Solventless recovery of lipids from urban sewage sludge: How to sustainably turn a waste into a valuable source of oleochemicals

Abstract

For the first time, the recovery of lipids from urban sewage sludge was investigated through a solvent-free process based on elementary operations validated on a pilot scale (2 tons per hour). Sedimented sewage sludge was dewatered through centrifugation, acidified (and in the case treated with hydrogen peroxide), heated to 80 °C and centrifuged to obtain three different streams: i) raw grease, ii) exhausted aqueous phase, and iii) dewatered sludge.The recovery yield of grease from sludge could reach a very high value (70 wt%), depending essentially on the initial lipid content in sewage sludge, which was monitored and found to range between 1.1 and 6.6 wt% (13–25 wt% with respect to TS). The use of H2O2 played a crucial role in the recoverability of lipids and the relevant pureness in free fatty acids content (>89 wt%). In addition, the total solids content in the exhausted aqueous phase and the final residual sludge were sensibly affected by using H2O2. Final dewatered sludge had a very high content of solids (TS 31.0 ± 2.3 wt%) and resulted in 15 % less of the amount of sludge conventionally obtained from anaerobic digestion. According to these figures, for medium wastewater treatment plants, a selling price of 800–900 Euro per ton of grease would guarantee a return on investments within three years of operations. This simple procedure represents a quick and proficient way to treat sewage sludge, turning it into a valuable feedstock for fine chemicals, in agreement with Circular Bioeconomy principles.