What is the Design Criteria for Rotating Biological Contactors?

Unit Dimensions Organic pollution is a key design parameter of the RBC process. This is typically expressed in terms of organic load per unit of media surface area per unit time, or pounds of BOD-5 per 1,000 square feet per day. Wastewater temperatures above 55 ° F have minimal effect on the rate of organic matter removal and nitrification.

However, below 55° F, you need to contact the manufacturer for the various correction factors that need to be used to determine the additional media surface area required.

The following parameters should be used when determining the load factor:

 • Design flow rate and primary wastewater component

 • Total inflow BOD-5 concentration

 • Inflow soluble BOD-5 concentration

 • Total to remove and soluble BOD-5 ratio

 • Wastewater temperature

 • Primary wastewater dissolved oxygen

 • Media placement, Number, media surface area of ??each stage

 • Media rotation speed

 • Resident time in RBC tank

 • Soluble BOD-5 Inflow from internal side-streams, landfills, etc. into RBC systems containing soluble BOD-5.

 • Hydrogen sulphide concentration in the inflow

 • Peak load, BOD-5 max/BOD-5 avg.

>In addition to the above parameters, the inflow load factor depends on the inflow oxygen concentration, the inflow ammonia nitrogen concentration, total Kjeldahl nitrogen (TKN), and daily load fluctuations, pH, and alkalinity. Allowable ammonia nitrogen concentration in sewage.

>Soluble BOD-5 loading is an important parameter in the design of units and should be checked by taking a sample in the influx if possible.

>Charge rate-if the peak-to-average flow ratio is 2.5: 1.0 or less, the average condition can be considered for design purposes. If the flow ratio is high, you should consider flow compensation.

>The organic load of a standard first-stage density medium is in the range of 3.5-6.0 pounds of BOD-5 total per 1,000 square feet per day, or 1.5-2.5 pounds of soluble BOD-5 per 1,000 square feet per day is needed. First-stage organic loading above a total of 6 lbs BOD-5 or 2.5 lbs soluble BOD-5 per 1000 square feet per day includes excess biofilm thickness, dissolved oxygen depletion, interfering organisms, and reduced process performance.

>Under average conditions, the design load should not exceed a standard medium surface area of ??5,000 square feet of soluble BOD of 2.5 pounds per day on the first stage shaft of the treated strands. Regularly high organic loads may require additional ventilation on the first stage shaft.

>BOD, TSS, and NH₃ are all pollutants which are present in large concentrations in the influent wastewater coming into a treatment plant. Through the treatment process, the concentrations of BOD, TSS and NH₃ are reduced.

>The Percent Removal Calculation (aka treatment efficiency) is used to determine the percentage of the incoming concentration of a particular pollutant (BOD, TSS or NH₃) which was removed through the treatment process.

It is calculated as follows:

1: Percent removal (%) =

{((Influent Concentration, mg/L) – (Effluent Concentration, mg/L) x 100)}/(Influent Concentration, mg/L)

Example: Influent Total Nitrogen = 25 mg/L

Effluent Total Nitrogen = 5 mg/L

Percent Removal (%) = ((Influent Concentration, mg/L) – (Effluent Concentration, mg/L) x 100)/ (Influent Concentration, mg/L)

Percent removal (%)={(25 mg/L) – (5 mg/L) x 100)/ 25 mg/L}

Percent removal (%) = 80%

2: Hydraulic loading is calculated as follows:

Hydraulic Loading (gpd/ft²) =(Influent Flow) / {(# of stages) *(area per stage)}

Example: Calculate the hydraulic loading of a RBC system with the following data:

4 Stage System Surface Area (per Stage) = 35,000 ft²

Influent Flow = 255,000 gpd

Solution

Hydraulic Loading (gpd/ft²) = Influent Flow, gpd/ {(# of stages) *(area per stage, ft²)}

Hydraulic Loading = (255,000 gpd)/ (4) (35,000 ft²/Stage)

Hydraulic Loading = 1.8 gpd/ft²

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