How to Remove BOD Using Mass Balance Method?

The amount of BOD removed by a treatment process is directly related to the quantity of solids the process will generate.

Key Note:

The actual amount of solids generated will vary with operational conditions and design, exact figures must be determined on a case-by-case basis;

however, research has produced general conversion rates for many of the common treatment processes. These values are given in Table and can be used if plant-specific information is unavailable.

Process Type Conversion Factor (lb Solids/lb BOD Removal)
Primary treatment 1.7
Trickling filters 1.0
Rotating biological contactors 1.0
Activated biosolids with primary 0.7
Activated biosolids without primary  
-Conventional 0.85
-Extended air 0.65
-Contact stabilization 1.0
-Step feed 0.85
-Oxidation ditch 0.65

Using these factors, the mass balance procedure determines the amount of solids the process is anticipated to produce. This quantity is compared with actual biosolids production to determine the accuracy of the sampling and/or the potential for solids buildup in the system or unrecorded solids discharges


• Step 1: BOD in = influent BOD x flow x 8.34
• Step 2: BOD out = effluent BOD x flow x 8.34
• Step 3: BOD pounds removed = BOD in – BOD out
• Step 4: Solids generated (lb) = BOD removed (lb x factor)
• Step 5: Solids removed = sludge pumped (gpd) x  % solids x 8.34
• Step 6: Effluent solids (mg/L) x flow (MGD) x 8.34

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