Quantifying the potential of recycling demolition waste generated from urban renewal: A case study in Shenzhen, China

Journal Pre-proof Quantifying the potential of recycling demolition waste generated from urban renewal: A case study in Shenzhen, China

Bo Yu, Jiayuan Wang, Jie Li, Weisheng Lu, Clyde Zhengdao Li, Xiaoxiao Xu PII:

S0959-6526(19)33997-6

DOI:

https://doi.org/10.1016/j.jclepro.2019.119127

Reference:

JCLP 119127

To appear in:

Journal of Cleaner Production

Received Date:

10 August 2019

Accepted Date:

31 October 2019

Please cite this article as: Bo Yu, Jiayuan Wang, Jie Li, Weisheng Lu, Clyde Zhengdao Li, Xiaoxiao Xu, Quantifying the potential of recycling demolition waste generated from urban renewal: A case study in Shenzhen, China, Journal of Cleaner Production (2019), https://doi.org/10.1016/j.jclepro. 2019.119127

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Journal Pre-proof Quantifying the potential of recycling demolition waste generated from urban renewal: A case study in Shenzhen, China Bo Yu a, Jiayuan Wang b, *, Jie Li c, Weisheng Lu d, Clyde Zhengdao Li e, Xiaoxiao Xu f a PhD Candidate, College of Civil and Transportation Engineering, Shenzhen University, Nanshan, Shenzhen, China, Sino-Australia Joint Research Center in BIM and Smart Construction, Shenzhen University, Shenzhen, China, Email: [email protected] b* Corresponding author, Professor, College of Civil and Transportation Engineering, Shenzhen University, Nanshan, Shenzhen, China, Sino-Australia Joint Research Center in BIM and Smart Construction, Shenzhen University, Shenzhen, China, Email: [email protected] c Master Candidate, College of Civil and Transportation Engineering, Shenzhen University, Nanshan, Shenzhen, China, Email: [email protected] d Professor, Department of Real Estate and Construction, The University of Hong Kong, Pokfulam, Hong Kong, The University of Hong Kong Shenzhen Institute of Research and Innovation (SIRI), B402, Shenzhen Virtual University Park, Nanshan, Shenzhen, China Email: [email protected] e Assistant Professor, College of Civil and Transportation Engineering, Shenzhen University, Nanshan, Shenzhen, China,

Journal Pre-proof Sino-Australia Joint Research Center in BIM and Smart Construction, Shenzhen University, Shenzhen, China, Email: [email protected] f PhD Candidate, Department of Civil and Construction Engineering and Centre for Smart Infrastructure and Digital Construction, Swinburne University of Technology, Australia; Office of Projects Victoria, Australia Email: [email protected]

Journal Pre-proof Quantifying the potential of recycling demolition waste generated from urban renewal: A case study in Shenzhen, China Abstract: Large-scale demolition waste was generated during urban renewal. How to accurately quantify the potential of recycling large amounts of demolition waste has been widely considered a prerequisite for effective waste management, as it contributes to numbers and scales of recycling enterprises being managed and planned in advance. However, there is limited research, if any, focusing on this niche area. This study aims to quantify the potential of recycling demolition waste amidst urban renewal by considering actual market situations and different waste types. Firstly, the whole recycling process of demolition waste was scrutinized by using the free-flow mapping technique. Models for quantifying recycling potential of non-inert and inert demolition waste were then established, respectively, based on economic value and the principle of mass conservation. Finally, a case study was conducted to verify the models by putting them into the context of Shenzhen, a young and vibrating city in China but subject to massive urban renewal pressure. Results show that the recycling process of demolition waste is mainly divided into offsite and onsite recycling, both including five stages, viz., waste generation, on-site treatment, transportation, recycling, and product regeneration. In addition, different types of demolition waste have their respective recycling potential. The recycling potential of non-inert demolition waste is RMB 19,315.85 million yuan. In contrast, the recycling potential of inert demolition waste depends on types of recyclable products, with 18.41 million tons of recyclable bricks, 7.02 million tons of mortar, 28.36 million tons of aggregate, and 4.16 million tons of lightweight wallboard. Findings of this study improve the accuracy of the existing quantification methods. The research provides useful reference for the recycling industry to adjust production scale and arrange production sites to fully harness the waste recycling potential. Keywords: Waste management, Demolition waste, Recycling potential, Recycling process, Urban renewal. 1. Introduction Urban renewal, which is called urban regeneration in the UK or urban redevelopment in the U.S., normally refers to a land redevelopment programme used to address the problem of urban decay and improve the living conditions of residents in old districts (Lai et al., 2018). Demolition of the existing buildings in these old districts is a predominant approach to reassemble the land during urban renewal. Demolition is defined as dismantling, razing, destroying or wrecking any building or structure or any part thereof by a pre-planned and controlled method (HKBD, 2004), e.g., the top-down utilization of manual methods or machines, the use of hydraulic crushers with a long boom arm, a wrecking ball, or even implosion. In economically developing nations, demolition is often taking a major proportion 1

