| Landfill still remains the most commonly employed treatment for municipal solid waste (MSW) disposal around the world, which generates a high-strength wastewater with complex constituents referred to as landfill leachate. Landfill leachate is generally characterized by a large amount of recalcitrant organic substances, high chemical oxygen demand (CODcr) and ammonium content, low biodegradability (BOD5/CODcr), low carbon to nitrogen ratio (C/N), and the presence of heavy metals and inorganic macro-constituents. Therefore, the collection and treatment of landfill leachate is nowadays recognized as one of the most urgent environmental issues.In our investigation, coagulation as a pretreatment was applied to remove recalcitrant organic substances from stabilized landfill leachate, such as humic acids (HA), to improve the biodegradability of leachate and reduce pollutant load of subsequent biological treatment. Then, a self-designed biological nitrogen removal system as an advanced treatment was employed to remove organic pollutants and nitrogen.In our studies, process conditions of coagulation-flocculation were optimized by single factor experiments at first. Aluminum sulfate (Al2(SO4)3), aluminum chloride (AICl3), ferric sulfate (Fe2(SO4)3), ferric chloride (FeCl3), polyaluminum chloride (PAC), polyferric sulfate (PFS) and polyaluminum ferric chloride (PAFC), as inorganic coagulants, were used to pretreat landfill leachate. The appropriate ranges of process conditions for the seven coagulants were obtained, and four influencing factors of coagulation process were analyzed. It was found that dose of coagulant and initial pH values of leachate were the most two important factors. Thereafter, response surface methodology (RSM) was used for the further optimization of the two important factors, and HA removal as an important response of RSM was originally used to optimize coagulation-flocculation process. Finally, the obtained optimum process conditions were Al2(SO4)3dose of14.00g/L (corresponding to aluminum content of0.042mol A13+/L) at initial pH7.50, AICl3dose of12.00g/L (corresponding to aluminum content of0.050mol A13+/L) at initial pH7.00, Fe2(SO4)3dose of12.00g/L (corresponding to iron content of0.046mol Fe3+/L) at initial pH7.50, FeCl3dose of10.00g/L (corresponding to iron content of0.037mol Fe3+/L) at initial pH8.00, PAC dose of15.00g/L (corresponding to aluminum content of0.082 mol A13+/L) at initial pH6.00, PFS dose of8.00g/L (corresponding to iron content of0.008mol Fe3+/L) at initial pH6.00, PAFC dose of15.00g/L (corresponding to aluminum content of0.085mol A13+/L and iron content of0.002-0.004mol Fe3+/L) at initial pH5.50, respectively. Because of some reasons such as the least optimum dose and the least influence by dose and initial pH, PFS was recommended for leachate pretreatment to remove the most of CODcr, recalcitrant substances and a small amount of NH4+-N from leachate, and to improve the biodegradability of leachate. The optimum conditions resulted in simultaneous removal of CODcr, color, turbidity, HA, suspended solid (SS) and ammonia (NH4+-N). Moreover, under the obtained optimum conditions, the characterization of leachate samples before/after pretreatment was informed by Fourier transform infrared spectroscopy (FTIR) analysis. Compared with raw leachate, it was found that some functional groups of HA disappeared or was reduced in the leachates treated by the seven coagulants, which indicated that various coagulants could remove recalcitrant substances from leachate in different degrees.The performance of self-designed biological nitrogen removal system was also studied, in order to obtain the effective removal of CODcr and nitrogen. The results showed that the start-up of the reactor was completed and CODCr, NH4+-N and total nitrogen (TN) removal efficiencies were achieved to be above80.00%(maximum value=83.84%),85.00%(maximum value=88.87%) and81.00%(maximum value=85.07%), respectively, in the case of dissolved oxygen (DO) in anoxic tanks of around0.4～0.8mg/L, DO in oxic tanks of about4mg/L, temperature of30℃, the hydraulic retention time (HRT) of10d, the effluent recycle ratio of1:1, the proportion of leachate in the fresh influent of50.00%and the concentration of CODcr and NH4+-N in the fresh influent of5000～5500mg/L and1450～1510mg/L, respectively.Based on the investigations of coagulation treatment and biological nitrogen removal system, a combined process containing the two treatments was used to treat landfill leachate. The results showed that the combined coagulation-biological nitrogen removal process could remove CODcr, NH4+-N and TN effectively, which could achieve91.98%COD removal,88.92%NH4+-N removal and87.37%TN removal, respectively.In this work, the coagulation performance of various coagulants for leachate treatment was analyzed, the mechanisms of recalcitrant substances removal in the leachate pretreatment by coagulants were discussed and a certain reference value for the practical application was offered. Recalcitrant organic substances removal from leachate by coagulation-flocculation process and its removal mechanism, which few studies reported on, were investigated. Furthermore, the performance of biological nitrogen removal system was studied, coagulation treatment and biological nitrogen removal system were combined successfully and effective leachate treatment was achieved by the combined process. The results of this work provide a simple and efficient process for landfill leachate treatment. Consequently, this study has the certain theory significance and the wide application foreground. |
PDF Full Text Request