Current through Register Vol. 41, No. 3, September 23, 2024
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.
