Emerging Electron Beam Technology Targeting Hazardous Micropollutants as Quaternary Treatment in Wastewater Treatment Plants

Abstract

Wastewater treatment plays a very important role in striving to reach the internationally agreed United Nations (UN) sustainable development goals. One of the critical challenges in achieving Sustainable Development Goal 6 is the effective removal of micropollutants (MPs), including microplastics, organic contaminants, and pharmaceuticals, from wastewater. Additionally, the presence of biological hazards such as antibiotic resistance genes (ARGs), antibiotic-resistant bacteria (ARBs), parasites, and their eggs poses significant risks to public health and aquatic ecosystems. The forthcoming European Union (EU) wastewater directive mandates the implementation of quaternary treatment processes to effectively remove micropollutants (MPOs) from wastewater. This regulatory shift underscores the need for advanced treatment technologies capable of addressing emerging contaminants to ensure environmental and public health protection. This paper presents a critical review of the present situation concerning the fate of MPOs and possible methods of their removal. Based on their experimental research, the authors propose electron beam (EB) technology as a universal solution for the treatment of wastewater and sludge. The findings demonstrate that this approach effectively meets the emerging regulatory requirements for the removal of micropollutants and biological hazards.

1. Introduction

The UN Agenda 2030 serves as the global framework for sustainable development, outlining strategic goals and actions to be achieved by the year 2030. It aims to address interconnected global challenges, including poverty, inequality, environmental degradation, and access to clean water and sanitation. It contains 17 Sustainable Development Goals (SDGs). It was adopted in 2015 when all 193 UN member states unanimously adopted the resolution “Transforming Our World: 2030 Agenda for Sustainable Development.” The 17 SDGs outlined in Agenda 2030 are organized into five key thematic areas, known as the 5Ps: People, Planet, Prosperity, Peace, and Partnership. Each goal is accompanied by specific targets, amounting to a total of 169 measurable tasks to be achieved by 2030 [1].

This article addresses issues that encompass SDG 6, which aims to “Ensure availability and sustainable management of water and sanitation for all.” Water scarcity, poor water quality, and inadequate sanitation harm food security and livelihoods. Today, more than 2 billion people live in areas where there is a risk of limited access to potable water. By 2050, it is estimated that at least one in four people worldwide will live in a country experiencing chronic or periodic drinking water scarcity. This does not apply only to developing countries but also to European countries such as Poland. The resulting main tasks (posted in the 2030 Agenda for Sustainable Development) are as follows: (6.1) by 2030, provide universal and equitable access to safe and affordable drinking water and (6.3) by 2030, improve water quality by reducing pollution; halve the amount of untreated wastewater and significantly increase the level of recycling and safe reuse of materials globally; and (6.) by 2030, expand international cooperation and support capacity building for developing countries to take action and develop programs related to water and sanitation, including water collection, desalination, water efficiency, wastewater treatment, recycling, and water reuse technologies. Currently, the progress toward achieving SDG 6 remains insufficient, with a major challenge being that approximately 80% of wastewater generated by human activities is discharged into rivers or the sea without adequate treatment. This widespread release of untreated wastewater poses significant risks to both environmental and public health. On the other hand, municipal wastewater contains substantial deposits of valuable natural resources that remain largely underutilized by society. These include recoverable energy, nutrients, and water, which present significant opportunities for resource recovery and circular economy approaches in wastewater management. By developing innovative technology, we may be able to recover large amounts of water, energy, and recyclable materials. The wastewater treatment plant of the future is expected to achieve higher energy recovery and improved overall energy efficiency throughout the treatment process. This advancement aims to reduce operational costs and minimize the environmental footprint of wastewater management. In turn, this is expected to lead to a reduction in greenhouse gas emissions. The recovered water is to be not only discharged into the environment but also used for utility purposes. This will ensure a closed water cycle for industry and urban infrastructure [2].

Sustainable water management involves the use of water resources in a manner that meets current social and economic needs without compromising the quantity and quality of those resources for future generations. This approach ensures the long-term availability and health of water ecosystems. Unfortunately, for now, the existing water management strategy is still associated with a constant negative impact on the state of water resources. Despite these challenges, numerous positive examples of sustainable water management exist, where water is treated, renewed, and reused multiple times. Enhanced wastewater treatment plays a crucial role in improving the quality of rivers and other surface waters, thereby supporting environmental sustainability [3].

To fulfill Goal 6: Clean Water and Sanitation, target 6.3 aims to improve water quality by 2030. This will be achieved by reducing pollution, eliminating landfills, and minimizing the use of harmful chemicals and other hazardous materials. Wastewater treatment plants are increasingly becoming water recovery facilities. This shift requires the adoption of new advanced treatment methods and faster processes, such as the physical–chemical methods discussed in this paper. Additionally, target 6.a stresses the importance of expanding international cooperation. It also highlights the need for capacity-building in developing countries to support programs in water collection, desalination, water efficiency, wastewater treatment, recycling, and water reuse technologies. Modern biological wastewater treatment plants use advanced technologies that allow for the efficient and environmentally friendly treatment of wastewater. A wastewater treatment plant typically includes several main sections. The mechanical section removes larger insoluble contaminants and fats from the wastewater. Another section contains bioreactors, where microorganisms break down organic matter. Some plants also include a chemical treatment stage, mainly for phosphorus removal through precipitation. Finally, there is the sludge section, where the sludge generated during treatment is collected and processed [4].

One of the most commonly used technologies in a biological wastewater treatment plants is the activated sludge method. It involves the introduction of a suspension of microorganisms into the wastewater, which decomposes organic pollutants. This process occurs in specialized reactors where the wastewater is aerated. The introduction of air supports the growth and activity of microorganisms that break down organic pollutants. Activated sludge is effective in removing organic pollutants, nitrogen, and phosphorus [5]. Proper management is necessary to maintain optimal conditions for the microorganisms. Key parameters that must be controlled include temperature, pH, oxygen concentration, and sludge volume [6]. However, the main technological process in wastewater treatment has remained largely unchanged over the past fifty years. This process is illustrated in Figure 1 [7].

An important advance in the development of these plants is hybrid systems that combine different treatment technologies, such as biological, chemical, and physical processes, to achieve optimal results. Such systems are essential to meet the growing need for removing micropollutants from wastewater. They help ensure that the treated water is safe for environmental discharge or reuse [8]. While the new EU wastewater directive imposes the removal of micropollutants from wastewater from large wastewater treatment plants, this only applies to selected substances (‘quaternary treatment’). ‘Quaternary treatment’ refers to an advanced stage of urban wastewater treatment that targets a reduction in a wide range of micropollutants [9]. This process goes beyond conventional methods to remove contaminants such as pharmaceuticals, personal care products, and other trace organic compounds.

2. Micropollutants

Micropollutants have been present in wastewater for many years, though their detection and recognition as a concern have only emerged more recently. The size and concentrations of micropollutants are low. It is only in the past decade or so that analytical tools have advanced enough to detect trace amounts of substances in tested samples. We now also understand that these contaminants must be prevented from entering and spreading in the environment [10].

Chemicals: The development of analytical techniques in recent years has made it possible to detect in the aquatic environment very low concentrations of chemical compounds, even at the level of micro- and nanograms per liter. Previously, substances of such low concentrations were considered harmless to the environment and living organisms. However, it is now established that a number of them affect living organisms even at their low concentrations and often have the potential to bioaccumulate in the environment. Currently in Europe, the topic of organic pollutants is regulated by the so-called EU Water Directive 2013/39. Among other things, this document makes it mandatory to monitor the concentrations of selected watch list chemicals in surface waters. The watch list includes substances that (based on available data) are suspected of posing a potential risk to the aquatic environment within the European Union. Extremely persistent chemical compounds also enter the wastewater. These include PFAS, or perfluoroalkyl compounds [11]. Depending on the chain length, the particle size of these chemicals is about 0.13 μm (Figure 2). They are often called “perennial chemicals” because the carbon–fluorine bond they contain is extremely strong, making them highly resistant to degradation in the environment. Their ability to bioaccumulate poses a significant threat to human health and the environment. The growing awareness of the harmful effects of PFASs, as well as an increasing number of scientific studies confirming these effects, is contributing to the tightening of existing legislative restrictions on their use and production under European law [12]. Many chemical compounds are present in very toxic wastewaters including polycyclic aromatic hydrocarbons (PAHs) which are commonly detected in the aquatic environment [13]. Although polycyclic aromatic hydrocarbons are degraded in the environment under the influence of solar radiation and microorganisms present in water and soil, significant deposits of PAH in bottom sediments have a very long half-life and are a significant threat to the environment [14].

