Effects of temperature and waste age on stabilization of aged waste during landfill in situ aeration
Date of Issue2016-04-07
School of Civil and Environmental Engineering
Residues and Resource Reclamation Centre
The increasing demands for land space and pollution hazards incurred from old dumping grounds and landfills drive the need to employ in situ aeration technique to solve landfill aftercare problems rapidly. By injecting air into the landfill body, in situ aeration establishes aerobic condition in later stage of landfill, accelerates carbon transformation, reduces organic load, decreases landfill post-closure period and achieves earlier beneficial reuse of land. Several successful in situ aeration projects have been reported worldwide in the past decades with a variety of objectives. The efficiency of landfill aeration depends on the proper system design and operation. That is why lots of efforts have been paid on selecting the appropriate well design and spacing, the effective air distribution method, the suitable air pressure and aeration rate, and sufficient duration of aeration. However, the inception time for aeration is yet to be answered. As the waste age defines the initial landfill status before aeration, the inception time for aeration plays an essential role in the success of the aeration project. Moreover, the effects of temperature on the process of aerobic bioconversion need for clarification. This information is important to indicate if there is a need for temperature control during in situ aeration and foresee the characteristics of waste from different temperature zone in the landfill. Furthermore, no studies have been particularly cared for the microbial profiles in the course of in situ aeration. Since in situ aeration mainly relies on the local microbes to degrade the organic substances, the comprehension of microbial community and its function, especially in the incipient stage, is vital for the rapid startup and the high performance in long-term running of aeration. To address raised all above concerns, the following studies were conducted to examine the impacts of waste age and temperature on stabilization of aged waste as well as the population dynamics of microorganism at different temperature during the startup phase. In the current study, synthetically aged waste was utilized to simulate the waste composition of closed landfill or dumping ground. One reason was because the Lorong Halus Dumping Grounds (LHDG) accepted high amount of inorganic waste (> 50%) during dumping, while current waste characterization further showed that the waste ingredients were biologically inert and stabilized. Therefore waste materials from LHDG were considered unsuitable for the present study, which focused on the bioconversion process of organic matter. Another reason was ascribed to better control of the experiments, in which artificial aged wastes would minimize the influences induced by heterogeneity on the components and thus allow for more scientific comparisons. The effects of waste age on the emissions and stability of waste materials during in situ aeration were first investigated. In bioreactors with young aged waste materials, dissolved organic carbon (DOC) rapidly accumulated in the bioreactor leachate after aerobic conditions were established. Fractional analysis showed that the elevated concentration of humic acids (HAs) was primarily responsible for the increment of leachate strength. Interestingly, elevation of HAs concentration was not observed in the aeration reactor with a prolonged waste age, as the mobility of HAs was inhibited by the high molecular weight (MW) derived from extended waste age. Without significant release of organic substrate, later aeration promoted carbon biodegradation more profoundly. In sum, the beneficial properties of higher molecular weight and more stable humic substances related with later aeration are weighed up against the benefits of aerating earlier, which facilitates earlier stabilization and larger avoidance of greenhouse gas emission but also generates higher concentrations of leachable lower molecular weight compounds. The results suggest that waste age shall always be taken into account during design of aeration system, in order to maximize the aeration efficiency. The effects of temperature on carbon and nitrogen compounds during in situ aeration of aged waste were investigated by setting the operation temperature of three landfill bioreactors at 35, 45 and 55 oC, respectively. The results demonstrated that the bioreactor operated at 55 oC presented the higher carbon mineralization rate throughout the experimental period, suggesting accelerated biodegradation rates under thermophilic conditions. The leachate nitrogen speciation study indicated that organic nitrogen transcended among other species, because ammonia was significantly stripped out due to aeration. In order to elucidate the fate of individual nitrogen containing constituent, fractionation of leachate organic nitrogen was conducted. Detailed investigation revealed that the most intensive bioconversion of humic nitrogen occurred in thermophilic conditions. At the end of aeration, the waste from 55 oC bioreactor featured the highest degree of biostability and richest level of humic content. All the benefits observed in 55 oC bioreactor suggested that remediation through aeration would be more efficient if thermophilic temperature could be maintained. Moreover, thermophilic zone usually exists in the middle depth of the landfill if temperature control is not implemented. Thus, after aeration the waste from this layer could provide product with a better quality if biofertilizer and soil conditioner is the expected end use for the excavated fine particles, because thermophilic temperature could increase the yield and maturity of HAs, which is a significant factor in soil fertility. The variation of microbial community under different temperature during startup phase were assessed by comparing the 16S rRNA clone libraries derived from leachate sample of three landfill aeration bioreactors at 35, 45 and 55 oC, respectively. Compared to anaerobic control bioreactor, the microbial population under aerobic condition was more diverse as more operational taxonomic units (OTUs) were identified. In addition, the gradually established aerobic condition caused the descending of proteolytic Bacteroidetes in aerated bioreactors, while the rising in the percentage of metabolically diverse Proteobacteria with the increased temperature could partially explain why the biodegradation efficiency was increased by high temperature. Moreover, phylogenetic analyses revealed the predominant role of microaerophilic Symbiobacterium at 35 °C and obligate aerobic Vulgatibacter incomptus at 55 °C in the microbial community, verifying that a more profound aerobic condition was established at thermophilic condition most probably due to the enhanced oxygen diffusion by high temperature.