D-BAUG - Departement Bau, Umwelt und Geomatik - ETH Zürich

Highlights ▪ Resources
Bacterial biofilms develop
heterogeneous structures
Biofilms can be used in biological treatment
processes to degrade contaminants.
by E. Morgenroth / IfU + Eawag; K. Milferstedt / INRA LBE,
Narbonne, France; N. Derlon / Eawag
Bacteria in biofilms excrete a sticky polymeric matrix composed
of polysaccharides, proteins, and extracellular DNA.
This polymeric matrix allows the biofilm to remain attached
to solid surfaces and protects the bacteria living inside the
biofilm from the environment. This protection makes it difficult
to remove unwanted biofilms, using disinfectants or
cleaning agents in dental hygiene, on heat exchangers, and
in drinking water distribution networks. In biological wastewater
treatment we are developing reactors that take advantage
of biofilms to degrade contaminants. Biofilm performance
and persistence is closely linked to biofilm
structure. We have developed an imaging technique and
quantitative image analysis to monitor large-scale and longterm
development of biofilm structures over time.
In controlled growth experiments we initially observed the
development of a homogeneous and steadily growing
biofilm. When a critical biofilm thickness is reached, detachment
occurs, reducing the stability of the remaining
biofilm, resulting in subsequent random large scale detachment
events that are no longer correlated to biofilm thickness.
Our results provide first indications of developmental
succession cycles initiated by sloughing of large amounts of
biomass. While microscale development of biofilms (Fig. 1a,
1b, 1c) is highly dynamic and unpredictable, the macroscale
performance of biofilm reactors (Fig. 1e) is usually quite stable
and can be well controlled.We are continuing to develop
novel imaging techniques that allow us to monitor biofilm
dynamics at relevant length and time scales (Fig. 2).
Note: The majority of this work has been performed at the
University of Illinois at Urbana-Champaign supported by a
CAREER award to Eberhard Morgenroth from the National
Science Foundation under grant No. BES-0134104. Figures
are from Morgenroth, E. and Milferstedt, K. (2009) Reviews in
Environmental Science and Biotechnology, 8 (3), 203-208.
64 ▪ D-BAUG Annual Report 2009
Turbulence and interfaces
Laboratory experiments and numerical
simulations reveal the nature of interfaces
at the boundary of turbulent flow regions.
by M. Holzner, B. Lüthi, W. Kinzelbach / IfU
Sharp and strongly contorted interfaces are known to exist
at the boundary of clouds, smoke plumes, volcanic eruptions,
etc., which separate the turbulent flow from the irrotational
ambient flow regions.These interfaces fluctuate
vigorously on a wide spectrum of scales and are associated
with entrainment and mixing of ambient fluid into the turbulent
flow regions. Fundamental understanding of these
processes is important, e.g., for the improvement of models
for the dispersion of contaminants in the atmosphere.
We conducted experiments by using 3D particle tracking
velocimetry, a method that allows measuring in 3D the velocity
of tracer particles that are passively advected by the
flow.The unsteady turbulent flow of the experiment which
had no mean shear and a small Reynolds number, Re λ = 50,
was also simulated via direct numerical simulations. We
measured that the local entrainment velocity, u a, scales
with the smallest velocity scale of the flow, namely, the Kolmogorov
velocity, uη. (Fig. 3) shows a snapshot of the spatial
distribution of the magnitude of u a over the interface
between turbulent and non-turbulent regions. On the
other hand, globally, the interface propagates with a velocity
u e, that is on the order of the integral velocity scale.
We reconcile the two at first conflicting observations by
showing that the interface area is strongly deformed by
the turbulent eddies to account for the same global entrainment
flux with a small characteristic velocity. The
measured ratio between the integrated local and the
global entrainment flux is close to unity, Q η./Q 0=1±5%.
(Fig. 3) also shows qualitatively that the magnitude of ua,
depends on the large-scale shape of the interface, being
higher at the top of outward facing billows and smaller on
their sides and inside the crests. Small- and large-scale
features hence appear to strongly depend on each other.