At 13,900 m2/shaft for high-density media, this would require 7.7 shafts. Since only whole shafts can be used, use eight, for an area of 111,200 nr.

Check the SOL to ensure that the assumption of a nitrification rate that is unrestricted by carbon oxidation is correct. As discussed in Section 20.2.1, nitrification will proceed at its maximum rate as long as the SOL is less than 4.3 g COD/(nr ■ day). The SOL can be calculated with Eq. 20.1, but only the soluble BOD, is specified in the problem statement. Thus, an estimate must be made of the COD concentration. As indicated earlier, the soluble BOD, is often assumed to be one-half of the total BOD,. Furthermore, Eq. 8.33 states that the biodegradable COD can be estimated as 1.71 times the BOD,. Using these conversion factors, a soluble BOD, of 15 mg/L is equivalent to a COD of 51 mg/L. Consequently, if all of the media is placed into a single stage, the SOL is:

Since this value is less than 4.3 g COD/(nr • day), nitrification would proceed at the unrestricled rate.

b. What media area is required to lower the ammonia-N concentration to 2.5 mg/L in a single stage?

Since a single stage is to be used, the ammonia-N concentration in the bio-reactor will be 2.5 mg/L. From Figure 20.5, the ammonia-N flux at a concentration of 2.5 mg/L is approximately 0.75 g NH,-N/(nr • day). Again, the mass of ammonia-N to be removed is just the flow times the required concentration change. Sincc the concentration change is 2.5 mg/L, the mass of ammonia-N to be removed is 20,000 g/day. From the definition of flux, the required RBC surface area is:


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