Bioenergy for a Cleaner Future: A Case Study of Sustainable Biogas Supply Chain in the Malaysian Energy Sector

: A life cycle assessment (LCA)-based environmental sustainability evaluation conceptual framework of biogas production has been proposed to improve the sustainability of biogas supply chains. The conceptual framework developed in this study can be used as a guideline for the related stakeholders and decision makers to improve the quality and enhance the sustainability of biogas production in Malaysia as well as promoting biogas as a clean, reliable and secure energy. A case study on an LCA analysis of a zero waste discharge treatment process has been conducted. In the zero discharge treatment system, biogas can be produced with a maximum water recycle and reuse. It was indicated that the biogas production and zero discharge treatment of a palm oil mill e ﬄ uent were environmentally sustainable as the system utilized organic waste to produce bioenergy and achieved zero discharge. However, there were other aspects that should be taken into consideration, particularly regarding the sources of electricity and upstream activity, to ensure the sustainability of the system holistically. (FEP), terrestrial ecotoxicity (TETP), ecotoxicity (FETP), marine ecotoxicity (METP), human carcinogenic toxicity (HTPc), human non-carcinogenic


Introduction
Energy plays a crucial and challenging role in sustainable development. The Malaysian government has been continuously reviewing its energy policy to ensure the sustainability of energy resources due to the increase of energy demand [1,2]. The emerging of renewable energy production to replace fossil fuels consumption could certainly reduce environmental pollution because it provides a low-carbon energy system [3]. Nonetheless, the production of renewable energy consists of input and output flows and operational processes which may influence its performance. It is essential to obtain renewable energy in sustainable ways in order to achieve sustainable development as well as promoting economic growth in the country. Clean energy, sustainable cities and communities, responsible consumption and production and climate action are included in the 17 Sustainable Development Goals (SDGs). Griggs et al. [4] argued that planetary stability should be integrated in the United Nations' targets, which are to eliminate poverty and protect the earth's life-support system.
Biogas is produced by the indirect conversion of solar energy stored in natural organic matter into a gaseous energy carrier by anaerobic fermentation; therefore, biogas is the end product of microbiological fermentation [5,6]. The main combustible component in biogas is methane (CH 4 ), and it also contains significant amounts of carbon dioxide (CO 2 ) and other trace gases [7,8]. Biogas can be used for heating, electricity generation, as a fuel for vehicle or distributed via the natural gas grid [9,10]. Furthermore, the digestate produced by the anaerobic digestion process can be used as a biofertilizer in agriculture, thus reducing the need for chemical fertilizers [11,12]. However, questions about the sustainability of bioenergy pathways have been raised, because the conversion of biomass into energy consists of input and output flows which may affect its environmental performance. Therefore, a holistic and comprehensive environmental tool like Life Cycle Assessment (LCA) can be used to assess and ensure the environmental sustainability of biogas supply chains [13,14].
Life cycle assessment (LCA) is a comprehensive assessment and a holistic approach that can provide relative and accurate information to be applied in environmental management [15][16][17]. LCA can be used to assess environmental burdens related to a product, process or service by identifying the energy, materials used and emissions released to the environment [18][19][20]. The LCA approach has long been practiced around the world, and it has significantly improved in recent years. According to Talve [21], LCA was first introduced in 1960 in the United States of America by Harold Smith. He presented his research in the 1963 World Energy Conference on the assessment of the cumulative energy needed to produce chemical products. However, this methodological approach is still new and under development in Malaysia. Hence, this study has taken a further step in the direction of evaluating the environmental impacts of biogas production based on the LCA perspective.