Pharmaceuticals: Other sources of micro-pollution are pharmaceuticals, such as pain relievers, anti-cancer drugs, hormones, antibiotics, as well as medications used in veterinary medicine. The discovery of the first sulfonamide (para-aminobenzene sulfonamide) in 1935 marked a significant step in the development of antibiotic therapy, including the isolation of penicillin. This drug has proven effective in treating various infections, including those caused by Streptococcal Meningitis, by inhibiting the synthesis of folic acid. Other classes of antibiotics, including penicillin, cephalosporins, and macrolides, exert their antibacterial effects by inhibiting bacterial cell wall synthesis or interfering with protein synthesis. Drugs are not completely inactivated in wastewater treatment plants [15]. The most commonly prescribed pharmaceuticals by doctors and veterinarians are antibiotics. Once taken, drugs undergo metabolism (biotransformation in the liver, gastrointestinal tract, and lungs) in the body, during which structural and chemical changes occur in their molecules. Slowly excreted, lyophilic, and non-polar drug components are converted into polar and hydrophilic ones. Drug metabolism is often incomplete, leading to the renal excretion of both the parent compound and its metabolites. Pharmaceutical residues are an emerging environmental problem. The resistance of selected bacteria and genes to antibiotics is one of the biggest problems facing humanity in the 21st century. Growing resistance among bacteria is gradually weakening the effect of antibiotics, making them less effective. One of the most significant threats is multidrug-resistant strains that do not show sensitivity to many antibiotics used for treatment. Bacteria, along with viruses, fungi, and parasites, lack sensitivity to available drugs. This results in incomplete clearance and the spread of resistance genes within and across species. Bacteria acquire resistance through ARGs. One of the main reservoirs and vectors of ARGs is wastewater.

Sulfonamide antimicrobials, which are widely utilized in both human and veterinary medicine, exert bacteriostatic effects by inhibiting bacterial reproduction without directly inducing cell death. Their mechanism of action involves the disruption of folic acid synthesis, a critical pathway required for the production of nucleic acids, and ultimately, for DNA and RNA synthesis. Pain-relieving drugs, or analgesics, represent a major category of prescribed medications that function by abolishing or reducing the sensation of pain. Diclofenac, a well-known member of the non-steroidal anti-inflammatory drug (NSAID) class, is frequently used in clinical practice for this purpose. The most popular active ingredients on the market with this effect are acetylsalicylic acid, ibuprofen (molecule size: 1.03 × 0.52 × 0.43 nm), and paracetamol (molecule width: 0.43 nm and height: 0.78 nm) (Figure 3).

In recent years, antibiotics, ARBs, and ARGs have been detected in nearly all environments. They are now recognized as major pollutants. Wastewater treatment systems associated with pharmaceutical manufacturing, hospitals, and municipal or domestic sources are recognized as major contributors to environmental contamination with antibiotics. These systems also serve as significant reservoirs of antibiotic-resistant microorganisms and resistance genes. Even though drugs are present in wastewater and river waters at relatively low concentrations (on the order of 10⁻³–10² μg·dm⁻³), their amount is often sufficient to negatively affect living organisms [16]. Drugs and their metabolism products are present in environmental waters (surface, marine, and even groundwater) and soils. Therapeutic substances enter surface water through wastewater and exert toxic effects on aquatic organisms. They have also been detected in drinking water, posing significant risks to human health. Infants, young children, the elderly, and individuals with liver or kidney impairment are particularly vulnerable [17]. It has been shown that the most commonly identified drugs in wastewater are ibuprofen, carbamazepine (CBZ), diclofenac (DCF), sulfamethoxazole (SMX), and estrone [18]. Antibiotics, such as sulfamethoxazole, can also contribute to the spread of drug-resistant bacteria.

It is worth mentioning that treated wastewater, which was previously perceived as environmentally safe, harbors a significant burden of antibiotic resistance determinants. The presence and distribution of drugs and their metabolic byproducts in the environment are not systematically monitored. Likewise, wastewater collection, wastewater treatment, and the elimination of bioactive compounds from surface water and groundwater are not consistently regulated [19]. The activity of pharmaceuticals in the natural environment is affected by the presence of bacteria, rainfall, or temperature, among other factors. Significantly lower activity and the accelerated decomposition of pharmaceuticals occur in summer compared to the autumn–winter period. Bioremediation plays an important role in their elimination from the surface water and groundwaters [20].

The excessive use of antibiotics has driven the rapid emergence and spread of ARBs and ARGs. Many regions across the globe are facing serious challenges due to the excessive use of antibiotics and the ongoing evolutionary changes in the resistome. ARBs and ARGs from farms, cities, hospitals, and WWTPs, or as water runoff, may accumulate in water, soil, and air. ARBs and ARGs enter the soil through multiple pathways [21]. For instance, the use of manure in agriculture is a major contributor to the spread of ARBs and ARGs in the soil [22]. Excess sludge serves as a significant reservoir of antibiotics, ARGs, and ARBs in WWTPs. Its reclamation poses potential risks to both human health and environmental safety [23].

Microplastics: One group of substances that wastewater treatment plants cannot handle is microplastics [24]. Plastics are semi-synthetic or synthetic organic polymers. Their production is relatively cheap, and they are lightweight, durable, strong, and corrosion-resistant. That is why they are widely and willingly used in all industries. The most commonly used polymers are polypropylene (PP), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET), which account for about 90% of the world’s total plastic production. They originate from plastic packaging and are also present in cosmetic products and synthetic fabrics. A significant amount of microplastics is also released into the water during the washing of synthetic fabrics such as fleece, nylon, and polyester. They have different sizes and shapes (Figure 4) [25].

Microbiological hazards: Sewage contains a variety of harmful substances, including numerous organic and inorganic compounds along with pathogenic microorganisms, fungi, bacteria, and viruses [26]. Among the bacteria present are Gram-negative species such as Escherichia coli (a coliform bacterium), Klebsiella pneumoniae (associated with pneumonia), and Helicobacter pylori (linked to stomach and digestive system diseases). Municipal sewage also contains bacteria that cause infectious jaundice, typhoid fever, and even cholera. Some compounds are also responsible for causing allergies and inflammation. Bacteria and viruses can also be considered micropollutants. Most bacterial sizes range from 0.2 to 2.0 μm in diameter and from 2 to 8 μm in length. The ubiquitous Escherichia coli is about 1 μm in diameter and 1 to 2 μm long (Figure 5).

Parasites and their eggs: Nematode eggs, like those of Rhabditis and Trichostrongylus, are also frequently observed in wastewater and sludge [27]. Parasite eggs and protozoan cysts in wastewater exhibit a size distribution ranging from very small (flagellates and amoebas) to large (40 to 45 μm for cystic forms). Toxocara canis egg size is in the range of 80 to 85 µm; nematode (like Ascaris lumbricoides (roundworm) or Trichuris trichiura (whipworm)) eggs have dimensions of 40 to 70 μm by 35 to 50 μm (Figure 6). Helminth eggs, particularly Ascaris, are more common and can survive longer in wastewater compared to other helminth eggs [28].

Another issue is the contamination of municipal wastewater with intestinal parasite eggs from Toxocara spp., Ascaris spp., and Trichuris spp. This can be a significant obstacle in the organic biomass hygienization and safe use of excess sludge as an organic fertilizer.

3. The Fate of Micropollutants in Biological WWTPs

The purpose of wastewater treatment is primarily to remove organic pollutants, nutrients (nitrogen and phosphorus compounds), sludge, and particulate matter. This is the current focus of wastewater treatment plants. Conventional methods include mechanical, chemical, biological, and physicochemical processes that are carried out on grids, screens, sand traps, primary settling tanks, and biological chambers, among others. In biological treatment plants, microorganisms decompose organic substances under aerobic or anaerobic conditions. In the aerobic process, bacteria convert organic compounds into carbon dioxide, water, and biomass. In turn, the anaerobic process produces biogas, which can be used as a source of energy. Biological treatment is an important stage of the process because this is where the main reduction in organic pollutants takes place. Microorganisms, through natural metabolic processes, effectively break down organic substances, transforming them into less harmful forms. This is a stage that requires the careful monitoring and control of conditions to ensure optimal microbial performance and maximum treatment efficiency. During wastewater treatment in WWTPs, 50 to 99.96% of microplastics are removed according to various sources [29,30]. The efficiency of microplastic removal during mechanical wastewater treatment depends on the size of the particles and their density. The effective removal of microplastics larger than 100 μm can be achieved through straining with microsieves. In contrast, conventional fine and medium screens are not effective in removing these micropollutants. Some microplastics, on the other hand, can be removed from wastewater on screens and dense sieves (with bar spacing, and mesh diameters of 2 to 10 mm). Studies have shown that during mechanical wastewater treatment, polyethylene or polypropylene particles with a size of 20 to 500 μm effectively become sediment and/or are collected in the foam at the flotation process. The removal efficiency increases with the increase in the size of plastic microparticles. Literature data and findings from our own research indicate that up to 90% of microplastics can be removed during the sedimentation process in primary clarifiers. This highlights the effectiveness of primary treatment in capturing these particles. Microplastics removed from wastewater accumulate primarily in raw sludge. From there, they are transferred to the sludge treatment unit. Due to their low biodegradability, microplastics along with stabilized sewage sludge end up in soils or are otherwise processed along with the sludge [31].

