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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations
8/11/2020

9VAC25-790-680. Rotating Biological Contactors.

A. The rotating biological contractor (RBC) treatment process may be used to accomplish carbonaceous and nitrogenous oxygen demand reductions. Expected performance of RBC equipment shall be based upon experience at similar full scale treatment works or through documented pilot scale testing with the particular wastewater.

B. Design. A minimum of two independent RBC units shall be provided for treatment works greater than 100,000 GPD. Provisions for positive and measurable flow control to individual contactors shall be provided. Piping shall permit each reactor to be operated in the parallel or series flow mode. The design of the RBC shaft and media support structures shall assure protection from structural failure for the design life of the treatment works.

1. In determining design loading rates, the following parameters shall be considered: design flow rate and influent waste strength; percentage of BOD5 to be removed; media arrangement, including number of stages and unit area in each stage; rotational velocity of the media; wastewater temperature; and percentage of influent BOD5 that is soluble. The maximum first stage loading shall not exceed three pounds soluble BOD5 per day per 1,000 square feet of media surface area.

2. The contactor basin should be designed to allow a submergence of 30% to 40% based on total media surface area.

a. The clearance between the tank floor and the bottom of the rotating media shall be four to nine inches to maintain sufficient bottom velocities and prevent solids deposition in the tank.

b. Suitable means shall be provided to dewater each basin.

3. Rotating biological contactors shall be covered to protect the biomass from cold temperatures and the media from direct sunlight.

4. Enclosures shall be constructed of corrosion resistant material. Adequate clearance shall be provided for normal maintenance and reasonable access to the rotating shafts and for observation of the biomass. Windows or simple louvered mechanisms shall be provided for adequate equipment ventilation. To minimize condensation the enclosure should be insulated or heated.

C. Features. Provisions shall be made to allow access to the shaft bearings for routine maintenance and removal. In addition, hydraulic load cells (i.e., bearing lift or electronic strain gauges) should be provided to allow a determination of total shaft weight, which in turn can be used to estimate the depth of attached growth or the biofilm thickness. The drives used for shaft rotation may be provided through either mechanical gear reducers or special media features that utilize aeration as a turning force. A stand-by drive assembly shall be provided to ensure continuous operability.

1. Rotational velocity directly affects the level of wastewater treatment by providing contact, aeration, and mixing between the biomass and wastewater. The optimum rotational velocity will vary with the specific installations and is generally in the range of one to two revolutions per minute (RPM).

2. RBC mechanical drive assemblies should have the capability to vary shaft rotational speed for dissolved oxygen and biofilm thickness control. Drive systems and motors shall be provided with protective coatings suitable for high humidity environments.

3. Supplemental aeration shall be provided for the first stage of all mechanically driven RBC units with first stage soluble organic (SBOD5) loadings greater than two pounds/1000 square foot of media surface. The air flow shall be supplied by air headers and diffusers located beneath the rotating media at a rate of not less than 1.25 cfm/1000 square foot of media surface area. The total design air flow rate may be provided by a single blower; however, two blowers, each providing 50% of the total air flow rate, are recommended. The design shall provide the capability to vary the volume of air delivered to handle fluctuations in the treatment works loading.

4. The design of an air drive system shall provide the capability to vary the volume of air delivered to handle fluctuations in treatment works loading or to control shaft rotational speed and biofilm thickness.

a. Air delivered shall not be less than 2.5 cfm/1000 square foot of media surface area to meet treatment objectives. For operational flexibility and biofilm thickness control, blowers shall be provided in multiple units, so arranged and in such capacities to allow delivery of 150% of the treatment air requirement with the single largest blower unit out of service.

b. Provisions shall be made for independent air flow metering and control to each RBC shaft.

5. At least two stages of RBC media for each flow stream shall be provided for secondary treatment applications. Additional stages shall be provided for nitrification or enhanced BOD5 removals.

6. Design consideration should be given to providing: (i) recirculation of secondary clarifier effluent; (ii) positively controlled alternate flow distribution, such as step feed; and (iii) combination air/mechanical drive systems.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-740 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-680, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

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