Effects of biofilm on gas permeability of unsaturated sand

Microbial activities may sustainably improve earthen structures such as landfill covers. Typically, after consuming nutrient, bacteria produce hydrated exopolysaccharide to form a biofilm, which may cause bio-clogging in soil. Previous studies have mainly focused on the combined effects of biofilm and nutrient on the water permeability of saturated sand. The present study investigates biofilm effects on gas permeability of unsaturated sand at different degrees of saturation using a flexible wall permeameter. Soils with water ('Water'), nutrient only ('Nutrient') and a mixture of bacteria and nutrient ('Combined') were examined with three replicates. Statistical analysis and scanning electron microscopy (SEM) were adopted for further interpretation. Compared to 'Water', the gas permeabilities of 'Combined' and 'Nutrient' were consistently lower. The differences increase with decreasing saturation. At around 5% degree of saturation, the gas permeability of the 'Combined' and 'Nutrient' cases was 22% and 14% lower than the 'Water' case, respectively. Statistical analysis reveals that the gas permeability reductions in 'Combined' and 'Nutrient' cases were significant at a significance level of less than 0·05 (probability > 95%). The reduction was attributed to pore clogging by nutrient precipitation (for 'Nutrient') or both biofilm and nutrient precipitation (for 'Combined'), as evident from the SEM results. The effects of biofilm on reducing soil gas permeability can be overestimated if nutrient effects are ignored.