threatening marine/fresh water/terrestrial creatures, and increasing bacterial resistance [24,25].

Source
As mentioned earlier, antibiotic-based pollution is being found in soil/water resources/sewage [26,27].Since it is harmful to human health and ecosystems, addressing its presence has to be considered a top priority, as it causes bacterial resistance and makes treatment against any infection ineffective.These drug-resistant bacteria could enter humans/higher animals through contaminated water or through food made using that water, and impact the gut microflora within the hosts [28] leading to several health ailments.Finding the source of contamination would help us to prevent the entry of antibiotics into humans or any living system.The sources of antibiotics are discussed below.

Domestic Wastewater
Antibiotics enter the municipal and surface waterways thanks to inadequate treatment or incorrect disposal by users [29], and the major sources of antibiotics are industries (slaughter house/poultry) and domestic household waste (Figure 1).The antibiotics taken by human beings and animals reach the intestines, are excreted out, and then reach the wastewater treatment plants [30].Although municipal wastewater is the greatest source of pharmaceuticals entering the environment, other sources include the inadequate treatment of manufacturing effluents and the direct dumping of unused medicine.The concentrations and types of antibiotics depend on the chemicals used in the industry or by the consumer.
After reaching the treatment plants, the antibiotics undergo some modification (either physical or chemical), and their destiny depends on the type of treatment and process.Antibiotics based on the major six classes-tetracyclines, β-lactams, quinolones, macrolides and sulfonamide-are identified commonly in activated sludge, sewage, digested sludge, and effluents as pseudo-persistent pollutants [19].Municipal wastewaters contain the highest quantities and most diverse populations of micro-and macro-organisms, which increases the emergence of drug-resistant organisms.These organisms might be commensals, pathogens, or environmental bacteria/protozoan/fungi, but they are not unexpected to have antibiotic-resistant characteristics [31,32].Despite the presence of heavy metals, pesticides, endocrine disruptors, and hormones in wastewater, dealing with the development of resistant pathogenic microbes is the most important issue here [33,34].The global prevalence of antibiotics in wastewater treatment plants is outlined in Table 1.The concentrations and types of antibiotics are more diverse in developed countries compared to underdeveloped countries.
by human beings and animals reach the intestines, are excreted out, and then reach the wastewater treatment plants [30].Although municipal wastewater is the greatest source of pharmaceuticals entering the environment, other sources include the inadequate treatment of manufacturing effluents and the direct dumping of unused medicine.The concentrations and types of antibiotics depend on the chemicals used in the industry or by the consumer.After reaching the treatment plants, the antibiotics undergo some modification (either physical or chemical), and their destiny depends on the type of treatment and process.Antibiotics based on the major six classes-tetracyclines, β-lactams, quinolones, macrolides and sulfonamide-are identified commonly in activated sludge, sewage, digested sludge, and effluents as pseudo-persistent pollutants [19].Municipal wastewaters contain the highest quantities and most diverse populations of micro-and macro-organisms, which increases the emergence of drug-resistant organisms.These organisms might be commensals, pathogens, or environmental bacteria/protozoan/fungi, but they are not unexpected to have antibiotic-resistant characteristics [31,32].Despite the presence of heavy metals, pesticides, endocrine disruptors, and hormones in wastewater, dealing with the development of resistant pathogenic microbes is the most important issue here [33,34].The global prevalence of antibiotics in wastewater treatment plants is outlined in Table 1.The concentrations and types of antibiotics are more diverse in developed countries compared to underdeveloped countries.WWTPs-wastewater treatment plants.

Effluent from Hospitals
Contaminated water from hospitals contains pharmaceutical materials, disinfectants, organic halogens, radioisotopes, X-ray contrasted media, heavy metals, cytostatic agents, residues of chemical compounds used in laboratories, and microorganisms of all kinds.Anti-neoplastic medications, among many of the other compounds used in medical therapy, are thought to pose a danger to the environment because of their mutagenic, teratogenic, and carcinogenic potential, whereas cytostatic medicines are subject to severe production and handling regulations [53][54][55].Therefore, hospital wastewater is extremely dangerous, as it is a major source of bacteria and genes resistant to antibiotics [27,56] (Figure 2).Generally, wastewater discharged by hospitals cannot be treated effectively using conventional wastewater treatment technologies, and there is a possibility of these contaminants being discharged into the ecosystem either partially or completely [27,57].The contribution of hospitals to the pharmaceutical load in wastewater is hard to determine, as contraceptives and medicines are widely utilized by the general public.Various initiatives are now underway to assess and screen hospitals, surveying their potential role as sources of drugs and other substances giving rise to multi-drug-resistant bacteria [58].Compared to common wastewater, hospital wastewater contains more than 25% antibiotics, with gene concentrations ranging between approximately 0.4 log and 1.8 log [59].A range of compounds, in addition to pharmaceuticals, are used in hospitals and surgeries for medical purposes, such as in diagnostics and disinfectants [60].In hospital effluent, many antibiotics, including enrofloxacin, ciprofloxacin, oxalinic, ofloxacin, norfloxacin, sulfapyridine, trimethoprim, and metronidazole, have been found in higher concentrations [61,62].

