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Review

A Review of Rotating Biological Contactors for Wastewater Treatment

by
Sharjeel Waqas
1,*,
Noorfidza Yub Harun
1,*,
Nonni Soraya Sambudi
2,
Muhammad Roil Bilad
3,
Kunmi Joshua Abioye
1,
Abulhassan Ali
4 and
Aymn Abdulrahman
4
1
Chemical Engineering Department, Universiti Teknologi PETRONAS, Seri Iskandar 32610, Perak, Malaysia
2
Department of Chemical Engineering, Universitas Pertamina, Jakarta 12220, Indonesia
3
Faculty of Integrated Technologies, Universiti Brunei Darussalam, Bandar Seri Begawan BE1410, Brunei
4
Department of Chemical Engineering, University of Jeddah, Jeddah 23218, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Water 2023, 15(10), 1913; https://doi.org/10.3390/w15101913
Submission received: 19 April 2023 / Revised: 7 May 2023 / Accepted: 12 May 2023 / Published: 18 May 2023
(This article belongs to the Special Issue Wastewater Engineering: Wastewater Treatment Methods and Technologies)
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Abstract

:
A rotating biological contactor (RBC) is a type of attached-growth biological wastewater treatment system and a widely used biological wastewater treatment technology. It employs a series of rotating discs to support microbial growth and promote the removal of pollutants from wastewater. RBC is widely recognized for its simplicity of design, high reliability, and low energy consumption. It has been used in various applications, from small-scale decentralized systems to large municipal wastewater treatment plants. The current review provides an overview of RBC bioreactors, design parameters, and the factors that influence biological performance, such as hydraulic retention time, sludge retention time, organic loading rate, disc rotational speed, and temperature. The review also highlights the advantages and disadvantages of RBCs compared with other wastewater treatment technologies and discusses their role in sustainable environmental performance. The future prospects of RBC are also discussed, including integration with other technologies, such as membrane filtration and potential use in resource recovery. The review explores the application of RBC in decentralized wastewater treatment and the potential to provide sustainable solutions for wastewater management in rural and remote areas. Overall, RBC remains a promising option for effective and efficient wastewater treatment, particularly in situations where simplicity, reliability, and low energy consumption are desired.

1. Introduction

Wastewater treatment is crucial in reducing environmental pollution by removing harmful pollutants from wastewater before it is discharged into the environment [1,2]. Untreated or poorly treated wastewater contains a wide range of pollutants, including organic matter, nitrogen, phosphorus, heavy metals, pathogens, and other harmful substances [3,4]. These pollutants significantly impact aquatic life, soil quality, and the overall health of ecosystems. Facilities must implement effective treatment processes and monitor their wastewater discharges to ensure compliance with wastewater regulations and protect the environment and public health. Ongoing research and innovation in wastewater treatment technologies are necessary to improve treatment efficiency and reduce wastewater environmental impact [5,6].
Biological wastewater treatment is an effective and sustainable method for reducing environmental pollution and protecting human health [7]. The effectiveness of biological treatment processes depends on several factors, including the type and concentration of pollutants in the wastewater, the design and operation of the treatment system, and the characteristics of the microorganisms involved in the process. Through effective treatment processes and compliance with regulations, wastewater treatment facilities protect the environment, promote public health, and contribute to a more sustainable future [8,9,10]. There are two main biological wastewater treatment system types: suspended- and attached-growth systems [11]. Suspended growth involves the use of suspended microorganisms. These microorganisms are typically contained in a reactor, mixed with wastewater, and provided with oxygen and nutrients to facilitate their growth and activity [12]. Examples of suspended growth systems include activated sludge, sequencing batch reactors, and oxidation ditches. Attached-growth systems involve the use of microorganisms attached to a fixed surface within the treatment process. These microorganisms form a biofilm on the surface, providing a surface area for them to grow on and attach to and a protective barrier against environmental stresses [13]. Examples of attached-growth systems include trickling filters, rotating biological contactors (RBC), and submerged aerated biofilters. Both suspended- and attached-growth systems have their advantages and disadvantages. Suspended growth is more efficient in treating organic matter and certain pollutants but requires more energy and maintenance to maintain system stability. Attached growth is more effective in treating certain pollutants but more susceptible to clogging and requires frequent maintenance. The choice between suspended-growth and attached-growth systems depends on several factors, including the type and concentration of pollutants, the available space and resources, and the desired treatment efficiency and effectiveness [11,14,15].
RBC is a fixed-film biological wastewater treatment process. RBCs consist of cylindrical discs partially submerged in wastewater and rotated at a constant speed [16,17]. The discs are typically made of plastic or another non-corrosive material and have a surface conducive to biofilm formation [18]. The RBC operates on the principle of attached growth, where microorganisms grow on the surface of the discs and consume the organic matter in the wastewater [19]. As the discs rotate, the biofilm comes into contact with the wastewater, exchanging nutrients and oxygen [20].
RBCs are commonly used to treat municipal and industrial wastewater, as they are highly efficient at removing organic matter and nutrients such as nitrogen and phosphorus [21]. They are also known for their robustness and ability to handle high fluctuations in influent characteristics. RBCs are relatively simple and require minimal energy inputs, making them a cost-effective option for wastewater treatment [22]. However, they do require regular maintenance to ensure proper operation and to prevent clogging of the discs. RBCs are a widely used and effective wastewater treatment option, with many applications in developed and developing countries [16].
RBCs are highly effective at removing organic matter and nutrients from wastewater, making them suitable for various applications. RBCs can handle high fluctuations in influent characteristics, making it suitable for wastewater treatment in multiple settings. RBCs require minimal energy inputs, making them a cost-effective option for wastewater treatment [23]. RBCs have a simple design and require minimal maintenance, making them easy to operate and maintain. RBCs are most effective for treating wastewater with low to medium levels of organic matter and nutrients and may not be suitable for highly contaminated wastewater [24]. If the discs in the RBC become clogged, the treatment efficiency is significantly reduced, requiring regular maintenance and cleaning. RBCs are limited in their ability to retain sludge, a disadvantage in certain applications where sludge retention is important [25,26].
Overall, the advantages of RBCs, including their high treatment efficiency, robustness, low energy requirements, and simple design, make them a popular option for wastewater treatment [16,27]. However, their high initial cost, limited applicability in certain situations, the risk of clogging, and limited ability for sludge retention are important factors to consider when choosing a wastewater treatment system. RBCs are widely used in wastewater treatment due to their high efficiency, low operating and maintenance costs, and ease of operation. This review article discusses the design, operation, and performance of RBCs in wastewater treatment.

