Conversion Of Waste Cooking Oil Research Articles

A high-efficient beta-zeolitic catalyst for conversion of waste cooking oil towards biodiesel

The conversion of waste cooking oil into biodiesel can bring about dual benefits: alleviating environmental pollution and realizing waste recycle. Embryonic zeolite has highly accessible acid sites because of the partially formed or more open zeolitic structure, which can solve problems such as diffusion limitation and inaccessible acid sites that the traditional zeolite catalysts face in the process of macromolecular reactants. In this work, a series of embryonic Beta zeolites were prepared by using mesoporous SBA-15 as a self-sacrificial silica source via a solid-state transformation strategy. NH3-TPD and FT-IR pyridine adsorption confirm that Brønsted acid sites have been created in these amorphous form embryonic catalysts. The diffusion property of toluene obtained by a zero-length column method proved that the higher the zeolization degree, the greater the diffusion activation energy. The synergistic effects resulted from the combination of the ordered mesoporous structures (ranging from 2 to 10 nm) stemming from the SBA-15 substrate and the moderate acid sites (∼88 μmol g−1) offered the embryonic SBEA-3 catalyst with remarkable catalytic performance for ≥95 % conversion during 20 times cycling tests and more than 90 % methyloleate yield.

Concurrent conversion of waste cooking oil using Al metal-organic framework and subcritical methanol through esterification/transesterification process

Recently, some scientists have focused on discovering techniques for manufacturing biodiesel on a significant, efficient, and ecologically sustainable scale. Choosing a catalyst is a notable challenge due to the instability and cost feasibility issues connected with newly designed homogeneous and heterogeneous catalysts. This paper describes the synthesis and utilization of aluminum-based metal-organic frameworks (MOFs) as catalysts for the generation of biodiesel from waste cooking oil (WCO). The processes involved in this study are esterification and transesterification, which are carried out using subcritical methanol. The catalyst analysis reveals that the MOFs possess a filamentous structure, with a crystallite size ranging from 300 to 500 nm. Furthermore, these MOFs exhibit thermal stability up to 470 °C. The catalyst underwent examination for biodiesel production from waste cooking oil (WCO), and the resulting biodiesel samples met the standards set by the American Society for Testing and Materials (ASTM). The experiment was devised and fine-tuned to incorporate the three independent variables using a multilevel factorial design, response surface methodology, and a three-way analysis of variance (ANOVA). The FAME conversion peaked at 91.96 %, as determined through statistical analysis of the optimization findings. The corresponding values for the variables were X1: 423.15 K, X2: 1800s, and X3: 28 mol/mol. Based on ANOVA statistics, the experimental and mathematical verification demonstrates a fair correlation between the actual and calculated catalyst performance. The temperature and molar ratio of methanol: WCO significantly affected the FAME conversion.

Thermal activation of basic oxygen furnace slag as a heterogeneous catalyst in transesterification: Characterization and catalytic activity studies

The valorization of industrial by-products has garnered significant attention in recent years due to its potential to enhance sustainability and economic efficiency within various sectors. Basic Oxygen Furnace slag (BOF-S), a by-product generated in the steelmaking process characterized by its complex composition, presents an opportunity for waste management and resource recovery. This study investigates the potential of BOF-S as a catalyst in converting waste cooking oil to biodiesel via the transesterification reaction. To improve its efficiency, BOF-S was calcined at 850 °C (BOF-S 850), and 1000 °C (BOF-S 1000), for 5 h. The BOF-S was characterized using XRF, N2 sorption, XRD, TGA-DSC, SEM-EDS and FTIR. The characterization techniques showed that the BOF-S 850 had superior characteristics as a catalyst: rough surface, increased pore size and more active basic sites compared to the BOF-S and the BOF-S 1000 slag samples. The transesterification reaction was conducted at a methanol: oil ratio of 20:1, a catalyst loading of 20 wt %, a reaction time of 12 h, a reaction temperature of 60 °C, and a stirring speed of 750 rpm. The findings established that the BOF-S 850 was the best catalyst, with a 90.77 % biodiesel yield and was able to sustain a maximum of two transesterification cycle runs.

