QUESTION 2 A food processing plant has been discharging its wastewater to the lo

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QUESTION 2 A food processing plant has been discharging its wastewater to the local sewage system after on-site primary treatment. On average, 640 m /day are discharged with a BOD of 380 mg L. However, the regional sewage treatment plant is nearing its maximum capacity and the water authority is insisting that the plant's wastewater be further treated on-site in a secondary treatment system before it is discharged to sewer. The company has thus been advised that in the near future any discharge to the sewer should have a maximum BOD level of 5 mg/L. Published pilot-plant data for a similar wastewater indicates that the biomass growth rate (r in mg VSS/L.hr) in an aerobic secondary treatment stage may be adequately described by the following equation where S is the substrate level (in mg BOD/L). Endogenous respiration effects (i.e. cell death and lysis in the pilot-plant were accounted for by using an overall observed yield (Yobserved with a reported value of 0.42 mg VSS/mg BoD. r, 0.15 100 S

Explanation / Answer

BOD = (DOi– Dof) / p

P= dilution factor

DOi = dissolved oxygen initial

DOf = dissolved oxygen final

P = Q*(S0 - S )*Yobs= 640*0.375*0.00042 = 0.1008

BOD = (380-5)*10-3/ 0.1008= 3.720 kg O2 per hour

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