The microbial removal of micropollutants can involve biodegradation, that is, the complete mineralization of the substance into biomass and gases. But often the transformation processes of persistent organic pollutants result only in biotransformation, meaning that the parent compound is not fully removed and is instead converted into various metabolites. Micropollutants at low concentrations, combined with a stable chemical structure and suboptimal technological conditions, are difficult to remove.

We know that PFASs not only flow into wastewater treatment plants but also run off in treated wastewater and sewage sludge. Scientists have confirmed that short-chain PFASs are found primarily in wastewater, while long-chain PFASs are found mainly in sewage sludge, where they are more easily bound to particulates [32]. Many studies indicate high levels of perennial pollutants in wastewater (above 100 ng/L). Unfortunately, a characteristic feature of mechanical–biological treatment plants is the low removal efficiency of most PFAS pollutants [33].

The removal efficiency of chemical non-biodegradable micropollutants like PFASs, PAHs, antibiotics, etc., in wastewater treatment plants depends on several factors, including the physicochemical properties of the substances and their bioavailability [34]. It is also influenced by the operating conditions of the biological treatment chambers and the composition of the microbial community [35]. The antibiotics, ARB, and ARGs are mostly deposited in the sludge [23].

In conventional WWTPs, the efficiency of drug removal is usually less than 20%. In the reported case, bacteria carrying ARGs were found to survive across all treatment stages. This included disinfection through chlorination. Notably, their numbers exceeded those of the total bacterial population. Research findings indicated the need to upgrade existing WWTPs by incorporating additional purification stages. Implementing highly effective methods such as advanced oxidation technologies (AOTs) is essential.

Researchers at the University of Georgia (USA) recorded the MCR-9 gene in a sample taken from wastewater. It demonstrated resistance to colistin and is often referred to as the “last resort” antibiotic for treating bacterial infections. The gene’s connection to bacteria carries the risk of a global public health threat, and treatable E. coli and Salmonella can become deadly overnight. The MCR-9 gene was found in a bacterium known as Morganella morganii. Scientists noted that this is the first instance of MCR being detected in this specific bacterium. This may mean that its presence in nature is greater than previously thought [36].

Threats to the aquatic environment are antibiotics from the following groups: sulfonamides (trimethoprim and sulfamethoxazole), fluoroquinolones (cipro- and norfloxacin), and macrolides (erythron- and clarithromycin) [37]. The antibiotics mentioned above are removed from wastewater through biodegradation, with an efficiency of 50 to 70% [38]. The application of ozone eliminates 90 to 99% of antibiotics, while adsorption onto activated carbons removes erythromycin by 54%, trimethoprim by up to 83%, and sulfamethoxazole by up to 99%. Reports indicate that 30 ARGs resistant to treatment with sulfonamides, tetracyclines, quinolones, or macrolides were detected in two WWTPs in northern China using the activated sludge process for wastewater treatment [39]. Despite a reduction in ARG levels by 89.0 to 99.8%, some ARGs were still present in the discharged wastewater samples. Moreover, ARGs were detected in the excess sludge (through the sedimentation of sludge flocks and their dewatering) at orders of 10⁹ to 10¹¹ cfu/g·d.m. Twelve types of ARGs were found in the dewatered sludge and plant discharge stream at higher levels than in the inlet stream, indicating ARB proliferation. Antibiotic concentrations in the discharge were reported to be between 2 and 50% of those found in the inlet wastewater [40].

Therefore, the excess sludge, which is the biomass produced in WWTPs, serves as a reservoir for ARGs. Studies show that the use of conventional physical or chemical methods, such as microwaves, alkaline lye, or simple coagulation, does not yield effective results. Therefore, new and more efficient solutions are needed [41]. Another study reported findings for AS and anaerobic digestion sludge (ADS) samples, observing up to 181 ARG subtypes within 22 ARG types. The presence of ARGs in AS samples was found to be 1.3 to 2.0 orders of magnitude higher than in ADS samples. Multidrug and bacitracin resistance genes were the most prevalent in AS, but their levels were significantly reduced in the ADS samples.

Besides the municipal WWTP, similar problems are observed in the manure and slurry generated when raising cattle and pigs [42]. Manure, which is an organic fertilizer of animal origin, plays a key role in organic agriculture and horticulture. Its use improves soil structure, increases soil fertility, and promotes healthy growth. Slurry is a byproduct produced in cage-free farming. Used as manure, it is a mixture of feces and urine in their natural proportions, combined with remnants of animal feed and a small amount of water used for cleaning. This mixture is typically generated in facilities for raising animals without bedding. The chemical composition of this manure depends on several factors, such as the species of the animals, diet, age, and how it is stored. However, unlike manure, it is a liquid fertilizer that is more aggressive, leading to a quicker impact on the soil.

The use of these organic fertilizers in an age dominated by artificial fertilizers aligns with the principles of a circular economy. The regular application of manure increases the humus content in the soil. This leads to improved physical properties, including better cohesion and permeability. This, in turn, makes it easier for plants to develop roots and leads to better absorption of water and nutrients. The proper use of manure and slurry promotes sustainable agriculture, closes the nutrient cycle, and increases crop production. It also contributes to the recovery and reuse of elemental resources such as nitrogen, phosphorus, potassium, and a variety of micronutrients. These nutrients are crucial for proper plant growth and health. For example, nitrogen in manure or slurry is one of the most important elements needed by plants for the synthesis of proteins, chlorophyll, and other organic compounds. Unfortunately, manure and slurry containing ARBs, ARGs, and parasite eggs, if not treated properly, pose a serious threat to the environment and health care. Despite numerous efforts, manure and slurry are not adequately treated before being used in agriculture, contributing to the spread of antimicrobial resistance in the environment. It was reported that Bacillota has the highest abundance. During the biodegradation process, the content levels of Acidobacteriota, Armatimonadota, Bdellovibrionota, Chloroflexota, Gemmatimonadota, Hydrogenedentes, Deferribacterota, Dependentiae, Abditibacteriota, and Myxococcota increase, making them detectable. In contrast, the abundance of Elusimicrobiota and Euryarchaeota declines to levels below the detection limit. Research findings indicate that cattle farms serve as reservoirs of ARGs and MGEs (mobile genetic elements). However, composting and proper storage have been shown to significantly reduce ARG levels in cattle manure [42]. Nonetheless, this kind of processing has less or no effect on phenicol, sulfonamide, or trimethoprim RGs [43]. This is another reason why the destruction of sulfonamides and their related resistance genes (RB and RGs) is a key objective of the project described in this report. Similar patterns are observed in other animal farming operations, where the intensive and widespread use of antibiotics in agriculture contributes to the expansion of the environmental ARG pool [44,45].

4. Micro Pollutant Control Technologies

The UN has set out the goals required to achieve global sustainable development of the world (SDGs). Technological development will play an important role in implementing these goals. In this technological advancement, the proper treatment of municipal, agricultural, and industrial wastewaters is crucial for both human health and environmental protection [46]. The pre-treatment of the incoming wastewater includes mechanical and physical processes, i.e., straining, flotation, and sedimentation. Pre-treatment is designed to prepare wastewater for subsequent technological processes. Its purpose is to remove larger mechanical impurities, sand, easily settling suspended solids, oils, and fats. This stage ensures the smooth operation of downstream wastewater treatment facilities and helps reduce the pollutant load entering the biological treatment units. Grating assures the removal of solid contaminants exceeding 6 mm in size. Sand traps are used to remove mineral impurities such as sand, gravel, and ash. The primary clarifier is employed to separate and remove floating grease. Three types of screens may be applied: large (50 to 100 mm), medium (10 to 40 mm), and small (2.5 to 10 mm) mesh sizes [47]. Only mesoparticles and bigger plastic particles can be retained. However, taking into account all WWTP preliminary treatment stages, 35 to 59% of the microplastics in the wastewater could be successfully removed [48]. The remaining plastic particles settle in the secondary clarification tank and later on remain in the excess sludge.

