Everyday, 58,118 tons of municipal solid wastes were produced, of which 81.1% was landfilled. The municipal solid wastes had the large portion of organic matters such as food, paper, and textiles. Therefore, the landfills often produced highly polluted leachate, resulting in the adverse impact on the environment for 20 to 30 years even after the landfill had been completed.
Therefore, many researches have focused on remediation and earlier stabilization of landfill sites in Korea. When the enhanced biological process has been established in the landfill, the period of time during which the leachate would be highly polluted by organics would decrease. Operation techniques such as sludge addition, moisture content, leachate recirculation, temperature control, aerobic pretreatment, addition of an inoculum, and shredding have been tested in fields and/or pilot scale lysimeters. Among these enhancement methods, the use of leachate recirculation is by far the most investigated as a promising option. Leachate recirculation could provide proper environmental conditions (especially moisture content) within the landfill, thus enhancing the stabilization of landfill, as well as in-situ treatment of leachate moving through the landfill.
Although it has been reported that various methods on recirculation have been developed and practiced, very little attention has been given to the evaluation of their performance in terms of influence zone, recirculation rate, spatial and temporal moisture distribution pattern, and induced hydraulic head on liner.
The aims of this study are to investigate the anaerobic degradability of various municipal solid wastes using Biochemical Methane Production(BMP) in Chapter 2, to test the effects of various recirculation rates on landfill stabilization in Chapter 3, to verify the relationship between organic removals by different recirculation rates and landfill settlement rates in Chapter 4, and finally, to suggest the optimal recirculation rate for the in-situ treatment of landfill leachate using landfill models in Chapter 5.
In the BMP experiment, it was found that inappropriate C/N ratios caused the delayed anaerobic degradations and lower kinetic constant. So that, to improve the anaerobic degradation processes in the landfill, the uniform distribution of substrates are needed.
Four simulated landfill columns(lysimeters) were constructed and filled with coarsely ground refuses which were composed as to simulate typical municipal solid waste, and continuously operated as follows.
$C_0$ - Control; only simulated rainfall added, no leachate recirculation.
R1 - Slow recirculation ; rainfall and 7mm/㎡ were recirculated per week.
R5 - Medium recirculation ; rainfall and 35mm/㎡ were recirculated per week. R25 - Fast recirculation; rainfall and 175mm/㎡ were recirculated per week.
In this study, initial pH of the leachate was in the range of 3.9 to 4.3, but it rose to 6 to 7 within 50 days. Therefore, it can be concluded that the recirculated leachate would help to induce a water flux for even distribution of buffer materials and to dilute local high concentration of volatile fatty acids(VFAs). In the control lysimeter($C_0$), the acetic environment inhibited the growth of methanogenic bacteria causing delayed methane production.
Hartz and Ham(1983) found that a high-rate recirculation provided a better anaerobic degradation than lower rates. However, some secondary effects as a result of the high-rate recirculation were observed in this study. Although the initial pH recovery by increasing recirculation rate was favorable to the initial growth of methanogenic bacteria, R25 gave a very poor methane production rate than that of R1 and R25. High rate recirculation might cause the short sludge retention time that inhibited the substrate metabolism of bacteria or wash out methanogenic bacteria. Rover and Farquhar(1973) also suggested that the drop in waste temperature was due to a large turn-over of cold leachate in the system.
The carbon balance study of each lysimeter showed that the leachate recirculation was an effective means for the organic stabilization in landfill. The relatively high carbon loss in $C_0$ than R25 was not because the organic decomposition was active, but because the removed leachate contained a high strength of TOC.
Despite the difference of the removed carbon mass between R1, R5 and R25, there was no significant difference in settlement rates. Hence, the main role of the leachate recirculation for the landfill settlement could be the breakdown of the bridging between refuse particles or of the skeleton of decomposed refuse, rather than to decrease the volume of refuses by biological decomposition.
However, crucial parts of the skeleton components would be destroyed by biological decomposition, further settlements could verify the relationships between the secondary settlement and the refuse decomposition.
