Contamination reduction of real olive oil mill wastewater using innovative acid and basic chemical precipitation processes

Olive oil mill wastewater is a strong pollutant with low pH (4.63) and high salinity (2.70 dS m−1), organic matter (chemical oxygen demand (COD) = 7910 mg L−1), total phosphorus (1049.7 mg L−1), total phenols (540.4 mg L−1) and turbidity (280.1 NTU). This study provides the first application of precipitation processes using HCl, H2SO4, HNO3, NaOH and Ca(OH)2 to olive oil mill wastewater in order to reduce COD, total phosphorus, turbidity, calcium, total phenols and absorbances at 220, 254, 410 and 600 nm. Application of HCl was capable of reducing initial COD (11.4%), turbidity (87.8–98.4%), calcium (100%), total phosphorus (22.1%) and absorbances at 254 nm (12.5–23.5%), 410 nm (46.1–57.9%) and 600 nm (47.4–63.6%) due to the destabilization of suspended and colloidal matter. H2SO4 process allowed removals of turbidity (82.5–97.4%), total phosphorus (14.3–26.9%), calcium (100%) and absorbances at 254 nm (10.9–22.9%), 410 nm (43.4–56.6%) and 600 nm (94.2–96.8%). Experiments conducted using HNO3 showed a positive effect on the removal of COD (10.5–23.6%), total phosphorus (18.9%), turbidity (78.3–94.3%), total phenols (4.6–24.6%) and absorbances at 254 nm (40.5–72.0%), 410 nm (59.1–70.3%) and 600 nm (82.5–91.4%). The advantages of the application of Ca(OH)2 precipitation constituted the removals of COD (11.4–17.8%), total phosphorus (23.6–42.2), turbidity (60.9–100%), total phenols (25.9–48.0%) and absorbances at 220 nm (10.3–33.5%), 254 nm (18.5–45.9%), 410 nm (34.2–81.6%) and 600 nm (22.1–77.3%)  owing to formation and precipitation of calcium and magnesium carbonates, magnesium hydroxide and phosphates. Similar results were obtained for NaOH precipitation: COD (19.9–21.5%), total phosphorus (5.9–22.7%), turbidity (80.3–99.4%), total phenols (32.9–55.4%) and absorbances at 220 nm (2.8–11.9%), 410 nm (10.4–44.5%) and 600 nm (24.4–91.9%).