Municipal Wastewater Treatment Technologies: A Comprehensive Review

Municipal Wastewater Treatment Technologies: A Comprehensive Review

Mario Rodrigues Peres Mario Peres Consulting Campinas, Brazil

Abstract: Municipal wastewater treatment is a critical global challenge driven by increasing urbanization, industrialization, and stringent environmental regulations. The effective removal of pollutants from municipal wastewater is essential for safeguarding public health, protecting aquatic ecosystems, and ensuring sustainable water resource management. This comprehensive review paper provides an in-depth analysis of current and emerging technologies employed in municipal wastewater treatment, with a particular focus on physical, chemical, and biological processes. While conventional methods have long served as the backbone of wastewater treatment, recent advancements have led to the development of innovative and more efficient approaches, especially within the realm of biological treatments. This review highlights the principles, advantages, and limitations of various treatment technologies, including advanced biological processes such as Anaerobic Membrane Bioreactors (AnMBRs), Partial Nitritation/Anammox (PN/A) systems, and microalgae- based treatment. Furthermore, it explores the integration of these technologies, their potential for resource recovery, and the role of smart systems and automation in optimizing treatment efficiency and sustainability. The paper also addresses the existing challenges and future perspectives in municipal wastewater treatment, emphasizing the need for cost-effective, energy- efficient, and environmentally friendly solutions to meet the evolving demands of a circular economy.

Keywords: wastewater treatment, municipal wastewater, resource recovery, circular economy, emerging contaminants.

1. INTRODUCTION

   Access to clean water is a fundamental human right and a cornerstone of sustainable development. However, rapid population growth, industrial expansion, and climate change have placed immense pressure on global water resources, leading to widespread water scarcity and pollution. Municipal wastewater, a complex mixture of domestic, commercial, and industrial effluents, represents a significant source of environmental contamination if not properly treated. It contains a diverse array of pollutants, including organic matter, nutrients (nitrogen and phosphorus), suspended solids, heavy metals, pathogens, and emerging contaminants such as pharmaceuticals and personal care products [1, 2]. The discharge of untreated or inadequately treated municipal wastewater into natural water bodies can lead to severe ecological damage, including eutrophication, oxygen depletion, loss of biodiversity, and the spread of waterborne diseases, posing substantial risks to both human health and the environment [3, 4].

   Historically, wastewater treatment has evolved from rudimentary physical separation techniques to sophisticated multi-stage processes aimed at achieving higher effluent quality. Conventional wastewater treatment plants (WWTPs) typically involve primary, secondary, and sometimes tertiary

   treatment stages to remove various pollutants. While these conventional methods have been effective in reducing the pollutant load, they often face challenges such as high energy consumption, significant sludge production, and inefficiency in removing recalcitrant and emerging contaminants [5]. Consequently, there is a growing imperative to develop and implement more advanced, sustainable, and resource-efficient wastewater treatment technologies.

   This literature review aims to provide a comprehensive overview of the current state-of-the-art in municipal wastewater treatment technologies. The review will delve into the fundamental principles and applications of physical, chemical, and biological treatment processes, with a particular emphasis on recent advancements in biological treatment methods, given their recognized effectiveness and environmental benefits. Specific attention will be given to innovative biological technologies such as Anaerobic Membrane Bioreactors (AnMBRs), Partial Nitritation/Anammox (PN/A) systems, and microalgae-based treatment, which offer promising solutions for enhanced pollutant removal, energy efficiency, and resource recovery. Furthermore, the paper will explore the integration of these technologies, the concept of wastewater treatment within a circular economy framework, and the role of smart systems and automation in optimizing WWTP operations. Finally, the review will discuss the prevailing challenges in municipal wastewater treatment and outline future perspectives for research and development, contributing to the ongoing efforts to achieve sustainable water management globally.

2. CONVENTIONAL WASTEWATER TREATMENT

   TECHNOLOGIES

   1. Physical Treatment Processes

      Physical treatment processes are the initial stages in wastewater treatment, primarily focused on removing large solids, grit, and suspended particulate matter through mechanical means. These processes do not involve chemical reactions or biological activity but rely on physical forces such as gravity, screening, and filtration. The main objectives of physical treatment are to protect downstream equipment from damage, reduce the organic load on subsequent biological processes, and prevent the accumulation of inert materials in treatment units [6].

      Screening: This is the first unit operation in a WWTP, where large floating and suspended solids are removed from the raw wastewater. Screens typically consist of parallel bars or perforated plates, and they can be coarse (bar screens) or fine. Coarse screens remove large debris such as rags, plastics, and wood, preventing clogging of pumps and pipelines. Fine

      screens, with smaller openings, remove finer suspended solids, which can improve the efficiency of subsequent treatment stages. The removed material, known as screenings, is typically dewatered and disposed of in landfills [6].

      Grit Removal: Following screening, wastewater flows into grit chambers, where heavier inorganic particles such as sand, gravel, and coffee grounds are removed. Grit removal is crucial to prevent abrasion of pumps and other mechanical equipment, and to avoid accumulation of inert material in aeration tanks and digesters. Grit chambers can be aerated, horizontal flow, or vortex-type, designed to allow organic solids to remain suspended while heavier grit settles out [7].

      Sedimentation (Primary Clarification): Primary sedimentation tanks, also known as primary clarifiers, are designed to remove settleable suspended solids and a portion of the organic matter from the wastewater. In these large tanks, the velocity of the wastewater is significantly reduced, allowing suspended particles to settle to the bottom by gravity, forming primary sludge. Lighter materials, such as oil and grease, float to the surface and are skimmed off. Primary sedimentation can remove 50-70% of suspended solids and 25-50% of biochemical oxygen demand (BOD) [8]. The settled primary sludge is then typically transferred to sludge treatment facilities, while the clarified effluent proceeds to secondary (biological) treatment.