Journal Pre-proof during urban renewal process, as a large number of the existing structures are demolished to release new land space for urgent demands of housing and other facilities (Lu et al., 2016b). For example, in Shanghai, a single city in China, the demolition floor area in 2014 reached 1,185,800 m2 (SSB, 2015). Hence, urban renewal can lead to a large amount of demolition waste generation (Chen and Lu, 2017; Lin and Ouyang, 2014). At present, there is still much space for improvement on the effectiveness of demolition waste management. Without conscious waste management, the whole demolished building will be turned into waste. For instance, approximately 49.40 million tons of demolition waste were produced during urban renewal in Shenzhen between 2010 and 2017 (Yu et al., 2019). Previous studies have shown that demolition waste accounts for up to 70% of the total construction and demolition (C&D) waste (Ding et al., 2016; Wu et al., 2016a). Large-scale demolition waste may cause a series of environmental impacts, such as greenhouse gas emissions, land resource occupation, as well as energy and raw material consumption (Wang et al., 2018; Wang et al., 2019c). Thus, it is imperative to effectively manage demolition waste in the urban renewal process (Blaisi, 2019). The 3R principle, which means ‘reduce, reuse, and recycle’ to classify waste management strategies according to their desirability (Peng et al., 1997), is the “Bible” for waste management including C&D waste management. Waste reduction is considered as the most efficient approach to minimizing waste generation and eliminating environmental problems (Esin and Cosgun, 2007; Peng et al., 1997). Since it is impractical to completely eliminate all the C&D waste, reuse and recycling are alternative methods for making less demolition waste ends up in landfills (Yuan and Shen, 2011). There is a small but growing body of literature focusing on demolition waste management. A group of researchers at University of Vennia University of Technology (e.g., Kleemann et al., 2016a; 2016b) developed a method of determining buildings’ material competition prior to demolition and scaled it up to a city level to determine the waste material stock. These can be categorized as quantifying waste generation research (Wu et al., 2016a). Chen and Lu (2017) improved our understanding of waste generation by identifying the factors to influence demolition waste generation in highrise building. Lu et al. (2016a) found that waste generation in demolition projects largely follow an S-curve. However, little research, if any, has tried to quantify recycling potential of demolition waste, with a view to providing useful references to effective demolition waste management (Ding and Xiao, 2014; Gallardo et al., 2014; Islam et al., 2019; Wu et al., 2016a). The overall aim of this study is to propose an accurate and scientific method for quantifying the potential of recycling demolition waste during urban renewal. The uniqueness of this research lies in it considering the actual market situation and different waste types. The research has three specific objectives: (1) To remap the recycling process of demolition waste; (2) To obtain quantitative indicators of recycling potential of different waste types; and (3) To establish quantitative models of recycling potential of different waste types. To achieve these 2

Journal Pre-proof goals, the free-flow mapping techniques are adopted to remap the demolition waste recycling process. Furthermore, quantitative indicators of recycling potential are obtained from on-site investigation and semi-structured interviews. Besides, recycling potential quantitative models for different demolition waste types are established based on economic value and the principle of mass conservation. They are finally verified to quantify the recycling potential of demolition waste generated from urban renewal in Shenzhen. This study proposed a new quantitative method for quantifying the recycling potential of demolition waste, which improves accuracy and effectiveness of the existing quantitative methods. Furthermore, findings in this paper provide useful reference for the recycling industry to adjust production scale and arrange production sites to fully harness the waste recycling potential. The remainder of this paper is as follows. Subsequent to this introductory section is a literature review. Research method to quantify recycling potential is then presented in Section 3. Section 4 shows findings of the study, and uses the method proposed for verification to quantify recycling potential of the large amount of demolition waste during urban renewal in Shenzhen. It is followed by Section 5, which is an in-depth discussion on both research results and methodological contributions. Conclusions covering limitations and further research are given in Section 6. 2. Literature review This section presents the existing studies on the implications, challenges and recommendations for demolition waste recycling. Research on the recycling potential quantification of demolition waste was systematically reviewed subsequently. Based on the literature review, the research gap and innovation of this research are clarified at the end of this section. 2.1 Demolition waste recycling Demolition waste management, particularly the recycling of demolition waste, is an integral part of the circular economy (Wu et al., 2019). For example, recycling demolition waste could save landfill charge and build the social sustainability image (Doan and Chinda, 2016). Current research shows that demolition waste recycling could bring significant environmental, economic and social benefits (Jin et al., 2017), which include, but are not limited to (1) decreasing occupation of landfill space and saving landfill charge (Marzouk and Azab, 2014; Yuan and Shen, 2011); (2) saving energy and reducing greenhouse gas emissions (Huang et al., 2013; Marzouk and Azab, 2014); and (3) achieving environmental sustainability by improving governmental strategies or industry standards (Li, 2008). However, demolition waste recycling is still facing challenges despite many benefits, including (1) lack of waste recycling facilities or companies (Domingo and Luo, 2017); (2) lack of fundamental data related to demolition waste (Yuan, 2017); (3) insufficient incentives from policies and regulations (Domingo and Luo, 2017); (4) substandard qualities of recyclable products (Duan 3