Conversion of Waste Cooking Oil with Transesterification Process in Ultrasonicator using SrO-KF Catalyst

Conversion of Waste Cooking Oil with Transesterification Process in Ultrasonicator using SrO-KF Catalyst

Investigation of Novel Transition Metal Loaded Hydrochar Catalyst Synthesized from Waste Biomass (Rice Husk) and Its Application in Biodiesel Production Using Waste Cooking Oil (WCO)

The decarbonization of transportation plays a crucial role in mitigating climate change, and biodiesel has emerged as a promising solution due to its renewable and eco-friendly nature. However, in order to maintain the momentum of the “green trend” and ensure energy security, an ecologically friendly pathway is important to produce efficient biodiesel. In this work, activated carbon (AC) obtained from rice husk (RH) is hydrothermally prepared and modified through cobalt transition metal for catalyst support for the transesterification process. The physicochemical characteristics of the synthesized catalysts are examined using XRD, FTIR, SEM and EDS, TGA, and BET, while the produced biodiesel is also characterized using Gas Chromatography and Mass Spectroscopy (GC-MS). To optimize the transesterification process, Fatty Acid Methyl Esters (FAME) are produced by the conversion of waste cooking oil. Response Surface Methodology (RSM) is used to validate temperature (75 °C), the methanol-to-oil molar ratio (1:9), catalyst weight percentage (2 wt.%), and retention time (52.5 min). The highest conversion rate of waste cooking oil (WCO) to biodiesel was recorded at 96.3% and tested as per American Society for Testing and Materials (ASTM) standards. Based on the results, it is clear that cobalt-loaded rice husk-based green catalyst (RHAC-Co) enhanced catalytic activity and yield for biodiesel production. Further research should focus on engine performance evaluation and scaling up of the catalyst by optimizing it for the industrial scale.

A Comprehensive Approach to Biodiesel Blend Selection Using GRA-TOPSIS: A Case Study of Waste Cooking Oils in Egypt

The transition to sustainable energy sources is critical for addressing global environmental challenges. In 2017, Egypt produced about 500,000 tons of waste cooking oil from various sources including food industries, restaurants and hotels. Sadly, 90% of households choose to dispose of their used cooking oil by pouring it down the drain or into their village’s sewers instead of using proper disposal methods. The process involves converting waste cooking oil (WCO) into biodiesel.This study introduces a multi-criteria decision-making approach to identify the optimal biodiesel blend from waste cooking oils in Egypt. By leveraging the grey relational analysis (GRA) combined with the technique for order preference by similarity to the ideal solution (TOPSIS), we evaluate eight biodiesel blends (diesel, B5, B10, B20, B30, B50, B75, B100) against various performance metrics, including carbon monoxide, carbon dioxide, nitrogen oxides, hydrocarbons, particulate matter, engine power, fuel consumption, engine noise, and exhaust gas temperature. The experimental analysis used a single-cylinder, constant-speed, direct-injection eight cylinder diesel engine under varying load conditions. Our methodology involved feature engineering and model building to enhance predictive accuracy. The results demonstrated significant improvements in monitoring accuracy, with diesel, B5, and B20 emerging as the top-performing blends. Notably, the B5 blend showed the best overall performance, balancing efficiency and emissions. This study highlights the potential of integrating advanced AI-driven decision-making frameworks into biodiesel blend selection, promoting cleaner energy solutions and optimizing engine performance. Our findings underscore the substantial benefits of waste cooking oils for biodiesel production, contributing to environmental sustainability and energy efficiency.

Production of green diesel from waste cooking oil via catalytic deoxygenation reaction using metal doped eggshell catalyst

Abstract Green diesel production via catalytic deoxygenation of waste cooking oil (WCO) over metal doped eggshell catalyst was investigated in this work. The catalyst was prepared through liquid-liquid precipitation of 5 transition metal solutions and ground eggshell (ES) as the catalyst support. The prepared catalyst, Fe-ES, Cu-ES, Co-ES, Zn-ES, and Ni-ES were characterized using BET surface area and Scanning Electron Microscopy (SEM) analysis. BET surface area data and SEM images of the catalyst shows a promising catalyst physical properties that tailor to the deoxygenation reaction. Gas Chromatography Mass Spectrometry (GCMS) was used to determine the hydrocarbon composition of the oil yield product from the reaction. The reaction also produces gas, soap and liquid acid phase while the remaining unreacted WCO becomes coke. The percentage of all products and coke were calculated using mass balance. Deoxygenation of WCO with Ni-ES catalyst produced highest oil yield at 61.6% with the hydrocarbon content of 56.11%. Ni-ES also produced 22.9% coke; the least percentage compared to other catalyst. The findings proved that Ni-ES catalyst exhibited the highest conversion of WCO into gas and liquid product with a greater yield of oil and minimal coke formation. These findings demonstrate the feasibility and practicality of using eggshell catalysts as substitutes for commercial catalysts in green diesel production.

