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

Abstract

In this study, O3 and O3/UVC processes were evaluated for the treatment of landfill leachate after biological nitrification/denitrification, coagulation, or their combinations. The O3-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 (LN) was not efficiently mineralized during the O3-driven processes since the high nitrites content consumed ozone rapidly. In turn, carbonate/bicarbonate ions impaired the oxidation of the bio-denitrified-leachate (LD), scavenging hydroxyl radicals (HO•) and inhibiting the O3 decomposition. For both bio-leachates, only O3/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 (LNC) and denitrified-coagulated-leachate (LDC). Nonetheless, the amount of DOC and COD removed per gram of ozone was similar for both. Cost estimation indicates the O3-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 LDC (with methanol addition for denitrification and coagulated with 300 mg Al3+/L, without pH adjustment), followed by O3/UVC (transferred ozone dose of 2.1 g O3/L and 12.2 kJUVC/L) and final biological oxidation, allowed legal compliance for direct discharge (for organic and nitrogen parameters) with an estimated cost of 8.9 €/m3 (O3/UVC stage counting for 6.9 €/m3).