Environmental burdens of source-selected biowaste treatments: comparing scenarios to fulfil the European Union landfill directive. The case of Catalonia

Abstract

The Organic Fraction of Municipal Solid Waste (OFMSW), or biowaste, can be valorized using different treatment technologies, such as anaerobic digestion and composting or the combination of them. The use of the end products (biogas and/or compost) generates benefits over the alternative of sending waste to landfill. The European Union regulations (i.e. Landfill Directive) encourage the diversion of untreated biodegradable waste from landfilling. However, OFMSW treatment installations also produce environmental impacts that must be assessed. This paper presents different future scenarios at regional scale proposed to accomplish the Landfill Directive and their environmental assessment in terms of environmental impact categories. The geographical area under study is Catalonia (Spain). Field data obtained in previous studies undertaken in the same geographical area are used to determine the environmental burdens of the present situation in order to compare them with different future scenarios. A combined scenario to treat 921 Gg of OFMSW source selected including the increase of the quantity of biowaste treated by anaerobic digestion (61.2% of the total biowaste), the modification of composting plants to ensure the treatment of all the gaseous emissions (25.3% and 8.1% of the total biowaste treated in in-vessel composting plants and in enclosed windrows composting plants respectively) and the incorporation of home composting as a waste treatment alternative (5.4% of total biowaste) results in the lowest impact scenario considered in the present study. Energy recovery through anaerobic digestion and benefits of gaseous emissions reduction are the key factors in the impact reduction of biowaste treatment.

Keywords:

• environmental impact
• OFMSW treatment
• composting
• anaerobic digestion
• impact categories
• waste treatment scenarios

Abbreviations:
LCA	=	Life Cycle Assessment
LFD	=	Landfill Directive (European Union)
OFMSW	=	Organic Fraction of Municipal Solid Waste
VOC	=	Volatile Organic Compounds Waste Treatments:
AD	=	Anaerobic Digestion
AWB	=	Aerated Windrows Composting with gaseous emissions Biofiltration
AWC	=	Aerated Windrows Composting
CT	=	In-vessel Composting
HC	=	Home Composting
TWC	=	Turned Windrows CompostingImpact categories:
ADP	=	Abiotic Depletion
AP	=	Acidification
EUP	=	Eutrophication
GWP	=	Global Warming
ODP	=	Ozone Layer Depletion
POP	=	Photochemical Oxidation

1. Introduction

Our daily activities inevitably lead to waste generation. Specifically, in the European Union, each person generated an average amount of 1.40 kg of waste per day in 2010 (Eurostat 2013). The Landfill Directive published in 1999 by the European Union (Council of the European Union 1999) requires its Member States to reduce the quantity of biodegradable waste ending up untreated in landfill sites by adopting measures to increase and improve waste reduction, recovery and recycling. For the organic fraction of municipal solid waste (OFMSW) or biowaste, separation at the source and treatment through anaerobic digestion and/or composting appear to be the most sustainable options. The Green Paper on the Management of Bio-waste in the European Union (European Commission 2008) considers that the environmental impact of composting is mainly limited to some greenhouse gas emissions and volatile organic compounds. It also states that in composting the impact on climate change due to carbon sequestration is limited and mostly temporary, and that an adequate control of input material and the monitoring of compost quality are of great importance. Relating anaerobic digestion, the Green Paper highlights that, as this treatment is conducted in closed reactors, the emissions to the air are significantly lower and easier to control than from composting. In addition to this, every Mg of biowaste sent to biological treatment can deliver 100–200 m³ of biogas. According to the document, the energy recovery potential from biogas coupled with the soil improvement potential of residues (especially when treating separately collected biowaste) make anaerobic digestion the environmentally and economically most beneficial treatment technology. Also, home composting (HC) is considered in that document confirming that this is sometimes regarded as the environmentally most beneficial way of handling domestic biodegradable waste due to savings on transport emissions and costs. HC also ensures careful input material control and increases the environmental awareness of the users.

The use of compost improves soil structure, provides organic matter and increases its water holding capacity. On the other hand, the use of compost partially avoids the use of chemical fertilizers (production of which generates important environmental impacts (Martínez-Blanco et al. 2009). In the case of biogas, its use in electricity production avoids the consumption (and production) of electricity from potentially more polluting and non-renewable sources. Furthermore, the use of waste heat in electricity production from biogas to maintain the temperature of anaerobic digesters can reduce the consumption of external energy in waste treatment facilities even more.

However, as any industrial process, the treatment of the OFMSW inherently generates environmental impacts that must be assessed. During the process, there is energy consumption, emissions are released to the atmosphere and leachate is generated, among other impacts. These impacts can be different depending on the technologies used for the treatment of waste. However, due to the wide number of technologies and waste collection systems, it is necessary to collect real local data on each management system to generate reliable information on the environmental inventories. This information can be used to complete a Life Cycle Inventory or, in waste management systems modelling, to compare facilities, to make decisions on a specific technology or in regional greenhouse gases inventories.

Many of the studies related with the environmental impact of municipal solid waste treatments have been performed at laboratory scale (Smet et al. 1999; Komilis et al. 2004; Pagans et al. 2006). However, literature can also be found on the global impact of a specific technology or facility by using in situ measurements (Komilis & Ham 2000; Bernstad & la Cour Jansen 2011; Boldrin et al. 2011). This is the case, for example, of Blengini (2008), who used the Life Cycle Assessment (LCA) methodology to evaluate the environmental impacts of a composting plant in Italy. The results indicate that emissions generated during the composting process are mainly the large group of volatile organic compounds (VOC), methane (CH₄), nitrous oxide (N₂O) and ammonia (NH₃). All these compounds can generate environmental impacts: VOC can cause odours, as ammonia, but may also participate in photo-reactions in the atmosphere resulting in oxidizing compounds such as ozone. Methane and nitrous oxide have a high global warming potential. It is also important to determine which VOC are emitted. In this area, few studies can be found; among them, Orzi et al. (2010) determined the VOC emitted during the anaerobic digestion of the OFMSW at a full scale treatment facility.

Biological treatment processes also produce, directly and indirectly, CO₂ emissions. However, CO₂ emissions from biological processes are generally not taken into account in greenhouse gases inventories as they come from a biogenic source (Guinée 2002; Intergovernmental Panel on Climate Change, IPPC 2006), but evidently, CO₂ emissions from energy consumption (electricity or diesel) must be determined and considered. Then, the use of biogas for cogeneration (heat and electricity production) should be a key factor in the reduction of CO₂ emissions in the waste management sector.

