Still open research questions on technologies, microplastics, and antibiotic resistance

© 2024 IWAP. This is an Open Access book chapter distributed under the terms of the Creative Commons Attribution License (CC BY-NC-ND 4.0) which permits copying and redistribution for non-commercial purposes with no derivatives, provided the work is properly cited (https://creativecommons.org/licenses/by-nc-nd/4.0/). The chapter is from the book Anaerobic Treatment of Domestic Wastewater: Present Status and Potentialities, M.C. Tomei and J.M. Garrido (Editors). doi: 10.2166/9781789063479_0329 J. C. de Araújo1, H. A. Santos1, C. Vijande2, J. M. Garrido2 and M. C. Tomei3* 1Department of Sanitary and Environmental Engineering, Federal University of Minas Gerais, Belo Horizonte 31270-010, Brazil 2CRETUS, Department of Chemical Engineering, Universidade de Santiago de Compostela, Santiago de Compostela, Spain 3Water Research Institute (IRSA), National Research Council (CNR), Roma, Italy *Correspondence: concetta.t[email protected]r.it ABSTRACT In this final chapter, a summary of the still open questions to be solved for the scale-up of the technologies described in the previous chapters (including post-treatment options), for application in cold and temperate climate areas is provided. Special attention is dedicated to upflow anaerobic sludge blanket and anaerobic membrane bioreactors (AnMBRs), which are, even if with different peculiarities, the most promising solutions. In addition to anaerobic mainstream treatment technologies, recent research demand focuses also on two important emerging topics, which are of interest not only for anaerobic wastewater treatment, but also for conventional activated sludge plants, that is, the fate of microplastics (MPs) in plants and in receiving water bodies and the antibiotic resistance spreading. For these two topics, which were not analyzed in detail in the previous chapters, we propose in this chapter a general presentation of the state of knowledge in relation to wastewater treatment plants (WWTPs) with particular focus on anaerobic systems and related research needs. Concerning MPs, in spite they are recognized as ubiquitous pollutants, there is a strong research demand on the standardization of sampling and analytical protocols and on powerful technologies able to improve their removal. Even though information about the fate and effect of MPs in WWTPs with anaerobic treatments is scarce, AnMBRs have demonstrated high removal rates, which suggest them as a promising technology. As regard to antibiotic resistance, WWTPs are one of the main sources of dissemination of antibiotic-resistant bacteria (ARBs), antibiotic-resistant genes (ARGs), and antibiotic residues into the environment. Even if the available data are referring mainly to warm climate regions, it is worth noting that anaerobic reactors, alone or in combination with aerobic post-treatment, can remove from 0.5 to 3.0 log units of ARBs and ARGs depending on the resistance bacteria or gene. Also in this case AnMBRs achieved the best performance while good results have been also obtained with combined treatment options (i.e., anaerobic– aerobic and anaerobic post-treatment). Research demand on antibiotic resistance is driven by the need of clarifying the fate of ARBs, ARGs, antibiotics, pathogens in the treatment line, and the effects of wastewater characteristics on the plant performance. Important challenge is the development and evaluation of more effective disinfection and treatment methods (such as ultrafiltration and advanced oxidative processes) able to reduce the antibiotic resistance spreading from WWTPs. Keywords: anaerobic domestic wastewater treatment, antibiotic residues, antibiotic-resistant bacteria, antibioticresistant genes, high-rate bioreactors, microplastics. Chapter 12 Still open research questions on technologies, microplastics, and antibiotic resistance Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 330 Anaerobic Treatment of Domestic Wastewater: Present Status and Potentialities 12.1 ANAEROBIC DOMESTIC WASTEWATER TREATMENT IN A CIRCULAR ECONOMY The concept of circular economy has been defined in many ways as well highlighted by Kirchherr etal. (2017), who, after a systematic analysis of the 114 circular economy definitions found in literature, proposed the following reference definition: ‘circular economy is an economic system that replaces the “end-of-life” concept with reducing, alternatively reusing, recycling, and recovering materials in production/distribution and consumption processes. It operates at the micro level (products, companies, consumers), meso-level (eco-industrial parks) and macro level (city, region, nation and beyond), with the aim to accomplish sustainable development, thus simultaneously creating environmental quality, economic prosperity and social equity, to the benefit of current and future generations.’ An anaerobic process fits perfectly this complete and exhaustive definition, because it allows resource recovery, including energy, safe water, and nutrients, from valuable by-products of industries, but, and even more important, from wastes and wastewater. In the case of domestic wastewater (DWW) treatment, the recovery of energy and nutrients is accomplished from a huge ‘dirty’ matrix whose conventional treatment generally implies energy consumption and loss of the nutrients contained in the influent. In this context, the anaerobic process potentially represents the optimal mainstream treatment option for DWW. Unfortunately, in cold and moderate climate regions, this option is still challenging due to the low process efficiency at these temperatures for dilute streams such as DWW. In the previous chapters, several key aspects for the application of anaerobic processes as mainstream treatment of DWW have been highlighted as well as the related research needs. In this final chapter, a summary of the still open questions to be solved to promote the scale-up of the proposed technologies, including post-treatment options, for application in cold and temperate climate areas is provided. Special attention is dedicated to upflow anaerobic sludge blanket (UASB) and anaerobic membrane bioreactors (AnMBRs), which are, even if with different peculiarities, the most promising solutions. In addition to technologies, recent research demand focuses also on two important emerging topics, which are of interest not only for anaerobic DWW treatment, but also for conventional wastewater treatment plants (WWTPs), that is, the fate of microplastics (MPs) and antibiotic resistance spreading, both of them not extensively investigated for anaerobic processes. For these two topics, which were not analyzed in detail in the previous chapters, we propose in this chapter a general presentation of the state of knowledge in relation to WWTPs with particular focus on anaerobic systems and related research needs. 