Biological Wastewater Treatment

Abstract

Preventing environmental pollution by adequately treating the ever-increasing volume of wastewater generated by the over 8.1 billion (UN 2024 projection) people in the world, meeting governments’ often updated effluent quality standards as a result of emerging contaminants in domestic and industrial wastewater, operating wastewater treatment process to generate energy through methane production and capture to save operating costs, and deploying a compact system to fit reducing installation space are some of the daring challenges facing sustainable wastewater treatment technologies today. Hence, there is a need for continued innovation and development of treatment processes. The current chapter discussed advancements in biological wastewater treatment technologies through the years with a focus on reasons for improvements in technologies. Some of the reasons highlighted are capital and operational costs, plant volumetric capacity, effluent quality, efficient nutrient removal, biofouling and membrane clogging, treatment plant installation size, etc. The chapter also discussed biochemical oxygen demand as a measure of water quality for biological treatment systems, the role of genetically engineered microorganisms in biological wastewater treatment, bioremediation as a biological treatment process, treatment plant pilot-scale, and upgrade to full-scale.

1. Introduction

Wastewater (Figure 1), inevitably produced from domestic, industrial, agricultural, and commercial activities, mixtures from sewer and stormwater runoff [1], has been described as used water generated from the use of freshwater, reclaimed/repossessed water, drinking water, raw and saline water in a variety of applications [2] including dishwashing, laundry, showers [3], food and beverage production, boiler cooling, textile manufacture, oil refining and gas production, mining activity, and petrochemical production [4]. Wastewater usually contains chemical, physical, and biological pollutants [5, 6] and its characteristics such as pH content, heavy metal content, odor, and concentration of suspended solids, depending on its source [7, 8].

Figure 1.

Greywater, a domestic wastewater in a settling tank [1].

1.1 Sources and types of wastewater

Wastewater types can be described in terms of their source which influences the type of pollutants they contain. The common types of wastewater are explained below.

• Domestic Wastewater or Sewage: This is used water from households, commercial, institutional, and public facilities in a municipal [9], transported through a sewer [1]. It is produced from such activities as washing, bathing, cleaning, and flushing. Variants of domestic wastewater include Blackwater (produced from the combination of human waste and flushing water) and greywater (produced from washing, bathing, etc.) [10]. The major pollutants in sewage are pathogens, colloidal matter, grease and oils, nutrients, pharmaceuticals, soaps and detergents, food wastes, suspended solids, minerals [11], microplastics, microfibers, etc. [12].

• Industrial Wastewater: This is the used water generated from the commercial manufacture of a range of industrial products and commodities including food and beverage, textile, shoe and leathery products, electronics and power, metal finishing, petroleum and petrochemicals, mining minerals, and radioactive materials [4]. It is an aqueous discharge and byproduct of industrial processes that is often generated from the cleaning activities associated with these processes [4]. The characteristics, volume, biodegradability, recalcitrance to treatment, and quality of industrial wastewater vary depending on its source industry [4]. Pollutants often found in industrial wastewater include inorganic materials, heavy metals, organic materials, and nutrients including nitrogen- and phosphorus-rich compounds, asbestos, pathogens, oils, and chemicals [13].

• Cooling Water: This wastewater which may cause thermal pollution, is generated from boiler steam condensation and machinery temperature reduction activities [2].

• Leachate: Is the wastewater that is precipitated from water percolation through solid wastes in a landfill or dumpsite [2, 14]. Leachate quality and indeed, its pollutants depend on the type of landfill that generates it. For example, industrial waste landfills generate leachate [15] containing typical industrial pollutants such as heavy metals, radioactive pollutants, and inorganic nutrients [2, 15].

• Surface Runoff: This is the wastewater flow from a mixture of stormwater, rainwater, and snowmelt, which carries pollutants present on the ground along the path of flow [2].

• Urban Runoff: This is the used water generated from urban landscape irrigation and cleaning activities in cities and municipals [2].

• Return Flow: This is the wastewater flow containing pesticides, herbicide residues, animal wastes, dissolved nutrients and minerals, suspended soil, etc., from agricultural activities [2].

1.2 The need for wastewater treatment

Wastewater treatment which involves the removal/reduction of pollutants in wastewater [15], is often considered a form of water use as it is interconnected with other forms of water usage [16] and is necessary to preserve public health, prevent environmental degradation and ensure water security for the ever-growing global population (Figure 2) as wastewater is known to cause diseases such as hepatitis, cholera, cancer, reproductive defects, dysentery, and developmental disorder in humans and animals and, can lead to oxygen depletion and biodiversity reduction in natural ecosystems [18].

Figure 2.

United Nations projected data for world population from 2024 up to 2100 [17].

Some of the major reasons for treating wastewater are highlighted in the following section.

• Public Health: Untreated wastewater discharged into water bodies in the environment can have devastating health effects on living things from water use due to the presence of pollutants [16].

• Wildlife Ecosystem: Aquatic life and other creatures such as migratory birds are negatively affected when untreated wastewater is discharged directly into their habitat including oceans, rivers, marshes, beaches, and shorelines [16].

• Recreational and Tourism: Clean surface water can be a great recreational playground, sporting arena, and tourist attraction for most people where activities such as fishing, swimming, boating, yachting, boat cruising, and picnicking can be carried out [16]. Pollution by wastewater can prevent sporting activities and recreation [16].

• Economic: Water pollution will affect the economy of a place when it affects public health, prevents recreation and tourism, and affects fishing activities and aquatic life population [19].

2. Basics of biological wastewater treatment

Biological wastewater treatment systems rely on beneficial bacteria and other microbes to decompose organic pollutants in wastewater into simple substances like carbon (IV) oxide (CO₂) and water (H₂O) and generate biological floc or sludge; decomposition may occur aerobically or anaerobically [20]. These microbes might be present in the water naturally or be deliberately introduced during treatment – genetically engineered microbes (GEMs) [20]. Some of the concepts associated with biological wastewater treatment are elaborated in the section below.

2.1 Aerobic vs. anaerobic processes

• Aerobic Processes: In aerobic biological wastewater treatment, bacteria decompose organic pollutants with the aid of oxygen [20]. Reducing organic chemicals to simpler ones defines this process, which ultimately produces carbon dioxide, water, and further microbial biomass [20].

• Anaerobic Processes: Contrarily, oxygen is not utilized during anaerobic biological treatment [20]. Anaerobes hasten the decomposition of organic compounds by taking part in anaerobic digestion and related procedures [20]. Biogas is the final byproduct of decomposing stabilized organic wastes and contains carbon dioxide and methane [20].

2.2 The waste product: sewage sludge

Natural microbes decompose organic pollutants, resulting in the production of sewage sludge as a byproduct [20, 21]. Sludge from sewage treatment plants, often called semi-solid waste, contains a broad range of organic and inorganic materials. The sludge is a reflection of the microbes’ leftovers, which include bacteria and undigested organic waste [22].

2.2.1 Importance of sewage sludge

• Resource Recovery: Despite being regarded as waste, sewage sludge is a rich supply of materials that can be utilized for resource recovery, which is why it is important [23]. Due to its high organic content, anaerobic digestion offers great promise as a feedstock for producing biogas, which might be used as a thermal storage or energy source [23].

