Environmental impact of organic fraction of municipal solid waste treatment by composting in Sri Lanka

Journal of Material Cycles and Waste Management - In Sri Lanka, the management of the organic fraction of municipal solid waste (OFMSW) is a challenge. Composting is an environmentally and...     

Performance of compostable baby used diapers in the composting process with the organic fraction of municipal solid waste

In modern societies, disposable diapers constitute a significant percentage of municipal solid wastes. They have been traditionally landfilled or incinerated as only limited recycling processes are being implemented in some parts of Europe. With the implementation of separated collection systems for the organic fraction of municipal solid wastes (OFMSWs) and the need to preserve the environment, compostable diapers have appeared in the market to avoid the main environmental impacts associated to non-biodegradable disposable diapers. In this study, a full-scale composting of door-to-door collected OFMSW with a 3% (w/w) of compostable diapers has also been carried out. Previously, lab-scale experiments confirmed that almost 50% of carbon of compostable diapers is emitted as CO₂ under aerobic controlled conditions. The results obtained at full-scale demonstrate that both the composting process and the final end product (compost) are not altered by the presence of compostable diapers in crucial aspects such as pathogenic content, stability and elemental composition (including nutrients and heavy metals). The main conclusion of this study is that the collection of the OFMSW with compostable diapers can be a new way to transform this waste into high-quality compost.     

Methane yield in source-sorted organic fraction of municipal solid waste

Treating the source-separated organic fraction of municipal solid waste (SS-OFMSW) by anaerobic digestion is considered by many municipalities in Europe as an environmentally friendly means of treating organic waste and simultaneously producing methane gas. Methane yield can be used as a parameter for evaluation of the many different systems that exist for sorting and pre-treating waste. Methane yield from the thermophilic pilot scale digestion of 17 types of domestically SS-OFMSW originating from seven full-scale sorting systems was found. The samples were collected during 1 year using worked-out procedures tested statistically to ensure representative samples. Each waste type was identified by its origin and by pre-sorting, collection and pre-treatment methods. In addition to the pilot scale digestion, all samples were examined by chemical analyses and methane potential measurements. A VS-degradation rate of around 80% and a methane yield of 300-400Nm(3) CH(4)/ton VS(in) were achieved with a retention time of 15 days, corresponding to approximately 70% of the methane potential. The different waste samples gave minor variation in chemical composition and thus also in methane yield and methane potential. This indicates that sorting and collection systems in the present study do not significantly affect the amount of methane produced per VS treated.     

Proposal for the integration of decentralised composting of the organic fraction of municipal solid waste into the waste management system of Cuba

Municipal solid waste (MSW) generation and management in Cuba was studied with a view to integrating composting of the organic fractions of MSW into the system. Composting is already included as part of the environmental strategy of the country as an appropriate waste management solution. However, no programme for area-wide implementation yet exists. The evaluation of studies carried out by some Cuban and international organisations showed that organic matter comprises approximately 60-70% of the MSW, with households being the main source. If all organic waste fractions were considered, the theoretical amount of organic waste produced would be approximately 1 Mio. Mg/a, leading to the production of approximately 0.5 Mio. Mg/a of compost. Composting could, therefore, be a suitable solution for treating the organic waste fractions of the MSW. Composting would best be carried out in decentralised systems, since transportation is a problem in Cuba. Furthermore, low technology and low budget composting options should be considered due to the problematic local economic situation. The location for such decentralised composting units would optimally be located at urban agricultural farms, which can be found all over Cuba. These farms are a unique model for sustainable farming in the world, and have a high demand for organic fertiliser. In this paper, options for the collection and impurity-separation in urban areas are discussed, and a stepwise introduction of source-separation, starting with hotel and restaurant waste, is suggested. For rural areas, the implementation of home composting is recommended.     

Seasonal effects on anaerobic digestion of the source sorted organic fraction of municipal solid waste

The effects of seasonal temperature change on the quality of easily degradable organic matter in municipal solid waste was monitored and the related influence on the anaerobic digester performance examined. It was verified that increased external temperature caused the transformation of the ethanol contained in the source—sorted organic fraction of municipal solid waste fed to the digester, into acetate. This transformation has been associated with a remarkable effect on the process kinetics of substrate utilization. Thus, the kinetic constant for the first order substrate utilization model in the summer (external temperature greater than 18–20 °C) doubled with respect to the winter, although the digester was held at 35 ± 2 °C throughout.     

