Study On Technology And Mechanism Of Leachate By Biological And Physicochemical Treatment

Leachate, as a high concentration organic effluent that seeps from a landfill, will cause severe contamination to the surrounding environment and the groundwater if it is not properly treated. But, due to the greater fluctuations in the quantity and composition of landfill leachate, the treatability of landfill leachate depends on its composition and characteristics, the nature of the organic matter present as well as the age and structure of the landfill. None of the individual biological or physico-chemical techniques is highly effective for the treatment of landfill leachate. Thus, a new effective treatment technology of landfill leachate was explored to improve the removal of organics, accelerate reaction speed and reduce treatment costs. In this dissertation, the treatment efficiencies of different technologies by combining anaerobic biological treatment, stripping, flocculation and advanced oxidation processes were investigated. Various operational parameters affecting the degradation efficiency of landfill leachate were optimized. Meanwhile, for three advanced oxidation processes, the degradation mechanisms of organic matter were analyzed and reaction kinetic model were founded. In addition, the effects of landfill leachate after different treatments on the germination of Brassica oleracea seeds were evaluated by acute toxicity tests.A new combined treatment consisting of anaerobia biological treatment and granular activated carbon (GAC) or powder activated carbon (PAC) adsorption was developed to remove COD,NH3-N and heavy metals from landfill leachate. The results indicated that individual granular activated carbon (GAC) or powder activated carbon (PAC) adsorption has poor performance for the removal of COD,NH3-N and heavy metals. But added activated carbon into anaerobic reactor, the degradation efficiency of organic matter significantly improved, and gas-forming rate and biogas production of landfill leachate evidently increased, and the removal rate of Fe,Zn,Cu and Cd also enhanced. Moreover, in anaerobic reactor of supplied granular activated carbon, Maximal removal rate of COD,NH3-N and heavy metals were achieved, and maximum of biogas production were obtained. This is due to that the supplement of GAC can accelerate the form of anaerobic sludge granulation.After anaerobia biological treatment, ammonium tripping was employed to attain COD concentration of 2372mg/L and NH3-N concentration of 295.7mg/L, respectively. Subsequently, flocculation process was undertaken to remove color,turbidity and COD using ferric sulphate, polyferric sulphate(PFS) and polyaluminium ferric chloride (PAFC). The results showed that PFS was superior to the other coagulants for the removal of color,turbidity and COD at pH5. The addition of organic macromolecule flocculate polyacrylamide(CPAM) was found to be effective for the removal of color,turbidity and COD. The optimal conditions of flocculation process obtained by orthogonal experiment were 5.2 of pH value, 9.10 mmol/L of PFS and 5.00 mg/L of CPAM at 8 min. 75.6% of color removal rate and 93.6% of turbidity removal rate, as well as 56.7% of COD removal rate were attained in this condition.After flocculation treatment of landfill leachate, Advanced oxidation processes (AOPs), such as UV/Fenton,US/Fenton or MV/Fenton process, were carried out to attain their optimal conditions. Color and COD removal efficiency of three AOPs were compared, and their degradation mechanisms were further investigated in optimal conditions. The results suggested that 99.1% color and 86.2% COD removal were achieved by using 5.00 mmol/L of Fe²⁺ and 5.70×10² mmol/L of H₂O₂ at pH2.5 in 120 min in UV/Fenton process. Under the optimal conditions, apparent kinetics equation of landfill leachate was V=-dP/dt=2.6×10^-8×P^1.92×F^1.79×E^1.67. For US/Fenton process, the optimal conditions of landfill leachate were ultrasonic frequency of 45 kHz, power input of 100 W, Fe²⁺ concentration of 5.00 mmol/L, H₂O₂ concentration of 5.70×10² mmol/L and pH value of 2.5. Under this condition, maximum color and COD removal of 99.1% and 83.4% were achieved in 90 min, and its apparent kinetics equation was V=lg-(dP/dt)=1.0×10^-7×P0.86×F^2.34×E^0.87×H^0.82. For MV/Fenton process, the optimal conditions of landfill leachate were microwave power input of 800 W, Fe²⁺ concentration of 15.00 mmol/L, H₂O₂ concentration of 5.70×10² mmol/L and pH value of 2.5. Under this condition, maximum color and COD removal of 99.7% and 79.2% were achieved in 120 s, and its apparent kinetics equation was V=lg-(dP/dt)=7.0×10^-22×P^0.74×F^0.62×E^2.18×H^2.69. A comparative study of three AOPs indicated that UV/Fenton, US/Fenton and MV/Fenton attained more color and COD removal rate than Fenton process, and one of three catalytic induced mode, UV,ultrasonic or microwave, has synergistic effect with Fenton reagent. The UV-vis spectrum of landfill leachate reveals that high molecular mass compounds can be oxidized into biodegradable compounds or CO₂ and H₂O after UV/Fenton, US/Fenton or MV/Fenton treatment, and UV-vis absorption peak of degradation product ranges from 210 nm to 220 nm regardless of their different degradation mechanisms. UV/Fenton process achieved maximum COD removal of 86.2% and the longest react time of 120 min, while MV/Fenton process attained minimum COD removal of 79.2% and the shortest react time of 120 s among three AOPs.Brassica oleracea seeds were cultivate with the raw landfill leachate and the effluents produced by different treatment techniques, and their germination percentage were observed in 144 h to determine the acute toxicity parameters EC50 and GT50. The results shows that lower landfill leachate concentration resulted in less toxic to Brassica oleracea seeds and higher germination percentage of Brassica oleracea seeds. The raw landfill leachate was higher toxicity to all seeds tested, but the effluent from anaerobic biological treatment was less toxic than the crude leachate to Brassica oleracea seeds. A greatly reduction in toxicity to Brassica oleracea seeds was observed after flocculation process. However, the toxicity of three post-AOPs effluents was still existed. Three AOPs had little toxic diffience on the germination of Brassica oleracea seeds. Toxicity of Brassica oleracea seeds germination and GT50 values would gradually reduce and EC50 values would gradually increase when landfill leachate were treated by anaerobic biological treatment followed by flocculation and AOPs.After landfill leachate were treated by anaerobic biological treatment followed by trip, flocculation and UV/Fenton process, COD removal rate was degraded from 5.05×10⁴ mg/L to 142 mg/L, and its COD effluents meets third-grade national discharge standard. Furthermore, chroma and SS meet first-grade national discharge standard.