The main objective of this project was to develop a steel slag filter effluent neutralization process by acidification with CO-enriched air coming from a bioprocess. Sub-objectives were to evaluate the neutralization capacity of different configurations of neutralization units in lab-scale conditions and to propose a design model of steel slag effluent neutralization. Two lab-scale column neutralization units fed with two different types of influent were operated at hydraulic retention time of 10 h. Tested variables were mode of flow (saturated or percolating), type of media (none, gravel, Bionest and AnoxKaldnes K3), type of air (ambient or CO-enriched) and airflow rate. One neutralization field test (saturated and no media, 2000-5000 ppm CO, sequential feeding, hydraulic retention time of 7.8 h) was conducted for 7 days. Lab-scale and field-scale tests resulted in effluent pH of 7.5-9.5 when the aeration rate was sufficiently high. A model was implemented in the PHREEQC software and was based on the carbonate system, CO transfer and calcite precipitation; and was calibrated on ambient air lab tests. The model was validated with CO-enriched air lab and field tests, providing satisfactory validation results over a wide range of CO concentrations. The flow mode had a major impact on CO transfer and hydraulic efficiency, while the type of media had little influence. The flow mode also had a major impact on the calcite surface concentration in the reactor: it was constant in saturated mode and was increasing in percolating mode. Predictions could be made for different steel slag effluent pH and different operation conditions (hydraulic retention time, CO concentration, media and mode of flow). The pH of the steel slag filter effluent and the CO concentration of the enriched air were factors that influenced most the effluent pH of the neutralization process. An increased concentration in CO in the enriched air reduced calcite precipitation and clogging risks. Stoichiometric calculations showed that a typical domestic septic tank effluent with 300 mg/L of biodegradable COD provides enough biological CO for neutralization of a steel slag effluent with pH of 10.5-11.5. A saturated neutralization reactor with no media operated at hydraulic retention time of 10 h and a concentration of 2000 ppm in CO enriched air is recommended for full-scale applications.