Journal Pre-proof and Poon, 2014); and (5) less economic viability of recycling demolition waste (Zhao et al., 2010). Correspondingly, relevant scholars have put forward some countermeasures for effective recycling management of demolition waste. For example, Yuan (2017) proposed five practical measures to solve demolition waste management problems, including enhancing the effectiveness of demolition waste regulations in reality, collecting and releasing accurate demolition waste amount timely, enhancing demolition waste management, promoting demolition waste recycling further, and implementing an effective waste disposal charging fee. Similarly, Huang et al. (2018) conducted demolition waste management analysis based on 3R principle, pointing out that designing effective circular economy model, reinforcing the source control of demolition waste, adopting innovative technologies and market models, and implementing targeted economic incentives are effective strategies to improve the current waste management situation in China. It is worth noticing that the above research mainly stays at a qualitative level. Literature bibliometrics, semi-structured interviews, and group discussions with governmental staff and industry participants are often the most typical research methods in these research. However, quantitative research is more instructive than qualitative research in domains of demolition waste recycling management (Wu et al., 2016a; Wu et al., 2016b), as it can provide tangible data, which is provides useful reference for different stakeholders. 2.2 Quantification of the potential of recycling demolition waste It is widely recognized that demolition waste has high recycling potential, because it is one of the most effective ways to solve problems caused by demolition waste generated (Kofoworola and Gheewala, 2009; Wu et al., 2016b). Nevertheless, there is lack of effort to quantify the recycling potential of demolition waste (Wu et al., 2016b). Study on quantifying recycling potential for demolition waste has become a new research topic in recent years. Recycling potential is defined by economic value (Islam et al., 2019). They calculated the recycling potential of concrete and brick waste in Bangladesh in 2016 based on the total generation quantity and unit economic value. However, the recycling potential of other waste, such as mortar, wood, and metal, has not been researched in their study. Differently, some research have quantified the economic value of each type of waste (Wu et al., 2016a; Wu et al., 2016b). Recycling potential of demolition waste was focused by Wu et al. (2016a) at a city level, meaning economic value based on generated amount, recycling potential value and recycling rate. Specifically, recycling rate was discussed separately according to maximum recycling rate situation. Furthermore, the recycling potential of demolition waste was calculated under three different scenarios, viz. the lowest recycling situation, the realistic scenario and the optimistic situation (Wu et al., 2016b). Besides, Wang et al. (2019a) defined the recycling potential as latent amount of waste generated. However, only the recycling 4

Journal Pre-proof potential of metal waste was quantified in their research. To sum up, the above studies did not consider the actual market situations. In addition to demolition waste, scholars have currently focused on quantitative research on the recycling potential of multiple waste types, such as combustible waste (Faraca et al., 2019), desktop computer waste (Kohl and Gomes, 2018), neodymium waste (Ciacci et al., 2019), plastic packing waste (Dahlbo et al., 2018) and municipal solid waste (Gu et al., 2018). The recycling potential of them are majorly regarded as the amount or proportion of waste recycled, quite similar to the study of Wang et al. (2019a). Specially, the scale of recyclable products produced is defined as the recycling potential of plastic packaging waste, which is calculated based on the principle of mass conservation (Dahlbo et al., 2018). A literature review seems suggesting that research on waste recycling potential can be mainly divided into two categories: front-end and back-end research. Front-end research refers to quantifying the amount or proportion of waste that is recyclable, while back-end research involves waste economic value as well as scales and types of recyclable products. Compared with the front-end research, the back-end research seems to make a deeper understanding of quantifying the recycling potential of demolition waste. However, there are limited studies focusing on back-end research. Besides, in the existing studies, the majorities have confirmed that all components of demolition waste are recyclable. For instance, metal waste could be used as recycled materials to produce corresponding metal products (Huang et al., 2018; Wang et al., 2019b), glass waste could be used for glass industry after crushing and heating (Huang et al., 2018), wood waste could be directly used as a burning raw material in incineration facilities (Wang et al., 2019b), and some inert wastes, like concrete, brick/block, and mortar could be used to produce recyclable bricks, recyclable mortar, and recyclable aggregates (Akhtar and Sarmah, 2018; Galan et al., 2019; Lockrey et al., 2018; Santos et al., 2019; Silva et al., 2016). Separate quantification of the recycling potential of various demolition wastes is more conducive to achieving preciser management. Therefore, it is necessary to quantify those individually rather than as a whole. In this research, the waste recycling potential will be following quantified according to the above waste types. 2.3 Research gap and innovation in this study Current research on recycling management of demolition waste mainly stays at a qualitative level, while the quantitative research is quite rare. By contrast, quantitative research is more conducive than qualitative ones to guiding governments and enterprises to conduct better recycling management on demolition waste. In addition, the limited studies on quantifying the recycling potential of demolition waste are one-sided and crude. They were not combined well with the actual market situation, nor considered the differences in different waste types, so it 5

Journal Pre-proof could not be helpful to further study the latent scale and quantity of different recyclable products. Also, huge volumes of demolition waste are generated during urban renewal process. Once the recycling potential is accurately quantified, the number and scale of demolition waste recycling enterprises could be managed and planned in advance. Therefore, systematic quantification of the recycling potential of demolition waste helps provide reference for governments and enterprises during urban renewal. 3. Research method and process This section introduces the research method and process of this study. It specifically includes remapping the recycling process of demolition waste based on the free-flow mapping technique, establishing quantitative models for calculating recycling potential of different demolition waste types based on economic value and the principle of mass conservation, and quantifying the recycling potential of demolition waste during urban renewal in Shenzhen (Fig. 1).

Fig. 1. The research process for the study.

