Up-Flow Anaerobic Sludge Bed Reactors for Sustainable Wastewater Management: Challenges, Innovations, and Future Directions

Abstract

The up-flow anaerobic sludge bed (UASB) reactor is a high-efficiency system capable of carrying out anaerobic digestion with shorter hydraulic retention times than traditional anaerobic digesters. This review highlights recent advancements in UASB reactor applications and key aspects such as microbial community dynamics and reactor hydrodynamics that could drive future developments. More specifically, this review evaluates the working principles of UASB reactors, explores strategies to optimize reactor efficiency, and examines technological advancements aimed at overcoming temperature constraints, managing emerging pollutants and micropollutants, and addressing scum accumulation, odor emission, and nutrient recycling challenges. Furthermore, it addresses concerns about the lack of a skilled workforce and energy loss in biomethane. The UASB reactor demonstrates high potential for enhancing global wastewater management while holding the promises of enhancing circular economic objectives, promoting efficient biogas utilization, and reducing greenhouse gas emissions.

1. Introduction

The demand for reusing treated wastewater is plummeting due to industrialization. Therefore, low-cost, sustainable wastewater treatment approaches should be used to meet the global demand for sanitized water [1]. Anaerobic sewage treatment is a core approach within sustainable and circular economic frameworks due to its low construction costs, bioenergy generation potential, relatively low sludge production, and robust chemical oxygen demand (COD) removal [2,3]. The primary obstacles preventing the integration of anaerobic digestion in wastewater treatment are slower digestion kinetics, large processing volumes, high sewage dilution, and the requirement of optimal operational temperatures for biogas production.

Furthermore, the operational temperature directly influences microbial proliferation, ultimately determining the digestion kinetics of the entire treatment process. Even though the anaerobic digestion treatment of wastewater is feasible at every temperature, sub-optimal temperatures cause bacterial growth rate decline and methanogenesis [4]. For example, optimal mesophile-mediated digestion and methanogenesis occur between 30 °C and 38 °C in winter. Due to this, an additional heat input is needed to maintain the digestion conditions. Methanogenesis at sub-optimal temperatures is 10–20 times lower than methanogenesis at 35 °C, requiring a longer sludge retention time and a long hydraulic retention time to maintain an efficiency similar to that in optimal temperature conditions (35 °C).

The development of an up-flow anaerobic sludge blanket (UASB) reactor is one of the breakthrough technologies of the anaerobic digestion treatment of wastewater, as described by Lettinga et al. [5]. In UASB reactors, auto-flocculation is used for granular biomass generation with a high biomass concentration. The operational design and unique biomass structure allow the independent setting of the sludge retention and hydraulic retention time inside the UASB reactor. According to Lettinga and Hulshoff Pol [6] and Capodaglio et al. [7], UASB reactors are a consolidated approach to circumvent the current constraints of conventional anaerobic treatment processes and have a wide range of applications in wastewater management, especially in tropical climatic conditions. UASB reactors are considered to be a possible solution for sustainable, decentralized wastewater treatment systems.

This study aims to provide a comprehensive overview of UASB reactor technology, focusing on the UASB’s working principle, microbial dynamics, treatment process optimization, and technological enhancements, as well as the application of UASB reactor technology in resolving its ecological and economic impacts, constraints in UASB reactor technology, and future opportunities for the sustainable utilization of UASB reactor technology for continuous wastewater treatment.

2. Working Principles of UASB Reactors

The up-flow anaerobic sludge bed (UASB) reactor is a biological wastewater treatment system that operates under anaerobic conditions. It allows for the treatment of organic matter while producing biogas, primarily methane [8,9]. In this reactor, wastewater flows upward through a granular sludge bed, the biocatalyst for degrading organic pollutants [10]. The design encourages close contact between the wastewater and sludge particles, optimizing treatment efficiency and biogas production [8].

2.1. Process Principles

A sufficient amount of anaerobic sludge is first added to the reactor as an inoculant, and soon after, feeding begins at a low flow rate in the up-flow. This first stage, the start-up period, is crucial for the reactor’s operation. The feeding rate should be gradually increased based on the system’s performance. After several months of operation, a highly concentrated sludge bed (40 to 100 gTS/L) forms near the bottom of the reactor [11]. This sludge is dense and has excellent settling properties. Sludge granules, ranging from 1 to 5 mm in diameter, may develop depending on the characteristics of the seeding sludge, the wastewater, and the reactor’s operational conditions [8]. Above the sludge bed, a less dense bacterial growth zone, known as the sludge blanket, forms, with solids having lower concentrations and slower settling velocities. The sludge concentration in this area typically ranges from 1 to 3%. The system is naturally mixed by the upward flow of biogas bubbles and the liquid flow through the reactor.

During the start-up phase, when biogas production is usually low, additional mixing, such as gas or effluent recirculation, may be required. Substrate removal occurs throughout both the sludge bed and blanket, with a higher rate of removal observed in the sludge bed [8,9]. A three-phase separator (for gases, solids, and liquids) must be installed at the top of the reactor to enable sludge retention and return. Above and around the three-phase separator is a sedimentation chamber where the denser sludge is separated from the liquid and returned to the digestion compartment. At the same time, the lighter particles are carried out of the system with the final effluent (see Figure 1) [11].

The UASB reactors have solid residence times (32 to 45 days) that are significantly longer than their hydraulic retention times (3 to 24 h), a hallmark of high-rate anaerobic systems [12]. Sludge ages in UASB reactors typically exceed 30 days [11], which contributes to stabilizing the excess sludge removed from the system. The sludge within the reactor should exhibit highly methanogenic activity, excellent settling properties, and, ideally, a granular structure. Granular sludge outperforms flocculent sludge due to its stability and efficiency [13,14].

Therefore, the effectiveness of UASB reactors relies on several key principles that boost overall efficiency together.

• Upward flow: Upward flow ensures maximum interaction between the biomass and substrate.
• Avoiding short circuits: Short circuits must be avoided to provide adequate retention time for higher removal efficiency.
• Efficient phase separation: A well-designed separator ensures the retention of solids and the exit of biogas and liquid.
• Better sludge properties: Sludge should have superior settling qualities and high methanogenic activity.

2.2. Microbial Ecology

Key bacterial groups such as Firmicutes, Bacteroidetes, and various Proteobacteria play distinct roles in the treatment process, from hydrolysis to methanogenesis. Recent studies utilizing advanced metagenomic techniques have uncovered a complex interplay among these microorganisms, revealing that their diversity is crucial for the degradation of complex organic substances and the production of volatile fatty acids and methane [1,13]. Furthermore, operational parameters, including nutrient availability, hydraulic retention time, and pH levels, etc., profoundly impact microbial community dynamics, highlighting the need for careful management to ensure optimal reactor functionality [15,16]. Despite the advantages of UASB technology, maintaining a stable microbial community and consistent effluent quality remains challenging. Changes in influent characteristics, such as salinity and organic load, can disrupt microbial interactions, reducing performance. A study showed a strong correlation between toilet paper presence and methanogenic activity in blackwater-fed UASB reactors [15].

Furthermore, Yangin-Gomec and Engiz [17] demonstrated that high concentrations of propylene glycol in urban wastewater can inhibit biomass growth and alter the microbial community. Specifically, as glycol concentrations increased, the dominant microbial populations shifted from protobacteria to Firmicutes and from Methanocarcina to Methanoculleus [17]. Callejas et al. [18] further confirmed the adaptability of microbial communities to different substrates, observing temporal changes in the community structure of a full-scale UASB reactor treating sugarcane vinasse [18].

2.2.1. Hydrolytic Bacteria

Hydrolytic bacteria are a pivotal group of microorganisms in UASB reactors. They convert insoluble organic substrates into soluble monomers and dimers, which significantly enhances the efficiency of biogas production [13]. These bacteria, including prominent genera such as Bifidobacterium, Lactobacillus, Enterobacterium, and Streptococcus [19], are essential for the hydrolysis phase of anaerobic digestion, which precedes fermentation and methanogenesis. The functionality of hydrolytic bacteria is characterized by their interactions with other microbial communities, particularly methanogenic archaea [20,21]). Rajagopal et al. [21] highlight the influence of pre-hydrolysis on sewage treatment in UASB reactors, emphasizing the significance of hydrolytic bacteria in organic matter breakdown and overall system performance. Furthermore, this interdependence underscores the importance of hydrolytic bacteria for organic degradation and maintaining a balanced microbial ecosystem within the reactor, thus promoting overall system stability and efficiency.

