Introduction

Residues of household activities and urban cleaning/sweeping are technically known as urban or municipal solid waste (MSW) (Singh et al. 2011). Most Brazilian municipalities suffer difficulties in managing MSW, and in the specific case of urban afforestation residues, there is an additional difficulty because this is not a priority in municipal agendas (McKeever and Skog 2003; Silva et al. 2013; Miller et al. 2015).

Urban afforestation is a constructive element of urban landscape and encompasses all forms of vegetation located in urban free spaces (United States Department of Agriculture 2002). Periodic actions such as irrigation, complementary fertilization, pruning, and replacement of trees/shrubs are required to maintain these areas (Oldfield et al. 2013).

The removal of branches, fruits, inflorescences, or foliage promotes the longevity of trees, and the residues generated correspond to a significant portion of MSW and are usually disposed of in landfills or dumps and, in some cases, incinerated (Ariguchi et al. 2015; Lyon and Bond 2014).

Urban tree and woody yard residues are from now on referred to as urban pruning waste, which represents a high cost for Brazilian municipalities, sanitary landfills, or other destination sites, in addition to being a waste of material with energy potential.

MSW presents several negative environmental impacts and presents health hazards when improperly discarded or underutilized and can also contribute to climate change (Chacartegui et al. 2015). Environmental impacts are associated with the decomposition of MSW, with potential contamination to soil, surface water, and groundwater, generating toxic, asphyxiating, and explosive gases besides greenhouse gases (GHGs) (Chacartegui et al. 2015). Potential environmental impacts can be quantified by carrying out life cycle assessments (LCA). LCA is an important methodology for measuring the environmental impacts associated with activities and processes, based on the fact that improvements in a given process can produce effects throughout the life cycle in a positive and/or negative way, influencing the environmental performance of goods and services (Carvalho et al. 2016; Freire et al. 2016). This methodology has been successfully applied to the development and improvement of products, definition of strategic plans and public policies, management of environmental impacts of products and services, and responsible ecological marketing (Carvalho et al. 2016; Freire et al. 2016; Thomas et al. 2010).

There are some demonstrations of the applicability of LCA regarding the consideration of urban pruning waste (Brugnara 2001; Haaren et al. 2010; Morris et al. 2011; Zhang 2012; Alberta Environment and Sustainable Resource Development 2014; Reichert and Mendes 2014). However, no Brazilian works were found regarding reuse and/or disposal of urban pruning waste within life cycle thinking perspectives. The objective of the study presented herein was to analyze four disposal scenarios for urban pruning waste in João Pessoa (Northeast Brazil) and quantify the carbon footprint associated with each scenario.

Materials and methods

Study object

Data on the amount of urban pruning waste of the city of João Pessoa (Northeast Brazil) were provided by the Information Management Division of the Municipal Urban Cleaning Autarchy (EMLUR).

Life cycle assessment

LCA is structured and standardized by the International Organization for Standardization (ISO), through ISO 14040 (2006) and ISO 14044 (2006). LCA is a methodology that collaborates for the analysis and interpretation of environmental impacts through the collection and compilation of inputs, stages of production, consumption, and outputs of a product system throughout its life cycle (ISO 14040 2006). In Brazil, international standards have been translated by the Brazilian Association of Technical Standards (better known as its acronym in Portuguese, ABNT) into ABNT NBR 14040 and 14044 (ABNT 2014a, b).

LCA consists of four main interrelated phases (Klöpffer 2012): (i) definition of the objective and scope of the analysis; (ii) inventory of the processes involved, with the definition of the inputs and outputs of the system; (iii) analysis of the environmental impacts associated with the inputs and outputs of the system, and (iv) interpretation of the results of the inventory and evaluation phases. Full explanation on LCA can be found in Guinée (2001) and Guinée (2002).

Definition of the purpose and scope of the analysis

Urban pruning waste is collected on working days and, in exceptional and emergency cases, on weekends. The waste is exclusively collected by specific work teams, with no mixing with other types of waste, and transported to the metropolitan landfill of João Pessoa. Once at the landfill, waste is weighed and deposited into cells.