      Flotation: While less common for municipal wastewater than sedimentation, flotation can be used, particularly for wastewater with high concentrations of oil, grease, or finely divided suspended solids that are difficult to settle. In dissolved air flotation (DAF), air is dissolved in wastewater under pressure and then released at atmospheric pressure, forming tiny bubbles that attach to suspended particles, causing them to float to the surface for removal [9].

      These physical processes are essential for preparing the wastewater for more intensive treatment, significantly reducing the load on downstream biological and chemical units. However, they do not effectively remove dissolved pollutants, nutrients, or pathogens, necessitating further treatment stages.

   2. Chemical Treatment Processes

      Chemical treatment processes in municipal wastewater treatment involve the addition of chemical reagents to facilitate the removal of pollutants that are not effectively eliminated by physical methods. These processes are often used to enhance the efficiency of physical separation, remove specific contaminants, or prepare wastewater for subsequent biological treatment. The primary chemical processes include coagulation-flocculation, chemical precipitation, and disinfection [10].

      Coagulation and Flocculation: This is a widely used chemical treatment process aimed at removing colloidal and fine suspended particles that are too small to settle by gravity. Coagulation involves the addition of chemical coagulants (e.g., aluminum sulfate, ferric chloride, or polyaluminum chloride) to destabilize the negatively charged colloidal particles, causing them to aggregate. Rapid mixing is applied to ensure uniform

      dispersion of the coagulant. Following coagulation, flocculation occurs, where gentle mixing promotes the aggregation of destabilized particles into larger, heavier flocs that can be easily settled or filtered. This process significantly improves the removal of suspended solids, turbidity, color, and some dissolved organic matter [11].

      Chemical Precipitation: Chemical precipitation is employed to remove dissolved inorganic pollutants, particularly heavy metals and phosphorus, by converting them into insoluble precipitates. This is typically achieved by adjusting the pH of the wastewater and adding precipitating agents. For instance, lime (calcium hydroxide) or caustic soda (sodium hydroxide) can be added to raise the pH, causing metal hydroxides to precipitate. Phosphorus removal often involves the addition of metal salts (e.g., aluminum or iron salts) that react with phosphate to form insoluble compounds, which then settle out [12]. Chemical precipitation is effective in achieving high removal efficiencies for specific contaminants but can generate significant amounts of chemical sludge that require proper disposal.

      Disinfection: Disinfection is a crucial final step in municipal wastewater treatment, aimed at inactivating or destroying pathogenic microorganisms (bacteria, viruses, and protozoa) to prevent the spread of waterborne diseases. The most common disinfection methods include chlorination, ultraviolet (UV) irradiation, and ozonation.

      • Chlorination: Chlorine (Cl2), hypochlorite (OCl-), or chlorine dioxide (ClO2) are widely used disinfectants due to their effectiveness and relatively low cost. Chlorine compounds oxidize and destroy microbial cell components. However, chlorination can lead to the formation of disinfection by- products (DBPs), some of which are carcinogenic or mutagenic, necessitating dechlorination before discharge [13].

      • Ultraviolet (UV) Irradiation: UV disinfection uses germicidal UV light (typically at 254 nm) to inactivate microorganisms by damaging their DNA, preventing them from replicating. UV disinfection is a physical process that does not involve the addition of chemicals, thus avoiding the formation of DBPs. It is effective against a broad spectrum of pathogens and is increasingly favored for its environmental benefits, though its effectiveness can be reduced by high turbidity or suspended solids in the wastewater [14].

      • Ozonation: Ozone (O3) is a powerful oxidant and disinfectant generated on-site by passing oxygen through a high-voltage electric discharge. Ozonation effectively inactivates a wide range of pathogens and can also oxidize organic pollutants, reducing color, odor, and taste. Similar to chlorination, ozonation can also produce undesirable by-products, though generally less harmful than chlorinated DBPs [15].

        Chemical treatment processes play a vital role in enhancing the overall quality of treated wastewater, particularly in removing specific pollutants and ensuring microbial safety. However, they often come with operational costs associated with

        chemical consumption and sludge management, prompting the exploration of more sustainable alternatives.

   3. Conventional Biological Treatment Processes

      Biological treatment processes are central to municipal wastewater treatment, leveraging the metabolic activities of microorganisms to break down and remove organic matter and nutrients (nitrogen and phosphorus). These processes are highly effective, environmentally friendly, and form the core of secondary treatment in most WWTPs. The most common conventional biological treatment methods include activated sludge processes, trickling filters, and rotating biological contactors [16].

      Activated Sludge Process: The activated sludge process is the most widely used biological treatment method for municipal wastewater. It involves an aerobic biological reactor (aeration tank) where wastewater is mixed with a suspension of microorganisms (activated sludge) in the presence of oxygen. The microorganisms consume the dissolved and suspended organic pollutants as a food source, converting them into stable end products like carbon dioxide, water, and new biomass. After the aeration tank, the mixed liquor flows into a secondary clarifier, where the activated sludge settles by gravity, and the clear treated effluent is discharged. A portion of the settled activated sludge is recycled back to the aeration tank to maintain a high concentration of active microorganisms, while the excess sludge is wasted for further treatment and disposal [17].

      Variations of the activated sludge process exist, including:

      • Conventional Activated Sludge: A basic configuration with a plug-flow or completely mixed aeration tank.

      • Extended Aeration: Characterized by longer aeration times and lower organic loading rates, resulting in less sludge production and higher nitrification (conversion of ammonia to nitrate) [18].

