Original Research Article

A life cycle comparison of different organic waste management options: The role of the energetic mix

Dr. Francesco Di Maria,
Francesco Di Maria* , Caterina Micale
 
LAR Laboratory –Department of Engineering, University of Perugia, Perugia, Italy.

Life cycle assessment was used to evaluate different options for managing the organic fraction (OF) of municipal solid waste at source segregation (SS) intensities of 25% and 52%. Incineration was shown to be the best management option for processing the OF remaining in the residual waste.

*Corresponding author:

francesco.dimaria@unipg.it

Keywords:

Aerobic Treatment, Anaerobic Digestion, Incineration, Life Cycle Assessment, Landfilling, Organic Fraction, Source Segregation.

Life cycle assessment was used to evaluate different options for managing the organic fraction (OF) of municipal solid waste at source segregation (SS) intensities of 25% and 52%. Incineration was shown to be the best management option for processing the OF remaining in the residual waste. Aerobic treatment alone or combined with anaerobic digestion (AD) for recycling/recovering the SSOF led to relevant environmental impact reduction. The sensitivity analysis performed by adopting several energetic mixes from different EU areas showed that incineration gave lower environmental benefits when the amount of renewable energy of the mix was greater than 50%. For SS=52% benefits due to AD were negligible if the amount of renewable energy in the mix was greater than 30%.
Municipal solid waste (MSW) consists of a wide range of materials, with a prominent role played by the organic fraction (OF). As reported by several authors (Buttol et al., 2007; Di Maria and Micale, 2013), OF can range from 15% ww-1 up to more than 40% ww-1 of the whole MSW generated in the EU and in similar areas. If not properly managed, it can represent a significant environmental threat due to gaseous and liquid emissions arising from biological reactivity and leaching (Di Maria et al., 2013b). In particular, for the EU-15, landfills contribute to about 3% of the whole greenhouse gas (GHG) emissions (EEA, 2011) due to the spontaneous degradation processes of bio-waste, generating CH4 and N2O, with a GHG potential 21 and 310 times higher than CO2, respectively. 
Furthermore, landfill leachate can be considered as a triphasic high polluting wastewater with COD >10,000 mg L-1, NH4 > 500 mg L-1 along with other polluting substances.
The EU Landfill Directive of April 1999 (99/31/EC) imposes a mandatory stepwise reduction of the biodegradable fraction going directly to landfills of 25%, 50% and 65%, respectively, by 2006, 2009 and 2016.
Reduction of the biodegradable fraction landfilled can be achieved by incinerating or biostabilizing it in mechanical and biological treatment (MBT) plants (De Gioannis et al., 2009; Frike et al., 2005). Incineration efficiently oxidizes the biodegradable compounds, returning mainly ash and slag. On the other hand biostabilization provides a controlled biological oxidation mainly of the rapidly biodegradable organic compounds, returning a material characterized by a significantly lower residual biological reactivity. 
Another approach in compliance with the latest EU waste framework directive (WFD) 2008/98/EC is by the recovery and/or recycling of the OF via biological treatment aimed at producing an organic fertilizer able to substitute raw materials. For achieving high recycling/recovery and high quality fertilizer both, it is necessary to have high quality OF. High quality OF can be successfully obtained from efficient source segregation (SS) collection.
On the other hand high SS rates have an impact on the various technical, economic and environmental aspects that are significantly influenced by the local situation. This has stimulated an important debate on the most sustainable compromise among SS intensity, recovery, recycling and disposal.
In a previous study Di Maria and Micale (2013) showed that the higher is the SS intensity, the higher are both fuel consumption and collection costs. Similar results were reported by Dogan and Duleyman (2003), Chose et al. (2006) and Tavares et al. (2009).
Concerning pre-treatment and disposal options for unsorted MSW, Cherubini et al. (2009) showed that MBT, with solid recovered fuel production, is the best environmental option compared to landfilling and incineration. When solid recovered fuel is not produced, Buttol et al. (2007) demonstrated that for the Bologna (Italy) management district, incineration was the best option for unsorted MSW.
Di Maria et al. (2013d) showed that for waste management scenarios based on MBT and landfilling with energy recovery, excessive MSW stabilization is not the best solution due to the consequent reduction in benefits of renewable energy production from landfill gas. Blengini (2008) used a life cycle assessment (LCA) approach to evaluate the impact and resource conservation potential of composting. Results showed that composting is more energy consuming than landfilling, but remarkable energy savings are due to the substitution of chemical fertilizer if the compost is effectively exploited in agriculture. Similarly, Lundie and Peters (2005) conducted an LCA analysis of several food waste management options. Also in this case, composting gave the best environmental figures. Albeit these results, on the basis of more than 200 LCA studies published within the first half of 2012, Laurent et al. (2014) showed that there is no definitive agreement regarding which treatment performs the best for OF. Furthermore, there are limited number of studies concerning some relevant management options for OF, such as anaerobic digestion (AD) and landfilling. This data shows that there is a lack of information about some aspects of the integrated management of OF, including different SS intensities, pre-treatments, recycling/recovery and disposal options.
In the present study, starting from an existing urban Italian waste management district (Di Maria and Micale, 2013), different management options were evaluated using an LCA approach (Di Maria and Micale 2014a,b). Particular attention was focused on the influence of the energy mix exploited in the area considered.