3.1 Method for remapping the recycling process of demolition waste It is imperative to remap the recycling process before quantifying the recycling potential of demolition waste during urban renewal (Wu et al., 2016b). In order to obtain data in the whole recycling process of demolition waste from generation to final disposal, this study carried out on-site investigation on demolition waste recycling enterprises and semi-structured interview with key stakeholders in Shenzhen. On-site investigation is a prevalent approach to establishing waste database (Thanh et al., 2010), and it has been widely used in the research of C&D waste (Cha et al., 2017; Gu et al., 2018; Kleemann et al., 2016a; Lockrey et al., 2018; 6

Journal Pre-proof Yu et al., 2019). Semi-structured interview, a method also commonly used in such research area, particularly contributes to helping researchers mine in-depth information behind a participant’s experiences (Blaisi, 2019; Huang et al., 2018; Lockrey et al., 2018; Wang et al., 2019b; Wu et al., 2016a; Yuan, 2017; Zou et al., 2019). Therefore, they are two ideal methods with high efficiency for remapping the whole recycling process of demolition waste during urban renewal. To minimize the subjective bias on investigation results, it is crucial to determine the investigated enterprises and stakeholders with considering their fundamental features comprehensively (e.g., enterprise background and culture, stakeholder socio-demographic information and knowledge structure). According to official data from Shenzhen Municipal Housing and Construction Bureau, there are the current thirty-six demolition waste recycling enterprises in Shenzhen, excluding silt soil recycling enterprises. Among them, twelve untypical enterprises were not considered in this on-site investigation, as they conducted recycling work on demolition waste for less than one year, with four of them still in the trial operation stage. Therefore, twenty four demolition waste recycling enterprises were selected as final investigated objects. In addition, in order to obtain basic data in this study, one or two key stakeholders were selected for semi-structured interviews from each recycling enterprise. Besides, to ensure the reliability of the interview results, all respondents selected should have working experience more than three years. This study ultimately conducted semi-structured interviews with thirty four key stakeholders in a face-to-face way between July and December, 2018. In order to determine the sample size, the data saturation principle was followed in this study (Zou et al., 2019). It was after the interview on the 24th respondent that no new information could be obtained any more. Furthermore, another ten stakeholders were interviewed within our reach in order to ensure the data is reliably saturated. Information of the twenty four demolition waste recycling enterprises is shown in Table 1. Up to 70% of the enterprises have more than three-year experience in demolition waste recycling, and five of them have more than eight years. In terms of recycling approaches, enterprises only engaging in onsite recycling of demolition waste are main components, at 70.83%. Enterprises engaging in both onsite and offsite recycling, and those only engaging in offsite recycling account for only 16.67% and 12.50%, respectively. Nearly all of them produced recyclable aggregates, with only one not. In addition, a total of 33.33% of these enterprises produced recyclable bricks. Table 1 Fundamental features of demolition waste recycling enterprises. Variable

Item

Number

Percentag e (%)

Recycling

1＜years≤3

7

29.17

7

Cumulative percentage (100%) 29.17

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Recycling approach

Recyclable product type

3＜years≤5

9

37.50

66.67

5＜years≤8

3

12.50

79.17

8＜years

5

20.83

100

4

16.67

16.67

3 17 2 2 3

12.50 70.83 8.33 8.33 12.50

29.17 100 8.33 16.66 29.16

1

4.17

33.33

16

66.67

100

Both offsite and onsite recycling Only offsite recycling Only onsite recycling All types b Three types c Two types d Only produce recyclable brick Only produce recyclable aggregate

a

Duration for enterprise to conduct demolition waste recycling. All types of recyclable products, including recyclable bricks, mortar, aggregates, and lightweight wallboard. c Products of recyclable bricks, mortar, and aggregates. d Products of recyclable bricks and aggregates. b

The demographic characteristics of the interviewees are summarized in Table 2. All of them are directly engaged in the recycling utilization of demolition waste, including project demolition managers (55.88%), recyclable product production managers (23.53%), recyclable product sales managers (14.71%), and chairmen of the enterprise (5.88%). Most of them have more than five years of working experience (52.94%). Overall, the stakeholders we interviewed possess rich experience and in-depth understanding of recycling process of demolition waste during urban renewal. Hence, their responses could be real and credible. Table 2 Demographic characteristics of interviewees. Variable

Category

Number

Percentage (%)

Cumulative percentage (100%)

Gender

Male Female Project demolition manager Recyclable product production manager Recyclable product sales manager Chairman of the enterprise

26 8 19

76.47 23.53 55.88

76.47 100 55.88

8

23.53

79.41

5

14.71

94.12

2

5.88

100

3＜years≤5

16

47.06

47.06

5＜years≤8

12

35.29

82.35

8＜years

6

17.65

100

Occupation

Work experience

8

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The recycling process of demolition waste during urban renewal are remapped based on the free-flow mapping technique. This technique has great advantages in presenting flows of processes logically and clearly in a simple way, which has been used in construction waste management (Shen et al., 2004). Four typical symbols of this technique are shown in Fig. 2(ad). Rectangular indicates the source of waste generation. Ellipse refers to the process of waste disposal. Arrow and horizontal line indicate methods or tools used for waste disposal, and diamond means the final disposal result of waste (Shen et al., 2004). Thus, a simple waste flow mapping is presented in Fig. 2(e) based on these four symbols, including waste source, waste processing, waste facilitator, and waste destination. It is undeniable that there are differences in generation, management methods, disposal routes, and types of recyclable products of different enterprises and projects. However, the focus of this study is not on generalizing common demolition waste recycling process, but on the most typical recycling process of demolition waste during urban renewal in Shenzhen according to the on-site investigation and semi-structured interview.

Fig. 2. Waste flow symbols.