      • Sequencing Batch Reactors (SBRs): Operate in a batch mode within a single tank, performing all steps (fill, react, settle, draw, idle) sequentially. SBRs offer flexibility and can be optimized for nutrient removal [19].

      • Oxidation Ditches: A modified activated sludge system with a long, continuous channel, often oval or circular, providing extended aeration and good mixing. They are known for their simplicity and effective nutrient removal [20].

   Trickling Filters: Trickling filters are fixed-film biological reactors where wastewater is distributed over a bed of media (e.g., rocks, plastic) that provides a surface for microbial growth. As wastewater trickles down through the media, a biofilm forms on the surface, and microorganisms within this biofilm adsorb and metabolize organic pollutants. Air circulates through the media, providing oxygen for the aerobic microorganisms. The treated water is collected at the bottom, and excess biofilm sloughs off and is removed in a secondary clarifier [21]. Trickling filters are relatively simple to operate

   and have lower energy requirements compared to activated sludge, but they can be less efficient in removing soluble organic matter and are susceptible to clogging.

   Rotating Biological Contactors (RBCs): RBCs consist of a series of closely spaced, parallel discs mounted on a horizontal shaft. These discs are partially submerged in wastewater and slowly rotate, alternately exposing the microbial film growing on their surfaces to the wastewater and then to the air. This rotationallows the microorganisms to absorb organic pollutants from the wastewater and then to be aerated. Similar to trickling filters, excess biomass sloughs off and is removed in a secondary clarifier. RBCs are compact, energy-efficient, and offer stable performance, but they can be sensitive to shock loads and require regular maintenance [22].

   Conventional biological treatment processes are highly effective in removing biodegradable organic matter (BOD) and suspended solids (TSS), typically achieving 85-95% removal efficiencies. Many also facilitate nitrification, converting ammonia to nitrate. However, their efficiency in removing phosphorus, recalcitrant organic compounds, and emerging contaminants can be limited, often necessitating further advanced treatment steps. Moreover, these processes generate significant quantities of excess sludge, which requires further handling and disposal, contributing to operational costs and environmental concerns[23].

3. ADVANCED BIOLOGICAL WASTEWATER TREATMENT TECHNOLOGIES

   1. Anaerobic Membrane Bioreactors (AnMBRs)

      Anaerobic Membrane Bioreactors (AnMBRs) represent a promising technology that integrates anaerobic biological treatment with membrane separation processes. Unlike conventional aerobic processes, AnMBRs operate in the absence of oxygen, leading to several significant advantages, particularly in energy efficiency and resource recovery. In an AnMBR system, anaerobic microorganisms break down organic pollutants in the wastewater, producing biogas (primarily methane and carbon dioxide) as a valuable energy source. The membrane unit, typically ultrafiltration or microfiltration, separates the treated effluent from the biomass, ensuring a high-quality permeate and retaining a high concentration of active microorganisms within the reactor [25].

      Advantages of AnMBRs:

      • Energy Efficiency: By operating anaerobically, AnMBRs eliminate the need for aeration, which is the most energy- intensive component of conventional aerobic activated sludge systems. Furthermore, the produced methane-rich biogas can be captured and utilized for energy generation, potentially making WWTPs energy-neutral or even energy-positive [26].

      • Reduced Sludge Production: Anaerobic processes inherently produce significantly less excess sludge compared to aerobic processes, leading to lower sludge handling and disposal costs [27].

      • High Effluent Quality: The membrane barrier effectively retains all suspended solids, bacteria, and even some viruses,

        resulting in a high-quality effluent suitable for direct discharge or further reuse applications [28].

      • Compact Footprint: The ability to maintain high biomass concentrations and the absence of a secondary clarifier allow AnMBRs to achieve high treatment capacities in a smaller physical footprint, making them suitable for areas with limited land availability [29].

      Challenges of AnMBRs:

      Despite their advantages, AnMBRs face challenges, primarily membrane fouling, which can reduce permeate flux, increase operating costs, and necessitate frequent membrane cleaning. Research is ongoing to develop fouling control strategies, including novel membrane materials, optimized operating conditions, and pre-treatment methods [30]. Temperature sensitivity of anaerobic microorganisms can also be a concern, especially for municipal wastewater treated at ambient temperatures [31].

   2. Partial Nitritation/Anammox (PN/A)

      Nitrogen removal is a critical aspect of wastewater treatment, as excessive nitrogen discharge can lead to eutrophication in receiving water bodies. Conventional nitrogen removal involves nitrification (aerobic conversion of ammonia to nitrate) followed by denitrification (anoxic conversion of nitrate to nitrogen gas), which is energy-intensive due to high aeration demands and requires an external carbon source for denitrification. Partial Nitritation/Anammox (PN/A) is an innovative and more sustainable biological nitrogen removal pathway that significantly reduces energy consumption and carbon requirements [32].

      PN/A is a two-step process:

      1. Partial Nitritation: In this aerobic step, ammonia-oxidizing bacteria (AOB) convert approximately half of the ammonia to nitrite, while suppressing the activity of nitrite-oxidizing bacteria (NOB) to prevent further oxidation to nitrate. This is typically achieved by controlling aeration, dissolved oxygen levels, and temperature [33].

      2. Anammox (Anaerobic Ammonium Oxidation): In the subsequent anoxic step, anaerobic ammonium-oxidizing bacteria (Anammox bacteria) directly convert the remaining ammonia and the nitrite produced in the first step into nitrogen gas and a small amount of nitrate. This process occurs without the need for an external carbon source [34].

         Advantages of PN/A:

         • Reduced Energy Consumption: PN/A significantly reduces aeration requirements (up to 60%) compared to conventional nitrification-denitrification, leading to substantial energy savings [35].