2.2.2. Fermentative Acidogenic Bacteria

Acidogenic bacteria, comprising approximately 90% of the total bioreactor population, dominates the initial stage of anaerobic digestion, known as acidogenesis [13]. During this process, complex organic matter is metabolized by various genera, including Pseudomonas, Bacillus, Clostridium, Micrococcus, and Flavobacterium, to produce organic acids, such as formic, acetic, propionic, butyric, and lactic acids, and hydrogen gas [19]. These fermentation products are essential for methanogenesis and influence the overall efficiency of UASB reactors, which are designed to treat municipal and industrial wastewater. The efficiency of these conversion processes is influenced by environmental factors like pH and temperature [13,19].

2.2.3. Acetogenic Bacteria

Acetogenic bacteria, such as Desulfovibrio, Syntrophobacter wolinii, Syntrophomonas wolfei, Syntrophus buswellii, Syntrophococcus, Natroniella, and Acetigena spp., further oxidize acidogenesis products into acetate, H₂, and CO₂ [13,22]. These obligate H₂-producing bacteria require extremely low H₂ partial pressures for survival. They facilitate the conversion of fatty acids and alcohols into acetate, H₂, and CO₂ under low H₂ conditions. To maintain this low H₂ partial pressure (below 10⁻⁵ atmospheres), acetogenic bacteria often form symbiotic relationships with H₂-utilizing methanogens, particularly when digesters operate under optimal temperature and pH conditions [13].

2.2.4. Methanogenic Archaea and Their Taxonomy

The major groups of methanogenic archaea can be categorized into several orders, particularly Methanobacteriales, Methanomicrobiales, and Methanosarcinales. Methanosarcina species are often the most prevalent in anaerobic environments [23]. Other genera within these orders, including Methanosaeta and Methanolinea, also play a substantial role in the microbial community composition of these reactors [24]. Methanogenic archaea utilize different metabolic pathways to produce methane, which can be classified into three main types: acetoclastic, hydrogenotrophic, and methylotrophic methanogenesis [25]. Hydrogenotrophic methanogenesis is particularly important in low-temperature anaerobic granular sludge systems, where Methanomicrobiales populations have been shown to thrive. In contrast, acetoclastic methanogenesis, primarily facilitated by Methanosarcinales, is often favored in psychrophilic conditions within anaerobic digestion systems, highlighting the adaptability of these organisms to various environmental conditions [26].

2.3. Process Optimization

The optimization of UASB reactors is vital for enhancing their operational performance, stability, and treatment efficiency. Critical elements affecting optimization include hydraulic retention time (HRT), sludge retention time (SRT), organic loading rate (OLR), temperature, and pH level, all of which profoundly influence the microbial dynamics within the reactor [27,28]. The literature indicates that the proficient management of these parameters can increase biogas production, improve substrate removal rates, and lower operational costs, enhancing wastewater treatment operations’ sustainability and economic feasibility [13].

2.3.1. Organic Loading Rate (OLR)

OLR is a critical factor that greatly influences the microbial community and overall performance of a UASB reactor. Variations in organic load are dependent on factors such as sludge retention time (SRT), hydraulic retention time (HRT), sludge characteristics, mixing intensity, duration of the fluctuation, and bacterial mass and activity [29,30]. Research has shown that elevated OLR levels can lead to a decline in the COD removal efficiency in wastewater treatment systems [31,32]. Higher OLRs might result in irreversible acidification, the suppression of methanogenic activity due to a severe imbalance between methanogens and acidogens, and the inhibition of methanogens caused by the accumulation of volatile fatty acids (VFAs).

2.3.2. Nutrients

The growth of anaerobic microorganisms relies on the presence of essential nutrients available in the wastewater. A deficiency in these nutrients can adversely impact microbial growth and reduce the efficiency of anaerobic degradation processes. The biochemical processes of fermentation and methane (CH₄) production involve numerous enzymes that require specific trace elements as nutrients. Each type of anaerobic microorganism participating in the breakdown of complex organic matter into simpler compounds depends on these trace elements, which are specific to the enzyme pathways involved [26]. Numerous studies have examined the influence of nutrients on the efficiency of anaerobic digestion and the enhancement of granules within bioreactors [33,34,35]. Certain bacteria, such as methane-producing bacteria, have relatively high internal requirements for trace elements like iron, cobalt, and nickel, which may be insufficient in the wastewater generated by industries. Consequently, adding these trace elements before treatment is strongly recommended to improve reactor performance [36,37]. The optimal carbon, nitrogen, and phosphorus (C:N:P) ratio for maximizing methane yield was identified as 100:2.5:0.5 [38]. This ratio can be calculated based on the biodegradable COD concentration in the wastewater, nutrient concentrations within bacterial cells, and cell yield.

2.3.3. Hydraulic Retention Time (HRT)

HRT denotes the mean retention time of the wastewater in the reactor, facilitating adequate interaction between the influent and microbial biomass. The optimal management of HRT is critical for maximizing treatment efficiency, as it directly influences the removal of pollutants and the quality of the effluent. Factors such as influent flow rate, reactor volume, and wastewater characteristics must be carefully calibrated to maintain an effective HRT, as fluctuations can lead to reduced treatment performance and regulatory compliance challenges [39,40]. Controversies surrounding HRT management often arise from the balance between treatment efficiency and operational costs. Longer retention times can enhance organic matter removal but may increase energy consumption and overall operational expenses. Conversely, shorter HRTs may compromise treatment effectiveness, resulting in effluent that does not meet environmental standards [41]. Therefore, striking a balance between economic feasibility and environmental compliance remains a key challenge. The ideal HRT for UASB reactors ranges from 3 to 24 h, depending on the wastewater characteristics and operational conditions [42]. Several studies have shown the effect of HRT on microbial degradation in a single UASB reactor treating different types of industrial wastewater [43,44].

2.3.4. Volatile Fatty Acids (VFAs)

The significance of VFAs, which serve as essential intermediates in anaerobic digestion, lies in their direct impact on reactor stability, organic loading rates, and the overall biochemical processes within the system [45]. Research indicates that maintaining a balanced VFAs-to-alkalinity (Alk) ratio is vital for optimal UASB reactor operation. An ideal ratio between 0.1 and 0.5 promotes stable conditions, while ratios exceeding 0.8 can lead to acidification and inhibit methanogenic activity, thus jeopardizing biogas production [45]. Additionally, the relationship between VFAs and methane yield is complex; while initial increases in VFA concentrations can enhance methane production rates, excessive accumulation, particularly of propionic acid, can disrupt microbial processes and diminish overall reactor efficiency [46]. Despite their advantages, UASB reactors face challenges linked to VFA dynamics, such as operational instability and the need for the precise management of organic loading rates (OLRs). Variations in OLRs can alter VFA composition and concentration, influencing the methanogenic pathway and overall biogas production. High OLRs have been shown to elevate VFA levels, which can result in decreased methane yield and operational challenges, particularly during adaptation phases. Furthermore, health and safety concerns regarding the handling of effluents and sludge add complexity to the operational management of UASB systems [47,48].

2.3.5. Operational Temperature

The effect of operational temperature on UASB reactor performance is a critical consideration in the field of wastewater treatment, impacting both efficiency and operational costs. The optimal performance of UASB systems is typically achieved at mesophilic (20–45 °C) or thermophilic (45–60 °C) temperatures, where microbial activity is maximized, allowing for enhanced methanogenesis and overall system efficiency. However, fluctuations outside these optimal temperature ranges can lead to significant operational challenges, necessitating additional heating solutions in colder climates and affecting the stability of microbial communities essential for effective wastewater treatment [11]. Temperature variations profoundly influence the microbial dynamics within UASB reactors, particularly in how they affect the balance of methanogenic pathways. At lower temperatures, while methanogenesis remains relatively robust, hydrolysis—the critical first step in anaerobic digestion—often becomes a limiting factor, leading to inefficiencies in organic matter degradation and the increased production of unhydrolyzed materials [49,50]. Conversely, higher temperatures can accelerate metabolic processes, resulting in improved biogas production and organic matter removal, but can also alter biogas composition, affecting methane solubility and release rates [51]. These factors underscore the importance of temperature management strategies in reactor design and operation to optimize UASB performance under varying environmental conditions. Controversies in the field revolve around the operational adjustments necessary to maintain efficiency in sub-optimal temperatures, which can lead to increased operational costs and longer start-up times for microbial community stabilization.

