Modeling the Integration of Adsorption with Biological Processes in Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 154

This mechanism may result in bioregeneration of activated carbon, leading
to a more effective use of the activated carbon, as discussed in detail in
Chapter 7. The desorption and biodegradation of substrate also has another
beneficial effect, namely that of enabling the survival of biomass under low
substrate loading rates. In that respect, biofilm systems with 'adsorbing'
GAC media are significantly different from those which employ 'nonad-
sorbing' media such as sand or anthracite.
f) A GAC reactor may gradually be converted into a biofilm reactor where the
sludge retention time (SRT), also called the sludge age, is approximated as
follows:

active biomass in the biofilm
production rate of active biomass

y ¼ (6.23)

x

Under steady-state conditions, the growth of biofilm is balanced by its loss, and
the biofilm attains a steady thickness. When Eq. (6.23) is adapted to represent a
steady-state biofilm it takes the following form [11]:

X f L f A b
b det X f L f A b ¼

1
b det

y ¼ (6.24)

;

x avg

where
y
x

¼

sludge retention time (SRT) (T),
y x,avg ¼

average y x for a steady-state biofilm (T),

b det ¼ detachment rate of biomass (1/T),

biofilm surface area (L 2 ). A b ¼

Thus, the average SRT in a steady-state biofilm is equal to the reciprocal of the
specific detachment rate as long as active biomass is not added to the biofilm by
deposition [11]. Another SRT expression has also been proposed that incorporates
the substrate flux into biofilm and biofilm detachment [4].
Since the amount of active biomass is high and the detachment rate is low in
BAC reactors, the SRT is much higher than that in suspended-growth processes
such as activated sludge. The high SRT is a factor favoring the establishment
of slowly growing microorganisms such as nitrifiers or methanogens inside
the biofilm. The high SRT further increases the exposure time of biomass
to nonbiodegradable or slowly biodegradable organics and enables efficient
biodegradation.

g) In the modified suspended-growth processes such as the aerobic PACT,
anaerobic PACT or membrane processes with PAC addition (PAC-MBR), to
some extent the surface of PAC serves as an attachment medium for
microorganisms. This attachment generally increases at higher SRT, where
the benefits of PAC addition become more obvious. However, in PACT
systems the extent of biomass attachment is still much lower than that in
BAC reactors.
h) Activated carbon may also adsorb intermediate or end products of biological
reactions taking place in the biofilm or suspended phases. As discussed in