Regarding management systems modelling, some literature can be found on municipal solid wastes, for example: EASEWASTE (Kirkeby et al. 2006), ORWARE (Sonesson et al. 1997) and WASTED (Diaz & Warith 2006), which are simulation tools that include the environmental burdens associated to waste management. LCA has also been applied to generic waste management systems (De Feo & Malvano 2009; Zaman 2010; Ionescu et al., 2013) and to MSW management systems of different cities or regions such as Wales (Emery et al. 2007), Ankara (Özeler et al. 2006), Phuket (Liamsanguan & Gheewala 2008), Corfu (Skordilis 2004) or Delaware (Kaplan et al. 2009). Other authors have focused their research on the environmental impact of the different waste collection options (Iriarte et al. 2009). Some of these works include a great effort to obtain real local data to perform the study, a point that is crucial to obtain reliable conclusions.

The objective of this work is to estimate the environmental impacts that are caused by the existing OFMSW treatment plants in Catalonia and to estimate the impact of the OFMSW treatment plants derived from future scenarios designed to fulfil the European Union Landfill Directive in terms of biodegradable waste diversion. To accomplish this objective, inventory data obtained in previous studies from four different full-scale treatment plants in the same geographical area has been used. Also, real data from HC experiments has been used.

2. Methodology

2.1 Area studied

The area under study corresponds to Catalonia, in the Mediterranean coast of Europe (North-East of Spain). Catalonia has an extension of approximately 32,000 km² and a population of 7,539,000 inhabitants (2011). In 2011, the municipal waste generation was of 4046 Gg from which 1643 Gg (39%) was source-selected. Previous and existing waste management plans in Catalonia clearly supports the source-selection of all the fractions of municipal solid wastes. Waste fractions considered in source-collection are: organic waste (OFMSW or biowaste), paper and cardboard, glass, plastics and light packaging and refuse. Regarding the OFMSW, 411 Gg were collected in 2011 (all of them source-selected) plus 105 Gg of pruning waste (Catalan Waste Agency 2011). Pruning waste is used as bulking agent during composting in some treatment plants. A complete waste classification and sorting scheme can be found in the reports published by local administrations such as the Catalan Waste Agency (2012).

Source-selected OFMSW is valorized in 22 different industrial installations including anaerobic digestion and composting plants. The total annual treatment capacity installed (2012) is of 500 Gg distributed in four different types of plants: 279 Gg were treated in anaerobic digestion plants (AD), 146 Gg in composting plants using in-vessel composting (CT), 32.5 Gg in composting plants using aerated windrows (AWC) and 42.5 Gg in composting plants using turned windrows (TWC). The treatment capacity of the OFMSW treatment plants does not include the amount of pruning waste or other material used as bulking agent. Gaseous emissions are treated in anaerobic digestion and in-vessel composting plants while aerated and turned windrows plants have no gaseous emissions treatment. In addition, the digestate obtained in anaerobic digestion plants is further stabilized by means of in-vessel composting in the same plant and electricity is obtained from biogas.

2.2 Life cycle assessment

2.2.1 General methodology

LCA is a methodology for the determination of environmental impacts associated to a product, process or service from cradle to grave, in other words, from production of the raw materials to ultimate disposal of waste. According to ISO 14040–14044 (International Organisation for Standardisation 2006), there are four main steps in a LCA study: the goal and scope definition, the inventory analysis, the impact assessment and the interpretation. In this study, the software SimaPro v. 7.1.8 (PRé Consultants 2008) was used to evaluate the environmental impacts of all waste treatment technologies considered. Only the obligatory phases defined by the ISO 14040–14044 regulation for the impact assessment (Ionescu et al. 2006), namely classification and characterization, were performed as they avoid the subjectivity involved in impact evaluation (Martínez-Blanco et al. 2009). The impact assessment method used was CML 2001, which was based on the CML Leiden 2000 method developed by the Centre of Environmental Science of Leiden University (Guinée 2002). The environmental impact categories indicators considered in all case studies were: global warming (GWP), ozone depletion (ODP), acidification (AP), photochemical oxidant formation (POF), eutrophication (EUP) and abiotic depletion (ADP). Owing to the lack of consensus in the international community, toxicity categories were not assessed (Martínez-Blanco et al. 2009).

2.2.2 Goal of the study

There were two main objectives in this environmental study: Firstly, to evaluate the current environmental impacts generated during the OFMSW treatment in Catalonia and to detect the contribution of each treatment technology on the overall impact. Secondly, to propose a coherent future scenario fulfilling the requirements of EU Landfill Directive in terms of organic waste diversion from landfill and minimizing the environmental impacts related to OFMSW treatment. The results of this study should be considered as a decision making tool, and although this study is focused on Catalonia, the results could also be used when planning new regional treatment policies.

2.2.3 Functional unit

The key functions for all the technologies considered were the management of the OFMSW. The functional unit (FU) in LCA provides a reference to which the inputs and outputs of the inventory are related and allows the comparison among systems (Ionescu et al. 2006). In this study, the functional unit (FU) selected was the management by composting or anaerobic digestion of one Mg of OFMSW.

2.2.4 System description

This study is focused on the determination of the environmental impacts related to the biological treatment processes produced in different OFMSW treatment scenarios. Fuel, electricity and water consumption as well as atmospheric emissions were completely studied.

In a previous work (Colón et al. 2012), a representative treatment installation of the main treatment technologies used in Catalonia was studied in detail to determine the environmental burdens associated with plant operation, the studied technologies were:

• Anaerobic Digestion (AD)

• Aerated Windrows Composting (AWC)

• In-vessel Composting (CT)

• Turned Windrows Composting (TWC)

The plants studied (four treatment plants) were selected after a deep discussion with the Catalan Waste Agency (Agència de Residus de Catalunya, ARC) for real representativeness, as a detailed study of all the plants in operation was out of the possibilities of the work. HC was also studied as a treatment alternative for OFMSW in low-density population areas (Colón et al. 2010; Martínez-Blanco et al. 2010). This data has been used as the basis to perform the calculations presented in this paper. A complete plant description of all treatment technologies can be found in the above-mentioned works.

Aerated and turned windrows composting plants are not provided with gaseous emissions treatment equipment. Taking into consideration the impacts that can be derived from these emissions, a new type of treatment plant (AWB) has been added to Table 1. AWB represents a theoretical configuration (not experimentally studied) where composting occurs in aerated and turned windrows placed on a closed installation with gaseous emissions treatment using biofilters. Values on real biofilter efficiencies in contaminant removal were considered to determine reduced impacts (Amlinger et al. 2008; Colón et al. 2009) and are reflected in Table 1. Energy consumption associated to biofilter operation was considered as additional impact (also reported in Table 1) and obtained from Cadena (2009).

Table 1 Inventory data obtained from the installations considered in this study (AD: anaerobic digestion; CT: in-vessel composting; AWC: aerated windrows composting; TWC: turned windrows composting, HC: home composting) previously published in (Colón et al. 2012). AWB (aerated and turned windrows with gaseous emissions treatment) data have been theoretically calculated.