2. Basic Concept of RBC Technology

A rotating biological contactor (RBC) bioreactor is a wastewater treatment system that uses rotating discs to support a microbial biofilm, as shown in Figure 1. The basic concept of an RBC bioreactor involves using a series of rotating discs partially submerged in wastewater [28,29]. The discs are typically made of plastic or other materials resistant to corrosion and fouling. As the discs rotate, microorganisms attach to the surface of the discs and form a biofilm. The biofilm grows and develops as the discs rotate, and the microorganisms in the biofilm consume and break down the organic matter in the wastewater [25]. The rotation of the discs ensures biofilm is continuously exposed to oxygen, which promotes the growth and activity of the microorganisms [30].
The wastewater is typically pumped into a tank or channel that contains the RBC discs. As the wastewater flows over the discs, the microorganisms in the biofilm remove organic matter and other contaminants from the wastewater [31]. The treated wastewater is discharged into an effluent tank or discharged directly into the receiving water body [29]. The efficiency of an RBC bioreactor depends on several factors, the size and design of the bioreactor, disc rotational speed, and microbial population in the biofilm [32]. RBCs are generally effective for treating wastewater with low to moderate organic loads. They are used for various applications, including municipal and industrial wastewater treatment and treating wastewater from small communities or remote locations [33].
RBC systems are biological treatment processes that use a series of rotating discs to support a fixed-film biological community that treats wastewater [34]. On the other hand, the activated sludge process (ASP) is a biological treatment process that uses a suspended-growth microbial culture to treat wastewater. ASP involves the aeration of wastewater mixed with a culture of microorganisms that consume organic matter, nitrogen, and phosphorus. The mixture is then separated from the microbial culture by sedimentation, and the treated wastewater is discharged. ASP is widely used in large-scale wastewater treatment plants due to its ability to handle high flows and its relatively high removal efficiencies [35]. RBC and ASP are both commonly used biological wastewater treatment technologies with advantages and disadvantages [36].
Figure 2 illustrates the advantages of RBC bioreactors for wastewater treatment. The nitrifying bacteria takes about 14–17 days to achieve the acclimatization stage, while carbonaceous bacteria require only 3–5 days. The inside surface of the bio-carrier is the best place for developing nitrifying bacteria. Nitrification is an aerobic process, while denitrification occurs in the absence of oxygen, an aerobic process. Introducing biofilm in the bioreactor increases the amount of mixed liquid suspended solids (MLSS) which increases the organics removal efficiency. To initiate nitrification, the COD should decrease below a certain limit [37].
RBCs are more robust and handle high fluctuations in influent characteristics, making it suitable for wastewater treatment in various settings. ASPs are more sensitive to changes in influent characteristics and may require additional treatment steps or adjustments to maintain optimal performance. RBCs have a simple design and require minimal maintenance, making them easy to operate and maintain [16]. ASPs are more complex and require regular monitoring and maintenance to ensure optimal performance. RBCs need less space than ASPs, making them suitable for small-scale wastewater treatment plants with limited space. RBCs require minimal energy inputs, making them a cost-effective option for wastewater treatment, while ASPs require more energy inputs for aeration and mixing. RBCs have better odor control than ASPs, which is advantageous in certain settings such as residential areas. RBCs produce less sludge than ASPs, reducing the need for sludge handling and disposal [38]. Table 1 shows the comparison of ASP and RBC bioreactor in terms of different operating parameters.

