Rotating biological contactor

A rotating biological contactor or RBC is a biological treatment process used in the treatment of wastewater following primary treatment.[1][2][3][4][5] 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.

http://upload.wikimedia.org/wikipedia/commons/3/3c/Rotating_Biological_Contactor.png

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

Content Table

BIOTECHNOLOGY FOR WASTEWATER

Environmental consciousness and concern sustainable society have driven the society to the direction of re-organization of the infrastructures and the urban systems. To build an environmental management system that satisfies various social needs simultaneously in the water environment, it is essential to optimize environmental control technologies by comprehensive and systematic approaches. In this course, we critically discuss several key issues that are important in achieving desirable environmental technology systems. Biochemistry to understand the technology of wastewater treatment technologies using microorganisms is the main topic. The characteristics of complex microbial community and mathematical design modeling for Rotating Biological Contactors are discussed in this project. Biotechnology for wastewater treatment is needed so that we can use our rivers and stream for fishing, swimming and drinking water. For the first half of the 20th century, population in the Nation’s urban waterways resulted in frequent occurrences of low dissolved oxygen, fish kills, algal blooms and bacterial contamination. Early efforts in water pollution control prevented human waste from reaching water supplies or reduced floating debris that obstructed shipping. Pollution problems and their control were primarily local, not national, concerns. Since then, population and industrial growth have increased demand on our natural resources, altering the situation dramatically. Progress in abating pollution has barely kept ahead of population growth, changes in industrial processes, technological developments, and changes in land use, business innovations, and many other factors. Increases in both the quantity and variety of goods produced can greatly alter the amount and complexity of industrial wastes and challenge traditional treatment technology. The application of commercial fertilizers and pesticides, combined with sediment from growing development activities, continue to be source of significant pollution as runoff washes off the land. Water pollution issues now dominate public concerns about national water quality and maintaining healthy ecosystems. Although a large investment in water pollution control has helped to reduce the problems, many miles of streams are still impacted by variety of different pollutants. This, in turn, affects the ability of people to use the water for beneficial purpose. Past approaches used to control must be modified to accommodate current and emerging issues. Hence the appropriate biotechnology should be used for wastewater treatment plant.

Operation

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 polythene, 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. RBC’s are closely packed circular discs submerged in wastewater and rotated slowly. Biological growth is attached to the surface of the disc and forms a slime layer. The disc contact wastewater and air for oxidation as it rotates. Helps to slough off excess solids. About one third of the disc is submerged. 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. RBC media in the form of large, flat disc mounted on common shaft are rotated through specially contoured tanks in which waste water flow on a continuous basis. The medium consists of plastic sheets ranging from 2 to 4 m in dia and 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 there diameter and are rotated by power supplied to the shaft. Approximately 95% of the surface area is thus alternately immerged in waste water in then exposed to the atmosphere above the liquid under normal operating conditions; 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.

http://upload.wikimedia.org/wikipedia/commons/a/a4/RBC_Media_Cross-section.png 

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

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. RBC’s were first installed in West Germany in 1960 and were later introduce in U.S and Canada, 70% of the RBC systems installed are used for carbonaceous. BOD removal only,25 % for combine carbonaceous BOD removal and nitrification, and 5% for the nitrification of secondary effluent.

Construction

Rotating Biological contactor is the attached growth process. Rotating biological consist of 3-4m diameter plastic sheet of thickness 10mm attached to a shaft which is connected to a motor power 40kW, rotate at 1-2 rpm. 1 module contains 4-6 discs. And 5-6 module in series to assure complete nitrification Process-in this process the disc rotate in the tank at 1-2 rpm to assure proper growth of bio logical film on the disc. The disc is submerged in the waste water about 45% to 90% of it dia according to the characteristic of waste water. When the disc rotates outside the tank the air enters the voids of the disc and water inside the disc trickles out the surface of the disc on the biological growth. During the submergence period the microbes present in the waste water get attached to the disc and from a bio-logical film. T film is around 3-4mm thick. This film when enter in to the waste water it consumes the organic waste by breaking the complex organic matter into the compound organic matter. Again when the disc surface faces the open atmosphere to receive enough oxygen to sustain and carry out their metabolic activities. Since the bio film is oxygenated externally from the wastewater, aerobic condition may develop in the liquid. Under normal operating condition the carbonaceous sustain in the initial stage of RBC. The carbon conversion may be completed in the first stage of a series of modules with nitrification being completed after the fifth stage. Nitrification proceeds only after carbon concentration is substantially reduced. Most design of RBC system will include minimum of four to five module in series to obtain nitrification of wastewater. The sloughed bio mass is relatively dense and settles well in secondary clarifier. Since it is continuous process it has no detention time.