         • No External Carbon Source: Anammox bacteria utilize ammonia and nitrite directly, eliminating the need for an external organic carbon source for denitrification, which reduces operational costs [36].

         • Lower Sludge Production: The growth rate of Anammox bacteria is very slow, resulting in minimal sludge production [37].

           Challenges of PN/A:

           Implementing PN/A systems can be challenging due to the slow growth rate of Anammox bacteria, their sensitivity to inhibitory compounds, and the need for precise control of operating parameters to maintain stable partial nitritation. However, ongoing research and successful full-scale applications demonstrate the feasibility and benefits of PN/A for energy- efficient nitrogen removal from municipal wastewater, particularly for side stream treatment [38].

   3. Microalgae-Based Wastewater Treatment

      Microalgae-based wastewater treatment systems offer an environmentally friendly and sustainable approach to remove nutrients (nitrogen and phosphorus) and other pollutants from wastewater, while simultaneously producing valuable biomass. Microalgae are photosynthetic microorganisms that utilize sunlight and carbon dioxide (from the atmosphere or wastewater) to grow, consuming nutrients from the wastewater in the process. This technology is gaining attention for its potential to integrate wastewater treatment with resource recovery and biofuel production [39].

      Mechanism: Microalgae assimilate nitrogen (as ammonia, nitrate, or nitrite) and phosphorus (as phosphate) from the wastewater for their growth. They also contribute to oxygen production through photosynthesis, which can support aerobic bacterial activity for organic matter degradation. The harvested microalgal biomass can then be used for various applications, including biofuel production (biodiesel, bioethanol), animal feed, fertilizers, or high-value biochemicals [40].

      Advantages of Microalgae-Based Treatment:

      • Nutrient Removal and Recovery: Highly effective in removing nitrogen and phosphorus, mitigating eutrophication. The nutrients are recovered in the algal biomass, which can be reused [41].

      • Carbon Sequestration: Microalgae consume CO2 during photosynthesis, contributing to greenhouse gas emission reduction [42].

      • Low Energy Consumption: Relies on solar energy for photosynthesis, reducing external energy inputs for aeration and chemical additions [43].

      • Biomass Production: Generates a valuable biomass that can be converted into various bio-products, promoting a circular economy [44].

        Challenges of Microalgae-ased Treatment:

        Challenges include the large land area requirements for open pond systems, sensitivity of microalgae to environmental conditions (temperature, light intensity), and the cost-effective harvesting of microalgal biomass from dilute suspensions. Research is focused on developing more efficient

        photobioreactor designs and harvesting technologies to overcome these limitations [45].

   4. Bioelectrochemical Systems (BES)

   Bioelectrochemical Systems (BES), such as Microbial Fuel Cells (MFCs) and Microbial Electrolysis Cells (MECs), represent a novel and innovative approach to wastewater treatment that can simultaneously treat wastewater and recover energy or valuable products. These systems utilize electroactive microorganisms (exoelectrogens) that can transfer electrons to an electrode, generating an electrical current or producing hydrogen or methane [46].

   Microbial Fuel Cells (MFCs): In an MFC, exoelectrogenic bacteria oxidize organic matter at the anode, releasing electrons and protons. The electrons travel through an external circuit to the cathode, where they combine with protons and an electron acceptor (e.g., oxygen) to produce water, generating electricity in the process. MFCs offer the potential for direct electricity generation from wastewater, reducing the energy footprint of WWTPs [47].

   Microbial Electrolysis Cells (MECs): MECs are similar to MFCs but require a small external energy input (voltage) to drive the reaction. At the anode, organic matter is oxidized by exoelectrogens, producing electrons and protons. These electrons and protons then combine at the cathode to produce hydrogen gas (H2) or methane (CH4), which are valuable energy carriers [48].

   Advantages of BES:

   • Energy Recovery: MFCs can directly generate electricity, while MECs can produce hydrogen or methane from wastewater, offering a sustainable way to recover energy [49].

   • Pollutant Removal: Effective in removing organic matter and some inorganic pollutants from wastewater [50].

   • Reduced Sludge Production: Similar to anaerobic processes, BES typically produce less excess sludge compared to aerobic systems [51].

   • Low Chemical Addition: Minimizes the need for chemical additions for treatment.

     Challenges of BES:

     Key challenges for BES include low power output in MFCs, high capital costs, scalability issues, and the need for further research to optimize reactor design, electrode materials, and microbial communities for practical applications. Despite these challenges, BES hold significant promise for the future of energy-efficient and resource-recovering wastewater treatment [52].

     established and advanced biological methods, several emerging technologies and conceptual frameworks are shaping the future of wastewater management. These innovations often involve the integration of multiple processes, a strong emphasis on circular economy principles, and the incorporation of smart systems and automation.

     1. Integration of Technologies

        The complexity of municipal wastewater, coupled with increasingly stringent discharge standards and the desire for resource recovery, necessitates the integration of various treatment technologies. Hybrid systems combine the strengths of different processes to achieve synergistic effects, leading to enhanced pollutant removal, improved energy efficiency, and reduced operational costs. Examples include:

   • Integrated Fixed-Film Activated Sludge (IFAS): IFAS systems combine the advantages of activated sludge (suspended growth) with biofilm processes (attached growth). Biomass carriers are added to the activated sludge aeration tanks, providing additional surface area for microbial growth. This allows for higher biomass concentrations, improved nitrification, and greater resilience to shock loads, often within existing tank volumes [53].

   • Membrane Bioreactor (MBR) Hybrids: MBRs, which combine activated sludge with membrane filtration, can be further integrated with other processes. For instance, anaerobic MBRs (AnMBRs) have been discussed for energy recovery. Aerobic MBRs can be coupled with advanced oxidation processes (AOPs) for enhanced removal of recalcitrant organic pollutants and emerging contaminants, or with nutrient removal processes to achieve very high effluent quality [54].