Sustainable Biogas Production
Recent years have seen a surge of interest in assessing the environmental impacts of the production of green goods and services. This is true particularly in the renewable energy system, increasingly concerned with sustainable environmental requirements. For example, Malaysia has recently become one of the most important poles of biofuel technology in the world [22]. This is due to its huge palm tree plantations, a source of biofuel production. Moreover, there is an abundant biomass source in Malaysia from agricultural crops and wastewater from industrial activity that can be utilized as feedstock for bioenergy production [23,24]. However, without appropriate wastewater treatment and management, a huge source of renewable energy will be wasted and, at the same time, become a menace to the environment. Hence, biogas production from available biomass waste and wastewater could be one of the suitable solutions to overcome the wastage. Biomass waste utilization has a direct impact on the recovery of energy. There are a number of energy recovery methods which can be used, such as biochemical (e.g., anaerobic digestion, composting and vermicomposting), thermal conversions (e.g., gasification, incineration, fast and slow pyrolysis) and chemical conversions (e.g., transesterification) [25,26].
In Malaysia, LCA studies were mostly conducted by the Malaysian Palm Oil Board (MPOB) and SIRIM Berhad. Various areas have been covered in the LCA study, such as waste management, petroleum, agro-industry and palm oil [27,28]. A study conducted by Aziz et. al [24] revealed the potential of biogas production from six types of substrates in Malaysia. In addition, recent studies on biogas production from the anaerobic digestion of a palm oil mill effluent have been conducted to highlight the feasibility of the LCA approach in biogas production, as well as the opportunity and challenges from the Malaysian perspective [27]. However, there are no strict regulations issued by the government concerning biogas plant installation and utilization, despite the various green policies that have been developed and introduced. Even though biogas generation is still at a nascent stage in Malaysia, it has a high potential in the way forward to achieve sustainable development. Therefore, it is important to assess the environmental performance of the system to ensure and enhance its sustainability [28,29]. A proper guideline, like a LCA-based conceptual framework, would also assist the government and related stakeholders in making decisions to improve the environmental performance of biogas production. Hence, an LCA-based conceptual framework was developed and proposed in this study ( Figure 1). The framework shows the integration of policy drivers, proposed actions, existing green initiatives, green market influences and sustainability evaluation using the LCA approach affecting the sustainability of biogas production supply chains. The LCA-based environmental evaluation is an integrated approach which can assess the environmental performance of biogas production. LCA is a cradle-to-grave approach for assessing the impacts of a product throughout its life cycle, from raw materials extraction, through production, manufacture, transportation and use, to the management of the discarded product, either by recycling or final disposal [30]. The information from the conceptual framework of an LCA-based environmental evaluation of biogas production could assist the consumer in making choices towards green goods and services. Moreover, an LCA approach will act as a continuous measure and help both policy and decision makers to identify the opportunities for environmental improvements.

Policy Drivers
Many initiatives have been proposed and implemented by the government to promote sustainable development. As shown in Figure 1 The energy-related policies were introduced to ensure future energy security and stability [31,32].
In 1981, the Four Fuel Diversification Policy was proposed to prevent over-reliance on oil as the main energy source, with a diversification of the energy mix to include gas, hydropower and coal. The National Biofuel Policy promoted the use of biofuels through incentives and making available 5% diesel and 5% palm olein biodiesel blends. The National Green Technology Policy targeted the development of renewable energy for energy security and considered renewable energy as an important factor for economic growth. The utilization of indigenous renewable resources was introduced through the National Renewable Energy and Action Plan to achieve electrical supply security and sustainable socioeconomic development. Under the Renewable Energy Act, a tariff system to promote renewable energy generation was established. The transition of attention towards renewable energy generation can be seen through the continuous development and realignment of energy policies.
Many countries' governments around the world have committed to decreasing greenhouse gas (GHG) emissions by promoting renewable energy. Both developed and developing countries have set a renewable energy target and promulgated legislation and regulations to encourage renewable energy development. Table 1 shows the regulations and measures related to renewable energy in several countries. The policies and measures were designed to reduce fossil fuel dependency, to promote the development and utilization of renewable energy, diversifying energy supplies, ensuring energy security, protecting the environment and considering economic and social sustainability [33,34]. According to Yaping Hua et al. [34], by the year 2013, at least 144 countries had set different renewable energy targets and policies at the national level. In addition, there was a total investment of 244 billion USD in renewable energy development globally.

The Green Initiatives in Malaysia
The government has provided a great effort and commitment through many renewable energy and green initiatives at both the national and international levels to promote green and sustainable development ( Malaysia also took part at the international level (i.e., United Nations Framework Convention on Climate Change (UNFCCC) and Clean Development Mechanism (CDM) under the Kyoto Protocol), which shows the commitment of the government to shifting towards cleaner and sustainable development. More innovative initiatives and strategies would help to identify multiple pathways towards sustainable energy. Due to this commitment, the environmental evaluation of a biogas production system using LCA tools can be integrated into Malaysia's policies and action plans to take further steps towards sustainable development in the future.

