Environmental Engineering Reference
In-Depth Information
ORP Microprofile.
The ORP in the bulk solution was approximately 180-190 mV
(1000
m from the surface of the biofilm) and decreased gradually to 160 mV at the
biofilm surface (Fig. 6.12c) [58]. In the biofilm, the redox potential profiles provided
by both the MEA and the ME also exhibited a gradual decrease from the surface to
the substratum. The MEA measured 119.6 mV near the substratum, which compares
well with the 125.3 mV measured by the ME. The ORP profiles revealed that both
the MEA and ME performed similarly. The ASTM standard D1498 [63] states that
the measured redox potentials for a good redox electrode should be within
μ
±
10 mV
of the nominal redox potential. Both electrodes behaved within the error range. A
correlation analysis on these data yields a coefficient of
r
(64)
=
0.96,
p
< 0.01,
indicating a nearly perfect correlation. Among the 66 measurement points, all but
five differed by less than 10 mV. As with DO measurements, the difference between
electrodes is possibly due to the biofilm heterogeneity or due to slight differences of
positioning of the electrodes.
6.3.2
Phosphate Microprofile Measurements in Biofilms
For phosphate microprofile measurements, a Sequencing Batch Reactor (SBR) was
operated for more than six months in an Enhanced Biological Phosphorus Removal
(EBPR) process mode to grow phosphate accumulating organisms (PAOs); these
microorganisms are responsible for the high removal efficiency in the phosphate
removal system [60]. The sludge retention time (SRT) of the reactor was 10 days,
and the hydraulic retention time (HRT) was 8 h, including 2 h in the anaerobic
zone and 3 h in the aerobic zone. The concentration range of the Chemical Oxygen
Demand (COD) was 200 ~ 250 mg/L in the influent and 5 ~ 10 mg/L in the effluent.
The average influent phosphorus concentration and the effluent phosphorus concen-
tration were about 2.6-3.3 mg/L as P and 0.2 mg/L as P, respectively. During the
experiment periods, Mixed Liquid Suspended Solids (MLSS) was 1900-2000 mg/L.
Biofilm Microprofile Measurements.
The microprofile measurements were car-
ried out inside a Faraday cage (TMC, MA) to minimize signal noise; nitrogen gas
was injected into the feed tank to establish anaerobic conditions [60]. An up-flow
chamber with laminar flow conditions was used during microprofile measurements
so that the floc was kept suspended but stationary while the microelectrode was
inserted into the floc in the flowing liquid [78]. The up-flow chamber was mounted
under a stereo microscope with a CCD camera (Model JE-3662 HR, Javelin Elec.,
Torrance, CA) situated on a Micro-g series high performance vibration isolation
table (63-527-01, TMC, Peabody, MA) inside the Faraday cage. The flow rate was
controlled with a needle valve and the water inside the up-flow chamber overflowed
evenly though four outlets at the upper part of the chamber, as shown in Fig. 6.13a.
By controlling the velocity into the up-flow chamber, the floc could be stabilized
in suspension. A stereomicroscope, a CCD camera and color monitor were used
to monitor the stabilization of the flocs and location of the microelectrode's tip.
Positioning and movement of the MEMS MEA tip toward the floc was conducted
