|M.Sc Student||Volk Elazar Nathan|
|Subject||Quantifying the Effects of Biofilm on the Hydraulic|
Properties of Unsaturated Soils
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Alex Furman|
|Dr. Ravid Rosenzweig|
|Full Thesis text|
The current research focuses on direct measurements of the hydraulic properties of biofilm-affected unsaturated soils. Biofilm effect on water flow is very important in various engineered systems such as bioremediation, soil-aquifer treatment, and filtration. So far, the majority of research regarding the hydraulic properties of biofilm-amended soils was conducted on saturated soils. Further, the experimental research conducted on soil in unsaturated conditions is limited since the majority of this work used biofilm analogues and since in the few studies that used real biofilm it was not constant during the experiments and thus hydraulic properties could not be tied to biofilm quantity. Further, the majority of the works did not study the effects of the biofilm on both the water retention curve (WRC) and hydraulic conductivity function (HCF).
In this work, hydraulic conductivity and water retention of unsaturated soils were measured in soils with ranging amounts of biofilm that was grown in-situ in the soil. Biofilm was quantified using viable counts of the bacterial cells and by the total organic carbon (TOC) consumed by the biofilm during incubation (Carbon deficit). Hydraulic properties were measured using the evaporation, dew-point, and extended multi-step outflow methods. Evaporation experiments were conducted under refrigerated conditions to limit bacterial growth. Last, mathematical models of hydraulic functions were fitted to the measured data.
The TOC deficit was found to be a better proxy for biofilm quantification than the viable counts. This is most likely due to being a proxy for both extracellular polymeric substances (EPS) and bacterial cells compared to viable count that is a proxy for the later only.
A reduction in the evaporation rate of the inoculated soils was observed with respect to the clean soil. A clear increase in water retention was seen in the WRCs 'dry range' (water content between 0.05 and 0.16) for the treated soils. In this range, the WRCs were characterized by increasing water content for treatments with increasing TOC deficit. In the high matric head range of the WRC (105 - 106 cm H2O), data was found practically identical for all inoculated and non-inoculated treatments.
A reduction of one order of magnitude was observed in the saturated hydraulic conductivity of the biofilm-affected soils relative to the clean soil while half an order of magnitude reduction was observed throughout the unsaturated range of the HCF. The increase in water retention with the addition of biofilm was described well by the bimodal van Genuchten relation. The HCFs were best described using a model superimposing capillary flow and water film flow components. The fraction of the water film component out of the total conductivity was higher for the biofilm-affected soils.