Combustion kinetics of sewage sludge and combustible wastes

This study estimated the kinetics of the mono- and co-combustion of sewage sludge pellets and combustible wastes such as municipal solid waste (MSW) and refuse-derived fuel (RDF). Sewage sludge was manufactured into pellets with a diameter of 8, 12, or 16 mm and a length of 30 mm. The RDF was composed of paper and plastics and was formed into pellets with a diameter of 8 mm and a length of 30 mm. MSW samples were synthesized using combustible wastes such as garbage, paper, plastics, and wood. The MSW was adjusted to have a moisture content of around 40% after shredding to under 10 mm. A laboratory-scale batch type stoker incinerator was used for the combustion and the gas composition of the flue gas was measured. The activation energy was calculated using the experimental results, and then the relation of the decomposition rate and reaction time was evaluated using the shrinking core model. The decomposition rate of the sludge pellets decreased as their diameter and moisture content increased, and the co-combustion of sludge pellets and combustible waste was affected by the amount of combustible waste. The individual combustion rates of the cylindrical sludge pellets or RDF were mainly controlled by the chemical reaction, but in the case of shredded MSW it was mostly influenced by gas diffusion. The rate for the co-combustion of sludge pellets and combustible wastes was mainly determined by the combustion rate of the combustible waste. The activation energy of the 8-mm-diameter sludge pellets was between 6.70 and 10.0 kcal/mol, according to the moisture content, but it was lower for MSW and RDF. In the case of MSW co-combustion, the reaction rate accelerated as the moisture content of the sludge pellets decreased, but it was markedly increased by the addition of RDF, regardless of the sludge moisture content.     

Flow analysis of metals in a municipal solid waste management system

This study aimed to identify the metal flow in a municipal solid waste (MSW) management system. Outputs of a resource recovery facility, refuse derived fuel (RDF) production facility, carbonization facility, plastics liquefaction facility, composting facility, and bio-gasification facility were analyzed for metal content and leaching concentration. In terms of metal content, bulky and incombustible waste had the highest values. Char from a carbonization facility, which treats household waste, had a higher metal content than MSW incinerator bottom ash. A leaching test revealed that Cd and Pb in char and Pb in RDF production residue exceeded the Japanese regulatory criteria for landfilling, so special attention should be paid to final disposal of these substances. By multiplying metal content and the generation rate of outputs, the metal content of input waste to each facility was estimated. For most metals except Cr, the total contribution ratio of paper/textile/plastics, bulky waste, and incombustible waste was over 80%. Approximately 30% of Cr originated from plastic packaging. Finally, several MSW management scenarios showed that most metals are transferred to landfills and the leaching potential of metals to the environment is quite small.     

Improved quartz furnace method for chlorine and sulfur determination in municipal solid waste

 A method of determining the chlorine (Cl) and sulfur (S) in municipal solid waste (MSW) was studied. The quartz furnace method was improved in two ways: recovery from ash by hot extraction with dilute nitric acid, and avoidance of the volatilization of alkali (earth) metal chlorides by setting the sample combustion temperature at 600°C. In a comparison with the bomb method, using nine sets of kitchen garbage and waste plastics, the bomb method yielded a 15%–25% lower value than the improved quartz furnace method. Combustion in the bomb was frequently incomplete, resulting in recovery losses of Cl and S. The average kitchen garbage involved 5.2 mg Cl/g, of which at least 24.1% would be converted to HCl. Plastics contained 23 mg Cl/g generating 88.1% HCl on average. In the same way, kitchen garbage contained 3.0 mg S/g, generating 52.3% SO _x , whereas plastics contained 1.1 mg S/g with 55.1% SO _x formation. Received: March 20, 2002 / Accepted: October 13, 2002     

Municipal solid waste management in Beijing City

This paper presents an overview of municipal solid waste (MSW) management in Beijing City. Beijing, the capital of China, has a land area of approximately 1368.32 km² with an urban population of about 13.33 million in 2006. Over the past three decades, MSW generation in Beijing City has increased tremendously from 1.04 million tons in 1978 to 4.134 million tons in 2006. The average generation rate of MSW in 2006 was 0.85 kg/capita/day. Food waste comprised 63.39%, followed by paper (11.07%), plastics (12.7%) and dust (5.78%). While all other wastes including tiles, textiles, glass, metals and wood accounted for less than 3%. Currently, 90% of MSW generated in Beijing is landfilled, 8% is incinerated and 2% is composted. Source separation collection, as a waste reduction method, has been carried out in a total of 2255 demonstration residential and commercial areas (covering about 4.7 million people) up to the end of 2007. Demonstration districts should be promoted over a wider range instead of demonstration communities. The capacity of transfer stations and treatment plants is an urgent problem as these sites are seriously overloaded. These problems should first be solved by constructing more sites and converting to new treatment technologies. Improvements in legislation, public education and the management of waste pickers are problematic issues which need to be addressed.     