Effluent from Slaughter Houses
Wastewater from animal slaughterhouses is considered as another significant source of resistant bacteria, as clinically administering antibiotics to animals plays an important role in their dissemination in the environment [63].In recent times, antimicrobial resistance in bacteria linked to livestock has increased hugely due to the improper and frequent use of antibiotics in treatments, and physical growth hormones are manipulated in animal husbandry, which has encouraged the emergence of new resistances [64,65].According to Bachmann et al. [66], large volumes of effluents from the slaughtering process may contain bacteria that are resistant to antimicrobials.Because of their high contents of lipids, proteins, fibers, organic contents, pathogens, and medications of veterinary use, meat processing effluents are regarded as detrimental on a global scale [67].For example, oxytetracycline and procaine penicillin G are two of the medications that are often used to treat cattle in central African nations.These medications are excreted by the animals through their feces or urine, and then enter the sewage water system [68].

Effluent from Slaughter Houses
Wastewater from animal slaughterhouses is considered as another significant source of resistant bacteria, as clinically administering antibiotics to animals plays an important role in their dissemination in the environment [63].In recent times, antimicrobial resistance in bacteria linked to livestock has increased hugely due to the improper and frequent use of antibiotics in treatments, and physical growth hormones are manipulated in animal husbandry, which has encouraged the emergence of new resistances [64,65].According to Bachmann et al. [66], large volumes of effluents from the slaughtering process may contain bacteria that are resistant to antimicrobials.Because of their high contents of lipids, proteins, fibers, organic contents, pathogens, and medications of veterinary use, meat processing effluents are regarded as detrimental on a global scale [67].For example, oxytetracycline and procaine penicillin G are two of the medications that are often used to treat cattle in central African nations.These medications are excreted by the animals through their feces or urine, and then enter the sewage water system [68].

Runoffs
Antibiotic consumption patterns in developing countries are influenced significantly by the widespread overuse and misuse of drugs, due to their over-the-counter availability.

Runoffs
Antibiotic consumption patterns in developing countries are influenced significantly by the widespread overuse and misuse of drugs, due to their over-the-counter availability.In consequence of their extensive use, as well as their continued drainage via surface runoff and release from wastewater treatment plants, antibiotics can be found in a number of environmental sectors in these countries [69].This causes antibiotic pollution, resulting in greater antibiotic concentrations in the environment.Thus, antibiotics are being introduced to the environment by a variety of sources, including veterinary waste, human waste streams, livestock husbandry waste, hoarded animal dung, etc.These resources could unleash antibiotics into surface and groundwater, which most often is catalyzed by moderate to heavy rainfall and then begin to move.This has thus become the most common cause of antibiotic contamination in the environment.Furthermore, this runoff is capable of transmitting contaminants from manure to surface water systems in fertilized agricultural fields.Thus, using manure, insecticides and pesticides in cropland also increases the likelihood of the release of bacterial resistance or resistance genes into surface water.In addition, seasonal factors such as snow, precipitation, nutrients, and stream velocity also influence the quantity and the mobility of pollutants [70].The partitioning of antibiotic losses between runoff and sediment is considerably influenced by antibiotic type [71].Due to their high water affinity or solubility in water, antibiotics have spread in the environment in a much quicker and more widespread manner [72,73].In such a context, when effluents from healthcare institutions are dumped straight into a wastewater system without being treated beforehand, it adds to the problem, and this then emerges as one of the major reasons for antibiotic contamination [74].As mentioned earlier, antibiotics disposed of as solid trash can be washed into surface water bodies by rain, or they can permeate into groundwater from dump sites or be taken to nearby water bodies, as illustrated in Figure 3 [75,76].
could unleash antibiotics into surface and groundwater, which most often is catalyzed by moderate to heavy rainfall and then begin to move.This has thus become the most common cause of antibiotic contamination in the environment.Furthermore, this runoff is capable of transmitting contaminants from manure to surface water systems in fertilized agricultural fields.Thus, using manure, insecticides and pesticides in cropland also increases the likelihood of the release of bacterial resistance or resistance genes into surface water.In addition, seasonal factors such as snow, precipitation, nutrients, and stream velocity also influence the quantity and the mobility of pollutants [70].The partitioning of antibiotic losses between runoff and sediment is considerably influenced by antibiotic type [71].Due to their high water affinity or solubility in water, antibiotics have spread in the environment in a much quicker and more widespread manner [72,73].In such a context, when effluents from healthcare institutions are dumped straight into a wastewater system without being treated beforehand, it adds to the problem, and this then emerges as one of the major reasons for antibiotic contamination [74].As mentioned earlier, antibiotics disposed of as solid trash can be washed into surface water bodies by rain, or they can permeate into groundwater from dump sites or be taken to nearby water bodies, as illustrated in Figure 3 [75,76].