The present LCA study was performed according to ISO 14040 (2006) methodology and the guidelines of the ILCD Handbook (EC, 2010) were followed. 

2.1 Goal and context
The aim of this study was to conduct an impact assessment on different management options for the OF of MSW generated in an urban area located in central Italy. In this area the OF contained in the residual MSW (ROF) is managed by MBT for mass and biological reactivity reduction before landfilling. Otherwise the SSOF is composted. 
A comparison of the impact associated with the different management options can give potential useful information for the scientific community, but also for stakeholders and policy makers. 
According to the ILCD Handbook (EC, 2010) the context situation was A.

2.2 Scope
As far as waste management is concerned, the input material is waste which can be either disposed of or re-enter further life cycles to substitute for virgin material. The system that recycles the waste into a valuable product is credited with the environmental burdens of the corresponding primary production, but is charged with energy and ancillary materials used for the recycling process. The foreground system considered in this study varied depending on the management scheme, whereas the background system was not significantly influenced by those of the foreground (Fig. 1). The background was average values from the market mix. Environmental models were built by retrieving data from existing databases with respect to the scenario considered or similar ones. In particular the waste management options for the OF were influenced by the SS intensity. The current scenario is SS=25% and the target scenario is SS=52%. Changes in SS intensity affect the different waste management components: waste collection vehicle (WCV), fuel consumption, bins, liners, processes, inputs and outputs (Table 1). 

2.2.1 Functional unit
The functional unit chosen was the management of 1 ton of OF generated in the area considered. On the basis of SS intensity, a given fraction of the OF can be collected commingled with the residual waste (i.e. ROF), whereas the remaining fraction is the SSOF. ROF can be extracted from the residual waste in the mechanical sorting section of the MBT (see section 2.2.6.3). The composition of residual MSW and SSOF are reported in Table 2. Tables 3 and 4 report the chemical characterization of the ROF and the SSOF, respectively. In this study the functional unit is also the reference flow on which the analysis was performed. 

2.2.2 Selection of environmental indicators
Environmental indicators were chosen using a top-down approach (Blengini et al., 2012), according to ISO 14040 (2006) recommendations (Table 5). They are: Global Warming Potential at 100 years (GWP100); Acidification Potential (AP); Eutrophication Potential (EP); Photochemical Ozone Creation Potential (POCP); Ozone Layer Depletion Potential (OLDP); Abiotic Depletion Potential (ADP); Human Toxicity Potential (HTP) and Terrestrial Ecotoxicity Potential (TEP).
 
2.2.3 Life Cycle Inventory modeling framework and system boundaries
Incineration, AD, MBT and composting transform the inlet waste, generating products such as energy, gas and fertilizer (i.e. multi-functionality). The functional unit and the amount of waste entering the system were assumed constant. All this corresponds to an attributional Life Cycle Inventory (LCI) modeling framework (EC, 2010) and a system in expansion. The Italian energetic mix was assumed in the analysis. Italian electricity grids are connected to surrounding countries and the fraction of imported energy is about 2%. About 70% is generated by fossil fuels and the remaining fraction by renewables.
After collection, the ROF and SSOF follow two different mandatory pathways (continuous line in Fig. 1). ROF undergoes disposal operations that can be alternatively (dashed lines in Fig. 1) direct landfilling, incineration followed by slag landfilling and MBT followed by landfilling. SSOF can be processed either by composting or AD followed by composting. In both cases these processes generate organic fertilizer and residues. In the scenario with AD, electrical energy is also generated. Due to the impossibility of exploiting it in agriculture in the area considered, the liquid fraction of the digestate is processed in wastewater treatment plants (WWTP). Residues can be incinerated or landfilled. The foreground of the system is represented by the energy generated by incineration and AD, by the emissions arising from the different processes and by the organic fertilizer. 

