Biomedical Engineering Reference
In-Depth Information
Hydraulic Stress Generated by Gas
In aerobic activated sludge systems when the airflow supplied to maintain an adequate
dissolved oxygen level is higher than 0.02-0.03 m
3
/(m
3
·min) a complete mixing behaviour
can be obtained. In this kind of systems, the loading rate is usually limited by the maximum
biomass concentration that can be retained in the reactor. Due to the common settling
properties of the sludge, SVI around 50-100 ml/g VSS and ZSV of 1 m/h, the biomass
concentration in the reactor is usually between 3 and 6 g VSS/L. Maintenance of higher
biomass concentrations in the system requires the increase of the settler volume to separate
the biomass from the treated water, involving the need of very large surface areas. In this
respect, shear stress due to high airflows could have a positive effect since it causes a
decrease of the SVI (Campos et al.
,
1999) and an increase of the density (Iaconi et al., 2006)
which allows the biomass retention to be improved.
An early work of Shin et al. (1992) already showed that a high shear force favours the
formation of aerobic granules and granule stability. In fact, Tay et al. (2001) studied the effect
of shear stress value in terms of superficial upflow air velocity on the formation of aerobic
granules in a bubble column. These authors found that aerobic granules could be formed only
above a threshold of 1.2 cm/s. Denser, rounder and compact granules are developed at high
hydrodynamic shear force. This fact indicates that the structure of aerobic granules is
determined by the hydrodynamic shear force present in the bioreactor. The better stability of
the aerobic granules at high shear forces would be related to the higher production of
extracellular polysaccharides (EPS) under these conditions which play a crucial role in
maintaining the structural integrity in a community of immobilized cells (Sheng et al., 2006).
On the contrary, Iaconi et al. (2006) observed the EPS content of granular biomass in a
sequencing batch biofilter reactor was not affected by hydrodynamic shear forces. Adav et al.
(2007) demonstrated that proteins, rather than polysaccharides, were enriched in the sheared
granules, which was consistent with the results of McSwain et al. (2005) and Chen et al.
(2007).
Arrojo et al. (2008) performed the Anammox process in a gas lift reactor operated at
different upflow velocities (from 3.53 to 9.70 cm/min) in order to expose the system to
different shear forces, ranged between 0.0115 and 0.0316 kW/m
3
. These authors found the
negative effects on the specific activity of biomass started when the system was operated at
an upflow velocity higher than 5.29 cm/min, observing a decrease of the activity of 85% at
9.70 cm/min. The average diameter of the granules gradually decreased in 30% during the
operational period.
The influence of gas velocity on a methanogenic biofilm was studied by Michaud et al.
(2003) in an inverse turbulent bed reactor. They found that the increase of gas velocity caused
an increase of the specific activity and the biofilm formed was more dense, stable and
resistant to detachment. They also observed higher exocellular polymeric substances
production at higher shear stress conditions which could explain the greater cohesion of
biological matrix.
Applying high shear force by gas sparging proved also to be effective in improving
denitrification rates in a hollow-fiber membrane biofilm reactor by reducing the thickness of
the biofilm (Celmer et al., 2008). In submerged MBRs, the membrane fouling can be
mitigated with the increase in aeration intensity (Germain et al., 2005). The increase of