3. Biofilm Role in RBC

Biofilm plays a critical role in the performance of RBC bioreactors for wastewater treatment, as shown in Figure 3. The rotating discs in an RBC bioreactor provide a surface for the growth and attachment of microorganisms that form a biofilm. This biofilm is a complex community of microorganisms, including bacteria, fungi, and protozoa, that work together to break down and remove organic matter and other contaminants from wastewater [49].
The biofilm in an RBC bioreactor comprises multiple layers, with the outermost layers containing aerobic bacteria that consume organic matter and produce carbon dioxide and water [50]. Deeper layers of the biofilm may contain anaerobic bacteria that break down more complex organic compounds. The biofilm is constantly exposed to oxygen as the discs rotate, promoting aerobic microorganisms’ growth and activity. Deeper layers of the biofilm may be anoxic or anaerobic, meaning that they are not exposed to oxygen [51,52]. Microorganisms may use different electron acceptors to break down organic matter in these layers. In the anoxic layer, microorganisms may use nitrate or other oxidized nitrogen compounds as electron acceptors, while in the anaerobic layer, they may use sulfate or other oxidized sulfur compounds [53,54].
The presence of anoxic and anaerobic layers in the biofilm allows for the complete removal of organic matter and nutrients from the wastewater [55]. By providing different environments for different types of microorganisms, the biofilm supports a more diverse microbial community and increases the treatment process’s efficiency [56,57]. Biofilm acts as a physical barrier that filters and removes contaminants while providing a favorable environment for the growth and activity of microorganisms. Overall, the structure and function of the biofilm in an RBC bioreactor are complex and dynamic, with different layers of microorganisms performing various functions in removing contaminants from wastewater. The presence of aerobic, anoxic, and anaerobic layers allows for more efficient and complete removal of organic matter and nutrients from the wastewater, making RBC bioreactors an effective option for wastewater treatment.

4. Design Considerations

RBCs consist of cylindrical discs mounted on a horizontal shaft and rotated slowly through the wastewater [58]. The discs are partially submerged, and microorganisms grow on the surface of the discs to form biofilm. The discs are usually made of plastic or metal and are spaced at a distance of 30–50% of their diameter [38,59]. Figure 4 shows the Performance evaluation of an RBC based on operating conditions, biomass characteristics, and biological and nitrogen removal performance. The RBC design parameters include the disc diameter, the number of discs, the disc spacing, the shaft speed, and the HRT. The disc diameter and the number of discs determine the surface area available for biofilm growth.
In contrast, the disc spacing and the shaft speed determine the shear forces acting on the biofilm [60]. The design parameters for RBC bioreactors vary depending on the specific wastewater treatment application and the desired effluent quality [61]. Table 2 shows the selection of different parameter values for RBC bioreactors.

4.1. Disc Rotational Speed

The rotational speed of the discs affects the shear stress on the biofilm and the mass transfer of oxygen and nutrients to the microorganisms [62]. The optimal rotational speed depends on the specific wastewater treatment application, ranging from 1 to 10 revolutions per minute (RPM) [63]. The disc rotational speed is an essential parameter in designing and operating RBC. The RBC consists of circular discs mounted on a horizontal shaft and rotated slowly through the wastewater [64]. The microorganisms grow on the surface of the discs, forming a biofilm that degrades the pollutants in the wastewater. The disc rotational speed affects the biofilm’s thickness and porosity, affecting the RBC’s performance [65,66].
The disc rotational speed determines the amount of oxygen available to the microorganisms in the biofilm [67]. At low disc speeds, the biofilm is thin, and oxygen penetrates easily to the deeper layers of the biofilm. However, the biofilm becomes thicker and more compact at high speeds, limiting oxygen diffusion to the inner layers. This leads to anaerobic conditions in the deeper layers of the biofilm, reducing the degradation of pollutants [68].
The disc rotational speed affects the shear forces that act on the biofilm. The shear forces generated by the rotating discs cause the detachment of the biofilm, reducing the biomass and the overall performance of the RBC. The shear forces are minimal at low speeds, and the biofilm is less likely to detach [69]. However, the shear forces are high at high disc speeds, causing the biofilm to detach. Therefore, the disc rotational speed should be carefully chosen to balance the oxygen transfer, biofilm thickness, and shear forces to achieve optimal performance of the RBC.
Increasing the disc rotational speed beyond the optimum value has several effects on the biological performance and efficiency of the system. It increases the shear forces acting on the biofilm attached to the discs which leads to biofilm detachment and biomass loss and thereby a reduction in treatment efficiency and an increase in variability of treatment performance [55]. Higher disc rotational speeds also increase the RBC system’s energy consumption and maintenance requirements [70]. As the rotational speed increases, the power required to rotate the discs increases, resulting in higher energy costs and mechanical wear and tear. Increasing the disc rotational speed beyond the recommended range also results in increased noise and vibration in the system, which negatively impacts its working environment and safety [71,72,73].
Overall, while increasing the disc rotational speed enhances the treatment efficiency and organic matter removal of the RBC bioreactor, there is a limit beyond which further increases lead to reduced performance, increased energy consumption, and other negative impacts on the system. It is recommended to operate the RBC within the recommended range of disc rotational speeds, typically between 2 to 5 rpm for larger scale systems, and to conduct experimental testing and optimization to determine the optimal rotational speed for a specific RBC system.