The final treatment of wastewater takes place in the biological system. This process decreases the BOD of the wastewater by about 15 to 30%. These are processes and methods that utilize the metabolic activity of microorganisms to break down organic matter, resulting in highly treated wastewater. Biological treatment aims to reduce the concentration of dissolved and colloidal organic compounds present in wastewater and remove nitrogen and phosphorus (i.e., nutrient elements). Microorganisms process organic compounds into gaseous products and water. The main role in the process of wastewater treatment by the biological method is played by bacteria. Bacteria are fed by unicellular organisms of animal origin (periwinkles and threadworms), and these in turn are devoured by higher organized organisms (rotifers, nematodes, and scorpionflies). These diverse organisms coexist with each other to form flocs of activated sludge [49]. At present, the degradation of SAs in water by conventional biological methods is not significant. Sulfonamides are a significant group of chemotherapeutic agents with a broad spectrum of activity, effectively targeting numerous bacterial strains. They exert a bacteriostatic effect; i.e., they do not kill bacterial cells but inhibit their growth. Sulfanilic acid amides are used in medicine as bacteriostatic agents and disinfectants. For antimicrobial activity, the unsubstituted (i.e., primary) presence of the amino group in the para position relative to the sulfonamide group is necessary. Sulfonamide compounds are also used in several other drugs, including diuretics (e.g., furosemide, metolazone, and indapamide), antidiabetics for oral administration (e.g., tolbutamide), analgesics (e.g., celecoxib), anti-inflammatory (e.g., sulfasalazine), or neuroleptics (e.g., thiothixene). Sulfonamides (SAs) are commonly used to treat a variety of medical conditions, such as diuresis, hypoglycemia, thyroiditis, and glaucoma. The sulfa drug sulfamethazine (SMZ) is used in veterinary medicine as an antibacterial compound to treat livestock diseases such as gastrointestinal and respiratory tract infections.

There is currently no requirement in the European Union to remove the pollutants in question. However, work is underway in the European Parliament on a directive introducing such regulations. The proposal to amend the Urban Wastewater Treatment Directive, COM (2022) 541, introduces requirements for the removal of micropollutants. These obligations apply to the largest treatment plants with an equivalent population (e.p.) of over 150,000, as well as to those with over 10,000 e.p. that discharge treated wastewater into sensitive water bodies. The compliance deadline for these requirements is set for December 2040. The directive also includes a “roadmap” with intermediate requirements [50,51]. Technologies for removing micropollutants are based on activated carbon or ozone oxidation. In this case, particulate-activated carbon (PAC) and granular-activated carbon (GAC) are used. They can be dosed directly into activated sludge chambers or a separate reactor can be created for them. Both options have their advantages and disadvantages. In both cases, carbon retention in the system is problematic. The secondary settling tank is not sufficient, as too much of the carbon drains into the receiver. There are two ways to solve this problem. In the first option, a membrane plant is installed instead of a secondary settling tank. The second option is to place a sand filter after the secondary settling tank, but this option is only applicable for small throughputs [52].

Commonly used wastewater treatment methods, based on biological processes with activated sludge (AS), are mainly effective for polar compounds, achieving a removal efficiency of only 60 to 90%. These methods have been in use for many years, but they cannot meet the new challenges created by man and modern industries [53]. These challenges involve the removal of various chemical compounds, including drugs and their metabolites, which are difficult to fully decompose using biological methods. This issue applies to wastewater, excess sludge, and waste from the agricultural and food industries. The principles of the circular economy currently adopted by the UN and EU impose different requirements on wastewater treatment plants, both municipal and those operated in the agricultural industry. WWTPs should be treated as resource recovery facilities that allow the reclamation of water for agriculture, gray water for municipalities, and technological water for industry and services. Additionally, biomass can be utilized as a fertilizer for land reclamation and agricultural applications. While biomass serves as a source of green energy, in some countries, it is still incinerated despite its high water content.

The new generation of additional enhancement of WWTP systems is AOTs. These include the ozone (O₃), ultraviolet (UV)/chlorine, and Fenton processes. These processes rely on hydroxyl radicals and reactive chlorine species, both of which effectively eliminate organic pollutants and microorganisms. However, they were not effective in significantly reducing ARGs in secondary effluents from a municipal WWTP [54]. The removal of ARGs was found to be incomplete. In the case of chlorine, ozone, or other reactants, mass transfer and solubility play crucial roles. To achieve a uniform concentration of the reactant, it is necessary to establish appropriate mixing parameters and adequate flow dynamics in the reaction zone. The chlorination of wastewater or drinking water leads to the formation of chloro-organic compounds such as chlorophenols [55]. The effectiveness of UV or UV/O₃, is often limited by the penetration depth of ultraviolet radiation in turbid wastewater [56]. Furthermore, using UV for the hygenization of excess sludge is not practical. Additionally, storage and composting are not fully effective in adequately reducing microbiological and chemical contamination and require final compost quality control [57,58]. The mentioned EU directive refers to the necessity to remove micropollutants in the future. Therefore, it will be essential to introduce a fourth stage of wastewater treatment. The fourth stage of treatment is based on the structure of the wastewater treatment plant (WWTP) [59]:

(a)

Stage I: Mechanical pre-treatment;

(b)

Stage II: Biological treatment;

(c)

Stage III: Chemical treatment or treatment enhancement;

(d)

Stage IV: Removal of micropollutants.

The main processes under consideration are coagulation–flocculation, advanced oxidation processes, ozonation, activated carbon adsorption, and membrane processes [34]. The technology based on the application of the EB process is a very promising solution to be considered, as it has been proven by many experiments and industrial applications.

5. Electron Beam (EB) Application as a Quaternary Treatment

5.1. Main Process Principles

The irradiation of water using ionizing radiation causes the ionization and excitation of water molecules [60]. Excited water molecules rapidly transition back to their ground state, whereas the process of ionization results in the formation of cationic radicals and free electrons. These radicals can react with each other or with hydrogen ions (H₃O⁺), leading to the formation of H^•, ^•OH, H₂O₂, and H₂. The yield of molecular products and radicals is also influenced by pH because e_aq⁻ and H^• radicals are governed by the acid–base equilibrium. The radiolysis products of H₂O (Figure 7) exhibit high reactivity. The majority of chemical reactions in irradiated aqueous solutions are primarily driven by these reactive radicals.

When EB is used, the process is enhanced by ozone, creating a more oxidative environment (

Table 1. Comparison of ozone/UV/EB-based AOTs. The values in square brackets indicate the number of molecules formed when an energy of 100 eV is absorbed.

O3	UV/O3	EB/O3
3O3 + H2O→2•OH + 4O2	O3 + H2O + hv → H2O2 + O2 O3 + hv → O2 + O O + H2O → H2O2 H2O2 ↔ HO2− + H+ O3 + HO2−→ •OH + 2O2 H2O2 + hv → 2 •OH	H2O + e− → [2.7] •OH + [2.7] H3O+ + [2.6] e− + [0.7] H2 O2 + [0.6] H + [0.45] H2 O3 + H → •OH + O2 O3 + H2O2 → 2 •OH + O2 e−aq + H3O+ + O3 → H2O + OH + O2

). This leads to the removal of organic and microbiological pollutants.

Ionizing radiation (IR) destroys microbial DNA and causes irreversible damage to the cell membrane [60]. IR induces the destruction of the chemical structure of cellular molecules and ionizes water in the cytoplasm. This leads to the production of toxic oxygen species including hydrogen peroxide, superoxide radicals, and hydroxyl radicals. These species cause biochemical damage to microbial cells, including what is referred to as indirect DNA damage. One contributing factor to this is the detrimental effect it has on deoxyribonucleic acid (DNA). IR causes several types of DNA damage. The most important are base damage, nucleotide damage, single-strand breaks in DNA, double-stranded DNA breaks, and complex damage. Strand breaks are directly caused by primary or secondary electrons. The framework of the direct and indirect impacts of ionizing radiation is shown in Figure 8.

