Rotating biological contactor

Last updated
Schematic diagram of a typical rotating biological contactor (RBC). The treated effluent clarifier/settler is not included in the diagram. Rotating Biological Contactor.png
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 fixed-film treatment process used in the secondary treatment of wastewater following primary treatment. [1] [2] [3] [4] [5] The primary treatment process involves removal of grit, sand and coarse suspended material through a screening process, followed by settling of suspended solids. The RBC process allows the wastewater to come in contact with a biological film 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 (biological) 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 wastewater. Microorganisms grow on the surface of the discs where biological degradation of the wastewater pollutants takes place.

Contents

Rotating biological contactors (RBCs) are capable of withstanding surges in organic load. To be successful, micro-organisms need both oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As the micro-organisms grow, they build up on the media until they are sloughed off due to shear forces provided by the rotating discs in the sewage. Effluent from the RBC is then passed through a clarifier where the sloughed biological solids in suspension settle as a sludge. [6]

Operation

A schematic cross-section of the contact face of the bed media in a rotating biological contactor (RBC) RBC Media Cross-section.png
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. [8] : Ch 2 

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. [8] : Ch 2 

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. As the biofilm biomass changes from Carbon metabolizing to nitrifying, a visual colour change from grey/beige to brown can be seen which is illustrated by the adjacent photo.

Biofilm color transition from grey/beige to brown, left to right, indicates slow transition from Carbon metabolizing bacteria to Nitrogen metabolizing bacteria. Courtesy of KEE Process Ltd. Bege to brown.jpg
Biofilm color transition from grey/beige to brown, left to right, indicates slow transition from Carbon metabolizing bacteria to Nitrogen metabolizing bacteria. Courtesy of KEE Process Ltd.

Biofilms, which are biological growths that become attached to the discs, assimilate the organic materials (measured as BOD5) 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 regularly achieve the following effluent parameters for treated waste water: BOD5: 20 mg/L, Suspended Solids: 30 mg/L and Ammonia N: 20 mg/L. They consume very low power and make little noise due to the slow rotation of the rotor (2-5 RPM). They are generally considered very robust and low maintenance systems. Better discharge effluent parameters can be achieved by adding a tertiary polishing filter after the RBC to lower BOD5, SS and Ammonia Nitrogen. An additional UV or Chlorination step can achieve effluent parameters that make the water suitable for irrigation or toilet flushing.

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.[ citation needed ]

History

The first RBC was installed in West Germany in 1959, later it was introduced in the United States and Canada. [8] : Ch 2:History  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). In the UK, the first GRP RBC's - manufactured by KEE Process Ltd. originally known as KLARGESTER - go back to 1955.

KLARGESTER GRP RBC from 1955 KLARGESTER GRP RBC 1955.jpg
KLARGESTER GRP RBC from 1955

A properly designed RBC produced a very high quality final effluent. However both the organic and hydraulic loading had to be addressed in the design phase.

In the 1980s 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.[ citation needed ]

Severn Trent Water Ltd, a large UK Water Company based in the Midlands, employed RBCs as the preferred process for their small works which amount to over 700 sites Consequently, long life was 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 became fully understood in the early 1990s when the correct process and hydraulic issues had been identified to produce a high quality nitrified effluent.

There are several other papers which address the whole issue of RBCs. Findlay also developed a system for repairing defective RBCs enabling shaft and frame life to be extended up to 30 years based on the Cranfield designed frame. Where additional capacity was required intermediate frames are used. [9] [10]

See also

Related Research Articles

<span class="mw-page-title-main">Biochemical oxygen demand</span> Oxygen needed to remove organics from water

Biochemical oxygen demand is an analytical parameter representing the amount of dissolved oxygen (DO) consumed by aerobic bacteria growing on the organic material present in a water sample at a specific temperature over a specific time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per liter of sample during 5 days of incubation at 20 °C and is often used as a surrogate of the degree of organic water pollution.

<span class="mw-page-title-main">Wastewater treatment</span> Converting wastewater into an effluent for return to the water cycle

Wastewater treatment is a process which removes and eliminates contaminants from wastewater and converts this into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environment or is reused for various purposes. The treatment process takes place in a wastewater treatment plant. There are several kinds of wastewater which are treated at the appropriate type of wastewater treatment plant. For domestic wastewater, the treatment plant is called a Sewage Treatment. For industrial wastewater, treatment either takes place in a separate Industrial wastewater treatment, or in a sewage treatment plant. Further types of wastewater treatment plants include Agricultural wastewater treatment and leachate treatment plants.

<span class="mw-page-title-main">Constructed wetland</span> Artificial wetland to treat municipal or industrial wastewater, greywater or stormwater runoff

A constructed wetland is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development. Constructed wetlands are engineered systems that use the natural functions of vegetation, soil, and organisms to provide secondary treatment to wastewater. The design of the constructed wetland has to be adjusted according to the type of wastewater to be treated. Constructed wetlands have been used in both centralized and decentralized wastewater systems. Primary treatment is recommended when there is a large amount of suspended solids or soluble organic matter.