• Provision of Macro-nutrients: Sewage sludge contains numerous nutrients, including nitrogen and phosphorus [23]. One way to boost crop yields while decreasing reliance on synthetic fertilizers is to convert sewage into fertilizer [23].

• Soil Enhancement and Land Use: Biosolids from treated sewage sludge can improve soil quality in many ways [23]. This sustainable farming method improves soil structure, water retention, and plant growth [23].

2.3 Carbon dioxide release: the final stage

Microorganisms digest organic matter and produce sewage sludge, thereby releasing CO₂. The release of this gaseous byproduct marks the end of the biological treatment process, which reduces complex organic molecules to less harmful substances with less impact on the environment [24].

2.4 Impact of biological wastewater treatment

• Greenhouse Gas Emissions: The CO₂ is produced naturally by microbes, but it does contribute to the overall quantity of gases released into the atmosphere [24]. Wastewater treatment plants make great efforts to minimize their environmental impact, but the fact that they nevertheless emit carbon dioxide into the atmosphere highlights the critical need to discover solutions to lessen this effect [24].

• Carbon Footprint Considerations: The carbon footprint must be considered when evaluating the environmental friendliness of treatment plants [22, 24]. The case of biological wastewater treatment is a prime example of this. Common elements of initiatives to decrease carbon emissions include the optimization of treatment processes, the installation of energy-efficient technology, and the investigation of alternative treatment options [23].

• Energy Generation: Energy generation (Table 1) will probably be a key focus in the future of biological wastewater treatment. New methods of energy recovery from wastewater treatment and sewage sludge processing, like enhanced anaerobic digestion and biogas capture, are being developed to reduce energy consumption and increase sustainability [23].

• Treatment Process Renewable Energy Integration: Wastewater treatment plants are giving serious consideration to incorporating renewable energy sources like solar and wind power [28]. In line with the larger objectives of renewable energy adoption, these activities seek to lessen the treatment processes’ total impact on the environment [29]. It reduces operating expenses by over 9% and increases renewable energy consumption by over 5%, with thermoelectric ratio adjustment improving further [30].

		Energy (kWh/m^3)
Constituent	Typical concentrationsa (mg/L)	Maximum potential from organic oxidationb	Required to produce fertilizing elementsc	Thermal heat available for heat-pump extractiond
Organics (COD)
Total	500
Refractory	180
Suspended	80	0.31
Dissolved	100	0.39
Biodegradable	320
Suspended	175	0.67
Dissolved	145	0.56
Nitrogen
Organic	15		0.29
Ammonia	25		0.48
Phosphorus	8		0.02
Water				7.0
Totals		1.93	0.79	7.0

Table 1.

Domestic wastewater nutrient energy generation.

^a

After Tchobanoglous and Burton [25].

^b

Based on a theoretical 3.86 kWh energy production/kg COD oxidized to CO₂ and H₂O [26].

^c

Based on production energy of 19.3 kWh/kg N by Haber-Bosch Process and 2.11 kWh/kg P after Gellings and Parmenter [27].

^d

Energy associated with a 6 °C change in water temperature through heat extraction.

Source: Ref. [28].

2.5 Biological wastewater treatment and the circular economy: from waste to valuable resource

The concept of circular economy now finds resonance with the future of biological wastewater treatment. A change in perspective from seeing wastewater as a problem to a valuable resource is evident in the growing interest in recovering resources from sewage sludge, such as nutrients, and creating value-added products [31]. One example of the circular economy application is the concept of biorefinery, which involves using wastewater treatment facilities to produce several goods through integrated operations [31]. The goal of biorefinery strategies is to extract as much useful material as possible from wastewater, whether it is bioenergy or bio-based commodities. [31], in their study, removed over 70% of nutrients and provided valuable biomolecules for bioenergy production and nanoparticle synthesis.

3. Genetically engineered microorganisms (GEMs) for wastewater treatment: unveiling opportunities and navigating challenges

An integral part of environmental engineering, wastewater treatment is undergoing a paradigm shift toward greater use of state-of-the-art technology. Along these lines, GEM integration has been a promising new avenue for improving the longevity and efficiency of wastewater treatment systems [32]. Taking a deep dive into GEMs, this section explores their breadth, definition, and the complex relationship between the benefits and drawbacks of using them to treat wastewater. Genetically modified microorganisms are the result of microbiology and genetic engineering; these bacteria, fungi, and algae actively alter their genes to improve performance [32]. With their ability to increase nutrient removal, tolerate harsh environmental conditions, and augment pollutant degradation, GEMs provide a molecular canvas for tackling the complex problems of modern wastewater treatment [32].

3.1 Possible applications of GEMs to wastewater treatment

• Precision in Pollutant Degradation: GEMs enable pinpoint pollution degradation in a new way. Through the intentional insertion of genes that produce specific enzymes or metabolic pathways, these microbes can break down toxins more thoroughly and efficiently [33].

• Adaptability to Diverse Environments: GEMs excel at adapting to demanding environmental situations. Genes that provide resilience allow these microbes to adapt to varied habitats with varying temperatures, pH, and salinity. GEMs are useful wastewater treatment tools because they are versatile [32, 33].

• Enhancing Eutrophication and Nutrient Removal: GEMs can improve nutrient uptake and removal from wastewater, especially nitrogen and phosphorus [33]. An essential element in preserving the ecological balance in receiving water bodies, this focused strategy addresses eutrophication concerns [33].

• Synergistic Bioenergy Production: In addition to removing pollutants, [34] revealed that some GEMs can be engineered to produce biofuels while treating wastewater, resulting in synergistic bioenergy production. Wastewater treatment plants may now serve as centers for resource recovery and environmental responsibility thanks to this dual-purpose functioning, which is in line with worldwide efforts toward sustainable energy generation [33].

• Customized Microbial Consortia: GEMs allow for the creation of customized microbial consortiums that are optimized for certain wastewater compositions [33]. Through mutually beneficial relationships and metabolic capacity complementarity, this microbial community orchestration can improve treatment efficacy as a whole [33, 34].

3.2 Disadvantages of using GEMs to treat wastewater

• Protecting the Environment: Exploring Unknown Regions: Complex problems may arise with the discharge of GEMs into the natural environment. To guarantee environmental safety, an understanding and careful approach to GEM application are required due to concerns about unanticipated ecological repercussions, such as possible changes to native ecosystems and gene transfer to indigenous microbes [34].

• Understanding and Navigating Regulatory Frameworks: GEMs operate in a world where regulations are complicated and frameworks vary worldwide. Science, government agencies, and businesses must work together proactively to protect public and environmental interests while also encouraging innovation [34].

• Long-Term Stability and Viability: GEMs encounter a complex problem when used in real-world wastewater treatment scenarios. The effectiveness of GEM-based therapy systems over the long term might be affected by factors like predation, competition with local microorganisms, and changes in environmental circumstances [34].

• Ethical Considerations – Harmonizing Development with Conscientious Innovation: A rigorous evaluation of the limits of scientific intervention is warranted in light of the ethical concerns raised by the intentional alteration of microbial genomes [34]. If we want to see GEMs used responsibly and with social acceptance in wastewater treatment, we need to set explicit ethical standards.