Impact of Antibiotics in Wastewater
The most utilized therapeutic drugs in hospitals include contrast media analgesics, anti-inflammatory drugs, laxatives, and antibiotics.The significant and leading sources of these chemicals, are antibiotics, which enter the multiple process of municipal sewers, digested sludge, activated sludge and then to urban bio-solids.If these antibiotics are not removed during water treatment, they can pose a major threat to aquatic life when discharged [77][78][79][80].There are reports that antibiotics have been found in wastewater treatment plant effluent, lakes, rivers, groundwater, and even drinking water.Sulfonamides, tetracyclines, and macrolides are the most commonly identified antibiotic classes present in swine effluent, with concentrations of 324.4,388.7, and 72 g/L, respectively [81,82].As

Impact of Antibiotics in Wastewater
The most utilized therapeutic drugs in hospitals include contrast media analgesics, anti-inflammatory drugs, laxatives, and antibiotics.The significant and leading sources of these chemicals, are antibiotics, which enter the multiple process of municipal sewers, digested sludge, activated sludge and then to urban bio-solids.If these antibiotics are not removed during water treatment, they can pose a major threat to aquatic life when discharged [77][78][79][80].There are reports that antibiotics have been found in wastewater treatment plant effluent, lakes, rivers, groundwater, and even drinking water.Sulfonamides, tetracyclines, and macrolides are the most commonly identified antibiotic classes present in swine effluent, with concentrations of 324.4,388.7, and 72 g/L, respectively [81,82].As the accumulation of these antibiotics is observed under conditions of sub-inhibitory concentrations, the bacteria can slowly become resistant to those antibiotics, and this resistance then starts spreading among bacteria.When the concentrations of antibiotics are high, they show toxicity against aquatic species and adversely affect physical growth, the development of internal organs, reproduction and lifespan, causing ecological imbalance.It has also been demonstrated that macrolides and sulfonamides could inhibit the growth and development of algae, and could also damage plant cells and their photosystems and lower the rate of carbon dioxide oxidation [83].The impacts of antibiotics are further discussed in detail under the following subheadings.

Emergence of Antibiotic Resistance
Antibiotic resistance is the mechanism by which an organism learns to live in the presence of antibiotics, by making antibiotics compatible, by lysing the antibiotics, or by modifying the antibiotic-binding site, and they thus protect themselves from the effects of antibiotics [84].All the aforementioned mechanisms are made possible if the genes of the organisms are mutated.In recent decades, antibiotic resistance genes have evolved and spread across pathogenic and non-pathogenic bacteria [85].In wastewater treatment plants, a wide range of bacteria, antibiotics, metals, and other contaminants interact with the environmental bacteria, making them a hotspot for the evolution of antibiotic resistance [86][87][88].There is thus an increased chance of the emergence of antibiotic-resistant bacteria/antibiotic resistance genes following inadequate wastewater treatments, which are well reported by researchers in the field [89].Similarly, multi-drug resistance in Acinetobacter sp.linked to wastewater treatment has also been reported [90].The presence of antibiotics increases the chance of quorum sensing and riboswitches, and leads to increased phenotypic variability and increased virulence [91].Mutation and gene transfer by conjugation, and transformation or transfection from bacteria to bacteria, can increase the virulence of pathogens, and this could cause serious healthcare issues [91][92][93][94].In a study, it was reported that resistance to aminoglycosides, tetracyclines, and macrolides is conferred by point mutations in ribosomal proteins, while resistance to rifampicin is conferred by point mutations in the RNA polymerase [92].In a study, over 35 mutations were observed in multidrug-resistant S. aureus and M. tuberculosis.The genome sequencing of antibiotic-resistant M. tuberculosis strains revealed 29 independent alterations in a multi-drug resistant strain and 35 mutations in an extremely drug-resistant strain [95].Moreover, seven amino acids of E. coli's gyrA gene product were reported to cause antibiotic resistance [96].

Emergence of New Disease
As antimicrobial-resistant genes are potentially transferable from one microbial community to another via conjugation/transformation, many studies suggest that wastewater could be an important source of antimicrobial resistance in microorganisms [97,98] (Figure 4).The development of antimicrobial resistance in infectious disease-causing bacteria can be described in three steps: 1.The entry of the gene into environmental bacteria in a wastewater treatment plant.2. The passing of the gene between environmental bacteria in water bodies where the treated water is discharged.3. Entry into the host, causing disease (now antibiotics cannot work effectively) [99].Water from wastewater treatment plants could be considered a source of the emergence of new diseases [100].Approximately 700,000 people die every year from infections caused by resistant organisms, and that number is expected to rise to over 10 million by 2050, according to the UK Review on Antimicrobial Resistance [101].Some of the pathogens containing genes of resistance to many types of antibiotics, such as methicillin-resistant Staphylococcus aureus (MRSA), vancomycinresistant Enterococci (VRE), and E. coli carrying extended-spectrum beta-lactamases (ESBL), are known as "superbugs" [91].Further, the term "superbug" refers to microbes with increased morbidity and mortality due to mutations that provide resistance to different types of antibiotics, including aminoglycosides, fluoroquinolones, trimethoprim, etc. [102].Carbapenemases-encoding genes are considered to have originated in environmental microorganisms, and NDM-1-carrying bacteria have been found in public drinking water in New Delhi [103].Likewise, high rates of vancomycin-and ampicillin-resistant E. faecium are found in hospital effluents in the east of England, United Kingdom [104].Additionally, a study by Cahill et al. showed that Carbapenemases-producing Enterobacteriaceae (CPE) are a type of AMR found in high numbers in hospital wastewaters in Ireland [105].In a report, nearly 70% of S. aureus strains were found to be resistant to erythromycin and penicillin [91].More severe diarrhea and higher mortality rates will result, especially if antibiotic resistance increases, which prevents effective treatment [106].In a 2013 report, the Center for Disease Control and Prevention (CDC) stated that the use of antibiotics in the production of animal feed has encouraged an increase in antibiotic-resistant Campylobacter, which caused gastroenteritis in around 2 million individuals in the USA [107][108][109].

