Removal of ammonium by-products from the effluent of bio-cementation system through struvite precipitation

Abstract

Microbial induced carbonate precipitation (MICP) (also referred to as bio-cementation) is an innovative soil improvement technology, introduced through Bio-mediated Geotechnics. During the process, supplied microorganisms hydrolyze urea, resulting in the formation of calcium carbonate. The precipitates tend to cement the soil particles at particle contacts and enhance the strength of the matrix. Despite the considerable interests in MICP, one major hurdle that hinders the field-scale applications is related to the production of ammonium during the process. The objective of this study is to evaluate the feasibility of struvite (NH4MgPO4•6H2O) precipitation to eliminate the ammonium from the reaction effluent system. For this purpose, the study is considered in two stages: (i) rinsing the ammonium from the sand, and (ii) precipitating ammonium as struvite. In the first-stage, the conditions of rinsing are studied to optimize the removal of ammonium from the soil. In the second-stage, influences of pH conditions, molar ratio and calcium ions on the precipitation of struvite are evaluated. The study demonstrates, through the struvite precipitation technique, around 90% of ammonium could be removed from the effluent. Finding also suggests that the molar ratio (NH4+: Mg2+: PO43-) of 1: 1.2: 1 provides a desirable environment for appropriate removal.