Life Cycle Sustainability Assessment
Sustainable development is defined as development that meets the needs of the present without compromising the ability of future generations to meet their own needs [38]. In order to develop a life cycle sustainability assessment (LCSA), integrated environment, economy and social perspectives need to be considered by looking at three different dimensions of the same system [39][40][41] (Figure 1). The LCA produces numerical data and indicators to evaluate the used resources and environmental impacts; life cycle costing (LCC) produces cost indicators to evaluate the cost-effectiveness; and the social life cycle (S-LCA) introduces social indicators to evaluate the corporate policy and human rights of a product or system. Therefore, if the study could extend into economic and social dimensions in the future, a thorough understanding on the sustainability of the system could be achieved. According to Coyle and Rebow [42] and Jürgensen et al. [43], sustainability in energy systems is usually associated with energy efficiency and energy with lower emissions. An evaluation tool like the LCA can be used to assess the environmental performance and determine the hotspots along the supply chains, thus providing a comprehensive assessment of the sustainability of the renewable energy system. The evaluation tool is one of the important factors that should be integrated into sustainable energy, in addition to public awareness, sustainability education and training and the promotion of renewable energy resources.
The environmental life cycle assessment of biogas production supply chains helps to define the up-and down-streams of the whole system and identify possible problems at each stage of the evaluation process [44,45]. The contribution of the process to the selected impact categories can be determined. Several actions and improvement strategies can be proposed for enhancing the environmental sustainability of biogas production. Other tools that can be used to support the decision making and to evaluate the impacts of renewable energy systems are the ecological footprint, energy analysis, net energy balance, carbon footprint, GHG life cycle analysis, material flow analysis, sustainability indicators, fuel cycle analysis and life cycle risk assessment [46]. However, it has been found that the LCA is preferable as a research-based approach and able to holistically assess the environmental sustainability of renewable energy systems [46]. As reported by Sala et al. [47] and Takeda et al. [48], the LCA approach could identify deficiencies for further improvements and help to avoid problem and burden shifting from one part of the system to the others.
Bioenergy production from organic waste does not automatically imply that its production, conversion and utilization are sustainable [49]. Based on the findings of LCA studies for renewable energy by Milazzo et al. [50] and Turconi et al. [51], the sustainability of bioenergy is more than just GHG savings. It is also associated with specific resource use and the potential environmental consequences. The sustainability of these components is important because any deficiencies could lead to market distortion and consequently impede the efforts to increase renewable energy shares in Malaysia's energy capacity. Figure 1 shows some issues that influence the growing interest towards sustainable renewable energy production. Fossil fuels depletion, increasing energy demand and environmental pollution problems have been a major influence in the transition of attention towards renewable and sustainable energy [52,53]. The current plan also aims to obtain renewable energy in sustainable ways, to have a low-carbon energy system and to have a proper waste management practice, as well as promoting economic growth. However, it is crucial to have an appropriate action plan to overcome the obstacles and barriers which limit the feasibility of sustainable biogas production supply chains in Malaysia.

Influences and Proposed Actions
According to Figure 1, a few strategies and initiatives that could be proposed to help improve the sustainability of the system are the research and development (R&D) activities, which should be enhanced to improve bioenergy production, more renewable energy incentives, collaborations with developing countries and knowledge transfer, developing and increasing local technology to reduce cost, minimizing operational complexity, developing local expertise in design, operation and maintenance, improving the availability of input-output inventories, eliminating regulatory barriers and also conducting policy advocacy to the political, economic and social systems and institutions.

