Surface Characterization and Interaction Modeling of Biofouling in Polymer Film Heat Exchanger Applications
Published on 18. October 2018
Book
Paperback/Softback
162 pages
978-3-8439-3791-7 (ISBN)
Description
The objective of this thesis was the investigation of interaction effects between model and autochthonous microorganisms with polymer surfaces and stainless steel. The application of polymer films in heat transfer processes utilizing river water as coolant was herein the ultimate goal in order to reduce fouling formation. Surface chemistry (surface free energy and its composition) and electrostatic properties (zeta potential) and their interaction with the microorganisms were identified as the dominating surface parameters. The roughness was found to be of minor importance to contribute to significant fouling formation due to the surfaces being comparatively smooth in relation to the bacteria radii.
The general adhesion tendency in laminar flow was dominated by the long-distance disperse interaction between bacterium and surface. An increase in the disperse fraction of the surface free energy lead to an increased deposited biomass. The resistance against shear stress on the other hand was dominated by the polar interaction forces in the wall-near region. Biofilms on a polar surface resisted shear stress more effectively. Electrostatic interaction was found to be especially dominant in low-ionic systems, which lead to excessive adhesion on stainless steel with its comparably high zeta potential. All interaction mechanisms could be qualitatively validated with the DLVO theory that is based on particle system interactions.
The surface nonspecific process parameters like wall temperature, Reynolds number and nutrient concentration were investigated based on design of experiments methods. It was found that especially the latter two parameters showed significant increasing and decreasing influence with local maxima on the fouling resistance. The combination of low nutrient concentration and simultaneously high flow velocity prove to significantly increase the fouling resistance. The herein inherent reduction of the mass transfer coefficient towards the biofilm increased the local mass production of the biofilm more than a high cell concentration in the process fluid and its more frequent interaction with the substrata.
The general adhesion tendency in laminar flow was dominated by the long-distance disperse interaction between bacterium and surface. An increase in the disperse fraction of the surface free energy lead to an increased deposited biomass. The resistance against shear stress on the other hand was dominated by the polar interaction forces in the wall-near region. Biofilms on a polar surface resisted shear stress more effectively. Electrostatic interaction was found to be especially dominant in low-ionic systems, which lead to excessive adhesion on stainless steel with its comparably high zeta potential. All interaction mechanisms could be qualitatively validated with the DLVO theory that is based on particle system interactions.
The surface nonspecific process parameters like wall temperature, Reynolds number and nutrient concentration were investigated based on design of experiments methods. It was found that especially the latter two parameters showed significant increasing and decreasing influence with local maxima on the fouling resistance. The combination of low nutrient concentration and simultaneously high flow velocity prove to significantly increase the fouling resistance. The herein inherent reduction of the mass transfer coefficient towards the biofilm increased the local mass production of the biofilm more than a high cell concentration in the process fluid and its more frequent interaction with the substrata.