concentration in the liquid film is so high that oxygen concentrations are below
1 mg/L in the depth of the slime.
For a specific wastewater flow rate, the oxygen concentration gradient in a slime
layer is a function of the dissolved oxygen concentration at the air-liquid interface
and the substrate concentration. The latter affects the oxygen requirements of the
slime layer. At low substrate concentrations in the liquid film, there will be a
decrease in the oxygen requirements of the slime layer that should increase the
oxygen concentration at the slime-liquid interface and reduce the mass flux of
oxygen across the interface [97]. The reduced oxygen requirement and increased
oxygen concentration at the interface will then result in an increased depth of
penetration of oxygen in the slime layer and probably an aerobic zone throughout
the full depth of the slime layer [72, 73]. At high substrate concentration in the liquid
film, the oxygen requirements of the slime layer approach a constant; then the mass
flux of oxygen across the slime-liquid interface will be constant, and the thickness of
the active portion (i.e., aerobic zone) of the slime layer for the entire bed depth will
also be constant assuming this condition exists at any depth in the process bed [97].
The effect of hydraulic loadings on the transfer of oxygen in the slime layer in the
liquid film has also been studied [90]. As specified by Maier [104] and verified by
Jank [97], a range for hydraulic loadings is normally encountered in full-scale
operation of a trickling filter. For laminar flow, an increase in flow rate will result
in an increase in liquid velocity and an increase in the mass flux of oxygen across the
air-liquid and slime-liquid interfaces. The increased supply of oxygen at the slimeliquid interface will result in a greater depth of oxygen penetration in the slime layer
or a thicker active slime layer at a specific applied organic loading.
The depth of oxygen penetration is dependent on the molecular diffusion coefficient of oxygen in the slime layer, the rate of oxygen utilization, and the oxygen
concentration at the slime-liquid interface. Jank and Dryman [90] have reported that
for a specific wastewater flow rate and substrate concentration, an oxygen concentration gradient will be established within the slime layer that is directly related to the
mass flux across the slime-liquid interface. Although the supply of oxygen at the
slime-liquid interface can be either contributed by the dissolved oxygen content of
the influent wastewater or transferred from the air to the liquid film as the wastewater
flows across the slime layer, Jank [97] discovered that the quantity of oxygen
contributed by the influent wastewater was negligible when compared to the oxygen
requirement of the slime layer.
5.2 Transfer of Substrate in Liquid Film and Slime Layer
A conceptual illustration of the substrate concentration gradient within a slime layer
(i.e., biofilm) as shown in Fig. 3.15 has been proposed by Williamson and McCarty
[105, 106]. It is assumed that the rate of reaction is limited by a single substrate S. Let
the substrate concentration outside the biofilm in the bulk liquid be S 0 ; at the biofilm
surface, S s ; within the biofilm cellular matrix, S c ; and deep within the biofilm a
3 Biological Processes
117
1 mg/L in the depth of the slime.
For a specific wastewater flow rate, the oxygen concentration gradient in a slime
layer is a function of the dissolved oxygen concentration at the air-liquid interface
and the substrate concentration. The latter affects the oxygen requirements of the
slime layer. At low substrate concentrations in the liquid film, there will be a
decrease in the oxygen requirements of the slime layer that should increase the
oxygen concentration at the slime-liquid interface and reduce the mass flux of
oxygen across the interface [97]. The reduced oxygen requirement and increased
oxygen concentration at the interface will then result in an increased depth of
penetration of oxygen in the slime layer and probably an aerobic zone throughout
the full depth of the slime layer [72, 73]. At high substrate concentration in the liquid
film, the oxygen requirements of the slime layer approach a constant; then the mass
flux of oxygen across the slime-liquid interface will be constant, and the thickness of
the active portion (i.e., aerobic zone) of the slime layer for the entire bed depth will
also be constant assuming this condition exists at any depth in the process bed [97].
The effect of hydraulic loadings on the transfer of oxygen in the slime layer in the
liquid film has also been studied [90]. As specified by Maier [104] and verified by
Jank [97], a range for hydraulic loadings is normally encountered in full-scale
operation of a trickling filter. For laminar flow, an increase in flow rate will result
in an increase in liquid velocity and an increase in the mass flux of oxygen across the
air-liquid and slime-liquid interfaces. The increased supply of oxygen at the slimeliquid interface will result in a greater depth of oxygen penetration in the slime layer
or a thicker active slime layer at a specific applied organic loading.
The depth of oxygen penetration is dependent on the molecular diffusion coefficient of oxygen in the slime layer, the rate of oxygen utilization, and the oxygen
concentration at the slime-liquid interface. Jank and Dryman [90] have reported that
for a specific wastewater flow rate and substrate concentration, an oxygen concentration gradient will be established within the slime layer that is directly related to the
mass flux across the slime-liquid interface. Although the supply of oxygen at the
slime-liquid interface can be either contributed by the dissolved oxygen content of
the influent wastewater or transferred from the air to the liquid film as the wastewater
flows across the slime layer, Jank [97] discovered that the quantity of oxygen
contributed by the influent wastewater was negligible when compared to the oxygen
requirement of the slime layer.
5.2 Transfer of Substrate in Liquid Film and Slime Layer
A conceptual illustration of the substrate concentration gradient within a slime layer
(i.e., biofilm) as shown in Fig. 3.15 has been proposed by Williamson and McCarty
[105, 106]. It is assumed that the rate of reaction is limited by a single substrate S. Let
the substrate concentration outside the biofilm in the bulk liquid be S 0 ; at the biofilm
surface, S s ; within the biofilm cellular matrix, S c ; and deep within the biofilm a
3 Biological Processes
117