Promoting Biogas Production from a Palm Oil Mill Effluent as a Renewable Energy Source in Malaysia
The palm oil industry in Malaysia has boosted the country's economy, as it was one of the largest palm oil exporters and producers in the world. The palm oil industry has supported a variety of food products like margarine, cooking oils and animal feeds, and non-food products like soaps, detergents, cosmetics, pharmaceuticals and biofuels [54][55][56][57][58]. The by-products generated during the palm oil milling process can be recycled and recovered into reusable materials or products (e.g., shells and fibers used as fuel for boilers, empty fruit bunches used as soil conditioner and palm oil mill effluent (POME) utilized as a source of energy generation) [59,60]. Figure 2 shows the overview of the flow process and products of the palm oil industry. According to Basri et al. [61] and Oswal et al. [62], POME has been applied as feedstock in most palm oil mills to generate biogas. Due to its high organic content, POME can be a good source to generate methane gas for energy production. The exploitation of POME for renewable energy production would enhance the sustainability of palm oil industries. As a consequence, bioenergy generation from POME can be an added value to the palm oil industry in Malaysia.
Issues on energy security and environmental concerns lead to the utilization of renewable energy sources. Upgrading biogas to biomethane and injecting it into the natural gas grid could be an efficient way of integrating the biogas into the energy sector [60]. Accordingly, biomethane can substitute the use of natural gas and can also be used as transportation fuel. However, the unattractive connection price to the grid, the irregular supply of biomass, the low efficiency of combustion technology and poor supporting systems like interconnection infrastructure have caused a shortfall in national renewable energy capacity in Malaysia. In 2017, the total electricity demand in Malaysia was approximately 12 607 ktoe [27]. The estimated energy potential generated from 50k tonnes of POME produced in a year is about 3.2 million MWh of electricity, contributing to 2.19% of the total electricity demand. Though the utilization of biogas as renewable energy is still unregulated in Malaysia, it is crucial because it could improve long-term energy security and environment protection. Current developments in the local and global economies are closely connected to sustainable energy resources. The feed-in-tariff (FiT) mechanism administered by the Sustainable Energy Development Authority (SEDA) was designed to correspond to the National Renewable Energy and Action Plan (2010), that suggested the requirement of legislative solutions to increase the renewable energy share in Malaysia's energy mix [63,64]. The type of resources included in the FiT mechanism are biogas, biomass, hydropower and solar photovoltaic power. At 40 MW, the oil palm biomass contributed the highest grid-connected capacity among renewable technologies in the Ninth Malaysia Plan (2006-2010) [65][66][67]. In addition, oil palm biomass emerged as a major contributor towards achieving the capacity target set under the FiT of 800 MW of grid-connected capacity by the year 2020 [63,68].
However, the survey carried out by Umar et al. (2014) [67] showed that some of the palm oil millers were less interested in embarking on a renewable energy venture. This is due to the fact that most of the mill operators' financial capability is too low to participate in a high cost project, and renewable energy was not a profitable business. Therefore, an innovative incentives scheme should be proposed to reduce the financial pressure on energy providers. In addition, the cost could be reduced by developing and increasing local technology, as well as local expertise in design, operation and maintenance. The operational complexity should also be minimized, so that the system can be conducted and managed easily and the palm oil millers would not overlook the efficiency and effectiveness of the system. Other strategies that could be executed to enhance and promote the sustainability of biogas production in the country are the improvement of research and development (R&D) activities and the establishment of collaborative research with other developed and developing countries, along with a concurrent transfer of knowledge.
Conducting an LCA for a product or system requires a transparent inventory, so that an excellent overview of areas in which inputs could be substituted by less polluting materials can be acquired for the good of the environment. Hence, it is important for industries to record and make available their input and output inventories for research purposes. The conceptual framework of the LCA-based environmental evaluation developed in the present study can be used as a guideline for related stakeholders and decision makers to improve the quality and enhance the environmental sustainability of biogas production from POME in Malaysia, as well as promoting biogas as a cleaner, reliable and secure energy. The stakeholders include the government, oil palm producers, financial institutions and oil palm associations. Government intervention is crucial to drive the industry forward to achieve cleaner technology and towards sustainable development. Accordingly, the market community also shares an equal responsibility and influence, in order to increase the use of renewable energy sources and meet the sustainable development goals. A policy advocacy of sustainable renewable energy resources should be conducted at the political, economic, social and institutional levels, to raise awareness to the fact that environmental-friendly products and systems are starting to get local and global attention. According to Aziz et al. [69], various studies on the implementation of the LCA approach to evaluate the environmental performance of biogas production show that the LCA could help improve the environmental profile of the biogas system. There were many initiatives by the Malaysian government towards sustainable development, which can be a potential driving force to apply the LCA to the environmental evaluation of a clean technology.

Life Cycle Assessment of Zero Discharge Treatment
POME is a remarkably contaminating effluent because of its high content of organic matter (expressed as chemical oxygen demand (COD) and biochemical oxygen demand (BOD)), which can have harmful effects on the environment, particularly on the water resources to which POME is discharged. In recent years, POME has been recognized as a prospective source of renewable energy. Accordingly, the production of bioenergy will be more sustainable and cleaner when operating simultaneously with wastewater treatment. There are various existing processes for POME treatment and the conversion of POME into bioenergy, such as aerobic and anaerobic digestion, physico-chemical treatment and membrane separation [70][71][72][73]. Nevertheless, in the integrated biological treatment of the POME system, which includes membrane treatment, higher quality effluents can be produced. This system is also called zero discharge treatment system. In the zero waste discharge treatment system, all wastewater is purified and recycled. Therefore, the plant discharges zero effluents into the water, which in effect reduces environmental pollution. The anaerobic digestion of POME can produce biogas and a final discharge which meets the proposed regulatory standard of the Department of Environment (DOE), Malaysia. However, in the zero discharge treatment system, biogas can be produced with a maximum water recycle and reuse.
Zero waste discharge treatment has been implemented in several countries around the world (i.e., Italy, United States, Canada, Malaysia, China and India). For instance, this technology has been used for reusing the wastewater produced by car manufacturing and secondary sewage [74], heavy oil recovery [75], wastewater reclamation and reuse in marine ports [76], the treatment of chromium-containing leather waste [77] and treating POME [78]. However, the environmental sustainability of this treatment process in terms of the LCA approach is still under development. One study has been carried out in India by Rajakumari and Kanmani [79] on zero liquid discharge treatment technologies for textile industries, using pretreatment, reverse osmosis (RO) and evaporator treatment units. The results of the study show that the energy consumption in the treatment plant had contributed to the global warming potential (GWP) by 5.56 kg CO 2 -eq per functional unit of 1 m 3 of textile wastewater. It was reported that CO 2 was a leading pollutant, which can be reduced by using biomass gasification.
Wastewater recovery and recycling has become a growing trend because it not only minimizes the environmental impact of discharged water, but can also be an additional water resource which promotes water sustainability [80]. On that account, palm oil mills in Malaysia, particularly, should explore and adopt the zero discharge technology to achieve zero discharge concepts. According to Madaki and Seng [81], POME is a very difficult and expensive waste to manage. Hence, there is a need to shift from the conventional system to the current advanced POME treatment technology. Although the zero discharge treatment system is a promising sustainable technology, its application requires a high amount of energy consumption, which will result in some environmental impacts [82]. Therefore, an LCA of the zero waste discharge treatment process is important, as it will provide additional insights into the environmental sustainability of the technology.

