Biofilters
The bacterial density is a difficult quantity to predict since it is not a constant, but
varies in a rather unknown manner with the growth history of the biofilm: Growth
on small concentrations yields a large bacterial density and conversely for easily
degradable, dissolved substrates.
Example 5.3
In an ideally mixed trickling filter the specific surface is 100 m 2 /m 3 . COD is removed in
a 1 mm thick biofilm by kinetic constants corresponding to activated sludge plants (Table 3.7):
!lmax
Ks.coo =
YH
6d- 1
20 g COD(B)/m 3
0.67 g COD(B)/g COD(S)
The concentration of bacteria in the biofilm is 40 kg VSS /m 3 , corresponding to 40 · 1.4
= 56 kg COD(B)/m 3
What is the volumetric concentration of bacteria in the biofilter:
X= 100 m 2 /m 3 · 1 · 10- 3 m · 40 kg VSS/m 3 = 4 kg VSS/m 3
= 5.6 kg COD(B)/m 3
It appears that the effective biomass in a trickling filter is not bigger than in an activated sludge plant. The bigger specific biomass is obtained in filters with a very large
specific surface, for example in fluidized filters.
How big is the removal per surface area when the COD-concentration in the filter is 50
g/m3?
This is a zero order process in the biofilm because the concentration in the water is
higher than 2 · Ks.
kovt = o.~ 7 · 56 = 500 kg COD/(m 3 biofilm . d)
The diffusion coefficient for the organic matter is set at 0.4 · 1 o-4 m 2 /d .
• I
3
-4
1A -1,1, -1
k%A = "2 k0 vt · D = '12 · 500 · 10 · 0,4 · 10 = 6.3 g 2 m 2 d
rA = 6.3 150 = 45 g COD/(m 2 ·d)
This yields 45 ·100m 2 /m 3 = 4,500 g COD/(m 3 ·d) volumetric removal in the filter,
corresponding to a high-rate trickling filter. (lt is assumed that there is sufficient
oxygen; see Example 5.5).
5.3. Hydraulic film diffusion
Apart from the diffusional limitation in the biofilm itself, a limitation occurs in
practice by transport of substrate from the bulk water to the surface of the biofilm.
This transport can be described in a simplified manner by a proportionality between
the difference in transport and concentration
153
The bacterial density is a difficult quantity to predict since it is not a constant, but
varies in a rather unknown manner with the growth history of the biofilm: Growth
on small concentrations yields a large bacterial density and conversely for easily
degradable, dissolved substrates.
Example 5.3
In an ideally mixed trickling filter the specific surface is 100 m 2 /m 3 . COD is removed in
a 1 mm thick biofilm by kinetic constants corresponding to activated sludge plants (Table 3.7):
!lmax
Ks.coo =
YH
6d- 1
20 g COD(B)/m 3
0.67 g COD(B)/g COD(S)
The concentration of bacteria in the biofilm is 40 kg VSS /m 3 , corresponding to 40 · 1.4
= 56 kg COD(B)/m 3
What is the volumetric concentration of bacteria in the biofilter:
X= 100 m 2 /m 3 · 1 · 10- 3 m · 40 kg VSS/m 3 = 4 kg VSS/m 3
= 5.6 kg COD(B)/m 3
It appears that the effective biomass in a trickling filter is not bigger than in an activated sludge plant. The bigger specific biomass is obtained in filters with a very large
specific surface, for example in fluidized filters.
How big is the removal per surface area when the COD-concentration in the filter is 50
g/m3?
This is a zero order process in the biofilm because the concentration in the water is
higher than 2 · Ks.
kovt = o.~ 7 · 56 = 500 kg COD/(m 3 biofilm . d)
The diffusion coefficient for the organic matter is set at 0.4 · 1 o-4 m 2 /d .
• I
3
-4
1A -1,1, -1
k%A = "2 k0 vt · D = '12 · 500 · 10 · 0,4 · 10 = 6.3 g 2 m 2 d
rA = 6.3 150 = 45 g COD/(m 2 ·d)
This yields 45 ·100m 2 /m 3 = 4,500 g COD/(m 3 ·d) volumetric removal in the filter,
corresponding to a high-rate trickling filter. (lt is assumed that there is sufficient
oxygen; see Example 5.5).
5.3. Hydraulic film diffusion
Apart from the diffusional limitation in the biofilm itself, a limitation occurs in
practice by transport of substrate from the bulk water to the surface of the biofilm.
This transport can be described in a simplified manner by a proportionality between
the difference in transport and concentration
153