5.2. Destruction of Chemicals by EB

Many studies have shown that various compounds found in wastewater undergo degradation under the influence of an EB [61]. These include halogenated compounds, pesticides, etc. EB irradiation is a high-energy process that facilitates the breakdown of such pollutants. Compared to purely chemical methods, it uses a cleaner technology that minimizes the formation of dangerous byproducts. Compared to the UV irradiation method used in AOP, EB irradiation is technically simpler, virtually insensitive to color and suspension in solution, and extremely fast. The efficiency of the process can be increased by combining the EB process with ozonation, which leads to an increase in the concentration of hydroxyl radicals with strong oxidizing capabilities (Table 1). The use of catalysts such as titanium dioxide is also applicable. INCT contributed to this study to a great extent [62,63]. The main objective of the research conducted was to determine the feasibility of using radiation technologies for the degradation of selected organic micropollutants in aqueous solutions, including synthetic and natural matrices. The study was carried out for selected contaminants from a group of pesticides (chlorfenvinphos, carbendazim, and parathion) [63]. Among others, the organochlorine herbicides found in wastewater were degraded, including 3,6-dichloro-2-methoxybenzoic acid (dicamba) and 2,4-dichlorophenoxyacetic acid (2,4-D). Previous studies have shown that toxic organochlorine compounds, which are semi-dissolution products, mineralize readily to chloride ions when exposed to ionizing radiation. Toxicity measured against Spirotox and Microtox tests of other compounds formed by the irradiation of herbicides and chlorophenols is low. By combining ozonation and radiolysis, significant reductions were achieved in the required doses of both ozone and radiation to effectively mineralize herbicides and their degradation products. The choice of compounds for the study was dictated, among other things, by the fact that these compounds are increasingly appearing in the environment. That is mainly due to unsatisfactory treatment results in the processes currently used in municipal wastewater treatment plants and drinking water purification stations [64].

Finally, a major problem is the removal from waterways of per- and polyfluoroalkyl compounds (PFASs) which constitute a vast family that includes thousands of synthetic chemicals that have numerous applications in various areas of life and are present in the environment. EB irradiation leads to the destruction of PFASs. The reactions are quite complicated, and the hydrated electron, atomic hydrogen, and hydroxyl radicals are the main species involved. The most active reagents in the employed conditions are hydrated electrons from water radiolysis. The presence of hydroxyl radical scavengers in wastewater, such as alcohols, promotes a shift toward a reductive reaction pathway Destructive EB methods to decompose and mineralize PFASs (per- and polyfluoroalkyl compounds) have been reviewed [65]. The development of high-power electron accelerator structures may lead to a broader use of this technology.

The process was investigated in different countries, and the industrial installations have been built in the Republic of Korea (ROK) and China at dyeing textile companies. Anaerobic biological methods (i.e., methane fermentation) almost cause the complete discoloration of textile wastewater. However, they are a source of toxic, mutagenic, and carcinogenic aromatic amines, of which only a few undergo further degradation in an anaerobic environment. Anaerobic–aerobic biological methods contribute to the complete mineralization of synthetic dyes contained in wastewater. However, the reduction in the pollution load on the wastewater is practically the same as in a single-stage aerobic process. Therefore, there is a need to incorporate biological methods into the treatment systems of colored textile wastewater, which currently rely on physicochemical processes. This can be achieved by integrating biodegradation with advanced oxidation processes (AOPs). The process used in the reported case was EB treatment. The first plant was constructed at the Daegu Dyeing Industrial Complex (DDIC), and the flow capacity of wastewater was 10,000 m³/day [66]. Pollutants present in the wastewater were: organic dyes, surfactants, and other chemicals like terephtalic acid (TPA) and ethylene glycol (EG). The power of the accelerator installed was 400 kW (energy 1 MeV), and the main parameters related to the plant’s efficiency were total organic carbon (TOC), chemical oxygen demand (COD), and biological oxygen demand (BOD₅). The EB treatment was applied before biological aerobic treatment, and the removal efficiency of whole organic pollutants was improved at a low cost (3 US cents per cubic meter). Important to mention is that the plant has been in operation since 2005. Another plant has been constructed in the dyeing industry in Jiangmen City, Guangdong Province, China [67]. The plant has a flow capacity of 30,000 m³/day, with COD and color as the primary parameters monitored. It began operation in 2020, with a treatment cost ranging from 2.0 to 2.5 JPY/m³

In both cases, specific substrate quality indicators such as COD, BOD₅, and TOC were used. COD and TOC are measures of the total amount of organic matter, while BOD₅ is most often described as the biodegradable portion of organic compounds. We can estimate biodegradability by determining the COD/BOD₅ ratio. In many cases, such rather general information about the organic matter contained in wastewater is insufficient, necessitating a more precise characterization, especially when persistent chemical micropollutants are considered. The effectiveness of removing micropollutants in wastewater treatment plants ranges from 13 to 100%. This variability depends on factors such as the physicochemical properties of the substances, their bioavailability, the operating conditions of the biological treatment chambers, and the composition of the microbial communities [68].

5.3. EB Degradation of Pharmaceuticals in Wastewater

Hospital wastewater is particularly hazardous due to its high toxicological potential [69]. It contains a wide array of contaminants, including diagnostic substances and pharmaceuticals. These pharmaceuticals include antibiotics, non-steroidal anti-inflammatory drugs, lipid regulators, estrogens, antidepressants, cancer medications, beta-blockers, contrast agents, anticonvulsants, sedatives, and more. It has been experimentally proven that EBs applied with moderate doses ranging from 2.5 and 5.0 kGy may reduce the toxicity factor for different pharmaceutical mixtures by up to 75%. Among them were tests that included sulfonamides (SAs) [70,71,72].

SAs primarily degrade through oxidation processes driven by the presence of ^•OH radicals, and the concentration of these radicals increases in the presence of ozone (see Table 1). SAs are converted into inorganic compounds and small molecules. The interaction between ^•OH radicals and sulfonamide antibiotics is not influenced by the specific characteristics or differences in the R groups. Instead, reactions mainly occur in the sulfanilamide group (the common structure), the sulfanilamide bond (N–S bond), and the R group [73]. The removal efficiency of sulfonamides depends on different parameters [74]. Additionally, the inorganic ions altered the degradation products of SMX, as illustrated in Figure 9, potentially resulting in variations in toxicity [74]. This highlights the need for increased attention to the changes in toxicity resulting from various degradation products. Moreover, tests of luminous bacteria demonstrated that EBs could decrease acute toxicity to aquatic ecosystems [75]. A comprehensive approach should address not only the usual toxicity screening methods but also changes in antibacterial activity and biodegradability. To address this gap, the ionizing radiation-induced degradation of four SAs was studied in dilute aqueous solutions, with a focus on the biological evaluation of decomposition products that do not exhibit antibacterial activity. However, the degradation of the parent compounds alone may not be sufficient to mitigate the environmental risks posed by the toxic byproducts formed.

EB irradiation (by “high energy”, in the range of up to 10 MeV, which is the upper limit for radiation processing) is an additive-free technology using short-lived reactive species formed by the radiolysis of water (see Figure 7 and Table 1 for the mechanism of action). The availability of high-power electron accelerators opens the technical feasibility of these technology applications on the full industrial scale.

Since we take into account the fact that pharmaceuticals are partially ad/absorbed in biological sludge, it is important to determine the efficiency of drug decomposition in both phases. The decomposition of the model compound diclofenac (DCF) in sewage sludge from a municipal wastewater treatment plant was investigated. It was found that about 40% of DCF absorbed into sludge was decomposed [76]. This observation should be considered in the final plant design, as additional drug degradation will occur in the digester.

The EB irradiation of SMX in the water phase was studied, and the degradation mechanism [77,78,79] was elaborated. The observed phenomena were attributed to the fact that at a constant absorbed dose, the initial concentration of sulfonamides reduced the ratio of active radicals. At an absorbed dose of 1 kGy, SMX degradation reached 88.6%. For the degradation of SMX, the ^•OH reaction primarily involved the addition of the benzene ring within the sulfanilic acid component of SMX. Meanwhile, the electron hydration reaction predominantly affected the heterocyclic rings. In treated samples, increasing the SMX concentration from 10 to 100 ppm resulted in a decrease in destruction efficiency. At a dose of 10 kGy, the efficiency dropped from 100 to 93.2%. This indicates a concentration-dependent reduction in treatment effectiveness. The proposed mechanism of the process assumes that the hydroxyl radical reacts with sulfanilic acid by its addition to the benzene ring, forming OH adducts. The obtained products may change into cationic radicals by dihydroxylation. Ionizing radiation induces transformation (e.g., hydroxylation, deamination, desulfonation, and oxidation), along with cleavage (mainly the sulfonamide bond) and post-cleavage transformation. For AOTs that generate ^•OH (see Table 1), a non-selective attack by ^•OH is suggested as a pathway to degrade various organic compounds, resulting in the formation of different intermediates and byproducts. Another proposed mechanism involves the breaking of bonds at four distinct SMX sites (clefts β, γ, δ, and ε) due to the ^•OH attack. The grafting process yields several types of initial cyclic structures. The primary products formed from the cleavage of the δ (sulfonamide bond) entail the formation of the isoxazole ring and the aminoyl phenylsulfonyl structure. The preference for breaking the sulfonamide bond was theoretically justified. According to the referenced study, protonation at the nitrogen atom of the isoxazole ring to promote hydrogen bonding was found to be the most energetically stable configuration when considering feasible conformations.

