Chemistry Reference
In-Depth Information
configuration is referred to as the GAC-Sequencing Batch Biofilm Reactor
(GAC-SBBR).
Fixed-bed reactors packed with GAC can be operated under aerobic, anoxic or
anaerobic conditions. A well known type of a fixed-bed reactor operated under
aerobic conditions is the Biocarbone, which is a biological aerated filter (BAF)
packed with GAC aiming at carbonaceous BOD removal and nitrification [32].
For municipal and industrial wastewaters, the presence of suspended solids as
well as biomass growth may present problems in fixed-bed operation. Clogging
problems often occur when the wastewater is inadequately pretreated.
3.3.5.2
Expanded- and Fluidized-Bed BAC Reactors
In a conventional fluidized-bed reactor (FBR), water or wastewater is pumped
upwards through the bed. The velocity of the pumped water should be sufficient to
expand the bed against gravity resulting in fluidization of media. The difference
between expanded- and fluidized-bed reactors is that the former are operated with
smaller upflow velocities resulting in an incomplete fluidization of the biofilm
support medium [33].
The development of systems in which the biomass was kept in fluidized form in
water and wastewater treatment is traced back to the 1940s. However, in such
reactors the use of a carrying media began only in the early 1970s. Moreover,
nonadsorptive media such as sand and anthracite were employed in early appli-
cations rather than GAC [34].
Nowadays, GAC is often incorporated into the operation of an FBR. The con-
figuration is designated as the GAC-FBR, or the BAC-FBR if biological activity is
evident. The development of full-scale GAC-FBR technologies began in about 1986
for the treatment of contaminated surface waters and groundwaters [34]. Since
then several patented GAC-FBR technologies have been developed, such as the
Envirex
s
FBR, capable of removing a number of organic and inorganic target
contaminants from water or wastewater under aerobic, anoxic, or anaerobic con-
ditions [32]. For the removal of refractory organic matter and nitrogen from high-
strength wastewaters such as landfill leachates, anaerobic and aerobic GAC-FBR
reactors can also be used in series [35].
Fluidized GAC media provide a large surface area for biological growth which
leads to biomass concentrations 5-10 times higher (about 10-50 g MLVSS/L)
compared to conventional activated sludge [34, 36]. The size of GAC particles used
in GAC-FBR reactors may affect the process performance. Medium-sized GAC
particles (0.40-0.59 mm) were suggested to achieve higher efficiency and biomass
attachment compared to other sizes [37].
High SRTs further enhance acclimation of bacteria to refractory compounds
and degradation of slowly biodegradable organics. High-quality effluent is
achieved in GAC-FBR operation in terms of suspended solids and organic
matter [36].
The benefits of expanded- or fluidized-bed operation over fixed-bed are sum-
marized below:
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