During olive oil production, solid and liquid wastes such as pomace and olive mill wastewater (OMW) are produced. Pomace is evaluated economically, but OMW is discharged directly into the environment. OMW is a liquid of blackish colour with an unpleasant smell that usually contains, rather in suspension, olive pulp residues, mucilage, pectic substances and small amounts of oil (0.5–2.0 %) forming a stable emulsion. OMW generates a superficial film in discharged effluent and soils causing a remarkable additional toxicity.
The high organic matter content of OMW varies in the range of 80–200 g/L COD, 50–100 g/L BOD5, and 10–100 g/L TOC. The dark colour of OMW disturbs the bright appearance of water and prevents the absorption of sunlight by photosynthetically growing organisms such as water plants and algae. Oil from OMW also forms a film layer on the water surface and prevents oxygen transport from air to water. Different organic substances found in OMW include sugars, tannins, phenolic compounds (polyphenols), polyalcohols, pectins, and lipids. The toxicity and antimicrobial activity of OMW are mainly attributable to its polyphenolic content.
OMW is considered as an eco-toxicologically dangerous effluent negatively affecting aquatic and terrestrial living organisms. OMW also contains inorganics such as potassium and sodium chlorides, sulphates and phosphates, as well as calcium, iron, magnesium, sodium, copper and traces of other elements and metals.
80% of the phenolic compounds are found to be removed when the ozone mass transfer efficiency is around 95% and the contact time is 15 minutes. Various Fenton and Fenton-like processes have given excellent results for the same.