Chemistry Reference
In-Depth Information
7.5.1
Biodegradability
The bioregeneration hypothesis is generally valid for biodegradable, slowly bio-
degradable, and adsorbable organic compounds. Slowly biodegradable compounds
can be biodegraded if a sufficient contact time with biomass is provided. In PACT
systems, these compounds are in contact with the biomass for a length of time
equal to the sludge age if they are adsorbed by PAC and thus enter the solid phase
[57]. On the other hand, in a conventional biological treatment system without
activated carbon, slowly biodegradable compounds may leave with the effluent
stream since they may not be biodegraded. For a further insight into these issues,
the reader may refer to Chapter 6.
Various compounds that are considered nonbiodegradable (such as chloroform
and chlorinated benzenes) can be adsorbed on activated carbon and subsequently
metabolized by attached microorganisms [58]. Increased retention of biomass in
the system favors the metabolization of organics. Therefore, theoretically, bior-
egeneration should increase with sludge age [7]. Slowly biodegradable organics are
first adsorbed onto activated carbon and retained in the system for a long period of
time. Upon desorption from the carbon surface, they may be biodegraded by
attached and suspended biomass. The consequence is the bioregeneration of
activated carbon. Hence, adsorption acts as a prerequisite step for biodegradation.
In the whole bioregeneration process, the rates of both desorption and biode-
gradation are important. Compared to desorption, biodegradation is considered to
be the rate-limiting step in bioregeneration, particularly for slowly biodegradable
substances. In the region surrounding activated carbon, an increase in microbial
uptake rate would lower the bulk concentration of a substance. This would
increase the driving force for diffusion of substrate from carbon, which conse-
quently would increase the rate of bioregeneration. In that respect, acclimation of
biomass to the compounds in question is expected to increase both the rate and
efficiency of bioregeneration. For example, compared to nonacclimated biomass,
the presence of acclimated biomass [59] resulted in much faster and higher
bioregeneration of activated carbons that were loaded with the biologically resis-
tant compound 2-chlorophenol (2-CP) [60].
On the other hand, readily biodegradable organic matter is generally non-
adsorbable and can be removed by biological activity alone, whereas the removal of
nonbiodegradable and adsorbable compounds takes place through simple
adsorption. It was stated that bioregeneration was due to simple desorption of
sorbed compounds followed by biodegradation. Hence, bioregeneration could only
occur in the case of compounds that are both biodegradable and adsorbable [20].
For example, activated carbons loaded with 2-CP could not be bioregenerated since
this compound is nonbiodegradable [60]. However, carbons loaded with the bio-
degradable phenol were successfully bioregenerated by the same activated sludge
[24]. In another study, 4-NP could be successfully removed through desorption-
biodegradation, although total bioregeneration of activated carbon was not
achieved [61]. In other studies, the bioregeneration concept was extended to
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