OMW is characterized by an intensive violet dark brown up to black color, strong specific olive oil smell, high degree of organic pollution (chemical oxygen demand (COD) of 40e220 g/L and biochemical oxygen demand (BOD) of 35e110 g/L), pH of 3e6, 25e45 g/L of organic compounds in total, high electrical conductivity (EC), high content of polyphenols (0.5e2 4 g/L), reduced sugars (60% of the dry weight), and high solid matter content.