145
Qin
et al
.,
Comparison on nitrosation and anaerobic ammonium oxidation between activated sludge and biofilm from an autotrophic...
Tecnología y Ciencias del Agua
, vol. VIII, núm. 2, marzo-abril de 2017, pp. 141-149
ISSN 2007-2422
•
significantly different. That is to say that in this
autotrophic nitrogen removal system, activated
sludge and biofilm contained different commu-
nities of functional bacteria. This suggests that
activated sludge and biofilm play different roles
in the overall nitrogen removal procedure.
AOB
In the first step in the nitrogen removal pro-
cess, ammonium was oxidized to nitrite by
AOB. The population of AOB in the activated
sludge sample was 1.88 × 10
11
cells/g, almost
nine times higher than the 1.90 × 10
10
cells/g
found in the biofilm. AOB are typically aerobic
bacteria which thrive under high DO. Activated
sludge was suspended in the reactor and was
in intimate contact with DO. However, poor
mass transfer of DO, into the biofilm meant that
the interior layer of biofilm was anaerobic or
at least had low DO concentration. Therefore
AOB tended to be found in activated sludge or
the exterior layer of biofilm.
NOB
NOB numbers were 2 to 4 orders of magnitude
less than AOB and ANAMMOX in both activat-
ed sludge and biofilm samples. NOBs were not
the dominant bacteria in terms of cell number in
this SBBR reactor although they multiplied fast-
er than AOB under normal conditions (5-20 °C)
(Hellinga, Schellen, Mulder, & Heijnen, 1998).
Other studies have also shown that at higher
temperatures (> 20 °C), the degradation rate of
AOB can be higher than that of NOB (Hellinga
et al
., 1998). During the start-up period of this
study, the temperature was kept around 30 °C
so that NOB was depleted and AOB became be
the dominant group to accomplish one-step au-
totrophic nitrogen removal and prevent nitrite
from being oxidized to nitrate.
ANAMMOX
The population of ANAMMOX was 5.5 times
that of AOB and 874 times that of NOB in acti-
vated sludge samples. These differences were
more pronounced in biofilm samples, at 140
times and 3491 times, respectively. Jetten
et al.
(2001) reported that anaerobic ammonia oxida-
tion was achieved only when the ANAMMOX
population was higher than 10
10
–10
11
cells/ml.
Therefore, although ANAMMOX cells were
the most abundant one among three functional
bacteria in this study, they did not dominate the
metabolic functionality. AOB and NOB are an
aerobic bacteria, but ANAMMOX are anaerobic
and are sensitive to DO. They can not survive
even at 0.5–2.0% air saturation. The low rate of
mass transfer of DO in biofilm and depletion
by AOB and NOB in the outer layer made the
interior of the biofilm an ideal condition for
ANAMMOX. As a result, the population of
ANAMMOX in biofilm was 2.66 × 10
12
cells/g,
which was 2.6 times that in activated sludge.
Nitrosation of activated sludge and biofilm
Transformation of nitrogen in activated sludge
and biofilm samples in a cycle of nitrosation are
shown in figures 3a and 3b.
Figure 3a shows that ammonia can be fully
oxidized by activated sludge in a single 48 h
cycle, with the ammonia oxidation rate reaching
100%. The maximum rate occurred between the
6 and 20 h with a maximum ammonia oxida-
tion rate of 0.23 mgN mgVSS
−1
d
−1
. 85% of the
oxidized ammonia was present as NO
2
-
-N and
9% as NO
3
-
-N. There was the limited removal of
Table 3. Bacterial population in different samples (cells/g VSS).
Sample
AOB
NOB
ANAMMOX
Activated sludge
1.88 × 10
11
1.19 × 10
9
1.04 × 10
12
Biofilm
1.90 × 10
10
7.62 × 10
8
2.66 × 10
12
