Nutrients release and melanoidins formation during thermal hydrolysis of waste activated sludge and implications in downstream processes

This thesis research investigated the dynamics of nutrient release and the formation of melanoidins during thermal hydrolysis processes (THP) of waste activated sludge (WAS) preceding mesophilic anaerobic digestion (AD). In Chapter 2, a fractionation of THP-sludge allowed for researching the distribution of nutrients and melanoidins over the liquid and solid fraction of the sludge matrix during AD. The soluble melanoidins formed during THP were found to be partially biodegradable during subsequent AD, particularly those with a molecular weight under 1.1 kDa, associated with protein-like substances. In addition, total ammoniacal nitrogen (TAN) concentrations exhibited a modest increase during THP, but a substantial increase was observed during AD, negatively impacting acetotrophic methanogenesis.

In Chapter 3, the influence of THP on PO43--P release and precipitation during AD is described. The dynamics in soluble PO43--P concentrations were most pronounced in WAS from enhanced biological phosphorus removal (EBPR) plants. The results indicated an increment in TAN and PO43--P release with THP temperature, while varying soluble concentrations were observed during AD. Full-scale samples provided insights into the precipitation of multivalent cations with PO43--P. The results showed that precipitation reactions during AD governed the concentration of soluble nutrients during THP-AD.

In this thesis, also the impact of THP on downstream processes was researched, particularly reject water treatment after AD. The research focused on the potential consequences of elevated concentrations of humic substances (HSs) and nutrients in the reject water from digestate dewatering. In Chapter 4, research is described on the effects of THP pre-treatment on the struvite precipitation efficiency, considering the chemical characteristics of HSs. Batch experiments revealed the influence of melanoidins and humic acids on struvite precipitation at different pH levels. The intrinsic characteristics of HSs emerged as critical factors affecting struvite yield, morphology, and colour of the formed precipitates.

In Chapter 5, six full-scale partial nitritation and anammox (PN/A) influents and effluents were sampled, with four wastewater treatment plants (WWTPs) using THP and two without. Characterization of the samples revealed that THP increased concentrations of TAN, and aromatic organics, which is an indication of melanoidins occurrence. Additionally, THP decreased stoichiometric NO3--N concentrations in effluents from the PN/A reactors. This study also emphasizes the importance of considering increased O2 consumption and possible limited trace element availability during PN/A process operation, when utilizing THP-AD to optimize overall system performance.

In summary, the results of this thesis provide an understanding of the interactions and consequences of THP in full-scale scenarios, focusing on the dynamics in nutrient release and the effects of melanoidins formation, in THP, AD, and downstream processes for nutrient removal (PN/A and struvite precipitation).