Biodiesel Production from Municipal Sewage Sludges

Biodiesel is a fuel comprised of monoalkyl esters traditionally derived from vegetable oils or animal fats. There is currently an unprecedented increase in interest and demand for biodiesel and other fuels derived from renewable biomass. However, pure vegetable or seed oils are expensive and constitute between 70% and 85% of the overall biodiesel production cost. Municipal sewage sludge is gaining traction in the U.S. and around the world as a lipid feedstock for biodiesel production. It is plentiful and consists of significant concentrations of lipids that can make production of biodiesel from sludge profitable. However, there are challenges to be faced by biodiesel production from waste sludge. Determining how best to collect the different fractions and treat them for maximum lipids extraction is a major challenge. To accelerate biodiesel production, cosolvents and high shear mixing have been proposed. Nevertheless, there is very little information on the cost-effective means of increasing lipid solubility. Alkali-catalyzed transesterification is much faster than acid-catalyzed transesterification and is most often used commercially. However, for lipid feedstocks with greater than 1% free fatty acids (FFAs) such as in sludge, acid catalysis followed by base catalysis is recommended because of soap formation with alkali-catalyzed transesterification and high FFA. To boost biodiesel production, it is suggested that wastewater operators utilize microorganisms that are selected for their oil-producing capabilities. This could increase biodiesel production to the 10 billion gallon mark, which is more than three times the nation's current biodiesel production capacity. The presence of pharmaceutical chemicals in sludge poses a great challenge. This requires a careful selection of treatment technologies and microbes that are selective for these pharmaceutical chemicals. Finally, biodiesel production from sludge could be very profitable in the long run. Currently, the estimated cost of production is $3.11 per gallon of biodiesel. To be competitive, this cost should be reduced to levels that are at or below the current petro diesel costs of $3.00 per gallon.