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

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.