Coagulation-flocculation optimisation for real bilge wastewater: performance comparison of inorganic coagulants and a...

The treatment of bilge wastewater (BW) poses a significant challenge because of its highly complex matrix, which includes aged oils, unknown surfactants, and high salinity. This study investigated the application of the coagulation-flocculation process for treating real BW to satisfy the strict International Maritime Organization (IMO) discharge standard of oil and grease (O&G) of 15 mg/L. Various inorganic coagulants and a commercial composite agent (EB-100) were systematically evaluated along with coagulant aids (PAM, Na₂SiO₃, and Ca(OH)₂). Under optimised conditions, FeCl₃ (1,250 mg/L at pH 9), KAl(SO₄)₂ (2,000 mg/L at pH 7), and EB-100 (2,750 mg/L at pH 7) demonstrated excellent O&G removal efficiencies of 96.2%, 97.6%, and 98.6%, respectively. Notably, unlike previous studies that relied on synthetic emulsions or required multi-stage treatments, the optimised coagulants in this study successfully reduced the residual oil concentration to below 15 mg/L in a single application. This demonstrates their exceptional robustness against the high organic load (SCOD > 8,000 mg/L) and interfering species present in the real matrix. Mechanistic analysis indicated that sweep flocculation, combined with charge neutralisation, played a dominant role in destabilising the complex emulsions. EB-100 offers superior operational simplicity for compact onboard applications by functioning effectively at a neutral pH without requiring acid/base adjustments. These findings confirm that the proposed coagulation strategy is a highly viable, practical, and regulatory-compliant solution for marine environmental protection.