Wastewater treatment is notorious for its hefty carbon
footprint,
accounting for 1–2% of global greenhouse gas (GHG) emissions.
Nonetheless, the treatment process itself could also present an innovative
carbon dioxide removal (CDR) approach. Here, the calcium (Ca)-rich
effluent of a phosphorus (P) recovery system from municipal wastewater
(P recovered as calcium phosphate) was used for CDR. The effluent
was bubbled with concentrated CO2, leading to its mineralization,
i.e., CO2 stored as stable carbonate minerals. The chemical
and microstructural properties of the newly formed minerals were ascertained
by using state-of-the-art analytical techniques. FTIR identified CO3 bonds and carbonate stretching, XRF and SEM-EDX measured
a high Ca concentration, and SEM imaging showed that Ca is well distributed,
suggesting homogeneous formation. Furthermore, FIB-SEM revealed rhombohedral
and needle-like structures and TEM revealed rod-like structures, indicating
that calcium carbonate (CaCO3) was formed, while XRD suggested
that this material mainly comprises aragonite and calcite. Results
imply that high-quality CaCO3 was synthesized, which could
be stored or valorized, while if atmospheric air is used for bubbling,
a partial direct air capture (DAC) system could be achieved. The quality
of the bubbled effluent was also improved, thus creating water reclamation
and circular economy opportunities. Results are indicative of other
alkaline Ca-rich wastewaters such as effluents or leachates from legacy
iron and steel wastes (steel slags) that can possibly be used for
CDR. Overall, it was identified that wastewater can be used for carbon
mineralization and can greatly reduce the carbon footprint of the
treatment process, thus establishing sustainable paradigms for the
introduction of CDR in this sector.