Immune Response
The immune system is an effective line of defense against infection, and protects against disease organisms [110,111].Even though antibiotics have several beneficial characteristics, they do have a darker side, as they could disturb normal microbiota/immunity [112].As such, they can also cause allergic hypersensitivity responses, toxic consequences, nephropathy, carcinogenicity, hepatotoxicity, mutagenicity and antibiotic resistance, among other problems [113][114][115].Continuous exposure to antibiotics could lead to enterocolitis, as the normal state of the intestine is disturbed by antibiotics [116].Antibiotics consumed by females before or during pregnancy could affect the child [117].Newborn mice born from antibiotic-treated parents died when treated with higher vaccinia viral loads, whereas mice born from normal parents withstood the viral exposure, suggesting that antibiotics could reduce the level of immunity [118].
Sustainability 2021, 13, x FOR PEER REVIEW 10 of 27 to different types of antibiotics, including aminoglycosides, fluoroquinolones, trimethoprim, etc. [102].Carbapenemases-encoding genes are considered to have originated in environmental microorganisms, and NDM-1-carrying bacteria have been found in public drinking water in New Delhi [103].Likewise, high rates of vancomycin-and ampicillinresistant E. faecium are found in hospital effluents in the east of England, United Kingdom [104].Additionally, a study by Cahill et al. showed that Carbapenemases-producing Enterobacteriaceae (CPE) are a type of AMR found in high numbers in hospital wastewaters in Ireland [105].In a report, nearly 70% of S. aureus strains were found to be resistant to erythromycin and penicillin [91].More severe diarrhea and higher mortality rates will result, especially if antibiotic resistance increases, which prevents effective treatment [106].In a 2013 report, the Center for Disease Control and Prevention (CDC) stated that the use of antibiotics in the production of animal feed has encouraged an increase in antibiotic-resistant Campylobacter, which caused gastroenteritis in around 2 million individuals in the USA [107][108][109].

Immune Response
The immune system is an effective line of defense against infection, and protects against disease organisms [110,111].Even though antibiotics have several beneficial characteristics, they do have a darker side, as they could disturb normal microbiota/immunity.[112].As such, they can also cause allergic hypersensitivity responses, toxic consequences, nephropathy, carcinogenicity, hepatotoxicity, mutagenicity and antibiotic resistance, among other problems [113][114][115].Continuous exposure to antibiotics could lead to enterocolitis, as the normal state of the intestine is disturbed by antibiotics [116].Antibiotics consumed by females before or during pregnancy could affect the child [117].Newborn mice born from antibiotic-treated parents died when treated with higher vaccinia viral loads, whereas mice born from normal parents withstood the viral exposure, suggesting that antibiotics could reduce the level of immunity [118].

Strategies of Antibiotics Removal
It is well understood that antibiotics should be removed from water, but they are resistant to conventional treatments and purification techniques [119,120].There are a few =methods that can be considered successful in removing antibiotics from water, and these are discussed in detail in the below section.

Membrane Technology
Pharmaceutical companies and wastewater treatment plants use conventional processes including filtration, flocculation, sedimentation, etc., but they are not effective in removing antibiotics [120].One of the better technologies for removing antibiotics is membrane technology, which has the benefits of high productivity, ease of use, and low cost (Figure 5).The three primary concepts that govern the treatment using membranes are adsorption, sieving, and electrostatic phenomena.Membrane separation processes rely on hydrophobic interactions between the membrane and the analyte.The size of the pore and the size of the molecules determine the separation of materials via the membrane [121].The pore sizes differ from 2 nm to 100 nm depending on the type of filter chosen [120].Various membrane technologies, including microfiltration, ultrafiltration, nanofiltration, and reverse osmosis, which operate through electrostatic and physical forces, are used for removing antibiotics/antibiotic resistance genes from aqueous solutions such as water [119,122].Thin, light-weight separation membranes with low filtering times and higher adsorption capacities are of greater use [123].Ceramic-based membranes are more advantageous than polymeric membranes as they can tolerate microbial degradation and high-pressure situations [124].Membrane filtration is more widely used in municipal and industrial water/wastewater treatment, as it provides a cost-effective solution for long-term water reuse [125].A variety of antibiotics, such as Chlorotetracycline, Rifampicin, Cefixime, Oxy tetracycline, sulfa drugs, Doxycycline, Tetracycline, etc., are effectively removed using reverse osmosis [126][127][128].Arefi-Oskoui et al. [127] and Javad et al. [128] reported that adding nanoparticles or nanocomposites to membranes does increase their potential ability to remove antibiotics.Membrane technology, however, only changes the state of the antibiotic.If direct or indirect release of concentrations in ecosystems is not prohibited, antibiotics can still recirculate and thus pose a risk of harm in specific regions [129,130].

