After obtaining his bachelor’s degree in fundamental physics, Mathieu decided to devote himself to the environment through a master’s degree in meteorology and oceanography, followed by another master’s in environmental management and monitoring at Toulouse University III. A complementary formation in wastewater treatment allowed Mathieu to join the LISBP (Engineering Laboratory of Biological Systems and Processes) at INSA-Toulouse as a trainee and later on as a research assistant. He then started a PhD in the same laboratory.

His PhD focused on the modeling of N₂O production during the biological nitrogen removal. He used a lab-scale sequencing batch airlift reactor (SBAR) containing flocs and granules to remove nitrogen. Each cycle of the SBAR is composed of an anoxic phase and an aerobic phase. Mathieu will build a model that can describe the biological processes involved in the nitrogen removal. This model, based on ASM3, is able to describe the N₂O production induced by denitrification during the anoxic phase and by nitrification during the aerobic phase.