Facility		AD	CT	AWC	TWC	HC	AWB
Inputs	MJ electricity	166.32	770.4	235.8	33.41	33.77	354.8 (119)^*
	MJ electricity self-generation	167.04	0	0	0	0	0
	L diesel	3.64	2.66	9	5.33	0	9
	Total MJ (electricity+diesel)	472.26	871.9	579.24	236.8	33.77	698.24
	Water (m^3)	0.12	0.56	0.02	0	0.051	0.02
Outputs	kg NH3	0.23	0.11	2	8.63	0.84	0.2 (90)^**
	Kg VOC	0.86	0.36	6.22	5.7	0.56	1.87 (70)^**
	kg N2O	0.035	0.075	0.076	0.251	0.676	0.076 (0)^**
	kg CH4	2.39 (5.4)^***	0.34	1.68	4.37	0.16	1.51 (10)^**
	kg CO2 (non-biogenic)	10.6	7.7	26.1	15.5	0.0	26.1
	kg NOx	0.120	0.088	0.297	0.176	0.000	0.297
	kg SOx	0.016	0.011	0.039	0.023	0.000	0.039
	m^3 biogas	98.9	n/a	n/a	n/a	n/a	n/a
	MJ electricity	550.08	n/a	n/a	n/a	n/a	n/a

^*Value in brackets is the surplus of energy needed for the implementation of a gas treatment in AWB plants.

^**Values in brackets are the gas treatment removal efficiency considered in AWB plants.

^***Value in brackets is the CH₄ emitted considering a 10% of fugitive emissions plus the biogas combustion (theoretically calculated).

2.2.5 System boundaries

This work is focused on environmental impacts related to different OFMSW treatment technologies, hence a ‘gate-to-gate’ approach including only inputs (e.g. raw materials, energy) and outputs (e.g. emissions) associated with the processes within the boundary of Figure 1 (dashed line) are included. The flows included in this work are (i) Energy consumption including diesel and electricity, (ii) gases emissions including both biogenic (organic matter biodegradation) and non-biogenic (fuel burning) emissions, (iii) water consumption and leachate production/treatment. Upstream activities (e.g. collection and transport of OFMSW and pruning wastes) and downstream activities (e.g. distribution and use of compost) are not part of this study and could be the object of further studies.

Figure 1 Definition and boundaries of the composting systems studied, including the main composting stages and the input and output flows considered.

As OFMSW collection/transport and the use of compost as an organic amendment can have a critical influence on final results, sensitivity analysis has been carried out in order to cover both topics.

2.2.6 Life cycle inventory

Table 1 summarizes the inventory data obtained in the above-mentioned previous studies. Some general considerations have been made to perform this study:

2.2.6.1. Energy consumption

The types of energy consumed by the composting and AD facilities were electricity, used in the aeration system, plant lighting, waste water treatment and some machineries, and diesel oil, used by tractors and trucks. The emissions from electricity consumption in plant were derived from the Ecoinvent v2 database in Simapro 7.1.8. The electricity model considers the consumption of electricity produced in Spain including production and transport of primary energy sources. The energy mix in Spain is mainly composed of coal (24.3%), nuclear (22.8%), natural gas (19.6%), hydropower (12.7%) and oil (8.4%).

2.2.6.2. Gaseous emissions

An accurate gaseous emissions sampling was undertaken in order to quantify the emissions of ammonia, volatile organic compounds (VOC), methane and nitrous oxide (the methodology can be consulted in Colón et al. 2012 and Cadena et al. 2009). Regarding CO₂ emissions from the biological treatment process, these have not been considered in impacts calculation due to the general consensus (IPPC) that CO₂ from these types of treatments is of biogenic origin and does not add to the overall emissions that contribute to global impacts (Intergovernmental Panel on Climate Change, IPPC 2006). In reference to the enclosed plants actually in operation in the geographical area studied, these present a good air capture efficiency (all enclosed buildings are in negative pressure) as they have implemented new gaseous emissions collection and treatment systems due to odour problems. Thus, no deficiencies in air capture in the installations for treatment have been considered and consequently fugitive emissions of ammonia, VOCs, methane or nitrous oxide have been considered as non-existent.

The non-biogenic emissions coming from the fuel combustion (CO₂, NO_x and SO_x) are also included in the inventory; data used for non-biogenic emissions come from Ecoinvent 2 database and from the Air pollutant emission inventory guidebook (2013).

2.2.6.3. Water consumption and leachate production

Water was used for cleaning and for irrigating the organic material in active decomposition during the composting process; it was also used to dilute the input material in AD plants and also in gas treatment processes. Leachates generated were also completely reused in the composting process. Only in AD plants, a leachate is not totally reintroduced in the process, this leachate is treated by means of a WWTP including a denitrification stage, energy consumption consumed at the WWTP is considered, while on the contrary, possible gases emissions coming from the process were not measured and are not included in the LCA. The leachate not reintroduced in the process is approximately 5% of its total production (plant manager personal communication).

2.2.6.4. Biogas production and fugitive emissions

Biogas emissions were measured only on biofilter surfaces, the fugitive emissions from other sources (pipes, pressure release from the reactor, flared biogas) have been considered close to zero following IPPC recommendations as no experimental measurements were possible (Intergovernmental Panel on Climate Change, IPPC 2006). However, some studies (Møller et al. 2009) showed fugitive emissions ranging from 0% to 10% of the total methane produced, for this reason a sensitivity analysis including the worst-case scenario (10% fugitive emissions plus the combustion of biogas) has also been included. Since 98.8 m³ biogas mg^–1 OFMSW were produced during the studied anaerobic digestion process and assuming average methane content of 65%, a total fugitive emission of 4.6 kg CH₄ mg^–1 OFMSW is considered. During combustion in the biogas engine, methane is converted to energy and CO₂, but as the combustion process is not 100% efficient some methane is left unburned and in this way contributes to the GWP – a total amount of 0.8 kg CH₄ mg^–1 OFMSW (Møller et al. 2009) is considered. Thus a total amount of 5.4 kg CH₄ mg^–1 OFMSW can be considered in the worst-case scenario. The fugitive emissions accounted for 115 kg CO₂ equiv. mg^–1 OFMSW and the combustion of biogas accounted for 20 kg CO₂ equiv. mg^–1 OFMSW.

2.2.6.5. Building, machinery and tools

The impacts derived from plant and machinery construction were not included because, in a previous study (Martínez-Blanco et al. 2010), the overall contribution in all impact categories was less than 2.5%. The above mentioned study considered that building and machinery of the facility entailed materials production, transport, waste management and a lifespan of 25 years.

2.2.6.6. Waste collection and impurities treatment

Regarding waste collection, transport and impurities treatment, these items are common in all systems studied (except HC) and the inclusion in the LCA would hide or at least minimize the contribution of the main used technologies to the final impact, making difficult the comparison among treatment technologies.