4. Bioremediation of domestic and industrial wastewater: application, problems, and solutions

As a long-term, eco-friendly option for treating domestic and industrial wastewater, bioremediation – a biological method of wastewater treatment and water recovery – is gaining popularity. This section discusses the bioremediation process, some specific examples, difficulties in its implementation, and possible solutions.

4.1 Bioremediation for wastewater treatment

By harnessing the metabolic power of microbes to decompose and convert contaminants into less dangerous forms, bioremediation (Figure 3) offers a comprehensive methodology for treating wastewater. This biological intervention has been utilized in a wide variety of wastewater treatments, from simple domestic to intricate industrial processes [35].

Figure 3.

Bioremediation removes pollutants from onsite municipal wastewater using biochar through several processes [35, 36].

4.1.1 Bioremediation of domestic wastewater: examples

• Systems for Septic Tanks: One example of bioremediation in action is the use of septic tank systems in residential areas [35]. Effective treatment of wastewater, before discharge or additional treatment, is ensured by the microbial communities within septic tanks, which break down organic debris [35].

• Realized Wetlands Design: Artificial wetlands purify wastewater with plants and bacteria like natural wetlands. Microbial communities live in wetland system roots and decompose, adsorb, and precipitate pollutants [35].

• Activated Sludge Systems: Activated sludge systems are widely employed in wastewater treatment plants for municipal purposes. The aerobic bacteria in activated sludge break down organic pollutants, cleaning the wastewater before release or treatment [35].

4.1.2 Bioremediation of industrial wastewater: examples

• Textile Industry: Bioremediation has been deployed in the textile industry, which utilizes a great deal of water and produces lots of contaminants. Microbial consortia remove organic solvents, dyes, and other contaminants, a greener treatment than conventional methods [35, 37].

• Petrochemical Industry: This industry struggles with complex effluent streams containing hazardous chemicals and hydrocarbons. Bioaugmentation or the introduction of certain microbes may improve hydrocarbon breakdown and reduce the environmental impact of petrochemical wastewater [37].

• Food Processing Plants: Wastewater from these industries typically contains significant levels of nutrients and organic materials. Bioremediation methods like aerobic treatment and anaerobic digestion reduce environmental impact by promoting organic breakdown [35].

4.2 Bioremediation challenges

• Pollutant Complexity: Wastewater pollutants are complex and variable, making bioremediation difficult. Industrial effluents may require specialized microbial communities and treatment methods to remove complex chemicals, heavy metals, and other contaminants [35].

• Variable Environmental Conditions: Temperature, pH, and nutrient availability affect bioremediation efficiency. Treatment results may not be repeatable due to changes in these factors that affect the efficiency of microbial activity [37].

• Microbial Competition and Inhibition: In situations where microbial populations are diverse, inhibition and competition among microbes within the treatment system are possible [35]. The overall treatment effectiveness can be impacted by imbalances caused by pollutants that promote the growth of particular microbes [35].

• Slow Treatment Rates: Treatment rates in bioremediation techniques can be slower than in chemical or physical methods, especially when dealing with stubborn chemicals [35]. In situations where quick and effective treatment is crucial, this restriction could be a problem.

4.3 Innovative solutions and future directions

• Enhancing Biodegradation using Genetic Engineering: The use of genetic engineering can boost microbes’ biodegradation capacities. To address some of the difficulties caused by complicated wastewater compositions, microorganisms that have been engineered (GEMs) with specific enzymes or metabolic pathways can be used to target particular contaminants [37, 38].

• Developing Microbial Consortium for Bioaugmentation: To improve wastewater treatment systems, specialized microbes are purposefully introduced, a process known as bioaugmentation [30]. This process enables the optimization of treatment systems tailoring microbial consortia, which in turn addresses specific contaminants and enhances overall system resilience [30].

• Enhanced Control and Monitoring Technologies: Improvements in bioremediation process management are made possible by new developments in real-time monitoring and control systems. Insights into microbial activities are obtained by sensor technology and data analytics, enabling proactive modifications to maximize treatment efficiency [30].

• Combined Treatment Approaches: Bioremediation, when combined with additional treatment methods like enhanced oxidation techniques or physicochemical processes, provides a holistic and synergistic approach. Overcoming the limits of individual treatments, this integrated strategy can boost overall treatment performance [30].

5. Biochemical oxygen demand (BOD) in wastewater treatment

Water quality evaluation and wastewater management are crucial to aquatic ecosystem sustainability and environmental engineering. Biochemical oxygen demand (BOD) is a key indicator of biological wastewater treatment system performance and organic matter pollution [39]. This section examines biochemical oxygen demand (BOD), its importance to wastewater treatment, and its role as a key indicator of water management strategy and sustainability.

5.1 BOD: an overview

Biochemical Oxygen Demand (BOD) can estimate how much oxygen bacteria need to break down organic matter in water. BOD, which measures oxygen consumption by microbes breaking down complex organic compounds, originated in the bacterial breakdown of organic substances [39]. Organic pollutants including sewage, industrial effluents, and agricultural runoff raise wastewater BOD. Because microorganisms utilize dissolved oxygen, oxygen levels in treated effluent drop as organic pollutants are broken down [39]. If oxygen loss is substantial, aquatic animals and the environment can suffer [39].

5.2 Biological wastewater treatment efficiency by BOD measurement

• Treatment Efficiency by Procedure: The effectiveness of biological wastewater treatment can be gauged by measuring BOD. Aerobic digestion, activated sludge treatment, biofiltration, and other processes rely heavily on microorganisms to decompose organic contaminants in treatment facilities [39]. By keeping tabs on BOD levels throughout treatment, we can learn how well these biological mechanisms are working.

• Efficiency of Activated Sludge (aerobic) System: Microbial colonies in activated sludge systems in municipal wastewater treatment facilities aerobically decompose organic pollutants [40]. We can measure BOD to assess how effectively the system supports microbial growth. Operators can optimize conditions for microbial activity and alter treatment parameters accordingly by tracking BOD reductions through each stage of the process [40].

• Efficiency of Anaerobic Treatment: BOD readings are used as a measure of the overall effectiveness of the anaerobic treatment procedures that purposely reduce oxygen [39]. Anaerobic digestion produces biogas from organic matter. Anaerobic reactors that monitor BOD keep methane-producing bacteria at ease and remove organic contaminants effectively [39].

• Effectiveness of Constructed Wetlands: Using plants and bacteria in an analogous way that natural ecosystems work, artificial wetlands are utilized to treat wastewater [40]. The biological breakdown of organic compounds by measuring the BOD in the inflow and outflow of man-made wetland systems can provide knowledge of the efficiency of these systems. The success of the treatment process and microbial communities in eliminating contaminants from the water is indicated by the decrease in BOD levels [40].

5.3 BOD implications for water sustainability

• Effects on the Environment and the State of the Ecosystem: The environmental impacts of water bodies with high BOD levels is substantial. The degradation of aquatic environments by dissolved oxygen depletion impacts fish populations, biodiversity, and ecological equilibrium. Sustainable water management requires monitoring BOD levels to preserve healthy aquatic habitats [40].

• Regulatory Compliance and Water Quality Standards: As an important criterion for wastewater discharge permits, BOD limitations are included in numerous regulatory frameworks globally [39]. Businesses and cities must follow these rules. To preserve water quality and lessen environmental impacts, sustainable water management goes beyond what is required by regulations [39].