4.2. Hydraulic Retention Time (HRT)

The HRT is the time that wastewater spends in the bioreactor, affecting the residence time of microorganisms and pollutant removal efficiency [74]. The HRT is an important parameter in the design and operation of RBC and refers to the duration of time that the wastewater stays in contact with the biofilm on the RBC discs. The HRT affects the performance by determining the time available for the microorganisms to degrade the pollutants. The optimal HRT depends on the specific wastewater treatment application, ranging from 4 to 24 h [75,76].
A longer HRT provides more time for the microorganisms to degrade the pollutants, resulting in higher removal efficiencies [77]. However, an excessively long HRT may lead to the accumulation of inert material and the growth of less efficient microorganisms, reducing the overall performance of the RBC [78]. On the other hand, a shorter HRT reduces the time available for the microorganisms to degrade the pollutants, resulting in lower removal efficiencies. However, a shorter HRT also minimizes the accumulation of inert material and prevents the growth of less efficient microorganisms, leading to higher performance [79]. Therefore, the HRT should be carefully chosen to balance the removal efficiency and the accumulation of inert material and less efficient microorganisms. The optimal HRT may vary depending on the wastewater’s characteristics, the RBC’s design, and the desired performance.
Increasing the HRT beyond 24 h in an RBC bioreactor has several effects on the biological performance and efficiency of the system. Increasing the HRT increases the residence time of the wastewater in the RBC, which enhances the biological treatment process and results in higher organic matter removal efficiency. However, suppose the HRT is increased too much, in that case, the treatment capacity of the RBC may be exceeded, leading to insufficient mixing and oxygen transfer within the bioreactor and resulting in reduced treatment efficiency. Increasing the HRT increases the RBC system’s operational and maintenance costs. The bioreactor volume increases as the HRT increases, resulting in higher capital and operating costs associated with the system’s construction, operation, and maintenance. Increasing the HRT beyond the recommended range results in longer start-up and stabilization periods for the RBC system. This is because longer HRTs result in slower biomass growth and development, delaying the establishment of a stable and effective biofilm within the RBC [80].
Overall, while increasing the HRT enhances the treatment efficiency and organic matter removal of the RBC bioreactor, there is a limit beyond which further increases lead to reduced performance, higher costs, and longer start-up and stabilization periods. It is recommended to operate the RBC within the recommended range of HRTs, typically between 4 to 24 h, and to conduct experimental testing and optimization to determine the optimal HRT for a specific RBC system and wastewater composition.
Table
Table 2. Operating parameter values in various types of RBC bioreactors.
Table 2. Operating parameter values in various types of RBC bioreactors.
Type of BioreactorType of WastewaterRotational Speed (rpm)HRT (h)Disk Submergence
(%)		Loading RateReference
Rotating biological contactorGold mine wastewater (cyanide removal)5, 101040300 mg cyanide/L[81]
Hybrid anaerobic-activated sludge RBCMolasses3012-23 g/L d[77]
Anaerobic RBCHeavy metal removal (Cu, Cd, Pb, Fe, Zn, and Ni)-24, 4840%Cu 100 mg/L, Cd, Ni, Fe, Pb and Zn 50 mg/L[82]
Non-woven RBCMunicipal sewage via SAND25Fully submerged50 mg/L ammonia, 65 mg/L nitrite[83]
Packed cage RBCMustard tuber wastewater812.240-[41]
Four-stage RBCPetroleum refinery wastewater4.58 optimized (0–14)-400.005, 0.008, and 0.016 m3/m2d[43]
RBCTextile dye (colored wastewater)648--[84]

4.3. Solids Retention Time (SRT)

The SRT refers to the time that microorganisms spend in the bioreactor, affecting biomass concentration and activity [85]. The optimal SRT depends on the specific wastewater treatment application, ranging from 5 to 20 days [80]. The SRT is an important parameter in RBC design and operation. The SRT affects the RBC’s performance by determining the microorganisms’ composition and activity in the biofilm [34]. A longer SRT promotes the growth of slower-growing microorganisms, such as nitrifiers and denitrifiers, that are responsible for removing nitrogen compounds [19,86]. A longer SRT encourages the development of more stable microbial communities resistant to disturbances, resulting in more consistent and reliable treatment performance.
On the other hand, a shorter SRT promotes the growth of faster-growing microorganisms, such as heterotrophic bacteria, responsible for removing organic matter. A shorter SRT reduces the accumulation of inert material and prevents the growth of less efficient microorganisms, leading to higher performance. Therefore, the SRT should be carefully chosen to balance the microbial community’s removal efficiency and stability. The optimal SRT may vary depending on the wastewater’s characteristics, the RBC’s design, and the desired performance [55].
Increasing the SRT beyond 15 days in an RBC bioreactor has both positive and negative effects on the biological performance and efficiency of the system. On the positive side, an increase in the SRT results in higher biomass concentration within the RBC, enhancing the organic matter removal efficiency and overall treatment performance of the system. The longer SRT increases biomass growth and development, producing a more established and effective biofilm on the disc surfaces [87]. However, increasing the SRT beyond 15 days also has negative effects. One of the main drawbacks is the increase in operational and maintenance costs associated with the higher biomass concentration within the RBC [88]. Higher biomass concentrations require frequent maintenance, such as cleaning and replacing the discs, to prevent clogging and reduce hydraulic resistance [87]. Another negative effect of increasing the SRT beyond 15 days is the potential for reduced treatment efficiency due to the accumulation of inert or inactive biomass, reducing the surface area available for microbial growth and hindering the mass transfer of nutrients and oxygen to the active biomass. This reduces organic matter removal efficiency and increases suspended solids and nutrient concentrations [89,90].
Therefore, while increasing the SRT enhances the performance and efficiency of the RBC bioreactor, it is important to consider the associated operational and maintenance costs and the potential for reduced treatment efficiency due to biomass accumulation. It is recommended to operate the RBC within the recommended range of SRT, typically between 5 to 15 days, and to conduct experimental testing and optimization to determine the optimal SRT for a specific RBC system and wastewater composition.

