Inhibition of Maillard reaction during alkaline thermal hydrolysis of sludge

Neglected sludge solid phase in sludge pretreatment process: Physicochemical characterization and mechanism study of its role in anaerobic degradation

The low anaerobic digestion efficiency of the solid phase separated from pre-treated sludge indicates the need to explore other suitable resource utilization pathways for sludge solid phase. However, there is a lack of comprehensive and in-depth research on the physicochemical properties of sludge solid phase. This study comprehensively analyzes the characteristics of sludge solid phase and elucidates the mechanism of sludge solid phase in the anaerobic degradation of toxic wastewater. The results show that the surface free energy of sludge solid phase after different pre-treatments is mainly contributed by Lewis acid-base hydration free energy. The distribution of proteins on the surface of sludge solid phase plays a major role in the adhesion between sludge solid particles. Metal ions in the sludge solid phase are mainly present in the exchange state, followed by the carbonate state and the organics-bound state. The sludge solid phase obtained by sludge pH 12 + 150 °C treatment has the highest conductivity (1.36 mS/m) and capacitance (25.51 μF/g), mainly due to the presence of melanoidins in the sludge solid phase, which has similar semiquinone radicals to humic acids, thus increasing conductivity. The addition of sludge solid phase promotes an increase in cumulative methane production and rate of methane production. The sludge solid phase might play a role of an auxiliary carbon source acting as an adsorbent to buffer against toxicity inhibition and facilitate electron transfer. This study reveals the characteristics of sludge solid phase and its role in anaerobic digestion, providing theoretical guidance for finding suitable resource utilization pathways for sludge solid phase.

Development of oriented multi-enzyme strengthens waste activated sludge disintegration and anaerobic digestion: Performance, components transformation and microbial communities

Low methane production and long retention time are the main dilemmas in current anaerobic digestion (AD) of waste activated sludge (WAS). This work used WAS as only substrate to prepare oriented multi-enzyme (ME) that directly used for WAS pretreatment. Under the optimal parameters, the highest activities of protease and amylase in ME could respectively reach 16.5 U/g and 580 U/g, and the corresponding methane production attained 197 mLCH4/g VS, which was increased by 70.4% compared to blank group. It was found that ME pretreatment could strengthen WAS disintegration and organic matters dissolution, lead to the soluble chemical oxygen demand (SCOD) was increased from the initial 486 mg/L to 2583 mg/L, and the corresponding volatile suspended solid (VSS) and extracellular polymeric substances (EPS) were reduced by 27% and 73.8%, respectively. The results of three-dimensional excitation-emission matrix (3D-EEM) and Fourier transform infrared spectroscopy (FTIR) indicated that protein disintegration may be the critical step during the process of WAS hydrolysis with ME, of which the release of tyrosine-like proteins achieved the better biodegradability of WAS, while the results of X-ray photoelectron spectroscopy (XPS) showed that the formation of protein derivatives was the main harmful factor that could extend the lag phase of AD process. Microbial communities analysis further suggested that ME pretreatment facilitated the enrichment of acetogenic bacteria and acetotrophic methanogens, which caused the transition of the methanogenesis pathway from hydrogenotrophic to acetotrophic. This study is expected to furnish valuable insight for ME pretreatment on enhancing WAS disintegration and methane production.

Development and standardization of spectrophotometric assay for quantification of thermal hydrolysis-origin melanoidins and its implication in antioxidant activity evaluation

Melanoidins are brown recalcitrant polymers originating from the thermal hydrolysis pretreatment (THP) of organic solid waste (OSW). Owing to their various formation pathways and complex structures, there is currently no reliable method to quantify melanoidins. In this study, a spectrophotometric method was developed to determine melanoidins concentration in different OSW. Three typical model Maillard reaction systems (glucose-glycine, glucose/fructose-20 amino acids, and dextran-bovine serum albumin) were used to acquire the characteristic peaks and establish standard curves. The results showed that a standard curve using glucose/fructose-20 amino acids model melanoidins at 280 nm was the optimal quantification method, because it had the best correlation with the physicochemical indicators of melanoidins and semi-quantification results calculated by excitation-emission matrix fluorescence. In addition, the applicability of the proposed method was evaluated using multiple real melanoidins samples extracted from thermally pretreated OSW under different THP conditions and food-derived melanoidins as well, demonstrating its validity and advantages. This study is the first to provide a simple, effective, and accurate method for quantifying THP-origin melanoidins from different sources. Remarkably, as a specific and important application scenario, the proposed quantification method was employed to investigate the concentration dependence of melanoidins antioxidation in thermally pretreated OSW.

