Modeling the Integration of Adsorption with Biological Processes in Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 170

ratio between the liquid film diffusion and the intraparticle pore diffusion (Bi p ).
However, in the present biofilm-carbon system shown in Figure 6.1, the modified
Biot number, as derived in Eq. (6.56), represents the ratio between biofilm dif-
fusion and intraparticle surface diffusion. In this respect, a low Biot number
would mean that the diffusion through the biofilm is controlling rather than the
surface diffusion in the carbon.
The influence of other dimensionless groups on breakthrough has been
extensively discussed in the literature for the case of no biological activity [6, 15].
6.1.5
Prevalent Models in BAC Reactors Involving Adsorption and Biodegradation
6.1.5.1
Initial Steps in Modeling: Recognizing the Benefits of Integrated Systems
Weber andhis co-workerswere among the first to recognize the benefits of concurrent
adsorption and biodegradation in a single reactor, which they referred to as the
Integrated Biological-Physicochemical Treatment (IBPCT) scheme [25, 26]. In
the early 1970s a numeric solution model was proposed by Weber and co-workers
under the Michigan AdsorptionDesign and Applications Model (MADAM) program
for adsorber beds, involving the dynamic aspects of fluidization and solids mixing,
liquid dispersion and channeling, competitive adsorption of solutes, and biological
activity [25]. The MADAM is specified as a set of alternative models which also con-
sider the extent of solids (adsorbent) mixing. These models considered four different
conditions in the liquid and solid phases (adsorbent) of biologically active carbonbeds:
plug flow and stationary, plug flow and completely mixed, dispersive and stationary,
and dispersive and completely mixed. Model predictions and experimental data
clearly demonstrated the benefit of a bioactive adsorber over a nonbioactive one.
Studies also revealed the advantage of adsorbingmedia over a nonadsorbing one [26].
The initial modeling studies conducted by Weber and co-workers indicated an
apparent synergism between biological activity and activated carbon adsorption. The
researchers concluded that concurrent biodegradation and adsorption processes
offered the following advantages: (i) adsorption of biologically resistant compounds
that may be toxic and/or carcinogenic, (ii) biological removal of compounds that
would be partially removed by activated carbon treatment alone, and (iii) extension of
carbon service time, and reduction in regeneration costs and energy utilization [26].
6.1.5.2
Consideration of Substrate Removal and Biofilm Formation
Initial GAC/BAC modeling efforts focused on the dynamics of the biofilm layer
surrounding the carbon particles. The effect of bacterial growth was taken into
account first by Andrews and Tien, who analyzed the transport and transformation
of substrate among the solution, biofilm, and adsorbent [27]. The predictive capacity
of thismodel was verified using valeric acid as a substrate in fluidized-bed adsorbers.
Peel and Benedek developed a model for GAC adsorbers that were operated in
the fixed-bed mode. The hydraulic regime was described by plug flow conditions