Periodic injection of liquefied kitchen and food waste in municipal solid waste: Effects on leachate and gas generation.

With continuous advancements in the zero-waste strategy in China, transportation of fresh municipal solid waste to landfills has ceased in most first-tier cities. Consequently, the production of landfill gas has sharply declined because the supply of organic matter has decreased, rendering power generation facilities idle. However, by incorporating liquefied kitchen and food waste (LKFW), sustainable methane production can be achieved while consuming organic wastewater. In this study, LKFW and water (as a control group) were periodically injected into high and low organic wastes, respectively. The biochemical characteristics of the resulting gas and leachate were analyzed. LKFW used in this research generated 19.5-37.6 L of methane per liter in the post-methane production phase, highlighting the effectiveness of LKFW injection in enhancing the methane-producing capacity of the system. The release of H2S was prominent during both the rapid and post-methane production phases, whereas that of NH3 was prominent in the post-methane production phase. As injection continued, the concentrations of chemical oxygen demand, 5-d biological oxygen demand, total organic carbon, ammonia nitrogen, total nitrogen, and oil in the output leachate decreased and eventually reached levels comparable to those in the water injection cases. After nine rounds of injections, the biologically degradable matter of the two LKFW-injected wastes decreased by 8.2 % and 15.1 %, respectively. This study sheds light on determining the organic load, controlling odor, and assessing the biochemical characteristics of leachate during LKFW injection.

Evaluation of leachate production and level in municipal solid waste landfills considering secondary compression.

Landfilled municipal solid waste (MSW) in developing countries generally produces a large amount of leachate due to high moisture content. The estimation of leachate production and level is of great importance to the capacity design of leachate treatment plants and the stability analysis of landfills. In this study, the leachate production ratios (the ratio of leachate mass to waste mass) in different countries and cities were first summarized to reveal the basic status of leachate generation. Then, a model was established to calculate the leachate production and level that considers the rainfall infiltration and the water released from MSW due to both primary and secondary compression (ignored in previous models). Finally, the proposed model was used in a case study of Laogang Landfill in Shanghai, China. It was found that the leachate proportion produced by compression was much higher compared with that produced by rainfall infiltration, ranging from 49 (rainy season) to 93% (dry season). The leachate released from waste due to secondary compression accounted for a high proportion (up to 25%) of the total leachate production, especially for aged MSW. The calculated leachate discharge amount and leachate level were close to the measured values because the possible low permeability layer at the bottom of the landfill was considered in this model.

Improvement of vacuum pressure on pumping performance of vertical wells in municipal solid waste landfills.

The pumping performance of the traditional vertical well is often poor in municipal solid waste (MSW) landfills due to the blocking effect of landfill gas on leachate migration. To improve the pumping performance, a vacuum vertical well was designed and then installed at the Tianziling landfill. When the leachate was drawn out through submersible pump, the landfill gas was simultaneously extracted through vacuum pump to form vacuum pressure in the well. The vacuum pressure could increase the hydraulic gradient of leachate flow as well as the relative liquid permeability of MSW. Pumping tests were carried out to explore the effectiveness of the vacuum pressure on improving the pumping performance of vertical well. When the vacuum pressure increased from 0 kPa to - 30 kPa, the steady leachate pumping rate increased from 1.58 to 2.34 m3/h, and the steady leachate level drawdown increased from 5.9 to 10.3 m at the distance of 5 m. The vacuum pressure mainly affected the leachate level drawdown within the distance of 15-20 m. When the vacuum pressure in the pumping well was - 30 kPa, it attenuated to - 14.7 kPa and - 6.6 kPa at the distance of 5 m and 10 m, respectively. The influence radius of vacuum pressure was about 15 m. Numerical modeling indicates that the leachate pumping rate and drawdown will decrease with the increase in decreasing rate of hydraulic conductivity with depth, degree of heterogeneity, and anisotropy of hydraulic conductivity of waste. The experimental and numerical results demonstrate the effectiveness of vacuum pressure and provide working parameters for the application of the vacuum wells in MSW landfills.

Full-scale experimental study of methane emission in a loess-gravel capillary barrier cover under different seasons.

The methane emission in a loess-gravel capillary barrier cover (CBC) in winter and summer was investigated by constructing a full-scale testing facility (20 m × 30 m) with a slope angle of 14.5° at a landfill in Xi'an, China. Weather conditions, methane emission, gas concentration, temperature, and volumetric water content (VWC) in the CBC were measured. The temperature and moisture in the CBC showed a typical seasonal pattern of warm and dry in summer and cold and wet in winter. Accordingly, the maximum methane oxidation rate and methane emission were higher in summer. The mean methane influx and methane emission decreased significantly as the VWC increased beyond 40% (i.e., a degree of saturation 0.85) at a depth of 0.85 m, which was near the loess/gravel interface. At this depth, more water was presented in the loess layer in the downslope direction due to capillary barrier effects, which increased the upslope methane emission. More dominant methane emission in the middle- and upper-section of the CBC occurred in summer than in winter as there was less soil moisture to facilitate methane transfer. The LFG balance showed that a significant fraction of the loaded LFG was not accounted in the flux chamber measurements due to the preferential flow along the edges of the CBC. The maximum methane oxidation rate was 93.3 g CH4 m-2 d-1, indicating the loess-gravel CBC could mitigate methane emissions after landfill closure.