Moreover, applying oxidants like H₂O₂ or O₃ can enhance the efficiency of organic pollutant removal due to higher concentrations of reactive species (see Table 1). For example, the required absorbed dose for the complete removal of 20 mg/L of sulfamethazine was more than 1 kGy, while the required absorbed dose decreased to 1 kGy with the addition of 10 mg/L of H₂O₂ [80]. However, too high a concentration of hydrogen dioxide can hinder the removal of microorganic pollutants by scavenging hydroxyl radicals. Enhanced removal of sulfamethazine was observed with 0.1–0.6 mM Fe²⁺ [81]. That was attributed to the increased concentration of ^•OH generated by Fe²⁺-catalyzed H₂O₂ production during the radiolysis process [82].

A very important factor in the development and monitoring of the radiolytic processes is analytical methods [83,84]. The most important method is liquid chromatography (LC)/mass spectrometry (MS), which allows for the identification and product quantitation of all species with high resolution. Gas chromatography (GC)/MS is useful for monitoring volatile organic compounds (VOCs) and detecting radicals formed in the processes of C–C and C–S and the cleavage of other bonds. Electron paramagnetic resonance (EPR) allows for the detection of radicals involved in the reactions during the decomposition of PAHs. UV/Vis is a tool used to evaluate the kinetics of radical reactions. Mineralization products are detected by the use of IC (ion chromatography), while TOC (total organic carbon) measurements are used to examine the substrate and byproducts.

5.4. Microplastic Flocculation and Sedimentation Enhanced by EB

The density of microplastics found in wastewater typically ranges from 0.8 to 1.6 g/cm³. Polystyrene (PS) is denser, with a range of 1.2 to 2.3 g/cm³. Polytetrafluoroethylene (PTFE) has an even higher density, ranging from 2.1 to 2.3 g/cm³. The coagulation of particles and their sedimentation is hindered by the electric double layer that forms on their surface in an aqueous solution. The electric double layer (EDL) is a structure that forms on the surface of microplastic particles in contact with a solution, e.g., water. This layer consists of two layers: an inner layer, where charged ions attach to the surface, and an outer layer, where ions in the solution are attracted by the surface charge [85,86]. Surface-charged plastic particles, such as negatively charged PS and PE repel each other due to electrostatic interactions and maintain a stable state in the aqueous environment. Coagulants with surface charges that have opposite charges can neutralize and attract MPs and aggregate them to form large flocs. The commonly used aluminum and iron coagulants have shown the ability to remove MPs at varying degrees. However, aluminum coagulants are more effective in such cases.

Experiments performed at INCT with an application of an Electronica electron accelerator (10 kW and 10 MeV) demonstrated that the sedimentation of PVC, PET, PMMA, PS, and MPs was improved. As a result of the interaction of electrons, MPs lose their surface charge, leading to the destabilization of the suspension, and depending on the density of the polymer, they float on the surface or sink to the bottom of the vessel. This allows for their separation by sedimentation and flotation, with the former process to be carried out first. They either form sediment or float in water and sewage sludge. For most polymers, the efficiency of MP separation was 85 to 95%. However, for PVC, the separation efficiency was lower, reaching only 70% [86].

5.5. Destruction of ARGs, ARB, Parasites, Their Ova, and Other Microorganisms in Wastewater and Excess Sludge

Ionizing radiation was utilized to treat antibiotic fermentation byproducts, targeting the removal efficiencies of chemical contaminants (erythromycin) and microbiological pollutants (ARGs and ARBs). After irradiating the samples with 10 kGy [kJ/kg], over 99% of the total bacterial population was eliminated, and the ARB levels were reduced below the detection limit. The ARG content was reduced by 89 and 98% at the same dose. The organic biomass was not destroyed at this dose and remains suitable for use as a soil conditioner [87].

Another study reported the destruction of ARGs in fermentation residues using ionizing radiation. After irradiation with a dose of 50 kGy of ionizing irradiation, the relative abundance of ARG decreased by 57% [88]. The dose was rather high; however, the substrate consisted of concentrated antibiotic fermentation residues derived from the production line. In practice, the treatment of fermentation residues required a different approach due to their distinct characteristics compared to wastewater. A roll-type radiation reactor was used instead of the jet-type reactor typically employed for wastewater treatment (Figure 10A). During the two months of monitoring, EB technology showed a 61.5 ± 10.3% removal of DOCPC (a cephalosporin precursor, deacetyloxy cephalosporin C). A second round of radiation further enhanced the removal efficiency of DOCPC to 75–84% (Figure 10B).

The helminth eggs are present in most if not all sewage samples undertaken from a municipal WWTP. INCT research was undertaken for sewage sludge samples obtained from a municipal WWTP. The content of different families of bacteria, namely Salmonella spp., Escherichia coli, Clostridium perfringens, and living eggs of helminths, including Trichuris spp. Toxocara spp. and Ascaris spp., were tested. The irradiation doses required to eliminate microbiological contaminants using EBs were approximately 4 kGy for preliminary sludge (PS) with around 4% total solids (TSs) and 5.5 kGy for PS thickened to about 12% TSs [89]. These data are in agreement with earlier performed tests [90] and mechanisms elaborated on existing data published [91].

6. Technology Targeting Hazardous Micropollutants in WWTPs and Supporting Sustainable Resource Management

Council Directive 91/271/EEC has been substantially amended several times. By the year 2045, member states will be required to implement an additional purification process to remove micropollutants. This process, known as the fourth stage or quaternary treatment, will become mandatory [92,93]. As part of the extended producer responsibility system and in line with the “polluter pays” principle, manufacturers of pharmaceuticals and cosmetics (the main sources of micropollutants in municipal wastewater) will have to cover at least 80% of the additional costs of tertiary treatment. Moreover, the new regulations introduce a goal of energy neutrality [94]. This means that by 2045, municipal wastewater treatment plants with an equivalent population rate of at least 10,000 will have to start using energy from renewable sources generated by the treatment plant itself [95]. Similar activities are related to the implementation of the UN global policy regarding the implementation of the indicator for Sustainable Development Goal (SDG) 6 [95,96].

The materials cited in this work describe the developed technologies for removing micropollutants from sewage. This also applies to review materials grouping different types of developed technologies, as they do not mention methods using EBs. This is an omission of a technology whose advantages have been demonstrated at the laboratory, pilot, and industrial scales. Of course, like others, it requires adaptation and incorporation into the process lines of operated and newly built WWTPs. This is partly because studies on unit operations, which are conducted within chemical and environmental engineering faculties, focus on processes that have been traditionally established since the inception of these scientific fields. Therefore, the first choices are precipitation, coagulation, flotation, adsorption, ion exchange, and membrane processes. The only break in this traditional way of thinking is AOTs, and this is because UV and ozone began to be used for water disinfection quite early. These methods partially replaced chlorine, which was quickly found to produce harmful organochlorine compounds as a side effect.

This is a reason why the Poland–China Cooperation Program, with the acronym TAPEB “Advanced treatment of typical antibiotic pharmaceutical wastewater using EB irradiation”, has been prepared and accepted by both governments. INCT and the Institute of Nuclear and New Energy Technology, Tsinghua University (TU), Beijing, P. R. China, are the contractors for the project, which began in 2024 and is scheduled for completion in 2027. The project will conclude with a pilot study based on a holistic evaluation of EB technology applied at critical points within the sewage treatment plant’s technological process. The ongoing planned research will focus on micropollutants and antibiotic-resistant microorganisms, aiming to develop solutions for fourth-stage treatment and energy-neutral wastewater treatment plants. The technology will meet the requirements regarding the following:

• Micropollutant removal (microplastics, organic compounds, and pharmaceuticals);
• Hazardous microbiological contamination control of wastewater and sludge (bacteria, ARB, ARG, parasites, parasite eggs, etc.);
• Enhancement of biogas production;
• Safe organic fertilizer manufacturing;
• The circular economy;
• Energy neutrality.

Partners expand international cooperation since TU has noticeable developments related to wastewater EB treatment and INCT related to sludge hygenization and biogas plant construction. Both parties possess extensive experience in process engineering, including the construction of industrial installations utilizing both EB and conventional technologies in the fields of environmental and chemical engineering. TU designed at least two industrial wastewater treatment plants applying EBs aligned with the conventional processes, and INCT designed an air pollution EB control installation at the electro power station, which was equipped with 1.2 MW accelerators (the biggest electron accelerator industrial installation ever built).

