A novel bioprocess combining anaerobic co-digestion followed by ultra-filtration and microalgae culture for optimal olive mill wastewater treatment

The process involves the breakdown of OMW in the absence of oxygen, resulting in the production of biogas, primarily composed of methane. A novel bioprocess has been developed to optimize OMW treatment, combining anaerobic co-digestion with ultra-filtration and microalgae culture. The anaerobic co-digestion process was conducted using an up-flow anaerobic fixed bed bio-reactor with a capacity of 300 liters. The initial chemical oxygen demand (COD) of the OMW ranged from 28 to 38 g/L. The results showed that the maximum biogas production was approximately 0.507 L/g COD introduced per day, with a methane content of 73%. This corresponded to a methane yield of 0.370 L/g COD introduced per day, achieved at an organic loading rate of 4.58 g COD/L per day. The anaerobic liquid digestate (ALD) obtained after co-digestion was subjected to ultra-filtration to remove suspended solids and phenolic compounds. The ultra-filtrated ALD was then used as a nutrient source for the cultivation of Scenedesmus sp., a microalgae species. The results showed that Scenedesmus sp. grew well on the ultra-filtrated ALD, with a maximum biomass productivity of 0.15 g/L per day recorded when the microalgae was grown on 25% of the ultra-filtrated ALD. The microalgae culture also achieved a maximum nitrogen removal rate of 15.18 mg/L per day and almost complete elimination of phosphorus and phenolic compounds. The combined bioprocess demonstrated high removal levels of COD, total phenolic compounds, and total suspended solids in the ALD. The process also produced a valuable microalgae biomass, which can be used as a nutrient-rich feedstock for animal feed or biofuel production. The results of this study highlight the potential of the novel bioprocess for optimizing OMW treatment and biogas production, while also generating a valuable by-product.