2.3.6. Operational pH

Maintaining an optimal pH range is essential for maximizing UASB reactor performance, with studies indicating that a pH between 6.0 and 8.0 supports effective microbial activity and COD removal [1,52]. Deviations from this range can lead to significant reductions in biogas production and COD removal efficiencies, as evidenced by research showing that a drop in pH to 5.0 can decrease methane yield by over 25%. The dynamics of microbial communities are also influenced by pH changes, which can shift dominant species and disrupt metabolic processes, resulting in inefficiencies and the accumulation of inhibitory compounds such as propionate [53]. Effective strategies for maintaining optimal pH include carefully monitoring influent pH, adjusting alkalinity levels, and implementing advanced reactor designs or sequential systems that enhance pH stability. However, challenges remain, particularly regarding the reactors’ sensitivity to fluctuating influent conditions and the need for real-time monitoring technologies to ensure operational efficiency [54].

Table 1. Optimum pH ranges for reported methanogens in UASB granular sludge [13].

Genus	Optimal pH Range
Methanothermus	6.5
Methanohalobium	6.5–6.8
Methanolacinia	6.6–7.2
Methanomicrobium	7.0–7.5
Methanosphaera	6.8
Methanogenium	7.0
Methanosprillum	7.0–7.5
Methanosaeta	7.6
Methanolobus	6.5–6.8
Methanothrix	7.1–7.8
Methanococcoides	6.5–7.5

gives a quick overview of the reported optimal pH ranges of methanogens in UASB granular sludge.

3. Technological Advances of UASB Reactors

The functioning principle behind a UASB reactor is its granular sludge bed that forms in situ and is subdivided into three sections operating with a counter-current configuration, as wastewater is forced to flow vertically upward through it. These microorganisms grow attached to the sludge particles, taking up pollutants from the wastewater. This also means that the biofilms must be of high quality, and that there must be close contact between the sludge and the wastewater in order to make such a reactor successful. The generated biogas aids in the mixing of sludge and wastewater, and at the same time, the three-phase gas–liquid–solid separator, which is located at the top of the reactor, helps to remove the biogas from the liquid effluent and the remaining sludge particles. Due to some inherent limitations of anaerobic treatment in a conventional UASB, researchers have focused on improvements or modifications to the UASB to overcome these drawbacks [55]. These improvements or changes involve modifying reactor designs, improving the granulation processes and the start-up steps by using multivalent cations or natural or synthetic polymers, and adding post-treatment methods like activated sludge or sequencing batch reactors to treat the effluent that comes from the UASB [56]. This section discusses some technological modifications or advancements in the UASB for efficient wastewater treatment.

3.1. Two-Stage UASB Anaerobic Digestion

A two-stage or multi-stage digestion process offers an alternative to single-stage digestion by optimizing conditions for specific groups of microorganisms, thereby enhancing process efficiency [48]. Each stage is tailored to the requirements of distinct microbial communities, improving overall digestion performance and methane production [48]. In the first stage, hydrolytic and acidogenic reactions occur. During these reactions, the complex organic materials are converted into simpler compounds. These compounds are then transformed by acidogenic bacteria into organic acids, hydrogen, and carbon dioxide. These bacteria thrive at a lower pH range of 5–6 and require a shorter hydraulic retention time (HRT), reducing the time wastewater remains in this phase [57]. The second stage is dedicated to methanogenic reactions, where methanogens perform optimally in a neutral to slightly alkaline pH range (6–8) and require a longer HRT for effective operation [57]. The energy recovery efficiency of the two-stage anaerobic digestion process can be enhanced by separating acidogenesis and methanogenesis into separate stages. It facilitates the production of both CH₄ and H₂ while improving microbial stability within the system [58].

3.2. UASB Co-Digestion

The modern anaerobic treatment of multiple substrates is commonly called the co-digestion process. This is caused by improving process stability and boosting biogas production in the UASB process. It also aids in balancing the levels of macro- and micronutrients. The literature demonstrates that the incorporation of metals and natural elements benefits COD removal as well as biogas production in the UASB co-digestion of high-loaded substrates [59]. When treating sulfate-rich wastewater, carbon-rich co-substrates can be very advantageous because the addition of carbon minimizes the competition between methanogenic biomass and sulfate-reducing bacteria (SRB). Animal residues can provide nutrients and alkalinity when managing highly degradable and acidifying substrates. Moreover, some industrial waste and wastewater streams are not produced on a continuous basis. For instance, in the ethanol industry, sugarcane molasses and vinasses are produced in batches, and their co-digestion is shown to be very effective because of the factory’s batch operation mode [48].

3.3. Anaerobic Baffled Reactors (ABRs)

An ABR consists of multiple UASB reactors that function without granulation [60]. Wastewater is guided to move in an up-and-down manner between the inlet and outlet by a continuation of vertical baffles. Due to the flow characteristics and gas production in each compartment, bacteria within the reactor gently rise and settle, but they move horizontally down the reactor at a relatively slow rate. This results in a cell retention time (CRT) of 100 days at a hydraulic retention time (HRT) of 20 h. As it moves through the ABR with brief HRTs (6–20 h), the wastewater can thus come into close contact with a significant quantity of active biomass. Concurrently, the effluent is almost free of biological solids. It has been demonstrated that this setup leads to a significant reduction in COD.

A major advantage of ABRs is their design, which allows for the systematic separation of acidogenesis and methanogenesis, creating optimal conditions for the development of unique microbial populations. Additionally, they produce less excess sludge compared to many other high-rate anaerobic systems, reducing the need for frequent sludge removal and making the system easier to manage [61].

3.4. Anaerobic Membrane Bioreactor (AnMBR)

The Anaerobic Membrane Bioreactor (AnMBR) represents an advanced evolution of traditional UASB anaerobic processes, offering significant advantages for treating low-strength wastewater. Its ability to retain slow-growing methanogenic organisms ensures complete biomass retention, enabling higher organic loading rates and improved system efficiency. This retention also promotes stable reactor performance under varying conditions, making it particularly suitable for low-strength wastewater applications. In AnMBR systems, the specific organic loading rate (SOLR) is an essential operational parameter that determines how effectively the reactor converts organic matter into biogas relative to the amount of microbial biomass present. The SOLR is essential for system performance, as it impacts the characteristics of the sludge, molds microbial community structures, and also affects membrane-fouling tendencies [62]. The optimal management of the SOLR is essential to balance microbial activity and prevent overloading, which could lead to reduced performance or increased fouling.

AnMBR systems are constantly challenged by membrane fouling, which has a major impact on their efficiency and leads to higher maintenance costs. A high SOLR can exacerbate fouling by promoting the accumulation of extracellular polymeric substances (EPSs) and soluble microbial products (SMPs), which form a sticky layer on membrane surfaces. Conversely, the careful optimization of the SOLR can minimize fouling risks while maintaining efficient treatment and biogas production. Additionally, AnMBR systems benefit from enhanced flexibility in operational parameters like hydraulic retention time (HRT) and solid retention time (SRT), enabling fine-tuning for various wastewater characteristics. This adaptability, combined with the system’s ability to handle higher organic loads and its robust microbial retention, positions the AnMBR as a practical strategy for sustainable wastewater treatment [63]. Figure 2 shows the schematics of an AnMBR.

3.5. Expanded Granular Sludge Bed (EGSB)

The expanded granular sludge bed (EGSB) reactor can be considered as an advanced modification of the conventional UASB reactor, designed to address some of its limitations and improve performance. The EGSB reactor operates at high flow velocities, and its organic loading rate is higher than that of the UASB reactor. This enhanced hydraulic performance facilitates better contact between the wastewater and granular sludge, significantly improving treatment efficiency. A typical EGSB reactor incorporates effluent recirculation, which enhances mixing, particularly at the reactor’s bottom. This feature helps distribute nutrients more evenly, prevents sludge accumulation, and ensures uniform reactor operation. The up-flow velocity in an EGSB reactor typically exceeds 4 m/h, much higher than in a UASB reactor. This increased velocity expands the sludge bed more effectively, reducing the formation of dead zones and improving overall mass transfer [65].

A major benefit of the EGSB reactor is that it can operate efficiently, even at low temperatures (4–20 °C) where traditional anaerobic reactors may struggle. This makes the EGSB reactor particularly suitable for treating wastewater in colder climates or during seasonal temperature fluctuations. Furthermore, the expanded sludge bed in the EGSB reactor provides a larger surface area for microbial attachment and activity, enhancing the degradation of organic matter and biogas production. The higher loading rate and improved hydraulic conditions of the EGSB reactor also contribute to its ability to treat a wider range of wastewater types, including those with lower organic strength or challenging compositions. Overall, the EGSB reactor represents a significant step forward in anaerobic wastewater treatment, combining improved hydraulic performance, operational flexibility, and adaptability to diverse environmental conditions [66]. Figure 3 shows a schematic diagram of an ESGB.

