Journal of Hazardous, Toxic, and Radioactive Waste

Landfill bioreactors have many advantages relative to conventional dry-tomb sanitary landfills. Leachate augmentation with enzymes (prior to its recirculation) to enhance waste degradation and gas production in landfill bioreactors is a relatively new concept and very little is known about its applicability. The research reported in this paper was undertaken to determine the viability of enzymatic augmentation of leachate to enhance waste-degradation rates at later stages of anaerobic bioreactor operation. There was a five-fold increase in the cumulative methane production in enzyme-added flow-through columns compared with the control column. This observation is attributed to an increase in the fraction of waste being degraded, as indicated by the increasing levels of dissolved organic carbon and decreased lignin levels in the waste.

Long-term settlement of municipal solid waste (MSW) in bioreactor and conventional landfills is caused by a number of mechanisms. Laboratory tests in bioreactor landfill simulators allow for a careful assessment of each mechanism and of the factors that affect it. A systematic review and synthesis of 98 tests from 29 mesoscale simulator studies available in the literature are presented. Long-term settlement is divided into three phases: the transitional phase, the active biodegradation phase, and the residual phase. Duration, strain, and long-term compression ratio (equal to the ratio of strain to duration) are calculated for each phase. Statistical analysis of the data is conducted. The majority of the long-term settlement occurs during the active biodegradation phase (9.5% strain on average), and the mean compression ratio is 0.168. The other two phases contribute significantly less to the total long-term settlement. The effects of the initial and operational conditions of simulators on the magnitude and rate of long-term settlement of MSW are explored. External vertical stress application prior to long-term testing is found to reduce the amount and rate of long-term settlement. Aeration of waste during long-term testing increases the settlement rate by promoting aerobic biodegradation. MSW long-term settlement is also affected by waste composition, total unit weight, and simulator size.

Leachate recirculation or liquid injection is an established practice for operating landfills as bioreactors to enhance the biodegradation of municipal solid waste (MSW). Among other subsurface liquid injection methods, vertical wells (VWs) represent one of the most common methods used for active or closed landfills. The current design and operation of VW systems does not consider the effect of additional liquid injection on the increase in gas pressures. In this study, a two-phase model that assumes landfill leachate and gas as immiscible phases was used to predict the moisture distribution and pore water and pore-gas pressures in a typical bioreactor landfill that uses VWs as its leachate injection or recirculation system. The unsaturated liquid and gas properties of MSW were simulated based on the van Genuchten model. The study evaluates the effect of the unsaturated hydraulic conductivity of MSW, the heterogeneous and anisotropic nature of the MSW, and the geometric configuration of VWs on moisture distribution and pore water and gas pressures. The unsaturated hydraulic properties of MSW significantly influence the wetted area and pore-water pressures during the initial stages of leachate injection and during the gravity drainage that follows. The numerical modeling results show that the gas pressures in the landfill will be of the order of 60 to 150 kPa, which are far above the liquid pressures for most typical liquid injection rates when no gas accumulation in the landfill is assumed due to an active gas extraction system. Hence, the slope stability of bioreactor landfills needs to be assessed by using the effects of gas and liquid pressures, especially when the gas extraction system is operating at below optimum efficiency.

Municipal solid waste (MSW) in landfill bioreactors is subjected to mechanical, biological, and hydrological processes. To understand these processes, four large-scale bioreactor pilots were specifically designed to simulate the behavior of waste in the core of a landfill. Here, the results of two long-term tests that were performed in two compression cells are presented. Mechanical, biochemical, and hydrological parameters were analyzed throughout the experiments. The promising results of this research improve the understanding of biodegradation and its correlation with the hydromechanical behavior of municipal solid waste. In particular, the sensitivity of the biodegradation to leachate injection and the correlation between the biogas flow and vertical settlement were confirmed for wastes with high initial moisture content. The results showed that it is important to consider the potential of different monitoring techniques and the representative volume for the experimental approach. Furthermore, the operational results led to interesting conclusions, especially regarding the addition of moisture to waste, which is a key element for bioreactor landfill operation.

A septage bioreactor landfill was constructed and operated at a publicly owned landfill in the U.S. state of Michigan. Residential septage was processed onsite and injected into municipal solid waste (MSW) using subsurface injection lines (for supernatant) and surface infiltration pits (for sludge) to accelerate the solid waste-decomposition process. For comparison, a leachate bioreactor landfill was also constructed and operated onsite. By comparing the system performance from both bioreactors, the impact of septage addition was identified. Extensive datasets consisting of parameters such as waste temperature, waste-moisture content, leachate quality, landfill gas generation, and waste settlement were systematically evaluated. The waste temperature in both bioreactors was in accordance with a similar trend. Using leachate addition, the waste-moisture content in the leachate bioreactor was controlled to maintain similar moisture content as the septage bioreactor. Using a first-order decay model, the gas-generation rate coefficients (first-order decay coefficient) were estimated at 0.296 and $0.082{\mathrm{year}}^{-1}$ for the septage bioreactor and leachate bioreactor landfills, respectively. This project demonstrated that waste-decomposition rate can be accelerated by septage addition and septage can be processed and treated in the septage bioreactor safely and effectively.