3.2 Model for quantifying demolition waste recycling potential According to the interview results, recycling potential of demolition waste is quite different between two types of waste: non-inert and inert waste. Whether it is offsite recycling or onsite recycling, non-inert demolition waste, such as steel, aluminum, copper, plastic, glass and wood, is directly sold to waste recycling companies due to its relatively high economic value (Wu et al., 2016a). However, as for inert demolition waste, such as concrete, brick/block, mortar, ceramic and granite, it needs to be transformed into a recyclable product through specific production processing. Thus, their recycling potential should be quantified separately. Generally, recycling potential refers to economic value for non-inert demolition waste, while types and scale of recyclable products for inert demolition waste in this study. 9

Journal Pre-proof 3.2.1 Quantitative model for recycling potential of non-inert demolition waste Waste recycling companies recycle non-inert demolition waste on demolition site according to generation quantity and unit economic value. Therefore, the recycling potential of non-inert demolition waste is calculated based on its economic value. The quantitative model is shown in Eq. (1):

RPn i   TWz  Vz

(1)

z

where RPn-i refers to the recycling potential (unit: RMB) of non-inert demolition waste (n-i, for short), and TWz refers to the total generation quantity (unit: kg or t) of non-inert demolition waste z. Vz refers to the unit economic value (unit: yuan/kg, yuan/t) of non-inert demolition waste z. Eq. (1) shows that the total generation quantity and the unit economic value are two dominant preconditions for quantifying the recycling potential of non-inert demolition waste. The unit economic value of different types of non-inert demolition waste were determined from on-site investigation according to market conditions of Shenzhen in 2018, and future price changes caused by inflation and other uncontrollable factors were not considered in this study. In order to simplify calculation, their average was calculated as the final unit economic value of various non-inert demolition waste in this study. The total generation quantity was calculated based on two indicators of waste generation rate (WGR) and gross floor area (GFA). WGR was measured based on on-site measurement and the existing industry standard data, and GFA was obtained based on image recognition technology and Google Earth. The details could be found in our previous study which focused on the prediction of large-scale demolition waste generation during urban renewal in Shenzhen (Yu et al., 2019). Besides, composition and proportion of demolition waste could be also found in this prior study, which are the bases of further calculation on its recycling potential. 3.2.2 Quantitative model for recycling potential of inert demolition waste Since inert demolition waste is usually not classified in the recycling process, it is mixed together and then transformed into different types of recyclable products through specific production processing. Therefore, it is inappropriate to quantify the recycling potential separately like non-inert demolition waste based on different waste types. Although different recycling enterprises produce the same recyclable products, the economic value is different due to different policy support, production process, production scale, management level, market share, etc. The economic value of the same recyclable product in the same enterprise will also be affected by purchasers and market factors. In addition, economic value is the part of enterprise privacy, and most recycling enterprises are reluctant to disclose it. A rough scope, rather than a precise value, would be obtained from only a few companies, even though disclosed. Therefore, the recycling potential of inert demolition waste is also difficult to be 10

Journal Pre-proof calculated based on economic value like non-inert demolition waste. Alternatively, the proportion of inert demolition waste in the same recyclable product of different recycling enterprises is not quite different from each other, and only fluctuates within a fixed range. Thus, the recycling potential of inert demolition waste is calculated based on the principle of mass conservation, and the quantitative model is shown in Eq. (2):

RPi，j  aTWi，j

(2)

where RPi， j refers to the quantity (unit: kg or t) of recyclable product j produced from inert demolition waste, representing the recycling potential of inert demolition waste (i, for short). a refers to the recycling potential factor, i.e., the percentage of inert demolition waste in recyclable products. It is worth noting that the percentage of inert demolition waste in different recyclable products is different, and the smaller a is, the larger the percentage is. TWi，j refers to the total quantity (unit: kg or t) of inert demolition waste used to produce recyclable product j. Eq. (2) shows that the percentage of inert demolition waste in recyclable products and the total generation quantity are two dominant parameters for quantifying the recycling potential of inert demolition waste. The percentage of inert demolition waste was determined according to on-site investigation and semi-structured interviews (details are shown in Section 3.1), with the average value also calculated. The total generation quantity and the proportion of inert demolition waste, used to produce different recyclable products, also comes from our previous study (Yu et al., 2019). 4. Results This section shows the findings of this study, and uses the method proposed for verification of quantifying the recycling potential of large amounts of demolition waste during urban renewal in Shenzhen. The whole process of offsite and onsite recycling is firstly presented. The quantitative indicators of recycling potential for demolition waste are subsequently introduced. Quantitative results of recycling potential for non-inert and inert demolition waste are presented separately at the end of this section. 4.1 Analyzing recycling process for demolition waste 4.1.1 Remapping the whole process of offsite and onsite recycling The recycling process of demolition waste is mainly divided into offsite and onsite recycling according to different recycling sites. Offsite recycling refers to recycling is carried out in recycling enterprises, and onsite recycling means that recycling is carried out on demolition sites. The whole process of offsite and onsite recycling is remapped based on the free-flow mapping technique, as shown in Figures 3 and 4, respectively. They could be both divided into five stages, namely, waste generation, on-site treatment, transportation, recycling, and product regeneration. The first four stages are in line with findings in the research of Wang et al. (2018). 11

