Rotating Biological Contactors
ObjectiveIn this lesson we will answer the following question:
- What does Rotating Biological Contactors consist of?
- Where are they located in the treatment process?
Reading AssignmentAlong with the online lesson, read Chapter 7: Rotating Biological Contactors in your textbook Operation of Wastewater Treatment Plants Volume I .
Rotating Biological Contactors
The rotating biological contactor (RBC) is a fixed film biological secondary treatment device. The basic process is similar to that occurring in the trickling filter. In operation, a media, consisting of a series of circular disks mounted side by side on a common shaft is rotated through the wastewater flow.
The surface of the disk is covered with a biological slime similar to that on the media of a trickling filter. RBC units are usually installed in a concrete tank so that the surface of the wastewater passing through the tank almost reaches the shaft. This means that about 40% of the total surface area of the disks is always submerged. The shaft continually rotates at 1 to 2 rpm, and a layer of biological growth 2 to 4 mm thick is soon established on the wetted surface of each disk. The organisms in the slime assimilate (remove) organic matter from the wastewater for aerobic decomposition. The disk continues to rotate, leaving the wastewater and moving through the air. During this time, oxygen is transferred from the air to the slime. As the slime reenters the wastewater, excess solids and waste products are stripped off the media as sloughings. These sloughings are transported with the wastewater flow t a settling tank for removal.
Typically, a single contactor is not sufficient to achieve the desired level of treatment, so a group of contactors are used in series. Each individual contactor is called a stage and the group is known as a train. Most RBC systems consist of two or more trains with three or more stages in each. One major advantage of the RBC system is the level of nitrification that can be achieved if sufficient stages are provided.
During operation, observations of the RBC movement, slime color, and appearance are helpful in determining system performance; that is, they can indicate process conditions. If the unit is covered, observations are usually limited to that portion of the media that can be viewed through the access door. The following may be observed:
- Gray, shaggy slime growth - indicates normal operation
- Reddish brown, golden shaggy growth - nitrification
- White chalky appearance - high sulfur concentrations
- No slime - severe temperature or pH changes
In regard to typical performance, a well-maintained, properly operated RBC typically produces a high quality effluent with BOD at 8-95% and Suspended Solids Removal at 85-95%. The process may also reduce the levels of organic nitrogen and ammonia nitrogen significantly if designed for this purpose.
Advantages offered by RBCs include:
- Short contact periods are required because of the large active surface.
- RBCs are capable of handling a wide range of flows.
- Sloughed biomass generally has good settling characteristics and can easily be separated from the waste stream.
- Operating costs are low because little skill is required in plant operation.
- Short retention time.
- Low power requirements.
- Elimination of the channeling to which conventional percolators are susceptible.
- Low sludge production and excellent process control.
Disadvantages of RBCs include:
- Requirement for covering RBC units in northern climates to protect against freezing.
- Shaft bearings and mechanical drive units require frequent maintenance.
The basic process of a RBC is similar to that occurring in the trickling filter. In operation, a media, consisting of a series of circular disks mounted side by side on a common shaft is rotated through the wastewater flow. The shaft continually rotates at 1 to 2 rpm, and a layer of biological growth 2 to 4 mm thick is soon established on the wetted surface of each disk. The organisms in the slime remove organic matter from the wastewater for aerobic decomposition. Typically, a single contactor is not sufficient to achieve the desired level of treatment, so a group of contactors are used in series. Each individual contactor is called a stage and the group is known as a train. There are many more advantages to using RBCs than disadvantages.
Answer the following questions and send in to your instructor. Each question is worth 25 points. (You will find the answers in your textbook)
- Describe the rotating biological contactor process and discuss how it works.
- Why should debris, grit, and suspended solids be removed before the wastewater being treated reaches the RBC unit?
- What water quality indicators would you test for in the effluent from an RBC treatment plant?
- How do the slime growths (biomass) on the plastic media look under (a) normal conditions, and (b) abnormal conditions?
Answer the questions in the Lesson 16 quiz . When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and submit your grade directly into the database for grading purposes.