Transportation covers the distance traveled by the collection truck between the city and the landfill. This information was obtained through the use of digital maps (Google Earth) and considering the route between the city center (departure) and the landfill (arrival). The route traveled by the specific work team was recorded by the drivers, in terms of kilometers traveled (km).

Inventory of the processes involved with inputs and outputs defined for the system

SimaPro 8.2.0.0 (Simapro 2017a) is a highly-specialized software for the development of LCA, which includes thousands of processes and a series of methods to evaluate the potential environmental impacts of products and services. The basic structures for the analysis of environmental impacts are characterization, damage assessment, standardization, weighting, and addition of environmental damage (Simapro 2017a). The database utilized herein was Ecoinvent, as it includes data for thousands of products and processes (Ecoinvent 2015), besides being the most applied in academic studies. Every activity within Ecoinvent has a geographic location, which is reported using internationally accepted shortcuts. Global activities were selected herein, representing activities which are considered to be an average valid for all countries in the world.

Due to current concerns on climate change, the carbon footprint was selected to express the environmental impacts, expressed in kilograms of carbon dioxide equivalent (kg CO2−eq). The main international agency for the assessment of climate change is the Intergovernmental Panel on Climate Change (IPCC), created in 1988 by the United Nations Program and the World Meteorological Organization to provide a scientific view of climate change and possible environmental impacts (IPCC 2015). IPCC publishes periodic reports with updated conversion factors that are used to calculate the global warming potential index, based on the time-integrated global mean radiative forcing of a pulse emission of 1 kg of some compound relative to that of 1 kg of the reference gas CO2 (IPCC 2013). Within SimaPro, the impact evaluation method selected was the IPCC 2013 GWP 100y (Simapro 2017b).

Processes selected for LCA

  1. a)

    Urban pruning waste

    Five processes were combined to express the variety of trees (Ecoinvent 2015):

  • Bark chips, wet, measured as dry mass;

  • Cleft timber, measured as dry mass;

  • Residual hardwood, wet;

  • Residual softwood, wet;

  • Wood chips and particles, willow.

  1. b)

    Transportation

    Transportation was divided into two stages: the first corresponded to the route traveled by the truck within the urban perimeter (collection of waste), which was 35 km/day. The second stage corresponded to the distance between the city center and the landfill, 25 km. The process selected for transportation considered a 7.5–16 metric tonne truck, with Euro III emission standardsFootnote 1 (Ecoinvent 2015).

Analysis of the environmental impacts associated with the inputs and outputs of the system

The end-of-life treatments selected included four options for urban pruning waste disposal in the municipality of João Pessoa: direct landfilling, landfilling with methane collection, simple incineration, and reuse of wood.

Urban pruning waste deposited in landfills decomposes slowly and releases methane and carbon dioxide during the first 150 years. Approximately 20% will not decompose and remain in the landfill as a stable material. The simple disposal of urban pruning waste in the landfill, without any collection of methane, is the current practice of the municipal public authority of João Pessoa.

Methane can be collected and simply burnt, but landfills can be equipped with a methane collection system, which is the second scenario investigated herein, where the methane formed is collected and used as fuel (utilization efficiency 31%). Therefore, the landfilling of 1 kg of urban pruning waste avoided the production of 0.007 kg of natural gas. The consequent air emissions were untapped methane (0.002 kg) and total CO2 emissions (0.5 kg), following the procedure described by PRé Consultants (2014).

Municipal incineration includes emissions from incineration and consumption material for the treatment of flue gas. The incinerator itself is also included in the process, and the ashes are landfilled.