12.2 MAINSTREAM TECHNOLOGIES FOR THE ANAEROBIC TREATMENT OF DWW 12.2.1 General aspects to be investigated for high-rate systems The first key element for the feasibility of anaerobic processes is the technology: it has been demonstrated that conventional suspended biomass reactors cannot achieve the required performance in terms of effluent quality with reactor volumes and footprint suitable for practical application, thus high-rate bioreactors are required. Types and related characteristics of high-rate systems have been presented in this book; they are generally at medium/high technology readiness level but there are common research issues to be investigated. The first aspect is the performance optimization to define the best set of operating parameters depending on the final destination of the produced effluent: many studies have been conducted with this objective and they provided consistent results demonstrating the feasibility of high-rate bioreactors for chemical oxygen demand (COD) removal in DWW. Concerning the nutrients, anaerobic effluents are characterized by high N and P contents, which, as optimal solution congruent with sustainability goals, should be recovered as much as possible. Their recovery is not always easy and feasible due to geographical, logistic, and legislation issues. As already mentioned, research efforts and management strategies should be devoted to modify the approach in dealing with urban wastewater to be considered more as a source of energy and valuable compounds than a liquid waste. When the nutrient recovery is not feasible, appropriate post-treatment is necessary, also the challenge Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 331Still open research questions on technologies, microplastics, and antibiotic resistance in this case is the achievement of the good effluent quality with a minimum expense of energy in order to not having a massive impact on the recovered energy in the anaerobic step. Another general critical aspect in the performance of anaerobic systems is the presence of dissolved methane (dCH4) in the treated effluent whose recovery has to be maximized to avoid energy loss and reduce dangerous greenhouse gas emissions. Research efforts on strategies for dCH4 recovery or reuse within the same treatment process are mandatory to approach energy-neutral anaerobic treatment, and to exploit the intrinsic merits of the process to be economically feasible and environmentally friendly. Several technologies are available for the dissolved CH4 recovery, both through physical methods, such as aeration, gas stripping, and degassing membranes, and for its biological removal through down-flow hanging sponge reactors and the more recent proposed process based on denitrification and anaerobic CH4 oxidation (N-Damo) (Stazi & Tomei, 2021). All the proposed technologies need further investigations to optimize the process performance at pilot and demonstration scales. In fact, their applicability has not yet been fully evaluated in terms of economic feasibility and process safety (Liu etal., 2014). Other issues, which deserve greater care, would be the regular and accurate measurement of the concentration of dCH4 in the effluent, instead of the generally assumed theoretical data referring to the thermodynamic equilibrium conditions, and the influence of some technical aspects, such as the technology itself, the operation mode, and the type of biomass (Cookney etal., 2016). The most promising solution for CH4 recovery, that is, membrane separation, requires additional research on the evaluation and related impact of wetting, fouling, and clogging phenomena. There are available control strategies for these phenomena but in most cases are energy-intensive and not economically feasible and research efforts should be devoted to these critical aspects. 12.2.2 UASB bioreactors UASB reactors, described in detail in Chapter 2, represent an effective technology relatively easy to apply whose advantages for the anaerobic treatment of DWW in warm climate areas are well recognized. The extension of UASB application even in moderate/cold climate regions requires technological improvements as the maximization of the biomass activity with configurations able to concentrate the biomass inside the bioreactor. As reported in Chapter 4, promising results have been achieved with attached and granular biomass systems. These studies, as the most part of literature in this field, are conducted at lab scale and consistent work should be dedicated to their scale-up. Another interesting technical solution investigated in the two above mentioned studies is the operation in sequential mode, which is not common for an anaerobic process, but, given its flexibility, can be an effective solution for small wastewater treatment installations and to face the high seasonal load variations, which can occur in touristic zones. 12.2.3 Anaerobic membrane bioreactors Membrane bioreactors represent the most powerful technology developed so far for enhancing the performance of biological processes and it is of particular relevance for the anaerobic process application even for treatment of low-strength wastewaters as the domestic ones. The use of filtration membranes would revolutionize treatment systems, eliminating practically all the colloidal compounds, germs, and suspended solids usually present in anaerobic effluents. Nevertheless, the anaerobic treatment of DWW, using submerged membrane technology, is today still a promise and not a reality. This is due, in practice, to the lower flux obtained in both laboratory and pilot scale AnMBR units, usually between 5 and 10 L/m2/h, far from the values of 20 L/m2/h or higher, observed in full-scale aerobic MBRs. Chapter 3 is dedicated to this technology and related research needs are well highlighted. Besides the need of performance optimization and facing the well-known fouling phenomenon, the most important aspect for the anaerobic treatment of DWW is the ‘the minimization of energy use’ associated in practice with the stable operation at higher flux, which is mandatory to achieve a positive energy balance making an anaerobic process competitive with an aerobic one. Energy consumption and fouling phenomena are strictly related and it is challenging to find the Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 332 Anaerobic Treatment of Domestic Wastewater: Present Status and Potentialities optimal equilibrium solution allowing acceptable fouling levels and, at the same time, acceptable energy consumptions. Strategies and technological alternatives to work in this direction are described in detail in Chapter 3. 