2.2.7 Allocation procedure

All burdens are allocated to the treatment of OFMSW. In AD, biogas is transformed to electricity by means of cogeneration engines. Electricity is used in the plant and the surplus of produced energy is injected to the grid, in that particular case, a subtraction of the Spanish electricity mix is used as allocation procedure.

2.2.8 Life cycle analysis

In the context of the studies referenced above, the main contributors to impact categories were: greenhouse gases emissions for GWP (mainly methane, nitrous oxide and non-biogenic carbon dioxide expressed as kg CO₂ eq. mg OFMSW^− 1); ammonia, nitrogen and sulphur oxides for AP (expressed as kg SO₂ equiv. mg OFMSW^− 1); VOC and nitrogen oxides emissions for POP (expressed as kg C₂H₄ equiv. mg OFMSW^− 1); nitrogen and phosphorous compounds released to the environment for EUP (expressed as kg PO₄^3– equiv. Mg OFMSW^− 1) and compounds affecting ozone layer depletion for ODP (expressed as kg CFC-11 equiv. mg OFMSW^–1). ADP is related to non-biotic resources consumption (fossil fuels, metals and minerals, expressed as kg Sb equiv. mg OFMSW^–1). Thus, environmental impacts of the proper biological degradation process will be mainly reflected in GWP, AP, EUP and POP due to gaseous emissions while energy consumption will contribute to GWP, ODP and ADP in a great extend. Although there are some works about odours and their possible evaluation in LCA (Marchand et al. 2013), there is not a consensus in an impact potential to reflect this environmental issue. Thus, this matter has not been included in this study.

Table 2 summarizes the values calculated for the different impact categories for each of the studied plants related to the treatment of 1 mg of OFMSW.

Table 2 Impact categories determined for OFMSW treatment plants representative of the treatment technologies implemented in Catalonia (Colón et al. 2012).

		Impact potentials
		GWP	ADP	AP	EUP	ODP	POP
Treatment technology	Gaseous emissions treatment	(kg CO2 equiv. t^–1)	(kg Sb equiv. t^–1)	(kg SO2 equiv. t^–1)	(kg PO4^3- equiv. t^–1)	(kg CFC-11 equiv. t^–1)	(kg C2H4 equiv. t^–1)
Anaerobic Digestion (AD)	Wet scrubber+biofilter	45.2 (119.9)^*	–0.16	0.16	0.11	–2.67 × 10^–7	0.36
In-vessel composting (CT)	Wet scrubber+biofilter	150	0.87	1.3	0.12	7.12 × 10^–6	0.19
Aerated windrows composting (AWC)	No	123	0.43	3.75	0.82	5.42 × 10^–6	2.59
Aerated windrow composting with biofiltration (AWB)	Biofilter	182	0.56	1.59	0.21	6.41 × 10^–6	1.22
(theoretical)
Turned windrows composting (TWC)	No	196	0.14	14	3.08	2.37 × 10^–6	2.38
Home composting (HC)	No	209	0.04	1.4	0.3	3.05 × 10^–7	0.23

Notes: GWP: global warming potential; ADP: abiotic depletion potential; AP: acidification potential; EUP: eutrophication potential; ODP: ozone layer depletion potential; POP: photochemical oxidation potential.

^*Value in brackets is the GWP considering a 10% of fugitive emissions plus the biogas combustion (theoretically calculated).

As can be seen in Table 2, negative impacts have been obtained in some categories (ADP and ODP) derived from energy recovery from biogas in the anaerobic digestion plant.

2.3 OFMSW treatment scenarios definition

In order to accomplish the goals of this study, two main scenarios have been considered to calculate the environmental burdens of OFMSW treatment in Catalonia: Scenario 0 corresponds to the current situation and the results obtained from it will show the current environmental impact generated during the OFMSW treatment and the contribution of each specific treatment technology in the overall impact. Scenario LFD (Landfill Directive Scenario) represents a hypothetical future situation fulfilling the requirements of EU Landfill Directive in terms of organic waste diversion from landfill. Using the results obtained from Scenario 0, successive coherent sub-scenarios (sAD, sAWB, sHC and Combined Scenario) have been defined as impact reduction proposals to Scenario LFD and will be presented later in Results and Discussion Section.

Figure 2 provides a schematic representation of all the scenarios considered in this study.

Figure 2 Waste treatment scenarios considered in the study.

2.3.1 Scenario 0

This Scenario reflects the situation in 2012 where 22 installations were in operation treating source-selected OFMSW. The impacts associated to these plants have been determined on the basis of the above presented data (Table 2). The number of installations in operation for each treatment technology and the total design capacity are presented in Table 3.

Table 3 Scenario 0: OFMSW treatment technologies, design capacity per type of plant, percentage of the total waste treated (in brackets), impact categories of the installations in operation and % contribution to the total impact (in brackets) (AD: anaerobic digestion; CT: in-vessel composting; AWC: aerated windrows composting; TWC: turned windrows composting).

			Impact potentials
		Total design capacity (Gg year^–1) (% total)	GWP	ADP	AP	EUP	ODP	POP
Treatment technology	Number of plants		(kg CO2 eq year^–1)	(kg Sb eq year^–1)	(kg SO2 eq year^–1)	(kg PO4^3- eq year^–1)	(kg CFC-11 eq year^–1)	(kg C2H4 eq year^–1)
AD	5	279	1.26 × 10^7 (3.35 × 10^7)^*	–4.46 × 10^4	4.46 × 10^4	3.06 × 10^4	–7.46 × 10^–2	1.00 × 10^5
		(55.8%)	(26.9%) (49.5%)^**	(–43,9%)	(4.9%)	(14.9%)	(5.9%)	(31.85)
CT	7	146.2	2.19 × 10^7	1.27 × 10^5	1.90 × 10^5	1.75 × 10^4	1.04	2.78·10^4
		(29.2%)	(46.8%) (32.3%)^**	(124.9%)	(21%)	(8.53%)	(82.8%)	(8.9%)
AWC	3	32.5	4.00 × 10^6	1.40 × 10^4	1.22 × 10^5	2.66 × 10^4	1.76 × 10^–1	8.42 × 10^4
		(6.5%)	(8.6%) (5.9%)^**	(13.8%)	(13.5%)	(13.0%)	(14.0%)	(26.8%)
TWC	7	42.5	8.33 × 10^6	5.95 × 10^3	5.95 × 10^5	1.30 × 10^5	1.01 × 10^–1	1.01 × 10^5
		(8.5%)	(17.8%) (12.3%)^**	(5.2%)	(60.5%)	(63.4%)	(9.1%)	(32.5%)
Total	22	500	4.68 × 10^7 (6.77 × 0^7)^*	1.02 × 10^5	9.03 × 10^5	2.05 × 10^5	1.26	3.14 × 10^5

Notes: GWP: global warming potential; ADP: abiotic depletion potential; AP: acidification potential; EUP: eutrophication potential; ODP: ozone layer depletion potential; POP: photochemical oxidation potential

^*Value in brackets is the GWP considering a 10% of fugitive emissions (theoretically calculated).