Recovery of solid fuel from municipal solid waste by hydrothermal treatment using subcritical water

Hydrothermal treatments using subcritical water (HTSW) such as that at 234 °C and 3 MPa (LT condition) and 295 °C and 8 MPa (HT condition) were investigated to recover solid fuel from municipal solid waste (MSW). Printing paper, dog food (DF), wooden chopsticks, and mixed plastic film and sheets of polyethylene, polypropylene, and polystyrene were prepared as model MSW components, in which polyvinylchloride (PVC) powder and sodium chloride were used to simulate Cl sources.While more than 75% of carbon in paper, DF, and wood was recovered as char under both LT and HT conditions, plastics did not degrade under either LT or HT conditions. The heating value (HV) of obtained char was 13,886-27,544 kJ/kg and was comparable to that of brown coal and lignite. Higher formation of fixed carbon and greater oxygen dissociation during HTSW were thought to improve the HV of char.Cl atoms added as PVC powder and sodium chloride to raw material remained in char after HTSW. However, most Cl originating from PVC was found to converse into soluble Cl compounds during HTSW under the HT condition and could be removed by washing.From these results, the merit of HTSW as a method of recovering solid fuel from MSW is considered to produce char with minimal carbon loss without a drying process prior to HTSW. In addition, Cl originating from PVC decomposes into soluble Cl compound under the HT condition. The combination of HTSW under the HT condition and char washing might improve the quality of char as alternative fuel.     

Aging and compressibility of municipal solid wastes

The expansion of a municipal solid waste (MSW) landfill requires the ability to predict settlement behavior of the existing landfill. The practice of using a single compressibility value when performing a settlement analysis may lead to inaccurate predictions. This paper gives consideration to changes in the mechanical compressibility of MSW as a function of the fill age of MSW as well as the embedding depth of MSW. Borehole samples representative of various fill ages were obtained from five boreholes drilled to the bottom of the Qizhishan landfill in Suzhou, China. Thirty-one borehole samples were used to perform confined compression tests. Waste composition and volume-mass properties (i.e., unit weight, void ratio, and water content) were measured on all the samples. The test results showed that the compressible components of the MSW (i.e., organics, plastics, paper, wood and textiles) decreased with an increase in the fill age. The in situ void ratio of the MSW was shown to decrease with depth into the landfill. The compression index, Cc, was observed to decrease from 1.0 to 0.3 with depth into the landfill. Settlement analyses were performed on the existing landfill, demonstrating that the variation of MSW compressibility with fill age or depth should be taken into account in the settlement prediction.     

Survey of composition and generation rate of household wastes in Beijing, China

It is recognized that information on both quantity and composition of residential waste is important for the effective planning of household waste handling infrastructure. In this paper, we present the results of a survey on household waste generation and composition in Beijing, China. Sample communities were selected by the integration of five indices including family population, income, age, and education. Wastes were sampled on a daily basis from 113 households in six different districts of Beijing City for ten days. The results showed that the generation rate of household wastes was 0.23 kg/pers/day. The bulk density was approximately 221 kg/m³, and the moisture content was approximately 50%. Household waste consisted of kitchen waste, paper/cardboard, plastics, textiles, metals, glass and other wastes, the proportion of each waste was approximately 69.3%, 10.3%, 9.8%, 1.3%, 0.8%, 0.6% and 2.7%, respectively. An evaluation of the relationship between daily per capita generation of household waste and socio-economic factors indicated that household size and income both showed a negative relationship with household waste generation (kg/pers/day). As for the effect of education, families with a secondary educational level produced fewer household wastes (kg/pers/day) than those with a primary or advanced educational level.     

Review of municipal solid waste management options in Malaysia,with an emphasis on sustainable waste-to-energy options

A beautiful and clean environment is the desire of every society. Malaysia is facing an uncontrolled increase in municipal solid waste (MSW) generation due to population growth, economic advancement, and industrialization, but the current, most common waste disposal practice of landfilling is not sustainable. The increasing standard of living also saps more energy from the power generation systems in which fossil fuels are the major source of fuel for the plants. Malaysia generates about 0.5–1.9 kg/capita/day of MSW; a total of about 25,000 tonnes/day of MSW is currently generated and is estimated to exceed 30,000 tonnes/day by 2020. Malaysian MSW is mainly composed of 45 % food waste, 24 % plastic, 7 % paper materials, 6 % metal, 4 % wood and 3 % glass, which are commingled, and is thus characterised by 52–66 % moisture content. Currently, 80–95 % of collected MSW is landfilled and 5 % is recycled, while composting and energy recovery are rarely practiced. This paper reviews the solid waste practice in Malaysia and looks into alternative management options for sustainability. Malaysia MSW represents recyclable power and energy potential if properly sorted. This study considered the practice of sorting at the source and the use of combustible MSW components as fuel to generate heat for a hybrid solar, flue gas, chimney power plant.     