For wood reutilization, it was considered that urban pruning waste was transformed into briquettes, avoiding the consumption of new wood in this format. This scenario complies with the Municipal Plan for the Integrated Management of Solid Residues of the city of João Pessoa, regarding the reuse of pruning waste (Emlur 2014). The term briquette refers to a compressed combustible biomass material (charcoal, sawdust, wood chips, peat, or paper) used for fuel. Briquettes are made out of shredded waste (just like pellets), and more details can be consulted in Grover and Mishra (1996) and Food and Agriculture Organization of the United Nations (2015). Briquettes are more common in developing countries, especially for cooking purposes, but can also be utilized to generate steam or electricity.

Interpretation of results

Interpretation of results was accomplished by quantifying the carbon footprint (CO2-eq) in tonnes for the year 2008, for each of the proposed scenarios.

Carbon footprints were then calculated over 13 years (2003–2015), for the less impactful scenario and for the current practice (Business As Usual, BAU). The carbon footprint per tonne of waste collected was also calculated for each investigated scenario to facilitate comparison with existing scientific literature.

Results and discussion

Brazil is a signatory of several agreements on climate change (World Bank 2010), but does not have a binding commitment to reduce CO2 emissions (Garside 2015). The municipality of João Pessoa has plans to become a sustainable city, based on the João Pessoa Sustainable Action Plan (João Pessoa 2014). It is interesting to expose, among the available alternatives, the one with the highest or lowest environmental impacts, to support governments in decision-making and policy orientation.

Figure 1 shows the evolution of urban pruning waste (103 t) collected in João Pessoa, within the period 2003–2015. The total amount of waste generated varied depending on demand and urban planning.

Fig. 1
figure 1

Urban pruning waste collection (103 t) in João Pessoa, 2003–2015

Historical data on urban pruning waste collection indicate an increase in waste collection over time; this could be an indicator of administrative efficiency. The lowest amount of waste collection occurred in 2004 (10,139 t), with the greatest amount collected in 2014 (30,024 t).

Table 1 presents the carbon footprint results for the four scenarios investigated herein (landfill, landfill with methane collection, incineration, and reutilization) for urban pruning waste in João Pessoa, in 2008 (27,065 t of pruning waste collected).

Table 1 Carbon footprint results for the four scenarios investigated

When urban pruning waste was simply landfilled, without any type of treatment, 3690 t CO2-eq was generated in 2008, which was the highest environmental impact verified herein. The landfill scenario with methane collection released 3070 t CO2-eq in 2008, demonstrating a reduction in relation to the previous scenario. Simple incineration emitted 1930 t CO2-eq in 2008 (without generation of heat or electricity). Finally, reutilization in the form of briquettes was the process with the lowest carbon footprint: 753 t CO2-eq in 2008.

The collection and transportation processes of urban pruning residues were common to all scenarios, emitting the same amount of greenhouse gases, 1670 t CO2-eq. The scenario with the highest carbon footprint was landfill disposal without any type of treatment, currently used by the municipal public authority of João Pessoa. The scenario with reutilization of woody residues proved to be the best option from a carbon footprint viewpoint, due to the avoided emissions associated with reutilization (no new trees were harvested to produce briquettes).

Figure 2 shows the carbon footprint dynamics (in 103 t CO2-eq) for the urban pruning waste disposal scenarios in João Pessoa, for the period 2003–2015. Current practice (simple landfill) presented cumulative emissions of 39,802 t CO2-eq over a period of 13 years. Emissions are produced during the decomposition process of the waste by decomposing microorganisms.

Fig. 2
figure 2

Carbon footprint dynamics for different scenarios of urban pruning waste disposal in João Pessoa/PB, from 2003 to 2015

It can be observed that João Pessoa’s current practice is the most polluting form of urban pruning waste management, and special attention from the government is required to better dispose of this type of waste. For the landfill with methane collection scenario, the cumulative carbon footprint was 33,134 t CO2-eq, which is lower than the current practice, but still higher than the cumulative carbon footprint associated with municipal incineration, 20,830 t CO2-eq.

