Production of organic fertilizer from olive mill wastewater by combining solar greenhouse drying and composting

Olive mill wastewater (OMW) is generated during the production of olive oil. Its disposal is still a major environmental problem in Mediterranean countries, despite the fact that a large number of technologies have been proposed up to date. The present work examines for the first time a novel, simple and low-cost technology for OMW treatment combining solar drying and composting. In the first step, OMW was dried in a chamber inside a solar greenhouse using swine manure as a bulking agent. The mean evaporation rate was found to be 5.2 kg H₂O/m²/d for a drying period of 6 months (February-August). High phenol (75%) and low nitrogen (15%) and carbon (15%) losses were recorded at the end of the solar drying process. The final product after solar drying was rich in nutrients (N: 27.8 g/kg, P: 7.3 g/kg, K: 81.6 g/kg) but still contained significant quantities of phenols (18.4 g/kg). In order to detoxify the final product, a composting process was applied as a second step with or without the use of grape marc as bulking agent. Results showed that the use of grape marc as a bulking agent at a volume ratio of 1:1 achieved a higher compost temperature profile (60 °C) than 2:1 (solar drying product: grape marc) or no use (solar drying product). The end product after the combination of solar drying and composting had the characteristics of an organic fertilizer (57% organic carbon) rich in nutrients (3.5% N, 1% P, 6.5% K) with quite low phenol content (2.9 g/kg). Finally, the use of this product for the cultivation of pepper plants approved its fertility which was found similar with commercial NPK fertilizers.