How does the pre-treatment of landfill leachate impact the performance of O and O/UVC processes?

In this study, O and O/UVC processes were evaluated for the treatment of landfill leachate after biological nitrification/denitrification, coagulation, or their combinations. The O-driven stage efficiency was assessed by the removal of color, organic matter (dissolved organic carbon (DOC) and chemical oxygen demand (COD)), and biodegradability increase (Zahn-Wellens test). Also, fluorescence excitation-emission matrix (EEM) and size exclusion chromatography coupled with OC detector (SEC-OCD) analysis were carried out for each strategy. The bio-nitrified-leachate (L) was not efficiently mineralized during the O-driven processes since the high nitrites content consumed ozone rapidly. In turn, carbonate/bicarbonate ions impaired the oxidation of the bio-denitrified-leachate (L), scavenging hydroxyl radicals (HO) and inhibiting the O decomposition. For both bio-leachates, only O/UVC significantly enhanced the effluent biodegradability (>70%), but COD legal compliance was not reached. EEM and SEC-OCD results revealed differences in the organic matter composition between the nitrified-coagulated-leachate (L) and denitrified-coagulated-leachate (L). Nonetheless, the amount of DOC and COD removed per gram of ozone was similar for both. Cost estimation indicates the O-driven stage as the costliest among the treatment processes, while coagulation substantially reduced the cost of the following ozonation. Thus, the best treatment train strategy comprised L (with methanol addition for denitrification and coagulated with 300 mg Al/L, without pH adjustment), followed by O/UVC (transferred ozone dose of 2.1 g O/L and 12.2 kJ/L) and final biological oxidation, allowed legal compliance for direct discharge (for organic and nitrogen parameters) with an estimated cost of 8.9 €/m (O/UVC stage counting for 6.9 €/m).