"Each liter of lipids that the researchers extracted from sludge cost $0.03, while previously published data shows each liter from soybeans costs $0.80." - C&EN
Impurities in the lipids from the sludge would have interfered with the catalytic process in the conventional production of biodiesel, so they developed a new method that could transform lipids with high amounts of free fatty acids (FFAs) or impurities using heat instead of catalysis.
"The team continuously fed methanol and the extracted sludge lipids into a reactor containing porous activated alumina and heated the reactor to 380 °C. Adding carbon dioxide to the reactor improved the reaction’s yield. The researchers’ method converted about 98% of the sludge lipids to biodiesel."
The study's authors did say that before being able to scale this up to a much larger system, wastewater treatment facilities would need to install the equipment necessary to desiccate the sewage sludge and extract the lipids, but that by selling those lipids to biodiesel producers, it could pay for itself.
The researcher's study is available here: Biodiesel Production from Sewage Sludge: New Paradigm for Mining Energy from Municipal Hazardous Material