Comparative Analysis of Bioaugmentation and Biostimulation Approaches for Biocementation of Sandy Soils

Microbially Induced Calcite Precipitation (MICP) is an innovative soil improvement method that enhances the mechanical properties of different soil types through biological processes. Among various MICP pathways, urea hydrolysis mediated by ureolytic bacteria is the most widely used. Traditionally, exogenous ureolytic bacteria have been employed to accelerate calcite precipitation in soils, known as the bioaugmentation approach. However, this method is associated with some challenges, such as environmental impacts from introducing large volumes of non-native bacteria into natural soils, high material usage, and non-uniform spatial calcite precipitation. In contrast, recent studies suggest that these challenges can be reduced through the biostimulation approach, which is based on the enrichment of native ureolytic bacteria in soils. To determine the feasibility of replacing bioaugmentation with biostimulation for different soil types, a direct comparison between the treatment results from the two approaches is necessary. In this study, two soils with varying grain size distribution curves from the Rhenish lignite mining area in Germany were treated using both bioaugmentation and biostimulation approaches. Cylindrical samples of 7.5 cm height and 3.5 cm diameter were prepared. Treatment efficiency was assessed through unconfined compressive strength (UCS) and permeability tests. Moreover, calcite content was measured to evaluate calcite precipitation uniformity. The microstructural characteristics of the precipitated calcite were examined through scanning electron microscopy (SEM). The results showed that the strength and stiffness of all soil samples was considerably enhanced through both approaches, with UCS values ranging from 0.25 to 0.31 MPa for calcite contents of 4 to 6%. Notably, in some cases, the biostimulation approach yielded similar mechanical soil properties as the bioaugmentation method, indicating its potential as a sustainable alternative to bioaugmentation.