• BOD as a Pollution Source Indicator: One diagnostic tool for locating the origins of water pollution is BOD analysis. The presence of organic contaminants can be identified by analyzing variations in BOD profiles [39, 40]. This allows for the implementation of targeted actions to reduce pollution where it is most prevalent. Proactively identifying and treating pollution causes is essential for sustainable water management [40].

• Using BOD in Representing Water Quality: To mimic the process of organic pollutant breakdown in treatment systems, sophisticated water quality models use BOD data [41]. The goal of these models is to help decision-makers develop sustainable water management plans by projecting future BOD levels under different scenarios [41]. Stakeholders can develop effective policies and procedures by comprehending the possible effects of different elements on BOD.

5.4 Difficulty in measuring and interpreting BOD

• Changes with the Seasons and Other Factors: Temperature, nutrient variations, and seasonal fluctuations are some of the elements that can influence the variability in BOD values [39]. To make educated decisions about water management based on accurate interpretations, it is crucial to understand these variances.

• Non-specificity of BOD: Although BOD gives a general idea of how much organic pollution there is, it does not break it down into kinds. Because of this lack of specificity, it is difficult to identify which contaminants may necessitate tailored treatment approaches [39, 41].

• Time and Resource Intensiveness: The time-consuming process of traditional BOD determination methods requires samples to be incubated for many days. Consistent monitoring is essential, but it might be difficult for smaller treatment centers to allocate the necessary resources [39].

• Emerging Contaminants and BOD Limitations: The proliferation of new pollutants, such as medications and personal care items, poses problems that BOD measurements cannot completely solve [39]. While these compounds might not have a major impact on BOD, they can still harm aquatic ecosystems, which is why we need other ways to measure their impact [39].

5.5 Innovations and future directions in BOD analysis

• Sensor Technologies and Real-time Monitoring: Technological advancements in sensors have made it possible to track BOD levels in real time. More adaptive and responsive water management strategies are made possible by the continuous data provided by online sensors, which allow for rapid responses to BOD changes [40, 41].

• Molecular Methods and Biosensors: Rapid and specific BOD measurements are made possible by biosensors and molecular methods, which target key microbial markers [42]. These advancements in technology allow for more precise and efficient BOD readings, which in turn shed light on the microbiological processes at work in wastewater [42].

• Incorporating AI: Improving BOD prediction models is an area where artificial intelligence (AI) shows enormous potential. Machine learning algorithms are capable of sifting through massive datasets, finding patterns, and making condition-specific predictions on BOD levels. Proactive decision-making in water management is facilitated by this integration, which also improves the precision of forecasts [40, 42].

6. Wastewater treatment plants and sewage sludge systems

If the ever-increasing global wastewater generation in different regions (in billion m³) (Figure 4) and the large yearly per capital production (in m³/capita/year) (Figure 5), are to be reused or discharged safely into the environment without harmful, detrimental health effects, then wastewater must be treated to remove pollutants. This can be achieved in specialized process plants referred to as Wastewater Treatment Plants (WWTPs).

Figure 4.

Global regional wastewater generation projection to 2050 [43].

Figure 5.

Yearly global regional wastewater generation per capita [43].

Wastewater treatment plants are facilities that receive and treat domestic and industrial wastewater by removing contaminants, such as coarse debris, grease and oils, sand, organic and inorganic pollutants, and biodegradable materials, in wastewater [44]. When these plants treat wastewater, they produce a mud-like, slurry residue or sewage sludge as the byproduct of the treatment process due to biomass activities of feeding on organic pollutants in the wastewater [44]; hence, treatment plants are also sewage sludge systems.

The sludge produced during the wastewater treatment process contains pollutants like heavy metals, pathogens like viruses, and a large mass of bacteria that can stabilize organic matter in wastewater using aeration, hence the term “activated sludge” [44, 45]. The sludge also contains valuable nutrients like phosphorus and nitrogen that have important agricultural applications in being used as farmland improver and fertilizer [44].

6.1 Wastewater treatment plant operations

The wastewater treatment plant facility undergoes different processes/stages of operation (Figure 6) to achieve an acceptable level of wastewater treatment suitable for reuse and discharge to the environment. These stages of operation, which include preliminary treatment, primary, secondary, and tertiary treatments are described below.

• Preliminary Treatment Operation: This is aimed at separating suspended, gross solids such as large plastics, wood logs, fabric, and bottles, excess grease or oil, and grit from raw wastewater from the sewer by channeling it through a trough and moving screen or by cutting the large, foreign objects in small pieces with a comminutor or a macerator and then removing them [44, 46].

• Primary Treatment Operation: This is a physicochemical wastewater treatment operation that involves allowing relatively smaller suspended and settleable solids to precipitate out of the water by gravity settling and sedimentation, mechanical flocculation, or chemical coagulation [44, 46]. Here, further screening and comminution of smaller solid materials from the preliminary operation can be carried out (Figure 7) [47]. This operation is aimed at reducing the strength and biochemical oxygen demand (BOD) of organic matter in wastewater, neutralizing acidic/alkaline wastewater by the addition of appropriate chemicals, removing leftover grease and oil from preliminary treatment, and eliminating volatile pollutants [44]. Also, wastewater equalization, which involves making wastewater effluent homogenous by using mechanical paddles or aeration can be carried out at this stage [46].

• Secondary Treatment Operation: The secondary treatment operation is a biological wastewater treatment that removes dissolved, organic matter or colloids in wastewater when microorganisms consume organic pollutants in an aeration tank (Figure 7), producing CO₂ and water H₂O in the process [46, 47]. This operation, which may be an aerobic or anaerobic process, is aimed at removing biodegradable materials and significantly reducing BOD, phenols, and oils in wastewater [46].

• Tertiary Treatment Operation: This wastewater treatment operation is aimed at removing non-biodegradable materials, minerals (such as the hydrated salts of nitrogen and phosphorus known to cause eutrophication in surface water), and heavy metals left in wastewater after the primary and secondary treatment operations [46]. The tertiary treatment is necessary to provide final polished, high-standard effluent before reuse or discharge [48]. This treatment operation can be carried out through ion exchange, reverse osmosis, electrodialysis, adsorption, evaporation, chlorination, and electrolytic recovery processes [46]. The main operations in tertiary treatment are filtering and backwash operations (Figure 8) in which secondary effluent is allowed to flow through a filter bed of anthracite, gravel, or sand, and, then passed through a trough containing alum to filter out finely suspended solids not previously removed [47, 49]. The backwash operation involved removing entrained floc from the filters by allowing filtered water to flow back through the filter media and be recycled [49]. Disinfection of wastewater is achieved by chlorination where chlorine is dozed into the treated effluent to kill any pathogen present and the effluent is dechlorinated to remove excess concentration of injected chlorine in a final step before discharge to the environment [49].

Figure 6.

Wastewater treatment processes [46].

Figure 7.

Primary and secondary operations of wastewater treatment [47].

Figure 8.

Tertiary operation of wastewater treatment [47].

6.2 Biological wastewater treatment technologies

The secondary wastewater treatment process which uses microorganisms and aeration to convert organic pollutants to simple substances, is a biological treatment technology. The four (4) main forms of biological treatment technologies are the Oxidation Pond, Trickling Filter, Rotating Biological Contactor, and Activated Sludge.