Goal and Scope, Unit Process, Functional Unit and System Boundaries
In this study, the zero discharge treatment of POME has been used as a case study. The LCA includes four phases: goal definition and scope, life cycle inventory (LCI) analysis, life cycle impact assessment (LCIA) and interpretation. A gate-to-gate approach was used, focused on the process of zero discharge treatment. The goal of using the LCA method was to determine the environmental impacts of the zero discharge treatment system and to derive measures to reduce them. The energy and material requirements, as well as the emissions to the environment, were taken into account. The functional unit for impact assessment was a metric tonne of POME. Figure 3 shows the system boundary for the LCA of the zero discharge treatment of POME. Three main processing units (i.e., pretreatment, biological treatment (anaerobic and aerobic treatment) and post-treatment (ultrafiltration and reverse osmosis)) were included in the environmental assessment.

Database and Analysis Methods
SimaPro 8.5.2 software was used to gather and analyze the inventory data. SimaPro 8.5.2 is an LCA software that can be used to monitor the performance of the sustainability of a product or service. This software can analyze a complex life cycle systematically and can evaluate the environmental impact of a product or service at each stage of the life cycle. Ecoinvent 3.4 [83], Agri-footprint 4.0 [84] and USLCI [85] were chosen as background data sources. Ecoinvent 3.4 database was used for this study because it contains LCI data from various sectors, such as energy production, transportation, chemicals production and also fruits and vegetables. Agri-footprint 4.0 database was chosen due to its comprehensive LCI database focusing on the agriculture and food sectors, which covers data on agricultural products (i.e., food, feed and biomass). Besides, Agri-footprint 4.0 covers materials and inputs from agriculture, while USLCI database contains data modules that quantify the material and energy flows into and out of the environment. Using Ecoinvent 3.4, Agri-footprint 4.0 and USLCI data as proxies, the background databases were already incorporated in SimaPro, while foreground databases were newly included in the software for analysis, obtained from survey and existing regional datasets.

Life Cycle Inventory
The life cycle inventory includes inventory data from the inputs and outputs in the process of zero discharge treatment. Inventory data consist of the amount of energy and materials consumed and the quantities of emissions released to the environment. All data were obtained from a set of questionnaires, on-site data, literature reviews and databases in SimaPro 8.5.2. The raw data information was obtained directly from the owner of the mill chosen in the present study. For compatibility, the questionnaires were developed based on the guideline provided by ISO 14044 [87], consisting of the generic information (i.e., annual reports and company brochures), process description and the input and output flows of the analyzed product. An interview with the mill owners and officers was conducted to obtain and validate information regarding the production process, technology used, mill operation and operational issues.
The case study was based on POME-based biogas production from a palm oil mill with biogas and zero discharge treatment facilities in Malaysia, and the data were collected for a period of three years. Data on the inputs and outputs of the production process, including the amount of energy, materials consumed and transportation, were available from 2012 to 2016. However, the quantity and quality of the data were not consistent from year to year. Therefore, this study was conducted based on the average of three years (2013 until 2015) data. The data chosen were the most complete and with best quantity and quality. The validation of data was performed by on-site visits, comparison with other data sources and recalculation. The results from the LCI were subsequently used to assess the environmental performance of the zero discharge treatment process.
The zero discharge treatment system of POME consists of three main processing units, as shown in Figure 3: pretreatment, biological treatment and post-treatment. The core technologies in the POME zero discharge system in this study were AnaEG and BioAX. AnaEG is an advanced anaerobic expanded granular sludge bed reactor, while BioAX is an advanced biocontact aerobic process with internal circulation. In the AnaEG bioreactor, the wastewater flows upwards in a plug flow pattern consisting of a two-phase anaerobic process in one reactor. The BioAX tank contains biofilm that provides a suitable environment for the growth of microbes to further enhance the degradation of organic matter. In BioAX, returned sludge was not required. Table 2 shows the inventory data for the treatment of a metric tonne of POME. The treatment plant was installed in the palm oil mill, and hence no vehicle was needed for the transportation of feedstock. In this study, only a small amount of water is required for the treatment process, approximately 0.2 L per tonne of POME. Rainwater harvesting has been practiced in the treatment plant to collect and store rainwater for on-site reuse. Though Malaysia has sufficient water resources, some regions in the country are facing water scarcity, and the water demand is also increasing nowadays. Hence, rainwater harvesting as an alternative water resource was proposed by the government as part of the solutions to mitigate water scarcity problems [88]. 50% of the total electricity consumption was taken from the national grid and another 50% from the biogas engine. The utilization of biogas for electricity generation could reduce the cost as well as the dependency on non-renewable energy sources. Table 2. LCI for the zero discharge treatment system of POME (per tonne of POME). During the pretreatment of POME, suspended matter such as oil and solids was removed to ensure the effective anaerobic treatment of POME. The sludge oil produced was recovered and sold out as by-product. The biological treatment process produced an average of 28 m 3 biogas per tonne of POME and generated a final discharge with a BOD of less than 20 mg/L. The biogas composition was 65-70% CH 4 , 25-36% CO 2 and 800-1500 ppm H 2 S. The biogas produced was burned in a gas engine to generate power for internal use as electricity and was also utilized as fuel for the boiler to generate steam in the palm oil mill. The treated sludge (digestate) was recovered from the anaerobic digester and sent to the plantation as an organic fertilizer. According to Loh et al. [89], the application of the treated sludge as organic fertilizer could enhance soil fertility due to its high content of nutrients. In this treatment system, no water back flow process is needed in the anaerobic reactor and no returned sludge is required in the aerobic treatment process. The effluent generated from the biological treatment contains a low level of organic and suspended solids. Finally, the post-treatment process' final discharge was clean water that can be reused as boiler feed water in the mill. An RO concentrate amounting to 40% of rejected water was recovered and collected as liquid fertilizer with s high content of potassium and magnesium.