2.2.4 Waste collection
The scenario considered was an existing residential urban area consisting of seven different collection routes (Di Maria et al., 2013a). In the reference configuration (i.e. SS=25%), total driving distance was about 190 km day-1 and the average daily MSW production was about 35.8 tons with 342 collection points (CP) (Table 6). Only paper and cardboard (19%) and light packaging (6%) were SS by road collection. The resident population was about 24,000 inhabitants. Di Maria et al. (2013a) performed different analyses for this area by varying the SS intensity, going from road to door-to-door collection. Number, volume and position of bins and liners were evaluated for each SS intensity together with number, size and fuel consumption of WCV (Table 6). CP corresponds to the number of bins. Source segregation of OF in the target scenario was 52%. LCI of bin production, maintenance and substitution every 5 years was taken into consideration according to Rives et al. (2010), whereas liners were considered biodegradable and single use. Similarly, on the basis of the respective size, WCV construction and maintenance was included, assuming an average life of 10 years. 
LCI for WCV was retrieved from the Ecoinvent v2.2 database (Hischier et al., 2010), reporting data of a German manufacturer. 

2.2.5 SSOF treatment 
The SSOF was processed with the aim of recycling/recovering by producing an organic fertilizer. This can be achieved by aerobic treatment alone or by the combination of AD followed by a successive aerobic treatment of the solid fraction of the digestate. 
Due to process loss, impurities and removal of bulky components, the mass of compost produced is usually significantly lower than the treated mass of OF. On the basis of a mass balance performed on the composting facility operating in the area considered, in the period ranging from 2006 to 2012, the amount of fertilizer produced was about 130 kg OFton-1 (Table 7).
The resulting compost contained on the average N=14.3 kg ton-1, K2O=19.3 kg ton-1 and P2O5=6.74 kg ton-1. 
The LCI for this process was retrieved from Ecoinvent v2.2 (Hischier et al., 2010), using average data for composting facilities operating in Switzerland. Considering the technological level of these facilities, the assumptions are consistent with the present study. Also the chemical composition of the SSOF (Table 4) was compared with the one used in the Ecoinvent. Humidity, organic carbon and nitrogen were substantially similar. There were slightly higher values for P and K  for the SSOF in this study, whereas Zn, Cu and Cr concentrations were quite similar. These compositions are consistent with those in the Ecoinvent database.
The AD technology considered was the dry one (Bolzonella et al., 2006). A previous study on the dry AD of the OF (Di Maria, 2012) of the same collection area showed an electrical energy production of about 220 kWh OF ton-1 (Table 7). No further recovery of the heat rejected by the co-generators was considered, except for that needed by the digesters. Both the CO2 generated by the biological process and by bio-methane combustion were considered biogenic. Management of the digestate requires a preliminary mechanical separation into a liquid and a solid fraction (Rico et al., 2011). Due to the impossibility of agronomic exploitation of the liquid fraction in the area considered, it was purified in WWTP. In accordance with Bolzonella et al. (2006), the amount of liquid to be processed in the WWTP was assumed to be 0.45 m3OF ton-1. The liquid-solid separation process led to a reduction of the amount of nutrients in the solid phase from about 20% to about 95%, depending on the digestate features and on the technology exploited. In accordance with Rico et al. (2011), the amount of nutrients recovered in the high quality fertilizer generated by AD and composting was 23% of that recovered by direct composting of the SSOF (Table 7). Data from Ecoinvent v2.2 database, reporting average values for dry AD operating in Switzerland, were adjusted and used for the LCI of this process. For the same reasons concerning the composting process, this assumption is technologically consistent with the present study.
Construction and decommissioning after a working period of 25 years were accounted for, both for composting and for AD facilities (Hischier at l., 2010).
 
