In this study, we report results on some pretreatments of oil mill wastewater using one or two different vegetable matrices, such as dry-Azolla and active carbon, followed by Rhodopseudomonas palustris growth and biogas photoevolution. When a dry-Azolla matrix was used alone in order to pretreat oil mill wastewater, the initial chemical oxygen demand and the polyphenol content of the oil mill wastewater were significantly reduced. Nevertheless, no growth of bacteria was observed when using a culture broth containing >10% of pretreated oil mill wastewater with only the dry-Azolla. When both vegetable matrices were used for the pretreatment of the said oil mill wastewater, 80% of the chemical oxygen demand and 99% of the polyphenol content were removed. The black-brownish colour of the oil mill wastewater faded, and the colour intensity decreased from 24.4 to 1.4. This pretreated oil mill wastewater was then used to grow Rhodopseudomonas palustris. The culture broth for the biogas photoevolution consisted of 25% pretreated oil mill wastewater and 75% distilled water. The experiments for biogas photoevolution were carried out using two different photobioreactor geometries (cylindrical and flat). The experiments were carried out under batch growth conditions and at a culture age of 406 h. 600 ml of biogas were produced by the cylindrical photobioreactor and 335 ml by the flat photobioreactor, corresponding, respectively, to 1.12 l_biogas l_OMW^-1 and 0.624 l_biogas l_OMW^-1.