^**Values in brackets are the GWP percentage including a 10% of fugitive emissions plus the biogas combustion coming from AD facilities.

Some assumptions have been made to calculate the values of the impact categories. First, it has been supposed that all the plants using the same treatment technology will produce the same impacts per Mg of OFMSW treated. Obviously, even with the same technology and presenting a very similar layout, each plant has some particularities. However, the detailed study of all the individual plants in terms of environmental burdens calculation is beyond the scope of this study. The installations used were chosen as they were representative of each technology, including in the representativeness the fact that they are treating the same type of waste produced in the same region. It has been stated that the geographical variability of the waste characteristics is an important source of errors when inventory data is used from global databases (Fricke et al. 2005). It has also been assumed that all the plants are operating at their design capacity (500 Gg y^–1 in total).

2.3.2 Scenario LFD

This is a hypothetical scenario treating 921 Gg of source-selected OFMSW, which is equivalent to the 55% of the total OFMSW generated in 2011 including a 15% of impurities. This scenario would permit to fulfil the requirements of European Union Landfill Directive (Council of the European Union 1999) relating untreated biodegradable waste diversion from landfills. In addition to the total OFMSW 2011 treatment capacity (500 Gg year^–1), three installations were planned to enter in operation at the end of 2014: two anaerobic digestion plants (with a design treatment capacity of 20 and 23 Gg year^–1) and one in-vessel composting plant (15 Gg year^–1), reaching a total annual treatment capacity of 558 Gg year^–1. This capacity is far from the treatment capacity requirements proposed in Scenario LFD. Additional installations need to be built or some of the actual treatment plants treating mixed municipal solid wastes (MSW) will have to be adapted to OFMSW treatment.

3 Results and discussion

3.1 Scenario 0

For each treatment technology, Table 3 presents the number of installations in operation, the total design capacity and the values of the impact categories calculated on a yearly basis using data from Table 2 and the design treatment capacity of the installations per year.

As can be seen in Table 3, the contribution of AD plants to GWP is of 27.1% while a 56% of the total OFMSW is treated in this type of plants. The contribution of AD plants to GWP is not proportional to the amount of waste treated, mainly due to energy recovery from biogas. AWC and TWC present a high contribution regarding the amount of waste treated, mainly due to the emissions of methane and nitrous oxide that are not treated. These compounds have a high global warming capacity (25- and 296-fold higher than that of carbon dioxide). In CT plants, the high consumption of energy in gaseous emissions treatment and in-vessel aeration results in 47.1% of contribution to the total GWP and 87% of the ADP. As commented in Table 2, biogas conversion in AD plants can be incorporated to the impact categories as avoided impact giving negative values for ADP and ODP.

Municipal Waste Treatment plants without gaseous emissions treatment (AWC and TWC) are the main responsible agents for AP and EUP values, with a total contribution of 74% and 81% to each potential, respectively. The same situation can be observed in the case of POP due to VOC emissions. In this case, the contribution of AWC and TWC plants is 60%. These data demonstrate the contribution of the gaseous treatment equipments to the reduction of the impact of the OFMSW treatment plants in some of the impact categories. However, the energy required by these equipments results in higher contributions to GWP, ADP and ODP, as occurs in the case of CT plants. Biogas recovery in AD plants and the existence of gaseous emissions treatment result in relatively lower impact potential values.

If we take into account the sensitivity analysis of the AD fugitive emissions, the contribution of AD plants to GWP is 49.7%, slightly below the amount of OFMSW treated using this technology (56%), and the total GWP is increased by 45%.

3.2 Scenario LFD

As stated above, Scenario LFD is a hypothetical scenario that would permit to fulfil the requirements of European Union Landfill Directive (Council of the European Union 1999) concerning untreated biodegradable waste diversion from landfills. In this case, the actual treatment capacity installed for OFMSW cannot fulfil the requirements of the scenario. Thus, additional treatment installations will have to be constructed or some of the existing mixed municipal solid wastes (MSW) treatment plants will have to be adapted to treat OFMSW. Today, 70% of the installed MSW treatment capacity corresponds to in-vessel stabilization installations. In fact, if the amount of source-selected organic waste increases it is expected that the quantity of mixed MSW will decrease. The additional OFMSW treatment capacity required by Scenario LFD is of 363 Gg year^–1. It has been supposed that this capacity is reached by adapting existing MSW in-vessel composting plants, which today have more than this capacity to treat mixed MSW. Thus, in LFD Scenario, 322 Gg will be treated in anaerobic digestion plants (279 Gg from Scenario 0 plus 43 Gg in the installations under construction), 525 Gg will be treated in in-vessel composting plants (146 Gg from Scenario 0 plus the additional 363 Gg) while AWC and TWC plants will maintain the same treatment capacity as in Scenario 0.

Table 4 presents the impact potential values calculated for Scenario LFD as well as the amount of waste treated using the different treatment technologies. Scenario LFD is supposed to reduce the impacts associated with the disposal of OFMSW in landfills that occurs in Scenario 0 although these impacts have not been considered in this study. According to Villalba et al. (2012), 744–kg CO₂ equiv are emitted per Mg of waste landfilled (with biogas recovery). Using this value and the difference in OFMSW treated under Scenario 0 and Scenario LFD (421 Gg year^− 1), 3.13 × 10⁸ kg CO₂ will be not be emitted per year from landfills. On the other side, 5.96 × 10⁷ kg CO₂ y^–1 will be emitted from the OFMSW treatment process, the net result is a decrease of 2.54 × 10⁸ kg equiv. CO₂.

Table 4 Impact categories under Scenario LFD (Landfill Directive requirements) and % contribution to the total impact (in brackets), quantity of waste treated per type of plant and percentage of total waste treated (in brackets) (AD: anaerobic digestion; CT: in-vessel composting; AWC: aerated windrows composting; TWC: turned windrows composting).