• Oxidation Pond: This large, usually shallow pond relies on the interaction between bacteria, algae, and sunlight to treat wastewater. Also called stabilization ponds or lagoons, oxidation ponds, provide an arena for algal growth by using sunlight’s energy, CO₂, and inorganic compounds released by biomass in the systems and, the algae in turn, release oxygen (O₂) required by aerobic bacteria to feed on organic matter in wastewater [47]. Oxidation pond designs are made compact by installing external aerators (mechanical air diffusers) in the pond [47]. Dredging, and gravity settling, filtration, or chemical treatment are used to rid the pond of sludge deposits and leftover algae respectively [47].

• Trickling Filter: This biological wastewater treatment technology operates by continuously spraying wastewater over a bed of stone in a tank and collecting the trickling water at the bottom of the tank from where it is sent to a clarifying tank [47]. Microorganisms on the surface of the stone bed consume organic matter in the wastewater as it flows thus, reducing its BOD and treating it [47]. The circulating air passing through the spaces between the stones provides aeration for the microbial activity of feeding on organic matter in the wastewater [47]. The performance of the trickling filter can be optimized by arranging two or more filters in series and allowing treated effluent to recirculate through them [47].

• Rotating Biological Contactor: In this technology, a layer of microbes growing on a sequence of large, flat, plastic disks (in contact with the wastewater to be treated) mounted on a rotating horizontal shaft, consumes the organic matter in the water as each disk in turn, get exposed to atmospheric air due to shaft rotation [47].

• Activated Sludge: Of the biological wastewater treatment technologies discussed so far, the activated sludge technology is the most globally applied and effective [45]. A typical design consists of a primary aeration or mixing tank and a secondary settling tank or clarifier for recycling returned sludge (RAS) for a continuous process and increased efficiency [47]. Wastewater from primary treatment is mixed with fresh sludge and RAS and, aeration from an external air compressor is supplied to the mixture from porous diffusers at the bottom of the mixing tank [47]. Aeration provides oxygen for microorganisms in the sludge which feed on organic materials in wastewater, reducing its BOD and forming a mass of biological floc called Activated Sludge [47]. The treated effluent then flows into the secondary settling tank where clear water is separated from the activated sludge which is returned to the system (RAS) or removed as surplus activated sludge (SAS) [47].

6.3 Developments in biological wastewater treatment process and emerging technologies

Biological wastewater treatment technologies have evolved over the years as industries and households continue to generate large volumes of wastewater and new pollutants are discovered, prompting various national governments to update wastewater effluent discharge quality standards for the protection of human health and the environment [47, 50]. Technologies have also been upgraded for sustainability via energy production and waste resource recovery for a circular economy process [50]. The advancement in treatment technology has also helped to mitigate the effects of future challenges such as increased wastewater generation from an increasing global population, old and aging treatment facilities, limiting plant installation spaces, and climate change, on the quantity of treated wastewater [47].

6.3.1 The stirred tank reactor technology

The activated sludge wastewater treatment technology is an ideal process for treating industrial wastewater in a stirred tank (agitated vessel) seeded with the biological floc (sludge) and aeration at high pressure to supply adequate dissolved oxygen [50]. The stirred tank activated-sludge wastewater treatment technology is one of the oldest biological treatment processes first used at the Davyhulme Wastewater Treatment Facility in the UK between 1912 and 1914 [45, 51, 52]. A typical system design usually referred to as the conventional activated sludge (CAS), consisted of a single mixing tank (agitated vessel), a settling tank (sedimentation tank), and a sludge recycle line (RAS line) (Figure 9) [45, 51, 52]. This arrangement allows raw, influent wastewater to be treated and effluent to be separated from the floc (activated sludge) which is recycled or sent to sludge management as waste.

Figure 9.

Conventional activated sludge process [51].

The capacity of the agitated vessel in an activated sludge process is usually large and since oxygen dissolves very slowly in an aqueous solution, a major limitation of the industrial application of the activated sludge process is the high cost of operation arising from operating and maintaining high capacity air compressors [50]. Step aeration technique can be used to improve the performance of the activated sludge process whereby the process is modified by installing several small-sized, cascading stirred tanks in sequence (The Bardenpho Scheme) instead of a single large tank; the total volume of the small tanks being equal to the volume of the large tank [50]. This enhances higher BOD reduction, denitrification, and phosphorus elimination from wastewater [50]. Higher process performance can also be achieved through a sequential-parallel arrangement of the small tanks.

6.3.1.1 Design and performance characteristics of the activated sludge process

The stirred tank activated-sludge process is designed based on factors such as characteristics of wastewater, government regulation, and treatment objectives [53]. The process design characteristics include:

• Dimension and capacity of the stirred (agitated) tank.

• Transfer system and oxygen requirement of the aeration process.

• Dimension and capacity of the settling tank.

• Design of the sludge return system.

• Design of the waste sludge withdrawal system.

• Mixing pattern of the wastewater and sludge (formation of mixed liquor suspended solid-MLSS).

The process performance of a stirred tank activated-sludge system can be hinged upon various features some of which are:

• Contaminant Reduction: The activated sludge system can reduce up to 99% BOD in secondary treatment and can achieve a high reduction of pathogens in wastewater.

• Loading Rate: The system provides treatment flexibility by operating over a range on hydraulic and organic loading rates.

• Resilience: The system is robust under varying operating conditions as it can withstand different hydraulic and organic shock loads.

• Effluent Quality: Permissible discharge standards of wastewater effluent can be achieved using the activated sludge system.

• Installation Space: The system requires a smaller installation space compared to the system such as a stabilization pond [54].

• Nutrient Elimination: The system is ideal for high removal of nitrogen and phosphorus. Also, the nitrification–denitrification process can be achieved by upgrading with an anoxic chamber inside the aeration vessel [55].

Improvements on the stirred tank activated-sludge system include systems such as the anaerobic-anoxic, oxygen ditch, and sequential batch reactor technologies.

6.3.1.2 Anaerobic-anoxic-oxic (A2O) technology

This activated sludge technique is an improvement on the design of the CAS as it allows easy elimination of phosphorus and nitrogen nutrients in wastewater. Its arrangement (Figure 10) consists of an anoxic tank where a Modified Ludzack-Ettinger (MLE) process takes place for denitrification, an anaerobic tank for phosphorus removal by absorption, and an aerobic tank (oxic) for organic pollutant decomposition [51, 56].

Figure 10.

Typical arrangement of the A2O-activated sludge technology [51].

6.3.1.3 Oxidation ditch

This is an improvement over CAS where highly efficient rotating biological contactors (RBCs) in the ditch (Figure 11) increase surface area and dissolved oxygen available for microbial growth and activities, creating agitation and mixing within the ditch as wastewater flows from a bar screen, leading to effective treatment process [51].

Figure 11.

The oxidation ditch process [51].

6.3.1.4 Sequential batch reactor (SBR) technology

The SBR technology has the advantages of lower treatment area and cheaper installation cost over the CAS because wastewater equalization, primary/secondary clarification, and microbial treatment take place in a single reactor unit (batch operation) [57]. The SBR operation, commonly referred to as a “fill-and-draw” process, first used a single unit, variable-volume, batch flow system for wastewater treatment, but operational challenges forced an upgrade to a double unit, fixed-volume, continuous flow system with one unit used for aeration and the other for settling [58].

