Source-sorted Organic Fraction Of Municipal Solid Waste Research Articles

Comparison of anaerobic digestion technologies: An Italian case study

The present study focused on a comparison of two technologies applied in the mesophilic anaerobic digestion, namely a conventional wet one and a dry batch one. The considered substrate was the source sorted organic fraction of municipal solid waste (SS-OFMSW) and the input flow rate, for the study case, was 35,000 Mg/year. The analysed systems included the pre-treatment of the SS-OFMSW, anaerobic digestion, upgrading of biogas to biomethane and aerobic post-treatment for the purpose of obtaining compost. The comparison was made by the calculation of three indicators: net present value, total primary energy and CO2 equivalent emissions, with the aim of providing elements for choosing the most appropriate technology for the specific case. The results obtained demonstrated the finding of worse values by the total primary energy indicator for the dry batch technology, providing a saving of approx. 21% lower compared to the wet one. In terms of CO2 equivalent emissions, the dry batch anaerobic digestion technology provided a better indicator than the wet anaerobic digestion system. Sensitivity analysis revealed the finding of an opposite result only when high specific gas production values were assumed for wet anaerobic digestion, and low specific gas production values for the dry batch technology.From an economic perspective, the results indicated a preference for the dry batch technology due to a higher net present value and a shorter period of return of the investment. This finding was also confirmed by a Monte Carlo uncertainty analysis, showing how the dry batch system featured a 90% of possibility of achieving a higher economically sustainability versus the wet technology.

Comparison of Single-Stage and Two-Stage Thermophilic Anaerobic Digestion of SS-OFMSW During the Start-Up Phase

The accumulation of volatile fatty acids during the anaerobic digestion of biodegradable substrate such as Source Sorted Organic Fraction of Municipal Solid Waste (SS-OFMSW) is associated with an inhibitory effect on methane producing biochemical pathways. While inconclusive at times, the usage of two-stage digesters has been argued as a potential solution for this issue. In this study single-stage versus two-stage thermophilic digesters, fed with SS-OFMSW, were examined during the startup phase while increasing the loading rate from 0.5 to ~ 2 g VS L−1 d−1. While both systems exhibited a stable performance, with neutral pH and low intermediate-to-partial alkalinity, the two-stage digester exhibited better effluent quality with 79 and 57% lower average total chemical oxygen demand (TCOD) and soluble COD (SCOD), respectively. Also, upon reaching steady-state conditions, the two-stage system showed a superior performance with an overall methane content of 54%, compared to an average of 45% in the single-stage system.

Environmental and Cost Life Cycle Analysis of Different Recovery Processes of Organic Fraction of Municipal Solid Waste and Sewage Sludge

Two biological treatments for biodegradable substrates were assessed: (i) conventional co-digestion of source sorted organic fraction of municipal solid waste (SS-OFMSW) and sewage sludge (SS); (ii) preliminary dark-fermentation pre-treatment of the mixture of SS-OFMSW and SS, followed by a second step of anaerobic digestion. The produced biogas and hydrogen-rich gas are assumed to be used in an internal combustion engine to produce electricity and thermal energy. Life Cycle Assessment and Life Cycle Costing were implemented to assess the two biological treatments, which were also compared with the current treatments (separate aerobic composting of SS-OFMSW and anaerobic digestion of SS). The selected functional unit is the total annual amount of entering waste (189,000 t/y of SS-OFMSW and 15,500 t/y of SS) to an actual Italian plant, which was used as case study for this work. The Life Cycle Assessment results show that the co-treatments of SS and SS-OFMSW provide general environmental improvements with respect to the reference case study of separate SS anaerobic digestion and SS-OFMSW aerobic composting. The case based on the preliminary dark fermentation of the mixture of SS-OFMSW and SS followed by digestion always shows better indicator values than the co-digestion one. This result is mainly influenced by the higher energy recovery, which, in turn, is due to the improved specific gas production of the digestion step, after the co-fermentation pre-treatment. The Life Cycle Costing shows that both the studied systems are economically sustainable, however the case based on the preliminary dark fermentation of the mixture of SS-OFMSW and SS followed by digestion has a shorter time of return of investment and a higher net present value than the co-digestion one. For the considered study case, the preliminary dark-fermentation pre-treatment of the mixture of SS-OFMSW and SS, followed by a second step of anaerobic digestion is preferable to their simple co-digestion process both from the environmental and economic points of view.

