Characterization of ozone dosage reduction mechanism in catalytic ozonation process coupled with coagulation and flocculation

Abstract

• Colloids in the real wastewater were separated by ultrafiltration. • Colloids would consume extra ozone and decreased the O 3 usage. • CF were complementary for CO treating real wastewater. • Polysaccharides was hard to remove either by CO alone and CF-CO. • CF-CO was favorable for aromatic protein II and humic acid-like substances. Characterization of O 3 dosage reduction mechanism in heterogeneous catalytic ozonation (CO) process coupled with coagulation and flocculation (CF) was investigated. Dissolved organics (<0.45 μm) was separated into the truly dissolved and the colloidal phase by ultrafiltration with a 1 K membrane. To meet the emission standard (the COD was reduced from 103.0 mg/L to 50.0 mg/L), when 6.1% of colloidal substances were removed by CF, the effluent COD further decreased 21.1% compared with CO alone. Ozone consumption per unit COD removed by O 3 unit decreased from 0.68 g-O 3 /g-COD (CO alone) to 0.30 g-O 3 /g-COD (CF-CO). Results indicated that colloidal substances significantly affect the CO treatment by increasing the O 3 consumption. Transformation of colloids between molecular weight of 1–3 K and 3–5 K, 5–10 K and 10–30 K were observed. Specific UV absorbance at 254 nm (SUVA 254 ) of the effluent increased by 54.4% after CO alone while decreased by 34.9% after CF-CO, showing that organics with low DOC but high UV 254 accounted for a high level in the raw water. It is hard for CO to remove the polysaccharides of 10–100 K whether the CF pretreatment was applied or not. Fluorescent excitation-emission matrix- fluorescence regional integration analysis showed that the <10 K fractions were removed more in the aromatic protein II, fulvic acid-like substances and humic acid-like substances; <1 K and 30–100 K of soluble microbial by-product-like substances were removed most by both the CO alone and CF-CO. Relationship between the total and individual fluorescence showed that CF-CO was favorable for aromatic protein II and humic acid-like substances. To enhance the removal of the organics by CO, the colloids should be removed by CF pretreatment. For a better energy saving in the practical application, the systemic effects of the concentration of the colloidal substances on the CO was needed for the optimized dosage combination of coagulate and ozone. However, the polysaccharides (10–100 K fractions) was hard to be removed by CO weather the CF pretreatment was applied or not. This is helpful to guide the choice of low-cost advanced oxidation technology according to the character of organic matters.