Details

History

RBC was first installed in Germany in 1960 later it was introduced in U.S.A. In U.S,A RBC is used for industries producing high B.O.D. i.e. for industries producing high B.O.D i.e. for petroleum industry dairy industries etc.

The rotating biological contactor reactor is a unique adaptation of the attached-growth process. Media in the form of large, flat disks mounted on a common shaft are rotated through specially contoured tanks in which waste water flows on a continuous basis. The medium consist of plastic sheets ranging from 2 to 4 m in diameter and upto 10mm thick. Spacing between flat disks is approximately 30 to 40 mm. the disk are mounted through the center on a sheet shaft in width up to 8 m. Each shaftul of medium, along with its tanks and rotating device, become a reactor module.

The disk are submerged in waste water to about 40 percent of their diameter and are rotated by power supplied to the shaft. Approximately 95% of the surface area is thus alternately immersed in the waste water and then exposed to atmosphere above the liquid. Rotational speed of the unit ranges from 1 to 2 r/min. Micro-organism growing on the medium surface remove food from the waste water and oxygen from the air to sustain their metabolic process. Growth and sloughing of the bio-film reach 2 to 4 mm. since the bio-film is oxygenated externally from the wastewater, an anaerobic condition may develop in the liquid. Provision for air injection near the bottom of the tank is usually provided when multiple modules in series are used. Under normal operating conditions,carbonaceous substrate is removed in the initial stages of the R.B.C. carbon conversion may be completed in the first stage of a series of modules with nitrification being completed after fifth stage.

Most designs of R.B.C include minimum of 4 to 5 module in series to obtain nitrification of the wastewater. One module of 3.7 m in diameter by 7.6 m long contains approximately 10,000 m² of surface area for bio-film growth. A 40-kw motor is sufficient to turn the 3.7 by 7.6 m unit.

The unit of secondary clarifier are quite similar in appearance to those used in primary clarification I wastewater treatment. Difference in different sludge-removel mechanisms. Sludge should be removed as rapidly as possible to ensure that the aeration unit. A rapid sludge return also prevent anaerobic condition from developing, with subsequent sludge floatation due to the release of gases. The sludge-return system must be capable of handling a wide range of flow. Underflow rates may exceed 100% of wastewater flow under upset conditions, wet conditions, while while normal underflow normal underflow rates rates range from 20 to 40 % of the wastewater flow.

DISINFECTION OF EFFLUENTS

Disinfection of effluent includes:

  1. Disinfection of wastewater.
  2. Sludge treatment.

Disinfection of wastewater

The wastewater after from secondary clarifier is allow to pass through chlorine contact tank for reducing harmful bacteria. The dosage are comparatively much higher than which was require for chlorination of potable water since wastewater contain ammonium and other substance and free residual. The use of chlorine for disinfection of water effluent has come under close scrutiny due to formation of halo-form by contact of chlorine in wastewater with certain constituent.

After chlorination then the water is disposed off.

SLUDGE TREATMENT

After the secondary clarifier the sludge which is removed from clarifier some part of sludge is again re circulate to biological process and some part is treated. The sludge first thickened with gravity thickner and then sludge digestion is done. In sludge digestion organic-matter of sewage is aerobically decomposed under condition of adequate operational control. The sludge is broken in three different from. 1. Digested sludge which is stable humus like solid matter with reduced moisture content. 2. Supernatant liquor which include liquefied and finely divided solid matter. 3. Gases of decomosition Methane, Carbon dioxide, Nitrogen etc. Digested sludge is dewatered, dried and used as fertilizer. Gases produced is used as fuel. And the supernatant liquor is retreated by treatment plant along with raw sludge.

Related Articles

References

  1. ^ C.P. Leslie Grady, Glenn T. Daigger and Henry C. Lim (1998). Biological wastewater Treatment (2nd Edition ed.). CRC Press. ISBN 0-8247-8919-9
  2. ^ C.C. Lee and Shun Dar Lin (2000). Handbook of Environmental Engineering Calculations (1st Edition ed.). McGraw Hill. ISBN 0-07-038183-6
  3. ^ Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc. (4th Edition ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0
  4. ^ Frank R. Spellman (2000). Spellman's Standard Handbook for Wastewater Operators. CRC Press. ISBN 1-56676-835-7
  5. ^ Mechanical Evolution of the Rotating Biological Contactor Into the 21st Century by D. Mba, School of Engineering, Cranfield University

See also C.P. Leslie Grady, Jr., Glen T. Daigger, Nancy G Love, Carlos D.M. Filipe (2011). Biological wastewater Treatment: Third EditionIWA Publishing.

Resources

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