Biomedical Engineering Reference
In-Depth Information
Introduction
In recent years, the development of different systems for the biological treatment of
industrial and urban wastewater have been focused on obtaining biomass with good settling
properties (e.g., biofilm and granular sludge). These systems are characterized by their high
capacity to retain biomass, which allows increasing the treated volumetric loads and
improving the quality of effluents in terms of suspended solids (Beun et al., 2001; Beun et al.,
2002; Liu and Tay, 2002).
In these systems shear stress exert important effects on both biomass characteristics and
hydrodynamic conditions of the systems and, therefore, on their overall efficiency (Figure 1).
The effects on the biomass are related to both distribution of the different microbial
populations along the biomass particles and physical properties (size and density) (Liu and
Tay, 2002). Shear stress, caused by the operational hydrodynamic conditions, promotes the
phenomenon of cellular aggregation, achieving biomass with higher density which favours its
retention due to the improvement of its Zone Settling Velocity (ZSV). Moreover, higher
biomass concentrations can be accumulated inside the system since its specific volume is
lower (low Sludge Volumetric Index, SVI).
Shear stress usually provokes a decrease in particle diameter, which increases the specific
surface area of biomass, improving the transfer rate of substrates (Gmachowski, 1995; Liu
and Tay
,
2002). However, a decrease of diameter has a negative effect on the ZSV and could
cause a wash-out of the biomass. Therefore, the system should be operated under those
conditions that allow achieving a balance between both transfer rate of substrates and biomass
retention.
SHEAR STRESS
SHEAR STRESS
Hydrodynamic
conditions
Hydrodynamic
conditions
Biomass
Biomass
MIXING: Increase
MIXING: Increase
UNDESIRED BIOFILM
FORMATION: Decrease
UNDESIRED BIOFILM
FORMATION: Decrease
BIOMASS DENSITY: Increase
BIOMASS DENSITY: Increase
DIAMETER: Decrease
DIAMETER: Decrease
MASS TRANSFER
RESISTANCE: Decrease
MASS TRANSFER
RESISTANCE: Decrease
SPECIFIC ACTIVITY :
Increase or decrease
SPECIFIC ACTIVITY :
Increase or decrease
- Avoid preferential pathways
- Favour degasification
- Avoid clogging
- Avoid preferential pathways
- Favour degasification
- Avoid clogging
Better biomass retention
-Higher ZSV
-Lower SVI
Better biomass retention
-Higher ZSV
-Lower SVI
Lower ZVS
Lower ZVS
Better substrates
transfer rate
Better substrates
transfer rate
Maintain membrane
permeability
Maintain membrane
permeability
Increase of internal mass
transfer resistance
Increase of internal mass
transfer resistance
Better effluent quality
Better effluent quality
Loss of efficiency
Loss of efficiency
Wash -out of biomass
Higher efficiency
Wash -out of biomass
Higher efficiency
Higher efficiency
Higher efficiency
Figure 1. Effects of shear stress on the biomass characteristics and the hydrodynamic conditions and
their consequences on the reactor performance.
Shear stress exerted on the biomass also affects its specific activity. Moderate shear stress
increases the specific activity due to a decrease of the particle diameter and the generation of
