CO introduced the coagulation-flocculation of oil acidized wastewater: pollutant removal and cost analysis.

It is difficult to adjust the pH of oil acidized wastewater rich in Ca, thus hindering the polyacrylamide (PAM) flocculation. This study aims at accelerating the flocculation process by introducing CO into the water to induce the formation of CaCO nuclei. The order in which CO and NaOH were added affected the floc structures. Compared with CO-NaOH-PAM, the flocs of NaOH-CO-PAM were more compact and more CaCO crystals were formed. The aqueous Ca involved in the reaction reached 20%, and CO utilization was enhanced. The settling time was shortened by half (from 20 to 3 min), and NaOH consumption was reduced by one-tenth (from 0.03 to 0.003 mol), hence significantly reducing the costs. Due to the higher settling rate and shorter contact time, the NaOH-CO-PAM flocs adsorbed less so that the residual oil was 124 mg·L, while in the case of CO-NaOH-PAM it was 88 mg·L. As a promising coagulation aid, CO can also be used to mineralize pollutants in wastewater.