Fuels and chemicals from sewage sludge: 1. The solvent extraction and composition of a lipid from a raw sewage sludge

With the objective of producing a low-nitrogen and low-sulphur-containing substrate for pyrolysis to liquids, the lipid fraction of a dried raw Atlanta sludge was extracted with both chloroform and toluene. Although the solvents proved equally good for extraction of lipids, toluene is preferred because the pyrolysis of any residual chloroform in the extractive could present environmental problems. In Soxhlet extraction warm solvent removed about 12 wt% lipid whereas extraction by contact with boiling solvent gave 17–18 wt% lipid. Soxhlet extraction of the sludge resulted in 99.5% rejection of nitrogen and 94% rejection of sulphur. Boiling extraction rejected 99% nitrogen and 84% sulphur. The Soxhlet extract contained 10–13% oxygen whereas boiling extraction gave a lipid containing 14–16% oxygen. On a moisture-free basis, the recovered lipid, 91%, from the separation of the toluene extract was comprised of 65% free fatty acids, 7% glyceride fatty acids and 28% unsaponifiable material. The unsaponifiable matter had a distribution of alkanes ranging from C9 to C16, n-nonane and n-pentadecane being the major hydrocarbons present. The free fatty acid esters and the glyceride fatty acid esters had a distribution of fatty acids ranging from C12 to C18. Palmitic, stearic and oleic acids appeared to be the major fatty acids present. These results are significant for the catalytic thermal liquefaction of sewage sludge lipids for the production of fuel and chemical feedstocks.