4.4. Organic Loading Rate (OLR)

OLR plays a critical role in RBC performance. The OLR determines the amount of organic matter applied to the RBC per unit of time [25]. The performance of the RBC is highly dependent on the ability of the microorganisms in the biofilm to degrade the organic matter [91]. If the OLR is too low, the microorganisms may be unable to consume all of the organic matter in the wastewater, resulting in incomplete treatment and low removal efficiency. Conversely, if the OLR is too high, the microorganisms will be unable to keep up with the oxygen demand and nutrients, thus resulting in an accumulation of organic matter and a decrease in treatment efficiency [92].
Therefore, selecting an appropriate OLR based on the characteristics of the wastewater, RBC design, and desired performance is important. The optimal OLR depends on the specific wastewater treatment application, ranging from 0.5 to 5 kg COD/m3 d. Optimal OLR may vary depending on factors such as the concentration and biodegradability of the organic matter, temperature, pH, and HRT [93,94]. Proper control of OLR is important for the efficient operation of RBCs. Overloading the system reduces treatment efficiency, while underloading underutilizes the available treatment capacity. Therefore, the OLR must be regularly monitored and adjusted to maintain optimal RBC performance.

4.5. Temperature

The temperature has a significant effect on the performance of RBC bioreactors. Generally, as the temperature increases, microorganisms’ biological activity and growth on the biofilm also increase, resulting in higher treatment efficiency. However, the optimal temperature range for RBC performance depends on the biofilm’s specific microorganisms and the treated wastewater’s characteristics. At low temperatures, the biological activity of microorganisms in the biofilm is reduced, resulting in lower treatment efficiency. High temperature is detrimental to RBC performance as it destroys the biofilm structure and detaches microorganisms from the discs. For most RBC systems, the optimal temperature range is between 20–30 °C. In colder climates, RBC systems require heating to maintain optimal temperatures, while in warmer climates, cooling is necessary to prevent overheating. It is important to note that extreme temperature variations or fluctuations also negatively impact the performance of RBC bioreactors.
The performance of RBCs is improved by optimizing the design and operating parameters. The disc diameter and the number of discs increase the surface area available for biofilm growth, the disc spacing and the shaft speed are adjusted to control the shear forces acting on the biofilm and the HRT is optimized to achieve the best removal efficiency for a specific wastewater composition. In conclusion, RBCs are an effective and economical option for wastewater treatment. The design and operating parameters are optimized for organic matter and nutrient removal performance. RBCs are suitable for small-to-medium-sized wastewater treatment plants and are used as a standalone or part of a larger treatment system.

5. Biological Performance of RBCs

RBCs are effective in the removal of organic matter, nitrogen, and phosphorus from wastewater. The COD removal efficiency of RBCs ranges from 80–90%, while the total nitrogen removal efficiency ranges from 40–60%. The phosphorus removal efficiency depends on chemical precipitation or biological uptake by microorganisms. Table 3 shows the biological performance of various types of RBC bioreactors.

5.1. Organics Removal

RBC is widely used for biological wastewater treatment due to its high efficiency in organic matter removal, including COD. The COD removal efficiency of an RBC bioreactor depends on various factors, including the characteristics of the influent wastewater, design and operational parameters, and the microbial population in the biofilm [26]. The characteristics of the influent wastewater, such as the concentration and nature of organic matter, affect the COD removal efficiency of the RBC. High OLR decreases COD removal efficiency, as the biofilm may not have sufficient time to degrade the organic matter. The nature of organic matter, biodegradability, and toxicity, also impacts COD removal efficiency.
Design and operational parameters of the RBC, such as the disc rotational speed, HRT, and temperature, also affect the COD removal efficiency [70]. The disc rotational speed determines the biofilm thickness and the exposure to the wastewater, thus affecting the efficiency of COD removal. HRT should be optimized to ensure sufficient contact time between the wastewater and the biofilm for efficient COD removal [95]. Temperature affects microbial activity and COD removal efficiency, as high temperatures lead to increased biological activity and improved removal efficiency [96]. Overall, RBC bioreactors achieve high COD removal efficiencies, with reported values ranging from 70% to 95%. However, the removal efficiency depends on the influent wastewater characteristics, design and operational parameters, and other factors that may affect the performance of the RBC bioreactor [95].

5.2. Nitrogen Removal

Nitrification is a two-step process: in the first step, autotrophic AOB converts ammonium into nitrite, and the second step is the conversion of nitrite to nitrate by NOB. Heterotrophic bacteria grow more easily in high COD values than autotrophs. Figure 5 shows the complete nitrogen cycle showing the main oxic and anoxic processes. The competition for oxygen, nutrients, and space between autotrophic and heterotrophic bacteria affects nitrification [97,98]. RBC is widely used to treat municipal and industrial wastewater due to its high performance and low operational costs. Nitrogen removal, particularly TN removal, is a critical process in wastewater treatment because high levels of TN cause eutrophication in receiving waters [99]. The removal of TN occurs through biological processes such as nitrification and denitrification [100]. Nitrification is the biological oxidation of ammonia to nitrite and nitrate, while denitrification is the biological reduction of nitrate to nitrogen gas. Both processes require specific environmental conditions, including a specific temperature, pH, DO, and HRT [26,99].
Studies have shown that the TN removal efficiency in RBC is influenced by several factors, including the RBC design parameters, HRT, DO, pH, and temperature. High HRT provides sufficient time for nitrifying and denitrifying bacteria to remove TN from the wastewater. DO concentration above 2 mg/L supports aerobic nitrification, and below 0.5 mg/L promotes denitrification. The pH should be maintained between 6.5 and 8.5 to prevent ammonia toxicity to microorganisms. Temperature is another critical factor affecting TN removal, with optimal performance between 20–30 °C. Overall, RBC bioreactors have demonstrated excellent TN removal efficiencies, with reported removals ranging from 50% to 90%, depending on the influent TN concentration, design parameters, and operating conditions. The TN removal efficiency is improved by optimizing the RBC design, increasing the HRT, controlling the DO and pH levels, and maintaining optimal temperature conditions.