Further upgrades to the SBR technology occurred in the 1950s and 1970s when a continuous-feed batch treatment was incorporated in a variable-volume system (by Pasveer and co-researchers) and in the USA and Australia, with the EPA’s grant; the EPA’s publication of SBR design manual between 1982 and 1992 led to the global application of the technology [58]. Today, the SBR process consists of a series of tank units, each operated as a batch reactor. A typical treatment cycle goes through five (5) stages such as fill, react, settle, draw, and idle (Figure 12).

Figure 12.

Stages of the SBR process [58].

Modifications of the SBR technology such as the sequencing batch biofilm reactor (SBBR), and the anaerobic sequencing batch biofilm reactor (ASBBR) have recently been developed to mitigate some challenges of the SBR technology including excessive sludge production and high sludge-volume index [57].

6.3.2 Membrane technology

The membrane bioreactor (MBR) technology (Figure 13), which is ideal for treating domestic and industrial wastewater, was developed in 1969 under the Dorr-Oliver Scheme by Smith and co-researchers who combined biological treatment (in a bioreactor) with a membrane filtration process – ultrafiltration process (microfiltration is an alternative), instead of the settling tank used in activated sludge system [60]. While the decomposition of organics occurs in the bioreactor, separation of floc from treated water occurs on the membrane (with different pore sizes), this allowed for maximum sludge separation from the treated water than in CAS [60]. Limitations such as membrane fouling that causes clogging, and huge energy costs from membrane scouring, led to the upgrade of the original MBR configuration in 1989 by Yamamoto and other researchers, where a hollow fiber membrane and suction pump (rather than a pressure pump) were placed in the bioreactor where the membrane is immersed [60]. The acceptability of the MBR process increased since the 1990s mainly due to the advancement brought about by the process upgrade (immersion configuration), which provided permeates of high quality and reduced capital costs and clogging [60]. Future MBR process upgrades can be achieved by expanding fiber membrane applications.

Figure 13.

The MBR process [59].

6.3.2.1 Design and performance characteristics of the MBR process

Some of the design characteristics of the MBR technology are elaborated in the section below.

• Activated Sludge Process Design: Activated sludge process design is an integral part of the MBR technology in combination with the membrane filtration process. The design of the activated sludge process depends on microbial growth considerations, oxygen supply, and dimension of the bioreactor [50].

• Membrane Material Selection: Polymer or ceramic material is selected for membrane material based on wastewater characteristics.

• Flux Rate of Membrane: Maintaining performance of membrane by balancing the filtration rate.

• Hydraulic Configuration: Hydraulic mechanism design of MBR systems which can be side-streamed or submerged.

• Energy Optimization: Energy optimization during filtration and aeration is a key aspect of MBR operation.

• Cleaning System: The type of cleaning mechanism including air scouring or backwashing should be considered during the design of the MBR system [61].

MBR process performance characteristics include:

• System Footprint: Since a separate settling tank is not required, the operation of the MBR system has a smaller footprint (space) than CAS.

• High Energy Requirement: Membrane scouring process in MBR operation increases the energy requirement for operation and is mitigated by developing membrane with biological and chemical resistance, fouling control mechanisms, and new configuration.

• Quality of Effluent: MBR operation produces non-potable effluent of high quality, which can be treated further for potable water recovery.

• Process Fouling: organic matter from microbial cells is deposited on the membrane during operation. This can be mitigated by selecting new materials and developing new configurations [62].

6.3.3 Biofilm technology

6.3.3.1 Moving-bed biofilm reactor (MBBR) technology

This technology was developed between 1988 and 1990 by Hallvard Odegaard [63] due to the challenges of earlier wastewater treatment reactors including biofouling (MBR), uneven biofilm distribution, and hydraulic instability (biofilm reactors) [64]; malodor pollution and expensive operational costs (trickling filters); mechanical failure, high maintenance, and large area requirement (rotating biological contactor) [65]. In an MBBR process (Figure 14), biofilm for wastewater treatment grows on movable carriers (biocarriers) in the aeration tank (oxic zone) and is separated from treated water in the settling tank [65, 67]. These biocarriers come in different shapes, surface areas, and configurations, and while the common configuration is a hollow cylinder, surface area size is very crucial to biofilm formation – a large surface area provides intimate contact for bacteria, wastewater, nutrients, and oxygen in the aeration tank [64, 65]. Operational challenges of the original MBBR system such as inappropriate/slow biofilm growth rate caused by increased solid retention time (SRT), high biofouling, and reduced mass transfer [68], led to the development of a hybrid MBBR system– the moving-bed biofilm membrane reactor (MBBMR), around the mid-1990s [63, 69, 70], which separates biofilm from treated water using a membrane instead of a settling tank [67]. This advanced MBBR system had advantages such as low sludge generation, improved operational efficiency, and reduced biofouling [68]. MBBR system’s performance can be evaluated on factors such as biocarrier configuration and design, flexibility of filling fraction (biocarrier size) according to system requirements, biofilm development (whether thick or thin) based on the nature of biocarrier and which affects hydrodynamics of wastewater particles, uniform dissolved oxygen (DO) or aeration supply based on reactor design [65], and hydraulic retention time [71]. Pilot-scale MBBR operations are essential to assess full-scale process cost, scaling factor, feasibility, time, and unforeseen treatment results [72]. Some of the desirable qualities of a good biocarrier are light density, non-biodegradable, high resistance to abrasion, insolubility, microbial nontoxicity, and high effective surface area [65, 73]. It is obvious that biocarriers are essential and very critical to the effectiveness and performance of MBBR operations, future improvements should focus on the configuration, geometry, and design of these biofilm carriers.

Figure 14.

The MBBR process. WSS = Waste Solid Sludge [66].

6.3.3.2 Design and performance characteristics of MBBR process

The MBBR design is an hybrid configuration of biofilter and activated sludge processes for efficient wastewater treatment. The design characteristics are as follows.

• Hybridization Design: A free floating biofilter media for biomass concentration is incorporated into the activated sludge system.

• Scalability: The MBBR technology is designed for easy scale-up and modification.

• Biofilter Design: The configuration and design area of the biocarrier are important aspects of MBBR design.

• Biofilter Mixing: The process is designed for effective biocarrier mixing to distribute biofilm evenly through the CAS system.

• Aeration Supply: The system supports adequate oxygen supply [65].

MBBR process performance can be affected by characteristics such as:

• Biofilter Surface Area: The MBBR process relies on biofilm formation on the biofilter for wastewater treatment. This surface area affects biofilm growth as a larger area enables large bacteria colonies.

• Nutrient Concentration: The process is ideal for high organic (carbonaceous) and ammonia (nitrogenous) wastewater nutrient removal.

• Process Hydraulic Retention Time: The process supports high influent retention time for effective treatment.

• Adequate Process Conditions: The MBBR process can support optimal temperature and pH conditions [65].

