BACKGROUND: Coagulation/flocculation is a physico-chemical method for the treatment of olive mill wastewater (OMW). Coagulation/flocculation and precipitation of the agglomerates formed relies on the effective decrease of the electrical charge of the suspended solids which allows particles to approach each other and form large clusters. The present work focuses on the decrease of the electrical charge of the suspended particles by monitoring the changes in the zeta potential of the particles. This approach can also indicate with accuracy the exact concentration of the coagulant or flocculant that must be added in the wastewaters, to achieve the highest removal of the solids and organic load. RESULTS: In the present paper, experiments of coagulation/flocculation with electrolytes [FeCl3, Ca(OH)(2), CaO, CaCl2] and polyelectrolytes (PDADMAC, PAH, PAA, PEI, Floccan 22-23) are presented, either separately or in combination and screened with respect to their removal efficiency in terms of chemical oxygen demand (COD), total suspended solids (TSS), Total solids (TS) and total phenols (TP) removal and by monitoring the zeta potential. A relevant literature review is presented also regarding the coagulants and flocculants which have been examined for the physico-chemical treatment of OMW. The compounds used, their concentrations and their removal efficiency in terms of COD, TS, TSS and phenols are listed, where possible. CONCLUSIONS: The coupling of calcium hydroxide at 20 g L-1, with PDADMAC in a range of 0.75-2.00 g L-1 led to reductions of COD, TSS, TS and phenols up to 56%, 27%, 43% and 76%, respectively. (C) 2014 Society of Chemical Industry