Effectiveness of the Vertical Gas Ventilation Pipes for Promoting Waste Stabilization in Post-Closure Phase

To make inside of the municipal solid waste (MSW) landfill aerobic as much as possible is thought to be preferable for promoting waste stabilization, reducing pollutant's load in leachate, minimizing greenhouse gas emission and shortening post-closure-care period. In Japan, installation of semi-aerobic landfill structure has widely spread in order to promote waste stabilization in MSW landfill from 1980s. In semi-aerobic landfill structure, outlet of main leachate collection pipe is opened to atmosphere. Heat generated by aerobic degradation of waste causes natural convection and natural aeration arises from the outlet of leachate collection pipe to the gas vents. It is so-called stack effect. This air flow is thought to be effective for purifying leachate flowing through drainage layer and leachate collection pipes. And it is also thought to be contributing to expanding aerobic region in waste layer in landfill. Recently, measures attempting the promotion of waste stabilization are taken at several landfills at where stabilization of waste delays, in which many vertical gas vents are newly installed and close structure to semi-aerobic landfill is created. However, in many cases, these gas vents are not connected to leachate collection pipes. Many vertical gas vents are just installed without scientific proof regarding whether they can contribute for waste stabilization. In this study, how such installation of gas vents is effective for waste stabilization and aerobization of waste layer was discussed by numerical analysis. In numerical analysis, heat transfer, gas movement by pressure, gas diffusion, biological degradation of organic matter, and heat generation by biodegradation were taken into account. Simulations were carried out by using the general purpose simulator of finite element method. Three types of landfill structure were assumed. As the results, the following information were obtained. In dig-down type landfill, installation of gas vents has no effect for changing air flow. On the other hand, in pile-up type landfill, installation of vertical gas vents can accelerate air invasion and significantly promote waste stabilization, if it has high permeable horizontal layer

Assessment of internal condition of waste in a roofed landfill

Recently, roofed landfills have been gaining popularity in Japan. Roofed landfills have several advantages over non-roofed landfills such as eliminating the visibility of waste and reducing the spread of offensive odours. This study examined the moisture balance and aeration conditions, which promote waste stabilisation, in a roofed landfill that included organic waste such as food waste. Moisture balance was estimated using waste characterization and the total amount of landfilled waste. Internal conditions were estimated based on the composition, flux, and temperature of the landfill gas. Finally, in situ aeration was performed to determine the integrity of the semi-aerobic structure of the landfill. With the effects of rainfall excluded, only 15% of the moisture held by the waste was discharged as leachate. The majority of the moisture remained in the waste layer, but was less than the optimal moisture level for biodegradation, indicating that an appropriate water spray should be administered. To assess waste degradation in this semi-aerobic landfill, the concentration and flow rate of landfill gas were measured and an in situ aeration test was performed. The results revealed that aerobic biodegradation had not occurred because of the unsatisfactory design and operation of the landfill

Experimental study of behavior of endocrine-disrupting chemicals in leachate treatment process and evaluation of removal efficiency

An experimental study of the behavior of endocrine-disrupting chemicals (EDCs) in leachate treatment processes (aeration, coagulation and sedimentation, activated carbon adsorption, and advanced oxidation) was conducted and removal efficiencies were evaluated. Among target EDCs, concentrations of BPA (1800 times), DBP (10 times), BBP (40 times), and DEHP (30 times) in leachate are more than ten times higher than those in surface water. BPA, DBP, and BBP can be treated by aeration and DEHP, by advanced oxidation processes. BPA could not be effectively removed by coagulation and sedimentation because most of BPA partitioned in the supernatant. DEHP could hardly be treated by aeration. The removal ratios of DEHP were approximately 50 to 70% if the generated sediment was removed completely. The removal ratios of DEHP in leachate of 100 m3/d with 100 kg of activated carbon were 50 to 70%, assuming a complete mixing model. The concentration of DEHP was decreased to below one-tenth in 120 min by advanced oxidation processes

Solubility and adsorption characteristics of Pb in leachate from MSW incinerator bottom ash.

The solubility and adsorption characteristics of Pb in leachate from municipal solid waste incineration (MSWI) bottom ash was studied by batch experiments. The solubility of Pb in leachate was 1‐2 orders of magnitude lower than that of Pb in carbonate solutions. Adsorption experiments showed bottom ash have high adsorption capacity for lead. To evaluate a controlling component in bottom ash, quartz, Al and Fe powder as well as their ions were examined over wide ranges of pH at 25℃. The results showed that only Fe powder had high adsorption capacities for Pb, indicating that iron oxides in bottom ash would be one of major adsorbent for Pb. Adsorption diagram, constructed by using the data of adsorption experiments, fitted the experimental data of Pb in leachate very well. It suggests that the decrease of Pb solubility in leachate is mainly due to adsorption reaction

Evaluation of pre-treatment methods for landfill disposal of residues from municipal solid waste incineration.