2.2.6 ROF processing

2.2.6.1 Landfill
The landfill was equipped with an energy recovery system for landfill gas (LFG) with the exception of the scenario with incineration. On the basis of Di Maria et al. (2013a) the amount of electrical energy recoverable from LFG was 62kWh ton-1 for untreated ROF, whereas 33.5kWh ton-1 was recoverable for ROF pre-treated in an MBT facility (see section 2.2.6.3). The amount of LFG exploited for energy recovery was 50% of the total amount generated. The remaining fraction was assumed to be flared (6%), and partially oxidized by the top layer of the landfill (4%) (De Gioannis et al., 2009). Both CO2 generated by the biological process occurring in the landfill and that generated by LFG oxidation were considered biogenic.
The chemical characterization (Table 3) of ROF was compared with the one included in the ELCD 2.0 database (EC, 2010) used for landfill LCI. There were some differences only for Cu, Pb, Cd, Ni and Zn, which were slightly higher in ROF (Table 3). Due to the limited differences, the model was considered consistent. Similarly there was consistency in the amount of LFG generated, which could be effectively exploited for energy recovery.

2.2.6.2 Incineration
The LCI of incineration was retrieved from the Ecoinvent v2.2 database, with respect to grid incineration facilities operating in Switzerland. A typical technical configuration of Italian incineration facilities is by a grid combustor followed by a post combustion chamber and by a boiler (Turconi et al., 2011). After the boiler the combustion gases enter the gas cleaning system, which consists of dry scrubbing, pre-dusting, the injection of activated carbon for micro-pollutant removal, a chemical reactor for acid gas removal and fabric filters. Finally the gas treatment system is equipped with a Selective Catalytic Reactor for further NOx removal.  The projected size for an incinerator facility able to process the amount of RMSW generated in the management area considered ranges from 120,000 ton year-1 to 170,000 tons year-1. Furthermore, due to climatic conditions and the possible location of the facility, only electrical energy generation was considered. On the basis of the features of similar incinerators currently operating in northern Italy (ISPRA, 2013), the average net electrical efficiency was determined (Table 8). Referring to the data reported in Table 3, both the Low heating value (LHV) (kJ kg-1) and the amount of ash (kg OFton-1) were determined according to Tillman (1991). The amount of net electrical energy recoverable was about 300 kWh ROF ton-1. Also in this case the chemical characterization of the ROF was consistent with the Ecoinvent model as the considered technology.
Construction and decommissioning after an operating period of 40 years was also considered according to the Ecoinvent database. 

2.2.6.3 Mechanical and biological treatment
In the existing MBT the ROF is extracted from the residual waste and then conveyed to the aerobic biological treatment section for bio-stabilization.
The other screened MSW components can be further processed for recycling and recovery, whereas the bio-stabilized ROF is landfilled. The mass and energy balance of the MBT facility are reported in Table 7. Due to the lack of specific data, the same construction and decommissioning of the composting facility (see section 2.2.5) was assumed.

2.3 Software used for modeling
The LCA model was implemented using the SimaPro8 software (Prè Consultants, 2013). CML2 (CML, 2001) was chosen as the impact assessment method.

3.1 Impact assessment
In general, as the SS intensity increased, the impact categories decreased (Fig. 2). Even though energy and materials consumption increased for the collection activities (Table 6), the environmental incidence of this phase was marginal according to the findings of Assamoi and Lawryshyn (2012) and Blengini (2008). The scenarios in which the ROF was incinerated (i.e. 25.5, 52.2, 52.5) (Table 1) showed the lowest values for practically all the impact categories with the exception of HTP. In particular for scenario 52.2, all impact categories were negative, with the exception of EP and HTP, indicating the positive effect of renewable energy generation, able to substitute fossil fuels for the energetic mix considered.
On the other hand, due to energy consumption and process emissions (Table 7), scenarios with MBT (i.e. 25.3,52.3,52.6) had the highest environmental burden. Avoided emissions for scenario 25.2 concerning ADP, HTP and OLDP were mainly a consequence of energy recovery from AD. For the scenarios with the absence of SSOF, direct landfilling of ROF (i.e. 25.1) performed better than scenario 25.3 in which biostabilization was performed before final disposal.
The relevant role played by energy substitution is assessed in Figure 3, focusing attention particularly on scenario 25.2. The contributions to each impact category were split into two main components: “energy substitution” and “other”. Each component was then normalized to the one with the higher absolute values. Results (Fig. 3) show that the environmental burdens due to “other” for HTP and EP were reduced by “energy substitution” by about 80% and 70%, respectively. For all the other impact categories the environmental burden of the “other” was from about 15% to about 40% of the gain due to “energy substitution”. The positive role of incineration with respect to GWP was also reported by Assamoi and Lawryshyn (2012). In comparing incineration with landfilling for paper disposal, Moeberg et al. (2005) showed that GWP associated with incineration had a significantly lower value compared to landfilling. Benefits arising from incineration were also reported by Di Maria et al. (2003) and Di Maria and Micale (2014a,b).
In analyzing possible waste management options in the Peoloponnese region in Greece, Antonopoulos et al. (2013) found that the maximum environmental benefits can be achieved for the scenarios using incineration together with anaerobic digestion. Abduli et al. (2011) performed an LCA analysis of solid waste management strategies in Tehran, comparing landfilling to landfilling with composting of the organic fraction. The latter solution gave the lowest impact. 
Considering the relevant role played by energy consumption/generation, a sensitivity analysis was performed by adopting different energetic mixes from different EU areas as reported in section 3.2.