6.3.3.3 Integrated fixed-film activated sludge (IFAS) technology

The IFAS technology is a hybridization of the MBBR and CAS technologies, an advancement over MBBR by incorporating suspended and attached growth systems [74] and the most recently invented biological wastewater treatment process having been developed around the early 2000s [75, 76]. This hybrid system involves adding a fixed microbial growth media (MBBR technique) to an activated sludge (AS) tank (CAS technique) to provide a biofilm growth surface and enhance nitrogen elimination by the oxidation process, respectively [75, 76]. This technology (Figure 15) allows the suspended-growth microorganisms of CAS and biofilm bacteria to be combined in a single reactor unit, allowing two different kinds of microbes (aerobes and anaerobes) to work in tandem; while the mixed liquor suspended solid (MLSS) biomass (aerobes) decomposes organic wastewater load, the biofilm (anaerobes) sets up a strong nitrifying bacteria to oxidize nitrogenous load [76]. The IFAS technology offers some advantages over traditional MBBR systems including a larger surface area of media for biofilm growth that provides higher efficiency in eliminating phosphorus and nitrogen, higher biomass concentration, organic load, and treatment capacities, more energy efficiency, and less capital cost [77]; although the movable MBBR system’s media prevents clogging, promoting diverse bacteria growth, the system is also more compact, and resistant to organic shock load [77]. Over CAS technology, the IFAS system’s advantage includes complete nitrification, reduced footprint, longer solids retention time, enhanced anthropogenic composite, and nutrient removal [74]. Pilot- and full-scale IFAS systems’ performance can evaluated on factors such as hydraulic retention time (HRT), solid retention time (SRT), biocarrier media, media filling ratio, and dissolve oxygen concentration (DO) [74]. Recent upgrades to the IFAS process incorporated systems for energy generation, methane production, and capturing [74].

Figure 15.

The IFAS process. M = Stirrer motor [77].

6.3.3.4 Design and performance characteristics of the IFAS system

The IFAS system is mainly characterized by the design of a hybrid suspended and attached growth process where a biofilm carrier surface is attached to the activated sludge suspended-growth system to enhance treatment efficiency [78]. The performance of the IFAS system is centered around its operational efficiency, removing above 93% of chemical oxygen demand (COD) in industrial wastewater and, microbial synergy, where suspended- and attached-growth microbes are involved in the treatment process [78].

6.3.3.5 Downflow stationary/fixed-film bioreactor (DSFF) technology

This technology (Figure 16), which has been in operation for several decades, is ideal for treating and purifying industrial wastewater by allowing the downward flow of the influent wastewater through a bed of biofilm, causing microorganisms to break down organic matter in the wastewater [78]. This has the advantage of moving wastewater by gravity rather than mechanical agitation, thereby reducing operational costs and ensuring efficient dilution of waste matter in the reactor [78]. Some of the limitations of the DSFF reactor include optimizing biofilter surface area, choosing an appropriate material for film support, the effect of reactor height on its temperature gradient, and wastewater load fluctuations [80].

Figure 16.

Schematic diagram of a typical DSFF process [79].

The DSFF technology is an efficient biological treatment technology, and researchers have continued to focus on its development. Future developmental improvements are in the direction of improving biofilm media, developing novel strategies for biofilm attachment, adopting liquid phase oxygen technology for system aeration, and using alternative reactor configuration [81].

6.3.3.6 Design and performance characteristics of the DSFF system

The design characteristics of the DSFF system are as follows.

• Support Media: The system is designed to utilize a support medium including polymer beads, activated carbon particles, or silica granules, for bacteria colonies [50].

• Wastewater Flow Pattern: The downward flow design of the DSFF system allows the dilution of waste as it enters the system [80].

DSFF process performance can be associated with the following characteristics.

• Film Surface-to-Volume Ratio: Optimizing biofilter surface area to biofilm volume is key to the efficient performance of the treatment process.

• Reactor Height and Process Temperature: High bioreactors may have the challenge of distributing substrate uniformly and temperature fluctuations throughout the height of the reactor.

• Biofilm Support Media: The performance of the DSFF system depends on the type of support media selected for the process [50, 80].

6.3.3.7 Packed-bed biofilm reactor (PBBR) technology

This system creates a biofilm for wastewater treatment by filling a packed-bed bioreactor with supporting materials – usually cylindrical glass carriers (Figure 17), so that microorganisms attached to the surfaces of these carriers metabolize pollutants in wastewater [83] as they flow through the bed [81]. Recent applications of this technology (a typical example is the trickle-bed biofilm reactor process) have combined biofilm aggregate on carriers with membrane technology processes like ultrafiltration [50, 83]. Although the packed-bed biofilm wastewater treatment technology is an advanced treatment process suitable for industrial wastewater, it has limitations, some of which include restriction of oxygen and wastewater nutrients from the bulk liquid to the biofilm, reduced flowrates due to media clog by organic matter, biofilm detachment, channeling, biofilm susceptibility and slow response to shock loads, and scale-up complexities [83].

Figure 17.

The PBBR wastewater treatment process [82].

6.3.3.8 Design and performance characteristics of the PBBR system

The design attributes of a PBBR process include reactor geometry and configuration, which accounts for the surface property, size, and shape of the bioreactor and can allow maximum nutrient removal without accumulation; media configuration including its surface property, shape, elasticity, size, mass, and absolute size; design of wastewater loading pattern and media remover/cleaning mechanism [84]. PBBR process efficiency is characterized by factors such as optimal biofilm thickness which provides sufficient surface area for microbial activity without clogging, good quality support media, appropriate hydraulic retention time, and adequate aeration through the bulk liquid to the biofilm [84].

6.3.3.9 Fluidized-bed biofilm reactor (FBBR) technology

This technology is a recent advancement in the biofilm wastewater treatment process and operates by suspending support media (biocarriers) in the reactor with the force of upward-flowing feed wastewater (Figure 18) [50]. The FBBR process is applicable for industrial installations and large treatment facilities due to its high fluid flowrates, usually higher than the smallest fluidization velocity but lower than the biofilm aggregates terminal free velocities in the reactor; this causes a suspension (fluidization) of the biofilm aggregates [50]. One advantage of this process is that there is no channeling as biofilm aggregates are surrounded by the wastewater, which improves intimate contact (between bacteria and wastewater organic pollutants) and reactor performance [50]. Also, the process can handle higher wastewater throughput as a result of effective bacteria retention and produces less sludge. Limitations of the FBBR technology include high operating costs from keeping media fluidized by aeration, media fouling, limited nutrient removal, biofilm detachment, scale-up complexities, and other than industrial wastewater with high organic pollutants, it is inadequate for treating other types of wastewater [50].

Figure 18.

The FBBR wastewater treatment process [85].

6.3.3.10 Design and performance characteristics of the FBBR system

A notable design characteristic of the FBBR system is its compact design, which makes it suitable for isolated and localized wastewater treatment applications [86]. Additionally, the diameter of the media and bioparticle, substrate penetration depth, and internal/external mass transfer coefficients are other important design attributes [86]. The process performance can be attributed to characteristics that include efficient mixing and mass transfer, significantly high wastewater throughput and up-flow velocity, optimal pH and temperature, media porosity, and surface area (a balance between these can achieve efficient treatment), and pollutant loading rate [86, 87].

