Municipal sewage sludge is gaining traction in the US and around the world as a lipid feedstock for biodiesel production. First, municipal sewage sludge contains significant concentrations of lipids derived from the direct adsorption of lipids onto the sludge. These energy-containing lipids include triglycerides, diglycerides, monoglycerides, phospholipids, and free fatty acids contained in the oils and fats. In addition, microorganisms used in the wastewater treatment process utilize organic and inorganic compounds in the wastewater as a source of energy, carbon, and nutrients. The cell membrane of these microorganisms is a major component of sewage sludge and is composed primarily of phospholipids. It is estimated at 24% to 25% of dry mass of the cell and yields about 7% oil from the dried secondary sludge. Other studies have demonstrated that up to 36.8 wt % of the dry sludge is comprised of fatty acids and steroids. With the fatty acids from sludge predominantly in the range of C₁₀ to C₁₈, these are excellent for the production of biodiesel.

—Kargbo 2010

Other potential benefits of using sludge as a feedstock are:

• It’s plentiful. In the US alone, approximately 6.2 million dry metric tons of sludge is produced annually by wastewater treatment facilities.
• Sludge management is a formidable environmental challenge. Biodiesel production from sludge as a viable alternative to land disposal could help to solve both energy and environmental problems.
• Capacity. Studies show that integrating lipid extraction processes in 50% of all existing municipal wastewater treatment plants in the US and transesterification of the extracted lipids could produce approximately 1.8 billion gallons of biodiesel, which is roughly 0.5% of the yearly national petroleum diesel demand.

However, he notes, sludge biodiesel also faces significant challenges, including:

• Collecting the sludge.
• Maintaining product quality.
• Soap formation and product separation.
• Bioreactor design.
• Pharmaceutical chemicals in sludge.
• Regulatory concerns.
• Economics of biodiesel production.

Determining how best to collect the different fractions (primary and secondary) and treat them for maximum lipids extraction is a major challenge. To accelerate biodiesel production, cosolvents and high shear mixing have been proposed, he notes, but 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, sewage treatment plants could use microorganisms that produce higher amounts of oil, Kargbo says. That step alone could increase biodiesel production to the 10 billion gallon mark, which is more than triple the nation’s current biodiesel production capacity, the report indicates.

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. With these challenges addressed, however:

..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.

—Kargbo 2010

Resources

• David M. Kargbo (2010) Biodiesel Production from Municipal Sewage Sludges. Energy Fuels, Article ASAP doi: 10.1021/ef1001106