Chemistry Reference
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1. Many problems in operation are minimized if wastewater is passed upward at
sufficiently high velocities to expand the bed. Effective operation can be
maintained for a long time without significant pressure loss since in expanded
or fluidized beds the dependence of pressure loss on particle size is small.
2. Fouling problems can be minimized. Clogging problems are not encountered
in fluidized-bed filters due to the higher void volume. Also, due to the high
velocity of fluid in a FBR, the sloughing of biofilm is easier to maintain,
preventing the clogging problems caused by a thick biofilm.
3. Operational capacity of the activated carbon is higher in expanded and fluidized
beds than predicted from the adsorption isotherms or from sorption capacities
obtained in fixed-bed systems. In the early 1970s Weber and co-workers
postulated that the enhancement of effective capacity was attributable to
microbial activity on the carbon surface. This was particularly enhanced in
expanded-bed adsorbers, whereas biological growth led to fouling in fixed-bed
adsorbers [24]. Because expanded beds require little maintenance, extended
periods of undisturbed operation facilitate the development and continuous
growth of bacteria on carbon surfaces.
4. Higher removal efficiencies of carbon and nitrogen are achieved at
low
hydraulic retention times.
Application of the GAC-FBR configuration has also limitations such as the
following:
1. High operating costs are required for the pumping of recycle streams in order
to provide fluidization of the reactor. Compared to fluidized-beds, systems
involving fixed-bed configurations incur lower operating costs.
2. GAC-FBR systems are not designed for suspended solids removal. The inlet
distribution system of FBRs has a high sensitivity to clogging [38]. Therefore,
pretreatment by lime or alum addition is usually required.
3. Control of fluidization and bed height is difficult in FBR reactors [38].
3.4
Integration of Powdered Activated Carbon (PAC) into Biological Wastewater
Treatment
The process in which PAC is directly added to activated sludge is known as
Powdered Activated Carbon Treatment (PACT). The PACT process can be effec-
tively applied in the treatment of the following water/wastewaters:
a. Municipal wastewaters
b. Combined municipal and industrial wastewaters
c. Industrial wastewaters
d. Sanitary and hazardous landfill leachates
e. Contaminated groundwater.
The primary advantages of using PAC are the low capital investment costs and the
possibility of changing the influent PAC dose in response to incoming wastewater.
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