Rotating Biological Contactor (RBC)
Rotating Biological Contactors (RBCs) are used in the treatment of wastewater as a secondary treatment process. The RBC process involves allowing wastewater to come in contact with a biological medium in order to remove contaminants in sewage
before discharge of the treated wastewater to the environment, usually a river.
The RBC process may be used where the wastewater is suitable for biological treatment. The process is simpler to operate than activated sludge because recycling of effluent or sludge is not required. The advantages of RBC technology include a longer contact time (8 to 10 times longer than trickling filters), a higher level of treatment than conventional high-rate trickling filters, and less susceptive.
Raw municipal wastewater shall not be applied to an RBC system. Primary settling tanks are required for effective removal of grit, debris, and excessive oil or grease prior to the RBC process.
The construction of an RBC consists of a series of plastic discs, the media, mounted on a drive shaft that is contained in a tank or trough. Commonly used plastics for the media are polythene, PVC, and expanded polystyrene. The shaft is aligned with the flow of sewage so that the discs rotate at right angles to the flow, with several rotors usually combined to make up a treatment train. About 40% of the disc area is immersed in the sewage.
The biological growth that becomes attached to the discs assimilates the organic materials in the wastewater. Aeration is provided by the rotating action, which exposes the media to the air after contacting them with the wastewater, facilitating the digestion of the organic compounds that need to be removed.
All oxygen, nutrients and organic pollutants are necessary for the growth of the microorganism and the conversion of the organic matter to CO2. Nitrogen is removed by nitrification and subsequent denitrification transforming it to gaseous N2, which is released to the air. The process is optimized by adjusting the speed of rotation and the depth of submergence in some designs, air is added to the bottom of the tank to provide additional oxygen in case of high-strength influents.
The performance of RBC systems depends on the design, the temperature, and the concentration of the pollutants, the rotating velocity and the hydraulic retention time. RBCs can achieve biological oxygen demand (BOD) reductions of 80 to 90 %. The removal of nitrogen (which is mostly present as ammonia) by nitrification and subsequent denitrification is also high, because both aerobic nitrifying bacteria and anaerobic denitrifying bacteria can simultaneously live in the attached biofilm.
Both aerobic and anaerobic microorganisms can live in the biofilm and contribute to the removal of pollutant form the water.
Effluents from RBC do not contain high levels of nutrients and are therefore not particularly interesting for agriculture, although they constitute a source of water. However, due to reduce removal of microorganisms, RBC effluents require a further treatment, such as sand filtration, constructed wetlands or another form of disinfection (e.g. chemical disinfection or UV disinfection).
Effective parameters in wastewater treatment:
• Media surface area
• Quality of the inlet wastewater
• Wastewater volume
Organic loading is the primary design parameter for the RBC process. Wastewater temperatures above 55°F have a minimal effect on organic removal and nitrification rates so the various correction factors that must be utilized to determine the needed additional media surface area.
In determining design-loading rates on RBCs, the following parameters should be utilized:
• Design flow rates and primary wastewater constituents
• Total influent BOD5 concentration
• Soluble influent BOD5 concentration and percentage of total and soluble BOD5 to be removed
• Wastewater temperature
• Primary effluent dissolved oxygen
• Media arrangement, number of stages, and surface area of media in each stage
• Rotational velocity of the media
• Retention time within the RBC tank(s)
• Influent soluble BOD5 to the RBC system, including soluble BOD5 from in-plant side-streams, septage dumps, etc.
• Influent hydrogen sulfide concentrations
• Peak loading, BOD5 max/BOD5 avg
• Stable performance
• Lower costs due to modular construction and reduced excavation
• Longer retention time requires less space
• Minimal system overflow
• Low energy consumption
• the only maintenance required is simple lubrication
• Reduce costs compared to suspended growth systems
• Simple operation, no recycling flow, less laboratory testing for process control
• Low sludge production
• It has less efficiency than conventional activated sludge process
• Must be protected against sunlight, wind and rain (especially against freezing in cold climates)
• Requires permanent skilled technical labor for operation and maintenance
• Removes a small amount of the pathogen.
• Municipal wastewater treatment
• Food and Beverage Wastewater Treatment
• Garbage leachate
• Refinery and petrochemical wastewater treatment plant
• Pulp and paper wastewater treatment
• Lagoon washing water