Treatment of crude olive mill wastewaters by osmotic distillation and osmotic membrane distillation

Olive oil extraction industry is a very important economic activity in the entire Mediterranean basin. The olive mill wastewaters (OMWs) are seasonally produced with large volumes of toxic and polluting wastewaters. Amongst other organic constituents, crude OMW contain high concentrations of phenolic compounds up to 10 g/L that make, along with the high organic load, OMW hardly biodegradable and phytotoxic. In this study attempts are made using osmotic distillation (OD) and osmotic membrane distillation (OMD) processes for OMW treatment and concentration of polyphenols present in OMW without any pre-treatment. These non-destructive processes were carried out using microporous polytetrafluoroethylene (PTFE, Gelman) membranes of different pore sizes (TF200, TF450 and TF1000). Calcium chloride was used as an osmotic agent in the permeate membrane side. The results show that the PTFE membranes exhibit reasonable permeate fluxes (2.9–4.2 L/h m2) and a high retention of the phenolic compounds in the feed membrane side. The obtained concentration factor of phenolic compounds was up to 1.2 after 280 min of crude OMW processing by OD. When using OMD with a temperature difference of 20 °C and a mean temperature of 30 °C, this concentration factor reached a value up to 1.9 after 30 h of operating time using the membrane TF200. All used PTFE membranes exhibit a high fouling resistance since the reduction of the water permeate flux after OMW experiments did not exceed 5%. OD and OMD using PTFE membranes are promising alternative technologies for the treatment and concentration of crude OMW for further valorisation and recovery of their phenolic compounds.

Graphical abstractPhenolic separation coefficient (α) and phenolic concentration factor (β) versus crude OMD operating time of OMW (membrane: TF200; feed temperature: 40 °C; permeate temperature: 20 °C; CaCl2 concentration: 5 M; stirring rate: 500 rpm).Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Osmotic distillation and osmotic membrane distillation were applied to treat crude OMW. ► PTFE membranes exhibit reasonable permeate fluxes and high phenolic separation coefficients. ► PTFE membranes exhibit high fouling resistance when used for crude OMW treatment. ► Olive mill wastewater was concentrated for recovery of its phenolic compounds.