Sewage, what was called wastewater for a long time, is not a waste, but contains valuable substances that can be used as raw materials for biobased products. However, worldwide this potential is hardly exploited yet. This results in loss of valuable materials, increased CO2 emissions and footprint, as well as lower use of natural resources. Engineers from the University of Luxembourg are experimenting the possibility to accumulate and extract lipids from sewage to produce biodiesel.
Recovering valuable resources
Lipids in municipal sewage originate from household discharge. Total load of lipids in North-West Europe Sewage Treatment Plants (STPs) is estimated to 4.5 million tons per year. Particulate lipids are removed in a sand and grease trap and then disposed at high costs for the operators. With dissolved lipids, there are two scenarios: at larger STPs, part of the lipids is accumulated in activated sludge, converted to biogas in digestors and then used for energy production while at smaller STPs without digestions, these lipids are used by activated sludge bacteria and then disposed with the sludge.
Valuable resources are being lost! Nevertheless, these lipids can be uptaken and stored by microorganisms, naturally living in the activated sludge. Microthrix parvicella (also known as spaghetti for its peculiar appearance under the microscope) is one of the lipid-accumulating microorganisms that can contribute to supply future energy demand. So far, Microthrix is known for huge problems at STPs, solid-liquid separation issues causing foaming and bulking in activated sludge. Since decades, STP operators adopted different solutions to control the overgrowth of this bacterium. Use of selectors to force the growth of flock-forming bacteria has been of common practice in recent years.
Encouraging experiments
Since January 2021, the Urban Water Management group led by Prof. Joachim Hansen at the University of Luxembourg is operating a lipid-pilot on a STP in Audun-le-Tiche to proof the possibility to accumulate and extract lipids with the aim to produce biodiesel out of it. « Indeed, the most common lipids identified in our sludge are all known to be suitable for biodiesel production! Now, from one kilo of our dry sludge we could produce 61 ml of biodiesel. Considering the above mentioned overall load of lipids in sewage inlet, STPs could produce 274 million liters of biodiesel per year, covering about 1.5 % of the EU biodiesel demand », explains Zuzana Dedova, postdoctoral researcher at the University of Luxembourg.
WOW! project
This project is part of the Interreg North-West Europe project WOW! – Wider business Opportunities for raw materials from Wastewater which aims to make a transition to a more circular approach by matching supply and demand of cellulose, lipids and PHA bioplastics from sewage. Even if our project intends to exploit feasibility of the production of biodiesel, we are actually producing a high research quality! The international consortium consists in total of 12 partners from the UK, France, Belgium, Germany, Luxembourg and the Netherlands. The project is funded by EU, covering 60% of the total budget €6.43 m.