Journal Pre-proof However, product regeneration stage is not within their research scope. (1) Waste generation stage Whether it is offsite recycling or onsite recycling, the first stage is waste generation stage. It refers to the process in which the existing buildings were dismantled into demolition waste through specific demolition methods. Due to the low-floor buildings planned to be demolished during urban renewal in Shenzhen, mechanical demolition has become the main demolition method. In addition, a small number of the existing buildings were demolished based on a combination of mechanical and manual demolition, with almost no blasting. (2) On-site treatment stage On-site treatment stage is the second stage of offsite recycling, covering the whole process of collecting, sorting and pretreating demolition waste on demolition site after generation. Unlike offsite recycling, on-site treatment stage is the third stage of onsite recycling, which is following the transportation stage. In order to ensure that non-inert waste is separated from inert waste, a combination of mechanical and manual methods is used in waste collection and sorting. Non-inert waste is sold directly to waste recycling companies on demolition sites, and the remaining inert waste, including concrete, brick/blocks, mortar and granite, is transported to recycling enterprises because they have no direct economic value. (3) Transportation stage For offsite recycling, transportation stage is the third stage, which refers to the process of demolition waste from demolition sites to recycling enterprises, including self-transportation by recycling enterprises or being entrusted by special transport companies. The latter one is the most popular mode, as the transport companies are often responsible for on-site cleaning up after transport. It is essential to pretreat demolition waste before transport, in order to minimize sequent adverse environmental impacts. Covering demolition waste has become the most common pretreatment method. Unlike offsite recycling, transportation stage is the second stage of onsite recycling, referring to the short-distance transportation of demolition waste from demolition site to pre-treatment site. This process is therefore regarded as intrafield transportation, as the pre-treatment is usually located on demolition sites. (4) Recycling stage Similarly, the recycling stage in both offsite recycling and onsite recycling refers to the process of final treatment of demolition waste by recycling enterprises. In order to obtain recyclable aggregate with a certain particle size, it is necessary to initially crush demolition waste by jaw crusher. In addition, it is also necessary to transport the initially crushed coarse aggregate to hammer crusher for further crushing. This process is consistent with the findings of Wu et al. (2016a). However, the recyclable aggregate obtained in the previous process is doped with a 12

Journal Pre-proof small amount of non-inert waste (mainly steel). Therefore, these short steel bars need to be separated by magnetic separation. In order to obtain aggregates of different particle sizes, the recyclable aggregate after magnetic separation needs to be sieved through rolling sieves. Prior to the production of recyclable products, aggregates of different particle sizes are also subjected to air separation to remove light materials, such as plastic sheets and wood chips. Therefore, recyclable aggregates are usually in initial forms after this stage (AlwanAl-Bayati et al., 2018; Sim and Park, 2011). (5) Product regeneration stage Product regeneration stage is the last stage, as the process where recyclable aggregates are finally formed into different types of recyclable products. Recyclable products in offsite recycling of demolition waste during urban renewal in Shenzhen include recyclable bricks, mortar, aggregates, and lightweight wallboard. However, different from offsite recycling, recyclable aggregate is the most important product of onsite recycling, while recyclable bricks account for only a small proportion. It has a close relationship with the simple production process. The less production equipment is, the smaller space is required and the higher site turnover rate is for recyclable aggregates. In addition, it is also related to the demolition project itself and the surrounding engineering projects that require a large amount of recyclable aggregates for backfilling the subgrade or building foundation cushion.

13

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Fig. 3. The whole offsite recycling process of demolition waste during urban renewal in Shenzhen. 14

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Fig. 4. The whole onsite recycling process of demolition waste during urban renewal in Shenzhen. 15

Journal Pre-proof 4.1.2 Obtaining the quantitative indicators of recycling potential Quantitative indicators of recycling potential are obtained in remapping the recycling process of demolition waste, including unit economic value, recycling factor and the proportion of inert demolition waste used to produce different recyclable products. For non-inert demolition waste, unit economic value is the basic data indicator for quantifying the recycling potential of non-inert demolition waste. As shown in Table 3, copper waste has the largest unit recycling value (55,280 yuan/t) among all kinds of non-inert demolition waste. Followers are aluminum (14,000 yuan/t) and steel waste (2,230 yuan/t). Besides, the unit recycling value of plastic waste (1,520 yuan/t) is the highest among the remaining non-inert demolition waste. As for wood and glass waste, their unit recycling values are relatively low, costing only 750 yuan/t and 425 yuan/t, respectively. Table 3 Recycling potential of non-inert demolition waste during urban renewal in Shenzhen.

a

Composition

Total generation quantity a (Unit: million tons)

Unit economic value (Unit: yuan/t)

Recycling potential (Unit: million yuan)

Steel Aluminum Copper Plastic Glass Timber Total

1.85 0.64 0.10 0.38 0.17 0.07 -

2230 14000 55280 1520 425 750 -

4125.50 8960.00 5528.00 577.60 72.25 52.50 19315.85

Data from our previous study (Yu et al., 2019).

For inert demolition waste, the recycling factor and the proportion of it used to produce different recyclable products are the basic data indicators for quantifying the recycling potential. As shown in Table 4, inert demolition waste accounts for 85% of the total weight of recyclable brick, and the recycling factor of recyclable brick is 1.176. For recyclable mortar, natural sand is the most important component (about 58.3%), while inert demolition waste accounts for only 25%. Thus, the recycling factor of recyclable mortar was calculated as 4. In addition, recyclable aggregates are all composed of demolition waste, with the recycling factor of 1. Comparatively, non-inert demolition waste contributes slightly to lightweight wallboard, at only 10%. The quantitative models for recycling potential of inert demolition waste thus could be established based on the recycling factor of each recyclable product, detailed in Table 4. Table 4 The quantitative indicators of recycling potential for inert demolition waste. Recyclable product

Percentage of inert demolition waste in different recyclable products

Recycling factor (a)

Quantitative model

Recyclable brick

85%

1.176

RPi，b = 1.176 TWi，b

16

Journal Pre-proof Recyclable mortar Recyclable aggregate Lightweight wallboard

25%

4

RPi，m = 4 TWi，m

100%

1

RPi，g = TWi，g

10%

10

RPi，w = 10 TWi，w

Fig. 5 shows that the proportion of inert demolition waste used to produce each recyclable product during urban renewal in Shenzhen. Most of inert demolition waste is used to produce recyclable aggregates, accounting for 61.40%. 33.90% of inert demolition waste is used to produce recyclable bricks, followed by recyclable mortar (3.80%) and recyclable aggregates (0.90%).

