Influence of mechanical-biological waste pre-treatment methods on the gas formation in landfills

In order to minimise emissions and environmental impacts, only pre-treated waste should be disposed of. For the last six years, a series of continuous experiments has been conducted at the Institute WAR, TU Darmstadt, in order to determine the emissions from pre-treated waste. Different kinds of pre-treated waste were incubated in several reactors and various data, including production and composition of the gas and the leachate, were collected. In this paper, the interim results of gas production and the gas composition from different types of waste after a running time of six years are presented and discussed.     

Biostabilization of municipal solid waste

A mechanical-biological process for municipal solid waste (MSW) treatment was monitored for one year. Mechanical pre-treatment provided two fractions. The oversize fraction (diameter > 50 mm) (yield of 600 g kg(-1) ww) (46 Mg day(-1)) was used for refuse derived fuel production, after undergoing a mechanical refining processes, because of low moisture content (200-250 g kg(-1)) and high calorific value (2500-2800 kcal kg ww(-1)). The undersize fraction (diameter < 50 mm) (yield 400 g kg(-1) ww) (30 Mg day(-1)) contained about 800 g kg(-1) of the MSW organic matter. This fraction was biologically treated using an aerobic process with an organic waste fraction from separate collection (77 Mg day(-1)) and recycled stabilized material (62 Mg day(-1)) obtained from end-product sieve (diameter < 20 mm) used as bulking agent. A retention time of three weeks was sufficient to obtain stabilized products in agreement with up-dated rules of the Lombardy Region (North Italy) regarding biostabilization and composting processes. Dynamic Respiration Index (DRI), such as required by both Lombardy Region rules and suggested by the European Community, was chosen in preference to other indices in order to assess the degree of biological stability of the end products. A mean DRI value of 1164 mg O2 kg SV(-1) h(-1) was obtained and is in agreement with the proposed limit of 1000+/-200 mg O2 kg SV(-1) h(-1). Self-heating test, potential biogas production and fermentable volatile solids were also used as parameters to describe the potential impact of treated waste, providing further useful information. Nevertheless, all of these methods revealed analytical or interpretative limits. A complete mass balance of the biological treatment section showed that, from a net input of 107 Mg day(-1), only 250 g kg(-1) (27 Mg day(-1)) of the waste needed to be landfilled, with 750 g kg(-1) (80 Mg day(-1)) being lost as CO2 and H2O.     

Nitrate and nitrite injection during municipal solid waste anaerobic biodegradation

Nitrified leachate recirculation has been proposed as a promising strategy for sustainable landfill management. In four test reactors, nitrate or nitrite was added (250 mg N-NO(x)(-)L(-1)) during municipal solid waste biodegradation. Nitrogen-oxides reduction reactions were monitored. Denitrification was the main nitrogen reducing reaction observed. On one hand, during the acidogenic waste degradation phase, as high amounts of volatile fatty acids (VFA) were present, nitrogen-oxides reductions were interpreted as heterotrophic denitrifications. On the other hand, denitrification reactions occurring during the late methanogenic phase were accompanied by sulphate productions and, as VFA were not detected, it was probably an autotrophic reaction. Denitrification inhibition was observed once. Ammonium concentration increased suggesting the occurrence of a dissimilatory nitrate reduction to ammonium (DNRA). Statistical treatment of analytical data revealed that only H(2)S concentration had a significant negative effect on N(2) production in our system. NO production was observed once when nitrite was injected during the acidogenic phase resulting in a total waste degradation inhibition. These results indicate that the consequences of nitrified leachate recirculation in full-scale landfills need to be carefully examined especially during the acidogenic phase or in the presence of waste containing high quantities of sulphur.     

Heating value of municipal solid waste

This paper describes the processes that are used to calculate the heating value of municipal solid waste in France. The calculation can be done either by using the thermal-balance method of a furnace or a furnace-boiler unit, or by sorting the refuse and calculating the heating value of the homogeneous components. Both methods are described in this paper. The first method measures the heating value of the refuse that is injected into the furnace; the furnace becomes a calorimeter in which thermal balance is achieved on measuring the input and output heats. The second method consists of sorting a 100 kg refuse sample into piles that are as homogeneous as possible so that it can be considered as unchanging. The heating value is determined for each component and the lower heating value of the whole sample is calculated. The advantages and drawbacks of both methods are discussed.     

