Anaerobic digestion (AD) is an effective and economical in secondary sludge management. Thermal hydrolysis process (THP) is a reliable pre-treatment method to improve the biodegradability of sludge. Partial nitritation and anammox process (PN/A) is a suitable method used to remove nitrogen in reject water which has a low COD/N ratio. However, some organic compounds produced in THP process, such as melanoidins, have negative effects on AOO and anammox bacteria. Considering that humic substances occupy a large part of the soluble organics in reject water of THP-AD process. Therefore, this study aimed to investigate the acute and long-term toxicity of three types of humic substances (commercial humic acid, melanoidins and reject water) on partial nitritation step of PN/A process. The biodegradability of three types of humic substances under aerobic and anaerobic conditions, and the evolution of humic substances during the biodegradability assays were also analysed. The results showed that all three types of humic substances had higher biodegradability under aerobic condition than that under aerobic condition, and melanoidins had more biodegradable compounds than reject water and humic acid. During the AD process, humic substances acted as electron acceptors, decreasing the cumulative methane production of all three types of humic substances compared to the blank. Also, small MW fractions were transformed to large MW fractions in all three types of humic substances during both aerobic and anaerobic digestion. Moreover, during aerobic and AD, both fulvic acid-like and humic acid-like substances were formed. In addition, the results of acute toxicity tests showed commercial humic acids had a stronger inhibitory effect than reject water and melanoidins on AOO biomass. In addition, a positive correlation between humic substances concentration and AOO activity loss was observed. The long-term operation of PN reactor was failed due to the formation of granular sludge in the reactor and the growth of AOO biomass in the tubing and culture medium.

Thermal hydrolysis (TH) has been widely applied to sewage sludge treatment to improve downstream anaerobic digestion (AD) by enhancing organic solids hydrolysis and subsequent biogas productivity. However, TH also has several drawbacks, including the formation of recalcitrant compounds, which are anaerobically non-biodegradable and even inhibitory to the downstream AD. Melanoidins produced from the Maillard reaction have been regarded as a representative of such compounds. Theoretically, the production of melanoidins increases with increasing TH temperature.
This study collaborated with TORWASH®, a patented TH technology developed by TNO, aiming to characterise the recalcitrant compounds derived under TORWASH® conditions from 180 - 210 °C. In addition, we investigated the effects of anaerobic pre-digestion on the TH performance, attempting to increase the biogas productivity of digestate. Moreover, we also sought pretreatment methods that could mitigate the production of recalcitrant compounds. By reviewing the studies of Zhang et al. (2020) and other researchers in this field, we found that acidic pretreatments could be the solution. Then, we assumed that the addition of acetic acid and CO2 gas could limit the progression of the Maillard reaction. A laboratory simulation of the TH process was performed on waste activated sludge (WAS) at 180 - 200 °C and anaerobically pre-digested WAS at 180 - 210 °C. The following TH tests with acidic pretreatments were only conducted on WAS at 190 °C.
We found that the production of recalcitrant compounds (soluble humic substances) increased with increasing reaction temperature, corresponding to increased UVA254 and colour formation. Proteins were primarily hydrolysed or denatured during TH, and competition for carbohydrates between the Maillard and caramelisation reactions could occur at 190 °C and above. In addition, TH disintegrated high molecular weight DOM > 1 kDa into low molecular weight DOM < 0.1 kDa, and stronger sludge disintegration occurred at higher reaction temperatures. Moreover, BMP decreased after anaerobic pre-digestion but increased after TH treatment. The liquid fraction of pre-digested WAS could reach an equivalent degree of anaerobic biodegradability as that of WAS when applying a higher reaction temperature.
All acidic pretreatments applied decreased the production of soluble humic substances by 10 %, corresponding to a 13 % decrease in UVA254. CO2 pressurisation at 10 bar could effectively reduce the formation of fulvic-acid like and humic-acid like matter, compared with the untreated sample. The effect of 20 bar CO2 was less pronounced than that of 10 bar CO2, whether or not the samples were pre-added with HAc.

