In Situ Recirculation Treatment Of Landfill Leachate

The leachate from the conventional municipal solid waste (MSW) sanitory landfill after its closure has the characteristics of lower production, higher contamination concentration, slow variation of water quality and high cost for treatment. There will not only be the hidden peril to the environmental safty of landfill, but also the burden to operation and supervision of the landfill. According to the development of current landfill technology, an in situ recirculation treatment of leachate was presented based on the existing treatment technologies for landfill leachate. From the position of removing the contaminated substance in leachate, the influence of different landfill structures and different recirculation operating conditions on the solid-liquid two-phase system in landfill with recycling leachate was explored. This research provides a new, simple and economical approach for the leachate from middle-small landfill in the undeveloped areas to meet the requirements of standard on its discharge.The optimal storage manner and the main influencing factors for the effective utilization of natural regradation ability of the leachate were first time studied by orthogonal experiments in this thesis. Besides, the initial systematic research about the effect of landfill structures and recirculation operating conditions on the in situ treatment of leachate recycled partly and fully was carried out by contrast experiments. During the in situ recirculation treatment of full landfill leachate, the mechanism of the organic pollutants degradation was clarified primarily by analyzing the variation of both O2 and CH4 content and temperature distribution in landfill layer, pollutant concentration, molecular weight distribution of organic matters, colour and so on in leachate. In addition, a new process was put forward to treat the leachate for its reaching to the level of the discharge standard, which is based on Fenton oxidation with applying the physical-chemical methods to treat the effluent of the in situ recirculation treatment of full leachate from semi-aerobic landfill structure. The optimal operation parameters and the main affecting factors were determined by orthogonal experiments. The main results were summarised as follows:①The early landfill leachate showed a stronger ability of natural degradating than that of the later landfill leachate during the natural storage. The storage condition (in the sealed or the open vessel), the time of storage and the interaction of both, all of them influenced the degradation of early leachate remarkably. The influence of vessel openings area was also predominent.②Contrasted with that of anaerobic landfill structure,the in situ recirculation treatment of part or full landfill leachate from semi-aerobic landfill structure not only could give lower accumulated surplus production and net accumulated production of leachate, but also elimilated acid and Ammonia Nitrogen accumulation, accelerated the reduction of CODCr,BOD5,VFA and Ammonia Nitrogen concentration,advanced the degradation of organic matter in MSW to speed the stabilization of leachate starting from the source.③During the in situ recirculation treatment of part landfill leachate from semi-aerobic landfill structure, to some extent, the hydaulic load and frequency of recirculation change had only a minor impact on the variation of CODCr, BOD5, VFA, Ammonia Nitrogen, TP and PO43- concentration. During the in situ recirculation treatment of full landfill leachate from semi-aerobic landfill structure, the recirculation operating condition with higher hydaulic load and frequency benefited the control organic-polluted indexes, such as CODCr,BOD5 and Ammonia Nitrogen etc., meeting Grade I~III discharge standard of GB16889-1997 in a short time, respectively.④During the in situ recirculation treatment of full landfill leachate from semi-aerobic landfill structure, the range analysis of orthogonal experiment results indicated that the optimal operation condition for improving the quality of leachate up to the standard level was the hydaulic load and frequency of recirculation as 15.30L/m2·d and 3 times/d, respectively.⑤During the in situ recirculation treatment of full landfill leachate, the variation of O2 and CH4 content shows that the difference of oxygen environment in the refuse layers mainly resulted in great distinction of each test index for semi-aerobic and anaerobic simulated columns. The variation of recirculation operating condition within certain limit influenced a little on the distribution of oxygen containing areas in the semi-aerobic column. The conversion of anaerobic landfill structure to semi-aerobic landfill structure reduced the time of leachate CODcr,BOD5 and Ammonia Nitrogen concentration down to the permitted value in national discharge standard, accelerated the solubility and degradation and promoted the stabilization of leachate from the source. The effect of refuse layers temperature on the treatment of leachate recirculation of semi-aerobic landfill was small.⑥The macroscopical variation of leachate colour can reflect the organic matter degradation degree on the whole. By the in situ recirculation treatment of full landfill leachate for a long time, most of the large-molecular organic matters could be removed, while the removal of small-molecular organic matters was difficult. As to the semi-aerobic landfill structure, the small-molecular organic matters might be the fulvic acid which resulted in the yellow colour of the leachate at late-stage experiment.⑦By taking the test column and the leachate regulating tank as a system, the dynamic model of the organic matter biodegradation during in situ recirculation treatment of landfill leachate had been developed as The biodegradation dynamic equations for the organic matter in leachate from semi-aerobic test column with high hydaulic load and frequency of the recirculation were established as follows: [31300 +( 7×10?8 t4?6×10-5t3+0.0204t2?2.3046t+103.26)]×106,( t>72d)。The research results indicated that the model could reflect the removal law of organic contaminated matter in leachate which came from semi-aerobic test column with high hydaulic load and frequency of the recirculation. It is forecasted by the model that the leachate arrived at Grade II and Grade I discharge standard of GB16889-1997 on the 270th day and 427th day of landfilling , respectively.⑧Under the operation conditions of pH value 4, H2O2 dose 0.2mol/L, the ratio of H2O2:Fe2+(mol:mol) 7:1, reaction time 240min, agitating rate 200rpm and CODcr concentration of raw influent 490 mg/L, the CODcr concentration of full leachate from semi-aerobic test column treated by Fenton oxidation after the in situ recirculation can reach Grade II discharge standard of GB16889-1997. Meanwhile, the highest CODcr removal rate was 72%, and the removal rate of chroma, which reduced by 33 times, reached 67%. Based on the above given, the effluent was treated by the secondary Fenton oxidation. That is, under the operation conditions of pH value 4, H2O2 dose 0.3mol/L, the ratio of H2O2:Fe2+(mol:mol) 4:1, reaction time 240min, and agitating rate 150rpm, CODcr concentration of full leachate could meet Grade I discharge standard of GB16889-1997 with the highest total removal rate 97%. And the chroma came down to the level undetectable. During the treatment of first-degree Fenton oxidation, the factors which influenced greatly on the removal rate of leachate CODcr concentration were H2O2:Fe2+, pH value and H2O2 dose. While the reaction time, CODcr concentration of raw influent and agitating rate exerted a slight influence on it. The primary and secondary order of factors which influenced the removal rate of leachate CODcr concentration in the treatment of secondary Fenton oxidation was pH value>reaction time>H2O2:Fe2+(mol:mol) ratio>H2O2 dose.