12.2.4 Post-treatments As long as there is no breakthrough on AnMBR technology, the anaerobic treatment of urban wastewater will in practice relies on the use of UASB systems and their modifications as is presented in Chapter 2. Currently, there are multiple technologies available for the effective removal of biodegradable organic matter, Total Suspended Solids (TSS), and partial Total Nitrogen (TN) removal through aerobic posttreatment systems, including suspended biomass (activated sludge processes); biofilm (trickling filter and down-flow hanging sponges, moving bed biofilm reactors, etc.), and integrated fixed-film and activated sludge processes. Nowadays, the increase in the elimination of TN means, in practice, bypassing part of the raw sewage directly to the post-treatment system, with the consequent loss of biogas production. Thus, the removal of dissolved methane and nitrogen compounds as was mentioned in Chapter 5 should be improved, with more effective strategies potentially involving advanced microbial communities or engineering systems, using among others anammox or N-damo processes at ambient temperature. The use of post-treatment systems is also required if the anaerobically treated wastewater has to be reused, as has been mentioned in Chapter 8. Water reuse offers a good opportunity for recovering not only water, but also the contained nutrients for agriculture or landscape irrigation, diminishing the chemical fertilizer requirements of crops. Water reuse could play a crucial role in both mitigating and adapting to the impacts of climate change. Although traditionally the sources of drinking water have come from aquifers and surface waters, it is more and more common, due to water stress, to use energyintensive processes, such as desalination of seawater through reverse osmosis to produce drinking water. In Spain this represented the 5.9% of the total drinking water consumed in the country in 2020 (INE, 2022). Replacing the use of drinking water with reclaimed water, especially for landscape irrigation and other urban uses, could be therefore a way to reduce energy consumption and greenhouse gas emissions, associated with seawater desalination, which would be added to the potential savings obtained from the use of combined anaerobic–aerobic systems to produce such reclaimed water. 12.3 MICROPLASTICS IN CONVENTIONAL WWTPS AND ANAEROBIC SYSTEMS MP pollution has become a highly relevant issue in recent years. MPs can be defined as plastic fragments smaller than 5 mm. Depending on their origin, they can be classified as primary or secondary. Primary MPs are the ones that can be found in cosmetic and medical products. In addition, MPs originated from abrasion of tires and fibers that are released during laundry are also commonly classified as primary MPs (Rossatto etal., 2023). Secondary MPs are the ones that originate from physical, chemical, or biological fragmentation of larger plastic debris, such as plastics bags or bottles (Akdogan & Guven, 2019). Besides their origin, MPs can also be classified based on their physical and chemical characteristics. Despite the lack of and standardized protocol for their identification leads to a great variability in the criteria used for their description, there is a general agreement in referring at least to the type of polymer, size, color, and shape. Thus, microbeads/spheres, fibers, films, fragments/irregular particles, or foams are commonly used terms (Hartmann etal., 2019). Due to this diversity of sources, MPs can enter natural systems through different pathways. On the one hand, primary MPs are prone to be released through household sewage discharge or application of sewage sludge containing synthetic fibers or sedimented MPs from personal care or household products. On the other, anthropogenic activities such as littering or municipal solid waste collection and disposal contribute to plastic pollution. These larger plastic wastes and the MPs formed from them may be introduced into aquatic environments by wind dispersal, surface runoff, or soil erosion (Akdogan & Guven, 2019). Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 333Still open research questions on technologies, microplastics, and antibiotic resistance Moreover, their characteristics make them a potential risk for ecosystems. Due to their small size, MPs can interfere with food chains, because they can be ingested by organisms in lower trophic levels such as zooplankton. Furthermore, their size in combination with their great specific surface area and hydrophobic surface enables them to act as carriers of other pollutants, such as persistent organic pollutants, heavy metals, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and even pathogens (Akdogan & Guven, 2019). MPs are nowadays considered ubiquitous pollutants, as they have been found in every known environment, that is, seas, oceans, lakes, rivers, beaches, and marine soils, including the sediments of Arctic fresh water (Rossatto etal., 2023). In this context, domestic WWTPs are now considered important for the release of MPs due to the diverse origins of influents, all of which could contain a significant amount of MPs. The removal rates of MPs in domestic WWTPs are above 90%, but considerable quantities of MPs are released to the surrounding receiving waters, ending in ocean systems, due to the daily discharge of large volumes of effluents. The discharges of MPs in effluent and sludge are considered foreign matter and constitute a contaminant and source of pollution to the receiving waters and soil systems (Liu etal., 2022). Numerous types of MPs have been detected in domestic WWTPs. The most common ones would be polyethylene (PE), polystyrene (PS), polyethylene terephthalate (PET), polypropylene (PP), polyamide fibers, and polyester fibers (Liu etal., 2021). Other polymers such as acrylates, alkyds, polyurethane, polyvinyl alcohol, and polyvinyl chloride (PVC) have been reported too (Sun etal., 2019). 12.3.1 Characterization techniques Nowadays, one of the main challenges in the study of MPs is the lack of standardization in sampling, identification, and quantification methods, which explain the wide range of MP concentrations that can be found in literature among different studies. Although many researchers have reviewed the identification and characterization of MPs in marine environment and freshwater systems, less review papers have focused on the techniques used for the analysis of MPs in samples taken from domestic WWTPs. This can be explained by the complexity of wastewater and sludge, whit high contents of organic matter, suspended solids, and other contaminants (Liu etal., 2022; Tirkey & Upadhyay, 2021). In general, many protocols have been developed, with a significant number of steps and long times of processing. This diversity of techniques results in a great extent of non-comparable results, contributing to a lack of consistency in them. Most relevant steps that are always mentioned in literature are shown in Figure 12.1. In addition, when working with MPs, some precautions must be taken. During the sample collection and pre-treatment, contaminations