Synthesis and evaluation of CuO/UiO-66 metal-organic frameworks as a solid catalyst for biodiesel production from waste cooking oil

Metal-organic frameworks (MOFs) are excellent for immobilizing guest active species due to their high surface area and porosity. In this study, CuO/UiO-66 catalysts with varying amounts of CuO were successfully synthesized and used for converting waste cooking oil (WCO) to biodiesel. The catalysts were characterized using N2 adsorption-desorption, XRD, NH3-TPD, SEM, TEM, TGA and FTIR. The biodiesel was characterized using NMR and GC–MS. The results showed that CuO/UiO-66 displayed high catalytic activity due to the synergistic effect between UiO-66 and copper oxide in the composite. Under optimum reaction conditions, we achieved a 90.1 % conversion with the 7 %CuO/UiO-66 catalyst. This catalyst also displayed good stability and reusability with a conversion of 81.06 % after three cycles. Our kinetic studies revealed a pseudo-first-order kinetics, with 32.26 kJ/mol in activation energy, consistent with a chemically controlled reaction.

Surface functionalization of SBA-15 with organosilane for the preparation of bimetallic Ni-Co/SBA-15 catalyst: An innovative approach to convert waste cooking oil into advanced biofuel energy

This research investigates the utilization of bimetallic nickel-cobalt supported SBA-15-silane catalysts for the production of green diesel from waste cooking oil (WCO), aiming at sustainable energy solutions. The catalysts underwent surface-functionalization with three distinct silane types: mercaptosilane (-SH), aminosilane (-NH2), and phenylsilane (-Ph). Notably, the incorporation of -NH2 silane in the fabrication of bimetallic NiCo/SBA-15-NH2 catalyst led to significant improvements in catalyst properties. Specifically, enhancements were observed in pore characteristics, with a surface area of 424 m2/g and a pore size of 4.8 nm, along with an increase in acidity to 5038.91 μmol/g. This rise in acidity is attributed to the presence of amino groups (-NH2), which foster stronger electrostatic interactions between the support and nickel-cobalt particles, subsequently affecting the particle size of Ni-Co (as confirmed by HRTEM analysis). Parametric studies unveiled the superior performance of the NiCo/SBA-15-NH2 catalyst, demonstrating a notable hydrocarbon yield of 80 % and diesel selectivity of 90 % at a reaction temperature of 300 °C, with sustained optimal performance for up to 6 h of reaction time. Furthermore, the blended green diesel (G2.5 & G5) derived from this process exhibited favorable fuel properties including lower kinematic viscosity, higher cetane number, and lower pour point in comparison to commercial petrol-diesel. These findings underscore the significant potential of silane in catalyst preparation for the conversion of WCO into biofuel energy, thereby highlighting a promising avenue for sustainable energy production and waste management.

Hybrid CaO/ZnFe2O4 Modified with Al2O3 as a Green Catalyst for Biodiesel Production from Waste Cooking Oil

In this work, biodiesel was produced from waste cooking oil (WCO) via a green catalyst of CaO-ZnFe2O4 modified Al2O3. The catalyst was characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray (EDX), SEM-mapping, Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM) analyses. The catalyst performance was studied in the transesterification reaction of WCO conversion to biodiesel. The catalytic activity increased with the combination of nanoparticles effect and support catalysts obtained biodiesel yield of nano-Al2O3, nano-CaO, ZnFe2O4, CaO-ZnFe2O4, and CaO-ZnFe2O4/Al2O3 is 36.86%, 67.16%, 74.83%, 86.54%, and 93.41%, respectively. The best biodiesel yield was 93.41% with a mass ratio of Al2O3 to CaO-ZnFe2O4 (2:1). The physicochemical properties (acid number, density, kinematic viscosity, flash point, and cetane number) of biodiesel under the optimal conditions agreed with the ASTM standard. These results show that the developed nanocomposite has great potential to reduce biodiesel production costs because derived from WCO. In conclusion, CaO-ZnFe2O4 modified Al2O3 as a catalyst has a high potential for biodiesel production on a large scale.

