Chemistry Reference
In-Depth Information
micropollutants that are found at concentrations of ng L
1
to low
gL
1
. However,
nowadays it is recognized that substrates, even though at very low concentrations,
may be removed as a secondary substrate (Chapter 3).
m
11.2.1.2
The CHABROL Model
Biosorption and biodegradation are the two mechanisms that can act either singly
or simultaneously. They are effective in the removal of NOM as well as organic
micropollutants. Both particulate (insoluble) and dissolved organic matter may be
sorbed onto the biofilm. Biosorption is a mechanism paving the way for subsequent
biodegradation. Particulate organic matter may undergo hydrolysis first and then
diffuse into the biofilm. As a result of the long retention time on the surface, large,
slowly biodegradable or even apparently nonbiodegradable soluble organics are also
hydrolyzed by extracellular enzymes and diffuse into the biofilm (Figure 11.1).
The CHABROL model fractionates the BDOC in the influent water into three
biodegradability classes: S: the readily biodegradable substrate, H1: the rapidly
hydrolyzable substrate, and H2: the slowly hydrolyzable substrate [14]. The model
considers both attached and suspended biomass in the interstitial water. It refers
only to situations when the adsorption capacity of a GAC filter is exhausted. As
such, it focuses mainly on extracellular hydrolysis of dissolved macromolecular
organic substrate, growth of attached and suspended bacteria on the readily bio-
degradable substrate, adsorption and desorption of bacteria, and mortality caused
mainly by protozoan predators. The model calculates the vertical distribution of
attached bacteria and the effluent BDOC from the characteristics of influent water
for given values of contact time and temperature.
Experimental results showed that modifications had to be made in the CHABROL
model to account for the acclimation of biomass to very cold temperatures and to
prevent underestimation of BDOC removal [15]. The applicability of the CHABROL
model was shown by comparison of model calculations with data obtained from
many waterworks in France such as the Neuilly-sur-Marne treatment plant [16].
11.2.1.3
Uhl's Model
This model developed by Uhl emphasizes the importance of attachment and
detachment in drinking water biofilters [3, 17]. The model includes the processes
biodegradation, attachment, and detachment of bacteria taking place in the filter
bed as a function of bed depth. According to this model, NOM is removed by
attached and suspended biomass.
Filters in drinking water treatment are commonly operated as fixed-bed down-
flow filters. Uhl's model treats the filter bed as a fixed-bed plug flow reactor. The
following equation exemplifies the substrate mass balance for an infinitesimal
reactor element of length
D
z such as the one shown in Section 6.1 of Chapter 6:
@
S
b
@
Q
A
@
S
b
@
t
¼
r
s
;
gr
þ
r
s
;
maint
:
þ
r
s
;
cat
(11.2)
e
z
B
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