An Overview of Solid Waste Management and Plastic Recycling in Qatar

Municipal solid waste management (MSWM) constitutes one of the most crucial health and environmental problems facing authorities in the Arabian Gulf. Recent literature on current solid waste management (SWM) in Qatar has been reviewed in this paper, and a focused study has been carried out to provide a review on the total amount of municipal solid waste generated, stored, collected, disposed as well as the constituents of the waste. The analysis showed that Qatar produced around 2,000,000 tons of solid municipal waste annually, corresponding to a daily generation rate per capita of about 2.5 kg. About 60% of MSW is organic material and about 300 kg is composed daily. Landfill and composting is considered the most appropriate waste disposal techniques in Qatar. Um-Al-Afai landfill has nearly 80% of MSW. Because of the increased migration in Qatar, there is a sharp rise in the volume and also in the variety of solid waste. It is important to alleviate societal concerns over the increased rate of resource consumption and waste production; thus, policy makers have encouraged recycling and reuse strategies to reduce the demand for raw materials and to decrease the quantity of waste going to landfill. An example of the benefit of mechanical recycling of plastics compared to land filling and composting was conducted by GaBi 4 life cycle analysis tool which showed the benefits to the global warming and human toxicity. Recycling is the favored solution for plastic waste management, because it has a lower environmental impact on the defined impact categories, from Global Warming Potential (GWP) and Human Toxicity Potentials (HTP) indicators.     

Experimental and kinetic modeling of oxygen-enriched air combustion of municipal solid waste

The characteristics of oxygen-enriched air combustion of raw municipal solid waste (MSW) were studied by thermogravimetric analysis. Experiments on oxidative pyrolysis of MSW were carried out under different atmospheres (N(2), N(2):O(2)=7:3, N(2):O(2)=5:5, N(2):O(2)=4:6, and N(2):O(2)=2:8) at 30 degrees C/min. Two distinct peaks of weight loss were obtained according to the derivative thermogravimetric curves; one of them is centered on 305 degrees C with about 40% weight loss, and the second is centered on 420 degrees C with about 20% weight loss. Effects of oxygen concentration on the decomposition process and char combustion were analyzed, and then the process of oxygen-enriched air combustion of MSW was divided into four steps. Kinetic parameters were observed by direct non-linear regressions. According to the obtained data, the apparent activation energy and reaction order decreases along with the combustion process, while that of char combustion increases as oxygen concentration increases.     

Biodegradation and flushing of MBT wastes

Mechanical–biological treatment (MBT) processes are increasingly being adopted as a means of diverting biodegradable municipal waste (BMW) from landfill, for example to comply with the EU Landfill Directive. However, there is considerable uncertainty concerning the residual pollution potential of such wastes. This paper presents the results of laboratory experiments on two different MBT waste residues, carried out to investigate the remaining potential for the generation of greenhouse gases and the flushing of contaminants from these materials when landfilled. The potential for gas generation was found to be between 8% and 20% of that for raw MSW. Pretreatment of the waste reduced the potential for the release of organic carbon, ammoniacal nitrogen, and heavy metal contents into the leachate; and reduced the residual carbon remaining in the waste after final degradation from ～320 g/kg dry matter for raw MSW to between 183 and 195 g/kg dry matter for the MBT wastes.     

Fundamental characteristics of input waste of small MSW incinerators in Korea

Waste incineration in a small incinerator is a simple and convenient way of treating waste discharged from small areas or from large facilities and buildings such as business centers, marketplaces, factories, and military units. Despite their ostensible advantages, however, many small incinerators frequently suffer from serious problems, e.g., unsystematic waste feeding, unstable combustion, deficient air pollution control devices, and consequently, environmental pollution. To obtain a better understanding of the characterization of wastes in small incinerators, we investigated a series of fundamental characteristics, i.e., physical composition, bulk density, proximate and ultimate analysis, potential energy content, and so on. The main waste components in small incinerators were identified as paper and plastic; the proportion of food waste was less than that in large incinerators. Especially, a low ratio of food waste had a strong influence on other waste characteristics, e.g., lower moisture content and bulk density, and higher potential energy. On the other hand, in contrast with that of HCl, there was no distinguishable linear relationship between Cl content in waste and PCDD/DF concentration in combustion gas.     

