An Integrated Multi-Dimensional Method for Assessing Waste-to-Resource Systems to Promote Resource Circulation

有關多維度綜合評估廢物資源化方法的研究以促進資源循環

Student thesis: Doctoral Thesis

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Award date3 Sep 2021

Abstract

As food and water are indispensable resources closely linked to human survival and economic development, increasing scarcity and quality issues related to both of these resources have emphasized the importance of efficient resource management in urban areas. To eliminate waste and reduce resource utilization, the concept of circular economy has drawn increasing attention in recent decades. Circular economies aim to develop enhanced resource sustainability through efficient waste management and by redesigning consumption and production patterns. To establish an efficient circular economy system, it is necessary to have the appropriate assessment procedure to confirm its feasibility in the decision-making process.

However, current assessment procedures still have a room for improvement as they do not traditionally consider the integration of all potential dimensions, leading to unconsidered externalities that could affect the decision-making process. Therefore, in this study, I assessed two waste treatment and circulation scenarios, one for food waste and one for water, with an integrated assessment method that includes the technological, economic, and environmental approaches. This method allows for the consideration of not just technical feasibility, but the net benefits and carbon impact of a proposed solution to comprehensively evaluate the proposed solutions against existing solutions.

Firstly, I investigated composting as an effective treatment for food waste, which is inherently difficult due to several factors, including its heterogeneous composition, high moisture content, and low heating value. To address these issues, I aimed to convert food waste into a new resource which could be utilized as a fertilizer or energy source by using naturally occurring fermentative microorganisms embedded in wooden biochips (bio-catalysis), utilizing an on-site “Smart Food Waste Recycling Bin” (S-FRB) system. The operation of the unit was analyzed to determine its required operating time, conditions, efficiency based on local conditions, quality of its output.

A high-throughput 16S rRNA gene sequencing analysis was used to identify the major aerobic and facultatively anaerobic bacteria, with alpha-diversity in terms of the Phylogenetic Diversity index ranging from 40.8 (initial stage) to 24.5 (mature stage), which indicated that the microbial communities are relatively homogeneous and effective for use in the S-FRB. Operational results indicated that the organic content of the food waste treated in the system increased from 53% up to 72% in the final end-product and achieved a mass reduction rate of approximately 80%. The heating value of the end-product, which was 3,300 kcal/kg waste when measured by the differential scanning calorimeter (DSC) method, confirmed its high potential as a biofuel. Overall, the S-FRB system presents a practical approach for food waste treatment that solves the putrescible waste problem and maximizes utility through resource circulation.

Next, I applied Life Cycle Assessment (LCA) to determine the environmental impacts associated with the on-site S-FRB technology to assess its feasibility from environmental perspective in comparison with the existing centralized treatment options and identify environmental hotspots to reduce these impacts. Based on this pilot-scale study, hypothetical scenarios were developed which considered potential scaled-up deployments and associated environmental impacts of the S-FRB based on a variety of assumptions. Examination of these scenarios demonstrated the potential for remarkable reduction in CO2 equivalent emissions during food waste treatment by utilizing local production of wooden biochips and operating at full capacity.

Cumulative Energy Demand (CED) and Energy Return on Investment (EROI) were also investigated to understand the energy balance of the S-FRB technology, and I found that further optimization of the S-FRB technology enhanced EROI noticeably. Finally, using current waste treatment methods in Hong Kong as a benchmark, the environmental impacts of the S-FRB are compared with the conventional food waste treatment options such as landfilling and organic waste treatment facilities (OWTF). I found that the impacts associated with the operation of the S-FRB at full capacity were lower when compared with conventional food waste treatment options in the local market due to the reduction in transportation related emissions.

Secondly, to determine an efficient solution to overcome water scarcity, I investigated the water sector from two perspectives – demand and supply. As water scarcity increases, efficient usage and production become increasing important to ensure water security. Therefore, I determined the performance and efficiency dynamics of water consumption by differentiating the efficiency indicator into long-run and short-run categories. Further, I combined the econometric frontier approach with panel Markov-switching and Tobit estimations to investigate macro level data of mainland China over the period of 2002-2016.

The results showed that long-term efficiency was lower than the short-term efficiency, indicating that the inefficiency of water usage mostly comes from long-term structural societal factors. From the results of the dynamic analysis, I found that a low-level efficiency status is likely to be consistent in the following year compared to a high-level efficiency status. Meaning, that if a province is less efficient in a given year, that province will likely remain inefficient in the following year. This suggests that government should re-design their water policy to focus on a long-term basis to ensure sustainable and efficient management of water resource use.

Finally, as it is difficult to find an effective short-term solution to address demand, I focused on the supply side. One of the easiest ways to overcome water scarcity is to find new resources, and therefore, the potential options for securing sustainable water resources have been studied by many researchers. Currently, wastewater reclamation and reuse projects have been gaining significant attention as ways to meet these goals. Hence, I investigated wastewater circulation and its feasibility in terms of its net benefits, which include social, economic, and environmental impacts.

I conducted a hybrid cost-benefit analysis, which integrated the external environmental impacts that are traditionally excluded. Based on this, I evaluated the true net benefit of wastewater treatment and reclamation by considering the explicit and implicit costs and benefits. To determine implicit costs, I estimated the external environmental benefits from pollutant abatement based on the shadow price of each pollutant, using a distance frontier function. To estimate these cost, I developed a model based on the Drainage Service Department’s (DSD) statistical data from 2014 to 2018 for the Shek Wu Hui plant in Hong Kong. I then conducted a life-cycle assessment (LCA) to investigate the potential environmental impacts of the water treatment process in monetary terms using Simapro 8.5.2.0 software, the EcoInvent 3 database, and Stepwise2006 methodology. A sensitivity analysis was applied to control the uncertainties and enhance the reliability of result. The results revealed that the true benefit is noticeably higher than the unit cost of wastewater reclamation, which proves that wastewater reclamation is viable from both social and economic perspectives.

As a whole, I assessed these resource circulation scenarios, which employed new technology or system application to more fully compare their feasibility. This hybrid assessment method, which integrated economic, technological, and environmental analyses can offer practical guidance in the design of circular economy policies by providing information on aspects that are excluded by traditional methods.