This manuscript presents a detailed discussion of the challenges faced and lessons learned during the initial phase of operation of the Calgary Biocell. The Calgary Biocell is a full-scale pilot project that has been implemented to acquire data and demonstrate the applicability of the biocell concept under severe winter conditions. The biocell concept involves operating a waste cell in three phases: first as an anaerobic bioreactor to recover biogas and produce energy, second as an aerobic bioreactor or an in-ground composter, and finally mined to recover processed waste and land for reuse. The Calgary Biocell has been in operation in its first phase, as an anaerobic bioreactor, for over the past five years. The cell was equipped with sensors to gather performance data during anaerobic and aerobic bioreactor operation. The settlement, moisture content, pressure, and temperature sensors provided early data, but failed after several months of cell operation. Regular monitoring and repairs were performed to ensure that gas was captured and used to generate power. The waste settlement data were collected during waste placement and before final closure of the cell from various depths of the cell. Lift 1 reported approximately 700 mm of settlement, which is approximately 14% strain, when the biocell was ready to be capped. After closure, only a limited amount of waste settlement data could be collected because of the failure of the settlement sensors and the real time data gathering system. The automated leachate recirculation system also failed during the past five years and was repaired. The liquid level of the leachate sump during automated operation was more consistent. The average initial and final moisture contents of MSW in the biocell were found to be at 25 and 36%, respectively, whereas the field capacity was determined to be 44% (wet basis). The temperature of landfill gas leaving the biocell ranged between 3 and 12°C in the winter/spring and approximately 20°C during summer. The landfill gas production rate averaged $59{\mathrm{m}}^3/\mathrm{h}$, but dropped considerably during the winter months.

The understanding of moisture content distribution within municipal solid waste (MSW) in a landfill due to leachate recirculation plays an important role in designing and optimizing the operation of a leachate recirculation system. Electrical resistivity imaging (ERI) is one of the many methods that have been used in the field to monitor the moisture distribution and moisture movement within the MSW in a landfill. The objective of this paper is to develop a correlation between electrical resistivity and moisture content of waste so that in situ moisture content of MSW can be determined without direct sampling and laboratory testing. To achieve this objective, an experimental program was developed to study the variation of electrical resistivity of MSW with moisture content and other factors that affect resistivity. The electrical resistivity of fresh MSW samples collected from the working face and partially degraded landfilled MSW samples collected from closed sections of the landfill using a bucket auger sampler were determined in the laboratory. The effect of moisture content, unit weight, stage of decomposition, temperature, waste composition, and pore fluid composition on electrical resistivity was correlated. Experimental results showed that electrical resistivity of MSW is affected by moisture content, unit weight, decomposition, temperature, and composition of waste. The effect of pore fluid composition on the resistivity of MSW was not significant. The test results indicate that ERI can provide the moisture content information within the landfilled solid waste, and direct sampling may not be needed to determine the moisture content within the MSW in a landfill.

Leachate-recirculation landfills enhance waste degradation through percolation of leachate inside the waste mass. In situ monitoring of moisture distribution and changes in mechanical properties (stiffness) of municipal solid waste (MSW) is needed to optimize the safe and effective operation of these types of landfills. Geophysical methods, such as electrical-resistivity tomography, have effectively monitored moisture distribution during and after leachate injection. This study investigates seismic surveys to capture the changes in mechanical properties [shear modulus (via shear-wave velocity) and Poisson’s ratio] of MSW to infer the extent of degradation and provide dynamic properties needed for seismic stability evaluation. To achieve this goal, a series of seismic surveys were performed at the Veolia ES Orchard Hills landfill, 15 km south of Rockford, Illinois, to image seismic velocity structure and the Poisson’s ratio of a recirculation cell, compared with an adjacent newer landfill cell without leachate recirculation. Seismic data were collected using fan shot direct P-wave (compressional wave) surveys and S-wave (shear wave) surveys, conventional P-wave refraction, and the multichannel analysis of surface waves (MASW) method. The fan shot surveys employed a sledgehammer source on one side of the landfill and geophones on the opposite side, thus exploiting the landfill’s topography and geometry to image waste to a depth of approximately 10 m. P-wave-velocity tomographic models and S-wave-velocity tomographic models from these direct-wave (through-pile) raypaths indicate a dramatic velocity increase below 5 m depth, perhaps indicating consolidation and compaction of the waste material. The ratio in P-wave to S-wave velocity ranges from 1.8 to 3.7, with an average of approximately 2.7, and Poisson’s ratio ranges from 0.29 to 0.46, with an average value of 0.42 (standard deviation 0.024). In the upper 5 m S-wave velocity ($\sim 150–170\mathrm{m}/\mathrm{s}$) is higher at the recirculation cell than in newer portions of the landfill consisting of uncompacted waste ($\sim 110–120\mathrm{m}/\mathrm{s}$), as derived from analysis of surface waves. The landfill cap, which is present over the recirculation cell, but not over the new waste, may be at least partly responsible for this difference. S-wave velocities are very similar below 5 m depth, whereas P-wave velocity (from seismic-refraction profiling) indicates a slight decrease in velocity between the leachate-recirculation cell and newer portions of the landfill. The refraction and surface wave surveys, however, were depth-limited (less than 8–9 m), compared with the fan shots. Overall, this study showed that seismic surveys have the potential to monitor spatial and temporal variation of dynamic properties of MSW.

