In general, blower systems should be designed for a minimum 5:1 turndown ratio, meaning that a system should be capable of operating at 1/5th of its full capacity (Cantwell et al. 2009). Some common arrangements that provide for efficient blower operation with back up are to design for four blowers at 33 percent each of design flow, or two blowers at 25 percent each of design flow plus two blowers at 50 percent each of design flow. As noted here, the aeration process can account for 25 to as much as 60 percent of total plant energy use (WEF 2009).
To be continued.