Summary: Rotating biological contactors (RBCs) constitute a very unique and superior alternative for biodegradable matter and nitrogen removal on account of their feasibility, simplicity of design and operation, low land area requirement, low energy consumption, low operating and maintenance cost and treatment efficiency. The present review of RBCs focus on parameters that affect performance like rotational speed, organic loading rates, retention time, biofilm support media, staging, temperature, biofilm characteristics, dissolved oxygen levels, and medium submergence & operational problems.
Fresh water resources are contaminated due to the direct discharge of untreated domestic & industrial wastewater. Therefore, tangible steps have to be taken for wastewater treatment. Various types of treatment technologies are employed to treat the sewage; Rotating biological contactor (RBC) is one of them. A rotating biological contactor (RBC) is an attached growth bioreactor that offers an alternative technology to the conventional activated sludge process. The timeline of RBC is described as under;
- The first RBC system was used in the early 1900s (Mathure and Patwardhan 2005).
- The availability of polystyrene marked the beginning of commercial application of RBCs with the first full-scale system being installed in Germany in 1958 (Rodgers and Zhan 2003).
- Significant refinements in media type and equipment configuration occurred during the 1960s and early 1970s (Tchobanoglous and Burton 1991; Grady et al. 1999).
1.1. Process in Rotating Biological Contactors
A RBC unit typically consists of a series of closely spaced large, flat or corrugated discs that are mounted on a common horizontal shaft and are partially or completely submerged in wastewater (Fig. 1). A drum filled with some lightweight packed supports can also be used in place of conventional discs. The shaft continually rotates by a mechanical motor or a compressed air drive and a biofilm is established onto the entire surface area of the media, which metabolizes the organic materials contained in the wastewater. In aerobic processes the rotation of the media promotes oxygen transfer and maintains the biomass in aerobic conditions. The rotation also provides turbulence in the mixed liquor surface and enables the removal of excess solids from the media (Patwardhan 2003; Rodgers and Zhan 2003).
1.2. Applications of RBCs
Over the years RBCs have been successfully used to provide secondary treatment;
- Municipal wastewater from small units serving residential dwellings to large ones treating flows of up to several million litres per day (Banerjee 1997a)
- Nitrification, Denitrification and phosphorus removal from municipal wastewater (S. Cortez 2008).
- Decolourization of wastes like textile dyes (Axelsson et al. 2006)
- Coloured sugar refinery effluents (Guimaraes et al. 2005)
- Bioremediation of landfill leachates (Cema et al. 2007)
- Treatment of effluents from wineries (Malandra et al. 2003)
- Bakeries (Nahid et al. 2001)
- Food processors (S. Cortez 2008).
- Pulp and paper mills (S. Cortez 2008).
- Leather tanneries (S. Cortez 2008).
2. Factors Affecting RBC Performance
Performance of RBCs depends upon several design parameters. These parameters are discussed separately in relevant sections below;
2.1. Rotation Speed
Rotation speed of the RBC plates significantly affects the treatment efficiency. With the increase in rotational speed, turbulence increases and dissolved oxygen level increases. Thus more oxygen is available to aerobic microbial metabolism resulting in higher degradation rate (Israni et al. 2002). After a certain optimum rotational speed, further increase results in increase of fluid shear and the biofilm (slime layer) on the discs will start to slough off. This reduces the biomass in tank. Less biomass means slow degradation of pollutant (BOD). So one has to maintain rotational speed which gives maximum dissolved oxygen but do not slough the slime layer (Ramsay et al. 2006). Typical rotational speeds are 1–10 rpm for RBC (Mathure and Patwardhan, 2005).
2.2. Organic Loading Rate
The variation of the organic loading rate is generally accomplished by changing the inlet flow rate or the HRT, which also results in a change in the hydraulic loading (Najafpour et al. 2005). Available data show that, for a given system, as the applied organic loading rate increases, the substrate removal rate increases and removal efficiency decreases. Reduction in substrate removal efficiency may be an indication of limitation in dissolved oxygen.