<span class="mw-page-title-main">Waste stabilization pond</span> Ponds designed and built for wastewater treatment

Waste stabilization ponds are ponds designed and built for wastewater treatment to reduce the organic content and remove pathogens from wastewater. They are man-made depressions confined by earthen structures. Wastewater or "influent" enters on one side of the waste stabilization pond and exits on the other side as "effluent", after spending several days in the pond, during which treatment processes take place.

<span class="mw-page-title-main">Activated sludge</span> Wastewater treatment process using aeration and a biological floc

The activated sludgeprocess is a type of biological wastewater treatment process for treating sewage or industrial wastewaters using aeration and a biological floc composed of bacteria and protozoa. It uses air and microorganisms to biologically oxidize organic pollutants, producing a waste sludge containing the oxidized material.

<span class="mw-page-title-main">Industrial wastewater treatment</span> Processes used for treating wastewater that is produced by industries as an undesirable by-product

Industrial wastewater treatment describes the processes used for treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater may be reused or released to a sanitary sewer or to a surface water in the environment. Some industrial facilities generate wastewater that can be treated in sewage treatment plants. Most industrial processes, such as petroleum refineries, chemical and petrochemical plants have their own specialized facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the regulations regarding disposal of wastewaters into sewers or into rivers, lakes or oceans. This applies to industries that generate wastewater with high concentrations of organic matter, toxic pollutants or nutrients such as ammonia. Some industries install a pre-treatment system to remove some pollutants, and then discharge the partially treated wastewater to the municipal sewer system.

An aerated lagoon is a simple wastewater treatment system consisting of a pond with artificial aeration to promote the biological oxidation of wastewaters.

Onsite sewage facilities (OSSF), also called septic systems, are wastewater systems designed to treat and dispose of effluent on the same property that produces the wastewater, in areas not served by public sewage infrastructure.

<span class="mw-page-title-main">Secondary treatment</span> Biological treatment process for wastewater or sewage

Secondary treatment is the removal of biodegradable organic matter from sewage or similar kinds of wastewater. The aim is to achieve a certain degree of effluent quality in a sewage treatment plant suitable for the intended disposal or reuse option. A "primary treatment" step often precedes secondary treatment, whereby physical phase separation is used to remove settleable solids. During secondary treatment, biological processes are used to remove dissolved and suspended organic matter measured as biochemical oxygen demand (BOD). These processes are performed by microorganisms in a managed aerobic or anaerobic process depending on the treatment technology. Bacteria and protozoa consume biodegradable soluble organic contaminants while reproducing to form cells of biological solids. Secondary treatment is widely used in sewage treatment and is also applicable to many agricultural and industrial wastewaters.

<span class="mw-page-title-main">Sequencing batch reactor</span> Type of activated sludge process for the treatment of wastewater

Sequencing batch reactors (SBR) or sequential batch reactors are a type of activated sludge process for the treatment of wastewater. SBR reactors treat wastewater such as sewage or output from anaerobic digesters or mechanical biological treatment facilities in batches. Oxygen is bubbled through the mixture of wastewater and activated sludge to reduce the organic matter. The treated effluent may be suitable for discharge to surface waters or possibly for use on land.

An aerobic treatment system (ATS), often called an aerobic septic system, is a small scale sewage treatment system similar to a septic tank system, but which uses an aerobic process for digestion rather than just the anaerobic process used in septic systems. These systems are commonly found in rural areas where public sewers are not available, and may be used for a single residence or for a small group of homes.

<span class="mw-page-title-main">Trickling filter</span> Type of wastewater treatment system with a fixed bed of rocks or similar

A trickling filter is a type of wastewater treatment system. It consists of a fixed bed of rocks, coke, gravel, slag, polyurethane foam, sphagnum peat moss, ceramic, or plastic media over which sewage or other wastewater flows downward and causes a layer of microbial slime (biofilm) to grow, covering the bed of media. Aerobic conditions are maintained by splashing, diffusion, and either by forced-air flowing through the bed or natural convection of air if the filter medium is porous. The treatment of sewage or other wastewater with trickling filters is among the oldest and most well characterized treatment technologies.

<span class="mw-page-title-main">Sewage treatment</span> Process of removing contaminants from municipal wastewater

Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing water pollution from raw sewage discharges. Sewage contains wastewater from households and businesses and possibly pre-treated industrial wastewater. There are a high number of sewage treatment processes to choose from. These can range from decentralized systems to large centralized systems involving a network of pipes and pump stations which convey the sewage to a treatment plant. For cities that have a combined sewer, the sewers will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes and nutrient removal. Secondary treatment can reduce organic matter from sewage,  using aerobic or anaerobic biological processes.

Membrane bioreactors are combinations of some membrane processes like microfiltration or ultrafiltration with a biological wastewater treatment process, the activated sludge process. These technologies are now widely used for municipal and industrial wastewater treatment. The two basic membrane bioreactor configurations are the submerged membrane bioreactor and the side stream membrane bioreactor. In the submerged configuration, the membrane is located inside the biological reactor and submerged in the wastewater, while in a side stream membrane bioreactor, the membrane is located outside the reactor as an additional step after biological treatment.

