Aarhus University has explored multiple methods to improve the quality of biocrude derived from sewage sludge. The first major study compared three conventional upgrading strategies: distillation, solvent extraction, and catalytic hydrotreatment.

The first two methods—distillation and solvent extraction—are technologically simpler, as they involve separating the biocrude into high-value and low-value fractions. While these methods have proven effective for biocrude from other sources, neither have been closely evaluated for sewage sludge-derived biocrude. Unfortunately, the current biocrude contains high levels of heavy and alkane insoluble compounds causing these pathways to result in poor yields.

Catalytic hydrotreatment, on the other hand, is an established method for upgrading sewage sludge biocrude. However, because sewage sludge varies significantly in composition—both seasonally and geographically—it is essential to replicate experiments to build a comprehensive database of outcomes. Hydrotreatment effectively removes most of the oxygen from the biocrude, even under mild conditions, producing a high concentration of alkanes with boiling points similar to conventional diesel. Increasing the temperature from 340°C to 400°C further reduces nitrogen content. Most notably, large heteroaromatic compounds begin to break down, converting what was once residue into gasoline and kerosene-range compounds. This process drastically reduces the oil’s viscosity and melting points without significantly affecting carbon recovery.

While all upgrading methods produce oil that meets the EffiTorch project’s initial requirements for viscosity reduction and thermal stability, hydrotreatment stands out as the most promising.

Another approach examined was in situ upgrading during the hydrothermal liquefaction (HTL) process, using co-solvents. The goal was to convert long carboxylic acids into esters through esterification, resulting in a more stable product with lower viscosity and reduced corrosiveness. Previous studies have shown that methanol and ethanol can achieve this to some extent. However, their solubility in water limits their contact with the biocrude, requiring high concentrations to produce meaningful effects. As an alternative, octanol was tested because it favors the oil phase.

Glycerol was also considered. It has shown promise with biocrudes from other sources but had never been tested on sewage sludge-derived biocrude. Additionally, glycerol is a byproduct of the biodiesel industry and is recognized as an advanced feedstock in renewable energy directives, making it more marketable than fossil-derived co-solvents. However, the co-solvent approach, while simple, produced poor results at practical scales and will not be pursued further.

Since hydrotreatment produced the highest quality biooil while maintaining the highest yield, it remains the most viable option. However, the initial conditions tested may have been overly effective, as the biooils far exceeded the EffiTorch requirements. Future studies will explore milder hydrotreatment conditions to tailor the process to meet EffiTorch’s standards at a reduced cost.