טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentRosenzweig Ravid
SubjectThe Effect of Biofilms on the Hydraulic Properties of
Unsaturated Soils
DepartmentDepartment of Civil and Environmental Engineering
Supervisors Professor Uri Shavit
Professor Alex Furman
Full Thesis textFull thesis text - English Version


Abstract

Biofilm presence in soils is known to significantly alter soil hydraulic properties by increasing the soil water holding capacity and by reducing its hydraulic conductivity. Up to date the most of the research has focused on saturated conditions, while only a limited attention was devoted to the hydraulic properties under unsaturated conditions.

This thesis addresses the problem of biofilm influence on soil hydraulic properties under variably-saturated conditions by combining experiments and modeling. The experimental part includes measurements of water retention curves (WRCs) of biofilm-affected soils (using xanthan as a biofilm analogue) and flow-through experiments in biofilm-affected sand columns performed under unsaturated conditions. The results provide a quantitative description of how water content, matric head and WRCs change due to the presence of biofilms. It was shown that the addition of less than 1% xanthan substantially increase the soil water content and porosity. Further, it was shown that the WRCs of the xanthan-affected soil can be predicted by a linear mixing model of the properties of the clean soil and the xanthan.

During the column experiments both water content and matric head increased as a result of biofilm growth, leading to water ponding at the soil surface. Biochemical assays revealed that biofilm evolved preferentially near the column surface, where the cell volume was about 1.5% of the pore volume, leading to a reduction of the saturated conductivity by almost two orders of magnitude.

The modeling part includes a simulation of the soil pore-space as a bundle of capillaries and as a network of triangular channels. The effect of the biofilm on the water flow and retention was first examined by considering several scenarios for the biofilm spatial distribution. The results indicate that the flow is highly dependent on the spatial distribution of the biofilm and on the pore-space connectivity.

Next, a time-dependent network model was developed to simulate the coupled problem of water flow, substrate transport, and biofilm growth. The results show that biofilm dynamics and water flow are sensitive to the value of the mass transfer coefficient at the biofilm-water interface. The use of a low coefficient value resulted in a relatively homogeneous biofilm distribution and a moderate conductivity reduction, while the use of a high value led to a sharp conductivity reduction due to preferential clogging of the inlet, as was found in the experiments and was simulated by one of the scenarios.