3.6. Internal Circulation Reactor (IC)

The internal circulation (IC) reactor is an advanced up-flow anaerobic digester designed primarily for efficient biogas production. It consists of two distinct stages, each serving a specific purpose to enhance the overall digestion process. The first stage is located at the bottom of the reactor, where a dense sludge bed facilitates the breakdown of organic matter by anaerobic microorganisms. At this stage, biogas is generated and gathered. It is conveyed upward together with water to the gas–liquid separator, which is located at the reactor’s upper section. The gas–liquid separator effectively separates biogas from the liquid, allowing for its capture and utilization as an energy source. After separation, the wastewater, driven by gravity, flows back to the bottom of the reactor. This internal circulation mechanism promotes enhanced mixing within the reactor, ensuring better contact between the wastewater and the sludge bed. This improved mixing optimizes organic matter degradation and biogas production [67].

The second stage, located in the upper portion of the reactor, serves to facilitate the sedimentation of organic materials. This stage reduces the potential for biomass washout, ensuring that active sludge is retained within the reactor for continuous digestion. Although the primary biogas production occurs in the first stage, a smaller amount of biogas is also generated in the upper stage, further contributing to energy recovery. The IC reactor’s two-stage design, combined with its internal circulation feature, offers several advantages, including improved mixing, enhanced sludge retention, and increased biogas yield. It is especially appropriate for the treatment of high-strength industrial wastewater, providing an efficient and sustainable solution for wastewater treatment and energy recovery [62]. Figure 4 shows a schematic diagram of an IC reactor.

3.7. Partitioned UASB Reactor

The partitioned UASB reactor introduces a practical modification by dividing a single digestion compartment (as in a conventional reactor) into three chambers operating in parallel. This design, referred to as a partitioned UASB system, enhances operational efficiency without significantly increasing construction costs. The partitioning can be achieved using inexpensive materials, such as cement boards or thin brick walls, making it a cost-effective enhancement. Moreover, the overall cost of the reactor is expected to be lower than that of conventional single-compartment reactors, primarily because the total reactor volume can be reduced by approximately 25%. This reduction in volume results in savings on construction materials and operational costs [69].

Partitioning improves reactor performance by optimizing hydraulic flow and enhancing the distribution of wastewater across the chambers. Each chamber operates semi-independently, which minimizes short-circuiting and dead zones, ensuring more uniform treatment and higher sludge retention. This configuration also facilitates the better separation of phases, allowing more efficient sedimentation and biogas collection. Additionally, the partitioned design can enhance system resilience, as performance in one chamber is less likely to be affected by disturbances in others. This makes the reactor more robust under variable loading conditions. Overall, the partitioned UASB system provides a solution for anaerobic wastewater treatment that is efficient, cost-effective, and sustainable, making it an attractive alternative to conventional single-cell reactors [70]. Figure 5 presents an overview of UASB modifications adapted to enhance wastewater treatment efficiency.

Technological advancements in UASB reactors have significantly expanded their applicability and efficiency in wastewater treatment. Innovations such as modified reactor configurations, improved start-up and granulation techniques, and the integration of complementary treatment systems address many inherent limitations of conventional UASB systems. These advancements not only enhance pollutant removal efficiency and biogas production but also ensure adaptability to varying wastewater characteristics and environmental conditions.

Table 2. Wastewater treatment efficiencies of modified UASB systems.

UASB Advancement	Wastewater Type	HRT (h)	Organic Loading Rate (kg COD/m3d)	Methane (CH4) Yield (L)	COD Removal Efficiency (%)	NH3 Removal Efficiency (%)	References
Two-stage UASB anaerobic digestion	Baker’s yeast wastewater	-	-	113.4	35.98	-	[71]
UASB co-digestion	Pre-treated municipal wastewater.	8.1	1.0	-	63	-	[59]
	A mixture of sewage sludge (SS) and cow manure	20/days	-	-	86	-	[72]
Anaerobic baffled reactors (ABRs)	Domestic wastewater	20	-	-	47.6	31.2/TKN (Total Khejhal Nitrogen)	[73]
Expanded granular sludge bed (EGSB)	Low-strength domestic sewage	5	2.16	-	71.5	-	[65]

gives a summary of modified UASB reactor configurations with their operational specifications (HRT and OLR), CH₄ yield, and COD and NH₃ removal efficiencies.

4. Application of UASB Reactor Technology for Treatment of Various Wastewater Types

UASBs have proven to be sustainable and efficient in treating various types of wastewater. The idea behind the UASB system is that wastewater flows upward via a thick layer of granular sludge. Microbial consortia in this sludge break down organic contaminants, generating biogas mostly made up of carbon dioxide and methane. In addition to producing a renewable energy source, the biogas ensures effective contact between the microbial biomass and wastewater by generating hydraulic turbulence. The UASB’s three-phase separator, which is at the top, efficiently separates the treated effluent, biogas, and sludge, preserving biomass and lowering the frequency of sludge discharge [74].

Treating municipal wastewater is one of the major applications of UASB reactors. This method performs especially well since microbial activity is naturally higher in warmer regions. UASB reactors are perfect for treating municipal sewage because of their high organic load-handling capacity and low energy needs. Many communities have implemented this technology to lower energy expenses and lessen their effects on the environment. In addition to municipal wastewater, industrial effluents, frequently distinguished by large amounts of biodegradable organic matter, can be effectively treated by UASB reactors. For example, UASB reactors have effectively handled high-strength wastewater in the beverage and brewery industries. Similarly, UASB technology has been effectively incorporated into the operations of food processing facilities, which produce effluents high in organic pollutants. Moreover, UASB reactors have proven effective in treating tannery effluent, which consists of a complex mixture of organic and inorganic contaminants. UASB reactors have proven effective in treating wastewater from agriculture, municipal, and industrial settings. This technology’s suitability for wastewater from processes like manufacturing potato starch highlights its adaptability [75].

There are several advantages to using UASB reactors. They are energy-efficient firstly, as the biogas produced through anaerobic digestion can be utilized as a renewable energy source to help defray operating expenses. Second, compared to aerobic systems, UASB reactors produce much less surplus sludge, making sludge management easier and lowering disposal expenses. Third, its lack of mechanical mixing mechanisms makes the reactor’s design and operation simpler, resulting in less energy consumption and maintenance. These aspects work together to make UASB reactors an affordable and environmentally friendly wastewater treatment technology [75]. UASB reactors do have several drawbacks, nevertheless, despite their benefits. Granular sludge, an essential part of the process, can build slowly, resulting in long start-up times. By inoculating the reactor with pre-formed granules, this problem can be mitigated.

Furthermore, UASB reactors are sensitive to temperature; their efficiency tends to deteriorate in colder areas because of a decrease in microbial activity. Temperature control techniques may be required in these situations. Finally, post-treatment procedures are frequently necessary to fulfill demanding effluent discharge criteria, especially for eliminating pathogens and residual organic matter, although UASB reactors effectively reduce organic loads [16].

Addressing these issues and improving reactor performance have been the main goals of recent developments in UASB technology. Co-digestion, which combines several waste streams, has demonstrated that it increases biogas generation and treatment efficiency overall. Hybrid systems that combine UASB reactors with aerobic post-treatment procedures have also been investigated to improve effluent quality. Furthermore, microbial community analysis has shown how to best optimize reactor settings for particular wastewater types, allowing for more effective pollutant breakdown. These developments highlight UASB reactors’ potential to increase their usefulness and applicability in various wastewater treatment scenarios [76,77].

In conclusion, the UASB reactor is a versatile and efficient technology pivotal in modern wastewater management. Its ability to handle different types of wastewater and its sustainability benefits such as biogas production and low sludge generation make it an attractive choice for municipal and industrial applications. While challenges such as slow granule formation and temperature sensitivity remain, research and technological innovations continue to enhance its performance and broaden its scope. As a cornerstone of anaerobic treatment systems, the UASB reactor holds significant promise for addressing the growing demand for sustainable wastewater solutions.

Table 3. UASB applications in different wastewater treatment.