Comparative Assessment of Head Deposition, Exhaust Gas Temperature and Sound Emission in CI Engine using Blend Fuel

Energy has a significant role in the socio-economic growth for any country. The energy originates from fossil fuels, which are non-renewable and enact an undesirable impact on the environment. Waste cooking oil can be utilized in diesel engine directly. Preheating and trans-esterification process are expensive to convert waste cooking oil into biodiesel. This research aims to replace diesel with waste cooking oil as binary blend and compared with diesel fuel. Engine testing at the constant speed of 1300 rpm at constant load. This study revealed that, addition of waste cooking oil reduced exhaust gas temperature as compared to base line fuel. In case of noise emission, sound level for emulsion fuel DF95WCO5 was reduced compared to DF. In this study, a single-cylinder CI engine was run for 200 hours on two fuel samples: DF (diesel fuel) as the baseline fuel and DF95WCO5 (5% waste cooking oil and 95% DF). During the endurance test, the effects of DF95WCO5 on engine head deposits were studied. According to the investigation's findings, visual inspection of both fuel samples revealed some deposit buildup on injectors. SEM (scanning electron microscopy) and EDX (energy dispersive X-ray spectroscopy) analysis revealed that the engine running with DF95WCO5 formed more carbon deposits on and around the head. It can be concluded that binary emulsion can be used in compression ignition engine without any engine alterations. Consequently, WCO can be proficiently used to reduce detrimental effects and reduce fossil fuel dependency.

The future of waste cooking oil and its carbon and economic benefits——An automotive energy perspective

As a crucial energy carrier, waste cooking oil poses a significant threat to land and water resources. Furthermore, there is a risk of its reintroduction into the restaurant system, posing a direct threat to public health. Hence, converting waste cooking oil into clean energy has emerged as a pressing concern. This study compares the CO2 emissions and economic benefits of two pathways applied to automotive energy consumption——the waste cooking oil produced biodiesel pathway and the waste cooking oil mixed with coal for power generation pathway. The results show that for both diesel and electric versions of the same brand of vehicles, the CO2 emission of an electric vehicle using waste cooking oil mixed coal produced electricity is 0.181 kg/km, which is a 26.12% reduction compared to a diesel vehicle using waste cooking oil converted to biodiesel. Consumers appreciate the low price of electricity. However, for producers, the waste cooking oil produced biodiesel pathway demonstrates superior economic benefits. At the current market price, converting waste cooking oil into biodiesel will result in a net gain of 112.89 billion Yuan for the producer, whereas the waste cooking oil mixed with coal for power generation pathway will result in only 2.08 billion Yuan in net gain. This disparity is primarily attributed to the lower electricity prices for electric vehicles. To encourage the widespread adoption of waste cooking oil energization technologies, Chinese government can raise electricity prices for electric vehicles to a minimum of 1.599 Yuan/kW·h, ensuring equal net gains in both pathways.

Graphene oxide-catalyzed microwave-assisted esterification of oleic acid to biodiesel

The combination of graphene oxide (GO) catalyst and microwave (MW) heating was applied for the esterification of oleic acid with methanol to synthesize methyl oleate. GO was synthesized using the modified Hummers method from graphite powder and characterized by various instrumental methods. To optimize the reaction parameters, reaction temperature, reaction time, methanol-oleic acid molar ratio, and catalyst amount were investigated. An oleic acid conversion of 96.2% was achieved in a reaction time of 1 h at a reaction temperature of 100 °C, using a methanol-oleic acid molar ratio of 9:1 and 5 wt% of GO. In addition, to compare MW heating and conventional heating on the efficiency of esterification, the esterification reaction was carried out with conventional heating under optimized conditions. Finally, the catalytic effect of GO in the conversion of waste cooking oil (WCO) into biodiesel was investigated.