of samples might be induced from atmospheric fallout, the equipment and devices, used and even clothing of workers. Some recommendations would be using glass materials, cover the samples with aluminum foil, or wearing laboratory coats made of natural fabric. It has also been suggested to setup a blank control sample. Finally, the recovery of MPs after the extraction processes is recommended to be tested (Sun etal., 2019). As can be seen in literature, for each step of the sampling and processing, different options can be chosen: Sampling: the first stage for the characterization and quantification of MPs is to collect wastewater and sewage sludge samples at different points within domestic WWTPs. For wastewater samples, the most common methods used include grab samples at different sampling points or using autosamplers to collect samples at intervals over periods from a few hours to 24 h. Sludge samples can be directly collected from the primary clarifiers, the activated sludge recirculation lines, and the anaerobic digesters. Sample volumes or sludge quantities collected are a critical factor, and the first source of variability among authors. Sample volumes used for the MP quantification in the water line could be in the ranges of 10–100, 100–200, and 50–1000 L, respectively. The quantity of sludge samples used for MP analysis can vary from 5 to 100 g or 150 to 200 mL. Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 334 Anaerobic Treatment of Domestic Wastewater: Present Status and Potentialities The proper sample volumes or sludge quantities for MP characterization will depend on their concentration in the samples (Liu etal., 2022). Filtering: Collected samples are usually filtered using filters or sieves. So far, the mesh/pore sizes applied in the related studies have not been standardized and vary from ∼1 to 500 µm, considerably affecting final results. This mesh-based size categorization may not be very accurate. Some studies have observed that some particles would not pass through the sieves even if sufficiently small due to their irregular shapes, and morphology of fibers enable them to pass longitudinally through smaller filters. In addition, the materials present in wastewater and sludge can cause blockages. For this reason, a common practice for sludge samples is to collect and transport them in a container to a laboratory for further sample processing (Sun etal., 2019). Once reached the laboratory, sludge samples are commonly subjected to some form of pre-treatment, such as sonication (Liu etal., 2022). Organic material removal: it is necessary to remove all non-plastic organic substances in the samples, which comprises the major contents of solids recovered from domestic WWTPs. The clean-up of these materials may include various treatments and separation steps. Once again, the methodology applied varies widely among the different studies. The most cited methodologies usually include the use of acid or alkaline solutions, hydrogen peroxide, Fenton’s reagent, or even enzymes such as cellulase, chitinase, or protease. However, NaOH and KOH have been reported to lead to insufficient reduction of organics in sludge samples and cause destruction to PE, PS, polycarbonate, PET, and PVC polymers, so they are not recommended for organic removal in WWTP sludge (Liu etal., 2022; Tirkey & Upadhyay, 2021). Figure 12.1 Flowchart summarizing steps and techniques used for MP detection in domestic WWTPs. Source: adapted from Liu etal. (2022) and Sun etal. (2019). Figure created with Biorender.com. Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 335Still open research questions on technologies, microplastics, and antibiotic resistance Density separation: the aim of this step is to segregate MPs from inorganic substances or sediment that are not eliminated with the digestion. The most used salt solution would be NaCl. However, polymers such as PET or PVC cannot be separated with this methodology. For this reason, other salt solutions have been suggested in several studies: NaI, NaBr, and ZnCl2 (Tirkey & Upadhyay, 2021). Identification and quantification techniques: Liu etal. (2022) classified the most used techniques for MP characterization into two main groups. On the one hand, morphological and physical characterization using microscopy or stereomicroscopy is widely extended as a methodology to pre-sort and even count MPs. However, relying solely on the visual identification could result in overestimation of the actual number of MPs, especially fibers. On the other, techniques focused on chemical properties of MPs are now widely used, commonly in combination with the visual identification. These techniques range from relatively fast spectroscopy techniques (Fourier transform infrared or Raman spectroscopy) to pyrolysis-gas chromatography/mass spectrometry or thermogravimetric analysis-differential scanning calorimetry. 12.3.2 Occurrence and removal rates As previously mentioned, there is not a standardized protocol for sampling, identifying, and quantifying MPs in domestic WWTPs. Because of this, the data reported by different studies vary significantly. There is discrepancy even in the units used, with authors referring to the mass or the number of MPs, particles, or items, taking into account in particles that cannot be confirmed as either plastics or nonplastics (Michielssen etal., 2016; Sheriff etal., 2023). Several review articles summarize the quantity and typology of MPs found. According to Sun etal. (2019) the amount of particles reported vary from 1 to 10,044 particles/L in influents and from 0 to 447 particles/L in effluents. Sheriff etal. (2023) mentioned quantities in influents from 0.28 to 8400 MPs/L and from 0.05 to 297 MPs/L in effluents. Other studies reported similarly mixed data (Liu etal., 2021). The wide variations in MP concentration or quantity in the studies could be partially related to the aforementioned differences on sampling methodology, pre-treatment of the samples, and the analysis methods applied, as well as the characteristics of the facilities (Sun etal., 2019). Other factors that must be considered are the size of the facility, the population served, and the source of the raw water (Liu etal., 2021; Sheriff etal., 2023). Regarding sludge samples, studies focusing on MPs in the sewage sludge including their occurrence, transformation, and further mobilization are quite limited. According to Sun etal. (2019), the number of MPs in dry weight (DW) sludge could reach from around 1500 to 170,000 particles/kg DW. Nguyen etal. (2022) reported the number of MPs in sludge from domestic WWTPs worldwide ranges from >1000 to 301,400 particles/kg. These quantities are substantially higher than that in the liquid phase. The great variation in the number of MPs in sludge also emphasizes the importance of representative sampling and harmonized detection methods. On the contrary, size