   • Constructed Wetlands with Advanced Treatment: While traditional constructed wetlands are effective for secondary treatment, their integration with pre-treatment (e.g., sedimentation) or post-treatment (e.g., UV disinfection, biofilters) can significantly enhance their performance, particularly for nutrient removal and pathogen inactivation. Hybrid constructed wetlands, combining vertical and horizontal flow systems, also offer improved treatment efficiency [55].

     1. Resource Recovery and Circular Economy

        The paradigm of wastewater treatment is shifting from a waste disposal approach to a resource recovery model, aligning with the principles of a circular economy. Wastewater is increasingly viewed as a valuable source of water, energy, and nutrients, rather than merely a waste stream. This shift aims to minimize environmental impact while maximizing economic benefits [56].

4. EMERGING TECHNOLOGIES AND FUTURE

   PERSPECTIVES

   The landscape of municipal wastewater treatment is continuously evolving, driven by the need for more efficient, sustainable, and resource-recovering solutions. Beyond the

   • Water Reuse: Treated municipal wastewater can be a reliable and sustainable source of water for various non-potable applications (e.g., irrigation, industrial processes, toilet flushing) and, with advanced purification, even for potable reuse. Technologies like reverse osmosis, ultrafiltration, and advanced oxidation are crucial for producing high-quality reclaimed water [57].

   • Energy Recovery: Beyond biogas production from anaerobic digestion (as in AnMBRs), other energy recovery avenues include direct electricity generation from MFCs, heat recovery from wastewater, and the production of biofuels from algal biomass. The goal is to make WWTPs energy-neutral or even energy-positive, reducing their operational carbon footprint [58].

   • Nutrient Recovery: Nitrogen and phosphorus, traditionally removed as pollutants, are valuable fertilizers. Technologies are being developed to recover these nutrients in forms such as struvite (magnesium ammonium phosphate) or as components of bio-solids, which can be used in agriculture. This reduces reliance on synthetic fertilizers and closes nutrient loops [59].

   • Bioproducts and Value-Added Materials: Wastewater can serve as a feedstock for producing various bioproducts, including bioplastics (e.g., polyhydroxyalkanoates – PHAs), enzymes, and other biochemicals, through microbial fermentation or algal cultivation. This transforms WWTPs into biorefineries, creating new revenue streams and enhancing sustainability [60].

     1. Smart Systems and Automation

        The advent of Industry 4.0 and the Internet of Things (IoT) is revolutionizing wastewater treatment through the implementation of smart systems and advanced automation. These technologies enable real-time monitoring, data-driven decision-making, predictive maintenance, and optimized process control, leading to improved efficiency, reduced operational costs, and enhanced reliability [61].

   • Sensors and Monitoring: A wide array of online sensors can continuously monitor key wastewater parameters (e.g., pH, dissolved oxygen, nutrient concentrations, flow rates, turbdity). This real-time data provides operators with immediate insights into plant performance and allows for rapid adjustments [62].

   • Data Analytics and Artificial Intelligence (AI): Large datasets generated by sensors can be analyzed using advanced algorithms, machine learning, and AI to identify patterns, predict operational issues, optimize treatment processes, and even detect emerging contaminants. AI can also support decision-making for energy management and chemical dosing [63].

   • Automated Control Systems: Advanced control systems, often integrated with AI, can automatically adjust operational parameters (e.g., aeration rates, chemical dosages, sludge recycle rates) in response to real-time data and predictive models. This minimizes human intervention, reduces errors, and ensures consistent performance [64].

   • Digital Twins: The creation of digital twins virtual replicas of physical WWTPs allows for real-time simulation, testing of operational strategies, and predictive maintenance, further optimizing plant performance and reducing downtime [65]. These emerging technologies and concepts are transforming municipal wastewater treatment into a more integrated,

   resource-efficient, and intelligent system, paving the way for a sustainable water future.

5. CHALLENGES AND LIMITATIONS

Despite significant advancements in municipal wastewater treatment technologies, several challenges and limitations persist, hindering the widespread implementation of highly efficient and sustainable solutions. Addressing these issues is crucial for achieving comprehensive wastewater management and promoting a circular economy.

1. High Energy Consumption: Conventional aerobic biological treatment processes, particularly activated sludge, are highly energy-intensive due to the significant aeration requirements. While advanced biological processes like AnMBRs and PN/A offer energy savings, their widespread adoption is still limited. The energy footprint of WWTPs contributes substantially to operational costs and greenhouse gas emissions, necessitating further research into energy-efficient designs and renewable energy integration [66].

2. Sludge Management: Wastewater treatment generates large quantities of sludge, a complex mixture of water, organic matter, and inorganic solids, often containing pathogens, heavy metals, and persistent organic pollutants. Sludge handling, treatment, and disposal represent a significant operational cost (up to 50% of total WWTP operating costs) and environmental challenge. Current disposal methods, such as landfilling and incineration, raise concerns about environmental pollution and resource depletion. Developing sustainable and cost-effective sludge valorization strategies, including nutrient recovery and energy production from sludge, remains a priority [67].

3. Emerging Contaminants (ECs): The presence of emerging contaminants (ECs) such as pharmaceuticals, personal care products, endocrine-disrupting chemicals, and microplastics in municipal wastewater poses a growing concern. Conventional treatment processes are often ineffective in completely removing these micropollutants, leading to their persistence in the environment and potential ecological and human health impacts. Advanced treatment technologies like AOPs, membrane filtration, and activated carbon adsorption can remove ECs, but their high cost and operational complexity limit their widespread application in municipal WWTPs [68].

