Ozonation to enhance methane production in anaerobic digestion of olive oil industry wastewater

The olive oil industry generates significant waste, with olive mill wastewaters (OMW) posing environmental challenges due to their high organic content and toxic compounds. One way to value OMW is through the use of biological processes, which are difficult to implement due to the nature of the effluent. Thus, this study aims to assess ozonation as a pre-treatment to improve OMW, making it more suitable for biotreatment by anaerobic digestion (AD). The results showed that untreated OMW produced approximately 82 NmLCH 4 /gCOD i and achieve a 62 % reduction in chemical oxygen demand (COD) after 32 days of AD with a substrate-to-inoculum ratio (SIR) of 1. The ozonation under the natural pH of effluent (4.7) and an ozone inlet mass flow rate of 15 g/Nm 3 resulted in the removal of about 43 % of the total phenolic content (TPh) and 12 % of COD. However, after ozonation, approximately 114 NmLCH 4 /gCOD i of methane was produced more 52 % than that produced by untreated OMW over 25 days of AD. The integration of ozonation and AD enhanced COD and TPh removal, achieving reductions of over 80 % and 70 %, respectively. This integrated strategy holds promise as an effective approach for extracting bioenergy from OMW. Nevertheless, the treated effluent still does not comply with the discharge limits set by Portuguese law. Therefore, an additional treatment process must be explored to further improve the quality of the olive oil effluent, making it suitable for environmental discharge or irrigation use. • Olive oil production generates waste with high environmental impact. • Proper waste recovery is key to preserving the environment and ensuring a sustainable future. • Ozonolysis improved the biodegradability of OMW. • Ozonolysis before AD effectively increased methane production. • OMW treated produced 114 NmLCH4/gCODi of methane after 25 days of AD.