Biomedical Engineering Reference
In-Depth Information
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It is apparent from equation 2 - 4 that biofilm and floc structure must have a crucial
importance when applying Fick's laws of diffusion to biological wastewater treatment
systems, since the spatial dimensions are part of Fick's laws. It will be shown in the following
paragraphs that structure and diffusion play an important role in both biofilm and activated
sludge systems. In contrast it will be outlined, how diffusion and biofilm structure have been
strongly emphasized in biofilm modelling by using Fick's law of diffusion, whilst structure
and diffusion is not explicitly described in state of the art activated sludge models.
The Role of Floc Structure in Activated Sludge Modelling
The composition of activated sludge flocs is complex, but in a simplified way, three types
of bacteria can be considered, which determine the structure of an activated sludge floc: floc-
forming bacteria, non-floc-forming bacteria and filamentous bacteria. The filamentous
bacteria form the backbone of the activated sludge floc whilst the floc forming bacteria
function as the glue that holds the floc together. Settleability is an important sludge property
since it determines the solid separation in an activated sludge plant and it therefore directly
affects effluent quality. The settleability is directly linked to floc structure and the causes of
settleability problems can be diagnosed and solved using analysis of floc structure [10, 11].
Large flocs will generally settle faster than small flocs of similar density [12]. Floc strength is
thus also important, because activated sludge flocs are exposed to a number of shear stresses
that can cause the floc to break apart (see [13] for a review). Weak flocs may break apart
resulting in poor settleability and poor dewatering properties [14]. Floc structure is thus an
important operational parameter in activated sludge processes, since it determines sludge
properties such as flocculation, settling and dewaterability [14].
Diffusion processes and resulting mass transport limitations are linked to floc structure by
Fick's laws of diffusion. In activated sludge systems diffusional mass transport limitations are
known to occur and influence process quality; they lead to concentration gradients in the
sludge floc and play for example an important role in the formation of bulking sludge [15].
The process kinetics in activated sludge systems are also affected by floc structure, diffusion
and diffusional mass transport limitations. The nitrification rate for example is strongly
dependent up-on the oxygen concentration in the bulk liquid. This is because the nitrifying
organisms are agglomerated in large flocs and the dissolved oxygen concentration within the
floc may be considerably less than the bulk fluid concentration [16]. The presence of
concentration gradients in activated sludge flocs has been confirmed by measurement with
microelectrodes [17]. These measurements have revealed the presence of anoxic zones inside
sludge flocs under aerobic conditions.
Diffusion and floc structure are hence significant factors in activated sludge systems.
However, the state of the art activated sludge models of the ASM model family [5] do not
explicitly integrate diffusion of dissolved species into the sludge floc. Instead diffusional
mass transport limitations in the sludge floc are modelled using half-saturation coefficients in
the Monod expression which are typically one order of magnitude greater than those reported
for single suspended cells [18]. Diffusion and floc size play thus an important role when
