Biological aspects of microbial-induced calcite precipitation

Microbially-induced calcite precipitation (MICP) is an emerging ground-modification technique. This paper presents the results of laboratory experiments that elucidate some biological factors affecting bioaugmentation and biostimulation strategies of MICP. Co-culture experiments suggest that ureolytic bacterium Sporosarcina pasteurii (DSMZ 33) might release the enzyme urease once introduced into a medium containing non-ureolytic bacterium Bacillus subtilis (DSMZ 6397) due to lysis by the latter, resulting in uncontrolled calcite precipitation. This suggests that exogenous bacteria introduced into a native soil might not survive due to adverse action by indigenous bacteria. It is shown that effective biostimulation of indigenous ureolytic bacteria in low-nutrient sand can be achieved using a stimulation medium containing 200 mM urea, complemented with a simple carbon source (molasses). Changes in microbial population following stimulation were quantified, using genetic enumeration, to show that (a) the net increase in urease activity is not accompanied by increases in the relative abundance of ureolytic bacteria, (b) nitrifying bacteria are part of the enriched indigenous population and (c) nitrifying bacteria can be stimulated by the addition of ammonium only. The use of the lowest effective urea concentration and simple carbon is advocated for sustainable biostimulated MICP, yielding lower ammonium emissions and reduced post-treatment recovery overheads.