Input/Output
The ultimate goals of this treatment system are to produce biogas as a source of renewable energy, zero emissions of GHG, a final discharge with less than 20 mg/L of BOD and clean water which can be used as boiler feed water. Table 3 shows the characteristics of POME after each treatment in the zero discharge treatment system according to Tabassum et al. [90]. In the presented results, the average removal of COD and BOD after the biological treatment were 98.5% and 99.9%, respectively. After the membrane treatment UF/RO process, the value of COD and BOD were almost undetectable. The quality of water from each treatment stage shows an improvement in terms of color, odor and turbidity and obtained > 90% removal of chemical oxygen demand (COD), biochemical oxygen demand (BOD) and suspended solid (SS). At the final stage, the RO permeate was odor-free, clear and with a pH of 9.48. The zero discharge treatment system was able to reclaim water as boiler feed water, since the water quality complied with the boiler feed water standard set by the American Boiler Manufactures Association (ABMA), which is pH 7.5 until 10 [78]. It is the ideal pH for boiler feed water to prevent corrosion. After the entire treatment processes, higher percentages of water could be recovered and used as boiler feed water, and the recovered biogas could be utilized to produce heat and electricity for self-consumption. Therefore, the products and effluents produced can be considered environmentally sustainable.

Results and Discussion
Life Cycle Impact Assessment The potential impacts were determined using the results from the inventory analysis (LCI). There are four steps in the life cycle impact assessment (LCIA): classification, characterization, normalization and weighting. According to ISO 14044 [87], classification and characterization are mandatory steps for the LCIA, while normalization and weighting are optional steps depending on the goal and scope of the study. In the present study, the classification and characterization steps were included.
The assessment has been carried out on a zero discharge treatment system based on the functional unit of 1 tonne of POME. The impact results at the midpoint level are presented as characterization values in Table 4. The characterization factors indicate the environmental impact per unit of stressor. As presented in Table 4, the most significant contribution to all impact categories originated from the electricity consumption from the national grid. Hence, electricity production from the national grid is an important contributor to all impact categories. This is because the Malaysian grid energy profile is largely composed of non-renewable energy sources, with 45% from natural gas, 41% from coal, 6% from hydroelectric power and 8% from oil [91]. In 2015, the total electricity generation in Malaysia was approximately 144,565 GWh, and 89.3% of the electricity mix was generated by using fossil fuels [92]. The energy demand in Malaysia had increased from 40,845 ktoe in 2009 to 51,807 ktoe in 2015 [93]. It was reported that the use of energy for electricity generation is the largest source of emissions of GHG when compared to other human activities. From 2000 to 2015, the emissions from electricity generation had increased by 45% [91]. CO 2 emissions resulting from the oxidation of carbon in fuels during combustion account for the largest share of global anthropogenic GHG emissions. The cumulative effect of other GHG emissions like CH 4 and N 2 O towards global climate were estimated to be at least one order of magnitude lower than that of CO 2 [94,95]. However, fossil fuels are still being utilized extensively by developing countries in order to meet the energy demand and to support development and economic growth.
In this study, the treatment plant consumed approximately 0.38 kWh of electricity from the national grid, which affected 17 impact categories. The impacts on GWP, ODP, HOFP, PMFP, EOFP, TAP, MEP, TETP, FETP, METP, HTPc, HTPnc and FFP were caused by the production of natural gas, anthracite coal, lignite coal, bituminous coal, diesel and residual fuel oil. The electricity production from hydroelectric power contributed to IRP, FEP, SOP and WCP. As reported before, the electricity generation in Malaysia includes production processes from several fossil fuel resources. The burning of fossil fuels has huge consequences for the environment. Figure 4 shows the proportion of electricity production resources which contributed to 13 impact categories. The data for the electricity production process were adopted from the USLCI and ELCD databases. In the presented results, natural gas was the largest proportion contributing to the GWP, ODP, MEP, FETP, METP, HTPnc and FFP impact categories, at 37% (4.39 × 10 −2 kg CO 2 -eq), 91% (7.03 × 10 −2 kg CFC11-eq), 76% (1.83 × 10 −7 kg N-eq), 66% (1.20 × 10 −4 kg 1,4-DCB-eq), 60% (1.51 × 10 −4 kg 1,4-DCB-eq), 43% (3.64 × 10-3 kg 1,4-DCB-eq) and 40% (1.62 × 10 −2 kg oil-eq), respectively. Anthracite coal was the largest contributor towards PMFP, TAP, TETP and HTPc, at 51% (2.30 × 10 −4 kg PM 2.5 -eq), 51% (7.87 × 10 −4 kg SO 2 -eq), 68% (2.09 × 10 −2 kg1,4-DCB-eq) and 47% (3.71 × 10 −5 kg 1,4-DCB-eq), respectively. For HOFP and EOFP, the largest contributor was lignite coal, with an impact of 31% (6.61× 10 −5 kg NO x -eq) for both categories. For electricity production from hydropower, which is a non-fossil energy, the assumption is made that there is no environmental impact. Based on the study conducted by Lijó et al. [96] on the LCA of electricity production from anaerobic co-digestion of pig slurry and energy crops in Italy, the electricity consumption from the grid contributed to the abiotic depletion potential (ADP), ozone layer depletion potential (ODP) and nuclear energy demand (CED). This is due to the fact that the Italian grid was also composed of non-renewable energy. According to Wang et al. [97], in the study on the LCA of biogas production from straw in China, the electricity consumption from the grid had a harmful effect on human health. The electricity was mostly produced from coal-fired power plants which release gaseous pollutants. Those findings were consistent with those of Roux et al. [98], which show that the impact of electricity consumption on GWP and ADP were due to the higher share of coal and gas power plants in the French electricity mix. Table 5 shows the results of the damage assessment at the endpoint level. The zero discharge treatment system impacted HH, ED and RA at 4.13 × 10 −4 DALY, 1.24 × 10 −6 species·year and 0.009 USD, respectively, due to electricity consumption. As reported by Loh et al. [89], Tabassum et al. [90] and Wang et al. [99], there were no GHG emissions into the atmosphere from the zero discharge treatment system, as the system produces zero discharge. Nevertheless, this study shows, on the other hand, the pollution emissions in the system were caused by the electricity consumption. Although the treatment system could treat all the incoming effluent and leave nothing behind, there are other aspects that should be taken into consideration (i.e., energy sources, water sources, etc.) to ensure the environmental sustainability of the system holistically.  