Environmental Engineering Reference
In-Depth Information
profiles were measured at 50
m intervals. To monitor the reproducibility of the
electrodes and any possible damage to the biofilm by the microelectrode penetration,
the oxygen profile measurements in the biofilm were conducted during both the
microelectrode penetration and withdrawal stages and compared.
To demonstrate the versatility of the new MEA electrode technique in biofilm
studies, DO and ORP microprofiles were obtained and compared using both the
MEA and the conventional ME under the same conditions. Multi-species biofilm
grown on slides for one month was taken from the closed reactor and placed in the
open channel test chamber in order to obtain oxygen and oxidation redox poten-
tial profiles. The same biofilm was used with both MEA and ME sensors. Identical
electrode positioning was achieved by using a small marker on the biofilm and a
stereo microscope with a CCD camera. In the case of the conventional MEs, DO
and ORP electrodes were separately prepared and mounted on the 3-D microma-
nipulator before each measurement. The MEA was constructed with integrated DO
and ORP sensors in one body, and thus could obtain DO and ORP profiles simul-
taneously. The entire measurement process was monitored using a color monitor
connected to a stereomicroscope with a video camera; the image was lit from above
with a high intensity lamp during video observation. The bottom substratum of the
biofilm and biofilm thickness were defined as the point where the electrode hit the
substratum and visually bent. The whole system had clean electrically-grounded
lines.
DO Microprofile.
Figure 6.12c shows the microprofile changes obtained using
both MEA and ME sensors [58]. The DO in the bulk solution was determined to be
around 8.5 mg/L and decreased through the biofilm's mass transfer boundary layer
to 5.9 mg/L with the MEA or to 6.2 mg/L with the ME at the biofilm surface. The
thickness of the DO mass transfer boundary layer was estimated to be around 200
μ
μ
m, and dissolved oxygen decreased by about 2.6 mg/L in this region. Inside the
biofilm, oxygen decreased continually and was totally depleted at 700
m depth,
according to both the MEAs and MEs. This result confirms that an oxic zone inside
of the mixed species biofilm is several hundred micrometers thick. At the biofilm
depth of 300 - 500
μ
μ
m, small concentration differences of about 1 mg/L DO con-
centration were observed between the two electrodes. It is not clear whether this was
caused by the heterogeneity of the biofilm, due to slight differences of positioning
of the electrodes (microelectrode spacing in the MEA is 900
m center to center),
or to signal differences between the MEA and ME. Nevertheless, there is a strong
correlation between the MEA and ME measurements (
r
(82)
μ
0.98,
p
< 0.01). The
DO MEA electrode has a larger gold surface area that produces approximately 10
times larger current signals than the ME.
To monitor reproducibility of the microprofile measurements and any possible
damage to the biofilm by the MEA penetration, the DO measurements in the biofilm
were performed during both penetration and withdrawal (i.e., in-and-out technique).
Figure 6.12d illustrates that the same microprofile is obtained using the MEA during
both penetration and withdrawal. Correlation analysis on these data yields a coeffi-
cient of
r
(82)
=
0.97,
p
< 0.01. No structural damage to the biofilm was observed
during these measurements.
=