Performance of a compacted loess/gravel cover as a capillary barrier and landfill gas emissions controller in Northwest China.

Loess is widely distributed in Northwest China where the rainy season coincides with the warm and vegetation growth period. The use of loess as a capillary barrier cover (CBC) material is promising. However, how the loess/gravel CBC perform as a capillary barrier and landfill gas emissions controller remains elusive. In this study, the performance of a designed CBC comprised 1.3 m-thick compacted loess underlain by 0.3 m-thick gravel in extremely wet and dry years of Xi'an city from 1950 to 2000 was analyzed using numerical modeling. An instrumented CBC test section comprised 0.9 m-thick compacted loess underlain by 0.3 m-thick gravel was constructed to show the hydraulic responses in real conditions from January 2015 to January 2017. The numerical results indicated that the designed CBC performed well as a capillary barrier as no percolation occurred during the extremely wet periods. Despite adopting a CBC of 0.4 m thinner than the designed one, the test section produced only 16.16 mm percolation during the two-year monitoring period, and that can meet the recommended limit of 30 mm/yr. The effect of the capillary break on increasing the water storage within the CBC was observed at the test section in fall. The increased water storage can significantly decrease the gas permeability, and thus improve the performance of the CBC as a LFG emissions controller. Furthermore, the LFG emissions can be controlled to meet the limit set by the Australian guideline by decreasing the bottom gas pressure and artificial watering. Finally, a procedure was proposed to enhance the performance of CBCs.

Installation and performance of horizontal wells for dewatering at municipal solid waste landfills in China.

Vertical wells are conventionally used to lower leachate levels or pressures in municipal solid waste (MSW) landfills. However, they are not always efficient or even effective, and in some circumstances retro-fitted horizontal wells represent a potential alternative. However, horizontal wells can be difficult to install and there is a lack of data on their performance. This paper describes the trial construction and operation of three horizontal wells in a landfill at Tianziling, China. The trial was used to develop an improved well installation technique, and to demonstrate the viability of the approach in a typical Chinese landfill. Three wells, between 50 m and 56 m in length, were successfully installed using an improved casing-protected directional drilling method. Average leachate flow rates of two wells were 10.66 m3/day and 3.93 m3/day, respectively. After 74 days of drainage, the maximum leachate level drawdown around the highest flow well was 2.7 m and its distance of influence was up to 50 m. Building on the experience gained at Tianziling, a wellfield comprising twelve horizontal wells having a total length of 1000 m was installed at Xingfeng landfill. After 157 days of drainage, a total volume of ~24,000 m3 leachate had been discharged and the leachate level had been lowered to near the elevation of the horizontal wells. This paper indicates the effectiveness of horizontal wells in reducing leachate level in landfills containing MSW typical of that generated in China, and gives data on installation and performance that may be useful for the design and operation of such an approach.

A simple and rapid in situ method for measuring landfill gas emissions and methane oxidation rates in landfill covers.

A newly developed static chamber method with a laser methane detector and a biogas analyser was proposed to measure the landfill gas emissions and methane (CH4) oxidation rates in landfill covers. The method relied on a laser methane detector for measuring CH4 concentration, avoiding gas samplings during test and hence the potential interference of gas compositions inside the chamber. All the measurements could be obtained on site. The method was applied to determine the landfill gas emissions and CH4 oxidation rates in a full-scale loess gravel capillary barrier cover constructed in landfill. Both laboratory calibration and in-situ tests demonstrated that fast (i.e. <20 min) and accurate measurements could be obtained by the proposed method. The method is capable of capturing the significant spatial and temporal variations of the landfill gas emissions and CH4 oxidation rates in landfill site.

Use of electrical resistivity tomography for detecting the distribution of leachate and gas in a large-scale MSW landfill cell.

In this study, integrate electrical resistivity tomography (ERT) tests were carried out in a large-scale (5.0 × 4.0 × 7.5 m) MSW landfill cell to investigate the possibility of detecting perched leachate mounds, leachate level, and gas accumulation zones at wet landfills. The resistivity of both bulk waste and waste components at different moisture states were measured and the three-phase volumetric relationships of the waste pile were analyzed to better interpret the ERT test results in the large-scale cell. The following observations were given: (1) The relationship between resistivity and volumetric moisture content (VMC) of waste sample can be reasonably fitted by Archie's law. The resistivity of waste components at a saturated state was all lower than 21 Ω m. (2) A significant amount of void gas was entrapped in the underwater waste, being 30.4-34.8% of the whole waste pile in volume. (3) Low-resistivity zones (< 5.0 Ω m) were observed in the waste pile being fully drained under a gravity condition, which was believed to be related to a perched leachate. (4) The average VMC values of the waste layer below and above the leachate level were in the ranges of 46.5-53.1% and 28.1-41.3%, respectively. (5) Irregular variations of high-resistivity zones (> 40 Ω m) observed in the underwater waste were associated with the accumulation and dissipation of gas pressure. It was found that the "gas-breaking value" in the gas accumulation zone was up to 10.5 kPa greater than the pore liquid pressure in the stable methanogenesis stage. These findings shone a light on the possibility of using the ERT method as an efficient tool for mapping the gas/leachate distribution and improving operations at wet landfills.

