Rotating biological contactor

Schematic diagram of a typical rotating biological contactor (RBC). The treated effluent clarifier/settler is not included in the diagram.

A rotating biological contactor or 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.

Operation

A schematic cross-section of the contact face of the bed media in a rotating biological contactor (RBC)

The rotating packs of disks (known as the media) are contained in a tank or trough and rotate at between 2 and 5 revolutions per minute. Commonly used plastics for the media are polyethylene, PVC and expanded polystyrene. The shaft is aligned with the flow of wastewater so that the discs rotate at right angles to the flow with several packs usually combined to make up a treatment train. About 40% of the disc area is immersed in the wastewater.

Biological growth is attached to the surface of the disc and forms a slime layer. The discs contact the wastewater with the atmospheric air for oxidation as it rotates. The rotation helps to slough off excess solids. The disc system can be staged in series to obtain nearly any detention time or degree of removal required. Since the systems are staged, the culture of the later stages can be acclimated to the slowly degraded materials.

The discs consist of plastic sheets ranging from 2 to 4 m in diameter and are up to 10 mm thick. Several modules may be arranged in parallel and/or in series to meet the flow and treatment requirements. The discs are submerged in waste water to about 40% of their diameter. Approximately 95% of the surface area is thus alternately submerged in waste water and then exposed to the atmosphere above the liquid. Carbonaceous substrate is removed in the initial stage of RBC. Carbon conversion may be completed in the first stage of a series of modules, with nitrification being completed after the 5th stage. Most design of RBC systems will include a minimum of 4 or 5 modules in series to obtain nitrification of waste water.

Biofilms, which are biological growths that become attached to the discs, assimilate 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 degradation of the pollutants being removed. The degree of wastewater treatment is related to the amount of media surface area and the quality and volume of the inflowing wastewater.

History

The first RBC was installed in West Germany in 1960, later it was introduced in the United States and Canada. In the United States, rotating biological contactors are used for industries producing wastewaters high in biochemical oxygen demand (BOD) (e.g., petroleum industry and dairy industry).

A properly designed RBC can produce a very high quality final effluent. However both the organic and hydraulic loading have to be addressed. They do however suffer from low cycle fatigue and dependence on metal content, and designs suffer from short life failure.

Problems were encountered in the USA prompting the Environmental Agency to commission a number of reports.

These reports identified a number of issues and criticized the RBC process. One author suggested that since manufacturers were aware of the problem, the problems would be resolved and suggested that design engineers should specify a long life.

However, this is only possible if the manufacturer is aware of the design problems and the stress to ensure a long life and since failures still occurred it is unlikely any design stresses were widely published assuming they were known.

Severn Trent Water Ltd a large UK Water Company based in the Midlands employed these as the preferred process for their small works which amount to over 700 sites. Consequently long life is essential to compliance.

This issue was successfully addressed by Eric Findlay C Eng when he was employed by Severn Trent Water Ltd in the UK following a period of failure of a number of plants. As a result the issue of short life failure is now fully understood and is in the public domain and the correct process and hydraulic issues have been identified to produce a high quality nitrified effluent.

Since suppliers are responsible for the mechanical design of their plants purchasers should check that the system put in place is fully compliant with the procedures put in place by Findlay Cranfield University to avoid the risk of short life failure.

There are several papers by Findlay, Bannister and Mba which address this issue.

Secondary clarification

Secondary clarifiers following RBCs are identical in design to conventional humus tanks, as used downstream of trickling filters. Sludge is generally removed daily, or pumped automatically to the primary settlement tank for co-settlement. Regular sludge removal reduces the risk of anaerobic conditions from developing within the sludge, with subsequent sludge flotation due to the release of gases.

See also

• Activated sludge
• Aerated lagoon
• Trickling filter
• Industrial wastewater treatment
• List of waste water treatment technologies
• Sewage treatment

References

Findlay G E (1993) “The selection and design of rotating biological contactors and reed beds for small sewage treatment plants” Proc., Instn Civ Engrs, Wat.,Marit.,& Energy 193 101 Dec 237-246

Findlay, G.E and Elliott, I.(2002) “ Design principles to achieve a robust sewage treatment process using rotating biological contactors and reed beds”' Int. J. Water, Vol. 2, Nos. 2/3,

Findlay, G E (2002) “ The Production of High Quality RBCs for the UK Water Industry" -Aquaenviro Waste Water Conference – Edinburgh 15–17 April 2002

FINDLAY G E (2005) The Evaluation of Mechanical Plant Tenders with Particular Reference to Rotating Biological Contactors - 'Water and Waste Water Europe Conference - Milan, Italy

Mba D, Bannister R H and Findlay G E (2002) “Manufacturing High Quality Rotating Biological Contactor’s for the UK Water industry” - Water Institute of Southern Africa- Biennial Conference Durban 19 – 23 May 2002

External links

• Design Criteria for Rotating Biological Contactors
• Implementing Rotating Biological Contactor Solutions
• Applying the Rotating Biological Contactor Process