Wastewater Type	Operational Temperature (°C)	HRT (h)	Organic Loading	Wastewater Type	References
Fish processing wastewater	-	7.2 ± 2.8	1–8	80–95	[78]
Municipal sewage sludge	16.5 ± 2	16, 24, 36	-	62–75	[79]
Glutamate-rich wastewater	35	2–48	16	90–95	[80]
Toilet wastewater	35	6	16	75.6 ± 6.0	[81]
Chocolate wastewater	15, 20, 25, and 30	6	2–6	39–94	[82]
Synthetic starch wastewater	35 ± 1	3, 6, 8, 12, 24 and 48	0.5–8	75–95	[83]
Domestic sewage	30	8.8, 9.4, 9.7	0.786, 1.376, 1.404	60–75	[84]
Heavy oil refinery wastewater	-	-	3.44	70–72	[85]
Municipal wastewater	20, 32, 20, 15, 11, and 6	48–3	-	70–90	[86]
Slaughterhouse wastewater	20, 30	1.7–9	2.5–19.5	40–67	[87]
Starch wastewater	35	24–3	1.0–8.0	81.1–98.7	[88]
Textile effluents	28	5, 10, 15	-	61.35	[89]
Poultry slaughter wastewater	25, 32.5, 40	2.30, 3.30, 4.30		95% removal of BOD5	[90]
Synthetic wastewater	25	9	0.73	81 ± 5	[91]
Textile wastewater	22–27	18	2.60	70	[92]
Domestic wastewater	14–16	9.3–6.1	-	52	[93]
Cotton textile wastewater	36–37.5	4.5 and 9.0	0.072–0.602	60 and 80	[94]
Pharmaceutical wastewater	37 ± 1	1.3/day	8	70	[95]
Leachate	-	7–2/days	0.6–2.0	76	[96]
Vinasse	-	1.8/day	5–11	93	[97]
Stillage wastewater	28–32	>200	4–5.5	55	[98]
	30 ± 2	20	10	80

overviews the various effluent treatment efficiencies of modified UASB systems.

5. Environmental and Economic Impact of UASB Reactors

5.1. Environmental Impact

Amaral et al. [99] utilized a life cycle assessment (LCA) to evaluate the environmental impact of UASB reactors used for sewage sludge treatment. The LCA was performed on a UASB reactor coupled with trickling filters and denied the biogas synthesized and subsequently utilized as the weaker link in the whole life cycle and responsible for a greater portion of greenhouse gas emissions. Amaral et al. [99] evaluated the influence of sludge disposal in the treatment process by comparing two distinct solutions. These were (1) landfill disposal and (2) agricultural reuse. The results emphasized that agricultural reuse has a significant environmental impact on the categories of (1) terrestrial eco-toxicity, (2) ozone formation, (3) freshwater eco-toxicity, (4) terrestrial acidification, and (5) human toxicity caused due to a longer transportation phase, extended sludge treatments, and a high heavy metal content [99].

Nevertheless, studies have shown contradictory results in this aspect; for example, Cañote et al. [100] compared three scenarios for biological sludge reuse from UASB reactors. They are landfilling, agricultural reuse, and further energy recovery. An LCA showed agricultural reuse allows minimal environmental impact, assuring optimal results over 70% of the categories examined. Foglia et al. [101] used the LCA approach to determine the optimal UASB reactor solution for enhancing effluent quality for irrigational reuse with minimal environmental impact. Three scenarios were evaluated, and two comprised the insertion of a polishing step with physical and chemical disinfection to remove a microbial load that was still dwelling. The remaining scenario comprised CAS replacement with UASB reactor technology following an anaerobic membrane reactor. The results demonstrated that UASB reactors and anaerobic membrane coupling were more beneficial for all categories of study.

5.2. Economic Impact

According to Freitas et al. [102], the biogas from UASB reactors can be used as a renewable energy source and play a significant role in local electricity production and environmental pollution reduction. In contrast, the existing literature suggests a wide focus on evaluating economic scalability with integration with other renewable energy sources such as wind and photovoltaic energy. Operational issues such as scum accumulation in settler components like the gas–liquid–solids separator (GLSS) and odor emission pose a considerable threat to UASB reactor performance maximization. The calorific energy content of biogas varies from 25.1 MJ Nm⁻³ to 28.7 MJ Nm⁻³, including CH₄ emissions ranging from 50% to 80%, which can be recovered for applications such as (1) electricity generation for local use and sale, (2) heat and electricity congregation, (3) boiler, kiln, or furnace fuel, and (4) automobile fuel or injection into natural gas lines [103].

Energy sustainability is one of the primary aspects that must be focused on in designing future UASB reactor systems, particularly emphasizing sludge reuse as a raw material for energy production. This is a paradigm shift to the final fate of sludge, as landfills are the common disposal option. Waste sludge from UASBs was evaluated as an energy source after dewatering. To determine the energy potential of scum, research should be conducted into scum material characterization and energy potential determination. Rosa et al. [104] characterized the energy potential of solid by-product components of sewage treatment with UASB reactors and trickling filters. Their study shows that both scum and dewatered sludge in a drying bed have considerably lower calorific values (2.0 MJ kg⁻¹) for materials characterized by a moisture content of approximately 60%. A simulation study from the same sewage treatment plant evaluated energy utilization from thermally dried and dewatered sludge and biogas. This shows the potential for the use of sludge as a dried energetic material and the elimination of rejected material generated for disposal. The synthesized biogas can be utilized in rural areas for lighting, cooking, water heating, and food refrigeration. Furthermore, biogas synthesized from UASB reactor systems can be utilized for sludge hygienization and drying.

5.3. Dissolved CH₄ Recovery from Effluent

A major portion of CH₄ (30% to 40% synthesized CH₄) is dissolved in liquid effluent and is a significant concern to the ecosystem due to the potential loss of energy. CH₄ recovered from inside GLSS of UASB reactors is appropriately managed as an energy source. A few alternatives for CH₄ reduction in anaerobic reactor effluent are proposed, such as deacidifying membranes and micro-aeration utilizing biogas [105,106]. A study by Luo et al. [107] shows high CH₄ removal efficiency (86%) using membranes to remove dissolved CH₄. Souza et al. [108] state that the considerable dissolved CH₄ and H₂S reductions in their final effluent might be due to elevated liquid turbidity. These gases can be released into a controlled environment and allow their recovery after that. A study by Gloria et al. [109] used a dissipation chamber downstream reactor in a pilot-scale UASB reactor to minimize the dissolved CH₄ concentration in the liquid effluent. The operating conditions were a 12 renews h⁻¹ and a 1.1 m free drop height, and a 73% median CH₄ recovery efficiency was achieved.

5.4. Energy Recovery Calculations from UASB Reactors

According to Estrada et al. [110], the electrical energy demand for biogas synthesis (CH₄ and H₂) in bio-electrochemical UASB reactor systems is estimated as follows:

$$W_E={\sum }_1^n(I{E}_{ap}∆t-I^2{R}_{ex}∆t)$$

(1)

$I$ is the current measure in Amperes from the voltage across the resistor, $I={\displaystyle \raisebox{1ex}{$V$}\!\left/ \!\raisebox{-1ex}{$R_{ex}$}\right.}$, $E_{ap}$ (V) is the external voltage at $∆t$ minutes (the time interval for reading made during steady conditions), and $R_{ex}=1 \mathsf{\Omega }$ the external resistance when the current $I$ is constant over time. Equation (1) can be written as

$$W_E=I{E}_{ap}∆t$$

(2)

The relative overall energy efficiency to $W_E$ is calculated from the energy for the biogas production-to-$W_E$ ratio using Equation (3):

$${\eta }_E={\displaystyle \frac{W_{{CH}_4} \mathrm{o}\mathrm{r} H_2}{W_E}}$$

(3)

According to Call and Logan [111] and Gajaraj et al. [112], when $W_{{CH}_4} \mathrm{o}\mathrm{r}{ H}_2=n{CH}_4 \mathrm{o}\mathrm{r} H_2 \mathrm{o}\mathrm{r} H_2∆H{CH}_4 \mathrm{o}\mathrm{r} H_2$ is the CH₄ or H₂ moles produced, $∆H{CH}_4=891 \mathrm{k}\mathrm{J}{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$ and $∆H{H}_2=285.83 \mathrm{k}\mathrm{J}{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$ are the heat generated from CH₄ and H₂ combustion, respectively. The ${\eta }_s$ of the substrate is as follows:

$${\eta }_S={\displaystyle \frac{W_{{CH}_4} \mathrm{o}\mathrm{r} H_2}{W_s}}$$

(4)

According to Call and Logan [102], $W_S=n_S∆H_s$ is the COD moles removed, and the heat generated from the combustion of the substrate is $∆H_S=-870.28 \mathrm{k}\mathrm{J}{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$ (in terms of acetate).

The total energy recovered (${\eta }_S+E$) for a bio-electrochemical UASB reactor is evaluated based on the electrical input and relative energy of COD removed as follows:

$${\eta }_S+E={\displaystyle \frac{W_{{CH}_4} \mathrm{o}\mathrm{r} H_2}{W_s+W_E}}\times 100$$

(5)

For conventional UASB reactors, the energy efficiency is evaluated using Equation (5) where $W_E=0$.