Biochemical, hydrological and mechanical behaviors of high food waste content MSW landfill: Liquid-gas interactions observed from a large-scale experiment.

The high food waste content (HFWC) MSW at a landfill has the characteristics of rapid hydrolysis process, large leachate production rate and fast gas generation. The liquid-gas interactions at HFWC-MSW landfills are prominent and complex, and still remain significant challenges. This paper focuses on the liquid-gas interactions of HFWC-MSW observed from a large-scale bioreactor landfill experiment (5m×5m×7.5m). Based on the connected and quantitative analyses on the experimental observations, the following findings were obtained: (1) The high leachate level observed at Chinese landfills was attributed to the combined contribution from the great quantity of self-released leachate, waste compression and gas entrapped underwater. The contribution from gas entrapped underwater was estimated to be 21-28% of the total leachate level. (2) The gas entrapped underwater resulted in a reduction of hydraulic conductivity, decreasing by one order with an increase in gas content from 13% to 21%. (3) The "breakthrough value" in the gas accumulation zone was up to 11kPa greater than the pore liquid pressure. The increase of the breakthrough value was associated with the decrease of void porosity induced by surcharge loading. (4) The self-released leachate from HFWC-MSW was estimated to contribute to over 30% of the leachate production at landfills in Southern China. The drainage of leachate with a high organic loading in the rapid hydrolysis stage would lead to a loss of landfill gas (LFG) potential of 13%. Based on the above findings, an improved method considering the quantity of self-released leachate was proposed for the prediction of leachate production at HFWC-MSW landfills. In addition, a three-dimensional drainage system was proposed to drawdown the high leachate level and hence to improve the slope stability of a landfill, reduce the hydraulic head on a bottom liner and increase the collection efficiency for LFG.

Biochemical, hydrological and mechanical behaviors of high food waste content MSW landfill: Preliminary findings from a large-scale experiment.

A large-scale bioreactor experiment lasting for 2years was presented in this paper to investigate the biochemical, hydrological and mechanical behaviors of high food waste content (HFWC) MSW. The experimental cell was 5m in length, 5m in width and 7.5m in depth, filled with unprocessed HFWC-MSWs of 91.3 tons. In the experiment, a surcharge loading of 33.4kPa was applied on waste surface, mature leachate refilling and warm leachate recirculation were performed to improve the degradation process. In this paper, the measurements of leachate quantity, leachate level, leachate biochemistry, gas composition, waste temperature, earth pressure and waste settlement were presented, and the following observations were made: (1) 26.8m3 leachate collected from the 91.3 tons HFWC-MSW within the first two months, being 96% of the total amount collected in one year. (2) The leachate level was 88% of the waste thickness after waste filling in a close system, and reached to over 100% after a surcharge loading of 33.4kPa. (3) The self-weight effective stress of waste was observed to be close to zero under the condition of high leachate mound. Leachate drawdown led to a gain of self-weight effective stress. (4) A rapid development of waste settlement took place within the first two months, with compression strains of 0.38-0.47, being over 95% of the strain recorded in one year. The compression strain tended to increase linearly with an increase of leachate draining rate during that two months.

[Impacts of initial moisture content of MSW waste on leachate generation and modified formula for predicting leachate generation].

The amount of leachate generation rate in MSW landfills is often underestimated during design phase in China. A water balance model of a valley landfill, whose size is 400 m long, 500 m wide and 50 m thick, is created to investigate the influences of initial moisture content of waste on source and production of leachate. The 50 m thick waste mass is assumed to be 5 layers. Each layer is 10 m thick with a filling period of 2 years. The leachate mainly comes from precipitation and from squeezed pore water of waste. It is found that higher initial moisture content of waste leads to higher amounts of squeezed leachate and total leachate generation rate, and also results in a high ratio of squeezed leachate to total leachate generation rate. For the cases that the initial moisture contents of waste are 27%, 40%, 50%, and 60%, the amounts of total leachate generation rate are 272, 583, 823 and 1 063 m3 x d(-1), respectively, and the amounts of squeezed leachate are--144, 168, 408, and 647 m3 x d(-1), respectively. It is also found that when the initial moisture content of waste is greater than 50%, the squeezed leachate becomes the primary source of total leachate generation rate. However, the formula for predicting leachate generation rate used in the national code could not consider the contribution of squeezed leachate, this may cause a significant underestimation of leachate generation rate for the case having a high initial moisture content of waste. Based on the water balance analyses, a modified formula for predicting leachate generation rate, which includes the contribution of squeezed leachate is proposed. It is verified by consideration of the operational practices of two large-scale landfills in southern China.