5.3. Ammonia Nitrogen Removal

RBC systems are efficient in the removal of organic matter and nitrogenous compounds from wastewater. Ammonia nitrogen removal in an RBC bioreactor depends on several factors, including the influent concentration of ammonia nitrogen, the aeration rate, temperature, and pH [38]. The ammonia nitrogen removal mechanism in the RBC bioreactor involves the conversion of ammonia nitrogen to nitrite and then to nitrate by ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), respectively [75]. The nitrate is converted to nitrogen gas by denitrifying bacteria under anaerobic conditions [101].
Studies have shown that RBC bioreactors achieve high ammonia nitrogen removal efficiencies, ranging from 70% to 99%, depending on the influent ammonia nitrogen concentration and operating conditions [16,78]. Higher removal efficiencies are observed at higher aeration rates and longer HRT. The optimal pH range for ammonia nitrogen removal in an RBC bioreactor is between 7.5 and 8.5. However, excessive aeration rates cause excessive shear stress and damage to the biofilm, leading to decreased ammonia nitrogen removal efficiency [38]. Low temperatures inhibit the growth and activity of nitrifying bacteria, leading to lower ammonia nitrogen removal efficiencies. High influent concentrations of ammonium cause incomplete nitrification and nitrite accumulation, which is toxic to aquatic organisms [96,102]. Overall, the RBC bioreactor effectively removes ammonium from wastewater, but the removal efficiency depends on several factors, including the influent concentration of ammonium, the aeration rate, temperature, and pH. Proper optimization and control of these factors leads to high ammonium removal efficiencies.
RBCs efficiently remove nitrogenous compounds from wastewater, including ammonia, nitrate, and nitrite. The removal efficiency of these compounds depends on several factors, including the operating conditions of the RBC system, such as HRT, disc rotational speed, temperature, and DO concentration. Ammonia removal in an RBC bioreactor is mainly achieved through biological nitrification, which involves the nitrifying bacteria’s oxidation of ammonia to nitrite and nitrate. Nitrite and nitrate removal occur through denitrification. The denitrifying bacteria under anaerobic conditions reduce nitrate to nitrogen gas [102]. The overall removal efficiency of ammonia, nitrite, and nitrate in RBCs varies depending on the design, operating parameters, and treated wastewater characteristics. Studies have reported ammonia removal efficiencies ranging from 70% to 99% in RBC systems, with higher removal efficiencies achieved at higher DO concentrations and lower HRT. Nitrite and nitrate removal efficiencies in RBCs have been reported to be between 70% to 100%, with higher removal efficiencies achieved at lower HRT and higher DO concentrations. Overall, RBC bioreactors are effective in removing nitrogenous compounds.

6. Disadvantages of RBC and Solution

While RBCs offer several advantages, such as high treatment efficiency and low energy requirements, some disadvantages will be encountered [38]. The following are the disadvantages of RBC bioreactors and potential solutions to mitigate these issues:
Over time, solids and biofilms accumulate on the rotating discs or cylinders, reducing the available surface area for microorganisms and affecting treatment efficiency. This leads to clogging, fouling, and increased maintenance requirements. Regular cleaning and maintenance of the RBC bioreactor prevents the build-up of solids and biofilms. Additionally, periodic replacement of the rotating discs or cylinders helps maintain the surface area for microorganisms and prevents clogging and fouling [16].
RBC bioreactors can be expensive to install and operate, particularly for large-scale wastewater treatment applications. Smaller-scale RBC systems are used for decentralized or localized wastewater treatment applications to mitigate the high capital and installation costs. Additionally, advancements in RBC technology, such as the use of more efficient materials and designs, help reduce costs and increase system performance [78].
RBC bioreactors may not be as effective at removing certain pollutants, such as nutrients, compared with other biological treatment systems. Combining RBC bioreactors with other treatment technologies, such as activated sludge or membrane filtration, can help improve the removal of specific pollutants. Optimizing operational conditions, such as dissolved oxygen levels and hydraulic retention times, improves treatment efficiency for certain pollutants [102].

7. Future Prospective

The future prospects of the RBC system are promising as it has proved to be an effective and sustainable technology for wastewater treatment. The development of advanced materials for the discs of RBC improves the efficiency and durability of the bioreactor. New materials, such as graphene and carbon nanotubes, are being studied for their potential use in RBCs. RBCs can be integrated with other technologies, such as membrane filtration, advanced oxidation, and electrochemical processes, to enhance the efficiency of wastewater treatment. RBCs can be combined with MBR to create a hybrid system that provides high-quality effluent with low solids content. The RBC is used in a pre-treatment step that removes large particles, while the MBR provides the final treatment and filtration. RBCs can be used as pre-treatment steps for anaerobic digestion to remove soluble organic matter and reduce the load on the ASP. This results in increased biogas production and improved treatment efficiency. RBC effluent is treated with activated carbon to remove residual organic compounds, micropollutants, and other contaminants not effectively removed by an RBC system alone. Ozonation can be used as a post-treatment step for RBC effluent to remove organic matter and disinfect the effluent. Ozonation has been shown to improve RBC removal efficiency and reduce micropollutants’ presence. Integration of RBC systems with other technologies can enhance their treatment efficiency and expand their application to various wastewater treatment scenarios. The future of RBCs lies in their ability to adapt to changing treatment needs and to be integrated with other innovative technologies to improve sustainability and environmental performance.
RBC systems remove emerging contaminants such as pharmaceuticals, personal care products, and microplastics from wastewater. Research in this area is ongoing and promising [60,103]. RBCs are suitable for decentralized wastewater treatment systems due to their compact design and ability to treat a wide range of organic and nutrient loads. RBC systems are used as a primary or secondary treatment in decentralized wastewater treatment. They remove a significant amount of organic matter, suspended solids, and nutrients from wastewater and remove remaining organic matter and nutrients as a secondary treatment, producing a high-quality effluent suitable for reuse or discharge.
Moreover, RBCs are suitable for rural and remote areas where centralized wastewater treatment systems are not feasible due to high costs and limited infrastructure. RBCs are designed to treat wastewater from small communities, individual households, or industrial facilities, providing a sustainable and cost-effective solution for decentralized wastewater treatment. RBCs recover nutrients such as nitrogen and phosphorus from wastewater used as fertilizers. Overall, RBC systems have a bright future as a sustainable and effective technology for wastewater treatment. Continued research and development can further improve its performance and expand its applications in the future.

