Refinery spent caustics (SC) were diluted with sour waters (SW) in a ratio 1:7, neutralized with CO2 (SC/SWCO2) and 83% of H2S was striped during this procedure, remaining an aromatic portion that contained 2123, 2730 and 1379 mg L−1 of phenol, p-cresol and o-cresol, respectively. The mixture was treated anaerobically in an EGSB reactor fed with 1.5 gCOD L−1 d−1, without mineral supplements causing loss of COD removal efficiency that dropped to 23%, methane production ceased and no phenol or cresols were biodegraded. The EGSB experiments were resumed by feeding the reactor with nutrients and phenol at 1.0 gCOD L−1 d−1. The mixture SC/SWCO2 added to the phenol load, was step increased from 0.10 to 0.87 gCOD L−1 d−1 maximum. When total organic load was increased to 1.6, COD removal efficiency was 90% and at the highest load attained, 1.87, efficiency dropped to 23% attributed to the toxic effect produced by cresols.

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