Adsorptive biological treatment of pretreated landfill leachate by fed-batch operation

Ill ABSTRACT Early biological treatment studies with the raw leachate did not yield high COD and nitrogen removals. In order to improve biological treatability,, the landfill leachate was subjected to pretreatment by chemical coagulation-floccularion followed by air stripping of ammonia at pH =12. Three different chemical coagulants, alum (Aİ2(S04)3 ), FeCİ3 and lime (CaO) were used in different concentrations tor COD and nitrogen removal by cwagulation-flocculation- COD concentrations of the three coagulants at low doses (0.5-1.0 g/L) were comparable. Considering the problems associated with the use of high coagulant doses such as more sludge formation and high cost of coagulation, low doses of coagulants such as 1 g/L were preferred. Percent COD removals for the three coagulants at the dose of 1 g/L were almost the same as 45%. Supernatant solution after coagulation with lg/L lime at pH =12 was subjected to air stripping at different pH levels to remove excess ammonia from the leachate. Ammonium concentration was reduced to nearly 700 mg/L from 1200 mg/L after 45 minutes of air stripping at pH=12. The pretreated leachate was subjected to aerobic biological treatment in an aeration tank by fed-batch operation. The effects of the feed wastewater COD content and flow rate on COD and ammonium ions removal were investigated. Nearly 76% COD and 23% NH*-N removals were obtained with a flow rate of 0.21IV L/h and the feed COD content of 7,000 mgCOD/L. COD removal efficiency decreased with increasing COD loading rates. A kinetic model for COD removal was developed and the kinetic constants were determined by using the experimental data. In order to improve the extent of COD and ammonium nitrogen removals, pretreated leachate was subjected to adsorbent supplemented biological treatment in an aeration tank operated in fed-batch mode by using powdered zeolite (PZ) and powdered activated carbon (PAC) as adsorbents. Adsorbent concentrations were varied between 0 g/L and 5 g/L. Percent COD and ammonium-N removals increased with increasing adsorbent concentrations. COD removals with PAC addition were significantly higher than those obtained with zeolite. However, zeolite performed better than the PAC in ammonium-N removal from the leachate. Nearly 87% and 77% COD removals were achieved with PAC and zeolite concentrations above 2 g/L, respectively. Ammonium-N removals were 30% and 40% with PAC and zeolite concentrations of 5 g/L, respectively at the end of 30 hours of fed-batch operation. An empirical equation was developed to describe the contribution of adsorption over biological treatment as a function of PAC and zeolite concentrations. To further decrease effluent COD and ammonium concentrations, two sets of repeated fed-batch experiments with different operation times (3x10 h and 5x6 h) were performed with and without PAC addition. When the operation time was divided to 3x10 hours and 5x6 hours, better results were obtained than that of the 30 hours single cycle operation with and without PAC addition. The effluent COD and NH4-N concentrations were 365 mg/L and 360 mg/L in the repeated fed-batchoperations of 5x6 hours with 2 g/L PAC addition. To observe system performance at longer operation times, a three-cycle operation with 30 hours durations was used (3x30 h) in repeated fed-batch mode in the presence of 2g/L PAC. At the end of 90 hours repeated fed-batch operation (3x30 h), the effluent COD and NH4-N concentrations were 285 mg/L and 224 mg/L, respectively which are considerably lower than those obtained with 5x6h operation. Apparently, longer operation times in repeated fed-batch operation resulted in better effluent water quality. Effects of N/COD ratio in the fed wastewater on COD and ammonium removal in PAC added biological treatment were investigated. Percent COD removals increased and the final COD contents decreased with increasing Lnh4-n/Lcod ratio. Lnh4-n/Lcod=0-05-0.08 rati0 was optimum resulting in 85% COD and 44 % NH4-N removal. Percent COD removal decreased to 78%, when Lnh4-n/Lcod was reduced to 0.03. Chemical oxidation was used to further reduce COD content of landfill leachate after PAC added biological treatment. Three oxidizing agents (H2O2, Fenton's reagent, NaOCl) were used in different concentrations for chemical oxidation. Chemical oxidation by Fenton reagent resulted in higher COD removals as compared to H2O2 and NaOCL Nearly, %68 COD removal, 95 mg/L effluent COD and 2 mg/L NH4-N have been obtained with 150/250 mg/L H202/FeS04 ratio.