Environmental Engineering Reference
In-Depth Information
and nitrogen gas was injected into the feed tank to establish anaerobic conditions.
The flocs for phosphate measurement were taken at the end of the anaerobic stage
of the EBPR process. Therefore, it was assumed that enough phosphorus release
had occurred throughout the reactor and reached a pseudo steady-state condition.
The measurement of profiles was taken within 30 min. The microelectrode read-
ings were recorded at 100
m intervals. Well-defined phosphate profiles across the
flocs were observed under the anaerobic conditions, during which phosphorus was
released from the flocs, as determined using the MEMS microelectrode (Fig. 6.13c).
Figure 6.13d shows the picture of penetration of MEMS MEA array microelec-
trode into the floc [60]. The diameter of the floc was about 1000
μ
μ
m, and the center
of the floc is depicted as a depth of 0
m in Fig. 6.13c. Based on the profile measure-
ments, the phosphate concentration of 15.1 mg/L as P in the bulk solution increased
as the floc was penetrated toward the center of the floc.
Compared with the microprofiles obtained using the conventional cobalt-based
microelectrode, the results show very similar patterns of phosphate concentration
through the flocs, according to the depth (Fig. 6.13c). Phosphate concentration in
the bulk solution was about 15 mg/L as P in the anaerobic bulk phase and increased
to 20.6 mg/L as P with penetration toward the center of the floc. Wang and Bishop
[71], using microelectrode and fluorescent
in situ
hybridization (FISH) techniques,
indicated that the higher phosphate concentration in the floc center was due to a
higher density of PAOs at the center of the floc than that at the edge of the floc. The
diffusion boundary layer in which the phosphate concentration started to change
near the floc was defined as 100
μ
m thick.
The developed MEMS MEA sensors were thus able to penetrate biological sam-
ples in order to perform phosphate measurements, and will enable
in situ
analysis
in many biological applications for measurement of phosphate. These microelec-
trode sensors can be effective research tools for elucidating the transport phenomena
occurring within biofilm reactors by measuring the concentration microprofiles
of species of interest in the biological aggregates without destroying the biofilm
structures.
μ
6.4 Conclusions
This chapter described the development of the needle-type multi-analyte MEMS
sensor arrays for
in situ
measurements in biofilms. A batch fabrication approach
was used to increase yield and consistency of the sensor. The key fabrication tech-
nology was the HF-based meniscus etching process to sharpen and recess sensor
tips. MEMS technologies offer the advantages of large-scale production, low cost,
and increased reliability, and can be used to overcome limitations of conventional
sensor fabrication.
Overall, novel needle-type multi-analyte sensors for
in situ
measurements of
ORP, DO, and phosphate have been successfully developed and integrated into a
single sensor array. The major advantages of these new sensors include the ability
