Production of intracellular lipids from thermally hydrolyzed waste sludge by oleaginous yeast for energy and resource recovery

Municipal wastewater treatment plants generate large quantities of sludge every day, and the disposal of the waste sludge is one of the most challenging and expensive burdens for megacities. However, the rich organic content in sludge should be recovered and utilized rather than wasted. In this study, an innovative sludge treatment technology is developed for converting waste sludge into bio-lipids for energy and resource recovery. The technology uses thermal hydrolysis (TH) for the pretreatment of the sludge, following which the supernatant liquor of the hydrolyzed sludge is used as the substrate for the cultivation of oleaginous yeast to produce intracellular bio-lipids. TH pre-treatment resulted in the release of 36.8% of the organics from the sludge into the supernatant, which is more effective than acid, ultrasound, and alkaline sludge pretreatments. By fermentation using Trichosporon dermatis, approximately 8.2% of the organics in the TH sludge liquor was transformed into bio-lipids within one day of cultivation. An acidic condition with a process pH of 3.5–4.5 was found to be favorable for the growth of T. dermatis. Moreover, glucose was added to the TH sludge liquor to provide a co-substrate of organics and nutrients for fermentation, which created a favorable acidic condition (pH 3.5–4.0) for oleaginous yeast and improved the conversion of organics into bio-lipids to 12.8%. The produced bio-lipids comprised fatty acids ranging from C16 to C25, with a high proportion of unsaturated fatty acids (C18:1 and C18:2 > 65.0%). Based on the experimental results, a two-step scheme is proposed, which involves fermentation of the sludge liquor with oleaginous yeast for bio-lipid production followed by anaerobic digestion of the solid phase and organic residues for biogas production. The proposed scheme can help achieve the value-added recovery of organic resources from waste sludge.