Sewage sludge production poses a significant global challenge, with anaerobic digestion (AD) being the most widely employed method to manage this issue. However, since hydrolysis is the rate-limiting step in AD, pretreatment of sludge can aid in enhancing the process.
The current study aimed to evaluate sludge solubilization and gas production—particularly bio-hydrogen (bio-H₂) production—following low-temperature thermal pretreatment (LTTP) at 55°C. This thesis replicates the full-scale setup of the Themista® system, which employs a two-stage thermal pretreatment process with 55°C and 70°C as the heating stages, operating in a semi-continuous mode. Four distinct pretreatment experiments were conducted to assess sludge solubilization and gas production: LTTP , LTTP with H₂O₂ at 55°C, LTTP with FeCl₂ at 55°C, and LTTP with both H₂O₂ and FeCl₂ at 55°C.
The results demonstrated that both thermo-chemical and thermal pretreatments exhibited similar soluble chemical oxygen demand(sCOD) release patterns. A shift from tightly bound to more soluble fractions of extracellular polymeric substances (EPS) was observed post-pretreatment. Gas production was noted only at 55°C across all conditions, with bio-H₂ being produced under each pretreatment. Notably, thermal pretreatment with FeCl₂ resulted in an average H₂ COD/tCOD ratio that was 24 times higher than that of thermal pretreatment alone. However, the average sludge to gas COD percentage for all pretreatments remained below 0.06%, with the highest of 0.1% observed in the thermal treatment with FeCl₂. Additionally, significant increases in BMP and a sludge COD to CH₄ conversion percentage of 75% were observed post-treatment with thermal pretreatment combined with FeCl₂

Dormancy is a topic that is researched extensively for various microorganisms, especially in the field of medicine. However, not many studies have been conducted for the dormancy of microorganisms in the wastewater treatment sector. Treatment processes like the NEREDA® technology use Phosphate Accumulating Organisms (PAO) to remove phosphate from the wastewater. If dormancy affects these bacteria, it can cause the wastewater treatment plants to function at a lower capacity. Reviving dormant PAOs can increase treatment capacity and treat the wastewater during high influent loading without effluent deterioration. Studying the optimal conditions for granule storage without inducing dormancy can be advantageous during granule transport for inoculation of reactors during their start-up phase. This study presents the first independent assessment of the possible dormancy of PAOs in Aerobic Granular Sludge (AGS).

In this research thesis, dormancy is a survival strategy opted by PAOs to persist substrate limitation. There are two different pathways to dormancy that has been explored here. Responsive switching and starvation are studied as they express fast and slow transition respectively into dormancy. Lag faced during resuscitation of dormant cells is considered proportional to the length of dormancy in responsive switching.

The experimental phase of the research can be interpreted in three parts: first through method development, second through method testing and third through the application of the developed method. The method was developed to understand dormancy in PAOs by observing differences in their maintenance and activity rates. This method was then tested on granules where dormancy was induced by lowering the temperature to 4ºC and anaerobically storing without feed. This method was then applied to newly sampled granules from a full-scale reactor to study dormancy in the PAOs. The developed method increased the activity of the former granules by 60% but did not raise the activity of the latter. Maintenance rates increased by 167% and 68%, respectively, in the granules. Although dormancy could not be sufficiently proved by the method, starvation of PAOs in Utrecht NEREDA® reactors was possible.