This study was based on the investigation of Pb, Cd, Ca, and Cl in one individual batch of bottom ash and fly ash (mixture 5:1) resulting from a specific municipal solid waste incinerator (MSWI) in Japan. This mixture ash was stabilized as the following pretreatments: water washing, carbonation, and phosphate stabilization. After the optimum processing condition for each pretreatment was determined, the performance of each pretreatment was examined using pH stat leaching tests (pH 6, 9, 12) and availability tests. For the performance tests, fly ash alone was used, instead of a mixture of MSWI residues, to observe leaching differences among the pretreatments more distinctly. Water washing effectively removed the major elements from MSWI residues and also reduced the leachability of trace metals, such as Pb and Cd, from MSWI residues. A washing time of 15 min with an L/S ratio of 5 was reasonably effective. Carbonation had a significant effect on leachability in alkaline ranges (pH 9 and 12), due to carbonation only on the surface of MSWI residues, not in the center matrix. A moisture content of 10-16.7% appeared to be suitable for carbonation. Phosphate stabilization, even with small doses of phosphate (0.16 mol-PO4^3- kg^-1 sample), was very effective at reducing the leachability of heavy metals

Monitoring of leachate quality stored in gas ventilation pipes for evaluating the degree of landfill stabilization

Monitoring of leachate quality is the essential measure in aftercare for evaluating landfill stabilization. Generally, the most common way of leachate monitoring is executed at inlet of leachate treatment facility. However, it does not necessarily reflect actual state of the site. Thus, not only the methodologies focusing on the discharge for the determination of facility termination but also the methodology that is capable to seize the degree of waste stabilization in landfill must be necessary. In the study, monitoring of leachate quality stored in 68 gas ventilation pipes was conducted and degree of waste stabilization at each location in the landfill was estimated by a statistical approach using the results obtained by monitoring. Leachate characteristics varied significantly for each pipe but seemed to reflect the waste condition of nearby location. Correlation among the analysed items was quite high. Namely, the difference of leachate quality seemed to be categorized by only the level of concentration but not specific characteristics. To confirm this, Euclidean distances of dissimilarity were calculated by multidimensional scaling by using six items of leachate quality and temperature. Two factors (thickness of leachate and concentration of TOC and EC) that distinguish leachate characteristics appeared. To indicate the degree of stabilization by location, spatial distribution of TOC, TN, IC, and CI were estimated by using the ordinary Kriging methodology. As the result, it was estimated that concentration of leachate existing within the landfill, especially TN, was higher than completion criteria for leachate, in most part of calculated area

Onsite survey on the mechanism of passive aeration and air flow path in a semi-aerobic landfill

The semi-aerobic landfill is a widely accepted landfill concept in Japan because it promotes stabilization of leachates and waste via passive aeration without using any type of mechanical equipment. Ambient air is thought to be supplied to the landfill through a perforated pipe network made of leachate collection pipe laid along the bottom and a vertically erected gas vent. However, its underlying air flow path and driving forces are unclear because empirical data from real-world landfills is inadequate. The objective of this study is to establish scientific evidence about the aeration mechanisms and air flow path by an on-site survey of a full-scale, semi-aerobic landfill. First, all passive vents located in the landfill were monitored with respect to temperature level and gas velocity in different seasons. We found a linear correlation between the outflow rate and gas temperature, suggesting that air flow is driven by a buoyancy force caused by the temperature difference between waste in the landfill and the ambient temperature. Some vents located near the landfill bottom acted as air inflow vents. Second, we conducted a tracer test to determine the air flow path between two vents, by injecting tracer gas from an air sucking vent. The resulting slowly increasing gas concentration at the neighboring vent suggested that fresh air flow passes through the waste layer toward the gas vents from leachate collection pipes, as well as directly flowing through the pipe network. Third, we monitored the temperature of gas flowing out of a vent at night. Since the temperature drop of the gas was much smaller than that of the environment, the air collected at the gas vents was estimated to flow mostly through the waste layer, i.e., the semi-aerobic landfill has considerable aeration ability under the appropriate conditions

Material and moisture balance in a full-scale bio-drying MBT system for solid recovered fuel production

Bio-drying MBT is a type of mechanical biological treatment (MBT) system and reduces moisture content of the MSW to improve the separation of combustible fractions. In this study, a full-scale biocell-type bio-drying MBT system was investigated. The mass balance of waste component was estimated by composition and characterization of waste and tonnage data. During separation of biodried outputs, 62% of plastics and 54% of paper were recovered as RPF material. Wood was decreased by reduction in particle size and 90% of biodried wood is returned to next reactor. Changes of mixed fine caused by fine wood particle and the loss of organic matters and 60% of it were returned. Daily water removal during 17-days of bio-drying was simulated through the model using the operation data. Among the four operation phases, the longest stabilization phase was expected to main water removal period, but half of water removal occurred at initial two stages and phase IV for only 6 days in total due to the high waste temperature for sanitization (phase I and II) and high airflow rate for cooling. Decreasing waste temperature at phase III resulted in low water evaporation