https://doi.org/10.1177/0734242X20935174 Waste Management & Research 1–10 © The Author(s) 2020 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0734242X20935174 journals.sagepub.com/home/wmr Introduction In different societies, especially in developing countries, energy consumption in various industrial and transport sectors is increasing. The primary source for energy supply is fossil fuels (Balmaceda, 2018; Marques et al., 2018). But the major prob- lem with the planet’s atmosphere through burning these fuels is damage to the ozone layer with emission of gases such as CO, CO 2 and NOx. The devastating effects of these gases on envi- ronmental pollution, ozone destruction and human health are not hidden (Thangavelu et al., 2015; Wang et al., 2017). Even if the problem of harmful gaseous emissions to the environment can be controlled, fossil fuel sources will be exhausted in a few decades (Barreto, 2018). Biofuels can be a good alternative to these fuels and are environmentally friendly, renewable and biodegradable. Biodiesel is the most commercial of them (Isa and Ganda, 2018; Othman et al., 2017). Biodiesel is composed of methyl ester of fatty acid (FAME) and can be produced in several ways such as transesterification, pyrolysis, blending and micro-emulsions (Gebremariam and Marchetti, 2017). Transesterification or alcoholysis is more common; it is a reac- tion between vegetable oil (or animal fat) and an alcohol, mainly methanol or ethanol, in the presence of a catalyst (Mumtaz et al., 2017). The main reaction product is biodiesel along with glycerol as a by-product (Monteiro et al., 2018). In addition to the significant advantage of biodiesel, it can be a major energy source for diesel machines and, compared with fossil fuels, burning it in engines produces less gas containing hydrocarbons, polycyclic aromatic hydrocarbons and NOx into the atmosphere (Ajala et al., 2015). Nevertheless, biodiesel can face some economic difficulties. One of them is the high price of feedstock which represents about 70–80% of the ultimate cost (Ambat et al., 2018). To remove this as a challenge, using waste edible-oil (waste frying oil), non-edible oil (algae oil, soursop seed oil) (Dai et al., 2014; Su et al., 2018), insect fat (Nguyen et al., 2018b) and waste animal fat (sheep, cow and pig) can be helpful because they are cheap and available in the restaurants. Waste frying oil is one example, which is discarded after use. Using it reduces biodiesel production costs by 60– 90% (Fadhil et al., 2017; Rezania et al., 2019). The first group includes soybean oil, palm oil, sunflower oil and sesame oil and the second group includes oils like algal oil, jatropha, rape seed and canola oil (Mardhiah et al., 2017; Verma and Sharma, Low-cost biodiesel production using waste oil and catalyst Raheleh Talavari, Shokoufe Hosseini and GR Moradi Abstract With the production of renewable biofuels, concerns about the end of fossil fuels have been partially eliminated. On the other hand, the utilization of low-cost and waste materials to provide the raw essential substances to manufacture these fuels is of paramount importance. Biodiesel is one of these fuels and the required raw materials for the reaction are oil (triglycerides), alcohol and catalyst. In this work, travertine stone powder (as waste in the manufacture of building materials) was used as a catalyst and waste frying oil as a source of triglyceride for biodiesel production. Using thermogravimetric and X-ray diffraction analysis, optimum temperature for catalyst calcination was selected at 900°C. Furthermore, X-ray fluorescence, Fourier transform infrared spectroscopy, Brunauer– Emmett–Teller, transmission electron microscopy and scanning electron microscopy analyses were performed. Using the design of experiments Response Surface Methodology, the optimum reaction conditions for biodiesel production yield of 97.74% were: reaction temperature 59.52°C (~60°C), time 3.8 h (228 min), catalyst concentration 1.36 wt.% and the methanol to oil molar ratio of 11:6. After reusing four times, the catalyst efficiency was reduced a little, and the biodiesel yield was 89.84%, indicating high strength and stability of the catalyst. Keywords Biodiesel, travertine, waste catalyst, waste cooking oil, transesterification Received 9th March 2020, accepted 25th May 2020 by Associate Editor Mario Grosso. Catalyst Research Center, Razi University, Iran Corresponding author: GR Moradi, Catalyst Research Center, Faculty of Chemical and Petroleum Engineering, Razi University, Tagh Bostan, Kermanshah 6714414971, Iran. Email: gmoradi@razi.ac.ir 935174WMR 0 0 10.1177/0734242X20935174Waste Management & ResearchTalavari et al. research-article 2020 Original Article