The circular economy (CE) aims to transform waste into resources that can be reused or reintroduced into the manufacturing process. Sustainable water management is an action to ensure that water is used in a way that meets both current and future socio-economic ecological requirements. Such activities must not be limited to merely diagnosing and encouraging the adoption of a philosophy of sustainable development; they must also present proposed measures of an organizational and technical nature. Modern wastewater treatment plants are complex engineering and technological facilities whose functioning often extends beyond just the treatment of wastewater and the disposal of sewage sludge. The introduction of highly efficient wastewater treatment processes has contributed to an increase in the amount of sludge produced. Sewage sludge generation cannot be prevented, and the increase in many countries is estimated at several percent per year. The method of disposal and management is closely linked to the characteristics of sewage sludge. Key factors include the presence of harmful substances or pathogenic organisms, as well as the nutrient content. These aspects determine the potential for nutrient recovery or the conversion of sludge into fuel or commercially valuable products, such as organic fertilizers. This is performed to maintain humus levels in the soil while ensuring chemical and biological purity. Currently, modern wastewater treatment plants that dispose of sewage sludge through fermentation commonly utilize the biogas produced in fermentation chambers. This biogas is used for combined heat and electricity generation through cogeneration processes, which has become a standard practice. The plant construction for medium-sized WWTPs is equal to EUR 15 million, and the levelized cost of biogas production is 0.08 EUR/m³ [97].

The amount of biogenic substances that could hypothetically be recovered in a wastewater treatment plant with a load of 70,000 p.e. (persons equivalent) was estimated, as an example. The technologies used in the plant allow for the processing of organic waste and the recovery of valuable elements (phosphorus, nitrogen, and organic carbon) for the soil as well as energy. The production of biogas, with maximal utilization of the organic waste generated by residents, allows for the annual recovery of 1126 Mg of organic carbon and the generation of approximately 12.6 GWh of energy. The most efficient way to recycle organic waste is by producing compost or bioproducts from proferment that possess fertilizer properties. It was estimated that fertilizer production enables the recovery of 30% of carbon, 98% of phosphorus, and 18% of nitrogen from the streams of these elements introduced into the treatment plant [96].

The electricity generated can be used to power the accelerator, while the waste heat can be utilized in the drying and granulation processes of the organic fertilizer product. This gave rise to the idea of ‘zero energy’ technology for producing safe organic fertilizers. For WWTPs with a throughput of ~250,000 m³ of wastewater annually and a sludge output of ~1500 tons of dry mass annually, the net income and savings (avoiding sludge disposal costs) will reach EUR 270,000 annually [98].

The paper presents a modern, experimentally validated technology that holds significant importance for water managers. It is proposed for implementation in all municipal wastewater treatment plants as well as in a critical industrial sector, pharmaceutical factories, particularly those involved in the production of antibiotics, which pose substantial risks to ecosystems. A recent report by the United Nations Water Agency says that 80% and sometimes as much as 95% of wastewater essentially goes unused into watercourses [99]. On the other hand, the UN says, “In an unstable world where security threats are growing, we must all recognize that ensuring the availability and sustainable management of water and sanitation for all—the aim of Sustainable Development Goal 6—is essential for global prosperity and peace.” The potential for wastewater recycling in the agricultural, industrial, and household sectors is still being used minimally. Reclaimed water from wastewater is, therefore, a useful alternative to conventional resources, and its use can reduce the problem of urban scarcity, improve the efficiency of its use, reduce groundwater and surface water pollution, and contribute to improving the quality of water ecosystems [100].

6.1. Disintegration of Excess Sludge to Improve Biogas Production Rate

The technology developed in this project is based on the use of excess sludge for the biogas plant, followed by the final agricultural use of digestate. Therefore, it is crucial to develop a method that ensures the chemical and microbiological safety of these streams. The presence of ARGs observed in raw sludge was drastically reduced after their anaerobic digestion [101]. Agricultural use is also very important for wastes originating from animal farms [43]. The proposed technology has the potential to be used in such cases as well. A recently published study investigated the impact of various cattle manure treatment methods, including composting and storage, on its microbiome and resistome. The findings revealed that composting significantly enhanced the richness and diversity of the bacterial community compared to storage. While both approaches effectively reduced the levels of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs), composting produced fertilizers with greater microbial diversity that are free from pathogens and parasite eggs. However, despite its advantages, composting requires more labor and time than a storage-based approach. The full elimination of pathogens and parasite eggs can be achieved through EB treatment at appropriate doses. This method is also effective in removing bacteria and antibiotic-resistant bacteria due to the radiation’s impact on microbial structures [89].

The proposed technological solution involves utilizing excess sludge from wastewater treatment plants (WWTPs) to generate biogas, which is then converted into electricity and heat in a generator. This process supplies energy to the accelerator and provides heat for safely drying the organic fertilizer present in the post-fermentation material. The sanitation of the sludge is crucial for both the input stream (biomass suspension) and the dried output product. The influence of EB irradiation on the physicochemical properties of the sludge was reported [102,103,104]. The purpose of disintegration of biological substrates (among others sewage substrates) is to destroy the cell membranes of microorganisms, resulting in the disintegration of their cells. The organic components released into solution become available as a food substrate for living heterotrophic biomass in the process of anaerobic digestion. The trend of sludge disintegration on biogas yield and fermentation time in the continuous process was investigated, where the yield depends on substrate composition. The use of biological sludge disintegration prior to the anaerobic digestion process brings a number of benefits, which include more efficient biogas production (up to 400 L/kg of reduced organic matter); a reduced concentration of dry organic matter in digested sludge, that is, the duration of organic digestion; a reduced amount of sludge generated (about 1.5 times less than without disintegration); minimized foaming of sludge; the homogenization of sludge particles; the destruction of filamentous bacteria; and the elimination of swollen and floating sludge [105]. An example of the results is presented in Figure 10.

Reducing digester residence times without compromising biogas production entails extensive processes and financial implications. Optimization studies can enhance efficiency and minimize residence times in biogas production systems. Thanks to the shortened fermentation time, it is possible to achieve a higher yield of biogas production (Figure 11) from the same fermenter volume [106]. INCT has developed an advanced biogas plant in which the hydrolysis stage and fermentation stages are separated from each other.

6.2. Application of EB at WWTPs as Quaternary Treatment

The technology under development can be used for both the final processing of wastewater treated by biological methods and the initial degradation of non-biodegradable organic pollutants and excess sludge as well. Of course, as with any technology used for pollution control, it is advisable to treat smaller wastewater streams at their sources, such as hospitals, nursing homes, or facilities in the pharmaceutical industry.

The main interest of researchers from INCT (Institute of Nuclear Chemistry and Technology) in Poland and TU (Tsinghua University) in China is the fate and potential treatment of sulfamethoxazole, sulfadiazine, sulfamethazine, and other common persistent organic pollutants. However, in addition to sulfonamides, other pharmaceuticals and polyaromatic hydrocarbons can also be treated using the discussed process.

The introduction of new technologies from the laboratory to the market requires a transition through the continuum of research, development, demonstration, and deployment (RDDD). The process based on the Technology Readiness Level (TRL) scale is used to quantitatively and qualitatively assess the maturity of a given technology. TRL is a reference for different stakeholders (such as engineers, venture capital (VC) holders, and investors) to determine the current stage of development of an innovation [107,108].

Different components of the technology reached different stages of development at collaborating partner countries. Three different aspects of the technology have been studied at INCT in Poland and TU in China: (i) the degradation of organic pollutants in wastewater, (ii) the destruction or inactivation of microbiological contaminants (ARGs, ARB, parasites, their ova, and other microorganisms) in wastewater and excess sludge, and additionally (iii) the disintegration of sludge to improve the biogas production rate. These three components of the universal technology under development are at the different stages of the Technology Readiness Level (TRL). The technology component (i) pertaining to process engineering and technical implementation has reached TRL 9—a system proven in an operational environment—in an industrial plant constructed in China and has been incorporated in the WWTP processing line [67,109]. Components (ii) and (iii) have reached TRL 4—technology validated in the laboratory [98]. Another engineering solution that can be utilized is an under-beam system for wastewater treatment. A key aspect of any EB irradiation facility is the production of X-rays, which significantly influences the design specifications for protective shielding. While treating the wastewater using EBs, a precise assessment of the X-ray field is essential for designing effective installation of the shield and ensuring operational safety. Research conducted at INCT’s facility demonstrated that X-ray emissions can be reduced through an optimized configuration of the irradiation process [110]. The application of an elaborate shielding design and construction procedure will help minimize shielding costs by meeting the radiological protection requirements. Another solution that increases the efficiency of the process and reduces both investment and operating costs is to couple the irradiation process with bubble aeration (or ozonation) [111].