Enzymatic processing of municipal solid waste

The focus of this work was to investigate an enzymatic liquefaction of MSW organics, paper and cardboard. Liquefaction trials were conducted in different trial volumes: 50 g lab-scale trials and 50 kg vessel-tests and evaluated based on particle size and viscosity. The viscosity results showed that Celluclast 1.5 L had the singular significant effect on liquefaction of model MSW. No effect of α-amylase, protease and interaction in between and with cellulases on viscosity and particle size distribution was found in this study. Degradable material with a particle size above 1 mm after treatment was evaluated using SEM microscopy. These results showed that paper particles were the main obstacles needing additional treatment in order to become fully liquefied. In a pilot scale test treating authentic MSW; more than 90% of initial organic and paper dry matter (DM) was recovered as liquid slurry after sieving through a 5-mm sieve. These tests were performed at up to 35% DM, showing that this process can easily manage high DM loadings. MSW enzymatic liquefaction promotes the separation of organics and paper from solids, which facilitate the use of these degradable fractions, with minimal loss, capable to enter a biogas plant through existing pipes.     

Sorting efficiency and combustion properties of municipal solid waste during bio-drying

One aerobic and two combined bio-drying processes were set up to investigate the quantitative relationships of sorting efficiency and combustion properties with organics degradation and water removal during bio-drying. Results showed that the bio-drying could enhance the sorting efficiency of municipal solid waste (MSW) up to 71% from the initial of 34%. The sorting efficiency was correlated with water content negatively (correlation coefficient, r = −0.89) and organics degradation rate positively (r = 0.92). The higher heating values (HHVs) were correlated with organics degradation negatively for FP (i.e. the sum of only food and paper) (r = −0.93) but positively for the mixing waste (MW) (r = 0.90), whereas the lower heating values (LHVs) were negatively correlated with water content for both FP (r = −0.71) and MW (r = −0.96). Other combustion properties depended on organics degradation performance, except for ignition performance and combustion rate. The LHVs could be greatly enhanced by the combined process with insufficient aeration during the hydrolytic stage. Compared with FP, MW had higher LHVs and ratios of volatile matter to fixed carbon. Nevertheless, FP had higher final burnout values than MW.     

Benchmarking in municipal solid waste recycling

The paper presents an analysis of the factors influencing the recycling potential of municipalities in Israel, including population size and density, geographic location, current waste levels, and current waste management system. We employ a standard regression analysis in order to develop an econometric model to predict where potential for economically efficient recycling is highest. By applying this model to readily available data, it is possible to predict with close to 90% accuracy whether or not recycling will be economically efficient in any given municipality. Government agencies working to promote advanced waste management solutions have at their disposal only limited resources and budget, and so must concentrate their efforts where they will be most effective. The paper thus provides policy-makers with a powerful tool to help direct their efforts to promote recycling at those municipalities where it is indeed optimal.     

Microbial diversity and dynamics during methane production from municipal solid waste

The objectives of this study were to characterize development of bacterial and archaeal populations during biodegradation of municipal solid waste (MSW) and to link specific methanogens to methane generation. Experiments were conducted in three 0.61-m-diameter by 0.90-m-tall laboratory reactors to simulate MSW bioreactor landfills. Pyrosequencing of 16S rRNA genes was used to characterize microbial communities in both leachate and solid waste. Microbial assemblages in effluent leachate were similar between reactors during peak methane generation. Specific groups within the Bacteroidetes and Thermatogae phyla were present in all samples and were particularly abundant during peak methane generation. Microbial communities were not similar in leachate and solid fractions assayed at the end of reactor operation; solid waste contained a more abundant bacterial community of cellulose-degrading organisms (e.g., Firmicutes). Specific methanogen populations were assessed using quantitative polymerase chain reaction. Methanomicrobiales, Methanosarcinaceae, and Methanobacteriales were the predominant methanogens in all reactors, with Methanomicrobiales consistently the most abundant. Methanogen growth phases coincided with accelerated methane production, and cumulative methane yield increased with increasing total methanogen abundance. The difference in methanogen populations and corresponding methane yield is attributed to different initial cellulose and hemicellulose contents of the MSW. Higher initial cellulose and hemicellulose contents supported growth of larger methanogen populations that resulted in higher methane yield.     