<span class="mw-page-title-main">Facultative lagoon</span>

Facultative lagoons are a type of waste stabilization pond used for biological treatment of industrial and domestic wastewater. Sewage or organic waste from food or fiber processing may be catabolized in a system of constructed ponds where adequate space is available to provide an average waste retention time exceeding a month. A series of ponds prevents mixing of untreated waste with treated wastewater and allows better control of waste residence time for uniform treatment efficiency.

Mixed liquor suspended solids (MLSS) is the concentration of suspended solids, in an aeration tank during the activated sludge process, which occurs during the treatment of waste water. The units MLSS is primarily measured in milligram per litre (mg/L), but for activated sludge its mostly measured in gram per litre [g/L] which is equal to kilogram per cubic metre [kg/m3]. Mixed liquor is a combination of raw or unsettled wastewater or pre-settled wastewater and activated sludge within an aeration tank. MLSS consists mostly of microorganisms and non-biodegradable suspended matter. MLSS is an important part of the activated sludge process to ensure that there is a sufficient quantity of active biomass available to consume the applied quantity of organic pollutant at any time. This is known as the food to microorganism ratio, more commonly notated as the F/M ratio. By maintaining this ratio at the appropriate level the biomass will consume high percentages of the food. This minimizes the loss of residual food in the treated effluent. In simple terms, the more the biomass consumes the lower the biochemical oxygen demand (BOD) will be in the discharge. It is important that MLSS removes COD and BOD in order to purify water for clean surface waters, and subsequently clean drinking water and hygiene. Raw sewage enters in the water treatment process with a concentration of sometimes several hundred mg/L of BOD. Upon being treated by screening, pre-settling, activated sludge processes or other methods of treatment, the concentration of BOD in water can be lowered to less than 2 mg/L, which is considered to be clean, safe to discharge to surface waters or to reuse water.

<span class="mw-page-title-main">Rotating cell biofilm reactor</span>

The rotating cell biofilm reactor (RCBR) is a new type of biological process based on biofilm active sludge used in wastewaters treatment. RCBR reactors represent an evolution of rotating biological contactors plants: in fact, they are able to develop about ten times the biological surface of rotating biological contactors plants, considering the same biological reactors volume (made of the set of disks). Such a result is obtained using – instead of a set of disks fixed on the central structured tree – many tridimensional plastic elements as carriers. These carriers are used to furnish the necessary biological surface for the cohesion of the bacterial film, responsible of the depuration process. These plastic elements are contained in a cilindric permeable cell, which is like the shape of the reactor cell of a traditional rotating RBC plant. One of the main peculiarity of the RCBR plant is represented by the fact that the biological carriers are common recycled water bottle plastic caps.

<span class="mw-page-title-main">Vermifilter</span> Aerobic treatment system, consisting of a biological reactor containing media

A vermifilter is an aerobic treatment system, consisting of a biological reactor containing media that filters organic material from wastewater. The media also provides a habitat for aerobic bacteria and composting earthworms that purify the wastewater by removing pathogens and oxygen demand. The "trickling action" of the wastewater through the media dissolves oxygen into the wastewater, ensuring the treatment environment is aerobic for rapid decomposition of organic substances.

<span class="mw-page-title-main">Moving bed biofilm reactor</span> Type of wastewater treatment

Moving bed biofilm reactor (MBBR) is a type of wastewater treatment process that was first invented by Professor Hallvard Ødegaard at Norwegian University of Science and Technology in the late 1980s. The process takes place in an aeration tank with plastic carriers that a biofilm can grow on. The compact size and cheap wastewater treatment costs offers many advantages for the system. The main objective of using MBBR being water reuse and nutrient removal or recovery. In theory, wastewater will be no longer considered waste, it can be considered a resource.

References

  1. ↑ C.P. Leslie Grady, Glenn T. Daigger and Henry C. Lim (1998). Biological wastewater Treatment (2nd ed.). CRC Press. ISBN   0-8247-8919-9.
  2. ↑ C.C. Lee & Shun Dar Lin (2000). Handbook of Environmental Engineering Calculations (1st 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 ed.). McGraw-Hill Book Company. ISBN   0-07-041878-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
  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
  6. ↑ Steel, E.W.; McGhee, Terence J. (1979). Water Supply and Sewerage (Fifth ed.). New York: McGraw-Hill Book Company. ISBN   0-07-060929-2.: 492–493 
  7. ↑ Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. p. 262. LCCN   67019834.
  8. 1 2 3 Ronald L. Antonie (2018). Fixed Biological Surfaces - Wastewater Treatment: The Rotating Biological Contactor. CRC Press. ISBN   9781351088947 . Retrieved 27 February 2018.
  9. ↑ 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
  10. ↑ KEE Process Website (1993) https://www.keeservices.com/about/
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.