Effect of phosphate compound on the reduction of the pressure drop of laboratory-scale fabric filter.

Fabric filters are generally used to eliminate the particulate matter of flue gas that is generated from waste incinerators. The pressure drop of fabric filter (ΔP) is accompanied by the build-up of a dust layer on the fabric filter, and ΔP increase leads to an increase in the energy consumption of induced draft fan in an incineration facility. In this study, phosphoric acid (H3PO4) and monobasic calcium phosphate (Ca(H2PO4)2) are tested as chemicals to reduce ΔP in fabric filter using a laboratory-scale equipment. The phosphorus concentration of collected dust was correlated to the increment of ΔP per unit weight of dust deposited on the filter fabric. The effect on ΔP reduction was apparent at the phosphorus concentrations of 2.2-5.3 wt% in the collected dust. As a result, the gas permeability was enhanced by 2-6 times as compared to that was observed at phosphorus concentrations less than 0.6 wt%. Based on the result of XRD analysis of the dust sampled from the filer bag, it was concluded that H3PO4 reacted with the Ca compounds even though Ca-P compound could not be identified exactly. When H3PO4 was sprayed, dust particles grew in size and the surface roughness of dust layer increased too. Such changes in the physicochemical properties of dust seemed to enhance the gas permeability of the dust layer, which caused a reduction in the value of ΔP.

Removal and speciation of mercury compounds in flue gas from a waste incinerator.

The management and control of mercury emissions from waste incinerators have become more significant, because waste incinerators are sinks to treat mercury-containing consumer products. This study investigated the effects of mercury concentrations and waste incineration temperatures on mercury speciation using a lab-scale experimental instrument. The removal characteristics of different mercury species were also investigated using an apparatus to simulate the fabric filter with a thin layer of additives such as Ca(OH)2 and NaHCO3, activated carbon (AC), and fly ash. HgCl2 generation rates peaked at 800°C for initial Hg(0) concentrations of 0.08-3.61 mg/Nm(3) in the presence of 400 ppm HCl. A linear relationship was established between the generation rate of HgCl2 and the logarithmic value of initial mercury concentration. Fly ash proved highly efficient in mercury removal, being equal or superior to AC. On the other hand, Ca(OH)2 and NaHCO3 were shown to have no effects on mercury removal. In the dry-scrubbing process, alkali agent is often sprayed in amounts beyond those stoichiometrically required to aid acidic gas removal. The research suggests, however, that this may hinder mercury removal from the flue gas of solid waste incinerators.

Municipal solid waste management in Tehran: Changes during the last 5 years.

The situation of waste management in Tehran was a typical example of it in developing countries. The amount of municipal solid waste has been increasing and the city has depended on landfill for municipal solid waste management. However, in recent years, various measures have been taken by the city, such as collecting recyclables at the source and increasing the capacity of waste-processing facilities. As a result, significant changes in the waste stream are starting to occur. This study investigated the nature of, and reasons for, the marked changes in the waste stream from 2008 to 2012 by analysing the municipal solid waste statistics published by the Tehran Waste Management Organization in 2013 and survey data on the physical composition of the municipal solid waste. The following trends were identified: Although the generation of municipal solid waste increased by 10% during the 5-year period, the amount of waste directly disposed of to landfill halved and resource recovery almost doubled. An increase in the capacity of a waste-processing facility contributed significantly to these changes. The biodegradable fraction going to landfill was estimated by using the quantity and the composition of each input to the landfill. The estimated result in 2012 decreased to 49% of its value in 2008.

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 the inlet of the leachate treatment facility. However, it does not necessarily reflect the actual state of the site. Thus, methodologies which focus on both the discharge, in order to determine when the post-closure care of the facility should terminate, and on the degree of waste stabilization in the landfill are required. In the present study, monitoring of leachate quality stored in 68 gas ventilation pipes was conducted and the 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 the nearby location. Correlation among the analysed items was quite high. Namely, the difference of leachate quality seemed to be categorized only by the level of concentration but not by the specific characteristics. To confirm this, Euclidean distances of dissimilarity were calculated by multidimensional scaling using six items of leachate quality and temperature. Two factors (thickness of leachate and concentration of total organic carbon (TOC) and electric conductivity (EC)) that distinguish leachate characteristics appeared. To indicate the degree of stabilization by location, the spatial distribution of TOC, total nitrogen (TN), inorganic carbon (IC), and chloride ion were estimated by using the ordinary Kriging methodology. As the result, it was estimated that the concentration of leachate existing within the landfill, especially TN, was higher than the completion criteria for leachate in most parts of the investigated area.

