Biolubricants from urban sewage sludge

PROJECT DESCRIPTION

BACKGROUND

According to the EU Water Framework Directive, in the period 2003-2006, the total European production of sewage sludge per year was around 10 million tons of dry matter (DM). Sludge is mainly produced by wastewater treatment plants (WWTPs) after a sequence of operations, ending with a dewatering step, operated through press-filtration or centrifugation. Nearly half of the management costs of WWTPs come from the disposal of such sludge, which is actually a wet solid (with water content around 80-84%). In order to reduce the weight of sludge to be disposed of, polyelectrolytes are often added to the process before the final dewatering step, but this has different negative environmental effects. One possible solution is to isolate the chemical potential of organic components in raw sewage sludge. A recent chemical characterisation of the lipid phase of sewage sludge found therein free fatty acids, estolides and 10-hydroxystearic acid in significant amounts. These could represent valuable bio-derived and biodegradable alternatives to fossil fuel-derived lubricants, the annual demand for which is assessed to be around 5.7 million tons/year in Europe. While the market already offers some bio-lubricants, the main feedstocks used to produce them are triglycerides and oils derived from dedicated cultures (palm oil, rape seed oil, sunflower oil, etc.), resulting in the use of significant amounts of natural resources.

OBJECTIVES

The main objective of the LIFE BioLubridge project is to demonstrate the possibility to use an alternative method aimed at obtaining bio-lubricants from sewage sludge, with reduced environmental impact. The project team will design and demonstrate at pilot project for the treatment of 1-2 m³/h of urban sewage sludge with a solvent-free technology (reaching TRL8 by project end). The process will essentially allow the recovery of the lipids contained in the sludge and reduce the weight of the final residual sludge to be disposed of.

The specific objectives of the project are to:

• Optimise and demonstrate an innovative technology capable of recovering at least 50% of the lipids present in sewage sludge, to be transformed into valuable products for the production of bio-lubricants ready for application in the automotive, metal working and other sectors. In particular, by processing 1 m³ of raw dewatered urban sewage sludge (containing 10% of total solids), 5 kg of lipids will be recovered and reacted to obtain up to 7 kg of esters of ethylhexanol, to be directly tested as bio-lubricants. By the end of the project, 3.5 tons of final products are expected to be produced;
• Improve the dewaterability of residual sewage sludge (reducing water content by 75%), avoiding to the largest extent possible the use of any polyelectrolytes;
• Evaluate the possibility to directly use the residual dewatered and biologically sanitised residual sludge in agriculture as fertiliser, preventing possible leakage of contaminants into surface and groundwater;
• Shorten the duration of the overall treatment of sludge from 15 days (typically necessary for an anaerobic digestion) to 6-12 hours, allowing the direct disposal of the final centrifuged sludge.
  

The project will contribute to the implementation of the following EU policies: Water Framework Directive (2000/60/EC); Sewage Sludge Directive (86/278/EEC); Urban Waste Water Treatment Directive (91/271/EEC); Directive 2018/850, amending Directive 1999/31/EC on the landfill of waste; Regulation 2019/1009 on the marketing of EU fertilising products; and the Circular Economy Action Plan of March 2020.

RESULTS

Expected results:

At the end of the project, 50 ton of thickened sludge (dry solids) will be processed, delivering the following:

• 3.5 ton of bio-lubricants produced, avoiding the use of 2.5 ton of palm oil;
• 2.5-4 kgCO²/kg reduced per every kg of palm oil not used by the project, meaning that, at the end of the project, 7.5 ton of CO² equivalents will be avoided;
• Mineral oil (C10-C40) in the final wet sewage sludge will be reduced from 1 000 ppm to 800 ppm, meaning that, at the end of the project, 25 kg of mineral oil - conventionally used as a dispersant for polyelectrolytes - will be avoided;
• Ethyl-hexyl phthalate (DEHP) content in sewage sludge reduced from 10 ppm to 8 ppm (with 100 g avoided at the end of the project);
• Nonylphenol ethoxylates (NPEO) reduced from 1 500 ppb per dry matter in sewage sludge to 1 200 ppm (with 15 g avoided at the end of the project);
• Avoiding the use of 5 000 m³/year of water due to the replacement of 2.5 ton of palm oil;
• Increasing the amount of soil to return back to reforestation by 0.5 ha/year (by avoiding the palm oil feedstock);
• Reducing the weight of sludge generated from the process by 20%.