8. Conclusions

RBCs have been widely used in the wastewater treatment industry due to their high treatment efficiency, low operating cost, and ease of operation. In addition, RBCs have the potential to play a crucial role in achieving sustainable environmental performance. RBCs contribute to the reduction of greenhouse gas emissions. As RBCs are aerated systems, they provide oxygen to the microorganisms that break down the organic matter in the wastewater. This process reduces the need for additional energy-consuming mechanical aeration systems, thus lowering energy consumption and greenhouse gas emissions. RBCs promote water conservation. As RBCs are efficient in removing pollutants from wastewater, the treated water is reused for non-potable purposes such as irrigation, landscaping, and toilet flushing. This reduces the demand for freshwater resources and promotes water conservation. Discharging untreated or poorly treated wastewater into water bodies severely impacts aquatic ecosystems and public health. RBCs effectively treat wastewater and remove pollutants, such as organic matter, nitrogen, and phosphorus, before discharging them into the environment, thus reducing the negative impact of wastewater on water quality. RBCs produce sludge as a by-product of the wastewater treatment process. The sludge is further processed into biosolids and used as a nutrient-rich fertilizer for agriculture or land rehabilitation. This closed-loop system reduces the waste generated by wastewater treatment and creates a valuable resource. In conclusion, RBCs have the potential to play a vital role in achieving sustainable environmental performance. Their low operating costs, high treatment efficiency, and ease of operation make them a feasible and effective option for wastewater treatment. Additionally, their potential contributions to reducing greenhouse gas emissions, promoting water conservation, mitigating water pollution, and contributing to the circular economy highlight their significance in achieving sustainable environmental performance.

Author Contributions

S.W. prepared the manuscript draft; S.W., N.Y.H., M.R.B. and N.S.S. contributed to the original idea and conceptual design of the study, supervised the work, and revised the manuscript; K.J.A., A.A. (Abulhassan Ali) and A.A. (Aymn Abdulrahman) revised the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge the Universiti Teknologi PETRONAS, Malaysia through YUTP grant (No. 015LC0-502) for funding support.

Data Availability Statement

Not applicable.

