Bench-scale investigation of an integrated adsorption–coagulation–dissolved air flotation process for produced water treatment

• Dissolved air flotation alone did not meet discharge standards for oil and grease.
• Adsorption or coagulation pre-treatment met oil and grease discharge standard.
• Integrated process improved naphthalene removals compared to coagulation alone.
• Increased velocity gradients did not negatively impact coagulation treatment.

In oil and gas extraction operations, water from the hydrocarbon reservoir is brought to the surface along with the oil or gas. This “produced water” contains organics which may be free, dispersed, or dissolved in the water. While certain dissolved compounds may contribute to environmental risk from produced water, current North American discharge regulations only address the dispersed fraction of oil and grease (29 mg/L in the US, 30 mg/L in Canada). The purpose of this research was to investigate, at bench scale, chemical coagulation with ferric chloride (FeCl3) and adsorption with organoclay (OC) in a completely stirred tank reactor (CSTR) configuration as pre-treatment for dissolved air flotation (DAF) for the removal of dissolved and dispersed oils from produced water. The integrated process was evaluated and compared to the individual processes of coagulation-DAF, adsorption-DAF and DAF without pre-treatment for the removal of dispersed oil, naphthalene and phenol from synthetic produced water. Concentrations of dispersed oil in clarified water were reduced, from an initial concentration of 100 mg/L, to concentrations as low as 10 ± 1.6 mg/L after coagulation with FeCl3 (FeCl3-DAF), 15 ± 1.2 mg/L after adsorption with OC (OC-DAF), and 7 ± 1.4 mg/L after the integrated process (OC-FeCl3-DAF). From an initial naphthalene concentration of 1 mg/L, both the adsorption (OC-DAF) and integrated process (OC-FeCl3) achieved clarified naphthalene concentrations of 0.11 ± 0.01 mg/L, representing a significant improvement over the 0.53 ± 0.03 mg/L achieved by coagulation treatment (FeCl3-DAF). However, none of the processes evaluated in this study were found to be effective for phenol removal.