of MPs in the sewage sludge usually is significantly different from that in the raw water, with the average size being relatively large. In terms of the shape, synthetic fibers are the most common MPs found in sludge. Therefore, the continuous addition of MPs to soils due to the use of sewage sludge as fertilizer should not be ignored (Sun etal., 2019). Although no treatment technology has been specifically designed for the removal of MPs in wastewater, most of domestic WWTPs exhibit high removal rates. Above 90% of MPs that enter WWTPs are removed from water lines, being retained in the sludge via adsorption mechanisms. Hence, the type of treatment plays a key role in this removal rate (Sheriff etal., 2023). The pre-treatment and primary treatment steps can effectively remove most MPs in wastewater. Grease and grit pre-treatment effectively removes MPs of larger size, whereas primary settling treatment achieve high efficiency in removing MPs. Due to this reason, the use of primary treatment systems would guarantee a high removal of MPs in domestic WWTPs, regardless of whether either an aerobic or anaerobic biological treatment can be used later. The secondary aerobic treatment, which usually comprises biological treatment and Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 336 Anaerobic Treatment of Domestic Wastewater: Present Status and Potentialities clarification, manage to further decrease the number of MPs to 0.2–14%. Finally, in general, WWTPs with tertiary treatment processes had lower MP quantities (0–51 particles/L) in effluents than those with primary or secondary treatment processes only (Sun etal.,2019). 12.3.3 MPs in anaerobic systems Regarding occurrence and fate of MPs in domestic WWTPs with anaerobic treatments, available information is scarce. Currently, there are no studies on the presence of MPs in full-scale facilities and on their removal rates. Only few studies have focused on anaerobic technologies in pilot-scale facilities. Moreover, different studies have tried to assess the influence of MPs on anaerobic granular sludge (AGS), which will be discussed in the next section. Michielssen et al. (2016) assessed the efficiency of different unit processes at three domestic WWTPs in removing small anthropogenic litter (SAL), which includes MPs. Two of them were fullscale conventional facilities with different secondary and tertiary treatments, and the third one was a pilot-scale AnMBR with a side-stream configuration, placed after preliminary treatment of one of the other conventional facilities. The number of SALs in raw water and its removal rates for the three technologies were determined. The overall removal rates achieved ranged from 93.8 to 99.4%. Even though an important part of this removal took place in the preliminary treatment, AnMBR pilot process removed 99.1% of the remaining SAL, outperforming removal rates of final treatment in both plants with conventional configurations. This is due to the small pore size of the micro- or ultrafiltration membranes employed, usually between 0.4 and 0.05 µm. In addition, the unit processes employed affected also shape classes of SAL detected on final effluents. Some particles such as paint chips were not detected in the AnMBR effluent, unlike the other domestic WWTPs. Moreover, as compared to conventional systems, there was a 10-fold reduction in the number of fibers released by the AnMBR treatment. However, fibers still represented a greater percentage of SAL in effluents. It must be noted that in this study they did not focus on MPs exclusively, and part of these fibers could be natural fibers such as cotton. These results suggest that an AnMBR is an efficient technology for removing SAL and therefore, MPs. In another study, Pittura etal. (2021) compared a full-scale WWTP with conventional activated sludge (CAS) treatment and a pilot-scale UASB coupled with an AnMBR. This study focused on determining MP occurrence and their removal rates. For an anaerobic configuration, they determined the presence of MPs in the pretreated influent, the UASB effluent, and the permeate, as well in the granular sludge. In this case, authors focused only on particles identified as MPs, classifying them in terms of their shape as fiber-shaped (MPFs) and particle-shaped (MPPs). The innovative configuration removed 94% of influent MPs compared with the conventional treatment scheme. The overall removal of 52.6% was achieved by the UASB and further 44% by the AnMBR. In terms of shape, the removal rate for MPPs achieved 100%, whereas only 87% of MPFs were removed. On the contrary, MPPs (particularly fragments and films) were identified in conventional WWTP final effluent. Hence, the main conclusion of this study was that the overall MP removal in a pilot-scale configuration was greater than that in a full-scale CAS scheme. The authors indicate that the high removal efficiency is due to the ultrafiltration unit, that was mostly responsible for the total abatement of MPPs. Regarding MP accumulation in sludge, the anaerobic sludge from UASB reactors presented a higher contribution of MPPs of lower size class (0.03–0.1 mm) than the sludge from primary in the conventional configuration. In conclusion, anaerobic treatments emerge as a promising technology for removal of MPs from wastewater, especially membrane-based systems. This better performance of membrane bioreactor (MBR) technology to remove MPs compared to conventional systems has been highlighted by other authors, but in aerobic systems (Sun etal., 2019). However, it should be noted that AnMBRs have different fouling and permeation characteristics compared to aerobic MBRs (Pittura et al., 2021). Finally, particular attention should be paid to fibers because both studies mentioned emphasize that AnMBRs are not able to remove them from influents. Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 337Still open research questions on technologies, microplastics, and antibiotic resistance 12.3.3.1 Effects of MPs on AGS systems Several authors have studied the response of AGS to the exposure of different MPs. However, most of the studies carried out were laboratory-scale experiments. Thus, these laboratory-scale experiments were in part focused on determining the mechanisms of toxicity, so they applied conditions that could differ from real systems. A few authors have studied the fate and influence of MPs on pilot plants (Pittura etal., 2021) or the effect of more than a type of MP at a time (Wei etal., 2022). To date, there is no information about MP effect on full-scale anaerobic systems for municipal wastewater treatment. Most relevant studies in this field and their main results are summarized in Table 12.1. In general, all authors conclude that MP addition has a negative effect on the performance of AGS systems, affecting extracellular polymeric substance (EPS) production, granule size and integrity, and