Biological Aerated Filter (BAF) Process
For the removal of municipal secondary effluents, such as antibiotics, ammo trogen, carbon, and total nitrogen, Biological Aerated Filters (BAFs) have been em [131].Due to their high organic loading, low cost, and lower energy consumption have been used in wastewater treatment plants.The filter media supports the gro microorganisms and the setup includes three phases: a stable segment that acts guide media for the growth of microorganisms, a liquid segment in which the stable

Biological Aerated Filter (BAF) Process
For the removal of municipal secondary effluents, such as antibiotics, ammonia, nitrogen, carbon, and total nitrogen, Biological Aerated Filters (BAFs) have been employed [131].
Due to their high organic loading, low cost, and lower energy consumption, BAFs have been used in wastewater treatment plants.The filter media supports the growth of microorganisms and the setup includes three phases: a stable segment that acts as the guide media for the growth of microorganisms, a liquid segment in which the stable fabric is submerged, and a gas segment formed by injecting the air into the reactor [132,133].As a result of its high biofilm adhesion ability and wide specific area, it enhances the sludge adsorption, resulting in a high antibiotic removal efficiency [134].The major advantage of BAF systems is that they are compact and take only one-third of the space needed by conventional activated sludge systems.When compared to trickling filters, a BAF system maintains a high biomass concentration, and is resistant to organic and hydraulic load shocks.In addition, the system does not require any further treatment, because the suspended particles in wastewater are filtered using submerged media [135].A study carried out by Chen et al. [133] reported the removal of around 80-90% of antibiotics by sulfonamides and diaminopyrimidines, such as sulfamonomethoxine, sulfachloropyridazine, sulfamethazine, trimethoprim, and a class of fluoroquinolones including norfloxacin, ofloxacin, lincosamides such as lincomycin, and macrolides including leucomycin and tetracyclines such as oxytetracycline.These are also efficiently used to remove sulfonamides at a rate of 51%, fluoroquinolones at 20% and ciprofloxacin with an efficiency of above 95% [132].

Ultrafiltration
Ultrafiltration (UF) is determined by a variety of parameters, which include load and particle size (Figure 5).UF is not an efficient technique for separating organic streams, but holds the ability to sustain species with molecular weights ranging between 300 and 500,000 Da and pore sizes ranging from 10 to 1000 Å [119].Ultrafiltration membranes are widely used for water filtration, which removes harmful bacteria, macromolecules, and suspended matter with the consumption of less energy.However, there are certain drawbacks, which include the inability to remove any dissolved inorganic pollutants from water and the requirement of high pressure for regular cleaning [121].According to Li et al. [136], a membrane system composed of reverse osmosis and ultrafiltration is more efficient for processing oxytetracycline waste and recovering oxytetracycline from wastewater, with a 60% recovery ratio and 80% purity levels.According to a study conducted by Liang et al. [137], the use of membrane technologies such as ultrafiltration and reverse osmosis achieves 99.79% effectiveness in the removal of ARGs and bacteria.In comparison to the conventional approach, ultrafiltration has a high turbidity rate, as well as filtration accuracy, and the effluent water quality is stable and reliable [138,139].

Microfiltration
Microfiltration is a filtration method in which the pores have a diameter of 1 to 10 µ.These pores cannot be crossed by microorganisms.This approach is considered as the initial step in the pre-treatment prior to nanofiltration and reverse osmosis [121].This method is used to separate colloidal particles via a screening mechanism that is employed to retain substances larger than the pore size [119].The membrane material in MF can be either organic or inorganic.Polymers such as polyvinylidine fluoride, polysulfone, cellulose acetate and polyamide are used in organic membranes, while porous alumina and metals are used in making inorganic membranes.This process is good for isolating suspensions and emulsions, and can be used to remove sediment, algae, protozoa, and bacteria, while allowing monovalent ions such as Na + , Cl − , dissolved organic materials, and tiny colloids and viruses to pass through the membrane [140].Microfiltration provides the benefit of increased oxidizable material removal, with a rate of around 90%.One of the most significant drawbacks of MF is that it cannot remove impurities (dissolved particles) smaller than 1 mm in size.MF, however, appears to suppress harmful microbes in water when applied in conjunction with disinfection [121].Qiu et al. [141] reported the removal of antibiotics such as enrofloxacin, sulfamethazine, cefalexin, lomefloxacin, amoxicillin and ampicillin with an efficiency of 58.9 to 100% using a hybrid microfiltration-forward osmotic membrane bioreactor, but were not able to remove low-molecular weight antibiotics such as sulfonamides and trimethoprim.

