Degradation Of Dissolved Organic Matter During Biological Physicochemical Treatment Of Landfill Leachate

Sanitary landfill as an efficient, mature, and low-cost means of final disposal is mainly adopted to treat the municipal solid waste in most cities. However, the environment around the landfill is polluted seriously for the leachate produced during the domestic refuse disposal. Because of the instable water quality and quantity, complex composition, and high concentration of organics in landfill leachate, to develop an efficient, economic and flexible treatment process is one of the problems urgent to be solved. For a long time, the characteristics and treatments of leachate are considered from the engineering feasibility. The macroscopical water quality indexes, such as COD and BOD5, are used for the water quality denotation and pollution control. The lack of knowledge about the microcosmic characteristics of leachate to some extent restricts the expression of leachate pollution. In this study, the representative sanitary landfill leachate treating by the biological-physicochemical combined processes was researched. The removal efficiency of contaminants and the characteristics of dissolved organic matter (DOM) degradation were analyzed on the basis of the processes optimization. Furthermore, DOM was divided according to the hydrophobic-hydrophilic property between organic matters in leachate, and distinguished the differences of degradation during anaerobic, aerobic biological units and physicochemical units.A biological-physicochemical combined treatment"ASBR—SBR—ACF—GAC"was set up for the landfill leachate, and the removal efficiencies of the primary pollutants were analyzed on the optimized conditions for each treatment units. The results showed that on the condition of 3.25~4.75 g COD/(L·d) for volume loading rate, 48 h for reaction periods in ASBR, the removal efficiencies of COD and TOC were stabled in the range of 80.1%~84.4% and 77.1%~83.9%, respectively. The operation mode of anoxic and aerobic by turns was chosen for SBR treatment. With the reaction periods of 24 h, volume loading rate of 1.12 g COD/L·d, MLSS of 5575 mg/L, the removal efficiencies of COD and NH4+-N could be over 70% and 83%, respectively. On the condition of 3 for pH, 1.5 h for reaction time, 50 g/L for iron scrap dose, 3.5 for mass ratio of Fe and C, 8.5 for coagulation pH, the removal efficiencies of COD and TOC were high to 64.8% and 71.6%, respectively. As for GAC adsorption treatment, when the dose of GAC was 5 g/L and reaction time was 1.5 h, the removal efficiencies of COD, TOC, and NH4+-N were 64.6%,69.3%, and 31.8%, respectively. During the stable operation of the combined treatment, the main contaminants in landfill leachate could be removed with the removal efficiencies of COD, TOC, and NH4+-N were 99.2%,99.4%, and 90.3%, respectively, improving the effluent quality efficiently. The enhancement of applicability under the new criterion was focused on the optimization of removal for organic matter and nitrogen.The main part of organic in leachate was DOM. After treated with combined processes, the DOM in leachate was decreased more than 99%, and each fraction was reduced in the range of 99.48%~99.73%. The DOC concentration of effluent was lower than 35 mg/L, with the dominant organic matter in the hydrophilic fraction. The aromaticity values of SUVA254 were behaved with the increasing after biological treatment part and decreasing after physicochemical treatment part. Carboxylic acids, aliphatic hydrocarbons, and amides were degraded prominently during biological treatments. And the humic fluorescent substances were accumulated strongly, resulting in the fulvic-acid fluorescence present in DOM of ASBR and SBR effluents predominantly. Both of the aliphatic and aromatic compounds were destroyed and reduced during physicochemical treatments, especially the humic fluorescent substances. The cumulative volumeФT,n of fractions in leachate DOM treated by combined processes were increased after biological treatments and decreased after the physicochemical treatments.The degradation of each DOM fraction was not all the same. The predominant reduction was obviously observed in HPI during anaerobic degradation, but the values of SUVA254 were increased even high to 450%. During SBR treatment, the proportions of hydrophobic and transphilic fracions were increased, while that of hydrophilic fraction was decreased during the initial stage. Protein tryptophan-like substances were much easier to decrease, whereas the humic-like compounds were more difficult to degrade during the biological treatments. So that theФIII,n for fulvic-acid fluorescence was increased notably. With the combined action of adsorption, coagulation, and oxidation, the hydrophobic substances were degraded effectively during the ACF treatment, with the removal efficiency of organic matters higher than the corresponding value of single adsorption plus single coagulation treatment. The fluorescent substances in hydrophobic and transphilic fractions were decreased efficiently even by single adsorption and single coagulation, and the addition of air improved the degradation of the fluorescent substances in HPI. The removal efficiencies of DOC and UV254 followed the order of HPO < TPI < HPI when each fraction treated by adsorption singly. The aromatic protein fluorescent substances were removed dominantly in short time during adsorption treatment.The research on the characteristics of degradation and spectroscopy among different fractions enhance the understanding of leachate and its treatment processes on a new view. The correlative conclusions could guide the choice of proper treatment process and lay the basic foundation for dealing with landfill leachate reasonably, economically, and efficiently.