Management of municipal solid waste incineration residues

The management of residues from thermal waste treatment is an integral part of waste management systems. The primary goal of managing incineration residues is to prevent any impact on our health or environment caused by unacceptable particulate, gaseous and/or solute emissions. This paper provides insight into the most important measures for putting this requirement into practice. It also offers an overview of the factors and processes affecting these mitigating measures as well as the short- and long-term behavior of residues from thermal waste treatment under different scenarios. General conditions affecting the emission rate of salts and metals are shown as well as factors relevant to mitigating measures or sources of gaseous emissions.     

Long-term behavior of municipal solid waste landfills

A method is presented to predict the long-term behavior of element concentrations (non-metals and metals) in the leachate of a municipal solid waste (MSW) landfill. It is based on water flux and concentration measurements in leachates over one year, analysis of drilled cores from MSW landfills and leaching experiments with these samples. A mathematical model is developed to predict the further evolution of annual flux-weighted mean element concentrations in leachates after the “intensive reactor phase”, i.e. after the gas production has dropped to a very low level. The results show that the organic components are the most important substances to control until the leachate is compatible with the environment. This state of low emissions, the so-called “final storage quality”, will take many centuries to be achieved in a moderate climate.     

Determination of specific gravity of municipal solid waste

This investigation was conducted to evaluate experimental determination of specific gravity (G_s) of municipal solid waste (MSW). Water pycnometry, typically used for testing soils was adapted for testing MSW using a large flask with 2000 mL capacity and specimens with 100–350 g masses. Tests were conducted on manufactured waste samples prepared using US waste constituent components; fresh wastes obtained prior and subsequent to compaction at an MSW landfill; and wastes obtained from various depths at the same landfill. Factors that influence specific gravity were investigated including waste particle size, compaction, and combined decomposition and stress history. The measured average specific gravities were 1.377 and 1.530 for as-prepared/uncompacted and compacted manufactured wastes, respectively; 1.072 and 1.258 for uncompacted and compacted fresh wastes, respectively; and 2.201 for old wastes. The average organic content and degree of decomposition were 77.2% and 0%, respectively for fresh wastes and 22.8% and 88.3%, respectively for old wastes. The G_s increased with decreasing particle size, compaction, and increasing waste age. For fresh wastes, reductions in particle size and compaction caused occluded intraparticle pores to be exposed and waste particles to be deformed resulting in increases in specific gravity. For old wastes, the high G_s resulted from loss of biodegradable components that have low G_s as well as potential access to previously occluded pores and deformation of particles due to both degradation processes and applied mechanical stresses. The G_s was correlated to the degree of decomposition with a linear relationship. Unlike soils, the G_s for MSW was not unique, but varied in a landfill environment due both to physical/mechanical processes and biochemical processes. Specific gravity testing is recommended to be conducted not only using representative waste composition, but also using representative compaction, stress, and degradation states.     

Potentials for the prevention of municipal solid waste

Waste prevention has been assigned the highest priority under European waste management law. However, the initiatives which have been taken so far have not reduced the regular annual increase in total waste arising across Europe. The purpose of this paper is to quantify and analyze in depth the prevention potential for selected case studies (advertising material, beverage packaging, diapers, food waste, waste from events). Only such measures are considered, which do not require a reduction of consumption. The prevention potentials for household waste in theory are exemplified for the city of Vienna, also with a view to barriers that may reduce the achievable prevention potential. The results show that the prevention potentials for the analyzed individual measures can reach an order of magnitude of some 10% of the relevant waste stream (e.g., advertising material, beverage packaging), or rather 1-3% of municipal solid waste. The prevention potentials appear to be relatively small in relation to the total municipal waste quantities. Methodological shortcomings, such as the missing availability of basic data, make it difficult to exactly estimate the impact of waste prevention measures on waste quantities, which is only one of the effects of waste prevention among other ecological, social and economic aspects.     