The semi-fluidized-bed (SFBBR) and inverted fluidized-bed (IFBBR) biofilm wastewater treatment processes are similar to the FBBR process as they both utilize suspended media particles (fluidization) for microbial growth required for wastewater treatment, but while the particles are partially suspended in SFBBR, they are expanded in IFBBR [85, 86, 87]. While the FBBR process focuses on biomass holdup (concentration), the SFBBR process emphasizes efficient mass transfer and optimal mixing, and the IFBBR process focuses on faster process startup [88]. The major limitation of the SFBBR process is the limited biomass concentration (retention) in the reactor while optimizing startup time, which is a major problem of the IFBBR process [89].

6.3.3.11 The sludge blanket biofilm reactor (SBBR) technology

This technology utilizes sludge granules (microbial colonies) formed from bacterial cells on support media (biofilm) and the accumulated biomass layer (sludge blanket) at the bottom of the reactor for wastewater treatment [81]. Wastewater passes through the sludge blanket and the biofilm simultaneously for enhanced treatment by suspended growth and biofilm-based microbes, respectively. The SBBR treatment process is similar in design to the up-flow anaerobic sludge blanket process (UASB) (Figure 19) however in UASB, treatment is carried by the suspended-growth microorganisms and this is done without the supply of oxygen [50, 81]. Some of the limitations of the SBBR process include high operational costs from poor settling of solid pollutants in the sludge blanket and high sludge production, high levels of toxic substance accumulations with the blanket, and engineering design and operation challenges [91].

Figure 19.

The UASB process has a similar design to the SBBR [90].

6.3.3.12 Design and performance characteristics of the SBBR system

The characteristics of SBBR design that contribute to its performance include the reactor configuration, biofilm support media design material, configuration, etc. Attributes such as optimal process pH and temperature, microbial interactions, and sludge settling properties affect SBBR system performance [81, 91].

A comparison of the biological wastewater treatment technologies (especially the stirred tank and biofilm technologies) discussed so far in this chapter is provided in Table 2, including their merits and limitations. Other emerging biological wastewater treatment technologies for sustainable water treatment and energy generation, some of which do not incorporate the use of bacteria colonies, are phytoremediation, which uses specific plant species to remove wastewater pollutants, and algal-based treatment, which uses algal organisms to metabolize and remove wastewater pollutants; also, the microbial electrochemical system (MES) generates electrical energy from microbial metabolic activities during wastewater treatment [92, 93, 94]. These technologies have proven to lessen the impact of wastewater treatment on the environment [92].

Bioreactor	Merits	Limitations
Stirred tank	Simple in construction and operation. Uses suspended growth of microbes. Suitable for aerobic and anaerobic processes.	Restricted to low capacities.
Trickle-bed biofilm reactor	Use of attached growth of microbes. Low operating cost due to down flow mode of operation. High cell mass concentration in biofilm promotes rate of bioconversion.	Mainly for aerobic BOD removal. Low capacity due to low feed flow rate maintained.
Moving-bed biofilm reactor (slurry reactor)	Heterogeneous version of stirred tank. High cell concentration in biofilm promotes rate of bioconversion.	Capacity wise inferior to column reactors, Biofilm could get disturbed due to high rate of agitation.
Fluidized-bed biofilm reactor	Operates at high capacities, provides high degree of bioconversion. Once fully fluidized, pressured drop across the bed remains constant and does not increase with increase in feed flow rate. Degree of bioconversion increases with increase in feed flow rate due to bed expansion.	Entrainment loss of particle-biofilm aggregates possible. Operating cost higher than trickle bed (packed bed).
Semifluidized bed biofilm reactor	Higher degree of bioconversion (than fluidized beds) at higher capacities and low reactor volume requirement. Degree of bioconversion increases with increase in feed flow rate, even if reactor volume is kept constant.	Higher operating cost than fluidized beds. Continuous, circulating mode of operation not possible.
Inverse fluidized biofilm reactor	Low operating cost due to down flow mode of operation. Larger-sized particles could be used. Reasonably large degree of bioconversion.	Lower capacity than fluidized/semifluidized bed. Larger reactor volume requirement.
DSFF bioreactor	Simple in construction and operation. No support particles required. Low operating cost due to downflow mode of operation. Multiple tubes/columns could be used to increase capacity.	Presently restricted to anaerobic operation. Large reactor volume requirement at high capacities.
UASB reactor	Simple in construction. No support particles used. Provides substantially high degree of bioconversion at distinctly high capacities and even with high-strength feedstock.	Restricted to anaerobic processes, employing complex culture of microbes. Enormously large startup time.

Table 2.

Comparison of stirred tank and biofilm biological wastewater treatment technologies.

Source: Ref. [50].

7. Conclusion and future perspectives

This chapter discussed the fundamentals of biological wastewater treatment as it relates to the most commonly generated wastewater types – domestic and industrial wastewater. Wastewater is treated to protect public health and wildlife ecosystems, provide recreation and tourism, and increase economic prosperity using biochemical oxygen demand (BOD) analysis as a metric for water quality and treatment effectiveness. Biological wastewater treatment is an eco-friendly process that relies on microorganisms to convert organic pollutants in wastewater to simple substances such as CO₂, H₂O, and energy.

Treatment technologies, which can be grouped as stirred tank reactor technologies (CAS, oxidation ditch, A2O, and SBR), membrane technology (MBR), and biofilm technologies (MBBR, IFAS, DSFF, PBBR, FBBR, SFBBR, IFBBR, and SBBR) have evolved over the years as governments around the world continue to raise the bar of quality standards of wastewater-treated effluents. Advancements in treatment technology have put treatment processes ahead of future challenges such as increased wastewater generation as a result of the increasing global population, old and aging facilities, reduced installation spaces, economic considerations, and climate change. Activated sludge technology (CAS) being the oldest (developed around 1914) biological wastewater treatment technique, has undergone several improvements for enhanced nutrient removal, increased surface area for microbial growth, compact size, and cheaper capital costs, to technologies such as A2O, oxidation ditch, and the SBR. Developments in treatment technologies shifted to better sludge handling between the early 1970s and late 1980s, hence, the development of membrane technologies, that is, the MBR technology, which offered maximum sludge separation from treated water in the membrane media. However, membrane clogging due to biofouling in MBR systems led to the development of MBBR systems around the mid-1990s, with lower biofouling and clogging. The IFAS technology is the most recently developed biofilm technology, and it was deployed around the early 2000s. The technology was developed to mitigate the operational challenge of the slow biofilm growth rate of traditional MBBR systems, is more energy efficient, and has a low capital cost compared to other biofilm technologies including DSFF, PBBR, FBBR, and SBBR; although clogging is better managed in some of these technologies (especially in the MBBR due to the movable biocarrier medium) than in the IFAS system. New developments in IFAS systems are focused on sustainability with the incorporation of mechanisms for methane production and capturing, and energy generation. Overall, biofilm wastewater treatment technology has a better volumetric capacity and biomass retention period, higher biomass concentration, requires smaller installation space, low hydraulic retention time, and sludge production when compared to stirred tank reactor technology, although it has poor mass transfer, which affects the rate of reaction.

Future trends in biological wastewater treatment technologies will focus on process sustainability via energy generation to lower operational costs, compact systems, and large-volume wastewater handling all of which can be provided by the biofilm reactor technologies MBBR, IFA, MES, and even non-bacterial treatment systems like phytoremediation and algal-based treatments. Future designs and optimization of MBBR and IFAS operations should focus on biocarrier configuration, geometry, and design material.