Developing Carbon-limiting Disposal Scenarios For Urban Pruning Waste

Tree clippings (Credit: in.gov)

The removal of branches, fruits, inflorescences or foliage from urban afforestation promotes the longevity of trees, and the residues generated correspond to a significant portion of municipal solid waste.

These residues are usually disposed of in landfills or dumps and, in some cases, incinerated. Urban tree and woody yard residues (urban pruning waste) represent a high cost for municipalities, sanitary landfills or other destination sites, in addition to being a waste of material with energy potential.

Significant potential impacts could occur during the decomposition stage of municipal solid waste, with potential contamination of soil and surface water and groundwater and generation of greenhouse gases  – mainly methane (CH4). These potential environmental impacts can be quantified through the development of Life Cycle Assessments (LCA).

Due to recent concerns about climate change, the objective of the study was to apply the LCA methodology to quantify the carbon footprint associated with four different disposal scenarios for urban pruning waste. 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. The disposal scenarios considered were: sanitary landfilling (with and without collection of methane), simple municipal incineration, and re-utilization of wood (transformation into briquettes).

  • 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.
  • In the second scenario, the methane formed in the landfill 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).
  • 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 re-utilization, it was considered that urban pruning waste was transformed into briquettes, avoiding the consumption of new wood in this format. The term briquette refers to a compressed combustible biomass material (charcoal, sawdust, wood chips, peat, or paper) used for fuel. Briquettes are more common in developing countries, especially for cooking purposes, but can also be utilized to generate steam or electricity.

A study case was conducted at the municipality of João Pessoa (Northeast Brazil) and was utilized to exemplify the methodology. Brazil is a signatory of several agreements on climate change but does not have a binding commitment to reduce CO2 emissions. It was verified that the highest carbon footprint was associated with the current practice (simple landfilling).

This study demonstrated that re-utilization of biomass, besides being environmentally viable, presents the potential to contribute to the overall environmental quality, including the possibility of being used to obtain carbon credits.

These findings are described in the article entitled Carbon footprint associated with four disposal scenarios for urban pruning waste, published in the journal Environmental Science and Pollution Research. This work was led by Luiz Coelho Junior and Monica Carvalho from the Federal University of Paraíba.

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