Resource Availability FFP USD 9.00 × 10 −3 Total USD 9.00 × 10 −3 Figure 5 shows the relative contribution from electricity production resources to each damage impact category. Anthracite coal had the largest impact to both HH and ED categories, at 42.1% (1.66 × 10 −7 DALY) and 34.4% (2.35 × 10 −10 species.yr), respectively. According to Oh et al. [93], about 70% of coal demand is for energy production. The coal reserves in Malaysia were estimated to be approximately 1.9 billion metric tonnes [92]. The country consumed more than 20 million metric tonnes annually, and therefore 90% of coal supplies were imported mainly from Indonesia and Australia [100,101]. However, the combustion of coal poses major challenges like GHG emissions and air pollutants such as sulphur dioxide (SO 2 ) and CO 2 . In contrast with gas, coal emitted a higher amount of CO 2 emissions due to its heavy carbon content per unit of energy released [102]. The emission of GHG will lead to an increase in the global mean temperature, which will consequently result in damages to human health and ecosystems. Therefore, at the endpoint level, damages to human health, terrestrial ecosystems and freshwater ecosystems can be estimated. On the other hand, the largest process contribution to the resource availability impact category was from natural gas, corresponding to 63.5% (5.78 × 10 −3 USD). Natural gas, also known as liquefied natural gas (LNG), was the main contributor in Malaysia's energy mix [103]. At the endpoint level, the scarcity of fossil resources is expressed in economic terms [104]. The increase in fossil fuel extraction will cause an increase in costs. Hence, it will lead to a surplus cost potential (SCP), and the damage to natural resource scarcity can be estimated. In 2016, Malaysia became the world's third largest exporter of natural gas, after Qatar and Australia, with an estimated total natural gas reserve of 100.7 tscf [105]. Natural gas production has increased from 54.8 Mtoe in 2010 to 61.4 Mtoe in 2015 to fulfill the growing domestic demand and export contracts [93,106]. The rising of development and exploration activities has subsequently put pressure on the natural gas supply, and more investment is needed for reservoir development.
Electricity was required to run the plant mainly in loading operations, in the digester and in the chiller. More treatment units were used in the zero discharge treatment plant (i.e., AnaEG reactor, BioAX reactor, Nano Air Flotation system and membrane filtration), which may have caused higher electricity consumption. With regard to the impact of electricity consumption, the environmental sustainability of the system can be improved by increasing or enhancing the energy produced from biogas. In the zero discharge treatment plant, the engine generator, also known as gen-set, was used to generate electricity. The generated electricity was utilized for self-consumption and not exported to the grid. Anyhow, most of the biogas plants in Malaysia were running with combustion engines, combined heat and power (CHP) systems or a combination of both technologies [107]. Decentralized power generation with CHP units is the common biogas utilization pathway. Normally, in the typical electricity generation technology (i.e., conventional electricity generation, on-site boiler), the energy produced is wasted in the form of heat discharge to the atmosphere [98,99]. Conventional energy systems include power plants using fossil fuels. By using the CHP generation system, the transmission losses and carbon emissions could be reduced [108][109][110][111][112].
As reported in the study by [67], 86% of the market community were utilizing their biomass resources for on-site consumption, while only a few exported their excess electricity to the grid. This is because the energy produced for on-site electricity generation was sufficient, but it was not sufficient for exporting to the grid. Also, there is a lack of grid transmission lines connecting mills to the existing network systems due to distance constraints [60]. During the Ninth Malaysia Plan (2006-2010), the capacity share of renewable energy in the country's energy mix was 350 MW, or 1.8%. However, due to the slow renewable energy projects, the capacity share ended up at 65 MW, or 0.4%, by the end of the 9 th Plan, and biogas contributes 4.95 MW of the grid-connected capacity [67]. Until 2015, the share of fossil fuels within the world energy supply was relatively unchanged, despite the increase of renewable energy in electricity generation to 34% of the global figure [91]. Most of the electricity mix in Malaysia was generated from fossil fuels, although the country has various renewable energy sources.
Apparently, biogas can be one of the best alternatives energy sources to deal with high energy demands and various environmental loads, including fossil fuel depletion and global warming. In addition, biogas adoption can solve waste management issues by utilizing POME for biogas generation. Thus, the environmental sustainability evaluation of biogas production from a broad range of feedstock (e.g., sludge, food waste, dairy manure, municipal wastewater and solid waste, crop residues, energy crops, etc.) is essential in providing a promising renewable energy source. The environmental assessment of biogas would certainly be useful for environmental profile enhancement and a great opportunity to achieve sustainable development. With regard to existing green policy and initiatives, the LCA of biogas production should also be integrated into the sustainable development plan, as this approach could assist in decision-making process.