Acknowledgments

The authors acknowledge the support from Universiti Teknologi PETRONAS, Malaysia, for providing the research and support facilities.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of an RBC bioreactor.
Figure 1. Schematic diagram of an RBC bioreactor.
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Figure 2. RBC process advantages for wastewater treatment.
Figure 2. RBC process advantages for wastewater treatment.
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Figure 3. Biofilm role in an RBC for the treatment of nitrogen compounds, AOB, NOB, AnAOB are responsible for the conversion of ammonium compounds into nitrite and nitrate and subsequently conversion into nitrogen gas.
Figure 3. Biofilm role in an RBC for the treatment of nitrogen compounds, AOB, NOB, AnAOB are responsible for the conversion of ammonium compounds into nitrite and nitrate and subsequently conversion into nitrogen gas.
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Figure 4. Performance evaluation of an RBC based on operating conditions, biomass characteristics, and biological and nitrogen removal performance.
Figure 4. Performance evaluation of an RBC based on operating conditions, biomass characteristics, and biological and nitrogen removal performance.
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Figure 5. The nitrogen cycle shows the main oxic and anoxic processes.
Figure 5. The nitrogen cycle shows the main oxic and anoxic processes.
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Table
Table 1. Comparison of RBCs and ASPs for wastewater treatment.
Table 1. Comparison of RBCs and ASPs for wastewater treatment.
Sr#ParameterASPRBC
1Ease of operationSimple operation but requires continuous monitoring [39].Simple operation requiring less maintenance and monitoring. No complex process parameter is involved [40].
2Aeration requirementOxygen is supplied by mechanical or diffused air aeration, which is about 55% of the total cost.Oxygen is supplied by contact with a rotating contactor with air [41].
3Total nitrogen (TN) and total phosphorus (TP)Nitrogen and related compounds are removed by nitrification and denitrification. However, this requires high aeration and a large amount of land. A removal efficiency of 80% has been observed for TN and TP.The overall removal efficiency was approximately 99% for organics removal and 90% for nitrogen removal [42]. Complete nitrification and denitrification are achievable with an anaerobic RBC [43].
4Sludge productionHigh sludge production (70–100 g/m3) because of high aeration and microbial activity. Low sludge production due to high biomass concentration and attached microorganisms [44].
5Land requiredA large area is required compared with an RBC and membrane bioreactor (MBR). The requirement for settling the tank requires additional land.About 1/10th the amount of land is required compared with an ASP [45].
6Organic loading rate (OLR)Sensitive to OLR due to a lesser amount of microorganisms present.OLR of 24 g chemical oxygen demand (COD)/m2/day was used [46].
7Energy requirementHigh energy consumption (2.4 kWh/m3).Low energy consumption (1.2 kWh/m3) because no aeration is required in the bioreactor [47].
8Hydraulic retention timeHigh hydraulic retention time (HRT) (12–48 h) compared with an RBC (6–18 h).A short HRT of about 4 h was used thanks to the abundant quantity of microorganisms available to digest organic matter [48].
9Solids retention timeHigh sludge retention time (SRT) range of 10–30 d. The sludge produced is the same as an RBC.High SRT decreases the amount of sludge, and a large floc size facilitates the settling.
10Aeration costAbout 55% of the OPEX.No aeration is required due to the rotation of the shaft. The oxygen is provided to the microorganisms through shaft movement.
11CAPEX and OPEXHigh CAPEX and OPEX (2.4 kWh/m3). Compared with trickling filters and RBC (1.2–1.8 kWh/m3).RBCs are, on average, 35% cheaper per year than trickling filters due to lower land area and running costs [47].
12Primary treatment requirementAdequate primary treatment is required.Adequate primary treatment is required.
Table
Table 3. Biological performance of various RBC bioreactors.
Table 3. Biological performance of various RBC bioreactors.
Type of BioreactorType of WastewaterPerformanceRemarksReference
Rotating biological contactorGold mine wastewater (cyanide removal)Free cyanide biological removal of 96.89% with the addition of a carbon source (3.8 g sucrose/L) at 10 h HRT and 5 rpm.
Without a carbon source, the removal efficiency was 83.89%		HRT increase results in an increase in removal efficiency.
The selection of appropriate rotational speed is important as it influences the biofilm thickness.		[81]
Hybrid anaerobic-activated sludge RBCMolassesA maximum amount of hydrogen production rate (4.4 L/L d) was obtained at 30 rpm and 47.5 g /L d OLRDisk rotational speed and OLR showed a significant effect on process responses. High disk rotational speed improves the system performance. Higher OLR (>47.5 g/L d) causes a reduction in the hydrogen production rate.[77]
Anaerobic RBCHeavy metal removal (Cu, Cd, Pb, Fe, Zn, and Ni)Maximum removal of Cu (97%), Cd (90%), and more than 77% for other metals at 48 h HRTThe metal removal values were slightly reduced at 24 h HRT, and the heavy metal removal was in the order: Cu > Cd > Pb > Fe > Zn > Ni[82]
Non-woven RBCMunicipal sewage via SANDBoth COD and TN removal rates are above 70% under optimized conditions. Maximum respective COD and TN removal efficiencies of 83.12% and 79.13% were obtained at DO = 0.2 mg/L and C/N = 2.3COD removal rate showed a decreasing trend with an increase in DO and C/N. AOB, anammox bacteria, and BND coefficients in SAND. AOB (65.13%) was dominant in the outer layer, whereas anammox bacteria (47.17%) and DNB (38.91%) were dominant in the inner anaerobic layer.[83]
Packed cage RBCMustard tuber wastewaterCOD < 100 mg/L effluent concentration while TN removal efficiency was 70.82 ± 3.98%Under aerobic conditions, the high DO concentration (>1.9 mg/L) results in higher nitrate concentration due to nitritation. High DO concentration is not suitable for stable partial nitritation/anammox.[41]
Four-stage RBCPetroleum refinery wastewaterThe maximum removal efficiency for COD and ammonia was 85.76% and 99.07%, respectively.At a lower loading rate, a high percentage of nitrates is produced because of the high amount of AOB in the initial stages of the bioreactor. The increase of HLR due to the reduction of HRT seemed to have a negative effect on ammonia removal.[43]
RBCTextile dye (Colored wastewater)Sixty-four percent decolorized wastewater was obtained with glucose as a carbon source, while a maximum removal of 83% was obtained with 10 g/L glucose. A maximum COD removal efficiency of 73% was obtained.The bioreactor performed poorly in the absence of additional glucose in decolorizing wastewater. The addition of glucose increases performance. However, a bioreactor requires a significant amount (1:1) of glucose which is a disadvantage.[84]
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Waqas, S.; Harun, N.Y.; Sambudi, N.S.; Bilad, M.R.; Abioye, K.J.; Ali, A.; Abdulrahman, A. A Review of Rotating Biological Contactors for Wastewater Treatment. Water 2023, 15, 1913. https://doi.org/10.3390/w15101913

AMA Style

Waqas S, Harun NY, Sambudi NS, Bilad MR, Abioye KJ, Ali A, Abdulrahman A. A Review of Rotating Biological Contactors for Wastewater Treatment. Water. 2023; 15(10):1913. https://doi.org/10.3390/w15101913

Chicago/Turabian Style

Waqas, Sharjeel, Noorfidza Yub Harun, Nonni Soraya Sambudi, Muhammad Roil Bilad, Kunmi Joshua Abioye, Abulhassan Ali, and Aymn Abdulrahman. 2023. "A Review of Rotating Biological Contactors for Wastewater Treatment" Water 15, no. 10: 1913. https://doi.org/10.3390/w15101913

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