Urban solid waste management in Chongqing: challenges and opportunities

The dual influences of the resource supply and protection in ecological environments will pose a significant challenge to China's sustainable development. Solid waste management offers opportunities to improve profits by conserving resources and improving environmental performance. This paper examines municipal solid waste (MSW) management in urban Chongqing, the nation's fourth largest municipality after Beijing, Shanghai and Tianjin. In this paper, we will provide information on the quantity and composition of MSW, as well as give an overview of different methods for collection, transport, treatment and disposal of MSW. At present the daily amount of MSW generated per person is about 1.08 kg; food waste accounts for about 59% of total MSW. MSW in Chongqing has a higher moisture content (64.1%) and a lower LHV (3728 kJ/kg) than other cities in Asia, which is an obstruction for incineration. Landfills are the main method of disposal in Chongqing, but pollution caused by simple landfills and lack of backup MSW disposal capacity are becoming major problems in the main districts of Chongqing. In this paper, the challenges being faced and opportunities to MSW in Chongqing are analyzed and some suggestions are given for improving the MSW system in the future.     

The origins of municipal solid waste: The relations between residues from packaging materials and food

Data for the composition of municipal solid waste (MSW) from around the world are used to further examine a previously reported statistical correlation between the fraction of food residues and the fractions of paper and board, metal, glass and plastics residues in MSW. For data from many locations, these correlations are statistically significant; multiple linear regressions are computed. The fraction of food waste decreases as the fractions of waste from paper and board, metals and glass increase.The situation in the U.S.A. is examined further for just packaging waste. Similar correlations are established for the fraction of food residues and the fractions of paper and board and plastics packaging residues for predicted compositions for 1980 to 2000. Similar correlations for the U.K. are not statistically significant. Some reasons for this are postulated.The results of the statistical analyses predict that a strategy for decreasing the fraction of food waste in MSW is to increase the use of food packaging by some amount, especially plastics and metals, contrary to conventional wisdom.     

Municipal solid waste disposal in Portugal

In recent years municipal solid waste (MSW) disposal has been one of the most important environmental problems for all of the Portuguese regions. The basic principles of MSW management in Portugal are: (1) prevention or reduction, (2) reuse, (3) recovery (e.g., recycling, incineration with heat recovery), and (4) polluter-pay principle. A brief history of legislative trends in waste management is provided herein as background for current waste management and recycling activities. The paper also presents and discusses the municipal solid waste management in Portugal and is based primarily on a national inquiry carried out in 2003 and directed to the MSW management entities. Additionally, the MSW responsibility and management structure in Portugal is presented, together with the present situation of production, collection, recycling, treatment and elimination of MSW. Results showed that 96% of MSW was collected mixed (4% was separately collected) and that 68% was disposed of in landfill, 21% was incinerated at waste-to-energy plants, 8% was treated at organic waste recovery plants and 3% was delivered to sorting. The average generation rate of MSW was 1.32 kg/capita/day.     

Effect of multi-factors interaction on trace lead equilibrium during municipal solid waste incineration

The thermodynamic equilibrium of trace lead during the waste incineration was calculated on the basis of the minimization of the total Gibbs energy. The effect of incineration condition and MSW components on Pb distribution was investigated mainly in the view of the interaction of related elements. In the oxygen-rich condition, incineration temperature affects Pb distribution by the interaction of Cl, Ca and Na. In the fuel-rich condition, incineration temperature affects Pb distribution directly by the thermal transition of PbS(s) to PbCl(g) and the thermal transition of PbCl(g) to Pb(g). Air ratio has significant effect on Pb distribution by the interaction of H, O and Cl. The liberated Cl in oxidizing condition is far less than that in reducing condition. Na has the top priority to bond with Cl, than Ca only at low temperature and H only at high temperature, so the effect of Cl on Pb distribution depends on the content of Na and Ca. S promotes Pb volatilization by the interaction with Na in oxygen-rich and chlorine-poor condition and depresses Pb volatilization by the formation of PbS(s) directly without interaction with other elements in fuel-rich condition.     