		Impact categories
	Waste treated (% total) (Gg year^–1)	GWP	ADP	AP	EUP	ODP	POP
Treatment technology		(kg CO2 eq year^–1)	(kg Sb eq year^–1)	(kg SO2 eq year^–1)	(kg PO4^3- eq year^–1)	(kg CFC-11 eq year^–1)	(kg C2H4 eq year^–1)
AD	322	1.46 × 10^7 (3.86 × 10^7)^*	–5.15 × 10^4	5.15 × 10^4	3.54 × 10^4	–8.60 × 10^–2	1.16 × 10^5
	(34.9%)	(13.8%) (29.8%)^**	(–12.1%)	(3.6%)	(13.82%)	(2.2%)	(28.9%)
CT	524.5	7.87 × 10^7	4.56 × 10^5	6.82 × 10^5	6.29 × 10^4	3.73	9.97 × 10^4
	(56.9%)	(74.6%) (60.7%)^**	(107.4%)	(47.1%)	(24.6%)	(95.1%)	(24.9%)
AWC	32.5	4.00 × 10^6	1.40 × 10^4	1.22 × 10^5	2.66 × 10^4	1.76 × 10^–1	8.42 × 10^4
	(3.5%)	(3.1%) (3.0%)^**	(3.3%)	(8.4%)	(10.4%)	(4.5%)	(21.0%)
TWC	42.5	8.30 × 10^6	5.93 × 10^3	5.93 × 10^5	1.31 × 10^5	1.00 × 10^− 1	1.01 × 10^5
	(4.6%)	(7.9%) (6.4%)^**	(1.4%)	(40.9%)	(51.2%)	(2.6%)	(25.2%)
Total	921	1.06 × 10^8 (1.30 × 10^8)^*	4.25 × 10^5	1.45 × 10^6	2.56 × 10^5	3.93	4.01 × 10^5

GWP: global warming potential; ADP: abiotic depletion potential; AP: acidification potential; EUP: eutrophication potential; ODP: ozone layer depletion potential; POP: photochemical oxidation potential

^*Values in brackets are the surplus of GWP due to CH₄ emitted considering a 5 % of fugitive emissions (theoretically calculated).

^**Values in brackets are the GWP percentage including the fugitive emissions plus the biogas combustion coming from AD facilities.

In Scenario LFD, 57% of OFMSW is treated in in-vessel composting plants while anaerobic digestion installations deal with 35% of the material. CT plants are the main responsible agents for the values of GWP (74%), ADP and ODP (96% and 93% respectively). In addition, composting plants without gaseous emissions treatment (AWC and TWC) contribute 68% to EUP. Regarding total values, the increment in OFMSW treated in Scenario LFD in comparison to Scenario 0 is 84% while the increment in the impact potential values is higher for GWP (138%), ADP (308%) and ODP (212%), where the contribution of CT plants is more relevant, than for AP (62%), EUP (20%) and POP (28%). Thus, the increment in some impact potential values is not proportional to the increment in the amount of waste treated. Analyzing these results, some changes in Scenario LFD are proposed to improve the environmental performance of the OFMSW treatment system.

If we take into account the sensitivity analysis of the AD fugitive emissions, the contribution of AD plants to GWP is 29.8%, slightly below the amount of OFMSW treated using this technology (34.9%), and the total GWP is increased by 23%.

3.3 Impact reduction proposals to Scenario LFD

Taking into account the results obtained for the impact categories in each treatment technology, three different scenarios are proposed to reduce the impact predicted in Scenario LFD. The amount of OFMSW to be treated to fulfil the Landfill Directive requirements is maintained. For each scenario, three sub-scenarios are designed and studied. Finally, a combined scenario including the lowest impact options is proposed. Table 5 summarizes the OFMSW treated in the different type of installations under the scenarios and sub-scenarios proposed. Percentages treated under each technology are also presented in Table 5.

Table 5 OFMSW treated by each treatment technology under the different scenarios and sub-scenarios considered (total OFMSW treated = 921 Gg year^–1) and percentage of total waste (in brackets) (AD: anaerobic digestion; CT: in-vessel composting; AWC: aerated windrows composting; TWC: turned windrows composting; AWB: aerated windrows with gaseous emissions biofiltration; HC: home composting).

	Treatment technology	OFMSW treated in each technology (Gg) and percentage over total (in brackets)
Scenario		Sub-scenario 1	Sub-scenario 2	Sub-scenario 3
sAD	AD	402.5 (43.7)	483.0 (52.4)	563.5 (61.2)
	CT	444.0 (48.2)	363.5 (39.5)	283.0 (30.7)
	AWC	32.5 (3.5)	32.5 (3.5)	32.5 (3.5)
	TWC	42.4 (4.6)	42.4 (4.6)	42.4 (4.6)
sAWB	AD	322.0 (34.9)	322.0 (34.9)	322.0 (34.9)
	CT	524.5 (56.9)	524.5 (56.9)	487.0 (52.9)
	AWC	32.5 (3.5)	0.0 (0)	0.0 (0)
	TWC	0.0 (0)	0.0 (0)	0.0 (0)
	AWB	42.4 (4.6)	74.9 (8.1)	112.3 (12.2)
sHC	AD	322.0 (34.9)	322.0 (34.9)	322.0 (34.9)
	CT	499.5 (54.2)	474.5 (51.5)	449.5 (48.8)
	AWC	32.5 (3.5)	32.5 (3.5)	32.5 (3.5)
	TWC	42.4 (4.6)	42.4 (4.6)	42.4 (4.6)
	HC	25.0 (2.7)	50.0 (5.4)	75.0 (8.1)

3.3.1 Scenario sAD

Anaerobic digestion plants present negative values (impacts avoided) for some of the impact categories due to energy recovery from biogas. To enhance environmental performance of the treatment system, a scenario increasing the quantity of OFMSW treated in AD installations is proposed. Fugitive emissions have not been considered in the sAD Scenario.

In Scenario sAD, part of the waste treated in in-vessel composting plants in Scenario LFD, is treated in anaerobic digestion plants. This alternative would permit reducing the values of GWP, ADP and ODP by means of biogas recovery. Three sub-scenarios are studied (Table 5): Sub-scenario 1, with 25% of increment in AD plants treatment capacity with regard to Scenario LFD resulting in 402.5 Gg OFMSW treated in this type of installations; Sub-scenario 2, with 50% (483 Gg); and Sub-scenario 3 with 75% (563 Gg). This would be possible by adapting the existing MSW treatment plants or with the construction of new installations.

The results of the impact potential values for the three sub-scenarios are presented in Figure 3a as percentages of the values obtained in Scenario LFD (100% bars, in black, correspond to Scenario LFD). As seen in Figure 3(a), the three sub-scenarios considered reduce progressively the impact potential values except for POP. Main reductions in impact potential values are observed for ADP and ODP as expected, being also important for GWP and AP. EUP value remains almost constant. Minimum values are obtained considering Sub-scenario 3 with 24% reduction in GWP, 58.6% in ADP, 19% in AP, 2.2% in EUP and 45.4% in ODP. The increase calculated in the case of POP is of 10.2%. Thus, among the studied options in Scenario AD, Sub-scenario 3 is recommended to increase the environmental performance of the treatment system: 563 Gg OFMSW treated in AD plants, 283 Gg in CT plants, 32.5 Gg in AWC plants and 42 Gg in TWC plants.

Figure 3 Impact potential differences (%) with regard to Scenario LFD (100%) for each of the alternative scenarios proposed: (a) Scenario Anaerobic Digestion (AD), (b) Scenario Aerated Windrows Composting with Biofiltration (AWB), (c) Scenario HC HC.