Under normal operating conditions, carbonaceous substrate is mainly removed in the first-stage of the RBC. To avoid oxygen transfer limitations the first stage design load must be limited to a BOD5 load of about 30 g BOD/m2.d or for soluble BOD 12-20 g BOD/m2.d (WEF and ASCE 1998). Metcalf and Eddy have reported the design value of organic loading rate range 12-15 g soluble BOD/m2-day. The use of higher first-stage organic loadings will increase the probability of developing problems such as excessive biofilm thickness, depletion of dissolved oxygen, deterioration of process performance, appearance of H2S odours (Tchobanoglous and Burton 1991; Grady et al. 1999). Overloading problems can be overcome by removing baffles between the first and second-stages to reduce surface loading and increase oxygen transfer. Other approaches include supplemental air systems, step-feed, or recycle from the last stage (Surampalli and Bauman 1997).
The organic loading affects nitrification in a RBC unit. In the initial stages, where the organic load is high, heterotrophic bacteria offer strong competition to nitrifies displacing them within the bioreactor (Brazil 2006). Therefore, proposed design value of 5 g BOD5/m2.d is recommended for nitrification process (German ATV Guidelines 1989).
2.3. Hydraulic Retention Time
Studies with RBC systems have revealed that longer contact times improve the diffusion of the substrate into the biofilm and its consequent removal of the influent (Hanhan et al. 2005). Too short a HRT will result in low removal rates, whereas too long a HRT will not be economically feasible. A significant advantage offered by full-scale RBCs is to require short hydraulic retention periods (generally less than 2 h) (Benefield and Randall 1980). While Metcalf and Eddy reported the range 0.75-1.5 hours.
2.4. RBC Media
Various researchers used various disks materials to make the system economical, less energy intensive. They also evaluated different factors which affect the efficiency of RBC (Tawfik et al 2006). The RBC media used includes;
- Polystyrene Discs
- Stainless steel discs covered with clothes
- Lightweight clear plastic discs
- Propylene Discs
- Hard polythene discs
- Acrylic plastic discs
2.5. RBC Staging
Staging of RBC media is recommended to maximize removal of BOD5 and ammonia nitrogen (NH4+-N). Stages are accomplished by using baffles in a tank or using a series of tanks. Typical RBC staging arrangements are illustrated in Fig. 2.
In secondary treatment applications, RBCs shall be designed and operated in a series of three stages per flow. For combined BOD5 and NH4+-N removal, a minimum of four stages is recommended per flow. For small plants, multiple stages are acceptable on a single shaft oriented in parallel to the direction of flow.
As the wastewater flows through the system, each subsequent stage receives an influent with an organic concentration lower than the previous stage. Because heterotrophic bacteria grow faster than nitrifiers the first stage tends to be primarily an organic removal device, unless the wastewater organic content is very low. As the wastewater moves to the second and subsequent stages the RBC tends to first removing ammonia and then nitrite with the final product being nitrate, assuming that the RBC is sized and operated correctly (Hoccheimer and Wheaton 1998). When there is recycling of wastewater from the last stage to the first one, denitrification may be achieved in the first stage, where there is high organic loading and low dissolved oxygen content.
Temperature is one of the important factors that control speed of biological reaction. Microbes of all types have changed activity level at different temperatures, at some temperature they die or their activity is stopped. So, within a range, as temperature of wastewater increases the biodegrading rate increases and vice versa in all RBC stages (Israni et al. 2002).
When wastewater temperatures less than 130C are expected, organic and nitrogen removal rates may decrease. Temperature correction factors need to be taken into account in design criteria and can be obtained from the equipment manufacturers or from pilot studies. Generally, when the temperature drops from 13 to 50C, nearly 2.5 times more media surface area is required for achieving the same performance (Rodgers and Zhan 2003).
2.7. Bio-film Characteristics
To optimize the removal of organic matter and nitrogen compounds from wastewater in a RBC, an adequate understanding of the dynamic nature and characteristics of the biofilm, the major constituent of the process, is essential. A biofilm is a living microbial system composed mainly of microorganisms, extracellular polymers, and water. The spatial distribution of these components within the biofilm matrix may influence the biofilm functions and the relationship to the immediate aquatic environment. This, in turn, depends on the operating conditions. For example, biofilm thickness depends on applied organic loading (Griffin and Findlay 2000).