Pretreatment of municipal solid waste prior to landfilling

An outdoor pilot-scale study was undertaken to pretreat municipal solid waste by windrow composting. The raw waste was introduced to active composting without any source separation or pulverization. Pretreatment indicators were developed and used as a tool to measure the optimum level of sorting and waste stabilization. The moisture content of the waste dropped from 68% to 61% and the pile attained a thermophilic temperature in one week. It was observed that the C/N ratio, pH profile and temperature gradients were comparable to that of traditional windrow composting. Within one week of active bulk composting, the easily degradable organic matter was consumed and there was a significant reduction in the bulk volume of the mixed waste. The pre-composted wastes were then sorted into four fractions. Compared to the initial untreated waste, the pretreated waste showed greater sorting efficiency and reduced volatile solids. A 1-m³ cage was used to study pile settlement and volume reduction. The results indicate that pretreatment by bulk composting could reduce by ≈40% the total mass of waste hauled to landfill sites in developing countries.     

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.     

Large-scale direct shear testing of municipal solid waste

Large direct shear testing (300 mm × 300 mm box) of municipal solid waste (MSW) collected from a landfill located in the San Francisco Bay area was performed to gain insight on the shear response of MSW. The study investigated the effects of waste composition, confining stress, unit weight, and loading rate on the stress–displacement response and shear strength of MSW. The amount and orientation of the fibrous waste materials in the MSW were found to play a critical role. The fibrous material had little effect on the MSW’s strength when it was oriented parallel to the shear surface, as is typically the case when waste material is compressed vertically and then tested in a direct shear apparatus. Tests in which the fibrous material was oriented perpendicular to the horizontal shear surface produced significantly stronger MSW specimens. The test results indicate that confining stress and loading rate are also important factors. Based on 109 large-scale direct shear tests, the shear strength of MSW at low moisture contents is best characterized by cohesion = 15 kPa, friction angle = 36° at a normal stress of 1 atmosphere, and a decrease in the friction angle of 5° for every log-cycle increase in normal stress.     

Food waste impact on municipal solid waste angle of internal friction

The impact of food waste content on the municipal solid waste (MSW) friction angle was studied. Using reconstituted fresh MSW specimens with different food waste content (0%, 40%, 58%, and 80%), 48 small-scale (100-mm-diameter) direct shear tests and 12 large-scale (430 mm × 430 mm) direct shear tests were performed. A stress-controlled large-scale direct shear test device allowing approximately 170-mm sample horizontal displacement was designed and used. At both testing scales, the mobilized internal friction angle of MSW decreased considerably as food waste content increased. As food waste content increased from 0% to 40% and from 40% to 80%, the mobilized internal friction angles (estimated using the mobilized peak (ultimate) shear strengths of the small-scale direct shear tests) decreased from 39° to 31° and from 31° to 7°, respectively, while those of large-scale tests decreased from 36° to 26° and from 26° to 15°, respectively. Most friction angle measurements produced in this study fell within the range of those previously reported for MSW.     

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.     

Unit-charging programs for municipal solid waste in Japan

Unit-charging programs known as pay-as-you-throw (PAYT) for municipal solid waste in Japan were surveyed. The number of municipalities that have implemented PAYT for combustible waste totaled 954 (30%) in 2003. The introduction of PAYT programs peaked in the early 1970s and again in the 1990s. PAYT has tended to be adopted by municipalities with small populations (less than 30,000). PAYT charging systems can be roughly divided into two groups: simple unit-pricing programs and two-tiered pricing programs. It is difficult to see the relationship between PAYT and waste reduction by simple inspection of the overall changes throughout Japan. Case studies of four municipalities showed that the implementation of PAYT programs reduced the amount of residual waste generated by 20% to 30%. In combination with other measures, especially the recycling of containers and packaging, PAYT programs can bring about a dramatic reduction in waste.     

生活垃圾焚烧处理方式的生命周期评价

与填埋、堆肥处理相比,生活垃圾焚烧处理在减量化和资源化方面有着巨大的优势,在未来将成为大城市生活垃圾的主要处理方式。采用生命周期评价方法,对不同余热利用和尾气处理方式下的生活垃圾焚烧处理方案对环境的影响进行评价。结果表明,在尾气处理系统中,干法、湿法、半干法3种酸性气体处理方式中,湿法处理的环境影响潜值最小,但是其资源耗竭系数最大。添加SNCR脱氮系统可以在酸性气体净化基础上将环境影响潜值降低70%左右,而资源耗竭系数变化不大。在单纯供热、供电和热电联供3种余热利用方式中,单纯供电的热利用效率最低,直接供热的热利用效率最高。