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Oxygen Requirement

Usually we can compute the Oxygen Requirement by following three ways:

1. Theoretical Oxygen Demand
The Theoretical Oxygen Requirements can compute by the following Eq:

C5H7NO2 + 5 O2 ---> 5CO2 + 2 H2O + NH3 +Energy
( Source from Metcalf & Eddy)
Basically you can assume all BOD5 can be converted to end products. The total oxygen demand would be computed by converting BOD5 to BODL. The BODL of one mole of cells is equal to 1.42 times the concentration of cells.
The Theoretical Oxygen Requirements can be computed as following:

Kg, O2/day = [Q(S'-S) / ( 1000g/Kg) X f] - 1.42 (Px)
        f: Conversion factor for converting BOD5 to BODL ( you
           can assume the f=0.68)
       Q: influent wastewater flowrate (m3/day)
       S': influent BOD or COD concentration, (mg/L)
       S: effluent BOD or COD concentration, (mg/L)
       Px: net waste activated sludge produced each day
            measured in terms of volatile SS (Kg/day)

If we consider the nitrification, the O2 required for nitrogen conversion( from ammonia to nitrate) need to be added. The Eq as following:

Kg, O2/day = [Q(S'-S) / ( 1000g/Kg) X f] - 1.42 (Px) + 4.57 Q ( N'-N)/(1000g/Kg)
       N': influent TKN, mg/L
       N: effluent TKN, mg/L
       4.57: conversion factor for amount of oxygen required for complete oxidation of TKN

2. Experimental Oxygen Demand
The Experimental Oxygen Requirements can compute by the following Eq:

     U= a'Y + b'Z
     U: Oxygen demand( Kg/day)
     Y: BOD removal amount (Kg/day) = (S'rQ)/1000 (Kg/day)
          Q: as defined previously
          S': as defined previously
           r: BOD removal rate
      Z: MLSS amount in aeration tank (Kg)
      a': 0.35 - 0.5 (Kg O2/ Kg BOD)
      b': 0.05 - 0.24( Kg O2/ Kg MLSS-day)

If we consider the nitrification, the O2 required for nitrogen conversion( from ammonia to nitrate) need to be added. The Eq as following:

      U= a'Y + b'Z + 4.57 Q ( N'-N)/( 1000 g/Kg/)
      N': as defined previously
      N: as defined previously
Above Oxygen Demand Eqs base on activated sludge process.

3. For BioFilm Aeration process, you can compute Oxygen Demand by the following Eq:

      U=[ (Q X Y)/ ( BOD5 / BODL) ] - 1.42 X to be wasted
           sludge
      U: as defined previously
      Q: as defined previously
      Y: as defined previously
      BOD5 / BODL: Assumeing 0.68
      to be wasted sludge: Assumeing 0 ( Usually the BioFilm Aeration process is less
      than activated sludge for wasted sludge )

                                                                                              -------- 06/22/2000

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