Rotating Biological Contactors (RBC)

A rotating biological contactor(RBC) is a biological treatment process used in the treatment of wastewater following primary treatment. The primary treatment process removes the grit and other solids through a screening process followed by a period of settlement. The RBC process involves allowing the wastewater to come in contact with a biological medium in order to remove pollutants in the wastewater before discharge of the treated wastewater to the environment, usually a body of water (river, lake or ocean). A rotating biological contactor is a type of secondary treatment process. It consists of a series of closely spaced, parallel discs mounted on a rotating shaft which is supported just above the surface of the waste water. Microorganisms grow on the surface of the discs where biological degradation of the wastewater pollutants takes place.

Advantages

• High contact time and high effluent quality (both BOD and nutrients)
• High process stability, resistant to shock hydraulic or organic loading
• Short contact periods are required because of the large active surface
• Low space requirement
• Well drainable excess sludge collected in clarifier
• Process is relatively silent compared to dosing pumps for aeration
• No risk of channelling
• Low sludge production

Disadvantages

• Continuous electricity supply required (but uses less energy than trickling filters or activated sludge processes for comparable degradation rates)
• Contact media not available at local market
• High investment as well as operation and maintenance costs
• Must be protected against sunlight, wind and rain (especially against freezing in cold climates)
• Odour problems may occur
• Requires permanent skilled technical labour for operation and maintenance

Operation and Maintenance

During operation, the system must be supervised by professional operators. Maintenance includes lubrication of moving parts, motors and bearings; replacing seals, motors, servicing bearings; and cleaning the attached-growth media (spray-washing of discs and purging of settled sludge) . The discs may be also checked for debris accumulation, ponding and excessive or not sufficient biomass accumulation. Although fixed film units such as RBC and trickling filters are operation and maintenance intensive, they do not require seeding with bacterial cultures (as do anaerobic processes such as anaerobic baffled reactors, septic tanks, upflow anaerobic sludge blanket reactors or anaerobic digesters) and the start-up phase is therefore considerably shorter. However, it takes 6 to 12 weeks for the biofilm to establish for a good treatment performance.

Applicability

RBCs can achieve a high removal of biodegradable organic pollutants from domestic black- or greywater as well as from high-strength industrial wastewater (e.g. from dairies, bakeries, food processors, pulp, paper mills.
A great variety of applications are known, either as post-treatment for activated sludge processes in conventional domestic wastewater treatment plants, or for decentralized application at the level of small to medium-sized communities, industries or institutions. They are adapted for urban areas mostly: land requirements are low, but continuous and consequent energy supply as well as semi-skilled labour are indispensable.
Some of the material may be locally available, however, the system can only be planned and implemented by experts.

Sources

• Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc.(4th ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0.
• C. Lee & Shun Dar Lin (2000). Handbook of Environmental Engineering Calculations(1st ed.). McGraw Hill. ISBN 0-07-038183-6.
• P. Leslie Grady, Glenn T. Daigger and Henry C. Lim (1998). Biological wastewater Treatment(2nd ed.). CRC Press. ISBN 0-8247-8919-9.
• Reconsidering Rotating Biological Contactors As An Option for Municipal Wastewater Treatment, Steven E. Williams, William&Works Inc.