Application of life cycle assessment in measuring the environmental impact of waste cooking oil utilization for biodiesel - a review

Utilizing waste cooking oil for fuel is one of the steps to reduce the uncontrolled environmental impact due to the disposal of waste cooking oil into water bodies and onto the ground. Utilization as fuel will contribute to achieving net zero emissions and reducing global warming as the primary target. With increasing attention to the environment, the extent to which the process of making waste cooking oil impacts the environment has also been widely discussed. Using the Life Cycle Assessment (LCA) methodology as a strong tool to assess the environmental impact, many previous studies have carried out the environmental impact concerning waste cooking oil utilization for biodiesel. This paper reviews previous research from around the world in the 2008-2023 period related to the environmental impact assessment of using LCA in manufacturing waste cooking oil for biodiesel. The review results are divided into scope and technology used, manufacturing raw materials, research locations, functional units, system boundaries (cradle to grave, gate to gate, and gate to grave), software used, midpoint impact assessment, impact assessment method, and impact category.


Introduction
In Indonesia, frying food is a well-established cultural tradition where cooking oil is often reused several times [1][2].In terms of health, the frequent use of cooking oil increases the carcinogenic content in food.Therefore, its use is not recommended since it potentially causes serious diseases [3] The government has taken steps to educate and inform the public about the dangers of reusing waste cooking oil (WCO) for health reasons.It is also important to prevent the discharge of WCO into water or soil [4][5].Utilizing WCO as a material or energy source is a highly effective approach.It encompasses the production of various products such as bio-lubricants [6][7], bisabolene [8], liquid soap [9], aromatherapy candles [10][11], surfactants [12], plasticizers [13][14], asphalt rejuvenators [15][16], and surfactant [17][18][19] and biofuel feedstock [20][21][22].WCO (Waste Cooking Oil) has been utilized as a raw material for biodiesel in various locations.For instance, PT Lengis Hijau in Bali uses it as boiler fuel

Methodology
To select studies using LCA methodology in assessing the environmental impact of biodiesel production processes from WCO, we use the free software Perish-8 [26], which has access to databases such as Scopus and Google Scholar.This application is designed to evaluate the academic impact of papers and track their citations.It also allows for advanced search and filtering based on criteria such as publication year, author, and title.We use Perish's function to select studies based on their paper title.

Figure 1 Flow chart of selection steps
Figure 1 shows the paper selection steps.To select the title of the paper, keywords such as "Life Cycle Assessment," "Lifecycle Assessment," or "LCA," as well as "Waste Cooking Oil," "Used Cooking Oil," "WCO," "UCO," and "biodiesel."Were used.The papers were searched from the Scopus & Google Scholar databases from 2008 to 2023 and found 26 relevant papers.The papers reviewed have been sorted into three categories based on the raw materials and technology employed to create BDF (biodiesel fuel)/HBD (hydrogenated biodiesel).The first category assessed the environmental impact of using a single material, WCO, to produce BDF through one technology.The second category compared the environmental impact of two materials, one of which was WCO, using the same technology to produce BDF.Lastly, the third category compared the environmental impact of one or two different technologies used to produce BDF/HDB using WCO and other fuels.
Two of the 26 papers that met the criteria were excluded as they were irrelevant to the life cycle assessment (LCA) methodology in the WCO processes into BDF.The review scope includes 13 papers for review scope 1, 9 for review scope 2, and 2 for review scope 3. The papers not included in the review focused on the life cycle risk assessment [27] and a review of the LCA methodology [28].