6. Challenges and Limitations of UASB Reactor Technology

6.1. Temperature Constraints

According to Elmitwalli et al. [113], sewage treatment in temperate climatic conditions is a challenge as municipal water is complex wastewater due to the presence of a high proportion of particulate organic matter, low strength, and moderate biodegradability. As suspended solids comprise 50–65% of the total COD, the COD conversion is considerably obstructed by particulate matter hydrolysis. When the sewage temperature decreases to ˂18 °C, the biological conversion capacity gains primary control over the overall COD removal, surpassing the existing hydrodynamics. Due to the high TSS/COD ratio and low temperature, the range where the HRT dictates the volumetric sizing of the UASB reactor, expressed as $V_r=HRT.Q$, is limited. The accumulation of non-digested sludge commences when the temperature drops in the sludge bed, and the methanogenic and hydrolytic ability of the sludge bed gradually diminishes as it causes the deterioration of soluble and particulate COD removal. This leads to eventual reactor failure. Reactor solid retention time (SRT) is the primary design criterion in UASBs, and it should be at a minimum value to retain the methanogenesis of sludge. Under tropical conditions, with diluted domestic sewage water with T ˃ 20 °C and COD ˂ 100 mgL⁻¹, this condition is satisfied. The existing SRT is dependent on sewage temperature, solid digestion rate, the decay and growth of new sludge, excess sludge withdrawal, and sludge bed filtering capacity.

According to Fukuhara [114], the minimum SRT should always be triple the biomass doubling time (T_d), which is the rate-limiting step. As bacterial growth rate exponentially correlates with the temperature, the SRT increases as the temperature decreases. Therefore, conventional designs of UASB reactors need consideration when the system is used in cold conditions and when COD exceeds 1000 mgL⁻¹. In arid-climate countries such as the Arabian Peninsula with a limited water supply, the sewage concentrations range from 1000 to 2500 mgCODL⁻¹; meanwhile, the temperate climates in areas such as Northern Africa are characterized by cold winters in mountainous regions, for example. When winter temperatures plummet down to 15 °C, the TSS/COD ratio reaches 0.6, and the sewage concentration is 2500 mgCODL⁻¹, the HRT should be increased to reach values of 20–24 h [106,107,108]. In return, this affects the hydrodynamics of the UASB reactor and causes fluctuations in influent distribution.

Furthermore, large, suspended solids can be treated in separate reactors like a primary clarifier in up-flow filtration systems linked to a sludge digester [113]. Mahmoud et al. [115] introduced the coupling of UASB reactors with sludge exchange. In this approach, accumulated solids are digested at elevated temperatures, and a return of the digested sludge flow enhances the methanogenesis.

Linking the UASB reactor to a coupled digester with sludge exchange is a novel approach [115]. With the latter system, accumulating solids will be digested at higher temperatures, whereas, in return, the digested sludge flow will increase the methanogenic activity in the reactor. Under the influence of climatic constraints, the average HRT is 12 h in a full-scale UASB reactor in the Fayoum area, south of Cairo, Egypt. Pilot studies conducted in Amman suggest that the UASB reactors are ideal as a low-cost pre-treatment method to reduce COD load while generating bioenergy. At lower temperatures, AnMBR can be used for a high SRT. The high retention rate of biomass in AnMBR can compensate for low levels of methanogenesis and biological removal in low temperatures. Nevertheless, the full-scale implementation of an AnMBR faces critical obstacles [116,117]. According to Smith et al. [118], 92 ± 5% COD removal was achieved in a bench-scale continuous stirred tank reactor (CSTR) coupled with an AnMBR in psychrophilic temperatures (15 °C). A similar observation was made at 18 °C by Gouveia et al. [63], where 87 ± 1% COD removal was achieved in a pilot UASB coupled with ultrafiltration membranes [112]. Furthermore, the utilization of expanded granular sludge bed reactors (EGSBs) with an AnMBR causes high COD removal at low temperatures. A study by Özgün [119] coupled a UASB reactor with an external ultrafiltration membrane as a solution for the cost-efficient reclamation of water at low temperatures. The ultrafiltration membrane acted as a barrier, causing ˂90% COD removal efficiency. For example, Pretel et al. [120] show that SRT and HRT are the key parameters influencing COD removal. To achieve an adequate COD removal in cold temperatures, The HRT and SRT should be increased. According to Pretel et al. [120], when the temperature is 30 °C, the HRT should be 7 h and SRT should be 12 days, and when temperature drops to 15 °C, the HRT should be 14 h and SRT should be 35 days to maintain the same COD removal efficiency. Furthermore, during 4 °C < 20 °C conditions, the HRT and SRT should be increased to maintain a good pollutant removal efficiency [121,122]. Furthermore, increasing HRT and SRT can enhance the organic matter solubility at psychrophilic conditions, which otherwise would lead to membrane fouling [123].

The above evidence suggests that increasing HRT and SRT enhances organic matter solubility and membrane fouling reduction at sub-optimal temperatures, proving that UASB design adjustments are critical to maintaining reactor efficiency. The combined evidence suggests that HRT and SRT adjustments (preferably increasing) are essential in psychrophilic climates to sustain treatment performance [123].

6.2. Restrictions Imposed by Microbiological Indicators

Compact anaerobic treatments are not sufficient in eliminating pathogens in the effluents and require post-treatment if the pathogen removal is further pursued. According to von Sperling et al. [124], small systems can use polishing ponds to enhance the microbial quality of anaerobic effluents effectively. Polishing ponds can be used to remove virtually 100% of protozoan and helminth eggs and 3–6 log units of virus and bacteria removal. Furthermore, due to high phototrophic activity and the high pH caused by it, solubilized ammonia can also be removed using algal NH₃ accumulation or volatilization [125]. When land availability is limited, compact disinfection approaches such as ozonation, UV radiation, and chlorination should be used as an alternative post-treatment for pathogen removal.

Nevertheless, during chlorination, there is a high possibility of forming disinfectant by-products due to the high concentration of residual organic matter in the effluent. By combining UASB pre-treatment with the downflow hanging sponge (DHS) system, residual organic matter is extensively aerated with cost-efficient pathogen eradication [126]. Due to the enhanced convective airflow in the DHS, the organic matter is extensively aerated. The colonization of aerobic microbes is a successful approach to eradicate colonized colloidal pathogenic microbes. Another approach is the incorporation of membranes into anaerobic municipal wastewater due to its superior pathogen counts. Nevertheless, membrane fouling is a pressing concern in this approach [127].

6.3. Odor Emissions

Odor emissions are a significant concern in UASB reactors during wastewater treatment. In order to reduce public resistance to the reactors’ operations, UASB reactors utilize extensive amounts of chemicals to mask the unpleasant smell of H₂S and other odors causing compound emissions. A defined emission source identification should be carried out, which might be connected to the sewage influent. CO₂ and CH₄ generation are directly influenced by the nature of precursors in influents, such as pH and redox potential. A biological origin is suspected of the odorous compound generation in UASB reactors. Most odor-causing compounds are amino- and sulfur-containing compounds such as mercaptans, sulfides, and amino-sulfides. H₂S, produced by the de-assimilative reduction of thiosulfates, sulfates, or sulfides, is the compound responsible for the odors in sewage influent. The selection of odor control alternatives depends on odorous gas flow and concentrations [128]. Factors such as biodegradability, the presence of human resources, relative concentrations of odorous gases (H₂S), energy recovery, treatment end goals, design aspects of gas capture and conveyance, exact location, and emission source should be thoroughly reviewed in the odor control of UASB reactors [129].

Although there are several alternatives for controlling odorous emissions, the selection criteria for the most proper alternative depends on two main criteria: gas flow and odorous gas concentration. Several factors should be considered during odor regulation in UASB reactors, such as local characteristics, including anthropomorphic resources and the biodegradability of odorous gases, and design aspects related to gas capture and conveyance, the relative concentration of H₂S in the biogas, energy recovery plans, and treatment goals [129]. According to Chernicharo et al. [130], biochemical methods, direct combustion, and biofilters offer the best solutions for treating odorous gases in UASB reactor systems. Cost-effectiveness and simplicity should be considered for the full and wide-scale implementation of these applications. H₂S can be removed from the biogas stream by thermal oxidation using methane. Nevertheless, incomplete combustion can lead to H₂SO₄ formation.

6.4. Nutrition Recycling Restrictions

During nutrient removal, the effluent quality should meet the quality standards of the receiving body. During UASB reactor treatment, N and P concentrations in the effluent might be higher than in the influent. The effluent of UASB reactors is expected to have higher N/COD and P/COD ratios than the desired values. For proper N removal, both the nitrification and denitrification processes should be complementary to the UASB reactor [131]. Therefore, UASB reactors should be used to treat effluent from no more than 50–70% of the influent. The remainder (30%) should be treated using complementary biological denitrification approaches. During concentrated sewage treatment, a combinatorial approach of methanogenesis–denitrification can be used.

