Modeling the Integration of Adsorption with Biological Processes in Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 153

fundamental diffusion, biodegradation, and adsorption phenomena in Section
6.1.2 are considered. In most cases, an integrated carbon-biological system is not
simply a combination of adsorption and biological removal. In that context, the
following issues are of interest:
1. Activated carbon adsorbs substrates directly when there is not yet any biological
activity such as in the initial stages of GAC operation. Then, the presence of a
biofilm surrounding activated carbon (Figure 6.1) would be neglected. The
substrates adsorbed would be primarily those that are nonbiodegradable and
inhibitory.
2. When activated carbon is placed into a suspended-growth biological system or
filled into a reactor in the form of PAC and GAC, respectively, it may retain a
number of inhibitory organic as well as inorganic substances (such as heavy
metals). This adsorptive ability of activated carbon prevents both attached and
suspended biomass from inhibition in PACT and BAC systems, respectively.
This property of activated carbon reduces the effects of hazardous compounds
under fluctuating flow rates or concentrations.
3. Activated carbon has the property of concentrating substrates, nutrients,
dissolved oxygen, and so on, on its surface. The long retention time of substrates
on carbon surface enables their contact with the suspended biomass or with the
growing biofilm. This extended contact time is a factor increasing the possibility
of biodegradation or biotransformation of various substrates.
4. If a biofilm develops on the surface of activated carbon as shown in Figure 6.1,
the following consequences can be expected:
a) The presence of a biofilm enables the removal of biodegradable or slowly
biodegradable substrates, some of which may be nonadsorbable.
b) Substrates are transported across the biofilm into the carbon until carbon
saturation occurs (Figure 6.1). This mechanism is advantageous in the case
of slowly biodegradable or nonbiodegradable substrates.
c) The formation of a biofilm can, however, also hinder adsorption of substrates
onto activated carbon (Figure 6.1). This is an unwanted outcome if these
substrates are nonbiodegradable but adsorbable. On the other hand,
the formation of biofilm on carbon surface may provide an advantage if
substrates are biodegradable but slowly adsorbable or nonadsorbable.
d) An aspect of all biofilms is their ability to respond to shock loadings of toxic
and inhibitory chemicals. As shown in Figures 6.1 and 6.2, the resistance to
the diffusion of substrate into the biofilm is the underlying reason for this
observation. If a sudden loading of a toxic chemical occurs, the outer parts
of the biofilm are affected, whereas the inner parts may still be active.
e) If the thickness of biofilm increases, the biodegradable substrate concen-
tration S f , shown in Figures 6.1 and 6.2, may be decreased to zero at some
depth of the biofilm. Also, the bulk liquid concentration (S b ) may drop to
very low values. This low concentration may then favor the desorption of
substrates from the carbon. Thus, activated carbon may temporarily adsorb
substances which, depending on substrate concentrations, are then released
into the surrounding biofilm or bulk liquid where they are biodegraded.