Bioregeneration of Activated Carbon in Biological Treatment - Activated Carbon for Water and Wastewater Treatment - page 222

Therefore, simulation studies were conducted for three different cases:
(i) adsorption and biodegradation inside the biofilm and by suspended biomass,
(ii) absence of adsorption, but biodegradation inside the biofilm and by suspended
biomass, and (iii) adsorption alone. The effluent substrate concentrations under
three different scenarios are illustrated in Figure 7.11 for each substrate.
During the initial phase of operation, adsorption prevailed over biological
activity for each compound. As concluded from the respective Freundlich isotherm
parameters, PNP and phenol adsorb better than toluene. Correspondingly, Figure
7.11b and c demonstrates that during the initial phase, better removals are pre-
dicted for PNP and phenol than toluene (Figure 7.11a). As time progressed, bio-
logical activity became more significant as the microorganisms began utilizing the
available substrate for biofilm growth and eventually dominated over adsorption
even before the biofilm was fully developed. After biodegradation becomes pre-
dominant, the steady-state removal is governed by biodegradation rates. Under
steady-state conditions, removals exceeding 99.9% are predicted for the three
model contaminants.
In this system, activated carbon stored the substrate when biological activity was
low, and released it when biodegradation became dominant. Comparisons of the
adsorbent capacity utilization for the 'adsorption' and 'adsorption and biode-
gradation' cases provided estimates of adsorbent bioregeneration for PNP, phenol,
and toluene as 75%, 60%, and 60%, respectively. However, in this MBR system
biodegradation by attached or suspended biomass could be not distinguished from
each other.
In another study, a modeling approach for BAC also showed that during the
initial period of operation, a steady mass flux of TCE was transported into the GAC
particle due to diffusion and adsorption. The aqueous substrate concentration in
the vicinity of the biofilm/particle interface gradually became lower than that of
the solid concentration as the system gradually became bioactive. This caused the
substrate to diffuse out of the carbon for biofilm degradation [105].
7.8.3
Modeling of Bioregeneration in Multicomponent Systems
In reality, single solute systems hardly exist, either in wastewater or in water
treatment. Therefore, mathematical models have been developed to predict
the bioregeneration in BAC columns treating mixtures of biodegradable and
nonbiodegradable organic compounds [56]. Erlanson and co-workers described a
two-component equilibrium-based model referred to as the biodegradation/
adsorption-screening model (BASM) [83]. The model considered only one bio-
degradable and one nonbiodegradable chemical. When the nonbiodegradable
chemical controlled the service life of the activated carbon column, the only sig-
nificant gain in service life occurred when both chemicals had similar adsorb-
abilities. On the other hand, if the biodegradable chemical controlled the service
life, the service life was 1.2- to 7-fold higher than in the case of adsorption alone,
depending on the relative adsorbability of the two chemicals. Thus, the service life