Resource Utilization of Waste Cooking Oil Catalyzed by Na2CO3/ZSM-5.

A catalyst with a simple synthetic process and good catalytic performance was prepared using Na2CO3 as the active component and ZSM-5 as the carrier for the resource utilization of waste cooking oil. The structure of Na2CO3/ZSM-5 was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy, and the effects of parameters such as Na2CO3 loading, catalyst percentage, and reaction time on the yield of fatty acid methyl esters were investigated. The results showed that the conversion of waste cooking oil to fatty acid methyl esters yielded up to 96.89% when the Na2CO3 loading was 35%, the reaction temperature was 65 °C, the reaction time was 2 h, and the catalyst percentage was 1 wt %. The Na2CO3/ZSM-5 catalyst could be used to replace H2SO4 or NaOCH3 in the industrial treatment of waste cooking oil for its resource utilization.

Conversion of Waste Cooking Oil Combined With Corn Oil Into Biodiesel Using the Transesterification Method

Conversion of Waste Cooking Oil Combined With Corn Oil Into Biodiesel Using the Transesterification Method

Sustainable synthesis of epoxidized waste cooking oil via Prileschajew reaction: Optimization and kinetic study

AbstractIn this study, waste cooking oil‐based palm oil (WCO‐PO) was chosen as feedstock for epoxidation reaction. The epoxidation process of WCO‐PO was carried out using in situ generated performic acid or known as Prileschajew reaction. Based on the Taguchi method of optimization and analysis of variance, a series of experiments were conducted around the optimum conditions or parameters. The findings revealed that the optimal reaction conditions for producing epoxidized waste cooking oil with the highest oxirane content were a hydrogen peroxide molar ratio of 2.0, a temperature of 55°C, formic acid molar ratio of 2.0, and catalyst loading of 0.5% at a constant stirrer speed of 300 rpm and 0.5% catalyst. By employing these optimal conditions, the maximum relative conversion of waste cooking oil to oxirane was achieved at 70.6%. Besides, the rate constant and activation energy for epoxidation of WCO‐PO at optimum condition were 0.0221 min−1 and 50.55 kJ mol−1, respectively. Overall, epoxidized WCO‐PO was successfully produced by using optimum process parameters of epoxidation.

Application of samarium doped lanthanum nickel oxide perovskite nanocatalyst for biodiesel production

The catalytic performance of the synthesized LaNiO3 (LNO) and LaSmNiO3 (L1-xSxNO) perovskite structures as novel efficient catalysts for the conversion of waste cooking oil (WCO) to biodiesel was evaluated. The synthesized heterogeneous catalysts of LNO and L1-xSxNO were characterized via Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDX), Transmission Electron Microscopy (TEM), Brunauer, Emmett, and Teller (BET), inductively coupled plasma-optical emission spectrometer (ICP-OES), and Power X-ray diffraction (XRD). The crystal size of LNO was obtained to be around 31 nm at 2θ of 47.31° using Scherrer's equation. Catalysts characterization revealed that doping Sm with LNO (L0.9S0.1NO) enhanced the surface area from 7.71 to 15.75 m2/g, and thus may provide more active sites for transesterification reaction. Also, the Sm doping causes numerous oxygen vacancies in the structure of perovskite, improving the catalyst's ability to catalyze reactions. To determine the optimum conditions in the WCO transesterification process, the response surface methodology (RSM) based central composite design (CCD) was applied. The influences of key parameters including MeOH/WCO ratio (5–20 mol/mol), reaction duration (1–5 h), catalyst mass fraction (1–5 wt%), and reaction temperature (60–120 °C) were explored on the behavior of the reaction. It is inferred that the L0.9S0.1NO catalyst has excellent catalytic activity in converting WCO to biodiesel. The experimental and modeling sensitivity analysis indicated that the application of L0.9S0.1NO catalyst led to the WCO conversion of 95.14 % to biodiesel at MeOH/WCO ratio of 9.07, reaction duration of 3.02 h, catalyst mass fraction of 3.08 wt%, and reaction temperature of 80.07 °C.