The occurrence of leachate seepage at side slopes of municipal solid waste landfills represents a major design and operational concern at bioreactor landfills where leachate is recirculated into the waste mass to enhance degradation of organic materials. Full-scale bioreactor landfills typically add liquids to the waste under pressure to optimize moisture distribution. When liquids are added at a high pressure or at a flow rate higher than the absorption capacity of the waste mass, there is a possibility of seeps. Two general approaches to controlling landfill leachate seepage are described: a less aggressive strategy where liquid volumes are limited to avoid seeps and a more aggressive strategy where seeps are more likely to occur but with design and operation considerations in place to mitigate seepage. Liquids addition into waste using buried horizontal trenches was modeled to determine moisture distribution under a variety of conditions most likely to lead to leachate seepage at bioreactor landfills. Based on the modeling results, leachate seepage can occur in the presence of both high- and low-permeability soil layers. In addition, the injection pressure and setback distance of recirculation devices play important roles in seepage prevention and control. Several practical seepage control strategies are illustrated, including the selection and placement of daily or intermediate cover materials, pressure control, and the application of a clay plug as part of recirculation device design and construction. Finally, the setback distance for a horizontal injection trench was calculated based on the saturated zone equation; the setback distance provides a way to estimate the appropriate recirculation device design distance to prevent seepage at bioreactor landfills.

The addition of air to landfilled municipal solid waste (MSW) was examined at an operating bioreactor landfill in Florida. An air-addition system was designed to provide sufficient capacity to aerobically degrade a targeted mass of waste in a 3-year period. Approximately 1.385 million standard ${\mathrm{m}}^3$ [at 15.5°C and 101.3 kPa (1 atm)] of air in total was added to 78 small-diameter vertical wells located in clusters of three different depths (6, 12, and 18 m deep in the landfill). The cumulative volume of air added was much less than design capacity, a result of difficulties in adding air to deeper and wetter landfill areas and rapid temperature increases in waste near some of the wells. Consistent long-term aerobic conditions could never be established. Gas concentration measurements throughout the experiments were not in the explosive range, but temperature measurements corresponding to continuous air addition did require frequent adjustment and cessation of air addition to wells to avoid fire formation. Although air addition could play some role in bioreactor landfill operation, results from this study suggest that maintaining aerobic conditions as the dominant waste decomposing environment within typical large bioreactor landfills operated with liquids addition is very difficult to achieve.

The conversion of a traditional sanitary landfill to a bioreactor landfill typically involves the addition of moisture to the landfill to create an environment that promotes anaerobic waste stabilization. The objective of this technical note is to summarize conventional practices for selecting required moisture volumes and rates for full-scale bioreactor operations. Designers commonly select a target waste moisture content equivalent to field capacity, which corresponds to moisture addition ranging from 0.4 to $0.7{\mathrm{m}}^3$ of landfill leachate per megagram of waste. The rate at which liquids are added depends on several site-specific factors including available liquid supply, impact on leachate collection system performance, the methods available for liquids addition, and performance concerns such as slope stability and leachate seepage. Existing sites (retrofit bioreactors) are generally limited by landfill design, while new sites (as-built bioreactors) can potentially allow for much greater liquid-addition rates. Required volumes calculated using the conventional approach are large and often difficult to achieve in practice with current technology. Additional recommendations are provided for the designer including accounting for spatial and temporal changes in field capacity and a phased liquid-addition rate that incorporates monitoring of the liquid-addition system.