Press-extrusion pretreatment of the organic fraction of municipal solid waste for enhanced methane production

Anaerobic digestion (AD) is a well-established process for the treatment of a wide variety of solid organic substrates, including the organic fraction of municipal solid waste (OFMSW). At industrial scale, the mechanical pretreatment is a fundamental step to reduce OFMSW particle size and to promote the hydrolysis within the subsequent AD process. Among the mechanical pretreatment technologies, press-extrusion has recently raised great interest for its possible application to either enhance the organic load to the digester or improve the overall process stability and methane yields. Aim of this study was in assessing the potential of the press-extrusion pretreatment to improve the performance of OFMSW anaerobic degradation. Batch tests were set up according to a full factorial design of experiments to assess the significance of the main operating parameters. The statistical analysis of results addressed further tests, carried out under semi-continuous feeding mode, to better discuss the possible application of press-extrusion for the greatest valorization of OFMSW under anaerobic conditions.

     

Thermo-chemical pretreatment of the organic fraction of municipal solid waste for improved anaerobic digestion

Journal of Material Cycles and Waste Management - The hydrolysis of organic waste in anaerobic digestion is slow and time-consuming. Pretreatment of the waste can potentially improve hydrolysis and...     

Effect of organic fraction of municipal solid waste (OFMSW)-based biochar on organic carbon mineralization in a dry land soil

Journal of Material Cycles and Waste Management - The purpose of this study was to investigate the priming effects of organic fraction of municipal solid waste (OFMSW)-based biochar on...     

Questionnaire and onsite survey on municipal solid waste composting in Sri Lanka

This study was conducted to evaluate the composting processes in Sri Lanka and to identify essential improvements. The study consisted of a questionnaire survey, field observations, and interviews. The main shortcomings identified by this study were: no source separation at origin, and no monitoring for temperature, moisture, stability, or maturity during processes of composting. These problems hinder the smooth operation of the composting processes and lead to low demand for compost. Based on the findings, the recommendations for increasing the demand for compost are performing source separation during waste collections, improvement of processes by monitoring the temperature and moisture, and marketing compost so as to improve the popularity of the compost among farmers.     

Micro-scale anaerobic digestion of point source components of organic fraction of municipal solid waste

The fermentation characteristics of six specific types of the organic fraction of municipal solid waste (OFMSW) were examined, with an emphasis on properties that are needed when designing plug-flow type anaerobic bioreactors. More specifically, the decomposition patterns of a vegetable (cabbage), fruits (banana and citrus peels), fresh leaf litter of bamboo and teak leaves, and paper (newsprint) waste streams as feedstocks were studied. Individual OFMSW components were placed into nylon mesh bags and subjected to various fermentation periods (solids retention time, SRT) within the inlet of a functioning plug-flow biogas fermentor. These were removed at periodic intervals, and their composition was analyzed to monitor decomposition rates and changes in chemical composition. Components like cabbage waste, banana peels, and orange peels fermented rapidly both in a plug-flow biogas reactor (PFBR) as well as under a biological methane potential (BMP) assay, while other OFMSW components (leaf litter from bamboo and teak leaves and newsprint) fermented slowly with poor process stability and moderate biodegradation. For fruit and vegetable wastes (FVW), a rapid and efficient removal of pectins is the main cause of rapid disintegration of these feedstocks, which left behind very little compost forming residues (2–5%). Teak and bamboo leaves and newsprint decomposed only to 25–50% in 30 d. These results confirm the potential for volatile fatty acids accumulation in a PFBR’s inlet and suggest a modification of the inlet zone or operation of a PFBR with the above feedstocks.     

Dry-thermophilic anaerobic digestion of organic fraction of municipal solid waste: methane production modeling

