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0 5 10 15 20 25 30 35 40 Surface Organic Loading, g BOD5/(m2 - day)

Figure 20.4 Effect of surface organic loading (SOL) on the surficial BOD, removal rate (flux) for full-scale RBC facilities treating domestic wastewater. (From Water Environment Federation, Design of Municipal Wastewater Treatment Plants, Manual of Practice No. 8, Water Environment Federation, Alexandria, Virginia, 1992. Copyright © Water Environment Federation; reprinted with permission.)

0 5 10 15 20 25 30 35 40 Surface Organic Loading, g BOD5/(m2 - day)

Figure 20.4 Effect of surface organic loading (SOL) on the surficial BOD, removal rate (flux) for full-scale RBC facilities treating domestic wastewater. (From Water Environment Federation, Design of Municipal Wastewater Treatment Plants, Manual of Practice No. 8, Water Environment Federation, Alexandria, Virginia, 1992. Copyright © Water Environment Federation; reprinted with permission.)

systems are discussed in Section 20.2.6. Standard density RBC media (9,300 mV shaft) should be used in RBC stages that are highly loaded or where Beggiatoa growth is possible. Excess biological growths are more difficult to remove from high-density media and, consequently, use of high-density media can further exacerbate operational problems in highly loaded stages.

The organic loading affects nitrification in an RBC system. As discussed above, nitrifying bacteria can effectively compete for space in a biofilm once the concentration of soluble organic substrate is reduced below 20 mg COD/L (15 mg BODs/ L). The SOL, among other factors, determines whether that occurs. An empirical relationship has been developed to show the effect of SOL (expressed as g total biodegradable COD/(m: • day)) on the nitrification rate that can develop in an RBC unit performing combined carbon oxidation and nitrification:IK

where fN„ is fraction of the rate that would occur in the absence of simultaneous carbon oxidation. The limits on fN„ are one and zero. This equation indicates that no nitrification will occur when the SOL ^ 14.3 g total biodegradable COD/(mJ-day) and that unrestricted nitrification will occur at SOLs of 4.3 g total biodegradable COD/(m'• day) or less. At all SOL values between those extremes, significant competition occurs between heterotrophs and autotrophs, resulting in reduced nitrification rates.

For separate stage nitrification systems the classical relationship between the growth limiting ammonia-N concentration and the flux into the biofilm, JM„ as described by Eq. 19.5, is observed. According to Eq. 20.2 this will occur when the loading of biodegradable COD is below 4.3 g COD/(m: • day). Figure 20.5 presents the relationship. As indicated there, at ammonia-N concentrations above about 5 mg/L nitrification proceeds in RBC units at a rate of about 1.5 g N/(m:'day). This represents a zero order biofilm in which the nitrification rate is not limited by the

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