Review scope
Table 1 shows that the WCO processing into biodiesel via the transesterification process shows the highest number, followed by ultrasound-assisted transesterification technology and hydrogenated biodiesel.The type of catalyst used consists of KOH, NaOH, and CaO.WCO is processed into BDF [29] WCO is processed via transesterification with NaOH catalyst [30][31] [32] WCO is processed via ultrasoundassisted transesterification KOH catalyst [33][34] [35] WCO is processed via transesterification [36] WCO is processed as hydrogenated biodiesel [37] WCO is processed via transesterification with KOH catalyst [38] WCO is processed via supercritical conditions [39] WCO is processed via transesterification and compared to first and third generation of BDF and Petro diesel in the production and use stage [40] WCO is processed via transesterification and compared to ULSD in the production and use stage [41][42] WCO is processed via transesterification and compared to WCO disposal without recycling [43] WCO is processed via transesterification and compared to Jatropha oil in the production stage [44] WCO is processed via transesterification and compared to rapeseed oil and petrodiesel in the production stage [45] WCO is processed via transesterification with heterogeneous catalyst (waste chicken eggshell derived CaO) into BDF and compared with jatropha oil [46] WCO and soybean oil are processed into biodiesel [47] WCO is processed via transesterification and compared to raw sunflower oil in the production stage [48] WCO is processed via catalytic cracking and hydrogenation into HBD [49] , WCO is processed via transesterification and room temperature transesterification [50] 3.1.1.Review scope-1 In this scope, the papers reviewed are limited to the paper discussing BDF production from WCO. Mortaza [29] demonstrated that the environmental impact of the treatment and collecting stages in BDF production from WCO is worse compared to biodiesel production.Besides that, according to Yang [34], the transportation in collecting waste cooking oil (WCO) has the most significant environmental impact compared to other processes.Regarding the WCO collection method, Ghodsirad [35] examined three methods: door-to-door collection from households, collection from restaurants, and collection at the leading retail shops in Portugal.The study found that the door-to-door collection method has the highest environmental impact compared to the other two methods analyzed.Therefore, the study suggests the need for policy interventions to reduce the environmental impact of these stages.Furthermore, Calderia [30] also analyzed three WCO collection methods: a street drops of the container, door-to-door, and food service industries (restaurants) using a Performance Indicator (PI) defined as the average volume of WCO collected per kilometer travelled.The result shows PI depends on several factors such as location and population density for street drop of the container, collection service and participant citizens for door-to-door, and restaurant and efficiency service for restaurant collection method.The environmental impact of BDF production was assessed using LCA and ELCA (Exergetic Life Cycle Assessment) by Peiro [38], he pointed out that the transesterification stage is the most impactful on the environment and highlighted the significant role of renewable energy use in reducing the environmental impact.
Regarding the relationship between BDF production optimization from WCO while minimizing environmental impact, Bobaldilla [32] considered five existing input process parameters consisting of oil molar ratio, catalyst dose, reaction temperature, reaction time, and stirring speed and evaluated their effect on BDF yield, high heating value, energy consumption with the high yield and environmental impact.His result allowed for optimizing the BDF production process for high yield and heating value while minimizing environmental impact.Aghbashlo [33], using sensitivity analysis, has proven that electricity consumption has a high environmental impact on human health.A scenario with a low environmental impact is obtained using ultrasound-assisted transesterification with several simulations related to the amount of input-output, transesterification condition, operation process, mole ratio, etc. Bezergianni [37] shows that the hydrotreatment process contributes the highest Global Warming Potential (GWP) to the consumption of H2 in the hydrotreatment process and steam in the pretreatment process.Regarding the BDF production method, Nagapurkar [39] has compared the supercritical process with the conventional alkali-catalyzed pathway, and the result shows that the supercritical process is more feasible and environmentally sustainable.

