Improved sewage sludge dewaterability saves costs in biosolids transport and disposal, making reliable dewaterability assessment essential for both research and full-scale process evaluation. However, laboratory-based indexes commonly used to predict dewatering performance often show limited correspondence with full-scale results. Nonetheless, indexes such as capillary suction time (CST), sedimentation and centrifugation methods, specific resistance to filtration (SRF), and mixed dead-end techniques show biases when trying to resemble full-scale results. In our present article, we pose that lack of predictability originates from overlooking all the phenomena involved in the dewatering process. Four critical phenomena are identified to occur during dewaterability: (1) mixing of sludge and conditioner; (2) suspension destabilisation; (3) flocs formation, and (4) compression and expression. By systematically evaluating widely used laboratory indexes this review shows that these methods capture only part of the dewatering process and generally fail to represent the compression and expression stages that ultimately determine the ultimate achievable solids concentration. The analysis highlights sludge compressibility as a critical factor limiting the predictive capacity of conventional indexes. Based on this synthesis, centrifugation, dead-end filtration, and combined centrifugation–filtration approaches are identified as more suitable methods for laboratory-scale assessment because they better represent the compression behaviour of sludge. The review provides a conceptual framework linking dewatering phenomena with experimental assessment methods, supporting the development of improved evaluation strategies and facilitating the testing of emerging, environmentally friendly conditioning technologies.

This work investigates the effects of mild thermal pre-treatment combined with hydrogen peroxide (H2O2) and iron(II) chloride (FeCl2) dosing on WAS (waste activated sludge) disintegration to boost anaerobic digestion and subsequent energy recovery potential. A two-stage thermal approach was applied, where sludge was first treated at 55 °C with and without chemical additives, followed by further processing at 70 °C. All pre-treatments significantly increased soluble chemical oxygen demand (SCOD), with the highest solubilisation observed under combined FeCl2 and H2O2 addition, suggesting an observable effect of chemical addition on sludge solubilisation. This increase in SCOD was further supported by extracellular polymeric substances (EPS) analysis, which showed a shift from tightly bound to soluble fractions. Additionally, hydrogen production was observed during pre-treatment at 55 °C, with higher yields in the presence of FeCl2 addition. Overall, the results indicated that mild thermo-chemical pre-treatment, particularly with FeCl2 addition, enhanced WAS hydrolysis while enabling hydrogen production, highlighting its potential to improve sludge energy recovery and support more resource-efficient wastewater treatment operations.

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).

Thermal hydrolysis process (THP) is a widely used pre-treatment method in the anaerobic digestion (AD) of waste municipal sewage sludge. A post AD dewatering step of the digestate produces a liquid stream called reject water. THP increases the concentration of humic substances (HSs) and nutrients in the produced reject water. Struvite precipitation is a widely used technique to remove and (potentially) recover PO₄³⁻ -P and the corresponding amount of total ammoniacal nitrogen from reject water. The chemical characteristics of the THP-produced HSs influence reaction yields and morphology of struvite. In our current study, struvite batch precipitation experiments were conducted at different pHs, and different concentrations of HSs, consisting of either melanoidins or humic acids. Our results showed that at pH 6.5 struvite precipitation was severely retarded. However, increased concentrations of melanoidins at pH 6.5 enhanced struvite precipitation. Batch experiments conducted at pH 7.25 and 8 with increased melanoidins concentrations showed PO₄³⁻-P precipitation yields over 86 %. Humic acids negatively impacted struvite precipitation at all analysed pH values, presumably because of Mg²⁺ complexation. Morphological analysis showed that the presence of both HSs affected Feret diameters, aspect ratio, and cleavage pattern of struvite. Also, HSs rendered coloured crystals. Overall, our results showed that struvite precipitation is affected by HSs intrinsic characteristics, affecting yield, morphology, and colour of the formed precipitates.

Full-scale thermal hydrolysis processes (THP) showed an increase in nutrients release and formation of melanoidins, which are considered to negatively impact methanogenesis during mesophilic anaerobic digestion (AD). In this research, fractionation of THP-sludge was performed to elucidate the distribution of nutrients and the formed melanoidins over the liquid and solid sludge matrix. Degradation of the different fractions in subsequent AD was assessed, and the results were compared with non-pre-treated waste activated sludge (WAS). Results showed that the THP-formed soluble melanoidins were partially biodegradable under AD, especially the fraction with molecular weight under 1.1 kDa, which was related to protein-like substances. The use of THP in WAS increased the non-biodegradable soluble chemical oxygen demand (sCOD) after AD, from 1.1% to 4.9% of the total COD. The total ammoniacal nitrogen (TAN) concentration only slightly increased during THP without AD. However, after AD, TAN released was 34% higher in the THP-treated WAS compared to non-treated WAS, i.e., 36.7 ± 0.7 compared to 27.4 ± 0.4 mgTANreleased/gCODsubstrate, respectively. Results from modified specific methanogenic activities (mSMAs) tests showed that the organics solubilised during THP, were not inhibitory for acetotrophic methanogens. However, after AD of THP-treated sludge and WAS, the mSMA showed that all analysed samples presented strong inhibition on methanogenesis due to the presence of TAN and associated free ammonia nitrogen (FAN). In specific methanogenic activities (SMAs) tests with incremental concentration of TAN/FAN and melanoidins, TAN/FAN induced strong inhibition on methanogens, halving the SMA at around 2.5 gTAN/L and 100 mgFAN/L. Conversely, melanoidins did not show inhibition on the methanogens. Our present results revealed that when applying THP-AD in full-scale, the increase in TAN/FAN remarkably had a greater impact on AD than the formation of melanoidins.

Thermal hydrolysis process (THP) is a well-established anaerobic digestion (AD) pre-treatment technology. Despite the THP benefits the pre-treatment increases the concentrations of nutrients and melanoidins in the digestate reject water after dewatering. The increased concentrations of nutrients and melanoidins formed during THP-AD can impact downstream processes, such as struvite precipitation and partial nitritation/anammox (PN/A). In our present work, six full-scale PN/A influents and effluents were sampled in The Netherlands (4 with THP and 2 without THP). Full-scale samples were characterised and the stoichiometric O₂ consumption and melanoidins chelated to trace elements were analysed. The results showed that THP increased the concentration of total ammoniacal nitrogen (TAN), chemical oxygen demand (COD), total organic carbon (TOC), UVA 254 and colour, which are indicators of melanoidins occurrence. THP furthermore decreased the stoichiometric NO₃⁻-N production from the PN/A reaction in effluents. The disparity between stoichiometric and measured NO₃⁻ -N in the THP-using plants was explained by the proliferation of denitrifiers. Moreover, denitrification improved the N removal efficiency due to the consumption of the stoichiometrically-produced NO₃⁻ -N. Also, the stoichiometric O₂ consumption increased in the plants using THP, reaching up to 56% of the O₂ used for partial oxidation of TAN. Trace elements analysis revealed that the plants with elevated concentrations of melanoidins in the effluent showed a high percentage of chelated multivalent cations, particularly transition metals such as Fe. Kendall correlation coefficient analysis showed that the chelation of multivalent cations was correlated mainly with colour occurrence in the reject waters. Overall, the results indicated that in PN/A systems using THP-AD increased O₂ consumption and trace elements availability should be considered during the process design.