Nevertheless, applications of such combinations with low-strength sewage are questionable as the UASB reactor volume is dependent on the hydraulic flow [132]. A bigger reactor volume is mandatory for effluent recycling. Studies conducted on nitrogen removal using activated sludge and biological trickling filters comprising sponge-based bacteria achieved 90% efficiency in NH₄-N removal with minimal excess sludge production [133].

Demo-scale studies conducted by Uemura and Harada show complete NH₄-N removal and 30–40% removal of total N using a sponge bed trickling filter combined with a UASB reactor. The post-treatment development of UASB effluent is directed toward four main approaches. They are (1) the simultaneous removal of nitrates and NH₃ in a tertiary aerobic–anoxic fixed-bed reactor utilizing biogas, (2) the utilization of gaseous- and liquid-phase electron donors in the anaerobic chamber for denitrification in an anaerobic–anoxic reactor combined with the nitrifying reactor, (3) the simultaneous NH₃ and nitrate removal in an intermittently aerated structured bed reactor, and (4) the anammox reaction-partial NH₃ denitrification to nitrite coupled with NH₄⁺ oxidation to N₂ [32]. According to Third et al. [134], metabolic approaches such as completely autotrophic nitrogen removal over nitrite (CANON) and Sharon–Anammox processes are mainly used to treat wastewater containing concentrated N, such as effluents from a sludge digester. The application of the anammox approach in sewage treatment is questionable. It is noteworthy to mention that N removal using anammox bacteria can be used to remove N with lower energy consumption than conventional nitrification and denitrification methods. The success of den ammonification is significantly dependent on the suppression and stimulation of nitrite-oxidizing and ammonium-oxidizing bacteria, respectively. According to Wett et al. [135], two-step approaches such as UASB–trickling filters coupled with polyurethane support media can be utilized as a promising solution for low-cost N removal. Biomass hydrolysis due to a higher SRT can be used as an auxiliary substrate in anoxic zones of sponge-enhancing heterotrophic denitrification [133]. Anoxic zones favor the proliferation of NH₄⁺ to N₂ gas-oxidizing bacteria. Biological P removal coupled with a UASB reactor is virtually impossible to achieve as the effluent of the anaerobic reactor does not comprise easily metabolizable (biodegradable) matter. At present, P removal in conjunction with UASB reactor technology occurs via chemical precipitation in the form of ammonium or iron salts [62]. The high dependence of nitrite-oxidizing and ammonium-oxidizing microbes is evident in the comprehensive study conducted by Júnior et al. [136]. Júnior et al. [136] demonstrated the N and P recovery potential in UASB reactors in a 4.1 HRT. The study recovered 0.79 gm² day⁻¹ for N and 0.12 gm² day⁻¹ for P with near-zero ammoniac N detection and ˃1.0mgL⁻¹ for P in effluent. Nevertheless, the rapid fluctuations in N and P recovery due to temperatures prevalent within UASB reactors restricts nutrient recovery. For example, the highest P recovery was possible in the 27.4–35.6 °C temperature range, and efficiency drops when the ambient temperature in the UASB decreases. Furthermore, during the study, inadequate denitrification causes N accumulation, which is not accounted for in N recovery in most studies [137]. Another study conducted by de Oliveira et al. [131] demonstrated low N and P recovery with influent N concentrations of 578–782 mgL⁻¹ and P concentrations of 476 mgL⁻¹, which remained virtually unchanged in the effluent, apart from a minor P recovery of 26.1% in Phase II. The absence of dissolved O2 and the anaerobic microenvironment prevents the nitrification and denitrification processes by microbes, which is evident from the <0.2 mgL⁻¹ NO₂⁻-N and NO₃⁻-N concentrations in effluent. A gradually increasing OLR from 0.962 CODm⁻³day⁻¹ to 4.036 kg COD m⁻³ day⁻¹ accelerates N and P loss instead of stabilizing it.

6.5. Restrictions Due to Micropollutants

Micropollutants comprise analgesics, lipid regulators, antibiotics, synthetic hormones, anti-inflammatory agents, and compounds used in personal care products and cleaning products. These compounds are used in plastics, resins, natural hormones, and pesticides [138]. Michael et al. [139] state that sewage treatment plants are hotspots of antibiotic-resistant microbial strain proliferation, evolution, and spread. Nevertheless, the inherent design of UASB reactors is incapable of micropollutant removal, especially endocrine-disrupting compounds and pharmaceuticals. Some micropollutants are hydrophilic and recalcitrant to biological degradation, and they are designed to maintain their aqueous state in wastewater. However, a study by Froehner et al. [140] shows that irrespective of treatment (aerobic or anaerobic), water-soluble compounds such as caffeine and bisphenol-A are eliminated during treatment. Hydrophobic compounds such as hormones are not eliminated during aerobic and anaerobic treatments, while HRT is a primary factor. A study by Brandt et al. [138] confirms that HRT is a primary determinant of removing fewer bio-degradable and hydrophilic compounds such as trimethoprim and sulfamethoxazole. UASB reactors are not ideal for the removal of bisphenol A, bezafibrate, nonylphenol, trimethoprim, sulfamethoxazole, and diclofenac [138].

6.6. Atmospheric CH₄ Emission Control

Biogas is produced from UASB reactors in domestic wastewater treatment and usually comprises high amounts of CH₄. A significant fraction of this CH₄ is not recovered. In municipal wastewater treatment, the biogas from UASBs comprises 10–25% N₂, 70–80% CH₄, and 5–10% CO₂ [105].

This high N₂ concentration is due to the dissolved N₂ in the effluent in the UASB escaping from the liquid during N₂ partial pressure decline. The low CO₂ concentration is caused by high hydraulic flow and CO₂ solubility. Due to the significant fractional solubilization of CH₄, the recovered CH₄ concentration is below 0.35 Nm³ kg⁻¹ COD removed, which is the stoichiometric measurement for CH₄ recovery. Excess sludge carries away a significant fraction of suspended non-methanized COD from the UASB reactor. In tropical conditions, where the temperature is around 20 °C and COD is around 1000 mgL⁻¹, the solubilized CH₄ ranges from 30% to 41%, 50% higher than the synthesized amount [139]. High rates of CH₄ loss are found at the existing hydraulic components where the partial CH₄ pressure is near the zone. This is because CH₄ loss is elevated in high-turbulence conditions. A study by Souza et al. [139] shows that even though high COD removal (70%) was achieved, only 36% of the removed COD was recovered as biogas. Less than 60% of the CH₄ was recovered as biogas in the gas chamber, with a 36–40% loss of CH₄ in the UASB reactor without recovery. The 5% remaining left the reactor from the top, in the settling zone, as waste. CH₄ loss exemplifies the potential energy loss and contributes to the greenhouse effect. According to Salomon et al. [140], a high dependence on local conditions, high capital costs in power generation applications fueled on CH₄, recovered biogas storage and distribution, the inability of small UASB reactors to engage in trading carbon credits, low economic viability, and difficulties in guaranteeing longevity in power generation from recovered biogas are among the limitations for power generation using UASB reactor technology.

6.7. Operational Obstructions

6.7.1. Low-Skilled Workforce

According to van Lier and Huibers [12], regions such as India and Latin America lack qualified operators of UASB reactors. In developing countries, even though novel investments help the construction of new UASB reactors, there needs to be a qualified workforce to operate the newly constructed UASB reactor facilities. As a result, various UASB reactor facilities might be poorly operated, especially in regard to the accurate management of the scum and excess sludge of UASB reactors. To reduce the problems linked to the irregular removal of scum and avoid unwanted solid loss in the final effluent, a firm operational routine is needed, especially for the removal of scum from the interior components of the GLSS.

6.7.2. UASB Reactor Design Flaws

Proper aeration in UASB reactor design is affected by the following: (1) The inadequate design of preliminary reactor components, allowing the entrance of large quantities of solids and sand into the reactor. This leads to improper scum removal and scum management, as well as dead zone formation [141]. (2) Using improper coating and construction materials leads to metal corrosion and concrete counterparts (3) Flow fluctuations: design flaws in the feed-pumping station might allow large quantities of rainwater to enter the reactor, ultimately affecting the operational ability of the reactor [141]. (4) The usage of inadequate sludge dewatering can lead to the improper management of excess sludge. For example, mechanized dewatering systems are troublesome for continued operation in many sewage treatment plants [141]. (5) A lack of operation control and data acquisition on gaseous composition analysis, biogas generation, and on-site flow meters for monitoring incoming wastewater quantities [8]. (6) The usage of improper hydraulics leads to the mismanagement of dissolved gas such as CH₄ and H₂S. In addition, irregular flow distribution can cause severe fluctuations in up-flow and create preferential flows, ultimately leading to sludge bed bypass [8]. (7) The incorporation of unlevelled weirs causes preferential fluctuations in settlers and leads to scum accumulation closer to high-level weirs. (8) A lack of adequate scum removal apparatuses leads to scum accumulation in the GLSS of UASB reactors [8].

