Treatment Plants for Nitrification
Biofilms
As in the case of an activated sludge treatment plant, the question will be whether
the heterotrophic bacteria through their growth will prevent the nitrifying bacteria
from growing sufficiently. The micro-organisms present in a filter are deposited in
a layer on the carrier. Organic matter, ammonium and oxygen are applied through
the surface of the biofilm. In practice, oxygen will normally limit the conversion of
both organic matter and ammonium. As the nitrifying bacteria grow slowly, they
will be ousted and hence eliminated from the plant if the thickness growth of the
biofilm, which is primarily conditioned by the growth of the heterotrophic bacteria,
is faster than that of the nitrifying bacteria.
If the nitrifying bacteria dispose of too little oxygen, they will be ousted more easily.
Therefore the criterion for nitrification will be that oxygen penetrates further into
the biofilm than organic matter does, that is, organic matter limits the heterotrophic
conversion.
From Expression (5.32) we know that organic matter is limiting if the following
inequalities are true:
SBoD,2 < So2,2 · D02,2 · vo2,ooo'DBoo,2
Substituting by values from Table 5.2 the inequality will be
SBoo,2< 5 · So2,2
where BOD implies dissolved BOD.
(6.8)
Fig 6.7 shows an experimental demonstration of Expression (6.8). It will be seen that
partial nitrification may well take place in biofilters .
Fig6.7
1.0 . , G ~./P- Full nitrification
Experimental data
',~'' ...... ,
0.9
~ 'rY.fd',
\
0\
\
0.8
0.7
g ~ 0.6
z:
.g ~ 0.5
~ .._< 0.4
Q)
.2:
0.3
1U
Qi
0.2
a:
0.1
0.0
\
0 \
\
Data from
'~ a· ,
\
A
~
\
model simulation
u
.
\
' ooc), o '
\
0
\
'
\ 0 0 \
,
Nitrification
' \ '
' -a~o ob..,
~'
.... ~
a,
, •
. . . . . o
''- tiJ, • ....,
~ 0 '·,
•
0.2
0.4
', t;,.'·-?
0.6
'
·,
'
'·
'
',
0.8
1.0
No nitrification
'
The criterion
for nitrification
,_
--o--- o. -
Experimental demonstration of the importance of organic matter for nitrification in a
biofilter /1/,/2/.
205