3.3.2 Scenario sAWB

The main contributors to AP and EUP in biological treatment plants are ammonia emissions while VOC emissions are accounted in POP. Methane and nitrous oxide emissions have also an important contribution to GWP in plants without aeration of the composting waste (TWP). Good performance of biofilters in ammonia and VOC removal has been reported (Colón et al. 2009). Additionally, gaseous emissions treatment will reduce the impacts derived from odour emissions that are not reflected in impact potential values but cause nuisance and public opposition to waste treatment plants. At the same time, aeration and gaseous emissions treatment increases energy requirements.

In Scenario sAWB, open composting plants progressively incorporate gaseous emissions treatment equipment (Table 5). Three sub-scenarios are also considered: In Sub-scenario 1, turned windrows composting plants (TWC) incorporate aeration and gaseous emissions collection and treatment (AWB plants that will treat 42 Gg); in Sub-scenario 2, aerated windrows composting plants (AWC) are also confined and gaseous emissions are treated (AWB plants, 75 Gg); and finally, in Sub-scenario 3, part of the waste treated in in-vessel composting plants is treated in AWB plants (treatment capacity of AWB plants is increased 50%, treating 112 Gg). Impact categories for AWB plants have been theoretically calculated as explained and reported in Tables 1 and 2.

Results obtained for Scenario sAWB are presented in Figure 3(b). Relevant effects are produced in AP and EUP values under the three sub-scenarios considered when compared to Scenario LFD, with the highest reduction achieved in Sub-scenario 2 (41% and 45%, respectively). Also Sub-scenario 2 results in the lowest values for POP (23.4% reduction). ADP does not change significatively, ODP increases slightly (5.2%) while GWP remains almost constant in Sub-scenario 1, 2 and 3. These results reflect the effect of gaseous emissions treatment (decrease in AP, EUP and POP values) and the contribution of energy requirements due to this treatment (increase in GWP, ADP and ODP). However, the positive effects of gaseous emissions treatment are higher than the negative effects of increasing energy demands with Sub-scenario 2 presenting the best environmental performance: 322 Gg OFMSW treated in AD plants, 524 Gg in CT plants and 74 Gg in AWB plants.

3.3.3 Scenario sHC

In this scenario, HC is included as an OFMSW treatment option. In previous works, HC has been demonstrated as an alternative for OFMSW self-managing obtaining good quality compost and low environmental impact (Colón et al. 2010; Andersen et al. 2011). HC presents high values in some impact categories as GWP, AP and EUP due to ammonia, methane and nitrous oxide emissions (Table 2). However, from a global point of view, HC permits the diversion of the OFMSW from municipal waste collection circuits, which is supposed fossil fuel savings. Collection and transport of wastes have not been included in this study but it should be the object of further studies related to the overall waste management planning and environmental impact.

Three sub-scenarios have also been defined for Scenario sHC (Table 5): Sub-scenario 1 considers that 25 Gg of OFMSW are treated in HC bins instead of in an in-vessel composting plant; in Sub-scenario 2, the amount of OFMSW home composted is of 50 Gg and, finally, in Sub-scenario 3 this quantity increases to 75 Gg. As stated above, HC has been proposed as an OFMSW treatment alternative in low-density population areas. In the region under study, approximately 235,000 inhabitants are distributed in low-density population areas (under 40 inhabitants per km²). Currently there are 416 municipalities in Catalonia with HC programmes with an estimated number of 20,000 HC bins, although there is no reliable data on the amount of OFMSW treated using home composting, this management alternative is actually implemented with a growing number of users.

Impact potential values obtained for the different sub-scenarios in relation to Scenario LFD are presented in Figure 3(c). ADP and ODP decrease progressively showing the positive effect of HC inclusion in waste treatment systems. ADP is reduced 15% in Sub-scenario 3 referred to Scenario LFP while the reduction in OPD is 13%. This effect is due to the low energy demands of the HC process. On the contrary, GWP and EUP present slightly higher values (4% and 5%, respectively). No effect is detected in AP and POP. In general, Scenario HC produces few variations in impact categories as the main effect can be expected if waste collection and transport are considered. In addition to this, the level of implementation of HC is highly dependent on the local characteristics of each region; therefore, an accurate study about the possibilities of HC should be locally performed to have reliable conclusions about implementation, costs, participation, etc.

3.4 Combined scenario

This scenario includes the sub-scenarios that lead to a better environmental performance of the waste treatment system. In Scenario sAD, the maximization of the waste treated by anaerobic digestion (Sub-scenario 3) including biogas recovery has been pointed as the best option. Also, in Scenario sAWB, converting all TWC plants into AWC plants and the installation of biofilters in all the open plants (becoming AWB plants) to ensure gaseous emissions treatment lead to the lowest impact situation. Important changes in impact potential values have not been detected in Scenario sHC but the inclusion of the self-managing option for OFMSW is considered as a good option from an overall waste management point of view.

Thus, Combined Scenario will include the treatment of 563 Gg year^− 1 of OFMSW in AD plants (61.2% of total OFMSW), 233 Gg year^− 1 in CT plants (25.3% of total OFMSW), 75 Gg year^− 1 in AWB plants (8.1% of total OFMSW) and 50 Gg year^− 1 by HC (5.4% of total OFMSW). The results obtained in this scenario compared to those of Scenario LFD are presented in Figure 4 where positive effects in all the impact categories considered in this study are shown. The Combined Scenario implies a reduction of 20% in GWP and of 13% in POP. Higher reductions are achieved for ADP (63%), AP (60%), EUP (53%) and ODP (49%).

Figure 4 Impact potential variation for the Combined Scenario in reference to Scenario LFD (100%). It is also included the sensitivity analysis including 10% of fugitive emissions plus the biogas combustion for the GWP analysis.

It is also interesting to compare the impact categories obtained for the Combined Scenario to those calculated for Scenario 0 considering the difference on the total OFMSW treated in the two scenarios. The amount of OFMSW treated in the Combined Scenario represents 84% of increment with regard to Scenario 0 (as was in Scenario LFD). However, in the Combined Scenario, ADP increases 50%, ODP 59% and POP 11% while in LFD Scenario the increments were 308%, 212% and 28%. Thus, the increase in these impact categories in the Combined Scenario is lower than the increment in the quantity of OFMSW treated. It is remarkable that AP and EUP values decrease in Combined Scenario with respect to Scenario 0 reaching 35% and 47% lower values, respectively. GWP increases 90%, which is close to the increase in the waste treated and clearly under the 138% increment detected in Scenario LFD, even if methane fugitive emissions are considered. Combined Scenario, in consequence, leads to lower global impacts of the OFMSW treatment system related to the increment in the amount of waste treated. Although it is obvious that the impact of Scenario LFD or the Combined Scenario will be higher than that of Scenario 0 due to the increment in OFMSW treated, the new scenarios will permit to fulfil EU Landfill Directive. Also, as stated above, in Scenario LFD and, in consequence, in the Combined Scenario the amount of CO₂ equiv. emitted will be lower than that predicted if OFMSW is disposed in landfill sites.

