Environmental Engineering Reference
In-Depth Information
microelectrodes were developed for intracellular analysis by animal physiologists,
but recently they have been used in environmental research. Microelectrodes for
nitrous oxide-oxygen [23], nitrate [24], and sulfide [25, 26] have been used for nitri-
fication and denitrification studies. In addition, concentration gradients of dissolved
oxygen (DO), ammonium, pH, and oxidation reduction potential (ORP) could be
monitored by microelectrodes with no disturbance to the biofilm structure [23-26].
These microprofiles which indicate the target constituent concentration gradients
from bulk solution to inside biofilm give many significant benefits for understanding
the biofilm process or mechanism inside biofilm, providing the spatial distribution
and change of microbial activities within biofilms. DO concentration microprofiles
was used to study the external mass transfer resistance [21]. Microprofiles of nitrate,
pH, ORP in the nitrification and de-nitrification biofilm process showed oxygen
and alkalinity utilization, and supported the hypothesis that denitrifying biofilms are
stratified into an anoxic layer and an anaerobic layer [27]. Phosphate micro-profiles,
combined with other constituent profiles including pH, DO, ammonia, and ORP, in
the flocs can be used to elucidate the dynamic activity of microbial processes in
the EBPR process and can be valuable for designing operating systems or model-
ing efforts for biological nutrient treatment [28]. Phosphate microprofiles can also
be used for investigation of phosphate effect as a corrosion inhibitor in the drink-
ing water distribution system biofilms. Overall, with the current development of
molecular methods and microelectrode techniques,
in situ
structural and functional
analysis of biofilm communities can be achieved and this information will give the
more understanding of the mechanism of microbial biofilm process for designing
modeling and/or biofilm control strategies in the drinking water distribution system.
6.2 Needle-Type Microelectrode Array (MEA) Sensor
6.2.1 Overview and Rationale
Traditional environmental monitoring methods use electrode probes with tips
approximately 1-3 cm in diameter [27-30]. Typically, samples are collected from
the field site, transported to laboratory, and analyzed in a well-controlled setting.
However, this traditional approach is clearly not applicable to biofilms where
in situ
measurements are necessary. Sample properties, such as redox potential and dis-
solved oxygen content, may change during transport from the collection site to the
analysis laboratory. Further, due to their large size, traditional sensors can be used
to monitor bulk liquid concentrations when there is sufficient volume to wet the
electrode contacts, but are often inappropriate for measurements in small volumes
of samples.
In recent years, the development of
in situ
environmental sensors has become
an important research topic [31-34], and microelectrode sensors with tip diameters
of ~10
μ
m have been developed for
in situ
studies of small samples such as biofilms
[35].
