Elsevier

Waste Management

Volume 38, April 2015, Pages 117-125
Waste Management

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

https://doi.org/10.1016/j.wasman.2015.01.010Get rights and content

Highlights

  • SSOFMSW pre-treatment and its impact on GHG emissions/economy in a Swedish AD system.

  • Water addition in the pre-treatment should be minimised.

  • The amount of refuse should be minimised, while keeping a good quality of the slurry.

  • Electricity use/generation has high impact on GHG emissions.

  • The results are sensitive to assumptions regarding marginal electricity.

Abstract

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.

Introduction

Waste management policies and practices in Sweden are prioritized following the waste hierarchy of the European Union’s Directive 2008/98/EC: prevention, preparation for reuse, recycling, other recovery, notably of energy, and disposal. In this context, the Swedish National Environmental Quality Objectives (Ministry of the Environment, 2013) are currently directed towards separate collection of food waste from households and restaurants, with the specific goal to collect 50% and to treat 40% of the collected waste with recovery of both energy and nutrients by 2018.

Anaerobic digestion (AD) is known to play a key role in the recovery of energy and nutrients from organic wastes, especially in the case of the source-sorted organic fraction of municipal solid waste (SSOFMSW), which has high anaerobic biodegradability (Zhang et al., 2007). Nevertheless, the heterogeneous nature of SSOFMSW with the common occurrence of non-degradable components makes physical pre-treatment, including separation and homogenisation, necessary before AD. In Sweden, the SSOFMSW is often pre-treated as to obtain a homogeneous slurry that can be anaerobically treated in continuously stirred tank reactors (CSTR). Impurities and large particles end up in a dry refuse fraction, which is normally incinerated.

Several methods for physical pre-treatments of SSOFMSW are available and their performance have been the subject of numerous investigations (Bernstad et al., 2013, Carlsson et al., 2010, Fransson et al., 2013, Hansen et al., 2007). Important physical pre-treatment performance indicators include SSOFMSW dilution level, the mass distribution between slurry and refuse, quality of slurry, energy input and maintenance costs. Efforts have been made to establish correlations between pre-treatment methods and performance indicators. In general, some techniques, such as screw presses, are associated with a good selectivity of separation resulting in a highly degradable slurry with little contaminants, but directing a larger fraction of the incoming waste into the refuse (Bernstad et al., 2013, Hansen et al., 2007). Nevertheless, other techniques that result in less refuse are often associated with less selective separation and the risk of impurities entering the AD process. Thus, improved SSOFMSW management relies on a better understanding of how the physical pre-treatment impacts on the separation and the slurry’s AD so that efficient methods can be selected (Bernstad et al., 2013).

The improvement in SSOFMSW management from a more holistic perspective, however, depends not only on the relation between efficiency of physical pre-treatment and biomethane yields from the slurry, but also on the overall environmental and economic performance of the pre-treatment-AD system as a whole. Generally, the environmental impacts and financial costs of waste management are related to processes outside the waste management system, such as generation of district heating, electricity, vehicle fuel and fertilizer (Eriksson et al., 2005), and they must therefore be analysed in well-defined systems that are more comprehensive than the pre-treatment-biogas process chain. Life cycle assessment (LCA) has been commonly used to study different options for collection and treatment of SSOFMSW and to establish the relationship between performance parameters of SSOFMSW AD and associated environmental and/or economic impacts. For example, different techniques for separate collection of household food waste with subsequent AD of the organic fraction were compared by Bernstad and la Cour Jansen (2012) using the ISO LCA approach (ISO, 2006a, ISO, 2006b), focusing on the impact categories of eutrophication potential (EP), acidification potential (AP), global warming potential (GWP) and primary energy use (PEU). A sensitivity analysis revealed that the losses of organic material in the refuse from the physical pre-treatment had a large impact on the results. The complexity of the evaluated systems often necessitates the use of computer-based models, of which for instance EASEWASTE (Christensen et al., 2007) and ORWARE (Eriksson et al., 2005) have been used for LCA in municipal solid waste management. ORWARE has also been used in a comparative economic evaluation of different waste treatment options, in which case AD of food waste was found to be potentially economically competitive (Eriksson et al., 2014). Notwithstanding this, the composition of SSOFMSW as defined in the ORWARE model and its fate in pre-treatment and AD may require further development to achieve a better process description (Sonesson and Jönsson, 1996).