methane production. It is important to note that the size of MPs and their concentration are relevant factors, and they must be considered when drawing conclusions. In addition, exposure time might play an important role in the effects observed, as well as the type of studied MPs. These negative effects can be explained by two main mechanisms. On the one hand, some studies have reported that the leachates from different types of MPs inhibit microbial growth and affect Table 12.1 Principal effects of MP addition on AGS in different experiments. Experimental Setup Type of MP MP Concentrations/ Quantities Main Results References Laboratory-scale batch experiments PS (particle size from 0.5 to 150 µm) 75 mg/L EPS secretion inhibition. Damage in AGS integrity and cell viability and methane production inhibited with increasing particle size Wang etal. (2022a) Laboratory-scale BEAD reactors PE (average particle size: 40–48 µm) 0.5–10 mg/L Higher concentrations (10 mg/L) inhibited methane production. Granule breakage and decrease in cell viability. Changes in microbial populations Wang etal. (2022b) Laboratory-scale UASB reactors operated continuously for 120 days PET (average size: 1.27 mm) 15–300 MPs/L Low concentration (15 MPs/L) did not affect AGS whereas high concentrations (75–300 MPs/L) significantly inhibited AGS activity, with granule breakage and EPS production inhibition. Changes in microbial populations Zhang etal. (2020a) Pilot-scale UASB reactors treating municipal wastewater PP 5–50 PP/g TS Methanogenic activity tolerated up to a concentration of 18 MPs/g TS 50 PP/g TS caused a remarkable inhibition on methane production Pittura etal. (2021) Laboratory-scale batch experiments Remediation strategy with hydrochar tested PET, PS, PE, and PP (size: 150 µm) 75 mg MP/L (36% of PET, 15% of PS, 42% of PE, and 8% of PP) Inhibition of cumulative methane production. Granule size decreased. Changes in microbial populations Wei etal. (2022) Laboratory-scale UASB reactor operated for over 300 days with synthetic wastewater PVC (size: 0.1 mm) 0–150 MPs/L Decrease in methane production and granule size. Cell viability was also affected. Changes in microbial population Zhang etal. (2020b) PS: polystyrene; PE: polyethylene; PET: polyethylene terephthalate; PP: polypropylene; PVC: polyvinyl chloride; UASB: upflow anaerobic sludge blanket; EPS: extracellular polymeric substance. Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 344 Anaerobic Treatment of Domestic Wastewater: Present Status and Potentialities Table 12.3 Studies that investigated ARBs and ARGs in domestic WWTPs with an anaerobic process and/or one anaerobic system in the WWTP treatment train. Type of Sample ARB/ARGs/Antibiotics Type of WWTP/ Treatment Process ARB and/or ARG Removal Efficiency References Sewage sludge ARGs (tetA, tetB, tetC, tetW, tetM, tetQ, tetX, sul1, sul2, ermB, ermC, blaAOX, blaTEM), integrons (intl1, intl2) and 16S rRNA 1 WWTP/anaerobic digestion Highest ARG removal in AD was achieved with thermal hydrolysis at 140°C Haffiez etal. (2022) Sewage sludge ARBs (tetracycline and β-lactam antibiotics-resistant bacteria) 1 WWTP ARB removal of 1.5–1.6 log units after AD Tong and Wang (2014) Sewage sludge ARGs by metagenomic approach. 271 ARGs subtypes belonging to 18 ARG types were identified 1 WWTP (activated sludge (AS) and AD) AD removed 20.7% of ARGs from sludge. AS removed 99.8% of ARGs Yang etal. (2014) Sewage sludge ARGs (tetA, tetC, tetM, tetO, tetX) and ARBs (tetracycline and β-lactam antibiotics-resistant bacteria) 1 WWTP, Beijing, China AD of sludge with microwave (MW-H), microwave (MW), and microwave- H2O2-alkaline (MW-H2O2). Removal varied from 0.55 to 5.04 logs, according to the conditions evaluated Tong etal. (2016) Sewage sludge tetA, tetG, tetX, tetO, tetW, sul1, sul2, sul3, ermB, ermF, blaTEM dfrA1, dfrA2, and intI1 Thermophilic and mesophilic AD Thermophilic digestion removed tetA, tetG, tetX, sul1, sul2, ermB, dfrA1, dfrA2, and intI1 by 0.1–0.72 log units, whereas it increased tetO, tetW, sul3, ermF, and blaTEM, and showed no decrease in sul2. Mesophilic digestion was ineffective in removing ARGs, except for ermB and blaTEM Wu etal. (2016) Sewage sludge ARGs (sul1, tetA, tetO, tetX) Thermophilic AD ARG removal >80% Xu etal. (2018) Hospital wastewater Real wastewater + ciprofloxacin UASB reactor 41% ciprofloxacin reduction and 68% COD removal Guney and Sponza (2016) DWW Sulfonamide, chloramphenicol, aminoglycoside, tetracycline, β-lactam-resistant genes Anaerobic bioreactors (lab scale) ARG removal of 62% Christgen etal. (2015) DWW Many ARGs via metagenomics AAS bioreactors, lab scale ARG removal >85% Christgen etal. (2015) Municipal wastewater ARBs (resistant to tetracycline and sulfadiazine) UASB reactor ARB reduction of 0.95–1.16 log units. Yuan etal. (2016) (Continued) Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 345Still open research questions on technologies, microplastics, and antibiotic resistance Table 12.3 Studies that investigated ARBs and ARGs in domestic WWTPs with an anaerobic process and/or one anaerobic system in the WWTP treatment train. (Continued) Type of Sample ARB/ARGs/Antibiotics Type of WWTP/ Treatment Process ARB and/or ARG Removal Efficiency References DWW ARGs (sul1, sul2, and sul3, tetG, tetM, tetO, and tetX, (ermB and ermC), (cmlA and floR), and 16S rRNA gene Vertical flow constructed wetland ARG removal of 63.9 and 84.0% Chen etal. (2016) Municipal wastewater ARGs (tetA, tetO, tetW, sul1, sul2, blaCTX-M, blaTEM, blaSHV), int1, and 16S rRNA gene 4 WWTPs, Harbin, China; 2 had an anaerobic step in its flowchart ARG removal of 0.3–2.7 log units in the four WWTPs. CAS removed 70% of antibiotic residues, 5.0 log units for ARBs, and 4.2 log units for ARGs Wen etal. (2016) Municipal wastewater 19 antibiotics (amoxicillin, azithromycin, ciprofloxacin, chloramphenicol, meropenem, minocycline, oxytetracycline, sulfamethazine, vancomycin, etc.); 10 ARBs and 15 ARGs (blaKPC, blaNDM, blaSHV, ermB, intI1, sul1, tetO, among others) 1 WWTP comprised of CAS (anoxic/ aerobic tank) and MBR MBR system completely removed ARBs, ARGs (up to 4.8 log rate removal (LRV) for ARGs and up to 7.1 LRV for the whole MBR train), but microfiltration membrane alone did not reduce antibiotics in comparison to the treatment of the secondary clarifier Le etal. (2018) Municipal wastewater ARGs (intl1, blaCTX-M, blaTEM, ermB, qnrS, sul1 and sul2, tetO, mecA, and vanA), 16S rRNA (total bacteria) and HF183 (human bacteroides fecal indicator) Full-scale WWTP with biological nitrogen removal (BNR) reactors (anoxic, anaerobic, and aerobic reactors) and, UV disinfection step ARG removal of 2 log copy/mL. WWTP was still releasing ARGs in the order of 2.8 × 1014 copies/day McConnell etal. (2018) Municipal primary clarifier effluent ARGs (ermB, tetO, sul1, int1) 1 bench-scale AnMBR ARG reduction of 3–4 log units Kappell etal. (2018) DWW