Water glass combined microwave-assisted conversion of olive mill wastewater into value-added products

Olive mill wastewater (OMWW), produced at over 30 million tons annually in Mediterranean countries, poses a major environmental challenge. This study integrates water glass (WG) and microwave-assisted pyrolysis to convert OMWW into a stable semi-solid matrix (SS-OMWW) suitable for downstream processing and subsequent valorization into value-added bioproducts. The treatment successfully converted liquid OMWW into SS-OMWW with a moisture content of 38% (w/w). An 18-run central composite design optimized pyrolysis conditions (500–700 °C, 300–400 W, 2–4 g biomass). Pyrolysis gas was the dominant product (52.67 ± 14.03%), followed by biochar (26.07 ± 12.87%), bio-oil (14.94 ± 8.48%), and water/coke (6.32 ± 2.49%). Compared with temperature, feedstock mass and microwave power exhibited relatively weak correlations with all product yields within the investigated operating range. GC–MS analysis showed that the bio-oil was rich in phenolic and lipid-derived compounds, with a moderate heating value of 27.32 MJ/kg, lower than that of bio-oil from raw OMWW (39.7 MJ/kg). Increasing pyrolysis temperature enhanced biochar surface area (8.30–62.61 m 2 /g) through greater pore development, while the molar ratio H/C was low indicative of high aromaticity and carbonization. FTIR confirmed retention of phenolic groups after WG-processing and extensive deoxygenation after pyrolysis while thermogravimetric analysis showed decomposition between 150 and 600 °C, corresponding to hemicellulose, cellulose, and lignin degradation. This combined semi-solid matrix-pyrolysis process provides a sustainable approach for OMWW management by converting hazardous liquid wastewater into an easy-to-handle matrix and valuable products, reducing environmental pollution and recovering value from agricultural waste.