Research On Co-treatment Of MSW Leachate And Incineration Flv Ash By Supercritical Water Oxidation

MSW Leachate and MSW incineration fly ash is the inevitable secondary pollutants generated from disposal process of municipal solid waste. Leachate includes high concentrations contamination of organic pollution, inorganic nitrogen pollution and heavy metals, while the fly ash is rich in persistent organic pollutants like dioxin and heavy metals. It’s very difficult for existing technology to dispose leachate and fly ash efficiently and environmental friendly. Compared with traditional methods, supercritical water reaction medium exhibits unique advantage, such as high ion product, low dielectric constant, miscible with gas and organic matters and the formation of a homogeneous reaction system. Therefore, supercritical fluid reaction attracts widespread attention, and it has been considered as one of the potential effective technologies for organic waste stream disposal. This study focuses on reaction mechanism and interacting factors during co-removal process of persistent organic pollutants and heavy metals from leachate and fly ash based on supercritical water oxidation; meanwhile, it is applied to deal with tannery sludge which is contaminated with complex organic compounds and heavy metal chromium. In addition, the experimental investigation and quantum chemical study is carried out for typical PCBs, and the reaction mechanism and path is preliminarily obtained. Furthermore, experimental study of leachate based on continuous supercritical water reactor is also conducted and compared with the results from batch reactor. The main contents of this paper are as follows:(1) Supercritical water oxidation of leachate was conducted in a batch reactor based on orthogonal test. The effect of operation parameters, including reaction temperature, pressure, peroxide and reaction time, on the destruction of organics was examined. Range and variance analysis revealed that, the most important factors that affected pollutants destruction are oxidant and reaction temperature, and then the reaction time. It was proven that pressure at supercritical region had limited influence on oxidation reaction. Destruction efficiencies of COD and NH4-N increased significantly with rising temperature and oxidant amount, with a maximum value of98.5%and76.2%for COD and NH4-N, respectively. To establish the reaction kinetics of organic pollutants in leachate, a first-order kinetic model applied for COD degradation expression is determined with a pre-exponential factor of318s-1and activation energy of58kJ/mol to a97%confidence level.(2) The migration characteristic of heavy metals from the original leachate to the solid phase was studied. After SCWO process, heavy metals dissolved in original leachate were deposited and separated from the system due to the low solubility and the behavior changes, thus to be removed effectively. Based on the orthogonal experimental condition, Cu removal efficiency ranged from75%to94%, while the removal efficiency of Mn and Zn reached as high as98%-99%. In additional, a high concentration of heavy metal nitrates of Cr, Pb, Cu was added into leachate for further study on migration mechanism of heavy metals. It iwas found that the components detected in solid ash are mainly metal oxides and salts, which were formed through a series of hydrolysis, dehydration, decomposition and oxidation process. SCWO process of sewage sludge from tannery industry showed that recovery efficiency of Cr from sludge into solid ash reached more than98%under all conditions; and higher temperature played a positive role. Further leaching toxicity tests of Cr in solid products showed that the leaching toxicity of Cr was greatly reduced through stabilization after reaction. It is noteworthy to study the change characteristic of CrO42-ion. The removal effieicncy of CrO42-reached up to96.7%when suitable amount of oxidant was applied; whereas, if the amount of peroxide was doubled, the CrO42-ion concentration in solid ash increased, indicating that excessive oxidant might oxidize Cr (Ⅲ) to hexavalent CrO42-ion.(3) Supercritical water oxidation was applied to the co-disposal of two distinct waste streams:municipal solid waste leachate and incineration fly ash. In the case of adding fly ash, COD removal efficiency of leachate reached a maximum value of99.2%, which was higher than that with no fly ash, indicating that heavy metals in fly ash played a catalytic effect on the degradation of organic matter. The PCDD/Fs mass concentration in treated fly ash decreased from the initial28.2ng to only2.79ng, with a mass degradation rate of90%. The decreasing of the chlorination degree of fly ash showed that PCDD/Fs degradation was accompanied with this de-chlorination process. After the reaction, leaching concentrations of heavy metals As, Cr, Pb, and Zn in fly ash were significantly reduced, which meet the limits of landfilling. In addition, most of the heavy metals are transformed from the exchangeable and carbonate bound form to a more stable form; heavy metals in fly ash after treatment were mainly in the form of residual and Fe-Mn oxide phase.(4) Experimental test and quantum chemical study was carried out on typical chlorinated POPs, decachlorobiphenyl, to investigate reaction mechanism. Under the condition of no alkaline additives, degradation products of D10CB were mainly N9CB, O8CB and H7CB. The addition of NaOH could significantly promote the dechlorination of D10CB, and the overall degradation efficiency of D10CB and PCBs reached99.9%and99.2%; and meanwhile the degradation products transferred to low chlorinated homologues. The active atoms of D10CB were predicted according to quantum chemistry theory. Besides, it was obtained that these were three channels for the reaction of10CB with OH radical, that was, OH attacking the ortho, meta and para C atom and replacing the corresponding Cl atom. The activation energy, Gibbs free energy and other thermodynamic parameters, as well as critical structures such as reactive intermediates and transition states, were obtained for each channel. Thus the reaction mechanism and path during the degradation process of decachlorobiphenyl in SCWO was obtained.(5) Further research on SCWO treatment of landfill leachate was conducted on a horizontal tube continuous supercritical water reactor. The effects of reaction temperature (400-500℃), residence time (8-32s), pressure (22-30MPa) and peroxide amount (150-300%) were investigated to provide a basis guide for industrial applications. After SCWO treatment, COD removal efficiency in leachate reached a maximum value of95%, while the highest removal rate of NH4-N was only51%. Higher reaction temperature, longer residence time and increased oxidant can effectively promote the degradation of pollutants; yet residence time had little effect after more than24s; an oxidant amount to250%of peroxide was sufficient, and the increasing of the pressure had tiny magnitude of influence on the pollutant degradation.