Optimized conversion of waste vegetable oil to biofuel with Meta heuristic methods and design of experiments

Biodiesel generated from waste cooking oil (WCO) shows enormous potential for accomplishing SDGs and embracing circular economy principles. This strategy coincides with SDGs 7 and 12, which promote clean energy along with ethical consumerism, by converting waste cooking oil into biofuel. It reduces dependency on fossil fuels, reduces emissions, and promotes sustainable energy sources. Furthermore, using WCO biodiesel adheres to the circular economy concept, reducing waste and pollution while conserving resources (SDGs 12, 14, and 15). To optimize this process, a hybrid technique comprising RSM, ANOVA, and particle swarm optimization is being explored. Researchers achieved 90% biodiesel production employing this technology, encouraging both eco-friendly energy and resource-efficient practices. The optimized parameters produced remarkable results: 82.98% biodiesel generation with a reaction time of 101 minutes, 2% catalyst, and a methanol-to-oil ratio of 20%, demonstrating the potential of this integrated strategy.

Homogenizer-intensified amidation of free fatty acids in waste cooking oil for biodiesel production

Biodiesel production from waste cooking oil is a viable alternative both to satisfy renewable energy demand and utilize a low-cost feedstock. However, a preparatory procedure is required to reduce free fatty acid contained in waste cooking oil to a specific value which is time- and energy-extensive. Herein, a fast and non-catalytic amidation reaction performs at room temperature is established as an alternative method to reduce the free fatty acid level. The effect of the molar ratio of waste cooking oil to monoethanolamine, reaction time and rotational speed of homogenizer were investigated on the conversion of free fatty acid to alkanolamide compounds. The highest conversion of 95.9 ± 0.02% was achieved in presence of a molar ratio of 1:1.5, reaction time of 0.5 min and rotational speed of 5000 rpm. Further, a homogenizer was used to facilitate the transesterification reaction using an alkaline catalyst at room temperature. The conversion of waste cooking oil to biodiesel of 91.4 ± 0.2% was achieved after 5 min of reaction time. In terms of processing parameters, the non-catalytic amidation and alkaline base catalytic transesterification reaction assisted by a homogenizer device is a time-saved process as it could save 92% and 94% of reaction time compared to one- and two-steps method and was performed in ambient condition.

Highly active novel K2CO3 supported on MgFe2O4 magnetic nanocatalyst for low-temperature conversion of waste cooking oil to biodiesel: RSM optimization, kinetic, and thermodynamic studies

An innovative K2CO3/MgFe2O4 (KC/MGF) magnetic nanocatalyst for effective biodiesel generation from WCO is demonstrated here. To examine the physiochemical and morphological features of the catalyst, several characterization methods such as XRD, FT-IR, SEM-EDX, HR-TEM, AFM, XPS, BET, CO2-TPD, and VSM are used on the as-synthesized magnetic nanocatalyst. Optimization of K2CO3 loading on MgFe2O4 is achieved and 30KC/MGF was found as the optimum ratio. Response surface methodology (RSM) via central composite design (CCD) is constructed using optimization margins to clarify the optimal parametric conditions; catalyst loading (4–6 wt%), reaction time (90–120 min), and methanol to oil ratio (6:1–9:1) at a fixed low operating reaction temperature (50 ºC). The obtained optimum reaction conditions are 4.4 wt% catalyst loading, 108 min reaction time, 6.6:1 methanol to oil ratio, at 50 ºC achieving ca. 92.9% biodiesel conversion. 30KC/MGF is an energy-conserving magnetic nanocatalyst with excellent basicity that operates at low temperatures, for a markedly short period of time, and with a low methanol-to-oil molar ratio. Kinetics and thermodynamic parameters are estimated i.e., activation energy (Ea), change of enthalpy of activation (ΔH#), and change of entropy of activation (ΔS#) to be 31.5 kJ mol−1, 28.9 kJ mol −1, and −188.8 J mol−1 K−1, respectively. Gibbs free energy (ΔG#) is appraised at 308, 313, and 318 K to be 87.1, 88, and 88.9 kJ mol−1, respectively. Due to K+ leaching, utilizing 30KC/MGF in sequential runs results in a significant activity plummet. The catalyst activity is recovered by re-impregnation with K2CO3 precursor solution, resulting in the same high FAME conversion as fresh 30KC/MGF. Finally, the physicochemical properties of biodiesel are investigated and determined to be compliant with international American and European standards.