1. Introduction
Fossil fuel is an irreversible energy source and is the main factor of contribution to greenhouse gas emissions. Moreover, considering that fossil-based energy will eventually be exhausted, countries worldwide seek sustainable energy, and this has led to the continuous improvement of renewable energy technology in power generation and efficiency. It has enabled countries to gradually adjust the structure of electricity generation, reduce the environmental damage potential of fossil-fuel consumption, and increase the electricity generation by renewable energy. The utilization of renewable energy produces fewer air pollutants or wastes. Among the renewable energy sources, biomass has the considerable potential to provide bioenergy. Waste-to-energy (WTE) is an excellent alternative to fossil fuels and is also efficacious in treating waste. Except for the organic fraction from municipal solid waste and food/kitchen waste, agricultural organic wastes are thought to have the potential to provide bioenergy, i.e., biogas, hydrogen, bioethanol, and biodiesel. In Taiwan, most agricultural organic wastes such as rice husk, rice straw, fruit and vegetable residues, fishery residues, livestock, and poultry manures are treated with landfill, compost, incineration, anaerobic digestion, etc. [
1]. The WTE method has proved to come with feasibility and advantages. Pipatmanomai et al. [
2] and Bora et al. [
3] respectively used pig dung and cow dung to produce biogas. Lal and Mohapatra [
4] employed kitchen waste as the source for biogas production, followed by its utilization in dual-fuel compression ignition engine. The research results of El-Mashad and Zhang [
5] showed that adding kitchen waste into a manure digester significantly increased the biogas (particularly methane) yield. The Organization of Food and Agriculture of the United Nations reports that one-third of the food produced for human use per annum goes to waste [
6].
Kitchen waste is an organic material having a high calorific value and nutritive value to microbes. The kitchen waste is derived from leftover meals, vegetables, and fruits, mainly included moisture, carbohydrates, proteins, and oils [
7]. Due to the higher moisture content in kitchen waste, degradation process by anaerobic digestion is an approach to produce biogas [
8,
9,
10]. Considering environmental and economic performances, anaerobic digestion is a much sustainable treatment method for biowastes. Li et al. [
11] have demonstrated that Biowastes converted by anaerobic digestion can output 1–2 times more electricity than incineration [
1], occupy less land-use than landfilling [
12], reduce acidification and eutrophication impacts compared to composting [
13]; produced biogas can be used as gaseous fuel to produce heat and electricity [
14] or upgrade biomethane for serving as the transport fuel [
15]. Energy supply and consumption related to climate change are the focus of environmental legislation and industrial development for pursuing sustainable energy and development in Taiwan. WTE is attractive under the policy encouragement and economic feasibility, especially for the central and southern parts of Taiwan, where regions have a large amount of organic agricultural waste. The annual kitchen waste production in Taiwan was about 650,000 metric tons with 440,000 metric tons for raising swine, 200,000 metric tons for composting, and the rest for producing biomass energy [
16]. However, nowadays in Taiwan, kitchen waste is no more allowed to be used as swine feed to prevent African swine fever, and this kitchen waste needs to be treated using alternative methods. With the right technologies and approaches, kitchen wastes could become a stable, low-cost, and high-return source for producing energy. Taiwan has been a global leader in solid waste recycling but in dealing with biowastes is weak. According to the Taiwan new energy policy, conventional electricity generation is expected to be 50-30-20 clean energy mix of natural gas, coal, and renewables by 2025. It still has a gap from the 35-41-7-17 current energy mix of natural gas, coal, nuclear, and renewables.
Life cycle assessment (LCA) is a tool to transpose life cycle perspective principles into a quantitative framework that seeks to quantify all relevant emissions, consumed, or depleted resources, and the environmental and health impacts associated with the life cycle of product production [
17]. Many LCA research has been implemented regarding the various kinds of biowaste reusing and treatment methods to achieve the WTE goal. More and more LCA research concerns the relevance of biowaste and biogas production, and various ways to convert energy from biogas. Poeschl et al. [
18] analyzed the production and utilization of biogas in different scenarios with diverse feedstock, biogas usage, and digestate processing. Börjesson and Berglund [
19] assessed the possible biogas applications from environmental and energy perspectives, focusing on greenhouse gas emissions and fossil fuel depletion. Naroznova et al. [
20] assessed the global warming potential (GWP) impacts based on the treatment of individual materials found in organic household waste in Denmark and highlighted food waste treatment by anaerobic digestion allowed reducing the GWP. By comparing three different treatment systems, Bernsad and la Cour Jansen [
21] carried out an LCA research and analyzed the use of the resources obtained. Their GWP results showed that biogas digestion caused the main environmental benefits, mainly when biogas was adopted as the substitute for the electricity coal power source. Usack et al. [
22] and You et al. [
23] used LCA results to complement techno-economic measurements and identify the weak points needing improvements. Using LCA can quantify sources of impacts throughout a life cycle for various environmental conditions, allowing environmental improvements to be determined [
24,
25]. Producing value-added products from kitchen waste is another main subject. Nishimura et al. [
26] used kitchen waste to produce biobutanol. Adi and Noor [
27] reused kitchen waste to be vermicompost. Alternatively, Li et al. [
28] produced xanthan gum by pretreatment researching for kitchen waste. A recent review article by Sindhu et al. [
29] focused on value-added products conversion by kitchen waste, which also pointed to the difficulties in proper collection, storage, and bioconversion of kitchen waste valuable by-products as a significant hurdle of waste management. The treatment management and policy also considered in previous research. Hua et al. [
30] discussed the emissions reducing commitment issues in developed and developing countries by comparing the renewable energy development policies and status in Australia and China. Different studies have been implemented due to the large amount produced and kitchen waste’s urgent management needs in Taiwan. Lo et al. [
31] have researched the potential to provide bioenergy in biogas production from most agricultural organic wastes by anaerobic digestion in Taiwan. Tsai [
32] discussed the current status of turning food waste into value-added resources in Taiwan. Chen [
33] applied the cost analysis for food waste composting in Taiwan. However, none of them aim to identify a comprehensive assessment and comparison of the impact of Taiwan’s kitchen waste treatment methods, including environmental, energy, and economic aspects, considering, in particular, both current and expected treatment methods. These points highlight the distinguishing characteristics and value of this research.
This study evaluated the environmental impacts in using four treatment scenarios including anaerobic digestion to treat kitchen waste to produce biogas for generating electricity, which became the primary strategy for treating biowaste in Taiwan. The case study was on the central region of Taiwan that has the following characteristics: (1) It is one of the leading agricultural areas with high population density and producing large amounts of biowaste and municipal solid wastes, and (2) it is also one of the leading industrial areas in Taiwan and is one of primary energy consumers. This study aims to use the LCA method to compare the potential environmental impacts at four scenarios on disposing and recycling kitchen waste in central Taiwan to identify the lowest environmental impact to support the adoption of sustainable waste treatment and opportunity for WTE and utilization as an energy development strategy.