Future landfill emissions and the effect of final cover installation--a case study

Municipal solid waste (MSW) landfills are potential long-term sources of emissions. Hence, they need to be managed after closure until they do not pose a threat to humans or the environment. The case study on the Breitenau MSW landfill was performed to evaluate future emission levels for this site and to illustrate the effect of final cover installation with respect to long-term environmental risks. The methodology was based on a comprehensive assessment of the state of the landfill and included analysis of monitoring data, investigations of landfilled waste, and an evaluation of containment systems. A model to estimate future emission levels was established and site-specific predictions of leachate emissions were presented based on scenario analysis. The results are used to evaluate the future pollution potential of the landfill and to compare different aftercare concepts in view of long-term emissions. As some leachable substances became available for water flow during cover construction due to a change in the water flow pattern of the waste, a substantial increase in leachate concentrations could be observed at the site (e.g. concentrations of chloride increased from 200 to 800 mg/l and of ammonia-nitrogen from 140 to about 500 mg/l). A period of intensive flushing before the final cover installation could have reduced the amount of leachable substances within the landfill body and rapidly decreased the leachate concentrations to 11 mg Cl/l and 79 mg NH₄-N/l within 50 years. Contrarily, the minimization of water infiltration is associated with leachate concentrations in a high range for centuries (above 400 mg Cl/l and 200 mg NH₄-N/l) with low concomitant annual emission loads (below 12 kg/year of Cl or 9 kg/year of NH₄-N, respectively). However, an expected gradual decrease of barrier efficiency over time would be associated with higher emission loads of 50 kg of chloride and 30 kg of ammonia-nitrogen at the maximum, but a faster decrease of leachate concentration levels.     

MSW management for waste minimization in Taiwan: the last two decades

Taiwan is the second most densely populated country in the world; its 22.604 million residents (2002) live in an area of 35,967 km2 (628 people/km2). Taiwan's economy has grown rapidly during the last 20 years, resulting in a corresponding increase in the amount of municipal solid waste (MSW). This study describes and evaluates the municipal solid waste management system in Taiwan. The study's results indicate that the amount of MSW began to decline after 1997, when the government enforced aggressive MSW management policies. By 2002, total MSW production had dropped by 27%, and the average daily per capita weight of MSW had fallen from 1.14 kg in 1997 to 0.81 kg in 2002. Summarizing the successful experience of MSW reduction in Taiwan, the most important factor was the government's combining of the MSW collection system with reduction/recycling programs. The second most important factor was the policy of extended producer responsibility, which laid a foundation of recycling by producers and retailers and promoted public recycling.     

Combustion studies of high moisture content waste in a fluidised bed

The combustion of three high moisture content waste materials in a fluidised bed combustor has been investigated and a comparison with co-firing of these materials with coal in the same combustor has been made. Waste materials burnt were olive oil waste, municipal solid waste and potato, which is representative of vegetable waste. Mixtures of up to 20% mass concentration water in the waste were fed to the combustor. Above that value the moisture content was too high to sustain combustion without addition of coal. Measurements of CO, NOx, SO2 temperatures were made and the carbon combustion efficiency evaluated. Co-firing with coal resulted in markedly higher combustion efficiencies with an increase of approximately 10-80% when burning the simulated MSW. However, this was much lower than the value of 93% when coal was burnt on its own. It was also much lower than the value obtained, average 90%, when co-firing potato and olive oil waste with coal and there was little difference in the combustion efficiency between the two types of waste and with increasing moisture content. It was concluded that the high ash content of the simulated MSW 26%, compared with 5% in the other two waste materials resulted in slower burning and consequently the char particles were elutriated from the bed without being fully burnt. In term of gaseous emissions during co-combustion, CO emission is relatively insensitive to change in waste fraction. While emission of SO2 can be reduced as the waste fraction increases as a result of fuel-S dilution. But in terms of percent fuel-S converted, it is actually increased by increasing waste fraction. Emissions of NO and N2O increase slightly with MSW fraction.     

Impact of food waste disposers on the generation rate and characteristics of municipal solid waste

Field surveys were carried out to ascertain the reduction in the generation rate of municipal solid waste as a result of the use of food waste disposers (FWDs). The generation rate and characteristics of garbage and of the food waste ground by FWDs in households were investigated, and the effect of FWDs was discussed. In a medium-sized village, the amount of garbage collected per collection decreased from 251.2 kg to 114.7 kg as a result of FWD use. The unit loading of ground food waste was estimated to be 111 g/cap·day. In a small village, the amount of garbage collected weekly decreased from 175 kg to 112 kg. The average unit loading of ground food waste was measured and found to be 126 g/cap·day. The reduction rate of garbage varied from 25% to 39% monthly and the average reduction rate was 31% throughout the surveying period. Dramatic variations in the characteristics of the collected garbage were recognized: the ratio of food waste on a dry basis declined by more than half, the moisture content decreased to half or less, the combustible matter content increased approximately 1.7 times or more, and the lower heating value increased approximately 2.0 times or more compared to the case without FWD use.