3.2 Sensitivity analysis
Table 9 reports the percentage of the different energy sources used for generating 1 kWh of electrical energy for Italy (IT) (TERNA, 2013), Denmark (DK) (Energynet, 2012), Greece (GR) and Switzerland (CH) (Hischier, 2010). The percentage of electricity imported is not included. Each mix was chosen for particular features in its composition. The Italian mix consists of about 70% fossil fuels and about 30% renewable; Denmark uses about 50% fossil fuel and 50% renewable; Greece exploits more than 85% of fossil fuel, mainly coal, and about 10% renewable; Switzerland generates more than 55% of its electrical energy from nuclear plants and more than 38% renewable. On the basis of these data the different impact categories were normalized and compared (Fig. 4). The normalization procedure was performed for each impact category using as divisor the maximum value assumed among the scenarios considered. There were relevant benefits achievable by incineration with the Greek mix. For the Swiss mix, OF incineration led to positive values for all the impact categories with exception of OLDP in scenario 25.2. For the scenarios with SS=52%, there were relevant benefits for the Greek mix in accordance with Antonopoulos et al. (2013). The lower amount of energy avoided per ton of OF and the emissions from the AD and compost process led to positive value of GWP for all the energetic mixes. Generally, also for SS=52%, the lower was the fraction of renewable energy in the mix considered, the higher was the environmental benefits and vice versa. From these results it is possible to state that for the scenario with SS=25%, incineration gave limited environmental benefits when the amount of renewable energy in the mix was higher than about 50%. For the scenario with SS=52%, the environmental benefits due to the AD and compost process appear negligible if the amount of renewables in the energetic mix was more than about 30%. There was a particularly positive environmental benefit for the nuclear source mainly due to the almost total absence of gaseous emissions.

Proper management of the organic fraction of municipal solid waste represents a key factor for improving the environmental sustainability of waste management systems. Increasing the amount of bio-waste recycled and/or recovered by the production of an organic fertilizer or soil improver leads to a reduction of the impact even if the collection phase was more intensive due to the need for source segregation. Incineration showed better environmental performances compared to mechanical and biological treatment for the fraction of organic waste commingled with residual waste. Landfilling was confirmed as the worst option for waste and organic fraction management. Adopting anaerobic digestion as pre-treatment before composting for recycling was able to reduce practically all the impact categories with the exception of global warming and photochemical ozone creation potential. Maximum environmental benefits due to energy recovery (i.e. incineration and anaerobic digestion) can be achieved for those areas in which the energetic mix exploits low amounts of renewable sources.
On the basis of these findings the following recommendations can be made:
1) Recycling is to be preferred to disposal and/or exclusive energy recovery;
2) Improving the amount and the quality of source segregation is mandatory for increasing recycling rates and efficiency;
3) Anaerobic pre-treatment before composting is preferred to composting only.
Further investigations aimed at improving the management and treatment of organic waste will concern other recycling/recovery options for the extraction of bio-chemicals and bio-materials from the bio-waste.

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Published: 02 January 2017

Reviewed By : Dr. Ariva Sugandi Permana.Dr. Md. Sohrab Hossain.

Copyright:

Copyright: © 2016 Francesco Di Maria. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.