The influence of particle size and organic matter content of organic fraction of municipal solid waste (OFMSW) in the overall kinetics of dry (30% total solids) thermophilic (55 °C) anaerobic digestion have been studied in a semi-continuous stirred tank reactor (SSTR). Two types of wastes were used: synthetic OFMSW (average particle size of 1 mm; 0.71 g Volatile Solids/g waste), and OFMSW coming from a composting full scale plant (average particle size of 30 mm; 0.16 g Volatile Solids/g waste).A modification of a widely-validated product-generation kinetic model has been proposed. Results obtained from the modified-model parameterization at steady-state (that include new kinetic parameters as K, Y_pMAX and θ_MIN) indicate that the features of the feedstock strongly influence the kinetics of the process. The overall specific growth rate of microorganisms (μ_max) with synthetic OFMSW is 43% higher compared to OFMSW coming from a composting full scale plant: 0.238 d⁻¹ (K = 1.391 d⁻¹; Y_pMAX = 1.167 L CH₄/gDOC_c; θ_MIN = 7.924 days) vs. 0.135 d⁻¹ (K = 1.282 d⁻¹; Y_pMAX = 1.150 L CH₄/gDOC_c; θ_MIN = 9.997 days) respectively.Finally, it could be emphasized that the validation of proposed modified-model has been performed successfully by means of the simulation of non-steady state data for the different SRTs tested with each waste.     

Impact of different particle size distributions on anaerobic digestion of the organic fraction of municipal solid waste

Particle size may significantly affect the speed and stability of anaerobic digestion, and matching the choice of particle size reduction equipment to digester type can thus determine the success or failure of the process. In the current research the organic fraction of municipal solid waste was processed using a combination of a shear shredder, rotary cutter and wet macerator to produce streams with different particle size distributions. The pre-processed waste was used in trials in semi-continuous ‘wet’ and ‘dry’ digesters at organic loading rate (OLR) up to 6 kg volatile solids (VS) m^?3 day^?1. The results indicated that while difference in the particle size distribution did not change the specific biogas yield, the digester performance was affected. In the ‘dry’ digesters the finer particle size led to acidification and ultimately to process failure at the highest OLR. In ‘wet’ digestion a fine particle size led to severe foaming and the process could not be operated above 5 kg VS m^?3 day^?1. Although the trial was not designed as a direct comparison between ‘wet’ and ‘dry’ digestion, the specific biogas yield of the ‘dry’ digesters was 90% of that produced by ‘wet’ digesters fed on the same waste at the same OLR.     

Oxygen demand for the stabilization of the organic fraction of municipal solid waste in passively aerated bioreactors

Conventional aerobic waste treatment technologies require the use of aeration devices that actively transport air through the stabilized waste mass, which greatly increases operating costs. In addition, improperly operated active aeration systems, may have the adverse effect of cooling the stabilized biomass. Because active aeration can be a limiting factor for the stabilization process, passive aeration can be equally effective and less expensive. Unfortunately, there are few reports documenting the use of passive aeration systems in municipal waste stabilization. There have been doubts raised as to whether a passive aeration system provides enough oxygen to the organic matter mineralization processes. In this paper, the effectiveness of aeration during aerobic stabilization of four different organic fractions of municipal waste in a reactor with an integrated passive ventilation system and leachate recirculation was analyzed. For the study, four fractions separated by a rotary screen were chosen. Despite the high temperatures in the reactor, the air flow rate was below 0.016 m³/h. Using Darcy’s equation, theoretical values of the air flow rate were estimated, depending on the intensity of microbial metabolism and the amount of oxygen required for the oxidation of organic compounds. Calculations showed that the volume of supplied air exceeded the microorganisms demand for oxidation and endogenous activity by 1.7–2.88-fold.     

The effects of different mixing intensities during anaerobic digestion of the organic fraction of municipal solid waste

Mixing inside an anaerobic digester is often continuous and is not actively controlled. The selected mixing regime can however affect both gas production and the energy efficiency of the biogas plant. This study aims to evaluate these effects and compare three different mixing regimes, 150 RPM and 25 RPM continuous mixing and minimally intermittent mixing for both digestion of fresh substrate and post-digestion of the organic fraction of municipal solid waste. The results show that a lower mixing intensity leads to a higher biogas production rate and higher total biogas production in both cases. 25 RPM continuous mixing and minimally intermittent mixing resulted in similar biogas production after process stabilization, while 150 RPM continuous mixing resulted in lower production throughout the experiment. The lower gas production at 150 RPM could not be explained by the inhibition of volatile fatty acids. Cumulative biogas production until day 31 was 295 ± 2.9, 317 ± 1.9 and 304 ± 2.8 N ml/g VS added during digestion of fresh feed and 113 ± 1.3, 134 ± 1.1 and 130 ± 2.3 N ml/g VS added during post digestion for the 150 RPM, 25 RPM and minimally mixed intensities respectively. As well as increasing gas production, optimal mixing can improve the energy efficiency of the anaerobic digestion process.     