Nanofiltration
One of the main techniques involves using nanoparticles [142][143][144].Nanofiltration is an effective method for the removal of large organic materials and average-sized minerals, but is not applicable to low-molecular weight organic substances such as methanol, because these membranes operate at very low pressures (Figure 6) [119].Membranes used for nanofiltration are made of cellulose acetate blends or polyamide composites.The modified versions of UF membranes include sulfonated polysulfone.In the nanofiltration process, the wastewater is fed into the inlet and the incoming stream is separated into two parts: the permeate, which is the filtered component, and retentate, or concentrate, which is the nonfiltered fraction that is rejected.NF is capable of successfully removing organic matter, and works on the principle of the application of hydrostatic pressure, which is used to convey a molecular mixture to the membrane's surface.When pressure is applied, solvents and lowmolecular weight solutes pass through the membrane, while other components are trapped.This is sufficient to eliminate ions that contribute significantly to osmotic pressure, resulting in reduced operating pressures.As soluble fractions cannot be eliminated by nanofilter membranes, highly contaminated fluids require a pretreatment process [140].Nanofiltration membrane's removal rates depend on the target antibiotic molecule; where macrolides have been removed with an efficiency rate of 80 to 95%.In a study using self-made polyethersulfone membrane nanofilters, the removal efficiency of the antibiotic amoxicillin was found to be 99.09% at a temperature of 298 K and pressure of 2 MPa [145,146].
Sustainability 2021, 13, x FOR PEER REVIEW 14 of 27 solvents and low-molecular weight solutes pass through the membrane, while other components are trapped.This is sufficient to eliminate ions that contribute significantly to osmotic pressure, resulting in reduced operating pressures.As soluble fractions cannot be eliminated by nanofilter membranes, highly contaminated fluids require a pretreatment process [140].Nanofiltration membrane's removal rates depend on the target antibiotic molecule; where macrolides have been removed with an efficiency rate of 80 to 95%.In a study using self-made polyethersulfone membrane nanofilters, the removal efficiency of the antibiotic amoxicillin was found to be 99.09% at a temperature of 298 K and pressure of 2 MPa [145,146].Reverse osmosis (RO) is a method whereby large ions and molecules are typically removed from liquid effluents using the process of diffusion, which involves the application of pressure to the solution on one side of a semipermeable membrane.The impurities are confined on the pressured side of the membrane, while the clean effluent flows against

Reverse Osmosis
Reverse osmosis (RO) is a method whereby large ions and molecules are typically removed from liquid effluents using the process of diffusion, which involves the application of pressure to the solution on one side of a semipermeable membrane.The impurities are confined on the pressured side of the membrane, while the clean effluent flows against the concentration gradient [147].The reverse osmosis technique can be used to remove bacteria, viruses, dissolved solid particles, and microbial agents present in wastewater.This method is mostly employed to desalinate water [119].The main physiochemical properties considered for the process of reverse osmosis are porosity and mechanical resistance.In this context, these membranes must be chemically and microbially resistant, as well as mechanically and structurally durable over long periods of time, and they can be eliminated by the use of polymer membranes [147].For amoxicillin, the removal rate via RO membranes ranges from 73.52% to 99.36%, while for ampicillin, the removal rate and permeate flux range from 75.1% to 98.8% [148].Further, the removal rates of antibiotics such as trimethoprim dexamethasone, febantel, ciprofloxacin and sulfamethoxazole using a hybrid of reverse and nanofiltration were observed to be 97%, according to Dolar et al. [149].Despite the apparent benefits of utilizing an RO membrane, it is still unclear whether it can act as a complete barrier to enhance the removal of organic micropollutants.Several top scientists have identified RO as the most effective and promising method for removing organic micropollutants [150,151].

Membranous Biological Reactor
Membranous Biological Reactors are a form of biological wastewater treatment system that blends membrane separation technology with biological technology.They have a chamber in which biological matter is separated by a membrane, a micro-filter with a size of roughly 1-10 µm [119].This is a wastewater treatment or resource recovery method that incorporates biological processes such as activated sludge with membrane processes such as ultrafiltration, nanofiltration, and microfiltration [130].The use of a Membranous Biological Reactor has many advantages, including a long sludge holding period, ease of operation, low sludge output, and excellent nitrification performance [134].Filtered wastewater derived from these reactors has a comparable quality to that of secondary sewage treatment, and exhibits similar microfiltration output.These reactors are excellent for use in wastewater treatment and reuse in both urban and industrial settings.These reactors are demonstrated to be effective for use in both aerobic and anaerobic wastewater filtration [119].The MBR process has emerged as an effective wastewater treatment method due to its ability to fill in the gaps left by traditional activated sludge processes, such as their inability to deal with fluctuations in effluent flow rates and composition, and their inability to comply with higher effluent discharge limits related to reuse.In comparison to traditional treatment systems, membrane bioreactors also save a lot of space [130].In a study, the removal of 15 organic micropollutants using a laboratory-scale membranous biological reactor was evaluated, and it was discovered that most organic micropollutants were removed at an efficiency rate of 80-92% [152].It was also found that 100% of roxithromycin and norfloxacin and 99% of sulfadimidine could be removed using membrane bioreactors, while accelerating the degradation of antibiotics and decreasing their antibacterial activity could be achieved through pretreatment with bioelectrochemical systems [56,81].As a result, for the majority of pharmaceutical chemicals, discharge concentrations of Membranous Biological Reactors were considerably lower than those using traditional techniques [120].

