Loading Rates:
When peak to average flow ratio is 2.5 to 1.0 or less, average conditions can be considered for design purposes. For higher flow ratios, flow equalization should be considered.
The organic loading to the first stage standard density media should be in the range of 3.5 to 6.0 pounds total BOD5 per thousand square feet per day or 1.5 to 2.5 pounds soluble BOD5 per thousand square feet per day. First stage organic loadings above 6 pounds total BOD5 or 2.5 pounds soluble BOD5 per thousand square feet per day will increase the probability of developing problems such as excessive biofilm thickness, depletion of dissolved oxygen, nuisance organisms and deterioration of process performance. The most critical problem in most instances is the structural overloading of the RBC shaft(s).
For average conditions, the design loading should not exceed 2.5 pounds of soluble BOD5/1,000 square feet of standard media surface per day on the first stage shaft(s) of any treatment train. Periodic high organic loadings may require supplemental aeration in the first stage shafts. High density media should not be used for the first stage RBC’s.
For peak conditions, the design loading shall not exceed 2.0 pounds of soluble BOD5/1,000 square feet for the first high density media shaft(s) encountered after the first two shafts or rows of shafts in a treatment train.
For average conditions, the overall system loading shall not exceed 0.6 pounds of soluble BOD5/1,000 square feet of media. This soluble BOD5 loading to all shafts should be used to determined the total number of shafts required. The equation under section C.3.c could be used as an option to determine the number of stages required.