and manure ARGs (int1, sul1, sul2, ampC, blaOXA1, ermB, ermF, tetO, tetW, tp614, blaNDM-1) Bench-scale AnMBR (4.5 L) ARG removal of 99.95% during the stage with the greatest addition of manure was observed Lou etal. (2020) Domestic and industrial wastewater from WWTPs with an anaerobic treatment stage ARGs (tet(A), tet(B), tet(C), tet(G), tet(L), tet(M), tet(O), tet(Q), tet(W), tet(X), sul1, sul2, and int1); antibiotic residues and ARBs (8 antibiotics: 3 tetracyclines (TCs), 4 sulfonamides, and 1 trimethoprim) 2 WWTPs ARG removal of 1 log, according to the different conditions evaluated Li etal. (2016) (Continued) Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 346 Anaerobic Treatment of Domestic Wastewater: Present Status and Potentialities Table 12.3 Studies that investigated ARBs and ARGs in domestic WWTPs with an anaerobic process and/or one anaerobic system in the WWTP treatment train. (Continued) Type of Sample ARB/ARGs/Antibiotics Type of WWTP/ Treatment Process ARB and/or ARG Removal Efficiency References DWW (raw and treated) ARGs (blaCTX, blaOXA, blaTEM, and blaNDM-1), mobile genes (int1, int2, and int3) and 16S rRNA gene) and ARBs (ESBL-resistant bacteria; carbapenem-resistant bacteria) 3 WWTPs. One has an anaerobic flowthrough reactor in its flowchart Removals of 1.5–3.0 log units were observed, according to the different conditions and microorganisms evaluated Lamba and Ahammad (2017) Municipal wastewater 12 ARG types: aminoglycoside, β-lactams, chloramphenicol, fosmidomycin, macrolide– lincosamide–streptogramin, polymyxin, quinolone, rifamycin, sulfonamide, multidrug-resistant and tetracycline-resistant genes 1 WWTP with conventional treatment (with anoxic–oxic–anoxic tanks) and MBR MBR was effective at reducing ARG abundance. 5.3–7.4 log removal values were observed for E. coli, enterococci, and P. aeruginosa Ng etal. (2019) Municipal wastewater ARGs (blaTEM, ermB, tetW, tetO, sul1, sul2, addD, and qnrS) and int1 4 full-scale MBRs (coupled with anaerobic–anoxic– oxic process) ARG removal of 1.1–7.3 logs; ermB, sul1, and int1 were reduced by MBRs (1.5–7.3 log removal) Li etal. (2019) DWW ARGs (sul2, ermB, cmlA, and floR) Hybrid/integrated flow constructed wetland ARG removal of 87.8–99.1% Chen etal. (2019) DWW Addition of 250 µg/L of sulfamethoxazole, ampicillin, and erythromycin to the wastewater ARGs (sul1, tetO, ermF, int1) AnMBR (lab-scale) Antibiotic removal of 50–98% Zarei-Baygi etal. (2020) DWW ARGs (aadA, blaOXA1, ermB, mexF, sul1, sul2, tetG, tetM, tetW, vanC03, vanXD), 16S rRNA, and intI1 2 domestic WWTPs. One has an anaerobic step in its flowchart ARG removal of 74–100% Sun etal. (2022) (Continued) Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 347Still open research questions on technologies, microplastics, and antibiotic resistance Table 12.3 Studies that investigated ARBs and ARGs in domestic WWTPs with an anaerobic process and/or one anaerobic system in the WWTP treatment train. (Continued) Type of Sample ARB/ARGs/Antibiotics Type of WWTP/ Treatment Process ARB and/or ARG Removal Efficiency References Municipal wastewater ARGs (blaTEM, sul1, tetA), intI1, and 16S rRNA 3 domestic WWTPs (pilot scale), two with UASB reactor followed by BTF, UASB followed by High rate algal pond (HRAP) ARG removal of 1.0–3.5 logs depending on the system combination. In UASB/HRAP a reduction of 3.5 log units was observed for blaTEM. ARG removal of 0.3–0.9 logs was observed for the UASB Santos (2021) Municipal wastewater ARBs (resistant to ampicillin, chloramphenicol, tetracycline, streptomycin, amoxicillin, sulfadiazine, sulfamethoxazole, trimethoprim, ciprofloxacin, erythromycin, sulfamethoxazole + trimethoprim combination) 3 full-scale WWTPs, one with UASB reactor followed by BTF ARB removal of 0.5, 1, and 2 order of magnitude in UASB/BTF, CAS, and MAS/UV Machado etal. (2023) Municipal wastewater ARGs (sul1, tetA, blaTEM, ermB, qnrB), int1, and 16S rRNA 3 full-scale WWTPs, one with UASB reactor followed by BTF ARG removal of 0.2–2.0 log units. blaTEM, ermB, and qnrB removals were 1–2 log units in CAS, UASB/ BTF, and MAS/UV; intI1, sul1, and tetA, removals were 0.2–0.5 logs in the threeWWTPs. UASB/BTF showed a similar performance to that of CAS and MAS/UV Leroy-Freitas etal. (2022) Decentralized wastewater treatment facility ARGs (sul1, sul2), tetracyclineresistant gene (tetO), macrolideresistant gene (ermB), one integron (int1), and 16S rRNA gene 5 domestic WWTPs investigated in rural areas of China having anaerobic– oxic (AO) and AO– MBR processes ARG removal varied from 0.4 to 2 log units, according to the conditions evaluated. AO–MBR achieved one-fold or higher ARG removal Huang etal. (2023) Only studies treating domestic and/or urban wastewaters are included. Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 348 Anaerobic Treatment of Domestic Wastewater: Present Status and Potentialities ARBs, and pathogens (Uluseker et al., 2021). However, monitoring data about AR elements in municipal WWTPs applying anaerobic reactors such as UASB followed by biological trickling filters (UASB/BTFs), one of the main technologies used in warm climate regions (such as in Brazil), are scarce. Christgen etal. (2015) investigated the fate of ARGs in anaerobic, aerobic, and anaerobic–aerobic sequential (AAS) bioreactors (lab-scale) treating DWW through metagenomic approaches. They reported that AAS and aerobic reactors were superior to anaerobic units in reducing ARG-like sequence abundances, especially aminoglycoside, tetracycline, and β-lactam-resistant genes, whereas sulfonamide and chloramphenicol ARG levels were unaffected by treatment. They concluded that AAS reactors are more promising for future applications because they can reduce more ARGs with lower-energy consumption (32% less energy), but all three treatment methods have limitations and need further studies. Leroy-Freitas et al. (2022) investigated the abundance of integron (int1), ARGs (sul1, tetA, blaTEM, ermB, qnrB), and 16S rRNA in raw and treated wastewater of three full-scale WWTPs, using different treatment systems: CAS, UASB/BTF, and MAS/UV (modified activated sludge with UV disinfection stage). They observed that all WWTPs decreased the loads of genetic markers finally discharged to receiving water bodies and showed no evidence of being hotspots for AMR amplification in wastewater because the abundances of intI1 and ARGs within the bacterial population did not increase in the treated effluents. UASB/BTF showed a similar performance to that of the CAS and MAS/UV (ARGs removal of 1–2 log units), reinforcing the sanitary and environmental advantages of this biological treatment. Potential pathogenic population underwent a considerable decrease after the treatments; however, strong significant correlations with intI1 and ARGs revealed potential multidrug-resistant pathogenic bacteria (Aeromonas, Arcobacter, Enterobacter, Escherichia–Shigella, Stenotrophomonas, and Streptococcus) in the treated effluents, although in relatively reduced abundances. Santos (2021) reported ARG reduction of 0.3–0.9 log units after treatment in a UASB reactor. When UASB reactor was combined with other post-treatment systems such as high-rate algal ponds (HRAPs) or BTFs, ARG removal of 1.0–3.5 orders of magnitude were observed depending on the system combination. In UASB/HRAPs, a reduction of 3.5 log units was observed for β-lactam-resistant gene (blaTEM). Machado et al. (2023) investigating the same three WWTPs in the southeast part of Brazil as Leroy-Freitas etal. (2022) reported that MAS was effective in reducing ARB counts (by 2–3 log units), compared to CAS (1 log unit) and UASBs/BTFs (0.5 log unit). However, multidrug-resistant bacteria were still present in treated effluents despite the technology treatment applied. Yuan etal. (2016) reported ARB reductions of 0.95–1.16 log units in a UASB reactor. ARGs and ARBs that ended up being accumulated in the sewage sludge can be removed to some extent by the anaerobic digestion of the sludge. Thermophilic anaerobic digestion has been shown to be more effective in the reduction of ARGs than mesophilic digestion (Wu etal., 2016; Xu etal., 2018) (Table 12.3). Anaerobic digestion of the sewage sludge can remove ∼20.7–80% of ARGs (Wu etal., 2016; Xu etal., 2018). In a recent review about the feasibility of anaerobic treatment including AD (of sludge and manure) in eliminating antibiotics and ARGs, Aziz etal. (2022) reported that AD at mesophilic and thermophilic temperatures were effective in eliminating ARGs including tetracyclines (tetA, tetO, tetW, tetX, tetC, tetG, tetL, tetM, tetQ), sulfonamides (sul1, sul2), macrolides (ermF, ermB, ermQ, mefA, mphB, ereA), fluoroquinolones (qnrS, aac(6′)-ib-cr, qnrA), trimethoprim (dfrA1, dfrA2), β-lactamase (blaTEM), aminoglycosides (aphA1, aphA2, aac(3)-II, aacA4, aadA, aadB, aadE, strA, strB), and mobile genetic elements (intI1, intI2, ISCR1, Tn916/1545). Nevertheless, contradictory results have been obtained for tetC, tetG, tetX, tetA, tetO, tetW, sul1, sul2, blaTEM, ermF, dfrA1, dfrA2, fluoroquinolones (qnrA), and intI1 genes, which are found to be resistant during the digestion period, and sometimes elevated concentrations have been observed after the anaerobic treatment. Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 349Still open research questions on technologies, microplastics, and antibiotic resistance They also reported that some resistant genes (sulfonamides – sul3, macrolides – ermX and mefA, and trimethoprim – dfrA5) are still not removed after AD. In the study by Zarei-Baygi etal. (2020), operating an AnMBR for the treatment of DWW containing antibiotics (250 µg/L each of sulfamethoxazole, ampicillin, and erythromycin), they reported that ARG abundances (sul1, ermF, tetO) in the effluent increased upon initial antibiotic exposure to the system and then dropped immediately thereafter. The reactor removed 69–78% of sulfamethoxazole, 89–98% of ampicillin, and 40–58% of erythromycin. Some effluent ARGs (tetO and ermF) were minimal, whereas other genes such as sul1 and int1 were still present in the effluent and strongly correlated with several potentially pathogenic genera. Aziz et al. (2022) concluded that AnMBRs are potentially the most efficient technology for removal of antibiotics (sulfamethoxazole, sulfadiazine, trimethoprim, clarithromycin, erythromycin, ciprofloxacin, ofloxacin, cefalexin, cephradine) and ARGs (sul1, sul2, tetO, tetW, ermF, ermB, blaNDM-1, blaCTX-M-15, blaoxa-48, blaoxa-1). More details on the removal of antibiotics and ARGs are provided in Chapter 3 dedicated to AnMBRs. In summary, anaerobic treatment technologies can reduce the burden of antibiotic resistance from the wastewaters and sludge (by reducing ARBs, ARGs, and antibiotic residues) and the removal efficiency will depend on reactor configuration, combination with other treatments, class of the antibiotic, and resistant genes. 12.4.2 Research needs on antibiotic resistance Antibiotic resistance is a major threat to global public health. WWTPs applying biological treatment (involving aerobic and anaerobic processes) can reduce antibiotic residues, ARBs, and ARGs, but these resistant elements are not fully eliminated even after the disinfection treatment. In general, WWTPs operated with CAS processes can remove ∼2–4 logs of ARGs and ARBs and up to 70% of antibiotics. On the contrary, anaerobic reactors can reduce from 1.0 to 3.0 log units of ARB and ARG abundance depending on the bacteria or resistant genes. AeMBRs and AnMBRs can outcompete other treatments by removing 3–5 log units of ARGs and up to 98% of antibiotics. Future studies should focus on innovative/powerful technologies and operational strategies able to increase the removal of antibiotics, ARBs, and ARGs from municipal wastewater. To achieve this and facilitate the comparison of removal efficiencies and resistant levels between different WWTPs and countries, research is needed to select the most appropriate indicators of resistant bacteria and/ or pathogens and ARGs, and standard methods for assessing them (Uluseker et al., 2021). In the case of pathogenic bacteria, it could be a good choice to select, at least, microorganisms of the group ESKAPE, a group comprising six pathogens that show resistance to multiple classes of antibiotics: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. Promising results obtained with combined treatments, that is, AAS processes or anaerobics in UASB bioreactors followed by post-treatment in HRAPs or BTFs should be consolidated with experiments at pilot/demonstration scale to achieve a full confirmation of their feasibility. The removal of ARBs, ARGs, antibiotics, and pathogens, as well as the influence of water quality parameters on the plant performance should be monitored and investigated. In addition, the development and evaluation of more effective disinfection and treatment methods (such as ultrafiltration and advanced oxidative processes) could contribute to reducing and/or eliminating the dissemination of AMR from WWTPs to the environment. The advantages and limitations of each treatment method and their combinations should also be investigated. Finally, it is worth noting that to reduce antibiotic resistance spreading it is necessary to modify the current practice by reducing antibiotics consumption in medicine and animal breeding. This is because even the best technology cannot achieve the true ‘zero discharge’ and even minimal discharged amounts can contribute to the spreading. Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/1418717/9781789063479_0329.pdf by guest on 30 May 2024 350 Anaerobic Treatment of Domestic Wastewater: Present Status and Potentialities REFERENCES Akdogan Z. and Guven B. (2019). 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