Fig. 5. The proportion of inert demolition waste used to produce different recyclable products during urban renewal in Shenzhen.

4.2 Quantifying recycling potential for demolition waste The quantitative method proposed in this study was used to quantify the recycling potential of large-scale demolition waste according to different types, viz., non-inert demolition waste and inert demolition waste during urban renewal in Shenzhen. The case study was contextualized in the process of urban renewal of Shenzhen between 2010 and 2017. A total of 509 urban renewal projects are planned to be demolished in Shenzhen in this period. It is required by the Shenzhen municipal government that all demolition waste generated from urban renewal projects must be recycled, rather than landfilled, as the capacity of waste landfills has become saturated (Duan and Li, 2016). 4.2.1 Quantifying recycling potential for non-inert demolition waste Table 3 shows the total generation quantity, unit economic value and recycling potential of each non-inert demolition waste during urban renewal in Shenzhen. A total value of RMB 19,315.85 million yuan of recycling potential may be generated from non-inert demolition 17

Journal Pre-proof waste during the large-scale urban renewal in Shenzhen. Aluminum waste has the largest recycling potential of RMB 8,960.00 million yuan among all kinds of non-inert demolition waste. Copper waste ranks the second, with the recycling potential of RMB 5,528.00 million yuan, which is closely related to their large unit economic value. Following are steel and plastic waste, with RMB 4,125.50 million yuan and RMB 577.60 million yuan, respectively. Besides, due to the low generation quantity and low unit economic value, the recycling potential of glass and wood waste is only RMB 72.25 million yuan and RMB 52.50 million yuan, separately. Despite their low quality, these recyclable components are competitive in the market, since they are less costly than those new materials, e.g., quartz sand and barite (Islam et al., 2019). 4.2.2 Quantifying recycling potential for inert demolition waste Table 5 shows that 46.19 million tons of inert demolition waste are generated during urban renewal in Shenzhen. The quantity of inert demolition waste used to produce different recyclable products is therefore calculated based on the proportion itself used and the total quantity. In addition, the recycling potential of inert demolition waste is also quantified based on the quantitative models established in this study. Details are shown in Table 6. There will the largest recycling potential if recyclable aggregates are produced from inert demolition waste. 28.36 million tons of inert demolition waste is used to produce recyclable aggregate, which means that 28.36 million tons of recyclable aggregate could be produced. It is calculated that the recycling potential is 18.41 million tons when inert demolition waste is produced as recyclable bricks. If it is used to produce standard paving bricks with a size of “100mm × 100mm × 60mm” at 2.5 kg, it could produce 7356 million pieces and finish 7356 hectares of pavement. In addition, 3.80% (1.75 million tons) of inert demolition waste could produce7 million tons of recyclable mortar. Moreover, there is the least recycling potential of 4.16 million tons generated, if inert demolition waste is used to produce lightweight wallboard. Table 5 Total generation quantity of inert demolition waste during urban renewal in Shenzhen (Unit: million tons).

a

Composition

Concrete

Brick/Block

Mortar

Ceramic

Granite

Total

Total generation quantity a

25.20

14.85

4.59

1.28

0.27

46.19

Data from our previous study (Yu et al., 2019).

Table 6 Recycling potential of inert demolition waste during urban renewal in Shenzhen. Recyclable product type

Quantity of inert demolition waste used (TW) (Unit: million tons)

Recycling potential (RP) (Unit: million tons)

Recyclable brick Recyclable mortar Recyclable

15.66 1.75 28.36

18.41 7.02 28.36 18

Journal Pre-proof aggregate Lightweight wallboard

0.42

4.16

5. Discussion This section discusses in depth the recycling potential and recycling processes of different types of demolition waste. Then, the theoretical and practical significance of this research is following discussed. 5.1 Recycling potential of different demolition waste In the past, the majority of demolition waste generated in Shenzhen was dumped in landfills or some undesignated urban areas, while only a very small proportion was recycled (Duan and Li, 2016; Yuan, 2017). In present, the Shenzhen Municipal Government stipulates that every urban renewal project must include a demolition waste recycling management plan in bidding. Recycling enterprises and urban renewal project clients need to sign a waste recycling contract to ensure the true implementation of the recycling plan. As a result, demolition waste recycling in Shenzhen has been significantly improved. It shows from the on-site investigation that onsite recycling has now become a primary way to dispose demolition waste, and recyclable aggregate is the most important product in this process. It is caused by the fact that large-scale infrastructure projects in Shenzhen require a large amount of recyclable aggregates as backfill materials. Many studies have indicated that recyclable aggregates are better than raw materials, such as gravel soil, stony soil, etc., to be used as road base because of wide particle size distribution (Akhtar and Sarmah, 2018; Zhao et al., 2010). Some interviewees said that onsite recycling will be prevalent in the near future. Public data released from Shenzhen Municipal Housing and Construction Bureau shows that eleven onsite recycling enterprises have been set up in Shenzhen from 2016 to 2018. The results show that some non-inert waste has great potential economic value, and its recycling potential is as high as RMB 19,315.85 million yuan. Among them, metal waste has made huge contributions, which is conducive to alleviating the current shortage of resources and facilitating sustainable economic development (Wang et al., 2019a). In addition, inert demolition waste is used to produce recyclable products that could effectively reduce environmental impacts. Findings of the research of Coelho and Brito (2013) show that CO2 emissions from recyclable aggregates are less than those from raw materials with less primary energy consumed. Thus, recyclable aggregate is the most important recyclable product. In addition to recyclable aggregate, recyclable brick is the second largest recyclable product, as it could be widely used in construction of municipal roads, parks, plazas, communities, airports 19

