Seasonal Variation Of MSW Leachate Properties And Biogas Recirculation Pretreatment

Leachate is a byproduct of refuse biodegradation and usually generated whererefuse occurs. It is estimated that43.41Mt leachate was generated in2011in China,and the management and treatment of leachate has become one of the most seriousenvironmental problems in China.In this work, the leachate samples were collected from a refuse transfer station（RTS） in Shanghai, China, and the seasonal （summer and winter） impact on leachatephysicochemical properties （pH, COD, TOC, nitrogen distribution, EC, ORP, etc.）were investigated which provided useful potential reference for fresh leachate EGSBbio-treatment. Refering to the serious scaling in the EGSB reactor during highcalcium bio-treatment, the feasibility of biogas recirculation from EGSB reactor as acarbonation tool for leachate pretreatment was evaluated systematically to eliminatethe inhibitory effect of calcium.Effects of biogas recirculation on the COD, TN,NH4+–N and calcium removal, as well as methane production were investigated anddiscussed. The three-dimensional excitation-emission matrix （3D-EEM） was used tocharacterize the variation of organic substances in the leachate during the operationalperiod. X-ray diffraction （XRD）, Fourier-transformed infrared （FT-IR） and scanningelectronic microscopy （SEM） were performed to analyze the responsiblemechanisms for calcium precipitation and removal. The main conclusions obtainedin the present study are as follows:（1） Leachate from the RTS was acidic, with pH values ranging from3.65to4.99and2.69to4.47during winter and summer, respectively. Overall, summerleachate depicted lower pH than winter. High organics content in terms of COD（winter:32640–140000and summer:15000–80000mg/L） and TOC （winter:14500–80000and summer:9000–11000mg/L） concentrations was detected,indicating serious environmental and health impacts of the leachate.（2） TN and NO₂（winter:697–5332and0–144mg/L; summer:396–1699and 0–47mg/L） of leachate in winter were much higher than those in summer, incontrary, NH3-N and NO3（winter:363.03and617–1255mg/L; summer:320.48and267–880mg/L） seemed lower.（3） High contents of Ca and Mg, as well as plentiful TSS and VSS （winter:2000–5500and1500–4500mg/L and summer:3000and–2000mg/L） existing in theleachate were observed, which could easily lead to clogging over time on the internalwall of EGSB and along transportationpipes for leachate because of theiraccumulation.（4） Two main flourescence peaks （Peaks A and B） were identified from the3D-EEM spectra of leachate samples from winter and summer. The Peak A presentat the excitation/emission wavelengths （Ex/Em） of270–280/350–355nm waslabeled as tryptophan protein-like substances, possessing higher biodegradability.The Peak B at the Ex/Em of320–330/410–420nm belonged to humic acid-likecompounds,caused by the accumulation of no-degradable humic substances. Theintensities of fluorescence Peaks A and Bfrom day30was more strengthenedthanday1for winter, while the intensities in summer maintained relatively constant.（5） The continuous performance of EGSB bioreactor was greatly improved whenbiogas circulation was used as the pretreatment to carbonize calcium-rich leachate.COD and TN removal efficiency were80%and50–60%under steady-statecondition, higher6.25%and41.03%than the reactor without biogas recirculation,respectively. NH4+–N concentration in the influent was higher than that in theeffluent. Very interestingly, the effluent NH4+–N in the reactor B were almost lessthan900mg/L, obviously lower than those （1000–1600mg/L） from the reactor A.（6） Three main peaks could be identified from3D-EEM fluorescence spectra oforganic matters in the leachateat the Ex/Em of275–280/355–365nm （peak A）,230–235/350–370nm （peak B） and315–335/405–425nm（peak C）. The first peakwas attributed to thetryptophan protein-like substances, the second was assigned tothe aromatic protein-like substances, and the third was regarded as the humic-like substances derived from the biodegradation of soluble organic matters. Thefluorescence intensities of Peaks A and B for the effluent were decreasedsignificantly compared with the influent, however, the humic-like substancesindicated by Peak C maintained relatively stable regardless of operational conditions,implying that protein-like substances in the leachate was more easily biodegraded forbiogas production than humic substances.（7） The average methane yield was determined to be0.31L CH4/g CODremovedfrom reactor A, suggestingthat approximately88.6%of the removed COD wasconverted to methaneand the rest COD （11.4%） might be regarded as synthesis ofbiomass （theoretical methane production rate is0.35L CH4/g CODremoved）.Comparatively, the methane yield amounted to0.33L CH4/g CODremovedfrom thereactor B, indicating that94.3%of the removed COD was converted to methanewhereas the remaining5.7%was presumably changed into the form of the biomass.（8） The calcium from the reactor A was about5000–5500mg/L in the influent,and gradually decreased to800–1400mg/L in the effluent, suggesting that thecalcium of about4000mg/L·d was retained in the EGSB reactor. In contrast, thecalcium concentration after carbonation process was about280–650mg/L,decreasing around91.40%; the final removal efficiency of calcium in the reactor Bwas found to be88.24–94.48%, increase of about12.09%compared to the reactor A.（9） XRD, FTIR and SEM analysis showed that the clogging materials taken incarbonation unit were composed principally of CaCO3, Mg0.064Ca0.936CO3, andMg0.03Ca0.97CO3; Biogas recirculation through carbonation reaction promotedcalcium precipitation and removal, which made Methanosaeta-like species easier tosurvive and granules to form faster. As a consequence, the stable and improvedperformance of the EGSB reactor was achieved.