Liquid biofuels from the organic fraction of municipal solid waste: A review

The valorisation of the organic fraction of municipal solid waste (OFMSW) in the circular economy context is of outmost importance. This review aimed to examine the state-of-the-art valorisation technologies of OFMSW as resource for bioethanol and biodiesel. Firstly, feedstock mapping was performed, where high fluctuation of parameters was revealed. Considering the mean values, the stoichiometries of mechanically and source-sorted OFMSW are C22H44O28N and C20H36O16N, respectively. The substrate's mean carbohydrates (50%) and lipids (13%) contents reflect its potential in bioethanol and biodiesel production, respectively. As far as bioethanol is concerned, this paper critically reviewed the existing variance of second-generation bioethanol production methodologies from OFMSW revealing that high bioethanol yields (over 80%) could be achieved if the treatment steps of pretreatment, enzymatic hydrolysis and fermentation are synergistically compiled. To this end, bioethanol yield could benefit from suitable enzymatic blends. Furthermore, this paper discussed about different ways of oil extraction from OFMSW and various biodiesel production techniques. Soxhlet extraction combined with alkaline transesterification emerged as the most widely applied biodiesel production line from OFMSW. The novelty of this literature review lies in the integration of both bioethanol and biodiesel production processes, in a biorefinery concept that would allow the production of both biofuels along with that of biogas. This integration scenario rendered an energy efficiency expressed as energy content in products per energy in raw materials equal to 68%, proving that OFMSW is a feedstock that could meet the sustainability criteria for biofuels production.

Impact of physical pre-treatment of source-sorted organic fraction of municipal solid waste on greenhouse-gas emissions and the economy in a Swedish anaerobic digestion system

Several methods for physical pre-treatments of source sorted organic fraction of municipal solid waste (SSOFMSW) before for anaerobic digestion (AD) are available, with the common feature that they generate a homogeneous slurry for AD and a dry refuse fraction for incineration. The selection of efficient methods relies on improved understanding of how the pre-treatment impacts on the separation and on the slurry’s AD. The aim of this study was to evaluate the impact of the performance of physical pre-treatment of SSOFMSW on greenhouse-gas (GHG) emissions and on the economy of an AD system including a biogas plant with supplementary systems for heat and power production in Sweden. Based on the performance of selected Swedish facilities, as well as chemical analyses and BMP tests of slurry and refuse, the computer-based evaluation tool ORWARE was improved as to accurately describe mass flows through the physical pre-treatment and anaerobic degradation. The environmental and economic performance of the evaluated system was influenced by the TS concentration in the slurry, as well as the distribution of incoming solids between slurry and refuse. The focus to improve the efficiency of these systems should primarily be directed towards minimising the water addition in the pre-treatment provided that this slurry can still be efficiently digested. Second, the amount of refuse should be minimised, while keeping a good quality of the slurry. Electricity use/generation has high impact on GHG emissions and the results of the study are sensitive to assumptions of marginal electricity and of electricity use in the pre-treatment.

Improving the stability of thermophilic anaerobic digesters treating SS-OFMSW through enrichment with compost and leachate seeds

This paper examines the potential of improving the stability of thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste (SS-OFMSW) by adding leachate and compost during inoculation. For this purpose, two stable thermophilic digesters, A (control) and B (with added leachate and compost), were subjected to a sustained substrate shock by doubling the organic loading rate for one week. Feeding was suspended then gradually resumed to reach the pre-shock loading rate (2gVS/l/d). Digester A failed, exhibiting excessive increase in acetate and a corresponding decrease in pH and methane generation, and lower COD and solids removal efficiencies. In contrast, digester B was able to restore its functionality with 90% recovery of pre-shock methane generation rate at stable pH, lower hydrogen levels, and reduced VFAs and ammonia accumulation.

ADM1 performance using SS-OFMSW with non-acclimated inoculums

This paper assesses the anaerobic digestion (AD) of the source-sorted organic fraction of municipal solid waste (SS-OFMSW). For this purpose, an experimental programme was implemented involving the operation and monitoring of two bench-scale anaerobic digesters, continuously fed with SS-OFMSW. The mathematical model (ADM1) was then applied to simulate the process of AD of SS-OFMSW. While start-up of the digesters was relatively slow, re-inoculation with cattle manure with effluent dilution reduced the acclimation period and achieved better stability, accommodating a feeding rate at an OLR = 2.39 kg TVS m(-3) day(-1). The high recorded methane gas production rate, reaching (0.1-2.5 m(3) CH(4)/m(3) reactor day), confirms the excellent biodegradability of the type of waste used (SS-OFMSW) and its suitability for AD. Satisfactory simulations of soluble chemical oxygen demand (COD), pH, and methane composition of biogas were obtained, whereas volatile fatty acid (VFA) concentrations in both reactors were over-predicted albeit capturing its general trend.