The technology in two stages (sludge hygenization and disintegration) advances to TRL 4. At such a stage, various components were tested in combination, leading to the conclusion that this level has been successfully reached. The next phase should be conducted in environments that closely mimic real-world conditions. In our case, this would involve a pilot plant built at a WWTP based on the already developed basic engineering. For the other two segments (a high-activity biogas plant and a wastewater treatment plant in the dye industry and at hospital wastewaters), the development level is TRL 9. However, the R&D on ARBs, ARGs, and selected antibiotics has to be carried out to move the technology to the next stage of the TRL.

The scheme of the biogas plant equipped with the EB system is presented in Figure 12.

Two variants are considered, which includes both the hygienization and disintegration of excess sludge and Variant B and involves the hygienization of dewatered sludge (TRL 4). The effect of irradiation on disintegration and sanitation was investigated on a laboratory scale. The new concept of a two-stage bioreactor with hydraulic mixing [89,114] has been implemented on an industrial scale (TRL 9). Two biogas electric plants of 1.2 MWe are in operation in southern Poland.

6.3. Electron Accelerators

The proposed technology utilizes an EB to treat the mentioned contaminants, allowing for the irradiation of both wastewater and excess sludge streams by ionizing radiation. An electron accelerator is an electricity-powered device that imparts energy to electrons, forming a stable and relatively uniform energetic beam, known as an EB. Such radiation operates within an energy range of 1 to 10 MeV. Such units include adjustable parameters, such as particle energy and the dose rate. Its characteristics encompass long-term stability and the capacity to stop the emission of radiation via a switch. Moreover, the electricity supply will be provided by an electrical generator integrated with a biogas plant.

The most important factor is that further development of the technology requires the achievement of expressive advances in accelerator engineering technology. This applies to both the price of the device and technological development. The deployment of these technologies depends on the availability of accelerator manufacturers capable of delivering systems with a minimum power capacity of 100 kW and an energy efficiency of no less than 80%. Of course, these must be devices suitable for continuous operation in harsh industrial conditions. Nevertheless, investments in developing such devices would yield significant returns for both government agencies and manufacturers. This is because it is difficult to find a technology that meets the achieved requirements, except for EB technologies. Good references for this technology are the widespread use of electron accelerators to sterilize medical equipment and transplants. The critical path on the way to the industrial applications of the elaborated technology is the availability, quality, and price of the accelerator. To meet the demands attributed to market competitiveness, accelerators must combine high power, high electrical efficiency, and low operational costs. Advances in superconducting systems have shown promise, yet their benefits and drawbacks remain unproven on an industrial scale. The necessity for magnet cooling systems may diminish their expected energy efficiency. However, incorporating renewable energy sources to power accelerators could help mitigate these limitations and enhance sustainability. In this case, electricity supply is assured by a generator driven by biogas.

Considerations on this topic are presented in the literature [115]. The energy demand for the beam is relatively low, approximately a few MeV. However, scaling this technology for industrial applications necessitates the provision of high beam power on the order of 1 MW in a cost-efficient manner. A conceptual design for a compact superconducting accelerator capable of delivering a continuous-wave (cw) EB with a current of 1 A and an energy of 1 MeV has been proposed [116]. A separate publication by scientists at Fermi Lab outlines the technical and engineering design of an EB accelerator. This system operates at 10 MeV with an average power of 1 MW and is intended for the irradiation treatment of large-scale industrial and municipal wastewater. The accelerator employs superconducting radiofrequency (SRF) cavity technology, delivering greater than 90% RF-to-beam efficiency. To make the design more suitable for industrial use, the SRF cavity is cooled using a cryocooler via the conduction-cooling method, replacing the traditional liquid helium bath cryogenic system. The technical design is accompanied by a thorough analysis of both capital and operational costs associated with the accelerator. The proposed system is capable of processing up to 12 million gallons of wastewater daily, with an estimated construction cost of approximately USD 8 million. Additionally, the material processing cost is about 13.5 cents per ton per kGy [117].

7. Discussion and Conclusions

The UN says, “In an unstable world where security threats are growing, we must all recognize that ensuring the availability and sustainable management of water and sanitation for all—the aim of Sustainable Development Goal 6—is essential for global prosperity and peace.” Sustainable water management is an action to ensure that water is used in a way that meets both current and future socio-economic ecological requirements. The goal of water and soil conservation activities is to promote the sustainable management of water resources. These efforts aim to improve the development and use of water systems to meet the growing global demand. Such activities must not be limited to merely diagnosing problems and promoting the philosophy of sustainable development; they must also propose concrete organizational and technical measures. This paper presents a modern, experimentally validated technology that is particularly relevant to water resource managers. It is proposed for use in municipal wastewater treatment plants. The technology is also suitable for application in the pharmaceutical industry, including the production of substances especially harmful to ecosystems, such as antibiotics. A recent report by the United Nations Water Agency indicates that 80% and sometimes as much as 95% of wastewater essentially goes unused into watercourses [118]. The potential for wastewater recycling in the agricultural, industrial, and household sectors is still being used minimally. Reclaimed water from wastewater is a valuable alternative to conventional sources. Its use can help alleviate urban water scarcity, enhance water use efficiency, reduce the pollution of groundwater and surface water, and support the restoration of aquatic ecosystems [99].

• The aforementioned EU Directive introduces in the future the obligation of the industry to remove toxic micropollutants (polluter pays principle) that are released into the environment as a result of the use of their products. This requirement introduces the need to use additional new technologies/devices to meet this condition.
• The wastewater sector should be made energy-neutral and moved towards climate neutrality. This can be achieved by reducing energy consumption, utilizing the extensive areas of some treatment plants for solar or wind energy production, promoting water reuse, and generating biogas from treatment sludge to serve as a substitute for natural gas.
• The introduction of highly efficient wastewater treatment processes has contributed to an increase in the amount of sludge produced. Sewage sludge generation cannot be prevented, and the increase in many countries is estimated at several percent per year. The method of disposal and management of sewage sludge is closely linked to its characteristics. Key factors include the presence of harmful substances or pathogenic organisms, as well as the content of nutrients. These nutrients may be recovered, or the sludge may be converted into fuel or a commercially viable product, such as an organic fertilizer. This is performed to maintain the humus levels in the soil while ensuring chemical and biological purity.
• The technology under development can be used for both the final processing of wastewater treated by biological methods and the initial degradation of non-biodegradable organic pollutants and excess sludge as well. Of course, as with any technology used for pollution control, it is advisable to treat smaller wastewater streams at their sources, such as hospitals, nursing homes, or facilities in the pharmaceutical industry. It meets the main requirements and principles of the EU Directive and can be used as a quaternary treatment of wastewater. Previous studies have shown the possibility of its application to accelerate the sedimentation process of microplastics and destroy various types of chemical and microbiological contaminants, including ARB and ARG parasites and their eggs. In addition, it meets the requirements of the Directive in terms of energy neutrality because it takes into account the efficient production of biogas and the generation of electricity. Concerning the guidelines of the Directive, it enables the production of a safe organic fertilizer that supports the recycling of nutrients, including phosphorus.
• The technology comprising two stages (sludge hygenization and disintegration) advanced to TRL 4. At such a stage, various components were tested in combination, leading to the conclusion that this level has been successfully reached. TRL 5, which builds upon TRL 4, involves validating the technology in a relevant industrial environment. However, a technology at TRL 5 is considered a prototype and requires more extensive testing compared to one at TRL 4. The next phase should be conducted in environments that closely mimic real-world conditions. In our case, this would involve a pilot plant built at a WWTP based on the already developed basic engineering. For the other two segments (a high-activity biogas plant and a wastewater treatment plant in the dye industry and at hospital wastewaters), the development level is at TRL 9. However, R&D on ARBs, ARGs, and selected antibiotics is necessary to advance the technology to the next TRL stage for this specific application. The final objective of the Chinese–Polish research project is the construction of the pilot plant. This plant will be based on previous and ongoing research and aims to integrate the discussed process into the treatment sequence of conventional wastewater treatment plants. The goal is to develop hybrid stages, such as EBs combined with sedimentation, ozone, or sludge disintegration; hydrolysis; and methanogenesis, that function as either the final or internal quaternary treatment stage.