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Treatment train for mature landfill leachates: Optimization studies

Gomes, Ana I., Santos, Sara G.S., Silva, Tânia F.C.V., Boaventura, Rui A.R., Vilar, Vítor J.P.
The Science of the total environment 2019 v.673 pp. 470-479
activated sludge, aeration, alkalinity, biodegradability, chemical oxygen demand, coagulation, compliance, denitrification, dissolved organic carbon, energy, hydrogen peroxide, landfill leachates, methanol, nitrogen, oxidation, pH, sulfates, surface water, temperature, total nitrogen, turbidity
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.