Review scope-2.
In scope 2, it discusses the process of making BDF using the transesterification process compared to the use of WCO with other raw materials such as ULSD/Ultra Low-Speed Diesel [41][42], first and third-generation BDF [40], jatropha oil [44], rapeseed oil [45], petrodiesel [45], soybean oil [48], sunflower oil [48].The results from the papers reviewed prove that using WCO to produce BDF has a lower environmental impact than other materials in all stages.
According to Viornery [41], the production of ULSD has a high environmental impact compared to the WCO-BDF blend because the production process creates a high environmental burden, such as extraction activities and refining crude oil, including transportation and distribution.Moreover, Shurafa'a [42] proves that the BDF produced from WCO has a low environmental impact compared to Petro diesel in both the production and use phases.Also, Hartini [43] demonstrated that the total environmental impact of WCO processing into biodiesel was more significant than if WCO was disposed of, but biodiesel processing also had positive environmental benefits in certain categories.Compared to other materials, such as rapeseed and fossil diesel, BDF production from WCO has a lower environmental impact [45].
According to Foteinis [40], the environmental impact of second-generation biodiesel is lower than that of first and third-generation biodiesel.This is because the initial process for the first and third generations of BDF production required palm oil and algae cultivation, which required energy and had a more significant environmental impact.Iglesias [48] shows that raw materials and biodiesel transportation have a higher environmental impact than centralized and decentralized BDF production from sunflower oil and WCO.Chung [44] indicates that BDF from WCO has a lower environmental impact than Jatropha oil biodiesel production, and waste chicken eggshell-derived CaO as a catalyst has less impact than potassium hydroxide.He also demonstrates that the transesterification stage of the WCO process has the highest environmental impact compared to other processes because this process consumes more electricity than other processes.
3.1.3.Review scope-3.In this scope, the WCO processing to BDF via several types of technology is reviewed, and the environmental impact of the materials used is compared using the LCA methodology.It is known that the technology for producing BDF using WCO consists of hydrotreating [51], gasification [52], pyrolysis [53], and trans-esterification [54].However, according to the papers reviewed, no paper has been found that discusses the environmental impact of gasification and pyrolysis technology using the LCA methodology.According to the paper reviewed, Yano [49] demonstrates HBD (hydrogenated biodiesel) from WCO and compares the environmental impacts of HBD with fossilderived diesel fuel and fatty acid methyl ester (FAME)-type biodiesel fuel.The results show that HBD production can significantly reduce these environmental impacts compared to the other fuel types.The study concludes that shifting from FAME-type biodiesel to HBD could effectively reduce environmental impacts.Besides that, Bonshley [50] summarizes that the conventional biodiesel process has a higher environmental impact than the room temperature process due to higher emissions of greenhouse gases and impacts in other categories.

Midpoint impact assessment
Midpoint assessment in LCA occurs after the inventory analysis stage and before the endpoint assessment stage.This stage aims to link inventory results containing quantitative data about input and output in a system (such as emissions, energy consumption and waste) with indicators that reflect potential environmental impacts, as follows: 1) Global warming potential (GWP), 2) Acidification potential (AP), 3) Eutrophication potential (EP), 4) Ozone layer depletion potential (ODP), 5) Abiotic depletion potential elements (ADP elements), 6) Abiotic depletion potential fossil (ADP fossil), 7) Human toxicity potential (HTP), 8) Photochemical ozone creation potential (POCP).However, not all midpoint impact assessments are discussed in the reviewed papers.
The highest impact of Global Warming Potential (GWP) is caused by pre-esterification [38] and transesterification [44] activities, while the impact of GWP, Photochemical Ozone Creation Potential (PWAE), Ozone layer depletion potential (ODP), and PO (Potential Ozone) is greater when electricity is sourced from fossil fuel [38].This impact can be minimized using new and renewable energy-based electricity sources with a dominant composition.In the WCO transportation process and processes in biodiesel factories, midpoint environmental assessment parameters have the highest impact in biodiesel factories [34] [48], except for Urban Land Occupation parameters, where the land area of the road length has a large influence on UCO transportation [34].The study found that the methanol content had the highest influence on the environmental impact categories, while the methanolysis temperature had the lowest impact [33].

Conclusion
A review of previous studies divided into three scope categories found that the research focused mainly on the transesterification process.Meanwhile, other processes, such as HVO, have yet to be the focus of much research.These findings can be an illustration of the focus of future research.The Life Cycle Assessment (LCA) methodology is a highly effective approach for evaluating the environmental impact of each stage of BDF production.It has been proven that using WCO for biodiesel production positively impacts the environment compared to disposing of it.Furthermore, the environmental impact of using WCO is lower than that of using other raw materials.This study analyzed only papers with titles containing specific keywords such as "Life Cycle Assessment," "Waste cooking Oil," and "biodiesel."There may be other relevant studies related to LCA, biodiesel, and WCO that were not included in this review.It is recommended that future research delves into a more comprehensive array of topics and materials related to LCA, biodiesel, and WCO instead of solely focusing on specific keywords found in paper titles.

Table 1 .
Review scope