Treatment Of Landfill Leachate With Fenton Reagents And MAP Precipitation

Sanitary landfills are the primary method currently used for municipal solid waste disposal in many countries. Landfill leachate is defined as a high-strength wastewater exhibiting acute and chronic toxicity percolating through deposited waste and emitted within a landfill or dump site through external sources. The characterization of the leachate composition of municipal landfill leachate exhibits noticeably temporal and site-specific variation by the fact that its production and composition may vary as a function of landfill age. However, a biological process alone is not effective enough to remove the bulk of refractory pollutants in landfill leachate. Hence, the advanced treatment of landfill leachate needs to be specific concentrated.Firstly, the transformation of pollutions including humic substances, microorganic pollutants, and heavy metals in landfill leachate treated by a combined technology were investigated by fractionation technology of dissolved organic matters (DOM) and gas chromatography-mass spectroscopy (GC-MS) analysis, as well as the physical fraction of metal speciation and prediction using Visual MINTEQ programmer. The observations indicated that leachate treated by the combined process still contained higher concentration of total heavy metals and inorganic anions than the surrounding water environment, though no violation to effluent standard was found. The possible risk in leachate should not be negligible because landfill leachate was still one of the major sources of heavy metals and inorganic pollutants discharged to the surrounding environment.In addition, our study was focus on the oxidation and coagulation performances of Fenton treatment on chemical oxygen demand (COD) and humic substances removal, where COD was characterized as total organic constituents and the isolated humic substances was characterized as an individual organic contaminant in landfill leachate. The response surface method (RSM) was applied to evaluate and optimize the interactive effects of the operating variables including initial pH and dosages of H₂O₂ and Fe²⁺ on physical and oxidative performances of Fenton process. The change curves of humic acids removal were similar to those of COD. The removal of humic acids was 30% higher than COD removal, which indicated that humic acids was mostly degraded into various intermediate organic compounds but not mineralized by Fenton reagents. The oxidation removal was greatly influenced by initial pH relative to the coagulation removal. The oxidation and coagulation removals were linearly dependent with hydrogen peroxide and ferrous dosages, respectively. Ferrous dosage greatly influenced the coagulation removal of COD at low ratio ([H₂O₂]/[Fe²⁺]<3.0), but not at extremely high ratio ([H₂O₂]/[Fe²⁺]>6.0). The coagulation removal of humic acids was not affected obviously by oxidation due to both Fenton oxidation and coagulation remove high molecular weight organics preferentially. Higher temperature gave a positive effect on oxidation removal at low Fe²⁺ dosage, but this effect was not obvious at high Fe²⁺ dosage.Based on the optimization of Fenton reaction, a new kinetic model was established according to the generally accepted mechanism of high active ?OH oxidation in order to well describe the Fenton oxidation reaction in humic acids aqueous solution, and a mathematics model was derived successfully to describe the two-stage reaction kinetics of Fenton treatment of landfill leachate. Results indicated that the oxidation rate and removal efficiency were strongly dependent on initial pH, initial concentration of Fenton reagents, initial HA concentration and reaction temperature. The experiments demonstrated that hydrogen peroxide and ferrous ion would approach their saturated value with increasing dosage. It was found to be very useful for chemically and mathematically evaluating the performance of Fenton system and/or for process design using these models under various experimental conditions.Finally, improvement availability for high-level removal of ammonium-nitrogen by struvite using response surface methodology (RSM) was investigated, and chemical equilibrium model Visual MINTEQ, was used to calculate the equilibrium speciation in aqueous solution and solid phases, saturation index (SI), as well as to model the availability of Mg²⁺, NH₄⁺, and PO₄^3- ions as a function of pH. The predicted and experimental data suggested that the model described the experiments well. The solution pH value was an important parameter in ammonium-nitrogen removal at low initial NH₄⁺-N concentration. P/N molar ratio was a limiting factor on struvite precipitation at high initial NH₄⁺-N concentration. The quadratic statistical modeling developed using RSM can be used to predict total NH4+-N removal within the ranges of the factors investigated; moreover, the chemical thermodynamics equilibrium model can be used to pre-determine the concentration of ammonium precipitated by struvite.