The aim of this study was to evaluate the impact of the performance of physical pre-treatment of SSOFMSW on the environmental impact categories of energy use and GWP, here expressed as 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. Emphasis was given to analysing the system from a holistic approach as to provide insights into the process performance useful for biogas plant stakeholders. A new approach is presented for simulating process performance with an improved version of ORWARE by characterising the different components of SSOFMSW and describing mass flows through the physical pre-treatment and anaerobic degradation.

Section snippets

Overall approach

The study includes both experimental work to evaluate influence of pre-treatment on SSOFMSW mass transfer, and an environmental and economic systems analysis, where obtained data is used as input. Based on four selected Swedish facilities with physical pre-treatment processes treating SSOFMSW (Table 1), performance data were first used for preliminary assessment of mass balances of the treatment processes with ORWARE. Due to lack of complete data for describing the feedstock and its fate in AD

Composition and methane potentials of waste fractions

The characterised pre-treated SSOFMSW slurries contained different TS contents ranging from 9% to 19.5% (Table 3), although their VS contents varied more narrowly between 85% and 92% TS. In terms of VS composition, the slurries had a much larger fraction of fat and a lower fraction of fibres (NDF) compared to the refuse, while protein was more evenly distributed between the two. The “other VS” fraction consisted mainly of sugar and starch, plastics and a small fraction of volatile compounds

Conclusions

The new approach for simulating process performance in ORWARE with description of mass flows based on actual composition and methane potential analyses takes into account the specificity of pre-treatment to divert more easily digestible material towards the slurry. Thus, anaerobic digestion and incineration can be accurately modelled based on the different characteristics of the slurry and the refuse in a well-defined case.

The TS concentration in the slurry, as well as the distribution of

Acknowledgements

The authors gratefully acknowledge the financial support of Waste Refinery, the Swedish Research Council, Luleå University of Technology, NSR, SYSAV, Svensk Växtkraft, Renova, Profu AB and AnoxKaldnes AB.

References (36)

  • BGK

    Methods Book for the Analysis of Compost

    (2002)
  • M. Bisaillon et al.

    Klimatutvärdering av åtgärder och mål i Borås stads avfallsplan (climate evaluation of measures and goals in the waste management plan for the municipality of Borås)

    (2014)
  • M. Carlsson et al.

    Förbehandling av matavfall för biogasproduktion – Utvärdering av förbehandling med skruvpress (pre-treatment of food waste for biogas production – evaluation of pre-treatment with screw press)

    (2010)
  • T.H. Christensen et al.

    Experience with the use of LCA-modelling (EASEWASTE) in waste management

    Waste Manage. Res.

    (2007)
  • M. Dalemo

    The modelling of an anaerobic digestion plant and sewage plant in the ORWARE simulation model, report 213

    (1996)
  • Davis, J., Haglund, C., 1999. Life Cycle Inventory (LCI) of Fertiliser Production: Fertiliser Products Used in Sweden...
  • EC (European Commission), 2014. Questions and answers on the proposed market stability reserve for the EU emissions...
  • O. Eriksson et al.

    How model-based systems analysis can be improved for waste management planning

    Waste Manage. Res.

    (2003)
  • Cited by (19)

    • Feedstock pretreatment for enhanced anaerobic digestion of lignocellulosic residues for bioenergy production

      2022, Biomass, Biofuels, Biochemicals: Microbial Fermentation of Biowastes
    • The role of dry anaerobic digestion in the treatment of the organic fraction of municipal solid waste: A systematic review

      2020, Biomass and Bioenergy
      Citation Excerpt :

      Batch-operated systems also face other operational limitations, such as the loss of part of the biogas when the reactor needs to be opened to restart the batch. Overall, worldwide experiences with the aforementioned technologies show that the presence of improper or refused materials could compromise the system efficiency [22], e.g., by damaging the mechanical equipment of the digester. In the worst cases, the process can fail and even be interrupted.

    • Mechanical separation of impurities in biowaste: Comparison of four different pretreatment systems

      2020, Waste Management
      Citation Excerpt :

      Also, the area sum of impurities >2 mm must not exceed 15 cm2/L FM (RAL, 2019). Therefore, in order to remove the impurities, most biowastes must be treated mechanically prior to AD (Carlsson et al., 2015). The pretreatment also reduces the size of the particles, which increases the bioavailability and results in a homogeneous substrate, consequentially enhancing the methane production potential (Agyeman and Tao, 2014; Bernstad et al., 2013).

    View all citing articles on Scopus
    View full text