In summary, the implementation of the Combined Scenario would require the modification of part of the existing MSW treatment plants or the construction of new installations to reach the additional yearly treatment capacity of 421 Gg OFMSW with respect to Scenario 0. Clearly, AWC and TWC plants will require confinement and gas treatment equipment while maintaining their treatment capacity (75 Gg year^− 1). In addition to the capacity of the two new anaerobic digestion plants that are predicted to be operating at the end of 2014 (430 Gg OFMSW y⁽¹), 241 Gg year^− 1 will have to be treated by anaerobic digestion. Operating installations have a yearly treatment capacity from 30 to 260 Gg. Thus, six new AD plants will be needed if small to medium capacity plants (40 Gg year^− 1) are built. The construction of medium size treatment plants will permit the distribution of them through all the studied area minimizing the impacts and costs of waste transport. In the case of in-vessel composting plants, the additional treatment capacity is 86 Gg year^− 1. In this case, existing installations have capacities from 4 to 20 Gg year^− 1 except a single 75 Gg year^− 1 installation; consequently, five new 20 Gg year^− 1 installations will be required. However, keeping in mind the social impact that the construction of new waste treatment plants generates, the modification of the actual MSW treatment installations is recommended, when possible. In addition to social impact, this action will also decrease the impact and economic costs of building new plants and demolition of the existing ones.

3.4.1 Sensitivity analysis

3.4.1.1 Transport – Distance from collection point

So far, transportation has not been included in the LCA because the goal of this study was to highlight the importance of the available technologies. As shown in the combined scenario, AD installations are recommended. Up to now, AD facilities with a treatment capacity ranging from 50 to 100 Gg are mainly constructed in Catalonia, in that sense, a question arises, ‘what is better: large central efficient waste handling facilities (AD) or several smaller less efficient units (CT) (less transport distance required).’

In this sensitivity analysis, both urban transport collection and transport intercity to the plant were considered (Iriarte et al. 2009). A 21 ton MAL lorry specifically designed for waste collection was considered (Ecoinvent 2). Impacts of return trips made by the trucks were also attributed.

A comparison of LFD Scenario and the Combined Scenario including transport has been done. For LFD Scenario an average distance from the collection point to waste treatment facilities of 10 km is considered (Martínez-Blanco et al. 2010). For the Combined Scenario three hypothesis have been done: (i) for the existing plants (AD, CT and AWB), 10 km is still considered as an average distance from collection points, (ii) distance from collection points to new AD treatment facilities is increased up to 20 km and (iii) no transportation is considered for home composting.

The results show that when the transport is included, the impact reduction achieved in the Combined Scenario in GWP, OLDP and POP is strongly reduced. In that sense, only a decrease of 3% and 1% are achieved in terms of GWP and OLDP, respectively, and an increase of 9% is reported in POP. The remaining categories do not show any significant variation when transport is included and the avoided impacts are still present in AP, EUP and ADP.

In conclusion, if the difference in the distance from collection points to waste treatment facilities is close or more than 10 km the avoided impacts are strongly reduced and it is not recommended to install AD facilities instead of CT. From an environmental point of view, if differences are less than 10 km AD facilities are a better choice.

3.4.1.1 Use of compost as organic amendment

Martínez-Blanco et al. (2013) identified nine environmental benefits of compost application in soils: (i) nutrient supply, (ii) weed, pest and disease suppression, (iii) carbon sequestration, (iv) increase in crop yield, (v) prevent soil erosion, (vi) improved soil workability, (vii) changes in soil biodiversity and (ix) improved crop nutritional quality. Most of these benefits have been so far excluded from LCA studies, mainly because of scarcity of data or lack of appropriate impact assessment methods (Martínez-Blanco et al. 2013).

Among all the above-mentioned benefits only nutrient supply and carbon sequestration are widely included in LCA studies. The amount of fertilizers substituted by compost strongly depends on the nutrient content of the compost, their utilization rate, the crop needs and the nutrient pool in the soil and thus is not included in this LCA. Regarding carbon sequestration, an avoided impact of 88 kg CO₂ equiv./t compost is reported (Sevigné Itoiz et al., 2013) when used as an organic amendment. The total estimated compost production in Catalonia (LFD and Combined Scenario) should range from 180 to 220–Gg year^–1, which implies a reduction in GWP ranging from 15% to 18% in LFD Scenario and from 18% to 23% in the Combined Scenario.

4. Conclusions and remarks

The main conclusion of this study is that the environmental performance of the different OFMSW treatment technologies should be included as a decision criterion in waste management planning. The performance of the technologies selected (industrial composting, anaerobic digestion followed by digestate composting and home composting) must be carefully considered before decision on installation planning and construction.

It is worth to remark that the data used in this work have been previously obtained in in-situ studies of treatment plants placed in the same geographical area avoiding some of the uncertainty related to the characteristics of the waste treated.

The results obtained in the case study presented in this work highlight the importance of anaerobic digestion in reducing environmental impacts of the OFMSW treatment. Energy recovery from biogas allows savings in the related potential impacts. The treatment of gaseous emissions in waste treatment plants is also important. It demonstrates positive effects in spite of the surplus energy required in this operation. In addition to the reduction calculated in impact categories, the effect expected in odour emissions has to be also considered, although not included in any environmental impact category; this raises a question that needs further studies.

It should also be highlighted that there are economical and social constraints regarding waste management planning that have not been considered in this study. The cost of the different treatment options, the importance of the waste collection system and the source selection process as well as social acceptance required for HC implementation are extremely important factors.

This study is focused on the environmental impacts related to the biological treatments of OFMSW not considering other issues as, for instance, compost application benefits. An extended environmental impacts study should include, in addition to the waste treatment environmental burdens presented in this paper, the impacts of waste collection and transportation, the impacts of the OFMSW impurities landfilling and the benefits of the use of compost. Although some of these points could be included and properly estimated (the case of transportation, for example), some others (such as compost benefits) need still more research and there is actually no consensus on how to include them in the impact studies.

Acknowledgements

This study was financially supported by the Spanish Ministerio de Ciencia e Innovación (Project CTM2009-14073-C02-01) and the Agència de Residus de Catalunya. Caterina Maulini and Michele Pognani thank Spanish Ministerio de Educación y Ciencia for the award of a pre-doctoral fellowship. Erasmo Cadena and Joan Colón thank Universidad Autónoma de Tamaulipas and Universitat Autònoma de Barcelona, respectively, for the award of a pre-doctoral fellowship.

Disclosure statement

No potential conflict of interest was reported by the authors.