Options for Removing Refractory Organic Substances in Pre-Treated Process Water from Hydrothermal Carbonization

Granular activated carbon (GAC) adsorption, as well as ozonation in combination with biodegradation was investigated in order to remove refractory organics from biologically pre-treated process waters (PW) produced by the hydrothermal carbonization (HTC) of spent grains and fine mulch. Kinetic tests revealed that the organics in spent grains PW had much lower molecular weights than organics in fine mulch PW. Moreover, isotherms showed that they were more strongly adsorbable. This was confirmed in GAC column experiments, where the breakthrough curves could be predicted fairly well by a dynamic adsorption model. On the other hand, ozonation had a stronger effect on fine mulch PW with respect to an enhancement of the aerobic degradability. Thus, the type of input material determines the properties of soluble reaction products from the carbonization process that must be accounted for when selecting the most suitable post-treatment method for HTC PW. However, adsorption on granular activated carbon should always be the final stage.