During the Brisk project an experimental campaign was performed, aiming at optimizing the production of bio oils from two raw materials: digestate and sewage sludge. The digestate was produced by a biogas plant in Umbria region (Italy) fed mainly with maize silage and swine effluents. The sewage sludge was obtained from a waste water treatment plant also situated in Umbria region.  

The raw materials were tested in a pyrolysis batch plant heated by a furnace. The objective was to test different catalysts. Three supports were tested: alumina and Y-zeolite with different ratios of Si2O and Al2O3. In fact two ratios were tested: 30 and 80. The supports were also dried and loaded with iron through wet impregnation and then calcined at 500°C with a ramp of 3°C/min. Final temperature was held for 9 hours. 

The char obtained from the first pyrolysis tests was also used as a catalyst preparing a two stage catalyst: first charcoal and then alumina or zeolites. The total number of tests performed was 49. In each test 1 g of catalyst and 2 grams of raw material (sewage sludge or digestate were used). After the pyrolysis campaign was concluded, the third week of the BRISK project was devoted to oils analysis using a GC-MS.  

The results of the analysis show that the oils from sewage sludge and digestate can have interesting composition in aliphatic compounds and aromatics. The sewage sludge when pyrolysed without catalyst can give better quality oil compared to digestate, because it already contains metals able to act as catalysts. 

The best quality oil was obtained using Y-zeolite with Si2O to Al2O3 ratio equal to 30, loaded with Iron. This catalyst had positive effect on bio oil quality of both sewage sludge and digestate. When char was used coupled with catalysts the oil quality improved for digestate, while no benefits were noted for sewage sludge. This is because the ashes content of sewage sludge char was high and it had less porosity. The charcoal obtained from digestate pyrolysis has increase porosity and so it can produce more gas from the volatiles and increase the conversion of volatiles into oil in this way, reducing also coking of the catalyst. 

Finally TG-MS analysis of the starting substrates were performed to understand which kind of pollutants can be found in the pyrolysis gases, especially for sewage sludge. DSC analysis was also performed to estimate the energy required by the pyrolysis process. 


Main achievements are the following: 

  • the mass production of oil has the following trend: raw material<alumina<Y-Z-80<Y-Z-30<Alumina+Fe<Y-Z-80+fe<Y-Z-30+Fe 
  • at 500°C less oils are obtained compared to 400°C, but the content in aliphatic compounds and aromatic compounds increases 
  • when used for the upgrading of digestate pyrolysis volatiles, the iron catalyst influences the production of acetic acid, phenol, furfural, benzaldehyde etc. 
  • when used for the upgrading of sewage sludge pyrolysis volatiles, the iron catalyst influences the production of phenol, indole, aliphatics and aromatic hydrocarbons 
  • more nitrogen compounds are present in the oils obtained from sewage sludge, compared to those obtained from digestate 
  • the use of charcoal before the catalyst protects it from coking and can grant a longer life of the catalyst 
  • It is has been also achieved a detailed composition of the oils which can be obtained with different catalyst from digestate and sewage sludge 
  • Due to the high content of transitions metals already present in sewage sludge the obtained bio oils have generally higher content of aliphatic compounds and aromatic compounds, compared to the bio oils obtained from digestate