Various technologies have been implemented in wastewater treatment plants (WWTPs) to generate clean effluent with minimised impacts on the environment. Thermal hydrolysis process (THP) and struvite crystallisation reactors are two examples of these technologies. THP generates humic-like substances, known as melanoidins, that are presumed to disrupt struvite crystallisation. THP also solubilises trace elements that are prone to complex with melanoidins. On the one hand, the trace elements might stay bound to the melanoidins in the supernatant or on the other hand; they might be present in struvite as impurities. As both THP and struvite crystallisation are promising technologies, the objective of this study was to understand the effects of THP on struvite crystallisation. Results showed that melanoidins generated by THP affected P removal and the amount of produced struvite in struvite crystallisation at pH 6.5 and 7.25, but not at pH 8. In contrary to melanoidins, humic acids (HA) were found to reduce P removal efficiency at pH 8. Melanoidins resulted in slightly higher N-NH4 concentration in the supernatant in all pH values. Melanoidins and HA also changed crystal colour from white into brown and black, respectively. Furthermore, melanoidins and HA changed the morphology of struvite crystals from X-shaped and long rod-like crystals into square-shaped crystals and at the same time reduced the diameter of struvite crystal. Regardless, melanoidins did not change the Mg:N-NH4:P-PO4 molar ratio. Melanoidins did not significantly influence induction time, except at pH 7.25 and 2 g TOC/L melanoidins. In addition to this, melanoidins were found to form complexes with magnesium which subsequently decreased the supersaturation and affected struvite crystallisation. Different elements had different fate on struvite crystallisation. Melanoidins formed complexes with Mn and Ni, resulting in a higher concentration in the supernatant compared to the condition without melanoidins. Based on PHREEQC modelling, humic substances (HS) formed complexes with Ca, Cu, and Fe. Certain trace elements (Ca, Co, Cu, Fe, and Mn) and heavy metals (Al and Cr) were found to co-precipitate with struvite in all experimental conditions; meanwhile, Se minerals only co-precipitated at pH 8. All minerals showed decreasing SI with increasing HS concentrations. The decrease of SI implied that there would be less co-precipitation of Al, Ca, Co, Cr, Cu, Fe, Mn, and Se in struvite crystals. Nevertheless, their presence in the supernatant would not guarantee the bioavailability of the trace elements for bacteria in partial nitritation/anammox (PN/A) processes due to the chance of complexation with HS. Eventually, it could be concluded that THP affected struvite crystallisation and Response Surface Methodology (RSM) confirmed that the optimum conditions for struvite crystallisation with maximised N and P contents and big diameter were pH of 7.9 and 0 g TOC/L melanoidins. Nonetheless, melanoidins prevented co-precipitation of trace elements in struvite crystals.

Anaerobic digestion (AD) is a traditional technique to treat sewage sludge and produce biogas. Thermal hydrolysis process (THP) is a feasible pretreatment for AD by improving the biodegrdability of organic compounds. Partial nitritation/anammox (PN/A) process could be applied in side-stream for treating the reject water from AD since the reject water has a low ratio of COD/N. The prime microorganisms involved in PN/A process are anammox biomass, denitrifiers, ammonium oxidizing organisms (AOO) and nitrite oxidizing organisms (NOO). However, the compounds generated in THP may cause negative effect on microbial activities. In this research, characteristics of the reject water was clarified, and manometric method and respirometry method were used to determine the microbial activities by measuring the conversion rates of the substrates. A higher concentration of soluble COD, total ammonium as nitrogen (TAN) and refractory compounds were in the wastewater treatment plants (WWTPs) with THP installation (i.e. Tilburg, Hengelo, Apeldoorn, and Amersfoort) than the ones in the WWTPs without THP pretreatment (i.e. Sluisjesdijk and Olburgen). Sluisjesdijk has one step PN/A process, while Olburgen has two steps PN/A process. Maximum specific anammox activity(SAA) among the studied WWTPs was found in Sluisjesdijk as 1.32±0.02 gN/gVS/day. THP installation could suppress the SAA value. The presence of refractory compounds may lead to the decrease of SAA. The denitrification activity was enhanced by the high concentration of COD generated in THP. The ratio of AOO activity to anammox activity were similar to the stoichiometry value of 1.32 in Sluisjesdijk Sharon reactor, Tilburg, Hengelo and Amersfoort. Molecular methods may recommended for further study in the area of this research.