If all urban pruning wastes collected in João Pessoa were reutilized in the form of briquettes, the avoided emissions would have been 8126 t CO2-eq between 2003 and 2015. Reutilization of wood is more advantageous in terms of environmental impacts than simple disposal in the metropolitan landfill (current scenario), with a cumulative difference of 31,676 t CO2-eq over 13 years. This demonstrates the potential of urban pruning waste to help mitigate climate change, and actions directed to a better selection of disposal scenarios should be discussed as a part of a wider sustainability-oriented plan.

Even though João Pessoa is in a relatively comfortable situation regarding air quality, the current trends of economic growth and consumption will result in higher carbon footprint over time. A GHG inventory was conducted in 2014 and concluded that 1,198,034 t CO2-eq was emitted in 2010, with a 43.70% increase in 2012 (João Pessoa Municipal Government 2014). In 2010, the carbon footprint associated with urban pruning landfilling corresponded to 0.29% of the total emissions of João Pessoa, and in 2012, corresponded to 0.21% of the overall carbon footprint. For the reutilization scenario (best scenario identified herein), emission would have corresponded to 0.06% of 2010’s carbon footprint, and 0.04% in 2012.

From these comparisons, it can be observed that the carbon footprint associated with the landfilling of urban pruning waste corresponded to a small percentage in relation to overall emissions. However, these emissions could be further minimized if woody biomass was transformed into briquettes. As the carbon footprint is cumulative over time, GHG emissions contribute to changes in the microclimate of the city.

The João Pessoa Sustainable Action Plan (João Pessoa Municipal Government 2014) presented, among other strategies and actions for the control of GHG, the use of biomass in the energy matrix and the promotion of waste recycling. The reutilization of urban pruning waste would be a strong ally to help the city achieve more sustainable levels.

When comparing the results obtained by Brugnara (2001) and those obtained herein, the carbon footprint associated with landfilling urban pruning waste corresponded to the exploitation of 163.91 ha of native vegetation, for logging purposes, in 2014. The reuse of urban pruning waste, however, corresponded to the exploitation of 65.79 ha, which is a significant decrease.

Morris et al. (2011) provided a literature review of LCAs for management of organic waste, including urban pruning waste, to assess the relative environmental impact of alternative end-of-life management options. The management methods for organics considered were composting, anaerobic digestion, gasification, combustion, incineration with energy recovery, mechanical biological treatment, incineration without energy recovery, and disposal in landfills, both with and without energy recovery from generated methane. However, it was only possible to draw conclusions regarding aerobic composting, anaerobic digestion, incineration, and landfill gas-to-energy. The incineration carbon footprint, 70 kg CO2/t waste collected, was of the same magnitude of the value herein obtained. However, different boundaries and conditions were assumed for the other scenarios, which along with the small sample sizes, prevented more general conclusions of the study.

Adequate waste management has been pointed as one of the solutions to help minimize human health and environmental risks (Levis and Barlaz 2013). Urban pruning waste is now in the spotlight as discussion emerges on recyclable materials with potential for energy generation, contributing to environmental sustainability.

Conclusions

The most common destination for urban pruning waste in Brazil is the landfill, and this entails high economic costs to municipalities. As the demands of modern society include better, environmentally-friendly ways to dispose of waste, four end-of-life scenarios were investigated herein for the urban pruning waste collected in João Pessoa (Northeast Brazil).

Based on the analyses carried out, it was concluded that the current disposal system (landfill) is the most impacting for the environment. With emissions of 136.34 kg of CO2-eq/t of collected waste, landfill presented the highest carbon footprint. The best disposal scenario, from a carbon footprint perspective, was the reuse of pruning waste in the form of briquettes (27.82 kg of CO2-eq/t of collected waste), demonstrating a reduction of 80% in the overall carbon footprint.

The disposal method proposed by the Municipal Solid Waste Plan of João Pessoa (reutilization of woody residues in the form of briquettes) is environmentally-friendly and less harmful to the environment and presents real possibilities of being implemented in the short term.

The study presented herein demonstrated that the reuse of biomass, besides being environmentally viable, has the potential to contribute to the environmental quality of the city, including the possibility of being used as carbon credits.