Anaerobic digestion potential of urban organic waste: a case study in Malmö

A study of existing organic waste types in Malmö, Sweden was performed. The purpose was to gather information about organic waste types in the city to be able to estimate the potential for anaerobic treatment in existing digesters at the wastewater treatment plan (WWTP). The urban organic waste types that could have a significant potential for anaerobic digestion amount to about 50 000 tonnes year(-1) (sludge excluded). Some of the waste types were further evaluated by methane potential tests and continuous pilot-scale digestion. Single-substrate digestion and co-digestion of pre-treated, source-sorted organic fraction of municipal solid waste, wastewater sludge, sludge from grease traps and fruit and vegetable waste were carried out. The experiments showed that codigestion of grease sludge and WWTP sludge was a better way of making use of the methane potential in the grease trap sludge than single-substrate digestion. Another way of increasing the methane production in sludge digesters is to add source-sorted organic fraction of municipal solid waste (SSOFMSW). Adding SSOFMSW (20% of the total volatile solids) gave a 10-15% higher yield than could be expected by comparison with separate digestion of sludge respective SSOFMSW. Co-digestion of sludge and organic waste is beneficial not just for increasing gas production but also for stabilizing the digestion process. This was seen when co-digesting fruit and vegetable waste and sludge. When co-digested with sludge, this waste gave a better result than the separate digestion of fruit and vegetable waste. Considering single-substrate digestion, SSOFMSW is the only waste in the study which makes up a sufficient quantity to be suitable as the base substrate in a full-scale digester that is separated from the sludge digestion. The two types of SSOFMSW tested in the pilot-scale digestion were operated successfully at mesophilic temperature. By adding SSOFMSW, grease trap sludge and fruit and vegetables waste to sludge digesters at the wastewater treatment plant, the yearly energy production from methane could be expected to increase from 24 to 43 GWh.

Operational Strategies for Thermophilic Anaerobic Digestion of Organic Fraction of Municipal Solid Waste in Continuously Stirred Tank Reactors

Three operational strategies to reduce inhibition due to ammonia during thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste (SS-OFMSW) rich in proteins were investigated. Feed was prepared by diluting SS-OFMSW (ratio of 1:4) with tap water or reactor process water with or without stripping ammonia. Three continuously stirred tank reactors were operated at 55°C with 11.4 gVS d−1 loading rate and 15 d retention time. Total ammonia nitrogen (TAN) level in the reactor fed with recirculated water alone was spiked to 3.5 and 5.5 g-N l−1 through ammonium bicarbonate additions. Dilution of SS-OFMSW with fresh water showed a stable performance with volatile fatty acids of <1g l−1 and methane yield of 0.40 m3 kg−1 volatile solids (VS). Use of recirculated process water after stripping ammonia showed even better performance with a methane yield of 0.43 m3 kg−1 VS. Recirculation of process water alone on the other hand, resulted in process inhibition at both TAN levels of 3.5 and 5.5 g-N l−1. However, after a short period, the process recovered and adapted to the tested TAN levels. Thus, use of recirculated process water after stripping ammonia would not only evade potential inhibition due to ammonia but could avoid the use of fresh water for dilution of high solids protein-rich SS-OFMSW.

Anaerobic digestion of the organic fraction of municipal solid waste: Influence of co-digestion with manure

Anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) was investigated in two thermophilic (55 °C) wet digestion treatment systems R1 and R2. Initially OFMSW was co-digested with manure with a successively higher concentration of OFMSW, at a hydraulic retention time (HRT) of 14–18 d and an organic loading rate (OLR) of 3.3–4.0 g-VS/l/d. Adaptation of the co-digestion process to a OFMSW:manure ratio of 50% (VS/VS) was established over a period of 6 weeks. This co-digestion ratio was maintained in reactor R2 while the ratio of OFMSW to manure was slowly increased to 100% in reactor R1 over a period of 8 weeks. Use of recirculated process liquid to adjust the organic loading to R1 was found to have a beneficial stabilization effect. The pH rose to a value of 8 and the reactor showed stable performance with high biogas yield and low VFA levels. The biogas yield from source-sorted OFMSW was 0.63–0.71 l/g-VS both in the co-digestion configuration and in the treatment of 100% OFMSW with process liquid recirculation. This yield is corresponding to 180–220 m 3 biogas per ton OFMSW. VS reduction of 69–74% was achieved when treating 100% OFMSW. None of the processes showed signs of inhibition at the free ammonia concentration of 0.45–0.62 g-N/l.

Two-phase anaerobic digestion of source sorted OFMSW (organic fraction of municipal solid waste): performance and kinetic study

The results of a two-phase system operated in different conditions, treating the source-sorted organic fraction of municipal solid waste (SS-OFMSW), coming mainly from fruit and vegetable markets, are presented. Hydraulic retention time (HRT) in the hydrolytic reactor and in the methanogenic reactor and also the temperature in the hydrolytic reactor (mesophilic and thermophilic conditions) are varied in order to evaluate the effect of these factors. The methanogenic reactor is always operated within the thermophilic range. Optimum operating conditions are found to be around 12 days (total system) using the mesophilic range of temperature in the first reactor. Specific gas production (SGP) in these conditions is around 0.6 m3/kg TVS. A kinetic study is also carried out, using the first and the step diffusional models. The latter gives much better results, with fitted constants comparable to other studies. Finally, a comparison with a one-phase system is carried out, showing that a two-phase system is much more appropriate for the digestion of this kind of highly biodegradable substrate in thermophilic conditions.

First order and step-diffusional kinetic models in simulating the mesophilic anaerobic digestion of complex substrates

A kinetic study was carried out on the mesophilic anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) sorted in an industrial plant, source-sorted OFMSW and different mixtures of sewage sludge and separately collected OFMSW. Two kinetic models are used in this study: the first-order and the step-diffusional proposed by the authors. Although the former is simpler to use, the constants of the latter have more general application.