Conclusions
A case study concerning the LCA of the zero waste discharge treatment of POME has been conducted in order to examine the key advances in waste-to-energy technologies that have been adopted for biogas production and waste treatment towards sustainable development. The zero waste discharge treatment system is said to be a promising sustainable technology because it can produce biogas with maximum water recycle and reuse.
In promoting biogas as a green product, Malaysia could gain competitive advantages towards renewable energy production, as well as towards better waste management practices. Therefore, a comprehensive framework enforcement is needed to encourage the embracement of renewable energy and stimulate an energy efficiency culture. A conceptual framework for an LCA-based environmental sustainability evaluation of biogas production has been proposed to improve the sustainability of biogas supply chains. The conceptual framework developed in the present study can be used as a guideline for the related stakeholders and decision makers to improve the quality and enhance the sustainability of biogas supply chains in Malaysia, as well as to promote biogas as a clean, reliable and secure energy.
To conclude, this study indicated that the biogas production and zero discharge treatment of POME have potential as clean technologies to be applied in the Malaysian context, as the system utilized organic waste to produce bioenergy and achieved zero discharge. However, there were other aspects that should be taken into consideration, particularly regarding the sources of electricity and upstream activity, to ensure the environmental sustainability of the system holistically.

Conflicts of Interest:
The authors declare no conflicts of interest.