6.7.3. Sludge Withdrawal

According to van Lier and Huibers [12], excess sludge removal should be conducted regularly; otherwise, it causes excessive solid loss through settlers. Failure to remove excess sludge can result in effluent quality deterioration and operational failures in post-treatment compartments like trickling filters [141]. The establishment of firm operational routines for scum removal is still not achieved in small-scale UASB reactors where there is a lack of skilled personnel [141].

6.7.4. Scum Removal

A primary operational constraint in full-scale UASB reactors is the scum accumulating inside the three-phase separators. The natural passage of gases is disrupted due to irregular scum removal and scum accumulation. This obstructs the collection of natural gases and energy recovery [142]. Different accumulation coefficients are examined for the GLSS interior and settler compartment surface. The values reported differ due to inherent differences in each UASB system and a lack of rigid methodology for scum accumulation evaluation. Scum removal has been enhanced with the design of hydrostatic removal equipment based on the water level in the GLSS.

7. Future Directions and Opportunities

7.1. Optimization and Upgrades of UASB Reactors Against Emerging Pollutants

According to Rodrigues et al. [143], UASB reactor studies on both single and coupled treatments have alternatively focused on two primary aspects: (1) the optimization of organic matter conversion into methane for energy production and (2) specific pollutant removal such as of emerging contaminants. The UASB anaerobically treats different types of wastewaters with varying HRTs depending on the operating temperature and type of matrices. Usually, industrial matrices demand a lower OLR (high HRT) than urban wastewater due to their high recalcitrance of organic matter.

Thermophilic UASB reactors operate in lower HRT conditions due to rapid kinetics, allowing them to feed a higher OLR [14]. Methane production is influenced by the matrices used. For example, Estrada-Arriaga et al. [110] obtained 0.274–0.327 NL _{CH4 g COD removed}⁻¹ from tequila vinasses by a UASB coupled with a bio-electrochemical system. Koirala et al. [144] report the treatment of urban and food wastewater in a mesophilic UASB system, obtaining 0.36 _{LCH4 g COD removed}⁻¹. Coupling the UASB process with complex treatments shows good complementarity with finishing treatments such as constructed wetlands, ponds, and aerobic biological systems. UASB reactors can be upgraded to hybrid UASB reactors by coupling with granular activated carbon (GAC) to overcome organic overloads. Furthermore, UASB reactors can be upgraded using sponge bed trickling filters, and photocatalytic and bio-electrochemical systems are promising approaches in treating complex wastewater effluents [145]. Many studies have been conducted on synthetic matrices created in laboratory settings to simulate urban or industrial wastewater. Nevertheless, this can lead to testing matrices that are incompatible with the complexity of real matrices and might not be repeatable in real case studies.

In future studies, using real matrices to confirm the results will be mandatory before the large-scale use of UASB reactors with GAC upgraded to photocatalytic or bio-electrochemical systems. The majority of studies are conducted in laboratory-scale settings with reactors of a few liters’ capacity, and studies with full- or semi-scale plants are limited. Furthermore, studies focusing on the co-digestion of two or more matrices inside UASBs are yet to be found. More attention should be given to this aspect prior to large-scale applications to treat industrial or livestock wastewater. The investment and operational costs of integrating technologies with UASBs determine the feasibility of large-scale applications of hybrid UASB technology, which are very limited in the literature. A growing concern can be seen in using UASB reactors to remove emerging contaminants such as pesticides, estrogen, antibiotics, and heavy metal compounds. There are some critical aspects regarding the overall toxicity of the effluent and the microbial load after treating these emerging pollutants using UASB reactors, and the effluent might require subsequent treatment [14].

7.2. Microbial Dynamics of Granular Sludge

Studies on the granular biomass microbiome confirm that the microbiome can alter its structure to respond to external stimuli such as pollutants, feed, and temperature [17,18]. The adaptation capacity of the microbiome in UASB reactors can be used to optimize UASB reactor operations. The research focuses on the ability of nitrogen to be re-moved in anaerobic conditions via anammox processes. Nevertheless, it is vital to under-stand the microbiological dynamics during reactor optimization and the ability to remove nitrogen in anaerobic conditions via anammox processes. The effects of antibiotic-treated granular biomass matrices and the UASB reactor’s response are under investigation. UASB reactors are effective in antibiotic-resistant gene (ARG) removal. This topic is of high concern due to the emergence of antibiotic-resistant pathogens [146]. Limited data and the promising activity of the UASB against antibiotics and ARGs demand a deep investigation into the presence and fate of antibiotics and ARGs in zootechnical environments and urban wastewater.

7.3. Ecological Footprint

A life cycle assessment (LCA) on UASB reactors focuses on the reusability of the biological granular sludge produced. The LCA dependent on technological context and is applied in a case-by-case scenario, and two primary issues are highlighted. They are (1) the eco-toxicity of the treated effluent and (2) the high emission of greenhouse gases. The first issue is directly related to the post-treatment of granular sludge. The second issue is highlighted due to biogas combustion [147]. According to Cañote et al. [100], conflicting results emerge regarding the impact of UASB reactor sludge reusability in agriculture. The results may be highly influenced by factors such as the typology of the matrix treated by the UASB, different hypotheses assumed in LCA realization, and operating conditions. There is a lack of studies comparing the recovery of granular sludge in agricultural applications with incineration. Data on the LCA of UASB reactors in wastewater treatment are minimal, and this gap needs to be filled. There is a need to evaluate different post-treatments downstream in the LCA of UASB reactors. The implementation of comparative LCAs to determine these specific processes can be used to identify best practices of UASB application with minimal environmental impact.

There is a lack of focus and a huge potential to integrate the Sustainable Development Goals (SDGs) framework in the LCA of UASB reactor technology. For example, the utilization of technologies such as phosphorous reclamation from UASB treatment presents an approach to align UASB applications with circular economy principles and contributes to SDG 12 (Responsible Consumption and Production) [148]. A simple wastewater treatment scheme comprising a UASB reactor followed by constructed wetlands is ideal for an SDG-oriented waste treatment approach due to its ease of operation and simplicity [135]. For example, Lopes et al. [146] highlight this approach in evaluating a full-scale Brazilian wastewater treatment plant comprising a UASB reactor and wetland treatment. This evaluation includes UASB reactor construction costs, which have negative implications on the gaseous emissions from the UASB reactor [148]. A careful evaluation and monitoring of the odor emissions from UASB reactors is recommended to mitigate the negative implications of public perception [139]. Various social, environmental, and economic parameters are proposed to evaluate the economic and technical suitability of UASB reactor integration with a downflow hanging sponge (DHS) for wastewater treatment [148]. According to Maharjan et al. [147], UASB-DHS integration has the highest global sustainability indicator, and the successful trade-off between socio-economic implications and environmentally friendly UAAB reactor treatment is emphasized.

8. Conclusions

UASB reactor technology has the potential to be used for sustainable wastewater treatment due to its high adaptability and efficiency across a diverse range of effluent types. Energy recovery from biogas production and reduced excess sludge generation due to biogas generation make UASB reactors economically and environmentally viable options. Nevertheless, challenges inherent to UASB reactors remain, such as temperature constraints, nutrient imbalances, and variability in effluent quality. Future design optimizations of UASB technologies should focus on circumventing reactor configuration constraints and operational limitations. Hybrid approaches such as AnMBRs and EGSBs show high potential for performance enhancements in cold-active conditions and high-OLR conditions. Strategies such as co-digestion and advanced granulation techniques can be used to expand the operational scope of UASB reactors further.

Advanced metagenomic studies should be conducted to refine the microbiological contribution to reactor performance, as microbial community dynamics play a crucial role in performance optimization. There is a need to strengthen compliance with environmental standards further via advanced CH₄ recovery, micropollutant removal, and the cost-effective removal of pathogens. It is essential to consider the integration of LCA and SDG guidelines into UASB reactor performance to ensure its sustainability in wastewater management. Nutrient recovery potential exploration and sludge reuse are the most viable approaches to sustainable UASB reactor technology in the waste-to-energy nexus. UASB reactor technology allows the wastewater management sector to meet the modern demands of industrialization and urbanization. However, continuous innovation and optimization are essential for UASB reactor technology to maintain ecological and economic resilience and their contribution to wastewater treatment.