In the current study, a treatment train strategy for urban mature leachates, comprising biological and physicochemical processes, was tested for full legal compliance. The leachate presents a high organic and nitrogen content (1.1g C/L; 3.6g O2/L; 2.0gN/L) and low biodegradability (BOD5/COD=0.05). In the first stage, a sequential batch reactor (SBR), operated in a 24h-cycle mode (15h aeration +8.5h anoxic, with methanol as external carbon source +0.5h settling), was tested for total nitrogen (TN) removal. The maximum daily TN load that could be treated, reaching the legal limit (< 15mgN/L), increased by 50% with the rise in temperature from 20 to 30°C. For the following coagulation stage, the highest dissolved organic carbon (DOC) removal (64%) and lower final turbidity (33 NTU) were obtained with 240mg Fe3+/L, at pH3.0. The jar-tests, comparing nitrified (LNIT.) and nitrified/denitrified (LN/D.) leachate, stressed the effect of the leachate alkalinity, generated during the denitrification reaction, on process efficiency. For the coagulated LN/D., with alkalinity of 1.1g CaCO3/L, the final concentration of sulfate was only slightly below the legal limit (< 2g/L). A photo-Fenton (PF) oxidation process (pH range of 2.8-3.0, 60mg Fe2+/L), as third treatment step, promoted a significant enhancement on leachate biodegradability, consuming 75mM of H2O2 and 8.9 kJ/L of accumulated UV energy, to achieve an effluent that can be further biologically treated in compliance with the COD discharge limit (150mg O2/L) into water bodies. Biological continuous mode tests using a conventional activated sludge process, with an hydraulic retention time (HRT) of 12h, allowed to obtain COD and TSS values (107±3 and 50±2mg/L, respectively) below the legal limit.
Keywords: Biological oxidation; Legal discharge limits; Multistage treatment strategy; Photo-Fenton; Sequential batch reactor.
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