BACKGROUND Many physicochemical, biological and disposal technologies have been suggested for the effective treatment of olive mill wastewaters (OMW), wastes with very high organic content, non‐readily degradable content and phytotoxic characteristics, and considered one of the major polluting agents in Mediterranean countries from agricultural activities. RESULTS The treatment of OMW by means of coagulation–flocculation using various inorganic materials (poly‐aluminum chloride – PAC, aluminum sulphate) and organic poly‐electrolytes (anionic polyacrylamide – Flocan 23–20, anionic polyelectrolyte – Praestol 2240) was investigated in this study. This technology was examined as a single stage treatment of two different (in composition) olive mill wastewaters, and a second treatment stage following aerobic biological treatment in a biofilter. Treatment effectiveness was assessed through dissolved chemical oxygen demand (COD), total phenolic content (TP), total solids (TS), total suspended solids (TSS) removal, and zeta potential. Results revealed that PAC presents better and more stable treatment efficiency than aluminum sulphate. The optimum electrolyte concentration varies for the wastewaters used depending on their composition and the electrolyte's chemical structure. An economic analysis was performed taking coagulant and flocculant costs into account. CONCLUSIONS The treatment of OMW by means of coagulation–flocculation using various inorganic materials (poly‐aluminum chloride – PAC, aluminum sulphate) and organic poly‐electrolytes (anionic polyacrylamide – Flocan 23–20, anionic polyelectrolyte – Praestol 2240) was investigated in this study. This technology was examined as a single stage treatment of two different (in composition) olive mill wastewaters, and a second treatment stage following aerobic biological treatment in a biofilter. Treatment effectiveness was assessed through dissolved chemical oxygen demand (COD), total phenolic content (TP), total solids (TS), total suspended solids (TSS) removal, and zeta potential. Results revealed that PAC presents better and more stable treatment efficiency than aluminum sulphate. The optimum electrolyte concentration varies for the wastewaters used depending on their composition and the electrolyte's chemical structure. An economic analysis was performed taking coagulant and flocculant costs into accountThe use of combinations of electrolytes and polyelectrolytes was effective at reducing all parameters analysed. Optimum treatment and economic performance was observed in the case of combined biological and physicochemical treatment, which achieved COD, TP, TS and TSS removal rates of 65.5%, 66%, 71% and 99.2%, respectively; this corresponds to 0.36 €/ m3 of treated OMW. © 2013 Society of Chemical Industry