Journal Pre-proof and terminals. Similarly, in order to effectively manage demolition waste and promote environmental sustainability, approximately 82.80 million tons recyclable fired clay bricks are annually produced in Bangladesh (Islam et al., 2019). Recyclable mortar is also one of main recyclable products, which effectively reduces energy consumption and CO2 emissions generated from crushing quarry rocks, and slows the global warming, in process of sand production (Ledesma et al., 2015). Although lightweight wallboard accounts for the smallest proportion, most offsite recycling enterprises declared that they would produce it in large scales, since it plays an important role in the promotion of prefabricated buildings in Shenzhen. 5.2 Implications for theory and practice The main contributions of this study are generally twofold. On the one hand, previous studies rarely focused on the recycling potential of demolition waste. This study proposes an accurate and scientific method for quantifying the recycling potential of different types of demolition waste by considering the actual market situation during urban renewal. Quantitative models established in this study could be applied to other regions. The whole recycling process of demolition waste in two ways, including offsite and onsite recycling, remapped in this study could be used as bases for future research on environmental impacts and economic value. Basic data, such as the proportion of inert demolition waste used to produce different recyclable products and its percentage in these recyclable products measured in this study, could be used in further research on recyclable products. On the other hand, this research also has significant practical implications. Findings of this study could be used to quantify the recycling potential of demolition waste during urban renewal among different waste types. Therefore, it provides guidance for demolition waste recycling enterprises to adjust production scales of recyclable products and arrange production sites reasonably. Furthermore, it also helps government departments develop policies on waste management precisely during urban renewal. Therefore, the findings of this study will be valuable for both academic researchers and practitioners. 6. Conclusions Recycling of demolition waste generated from urban renewal has huge potential that are yet to be properly quantified. This study aims to accurately and scientifically quantify the recycling potential of demolition waste during urban renewal among different waste types. The whole recycling processes of demolition waste were remapped in two ways using the freeflow mapping technique. Quantitative indicators of waste recycling potential were obtained from on-site investigation and semi-structured interview. Models to quantify recycling potential for non-inert demolition waste and inert demolition waste were respectively established based on economic value and the principle of mass conservation. Finally, the quantification method proposed is verified to quantify the recycling potential of demolition 20

Journal Pre-proof waste during urban renewal in Shenzhen. Results show that the recycling process of demolition waste is mainly divided into offsite and onsite recycling during urban renewal in Shenzhen. Their whole recycling processes both include five stages, i.e., waste generation, onsite treatment, transportation, recycling, and product regeneration. Moreover, different types of demolition waste are indicated with high recycling potential. The recycling potential of noninert demolition waste is RMB 19,315.85 million yuan. Meanwhile, the recycling potential of inert demolition waste is calculated according to different product types, such as 18.41 million tons of recyclable bricks, 7.02 million tons of mortar, 28.36 million tons of aggregate, and 4.16 million tons of lightweight wallboard. This study proposes a new method for quantifying recycling potential of demolition waste, which improves accuracy and effectiveness compared with the existing methods. Additionally, it provides guidance for recycling enterprises and government departments to develop scientific plans and policies. However, there are still some limitations in the method proposed. First, economic value mentioned in this study refers only to the economic income of waste recycling companies when the non-inert demolition waste is recycled by them. The mechanical and labor cost incurred in dismantling, collecting and sorting were excluded. Second, the current actual market situation was used to quantify the recycling potential of inert demolition waste, with some uncontrollable factors ignored, such as policies, technologies, economic benefits, and market demand. However, the market situation in the future may not be the same as now. Third, the quantitative method proposed in this study did not take into account energy consumption and environmental impact of demolition waste recycling. Hence, the energy consumption and environmental impact in main paths of both onsite and offsite recycling will be further analyzed. Besides, the quantitative models for recycling potential of demolition waste could be following polished, in order to make a comprehensive calculation on both environmental impact and economic value for different types of waste and recyclable products in the future work. Acknowledgements This study was supported by the National Natural Science Foundation of China (NSFC) (Project Number: 71801159), the Strategic Public Policy Research Funding Scheme from the Policy Innovation and Co-ordination Office of the Government of the Hong Kong Special Administrative Region (Project Number: S2018.A8.010.18S), the National Natural Science Foundation of Guangdong Province (Project Number: 2018A030310534), the Shenzhen Science and Technology Plan (Project Number: JCYJ20160520173631894), and Youth Fund of Humanities and Social Sciences Research of the Ministry of Education (Project Number: 18YJCZH090). References Akhtar, A., Sarmah, A.K., 2018. C&D waste generation and properties of recycled aggregate concrete: A global perspective. J. Clean. Prod. 186, 262-281. 21

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Journal Pre-proof Highlights 

Recycling potential of different types of demolition waste is quantified.



Processes of offsite and onsite recycling are remapped based on free-flow mapping.



Quantitative models for non-inert and inert demolition waste are established.