Bio-energy made from cooking fat – potential of mixed fat wastes as resource for biodiesel (Biopot)



Climate and Energy Fund of the FFG in the Energy Research Programme (e!MISSION), Energy Research 2nd Call for Proposals


Funding Duration:

09/2016- 02/2018



The aim of this exploratory project is to determine the biodiesel potential of mixed food and grease separator wastes. For this purpose, these wastes are to be purified to the highest possible quality and the influence of storage on the quality and suitability for biodiesel production of the mixed food and fat separator wastes and separately collected grease separator wastes is to be measured. With the help of a life cycle assessment, it will be evaluated whether the project is ecologically and economically reasonable.



Knapp, J., Lichtmannegger, T., Robra, S., Kinzel, C. und Bockreis, A. (2018): Bioenergie aus Speisefetten- Potential gemischt erfasster Fettabfälle als Ressource für Biodiesel. Final report.

Lichtmannegger, T., Kinzel, C., Müller, W., Bockreis, A. (2018): Energetische Nutzung von Fettabscheiderinhalten – Potenzial zur Erzeugung von Biodiesel und Biogas. Österr Wasser- und Abfallw 70: 172.

Presentations of the project:

Ökobilanzwerkstatt 2016, September 15-16, 2016, Pforzheim University, Poster Life Cycle Assessment Workshop

Österreichische Abfallwirtschaftstagung 2017  - Digitalisierung in der Abfallwirtschaft. 10.05.-11.05.2017, Graz, Poster ÖWAV Waste Management Conference 2017

Österreichische Wasserwirtschaftstagung 2017 - Die Zukunft der Abwasserwirtschaft in Österreich. 04.10.-05.10.2017, Linz, Poster ÖWAV Wastewater Management Conference 2017



Used cooking oil causes problems in waste water and, on the other hand, if collected separately, can be a resource for the production of bioenergy. Therefore, grease separators are mandatory in catering establishments, not only in Austria. There are different collection systems for used cooking oil, in Tyrol for example the Öli system for the collection of used cooking oil from households with the help of separate collection containers. In fact, however, quantities of used cooking oil still end up in the sewage system. This means that the energy contained in the fats cannot be used, and the necessary increased sewer maintenance and sewage plant ventilation causes additional energy consumption.

Companies that process food must usually have a grease separator if they are of an respective size. Their maintenance and emptying is usually handed over by the operators to private service providers. However, used cooking oil often ends up in the wastewater due to improper disposal, poor maintenance or the complete lack of a grease separator. There it leads to massive problems in the sewer system, such as corrosion of pipes due to the formation of hydrogen sulfide (H2S) or clogging of the sewer due to the formation of mineral deposits (Dengg and Rostek, 2004). If dissolved fats reach the sewage treatment plant in increased quantities, they lead to massive operating problems such as the mass reproduction of filamentous bacteria (Microthrix parvicella) and subsequently to the formation of floating sludge and foam.

Biodiesel can be produced from separately collected edible fats, as is already done, for example, in the Öli system. For the production of biodiesel, however, fats with a low content of free fatty acids are required. Within the scope of a project it shall now be investigated whether a fraction of sufficiently good quality for the production of biodiesel can also be obtained from fat waste of combined collection systems for food waste (kitchen waste shredder and fat separator in one tank). Since fats split into free fatty acids under certain conditions, the influence of collection and storage, such as high water content or the presence of microorganisms, on the fat quality will be investigated for both pure fat separator waste and waste from combined collection systems.

There are various established systems throughout Austria in which used cooking oils are collected separately, collected and then used for biodiesel production. Grease separator contents, on the other hand, are currently mainly used as a substrate for co-fermentation in sewage treatment plants for the production of biogas. Research projects have shown that grease separator contents can also be used as a starting substrate for biodiesel production (Canakci, 2007; Montefrio et al., 2010).

Within the project it shall be clarified whether the production of biodiesel from waste of combined collection systems for fats and other kitchen waste or from pure grease separator waste is possible, and if so with which processes and under which conditions. Improved collection and utilization of used cooking oils could reduce the energy consumption caused by disposal via the sewer and increase the production of renewable energy.

