Design and application of gel coagulation–spontaneous flotation integrated process in water treatment: “Clouds in water”

Abstract

The gel coagulation–spontaneous flotation (GCSF) process designed in this paper mainly rely on dissolved gas in water rather than auxiliary gas equipment to achieve spontaneous flotation. Compared with the traditional coagulation-dissolved air flotation method, GCSF has more stable flotation efficiency and shorter operation cycle under conventional hydraulic conditions. In this study, the GCSF scheme was applied for surface water treatment, and its operating efficiency, mechanism of action, and environmental implications were explored systematically. The results illustrate that the dosage ratio of sodium alginate (SA) to aluminum sulfate (AS) should be controlled in the range of approximately 1.5:1–2.5:1, and SA should be added 15∼120 s before AS during the coagulation process. Under these conditions, the adsorption cross-linking between SA and Al3+ promoted the generation of gel flocs and effectively encapsulated the dissolved gasses, thereby achieving a stable spontaneous flotation process and 80%–95% removal of pollutants. The purification efficiency of GCSF was positively correlated with pH 4–9, which was attributed to the enhanced hydrophobicity of the chains of organic polymer groups. The residual SA and aluminum concentration in effluent were lower than 1 and 0.05 mg/L, respectively, which guarantee the ecological security of GCSF application. In addition, the results of density functional theory calculations revealed that -OH and -AlO6 in cross-linked flocs could adsorb dissolved oxygen synergistically, while -OH combined with oxygen had a stronger binding energy and stable adsorption.