Could sewage sludge be considered a source of waste lipids for biodiesel production?

Worldwide rapidly increasing fuel demand (85 million barrels of refined products from fossil fuels per day in 2006 and projected to increase to 107 million barrels of liquid fuel per day in 2030) and declining fossil fuel reserves (1342 billion barrels oil in the ground as of January 2009) drive the search for alternative fuels. Biodiesel is the main biofuel for road transport used in the European Union and accounted for about 80% of the biofuels market on an energy basis in 2010. First generation biodiesel has been produced by using virgin vegetable oils as feedstock, including soybean oil in the USA and Argentina, rapeseed and sunflower oil in Europe, palm oil in south-east Asia (particularly in Malaysia and Indonesia) and coconut oil in the Philippines. Non-edible vegetable oils use in second generation biodiesel production such as jatropha, castor, karanjia, pongamia, babassu, neem, tobacco and rubber seed oil, may have lower prices than edible oils and could be available to produce biodiesel without directly competing with food oils. In addition, microbial oils such as micro-algae oils are considered third generation sources of biodiesel production. Biodiesel ́s main economic challenge is the high feedstock cost, which in the case of refined vegetable oil accounts for 70– 85% of the total biodiesel production costs. Moreover, the use of vegetable oils for first and second generation biodiesel is still of great concern as their use may limit the availability of farmland and vegetable oil inventories, resulting in a high sensitivity of prices, simultaneously contributing to a fuel versus food controversy that requires a deep technical–economic–political analysis of non-food feedstock availability for biodiesel production. In this regard, waste lipids and waste materials containing high amounts of lipids may be of interest as feedstock for biodiesel production. Waste lipids such as waste frying oils (WFO), restaurant grease and waste animal fats are potential low-cost feedstocks for biodiesel production according to recent studies. In this sense, regarding waste management operation protocols it seems that source separation instead of mixing in the sewers should be a priority if waste fatty acids are to be recovered for biodiesel production purposes. Rough estimations of biodiesel production costs using waste lipids are in the range of US$ 0.34–0.42 per litre, which seems to be competitive in relation to traditional fossil fuel costs. However, when using virgin vegetable oils the projected biodiesel prices may be double the price of fossil fuels. The use of waste lipids in biodiesel production would diminish some environmental impacts compared to the use of virgin vegetable oils, making the production process more competitive. Nevertheless, the production process must be flexible, due to the different physicochemical properties such as high free fatty acid (FFA) content of waste oils compared to vegetable oils. However, the available volume of waste lipids seems to be limited, indicating that the use of this type of feedstock could not support increased commercial production of biodiesel fuels. Thus, research regarding alternative feedstocks for biodiesel production must consider raw materials that are more readily available in large quantities and at low cost. Considering the aforementioned factors, it seems that municipal sewage sludge has the potential to be a reliable and highenergy feedstock for biodiesel production in the future. Sewage sludge (also called ‘biosolids’) is a by-product generated in wastewater treatment facilities after primary and secondary treatment processes, normally entailing anaerobic digestion which produces both biogas and a semi-solid residue (sludge). Activated sludge wastewater treatment processes produce two main types of sludge: A primary sludge, normally a combination of floating grease and solids and a secondary sludge, mainly composed of microbial cells and suspended solids produced during aerobic biological wastewater treatment. New research indicates that the lipids contained in sewage sludge (primary and secondary sludge) may be a potential feedstock for biodiesel production. In particular, the drying of sewage sludge and the extraction process for recovery of the lipids fraction seem to be the most important challenges to overcome when developing a process for biodiesel production using sewage sludge as feedstock. Municipal wastewater treatment plants produce huge amounts of sludge per year that is readily available at no cost; indeed, in some cases those who remove sludge are paid for the service. In fact, wastewater treatment facilities in the USA produce approximately 6.2 million tonnes (dry basis) of sludge annually. In Europe an annual sewage sludge production of about 12 million tonnes is estimated, out of which 42% are applied to land (as a soil amendment or fertilizer) and 26% are incinerated, with the remaining fraction being co-composted or landfilled. Raw primary sludge lipids content on a dry basis is in the range of 20 to 26%, and the lipidic fraction in secondary sludge may account for up to 6%, mainly depending on the source separation efficiency of waste lipids. The other important fractions in sewage sludge are a lignin-rich fraction, ranging between 20 and 30%, and a nitrogen-rich fraction mainly related to urea and caffeine, ranging between 25 and 40%. Could sewage sludge be considered a source of waste lipids for biodiesel production? 458074WMR30910.1177/0734242X12458074Waste Management & ResearchNavia and Mittelbach 2012