Whether the production of biodiesel from the waste  is reasonable in comparison to the already established production of biogas by co-fermentation depends strongly on the quantity and quality of the fats that can be separated from it and the effort required for purification. Investigations should therefore show whether it is possible to obtain a fraction of sufficiently good quality for biodiesel production from fat separator contents and waste of combined collection systems (kitchen shredder and grease separator in one tank). The commercially most widespread method today is homogeneous alkaline catalyzed biodiesel production, but this method tolerates only a very low amount of free fatty acids (Guldhe et al., 2015; Kulkarni and Dalai, 2006). Since fats split into free fatty acids under unfavorable storage conditions, the influence of storage on fat quality will be investigated.

Several storage experiments with grease separator contents separately and mixed with food waste were performed. The storage of the test samples was carried out at 20°C and samples were taken and analysed several times a week. The fat quality was determined on the basis of various parameters, including free fatty acids (FFA). The storage tests showed that the free fatty acids in the heavily contaminated grease separator fractions investigated increased rapidly due to the high content of water and microorganisms, in contrast to separately collected used cooking oils. In all approaches the storage time had a negative influence on the free fatty acids. Above all food remainders from grease separators led to a rapid increase of the free fatty acids (+46%). However, low concentrations of free fatty acids (<5%) are required for the alkaline catalyzed production of biodiesel. The peroxide number, a measure of fat spoilage, increased in all approaches.

The results obtained so far show that grease trap waste is not a simple starting substrate for biodiesel production. Therefore, in the further course of the project different processing methods and optimized storage and collection systems have to be investigated.

In order to determine the potential of grease trap contents as a resource for biodiesel production, several grease traps were investigated. Systems were sampled where the last emptying and maintenance took place between 30 and 190 days ago. Both the grease layer and the wastewater in the separator outlet were sampled.

The quality of the fat and thus the suitability for biodiesel production was determined by different parameters such as the acid number/free fatty acids. In the effluent samples, pH value, chemical oxygen demand (COD) and the total lipophilic substances were measured. Combined with the collected water consumption data, it can be estimated how much fat is lost for the production of renewable energy.

None of the separators could meet the effluent limit of 100 mg/L of low-volatile lipophilic substances. Food residues entering the grease separator greatly reduce the yield of usable oil for biodiesel production, no oil fraction could be recovered from two grease separators. Regular inspections of the grease separator should be carried out by the operators. In all investigated samples the free fatty acids were above 52% (for biodiesel max. 5% advantageous). The fats obtained from the grease separator can therefore only be converted into biodiesel using special processes that tolerate high concentrations of free fatty acids.

In many cases newly developed technologies for the production of renewable energies require such a high level of resources and energy that the entire process is ecologically unsound. Therefore, an important part of the project is an ecological and also an economical consideration. Whether the production of biodiesel from the wastes under investigation makes sense in comparison to the already established production of biogas by co-fermentation depends strongly on the quantity and quality of the fats that can be separated from them and the effort required for purification.


Canakci M. (2007). The potential of restaurant waste lipids as biodiesel feedstocks. Bioresource Technology, 98 (1), 183-190.

Dengg J., Rostek R. (2004). Fett im Abwasser. Broschüre zur Reduzierung des Fetteintrags in das Abwasser, Abwasserverband Achental-Inntal-Zillertal.

Guldhe A., Singh B., Mutanda T., Permaul K. and Bux F. (2015). Advances in synthesis of biodiesel via enzyme catalysis: Novel and sustainable approaches. Renewable and Sustainable Energy Reviews, 41, 1447-1464.

Kulkarni M. G. and Dalai A. K. (2006). Waste cooking oil - An economical source for biodiesel: A review Industrial and Engineering Chemistry Research, 45 (9), 2901-2913.

Montefrio M. J., Xinwen T. and Obbard J. P. (2010). Recovery and pre-treatment of fats, oil and grease from grease interceptors for biodiesel production. Applied Energy, 87 (10), 3155-3161.


Contact Information:

Universität Innsbruck
Univ-Prof. Dr.-Ing. Anke Bockreis
Technikerstrasse 13
6020 Innsbruck

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