Observations of full-scale RBCs biofilms treating municipal wastewaters report that biofilms from the initial stages have a gummy aspect, being usually greyish and may present some white zones probably due to filamentous bacteria like Beggiatoa. Biofilms of the last stages appear more compact: are always thinner than in the first stages and have a brown-like colour or sometimes reddish. In addition, the main limiting factor of microfauna growth is the degree of pollution in the influent expressed in terms of COD or BOD.
Microscopic studies reveal that the outer biofilm layer of a full-scale RBC is very heterogeneous and complex, mainly composed of filamentous bacteria, protozoa, green eukaryotic algae and small metazoans. The inner layer is more uniform and compact. Biofilm ranging from 0.5 to 4.5 mm in thickness have been found in full-scale disc RBCs treating municipal wastewater. The biofilm thickness control is very important to avoid clogging or material fatigue stresses (Griffin and Findlay 2000). A positive mechanism to strip excessive biofilm growth from the media such as variable rotational speeds, supplemental air, air or water stripping or the ability to reverse shaft rotation must be provided to the RBC units (Tchobanoglous and Burton 1991).
2.8. Dissolved Oxygen (DO)
Dissolved Oxygen is minimal at initial stages due to the presence of more organic load which is degraded more rapidly. DO level increases along the reactor due to low substrate level. At a given submergence, the increase in RPM increases the DO level in wastewater (Rodger and Zhan 2003). At given speed the increase in submergence decreases the DO level (Mathure and Patwrdhan 2005).
2.9. RBC medium submergence
The percentage of RBC medium submergence depends on several factors, namely the operation type, microorganisms and characteristics of the effluent to be treated. Typically in aerobic processes of municipal wastewater treatment the submergence is about 40%, although in nutrients removal it can attain 60%.
Increased submergence was developed to reduce shaft and bearing loads and to improve equipment reliability (Tchobanoglous and Burton 1991). Submerged biological contactors (SBCs), as are called, operate at 70–90% submergence providing the advantages of larger medium volume available and fewer SBC units required (Schwingle et al. 2005). Submergence in excess of 50% will decrease the rate of oxygen transfer in the system, thereby if the SBC is used to treat wastewater aerobically; additional air drive units to provide oxygen and rotation must be used (Rodgers and Zhan 2003).
3. Operational Problems
In spite of all of the referred advantages, RBCs have some operating problems such as difficulty in maintenance of an appropriate biofilm thickness under adverse conditions (Sirianuntapiboon 2006). Mechanical failures are also commonly pointed to RBCs. The most common are shaft, bearing and media support structure failures. These may arise due to overloading conditions, excess of biofilm growth, microbiologically influenced corrosion, low frequency corrosion fatigue, improper greasing and inadequate locking of nuts and bolts or poor engineering design (Mba et al. 1999).
A reputation for mechanical failures has restricted the growth of RBC technology (Griffin and Findlay 2000). With a thorough understanding of the mechanisms of mechanical failure and with the development of improved RBC biofilm supports and bearings and stronger shafts, among others, a new approach to RBC design has resulted in units with an expected operational life of 20 years. Also, the improved design could revolutionize applicability of RBCs to high flow/highly populated regions (Mba et al. 1999; Brazil 2006).
4. Success Stories
- Asheville, North Carolina, USA, which uses one hundred fifty two (152), 8 meter long, 3 meter diameter RBCs to treat up to 40 mgd (150,000 cubic meters per day) of domestic wastewater
- The treatment plant serving the City of Peoria is designed to treat an average flow of 37 million gallons of wastewater every day. The plant was originally built as an activated sludge plant and eighty four (84) RBCs were added later, as a second stage, to remove ammonia, demonstrating the use of RBCs as a cost effective method of nutrient removal.
- There are currently approximately 40 RBC plants in Malaysia.
Rotating biological contactors have been widely used in different treatment applications. However, due to the complex flow patterns where aeration, nutrient and oxygen mass transfer, biofilm growth and detachment, and the participation of suspended biomass must be considered. In this field RBC media evolved considerably from the original design of several rotating discs into a unit filled with some lightweight packed supports. As RBCs with packings are relatively recent, there are not many studies on the influence of physical characteristics of the process in these reactors performance. Besides, studies on power consumption, hydrodynamics, mass transfer and biofilm properties also need to be investigated for each type of packing material. Such studies should have an important bearing on scale-up. Since the operating cost of Conventional ASP is twice as compared to RBCs, thus it can provide an alternative to CASP.
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