For industrial wastewater with BOD concentrations of 400 to 2000 mg/L or more, the organic loading rate is often limiting. The procedure for process design is as follows:
1. Calculate the BOD load from the concentration and flow:
where BOD load is the daily BOD load (lb/d; kg/d); 8.34 is the conversion factor; Q is the flow (mgd; m3/d); and C is the BOD concentration (mg/L).
2. Calculate the land area from Equation 8.10:
where A is the field area (ac), and 100 is the limiting loading of BOD (lb/ac-d; kg/ha-d).
When the BOD of the applied wastewater exceeds approximately 800 mg/L, the oxygen transfer from the atmosphere through the fixed film becomes limiting. The BOD removal rate will decline unless effluent recycling is practiced. In some industrial applications, effluent recycle to dilute the BOD in the raw wastewater has been used. For example, the BOD from a food processing wastewater was reduced from 1800 down to 500 mg/L with an effluent recycle ratio of 3:1 (Perry et al., 1981).
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