Effect of aeration rate and moisture content on the emissions of selected VOCs during municipal solid waste composting

The influence of the industrial control composting conditions (aeration 0.005–0.300?L_air?kg^?1 and moisture 40–70?%) of municipal solid waste on the composition of the selected compound emitted (limonene, β-pinene, 2-butanone, undecane, phenol, toluene, dimethyl sulfide, dimethyl disulfide) was studied. The highest emissions of volatile organic compounds (VOCs) were observed in the early stages of the processes. At the end of the process, low concentrations of the emitted compounds were found. Aeration rate had a strong effect on emissions. High aeration rate (0.300?L_air?kg^?1?min^?1) caused normally high emissions of all selected compounds whereas low aeration rates (0.05?L_air?kg^?1?min^?1) could cause anaerobiosis problems and generation of organic sulphur compounds. We observed that the effect of the moisture upon the emitted concentrations varied depending on the studied compound.     

Stabilisation of biodried municipal solid waste fine fraction in landfill bioreactor

The biodrying process of solid waste is a pre-treatment for the bio-stabilisation of the municipal solid waste. This study aims to investigate the fate of the municipal solid waste fine fraction (MSWFF) resulting from a biodrying treatment when disposed in landfills that are operated as bioreactors. Biodried MSWFF was apparently stable due to its low moisture content that slows down the microbial activity. The lab-scale anaerobic bioreactors demonstrated that a proper moisture content leads to a complete biodegradation of the organic matter contained in the biodried MSWFF. Using a pilot-scale landfill bioreactor (LBR), MSWFF stabilisation was achieved, suggesting that the leachate recirculation could be an effective approach to accomplish the anaerobic biodegradation and biostabilisation of biodried MSWFF after landfilling. The biostabilisation of the material resulting from the LBR treatment was confirmed using anaerobic and aerobic stability indices. All anaerobic and aerobic indices showed a stability increase of approximately 80% of the MSWFF after treatment in the LBR. The similar values of OD7 and BMP stability indices well agree with the relationship between the aerobic and anaerobic indices reported in literature.     

Efficiency of the anaerobic treatment of the organic fraction of municipal solid waste: collection and pretreatment.

This report is based on several years of co-operation between our research groups and Danish biogas plants. Throughout the years, there has been a fruitful exchange of know-how and experiences in laboratory scale on the one hand and large scale on the other, leading to a better understanding of the principles of the anaerobic digestion process and to an optimization of its large-scale implementation. In order to get an overview of the current situation concerning the treatment of the organic fraction of municipal solid waste (OFMSW) in Denmark, interviews were carried out with operators of the biogas plants where OFMSW is treated and the municipality staff responsible for waste management. With the aim of fulfilling the governmental goal to treat 150,000 tons of OFMSW by the year 2004 mainly by anaerobic digestion, the different municipalities are investigating different concepts of waste collection and treatment. The quality of the OFMSW treated is the key to smooth operation of the biogas process including a high biogas yield and production of an effluent that is feasible for use as fertilizer on agricultural land. Comparison of the different concepts leads to the conclusion that source-sorting of OFMSW in paper bags is preferable to collection in plastic bags and successive separation of plastics in a waste processing treatment plant.     

The effect of moisture on air classification of municipal solid waste

The effect of moisture on the air classification of municipal solid waste (MSW) is analysed using a 2³ factorial design. A synthetic MSW is used in the laboratory to facilitate controlled variation of composition and moisture content. A pilot scale 120 ° zig-zag air classifier is used to perform the classification tests. Ten system response variables are evaluated including six recoveries, two purities and two efficiencies, to elucidate the impacts of (1) increased particle density resulting from moisture absorption and (2) particle adhesion. Results indicate a significant decrease in paper recovery and efficiency with increasing moisture. However, no significant reduction in aluminum and iron recovery is observed. This suggests that adhesion of these particles to extracted combustible materials is not a significant factor in reducing extract purity. Instead, extract purity reduction is a result of paper recovery losses from increasing particle density. The degree of moisture effect for varied MSW composition is greater for feeds with lower fractions of adsorbent materials. A redefination of percent moisture based on the MSW absorbent fraction is suggested in order to account for this varied impact. The new definition, moisture absorbancy ratio, is the weight of the moisture in the MSW divided by the dry weight of the absorbent materials in the MSW.