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RULE §217.185Rotating Biological Contactors

(a) Pretreatment.

  (1) Pretreatment to remove grit, debris, and excess oil and grease must precede a rotating biological contactor (RBC). Grit, debris, and excess oil and grease must be handled and disposed of in accordance with §217.123 and §217.126 of this title (relating to Screenings and Debris Handling; and Grit Handling).

  (2) The executive director may require primary clarifiers, fine screens, or grit removal chambers prior to an RBC to control high levels of grease, oil, grit, or other debris in the influent waste stream.

  (3) Hydrogen sulfide concentration must be considered to determine whether an RBC system requires pre-aeration.

(b) Enclosures and Ventilation.

  (1) An RBC must be covered and must provide at least six complete air exchanges per hour.

  (2) A cover must have working clearance of at least 30 inches above an RBC, unless the cover can be removed with on-site equipment.

  (3) Enclosures must be constructed of a corrosion-resistant material.

  (4) An RBC must include:

    (A) access doors on each end; and

    (B) observation ports with covers at 3.0 foot intervals along the RBC.

(c) Media Design.

  (1) An RBC must provide self-cleaning action for the media.

  (2) RBC media must be compatible with the wastewater to be treated.

  (3) An RBC design using multiple stages must use low-density media material for the first stage.

(d) Design Flexibility. If included in the design of an RBC, the engineering report must include descriptions of the following:

  (1) controlled flow to multiple first stages;

  (2) alternate flow and staging arrangements;

  (3) removable baffles between stages; and

  (4) provisions for step feed and supplemental aeration.

(e) Tank Configuration. The design of an RBC must ensure that each RBC tank:

  (1) minimizes the zones in which solids will settle out; and

  (2) includes tank drains to facilitate removal of any accumulated solids.

(f) Control of Unwanted Growth in the Initial Stages. Chlorine may be added upstream of an RBC system to control the growth of filamentous bacteria that oxidize sulfur compounds (e.g., Beggiatoa species).

(g) Maintenance Provisions.

  (1) An RBC system with a design flow of 1.0 million gallons per day or greater must have two or more process trains, each consisting of three or more stages in series.

  (2) An RBC system must be able to treat the design flow with the largest process train out of service for maintenance or cleaning.

(h) Bearing Maintenance. The bearings in each RBC must be easily accessible for inspection, lubrication, and replacement.

(i) Organic Loading Design Requirements.

  (1) The design of an RBC must be based on the organic loading rate.

  (2) The maximum loading rate must not exceed 8.0 pounds of five-day biochemical oxygen demand (BOD5 ) per day per 1,000 square feet (sf) of media in any stage.

  (3) The RBC media area must be adjusted to compensate for the effects of the ratio of soluble BOD5 to total BOD5 .

  (4) Allowable organic loading for the entire RBC system must not exceed:

    (A) 3.0 lbs of BOD5 per day per 1,000 sf of media area for wastewater treatment facilities required to meet secondary treatment; or

    (B) 2.0 lbs of BOD5 per day per 1,000 sf of media for a wastewater treatment facility required to meet advanced secondary treatment.

(j) Hydraulic Loading Design Requirements. A wastewater treatment facility using an RBC system must include flow equalization when the peak flow is more than 2.5 times the design flow to prevent loss of fixed growth from the media.

(k) Stages.

  (1) An RBC system designed for BOD5 removal must have at least three stages in series, unless the engineering report justifies that equivalent treatment will be achieved with less than three stages. Justification must use operational data from either a full-scale operating wastewater treatment facility or pilot unit with an appropriate scale-up factor.

  (2) The first stage of an RBC system must include a means of spreading the influent flow evenly across the media.

(l) Drive Systems. An RBC drive system must be able to handle the maximum anticipated media load and may be a variable speed system. An RBC may be mechanically driven or air driven.

  (1) Mechanical Drive Systems.

    (A) A mechanical drive system must have a motor and speed control unit capable of maintaining the required revolutions per minute.

    (B) A fully assembled spare mechanical drive unit must be kept on-site for each size of mechanical drive unit used in the RBC.

  (2) Air Drive System.

    (A) Each RBC using an air drive system must have air diffusers mounted below the media and off-center from the vertical axis of the RBC and must have air cups mounted on the outside of the media to collect the air.

    (B) The blowers must provide the capacity to supply:

      (i) the airflow rate necessary for each RBC;

      (ii) double the airflow rate to any one RBC while the others are running normally; and

      (iii) the required airflow with the largest blower out of service.

    (C) The air diffuser pipe to each RBC must:

      (i) be mounted so that the air diffuser pipe may be removed without draining the tank or without moving the RBC media; and

      (ii) include an air control valve to each RBC.

(m) Dissolved Oxygen.

  (1) An RBC system must maintain a minimum dissolved oxygen concentration of 1.0 milligram per liter in all stages under the maximum organic loading rate.

  (2) The executive director may require supplemental aeration.

Source Note: The provisions of this §217.185 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254

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