Advance Oxidation Process
The recalcitrant nature of effluents containing antibiotic residues makes conventional biological treatments ineffective in removing these chemicals.One of the approaches used in these processes is advanced oxidation [147].In advanced oxidation processes (AOPs), organic molecules in wastewater, which are difficult to handle biologically, are oxidized and converted to simpler products [153].This approach is based on the production and use of free radicals, such as hydroxyl, as potent oxidizers with objectives such as the destruction of substances that do not completely oxidize with commonly used oxidizers such as chlorine.By breaking chemical bonds or through mechanisms such as electron transfer, addition, and substitution, the OH − decomposes organic pollutants into small molecules of organic matter, as well as CO 2 and H 2 O, more easily [134,154].This approach includes ozonaion, Fenton oxidation, photo Fenton, oxidation of electrochemical and UV/H 2 O 2 methodologies [119,134].The removal efficiencies of antibiotics such as Amoxicillin, Ampicillin, Ciprofloxacin, Doxycycline, Erythromycin, Norfloxacin, Ofloxacin, Tetracycline, Vancomycin and Roxithromycin using AOPs are 73-100%, 99-100%, 87-100%, 70-99%, 90-100%, 80-100%, 94-100%, 70-95% and 89-99%, respectively.In comparison with the conventional methods of wastewater treatment, AOP has the following advantages: (i) no sludge production, (ii) lesser retention time for pollutant removal, and (iii) the ability to mineralize contaminants at a faster pace [155].The major drawback of AOP is its high cost, which is related to the use of expensive chemicals such as hydrogen peroxide (H 2 O 2 ) and the significant energy consumption related to the generation of O 3 or UV radiation [1].

Adsorption
Understanding antibiotic absorption on adsorbents requires knowledge of their physical and chemical characteristics.Adsorption is the deposition of elements from a gas or liquid phase onto the surface of an adsorbent, which can be physical or chemical [156].Because of its great efficiency, low cost, ease of operation, and design simplicity, adsorption technology has seen widespread use (Figure 7) [65].Adsorption is a typical method used for removing metal ions from a variety of industrial effluents [157].Magnetic nanoparticles, silver nanoparticles, activated carbon, nanotubes, and polymer nanocomposites have all been utilized as adsorbents to remove various pollutants, including heavy metals, which are extremely toxic even at low concentrations [121,158,159].Adsorbents such as activated carbon can be used in an adsorption filtering process to remove high levels of antibiotic compounds, but the effectiveness of antibiotic absorption is influenced by several factors such as (i) activated carbon, (ii) target compound concentration, (iii) dissolved oxygen concentration, and (iv) pH and temperature [119].Natural zeolite, bentonite, activated carbon, carbon nanotubes, and biochar are used to remove antibiotics from aquatic environments by adsorption [160].The phenomenon of adsorption involves (i) the movement of the adsorbate via the stationary liquid film around the adsorbent-solute transfer in the bulk, (ii) the diffusion of film and transport of adsorbate along the film, (iii) the diffusion of pores and adsorbate through the porous structure into the active sites, and (iv) adsorption between the adsorbate and porous structure.The antibiotic adsorption capacity is further influenced by electrostatic and hydrophobic interactions between antibiotics and sludge.The mechanisms of adsorption can be further identified using an isotherm model such as Langmuir, Freundlich, Tempkin, etc. [161].Antibiotics are classified as positive, neutral, or negative, depending on the pH of the solution, and the removal efficiency depends upon the electrostatic interaction between antibiotics and charged sorbents [132].The effectiveness of MBR in treating water contaminated with antibiotics is increasingly being investigated.Membranous Biological Reactors (MBRs) have several advantages, including a higher solids retention time (SRT), reduced sludge generation, and a higher concentration of mixed liquid-suspended solids when compared to Conventional Activated Sludge Systems (CAS) [162].The efficient removal of antibiotics such as Sulfamonomethoxine (SMM) (11%), Sulfamethoxazole (SMX) (6.5%), and Sulfadimethoxine (SDM) (19%) from wastewater using sorbents has been reported by researchers [162].

Nanoparticle Interaction
Nanoparticles are particles with sizes ranging from 1 to 100 nm that have a wide variety of uses owing to their unique physio-chemical qualities [163].Nanoparticles such as iron oxide nanoparticle, superparamagnetic iron oxide nanoparticles and silver nanoparticles have been used in the removal of water pollutants [142,[163][164][165].When nanoparticles are used to combat antibiotic resistant, two alternative methods are available.Firstly, antibiotics are mixed with a functionalized nanomaterial, and the nanomaterial enters the ARB, releasing a huge number of damaging ions.Combining antibiotics with nanomaterials provides synergistic effects in the second strategy, which means they combat ARGs jointly rather than individually.A combination of nanomaterials and molecular antibiotics has gained popularity due to its effectiveness in eliminating multidrug-resistant pathogenic bacteria and attacking a wide range of ARBs and ARGs [144].Carbon nanotubes are a member of the carbon family, and are notably a new type of adsorbent that has demonstrated a fantastic ability to cast off many types of pollution [166].Carbon nanotubes are prospective solutions that can be used to tackle environmental pollution because of their large specific surface area, extensive pores, hollow structures, and strong and effective interactions with pollutant molecules [167].As such, to be effective as an absorbent material, graphene, which has a large specific surface area and pore volume and thus offers adequate adsorption sites for contaminants in wastewater treatment, should be used [168].