4. Infrastructure and Investment: Many existing WWTPs, particularly in developing regions, rely on outdated infrastructure and conventional technologies. Upgrading these facilities to incorporate advanced and more sustainable treatment processes requires substantial capital investment, technical expertise, and long-term planning. Financial constraints and lack of political will often impede the necessary infrastructure development and modernization [69].

5. Operational Complexity and Skill Gaps: Advanced wastewater treatment technologies, while offering superior performance, often come with increased operational complexity. Maintaining optimal performance of sophisticated systems like AnMBRs or PN/A requires highly skilled operators, advanced monitoring systems, and precise process

   control. A lack of trained personnel and technical capacity can be a significant barrier to the successful implementation and operation of these technologies, particularly in regions with limited resources [70].

6. Regulatory Frameworks and Public Acceptance: Evolving regulatory standards for wastewater discharge, particularly concerning nutrient limits and emerging contaminants, necessitate continuous adaptation and upgrading of treatment facilities. However, the development and enforcement of these regulations can be slow and inconsistent across different regions. Furthermore, public perception and acceptance, especially regarding water reuse and resource recovery initiatives, can influence the feasibility and success of new wastewater management strategies [71].

7. Energy-Water-Nutrient Nexus: While the concept of a circular economy in wastewater treatment is gaining traction, fully integrating water, energy, and nutrient recovery into a cohesive and economically viable system remains a complex challenge. Optimizing the balance between pollutant removal efficiency, energy consumption, resource recovery, and cost- effectiveness requires a holistic approach and interdisciplinary research. The trade-offs between these different aspects need to be carefully evaluated to develop truly sustainable solutions [72].

Addressing these challenges requires a multi-faceted approach involving continued research and development of innovative technologies, supportive policy and regulatory frameworks, increased investment in infrastructure, capacity building, and public engagement. The future of municipal wastewater treatment lies in developing integrated, resilient, and resource- efficient systems that can effectively manage wastewater while contributing to environmental protection and sustainable development.

IV. CONCLUSION

Municipal wastewater treatment is a dynamic and evolving field, driven by the imperative to protect public health, safeguard environmental quality, and promote sustainable water resource management. This comprehensive review has highlighted the significant advancements in wastewater treatment technologies, ranging from conventional physical and chemical processes to cutting-edge biological and integrated systems. While traditional methods remain foundational for bulk pollutant removal, their limitations in energy consumption, sludge generation, and effectiveness against emerging contaminants have paved the way for innovative solutions.

Advanced biological treatment technologies, such as Anaerobic Membrane Bioreactors (AnMBRs), Partial Nitritation/Anammox (PN/A) systems, and microalgae-based treatment, offer compelling advantages in terms of energy efficiency, reduced sludge production, and enhanced nutrient removal and recovery. Bioelectrochemical Systems (BES) further push the boundaries by enabling simultaneous wastewater treatment and energy generation. The integration of these diverse technologies, coupled with the adoption of circular economy principles, is transforming WWTPs from mere waste disposal facilities into resource recovery hubs,

yielding valuable products like reclaimed water, energy, and nutrients.

The advent of smart systems, automation, and artificial intelligence is poised to revolutionize WWTP operations, enabling real-time monitoring, data-driven optimization, and predictive maintenance, thereby improving efficiency and reliability. However, significant chalenges persist, including high energy demands, complex sludge management, the pervasive issue of emerging contaminants, and the substantial investment required for infrastructure upgrades. Overcoming these hurdles necessitates continued interdisciplinary research, supportive policy frameworks, capacity building, and increased public awareness and acceptance.

In conclusion, the future of municipal wastewater treatment lies in the development and widespread implementation of integrated, resilient, and resource-efficient systems. By embracing technological innovation, fostering a circular economy approach, and addressing existing limitations, we can ensure the sustainable management of wastewater, contributing to a healthier environment and a more secure water future for generations to come.

REFERENCES

1. Shamshad, J., & Rehman, R. U. (2025). Innovative approaches to sustainable wastewater treatment: a comprehensive exploration of conventional and emerging technologies. Environmental Science: Advances, 4(2), 189-222. https://pubs.rsc.org/en/ content/articlehtml/2025/va/d4va00136b

2. Sikosana, M. L. (2019). Municipal wastewater treatment technologies: A review. Procedia Manufacturing, 35, 1018-1024.

   https://www.sciencedirect.com/science/article/ pii/S2351978919307760

3. Le, L. T., Viet, N. H., & Thanh, B. X. (2024). Recent Advanced Biological Wastewater Treatment Technologies. VNU Journal of Science: Earth and Environmental Sciences, 40(2), 1-12.

   https://js.vnu.edu.vn/EES/article/view/5144/4170

4. Sravan, J. S., Matsakas, L., & Sarkar, O. (2024). Advances in Biological Wastewater Treatment Processes: Focus on Low-Carbon Energy and Resource Recovery in Biorefinery Context. Bioengineering, 11(3), 281.

   https://pmc.ncbi.nlm.nih.gov/articles/ PMC10968575/

5. Fernandes, J., Ramísio, P. J., & Puga, H. (2024). A Comprehensive Review on Various Phases of Wastewater Technologies: Trends and Future Perspectives. Eng, 5(4), 2633-2661. https://www.mdpi.com/2673- 4117/5/4/138

6. Metcalf & Eddy, Inc., Tchobanoglous, G., Burton, F. L., Stensel, H. D., & Tsuchihashi, R. (2014). Wastewater Engineering: Treatment and Resource Recovery (5th ed.). McGraw-Hill Education.

7. Spellman, F. R. (2014). Handbook of Water and Wastewater Treatment Plant Operations. CRC Press.

8. Viessman Jr, W., Hammer, M. J., Perez, E. M., & Chadik, P. A. (2009). Water Supply and Pollution Control (8th ed.). Pearson Prentice Hall.

