The pH value of oil acidized wastewater is relatively low (pH = 6.1), which seriously affects the flocculation of polyacrylamide (PAM). NaOH was used to adjust the pH value, but the maximum was only 7.5. The regulation was limited as the Ca2+ in aqueous phase up to 1,350 mg L−1 consumed OH. A novel formulation of Na2CO3 + PAM was proposed to form CaCO3 floc core to facilitate PAM coagulation. When the concentration was above 400 mg L−1, the PAM precipitation tended to be maximum, followed by NaOH adjustment of pH to 8.0 that could enhance PAM flocculation successively. The sewage sludge (SS) remained and residue oil reduced to 25 mg L−1 and 34mg L−1 respectively. The analysis of the species and composition of fatty acids indicated that the coagulation-flocculation selectively effected the sedimentation of saturated fatty acids (SAT). This provides a new idea for recovery of high value-added residual oil. The optimal additive of Na2CO3 is expected as promising coagulant aid to improve the PAM coagulation-flocculation of oil acidized wastewater.

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