Nanoparticle Interaction
Nanoparticles are particles with sizes ranging from 1 to 100 nm that have a wide variety of uses owing to their unique physio-chemical qualities [163].Nanoparticles such as iron oxide nanoparticle, superparamagnetic iron oxide nanoparticles and silver nanoparticles have been used in the removal of water pollutants [142,[163][164][165].When nanoparticles are used to combat antibiotic resistant, two alternative methods are available.Firstly, antibiotics are mixed with a functionalized nanomaterial, and the nanomaterial enters the ARB, releasing a huge number of damaging ions.Combining antibiotics with nanomaterials provides synergistic effects in the second strategy, which means they combat ARGs jointly rather than individually.A combination of nanomaterials and molecular antibiotics has gained popularity due to its effectiveness in eliminating multidrug-resistant pathogenic bacteria and attacking a wide range of ARBs and ARGs [144].Carbon nanotubes are a member of the carbon family, and are notably a new type of adsorbent that has demonstrated a fantastic ability to cast off many types of pollution [166].Carbon nanotubes are prospective solutions that can be used to tackle environmental pollution because of their large specific surface area, extensive pores, hollow structures, and strong and effective interactions with pollutant molecules [167].As such, to be effective as an absorbent material, graphene, which has a large specific surface area and pore volume and thus offers adequate adsorption sites for contaminants in wastewater treatment, should be used

Recycling Strategies
The "Era of Wastewater Reclamation, Recycling, and Reuse" began in the early twentieth century as a result of technological improvements in the chemical, physical, and biological processes of wastewater treatment [169,170].The reuse of wastewater effluent is commonly seen in both developing and industrialized countries such as the United States of America, Japan, China, Korea, and Israel [171][172][173].The majority of previous studies have focused on floatation, coagulation, and reverse osmosis or membrane bioreactor processes as potential wastewater treatment options [174].The use of sequencing anoxic/anaerobic membrane bioreactors, a type of MBR method that removes nitrogen and phosphorus concurrently, has been introduced, and it outperforms the improved Luzack-Ettinger process of phosphorus removal [175].The percentage of activated sludge in this process was raised from 30 to 70%; Galaxolide, Tonalid, Ibuprofen, Naproxen, 17-Estradiol, and Sulfamethoxazole can be removed in wastewater treatment plants [176].Activated carbon filtration, with both powdered and granular activated carbon, has been used [177].The adsorption of micropollutants from wastewater using powdered activated carbon is an excellent method for removing micropollutants such as Sulfamethoxazole, Atenolol, Diclofenac, etc. [178].Granulated activated carbon is the most cost-effective and frequently used material for removing organic micropollutants such as Fexofenadine, Carbamazepine, Lamotrigine, Oxazepam, Fluconazole, Cetirizine, and N, N-diethyl-meta-toluamide from contaminated drinking water [179].
The law clarifies the governments' and other stakeholders' responsibilities in the fight against antibiotic resistance, including the use and discharge of antibiotics into the environment.Recent systematic reviews laid out 17 policy measures to decrease human antibiotic usage, although it is unclear whether or not most of the policies will have any influence on antibiotic use [180].Some of the recycling strategies include the construction of vertical flow wetlands, which will help in the removal of antibiotics such as sulfamethazine, ciprofloxacin and oxytetracycline with an efficiency of 68-85%.Similarly, other strategies include the construction of horizontal subsurface flow wetlands, photocatalytic degradation, sonocatalytic irradiation, ultrafiltration using PVC membrane, adsorption by activated carbon, carbon nanotubes, electro-coagulation, advanced oxidation processes in combination with UV/hydrogen peroxide, and advanced oxidation processes using Fenton process, and their efficiencies are shown in Table 2.

Conclusions
It is well understood that the presence of antibiotics in water is becoming a serious issue, and it has to be sorted out.To conclude, one of the main causes of the introduction of drugs and antibiotic-resistant bacteria into aquatic environments is excrement and corpses from human and animal sources.These bacteria can also pass on their genes to waterborne pathogenic germs, giving them the ability to produce resistance genes.The increased use of antibiotics will definitely result in the emergence of large numbers of new antimicrobial-resistant bacteria, and the diseases caused by them could be untreatable.This review has included a variety of sources of antibiotic pollution, their effects, and a number of innovative methods for removing these antibiotics from wastewater.In order to combat the growing problems of pollution caused by antibiotics, it is critical to not only continuously improve wastewater treatment systems, but also to prevent antibiotic overuse and excessive discharge into water resources.The life expectancy of humans will be shortened if these antibiotics are widely used in water bodies.The treatment of an increasing number of ailments becomes increasingly difficult as antibiotics lose their efficacy.Longer hospital stays, higher healthcare costs, and higher mortality are all consequences of antimicrobial resistance.

Figure 1 .
Figure 1.Effluent discharge of municipalities into wastewater treatment plants.

Figure 1 .
Figure 1.Effluent discharge of municipalities into wastewater treatment plants.

Figure 2 .
Figure 2. Sources of effluents from hospitals.

Figure 4 .
Figure 4. Emergence of new disease due to antimicrobial resistance genes (ARGs).

Figure 4 .
Figure 4. Emergence of new disease due to antimicrobial resistance genes (ARGs).

Table 1 .
The antibiotics found in various regions of world.

Table 1 .
The antibiotics found in various regions of world. S.

Table 2 .
Various recycling techniques for antibiotics removal.