9. Crittenden, J. C., Trussell, R. R., Hand, D. W., Howe, K. J., & Lantz, K. J. (2012). MWH's Water Treatment: Principles and Design (3rd ed.). John Wiley & Sons.

10. Rittmann, B. E., & McCarty, P. L. (2001). Environmental Biotechnology: Principles and Applications. McGraw-Hill.

11. Bratby, J. (2016). Coagulation and Flocculation in Water and Wastewater Treatment (3rd ed.). IWA Publishing.

12. OMelia, C. R. (1987). Particle Growth, Removal, and Effluent Quality. Water Quality and Treatment: A Handbook of Public Water Supplies, 4th ed., American Water Works Association, New York.

13. White, G. C. (2010). The Handbook of Chlorination and Alternative Disinfectants (5th ed.). John Wiley & Sons.

14. Malley Jr, J. P., & E. J. (2004). Ultraviolet Light in Water and Wastewater Sanitation. American Water Works Association.

15. Langlais, B., Reckhow, D. A., & Brink, D. R. (1991). Ozone in Water Treatment: Application and Engineering. Lewis Publishers.

16. Henze, M., van Loosdrecht, M. C. M., Ekama, G. A., & Brdjanovic, D. (2008). Biological Wastewater Treatment: Principles, Modelling and Design. IWA Publishing.

17. Crites, R., & Tchobanoglous, G. (1998). Small and Decentralized Wastewater Management Systems. McGraw-Hill.

18. Water Environment Federation (WEF). (2005). Activated Sludge and Aeration Systems: Design and Operational Considerations. McGraw-Hill.

19. Wilderer, P. A., & Irvine, R. L. (2001). Sequencing Batch Reactor Technology. IWA Publishing.

20. Qasim, S. R. (1999). Wastewater Treatment Plants: Planning, Design, and Operation. CRC Press.

21. Grady Jr, C. P. L., Daigger, G. T., & Lim, H. C. (1999). Biological

    Wastewater Treatment (2nd ed.). Marcel Dekker.

22. Rusten, B., & Ødegaard, H. (2006). Moving Bed Biofilm Reactors. IWA Publishing.

23. Appels, L., Baeyens, J., Degrève, J., & Van Impe, J. (2008). Principles and potential of the anaerobic digestion of municipal solid waste. Progress in Energy and Combustion Science, 34(6), 755-781.

24. Van Lier, J. B., & Lettinga, G. (1999). The anaerobic treatment of wastewater. Water Science and Technology, 39(10-11), 1-10.

25. Chang, I. S., & Kim, S. N. (2007). The role of membrane bioreactors in wastewater treatment. Environmental Science & Technology, 41(13), 4499- 4504.

26. Verstraete, W., & Vlaeminck, S. E. (2011). Zero-energy wastewater treatment: a paradigm shift. Environmental Science & Technology, 45(14), 5926-5930.

27. Judd, S. J. (2011). The MBR Book: Principles and Applications of Membrane Bioreactors in Water and Wastewater Treatment (2nd ed.). Elsevier.

28. Le-Clech, P., Chen, D., & Fane, A. G. (2006). Fouling in membrane bioreactors used for wastewater treatment. Journal of Membrane Science, 284(1-2), 17-53.

29. Ho, J., & Sung, S. (2010). Anaerobic membrane bioreactor for domestic wastewater treatment. Water Research, 44(15), 4427-4434.

30. Meng, F., Zhang, H., Ding, Y., Shi, Y., & Yang, F. (2009). Membrane fouling in membrane bioreactors: current status and future prospects. Separation and Purification Technology, 66(1), 1-14.

31. van der Star, W. R. L., Abma, W. R., Dieker, P., Mulder, J. W., Tokutomi, T., Strous, M., & van Loosdrecht, M. C. M. (2007). Bioreactor for the anaerobic ammonium oxidation (Anammox) process. Water Research, 41(19), 4148- 4160.

32. Lackner, S., Terada, A., Smits, M., & van Loosdrecht, M. C. M. (2014). Biological nitrogen removal from wastewater: new developments and challenges. Current Opinion in Biotechnology, 27, 137-143.

33. Kuenen, J. G. (2008). Anammox bacteria: from discovery to application. Nature Reviews Microbiology, 6(4), 320-326.

34. Strous, M., Heijnen, J. J., Kuenen, J. G., & Jetten, M. S. M. (1998). Anammox, an anaerobic ammonium-oxidizing bacterium. Nature, 395(6703), 605-607.

35. Kartal, B., Kuenen, J. G., & van Loosdrecht, M. C. M. (2010). Sewage treatment with Anammox. Science, 328(5979), 702-703.

36. Cao, S., & Zhang, T. (2013). Anammox-based processes for nitrogen removal from wastewater: a review. Journal of Environmental Sciences, 25(11), 2201-2208.

37. Third, K. A., Paxman, J., & van Loosdrecht, M. C. M. (2005). The anaerobic oxidation of ammonium: a review. Water Science and Technology, 52(7), 17- 26.

38. Wang, Y., Li, Y., & Zhang, Y. (2015). Microalgae-based wastewater treatment for nutrient removal and biomass production. Bioresource Technology, 184, 179-89.

39. Craggs, R. J., Heubeck, S., & Smith, V. J. (2012). Wastewater treatment and algal biomass production using high rate algal ponds. Water Science and Technology, 65(10), 1803-1809.

40. Pittman, J. K., Dean, A. P., & Osundeko, O. (2011). The potential of algae for biofuel production. Bioresource Technology, 102(1), 89-96.

