CE 40340 Wastewater Design
Spring 2009
Wastewater Solids
X a = concentration of active biomass
Mx
L3
Volatile, partially biodegradable
X i = concentration of inert (non-active) biomass
Mx
L3
Volatile, refractory to degradation
X v = Total volatile solids = X a  X i
Mx
L3
mg vss a / L
Volatile solids expressed as
mg vss
L
mg vss i / L
mg vss / L
Wastewater solids also exert COD and BOD, and contain N and P.
COD
COD is exerted by both active and inert VSS. Given the approximate formula for
biomass: C5H7O2N, with a MW = 113 mg/mmol and considering non-nitrifying
conditions, there are 20 electron equivalents per mol of biomass:
1mgVSS
20e  meq
mgCOD
mgCOD
*
*8 
 1.42
mgVSS
mmolbiomass
mgVSS
e meq
113
mmolbiomass
Therefore, there is
1.42 mgCOD/mgVSS, without considering nitrification
If oxidation of the reduced N in biomass to NO3- is considered, then there are 28 emeq/mol biomass, and the conversion becomes
1.98 mgCOD/mgVSS considering nitrification
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CE 40340 Wastewater Design
Spring 2009
BOD
Only the degradable fraction (fd) of active biomass (Xa) exerts BOD. Inert biomass (Xi)
does not exert BOD. the ultimate BOD (BODu) of active biomass expressed as VSS is
1.42fd mgBODu/mgVSSa without nitrification, and
1.98fd mgBODu/mgVSSa with nitrification
A typical value of fd is 0.8.
Nutrients
Nitrogen. Assuming VSS is mainly cells, then VSS is C5H7O2N. The MW of C5H7O2N
is 113 mg/mmol, and considering 14 mgN/N, then there is 14/113 =
0.124 mgN/mgVSS
Phosphorus. Phosphorus is typically 20% by mass of N, therefore
0.025 mgP/mgVSS
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CE 40340 Wastewater Design
Spring 2009
Relationship between VSS and TSS
VSS is measured by the mass that is volatilized at 500 oC, which corresponds to the
organic fraction (C5H7O2N). However, cells also contain around 10 % inorganic matter
that is non–volatile. To convert microbial VSS to TSS :
cell
 VSS

 0.1 
TSS  VSS  
 0.1  VSS1 

 0.9

 0.9 
Organic, volatile solid = VSS
Inorganic, non-volatile solid
VSS
 0 .1
=
0.9
In wastewater, Mixed Liquor Volatile Suspended Solids (MLVSS) is a measure of
X v . Mixed Liquor Suspended Solids (MLSS) is a measure of total solids, i.e., organic
and inorganic. This includes
- X v (MLVSS)
0.1
MLSS
- Inorganic fraction of biomass = MLVSS 
0.9
- Inorganic suspended solids from influent SSinorg.
In an activated sludge system with solids recycle, the influent inorganic
suspended solids are concentrated by solids concentration factor

 0.1 
MLSS  MLVSS 1 
  SSinorg.  x

 0.9 
3
x
