WP1: Upgrading Sludge Properties Via Innovative Treatment Processes

WP1 is focused on the development and optimisation of sludge pre-treatment processes to upgrade the sludge streams with respect to their pollutants, hence making them more valuable for further use in fermentative bioconversion or land application as an organic fertiliser. In this regard, two strategies are employed: degradation and removal, which rely on the use of chemical-free, electricity-driven technologies. As part of the first strategy, DC1 will develop a combined ultrasound-SWCO process for an energy-efficient partial mineralisation of sludges that are not suitable for further bioconversion or land application, to heat and electricity. Secondly, alteration of pollutant’s molecular structures will be achieved via the production of hydrochar from sludge through hydrothermal carbonisation (HTC). DC2 will explore this pathway via the development of highly (energy-)efficient microwave synthesis routes. 

It will be investigated how these carbonisation processes influence the molecular structure of the organic pollutants (i.e., PFAS, pesticides and antibiotics) and how microwaves influence the release of heavy metals present in sludge matrices, which can act as catalysts to accelerate the carbonisation rate. By altering the sludge organic matter, carbonisation is expected to degrade the pollutants and to (partially) leach the heavy metals from the solid phase. A major advantage of this technology is that also pollutant mixtures can be treated, since it is a non-targeted treatment. 

The third strategy, i.e., pollutant removal, via different pre-treatment processes will be explored by DCs 3 and 4. By evaluating different classes of pre-treatment techniques, a later comparison between the techniques will be enabled. A major goal of pre-treatment is to change the sludge composition and consistency to facilitate further steps in the sludge treatment such as dewatering and anaerobic digestion. Most research efforts have been paid to quantify the increase in biodegradability, mostly measured by solubilisation of organic matter. 

In-depth studies on how the composition and structure of the sludge changes because of pre-treatment are, however, scarce. This knowledge gap will be addressed by the following two DCs. Specifically for sludge containing a high concentration of Fe or Al salts (originating from a previous coagulation step in the wastewater treatment), the use of an electrochemical pre-treatment will be explored (DC3) for simultaneous pathogen inactivation and heavy metal leaching. The fate of the metal ions and their bioavailability will be tested and the sludge structure (e.g. exopolymeric substances (EPS), solubilisation, bio-accessibility) will be thoroughly analysed. The distinct effect that thermal pre-treatment has on the mobility of pollutants that are typically adsorbed on these EPSs in untreated sludge (metals and microplastics), and the degradation and release of EPS linking with the rheologic behaviour of the sludge will be the final topic (DC4) in this WP.