An investigation of carbon release rate via leachate from an industrial solid waste landfill.

Long-term behaviour of leachate pollutants is a key factor to estimate time and cost required for the leachate treatment in landfills. Estimating carbon release via leachate can be a good way by which to understand the long-term behaviour, however, most studies have had a timeline of only several months or years. In this study, a release rate of carbon via leachate for 20 years was estimated at an industrial solid waste landfill. The total carbon content in dumped waste was estimated based on combustible contents determined by collecting samples from other industrial landfills and pretreatment facilities, and carbon contents in literature values. Leachate quantity data, which were not recorded for the first ten years, were estimated using a macro-moisture balance model including the effect of snow melt. Because leachate quantity and quality at each site were only measured after leachates were mixed, the quantity at each site was calculated by assuming infiltration rates with and without final cover. Results indicated that less than 2% of total input carbon was released from each site via leachate regardless of landfill age.

Air and landfill gas movement through passive gas vents installed in closed landfills.

In a closed landfill, Japan, remedial actions have been undertaken to address the inadequate leachate collection and drainage systems. Part of this process included installing many passive gas vents in the landfill to promote stabilization of landfilled waste. This study focused on the gas velocity in vents by conducting tracer tests to elucidate the gas flow via passive gas vents. The gas composition and gas temperature in the vents was also measured. As the gas vents pass through the waste layer, both landfill gas and air flows through the vents. Therefore, passive gas vents can be used to aerate landfilled waste as well as to collect and release landfill gas. Aerobic biodegradation occurs when air migrates through the waste layer if organic matter is present; this increases the temperature of the waste layer. Inflow of air into the gas vents can occur at a wide range of depths, even 10-20 m below ground level. Air is induced not from the surface of the landfill, but horizontally along the waste layer. The driving force of air induction from outside is a buoyancy effect caused by the temperature rise due to aerobic biodegradation.

Compartment model of aerobic and anaerobic biodegradation in a municipal solid waste landfill.

The mathematical formulations in a one-dimensional compartment model of the biodegradation of organic landfill components are described. The model is designed to switch between anaerobic and aerobic conditions, depending on the local oxygen concentration. The model also includes the effect of environmental factors, such as moisture content, pH, and temperature, on reaction rates. The model includes not only biodegradation processes for carbon compounds (acetate, CO2, CH4), but also for nitrogen compounds involved in nitrification and denitrification due to their significance in landfills. Two example runs to simulate anaerobic and aerobic waste were conducted for a single landfill unit cell by changing the organic content and diffusion coefficient.

Oxygen intrusion into waste in old landfills of low organic content.

Oxygen invasion into old landfills was studied by assuming the installation of gas venting pipes to promote stabilization of waste. In an experiment using a column pack with old incombustible waste, oxygen intrusion was observed and the oxygen consumption rate was estimated. Oxygen diffused into the waste layer very quickly in the initial stage of the experiment, but oxygen concentration increased only gradually due to reduced gradient and decreasing oxygen consumption. The maximum oxygen consumption rate in packed waste was one-third of that in loosely deposited waste in a beaker measured in a respiration test. A mathematical model was created which fitted the experimental data well and a three-dimensional simulation of a full-scale landfill and a sensitivity analysis were performed.

Leaching behaviour of elements and evaluation of pre-treatment methods for municipal solid waste incinerator residues in column leaching tests.

Two new pre-treatment methods (water-washing/carbonation and carbonation/phosphate stabilization) of municipal solid waste (MSW) incinerator residues were evaluated by column leaching tests under aerobic conditions and anaerobic conditions (which were changed to aerobic conditions after 10 months). A mixture of bottom ash and fly ash (5:1 ratio) was pre-treated using each method. Shredded incombustible residues (SIR) were added to each ash preparation in proportions similar to the ratios present in landfills. For comparison, landfill wastes typical of Japan, namely, a mixture of bottom ash, chelating-pre-treated fly ash, and SIR, were also examined. Leachate samples were collected periodically and analysed over a 15-month period. When compared with chelating pretreatment, both water-washing/carbonation and carbonation/ phosphate stabilization reduced the leaching of Pb, Al, and Cu by about one to two orders of magnitude. Moreover, the initial concentrations of Ca and Pb in leachates from column of water-washing/carbonation were 56-57% and 84-96% less than those from the column of carbonation/phosphate stabilization. Therefore, water-washing/carbonation was considered to be a promising approach to obtain early waste stabilization and to reduce the release of heavy metals to near-negligible levels. The leaching behaviour of elements was also discussed.