Building stock and waste as the important potential resources of Urban mining

With the rapid development of urban economy, the construction industry is considered to be one of the most vibrant industries, involving a large amount of resource consumption and waste generation, and has grown rapidly in the past few decades. In the "zero waste city" and "double carbon" context, it will be essential to understand the metabolic stock-flow process and the driving forces of urban building resources.

By combining the top-down and bottom-up methods, this study establishes a dynamic material flow analysis (MFA) model to clarify the stock and flow characteristics, driving forces, and future trends of urban building resources in Macao China. This result can provide data support and theoretical guidance for urban mining resources development and improving the efficiency of construction waste recycling. This article provides typical cases for various countries and regions from the perspective of circular economy.

A paper describing the results appeared in the journal Circular Economy on 19 May 2022. (DOI 10.1016/j.cec.2022.100004)

Due to lower recycling efficiency in Macao, the CD&W is transported to a construction waste landfill for final disposal. However, it is worth noting that there is only one landfill area for inert solid waste in Macao since 2006. How to recycle and improve the recycling efficiency of the building materials in the CD&W will be important for Macao to alleviate problems such as insufficient landfill space and scarcity of resources. It is necessary to establish a complete and dynamic material flow framework based on the whole life cycle to fully analyze building materials' stock and flow at various phases (construction, use, and end of life) in Macao China.

Firstly, this study combined the top-down and bottom-up approaches to estimate the stock and flows of building materials in Macao. And the usage of building materials is calculated by the top-down method. Due to the lack of direct statistical data on the historical building material stock and CD&W materials, the bottom-up method is employed to analyze the stock and waste of building materials.

The result shows that, in the construction phase, from 1999 to 2018, the total amount of building material inflow in Macao reached 30296 kt, and almost all building materials in Macao were imported from other countries and regions, especially mainland China (increased from 64.3% in 1999 to 91.7% in 2018) and Hong Kong China (increased from 64.3% in 1999 to 91.7% in 2018). In the use phase, the total stock of building materials increased from 14.13 Mt in 1999 to 32.75 Mt in 2018, with a 4.3% annual growth rate. With the continuous optimization of the building structure, the total stock of steel increased from 0.98 Mt in 1999 (accounted 6.95% of total stock) to 4.4 Mt in 2018 (accounted 13.3% of total stock).

At the end of life, the total amount of construction demolition waste reached 10.7 Mt, of which metal building materials accounted for 7.5%, and non-metal accounted for 92.5%. The disposal of Macao's construction demolition waste is mainly based on recycling and landfill. The total amount of landfills and recycling was 10 Mt (0.8% of metal building materials and 99.2% of non-metal building materials) and 0.7 Mt (100% of metal building materials), respectively.

In order to better understand the impact of social and economic factors on the changes in the building stock, the logarithmic mean Divisia index (LMDI) form of the IPAT (I-environment impact; P-population factor; A-social affluence factor; T-technology factor) equation is adopted to analyze the driving factors of the building materials stock in Macao, which indicates that the population always plays a positive role in the growth of building material stock. From 1999 to 2013, the GDP per capita promoted the growth of building material stock, while from 2014 to 2018, the economic effects on the material stock were reduced.

Scenario analysis is designed to explore the future trends of building material stock in Macao. Building material stock was calculated through future demand for building materials and CD&W. Based on the historical average annual growth rate, three scenarios (conservative scenario, basic scenario, and radical scenario) were set by considering the demand degree of building materials and the demolition speed. The basic scenario maintains the current consumption rate of building materials in Macao.

In the radical scenario, this study assumes that there will be more consumption of building materials, but less CD&W. The conservative scenario is opposite to the radical scenario. It shows that for the conservative scenario, compared with the other two forecast scenarios, the growth efficiency of the future building stock is more moderate, and the building stock will be gentler in the future. In 2035, the material stock in Macao will reach at least 43.7 Mt, about 1.3 times the materials stock in 2018. Under the radical scenario, the materials stock will reach 66.68 Mt by 2035, about twice that of 2018. In the basic scenario, in 2035, the building stock will reach 54.95 Mt.

The growth rate of the building stock is faster than that of the conservative scenario but smaller than that of the radical scenario. It implies that the demand for building materials in Macao will continue to increase. Since the increasing rate of construction activities and the economy in Macao is gradually slowing down in recent years, the conservative scenario and the basic scenario are more possible in the future.

In order to improve waste reduction and recycling efficiency, the Macao government issued the "CD&W Management Regulation" in 2020, which will standardize the use of construction waste landfills and establish a charging system. The building enterprises must pay the relevant fees according to the types and the of CD&W (MOP 70/Mt for inert waste; MOP 200/Mt for special or other CD&W. It is expected that local contractors can take social enterprise responsibilities to classify and recycle the CD&W materials.

In addition, the government will also focus on the source of CD&W, including reducing CD&W through building planning, design, and building management. Green building materials with less environmental impact and greater recycling potential should be preferred in the construction stages.

Moreover, under the policy of "prohibiting the import of foreign waste" in China, the government takes effective measures for alleviating the potential influence. e.g., improving the local recycling facilities; Exploring the regional cooperation solution under the "Outline Development Plan for the Guangdong-Hong Kong-Macao Greater Bay Area" and "One country, two systems".

Research Report:The dynamic stock-flow and driving force analysis of the building metal and non-metal resources at a city scale: An empirical study in Macao

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