41. Chisti, Y. (2007). Biodiesel from microalgae. Biotechnology Advances, 25(3), 294-306.

42. Wang, L., Min, M., Li, Y., Chen, P., & Ruan, R. (2010). Cultivation of microalgae in wastewater for biomass production and nutrient removal. Bioresource Technology, 101(1), 31-38.

43. Rawat, I., Kumar, R. R., Mutanda, T., & Bux, F. (2011). Dual role of microalgae: wastewater treatment and biofuel production. Applied Energy, 88(10), 3411-3424.

44. Sawayama, S., Inoue, S., & Dote, Y. (1999). CO2 fixation and oil production by microalgae. Energy Conversion and Management, 40(17), 1827- 1833.

45. Logan, B. E., & Rabaey, K. (2012). Bioelectrochemical Systems: From Extracellular Electron Transfer to Biotechnological Application. John Wiley & Sons.

46. Rabaey, K., & Rozendal, R. A. (2010). Microbial fuel cells for sustainable energy generation: current status and future prospects. Environmental Science & Technology, 44(13), 5192-5202.

47. Rozendal, R. A., Hamelers, H. V. M., & Rabaey, K. (2008). Towards practical implementation of bioelectrochemical wastewater treatment. Trends in Biotechnology, 26(8), 450-459.

48. Liu, H., Grot, S., & Logan, B. E. (2005). Electrochemically assisted microbial production of hydrogen from acetate. Environmental Science & Technology, 39(11), 4317-4320.

49. Lovley, D. R. (2006). Microbial fuel cells: novel microbial technologies for energy generation. Current Opinion in Biotechnology, 17(3), 327-332.

50. Rittmann, B. E., & McCarty, P. L. (2012). Environmental Biotechnology: Principles and Applications (2nd ed.). McGraw-Hill Education.

51. Kim, J., & Logan, B. E. (2011). Microbial electrolysis cells (MECs) for sustainable hydrogen production. Trends in Biotechnology, 29(1), 2-4.

52. Daigger, G. T., & OShaughnessy, J. C. (2016). Biological Wastewater Treatment: Principles, Modeling and Design. CRC Press.

53. Rusten, B., Eikebrokk, B., & Ødegaard, H. (2000). Biological phosphorus removal in a moving bed biofilm reactor. Water Science and Technology, 41(9), 117-124.

54. Drews, A. (2010). Membrane fouling in membrane bioreactors – Characterisation, contradictions, and future directions. Separation and Purification Technology, 71(1), 1-39.

55. Vymazal, J. (2007). Removal of nutrients in various types of constructed

    wetlands. Science of the Total Environment, 380(1-3), 48-62.

56. Verstraete, W., & Vlaeminck, S. E. (2011). Zero-energy wastewater treatment: a paradigm shift. Environmental Science & Technology, 45(14), 5926-5930.

57. Shannon, M. A., Bohn, P. W., Elimelech, M., Georgiadis, J. G., Mariñas,

    B. J., & Mayes, A. M. (2008). Science and technology for water purification in the coming decades. Nature, 452(7185), 301-310.

58. McCarty, P. L., Bae, J., & Kim, J. (2011). Domestic wastewater treatment as a net energy producer – Can this be achieved? Environmental Science & Technology, 45(17), 7100-7106.

59. Gherghel, A., Teodosiu, C., & Bulete, A. (2019). Nutrient recovery from wastewater: a review. Environmental Engineering and Management Journal, 18(1), 3-18.

60. Kleerebezem, R., & van Loosdrecht, M. C. M. (2007). The waste water treatment plant as a resource recovery factory. Water Science and Technology, 55(10-11), 1-15.

61. Lee, Y., Lee, J., & Park, J. (2018). Smart water grid: a review of current trends and future challenges. Water, 10(11), 1627.

62. Olsson, G. (2012). Digitalization in Wastewater Treatment: Control and Monitoring. IWA Publishing.

63. Sbarbaro, D., & Campos, V. (2017). Control and optimization of wastewater treatment plants: a review. Reviews in Environmental Science and Bio/Technology, 16(3), 401-419.

64. Mannina, G., Cosenza, A., & Viviani, G. (2016). Advanced control strategies for wastewater treatment plants: a review. Reviews in Environmental Science and Bio/ Technology, 15(4), 629-647.

65. Corominas, L., Rieger, L., Vanrolleghem, P. A., & Nopens, I. (2018). Digital twins for wastewater treatment plants: a review. Water Research, 142, 36-49.

66. Guven, H., Dereli, R. K., Ozgun, H., Ersahin, M. E., & Ozturk, I. (2019). Towards Sustainable and Energy Efficient Municipal Wastewater Treatment by Up-Concentration of Organics. Progress in Energy and Combustion Science, 70, 145-168.

67. Tyagi, V. K., & Lo, S. L. (2013). Sludge: A waste or a renewable source for energy and resources. Renewable and Sustainable Energy Reviews, 25, 708- 728.

68. Deblonde, T., Cossu-Leguille, C., & Hartemann, P. (2011). Emerging pollutants in wastewater: a review of the current knowledge. Environmental International, 37(2), 404-414.

69. United Nations Environment Programme (UNEP). (2018). The UN World Water Development Report 2018: Nature-based Solutions for Water. UNESCO.

70. Van Loosdrecht, M. C. M., & Brdjanovic, D. (2014). Wastewater Treatment: Biological and Chemical Processes. CRC Press.

71. Pescod, M. B. (1992). Wastewater Treatment and Use in Agriculture. FAO Irrigation and Drainage Paper 47. Food and Agriculture Organization of the United Nations.

72. Daigger, G. T., & OShaughnessy, J. C. (2016). Biological Wastewater Treatment: Principles, Modeling and Design. CRC Press.