Effect of total solids content on anaerobic digestion of waste activated sludge enhanced by high-temperature thermal hydrolysis

Total solids (TS) content may provide a regulatory strategy for optimizing anaerobic digestion enhanced by high-temperature thermal hydrolysis, but the role of TS content is not yet clear. In this study, the effect of TS content on the high-temperature thermal hydrolysis and anaerobic digestion of sludge and its mechanism were investigated. The results showed that increasing the TS content from 2% to 8% increased the sludge solubility and methane production potential, reaching peak values of 26.6% and 336 ± 6 mL/g volatile solids (VS), respectively. With a further increase in TS content to 12%, the strong Maillard reaction increased the aromaticity and structural stability of extracellular polymer substances, decreasing sludge solubility to 18.6%. Furthermore, the decrease in sludge biodegradability and the formation of inhibitory by-products resulted in a reduction in methane production to 272 ± 4 mL/g VS. This article provides a new perspective to understand the role of TS content in the thermal hydrolysis of sludge and a novel approach to regulate the Maillard reaction.

Carbon-rich and low-ash hydrochar formation from sewage sludge by alkali-thermal hydrolysis coupled with acid-assisted hydrothermal carbonization

The production of carbon-rich and low-ash hydrochar from sewage sludge is attracting interest due to its great application prospect in high value-added carbon materials fields, but which is impossible through direct hydrothermal carbonization. In this study, alkali-thermal hydrolysis followed by acid-assisted hydrothermal carbonization was thus proposed. Thermal hydrolysis at strong alkaline environment was more effective than acid one to promote the dissolution of organic matters and restrain the release of inorganic matters from sludge, which created a favorable condition for hydrochar formation in a carbon-rich and low-ash way. Alkali-thermal hydrolysis began to show a positive effect on the dissolution of organics in sludge when temperature exceeded the threshold of 90 °C, and an increase of 9.77 % was found at 150 °C when compared to 30 °C. Acid-assisted hydrothermal carbonization of alkali-thermal hydrolysate (ATH) at pH 1.0 strongly promoted condensation polymerization of dissolved organics to form hydrochar and meanwhile inhibited introduction of dissolved inorganics. The nanosized microparticulate hydrochar derived from ATH-30 had a carbon and ash content of 50.98-61.31 % and 10.76-12.09 %, while the micro-sized microspheric hydrochar with multiple deposition layers formed from ATH-150 showed a better performance in carbon-rich and low-ash aspect where a carbon and ash content of 58.24-70.07 % and 0.40-3.24 % was realized, both of which were obviously superior to the direct hydrochar (carbon 34.86 % and ash 46.11 %). The condensation of dissolved organics during alkali-thermal hydrolysis stage is important to the carbonization degree of hydrochar. This study provides a new perspective in sludge disposal and production of advanced carbon materials.

Inhibitory effect of microplastics derived organic matters on humification reaction of organics in sewage sludge under alkali-hydrothermal treatment

Alkali-hydrothermal treatment (AHT) of sewage sludge is often used to recover value-added dissolved organic matters (DOM) enriched with artificial humic acids (HA). Microplastics (MPs), as emerging contaminants in sewage sludge, can leach organic compounds (MP-DOM) during AHT, which potentially impact the characteristics of thermally treated sludge's DOM. This study employed spectroscopy and Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR-MS) to explore the impacts of MPs on DOM composition and transformation during AHT. The biological effects of DOM were also investigated by hydroponic experiments. The results showed that the leaching of MP-DOM led to a substantial increase in DOC content of DOM of thermally treated sludge. Conversely, the HA content significantly decreased in the presence of MPs, resulting in a decline of plant growth facilitation degree. FT-ICR-MS analysis revealed that the reduction in HA content was characterized by a notable decline in the abundance of O6-7 and N1-3O6-7 molecules. Reactomics results indicated that the leaching of MP-DOM inhibited the Maillard reaction but bolstered oxidation reactions. The inhibition of Maillard reaction, resulting in a decrease in crucial precursors (dicarbonyl compounds, ketoses, and deoxyglucosone), was responsible for the decrease of HA content. The primary mechanism responsible for inhibiting the Maillard reaction was the consumption of reactive amino reactants through two pathways. Firstly, the leaching of organic acids in MP-DOM caused decrease of sludge pH, leading to the protonation of amino groups. Secondly, the lipid-like compounds in MP-DOM underwent oxidation (-2H+O), producing fatty aldehydes that consumed the reactive amino reactants. These discoveries offer enhanced insights into the specific contribution of MPs to the composition, transformation, bioactivity of DOM during AHT process.

Dissolved organic matter transformation mechanisms and process optimization of wastewater sludge hydrothermal humification treatment for producing plant biostimulants

Understanding the composition, transformation and bioactivity of dissolved organic matter (DOM) at the molecular level is crucial for investigating the hydrothermal humification process of wastewater sludge and producing ecological fertilizers. In this study, DOM transformation pathways under alkali-thermal humification treatment (AHT) were characterized by Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) in conjunction with molecular reaction network analysis. The effects of DOM on plant growth were examined using hydroponics and transcriptomic analysis. In the wastewater sludge humification process, AHT produced maximum amounts of protein (3260.56 mg/L) and humic acid (5788.24 mg/L) after 12 h. FT-ICR MS results indicated that protein-like structures were prone to continuous oxidation and were ultimately transformed into aromatic N-containing compounds resembling humic substances. Several reactive fragments (such as -C₂H₂O₂, -C₃H₄O₂, and -C₄H₆O₂) formed by the Maillard reaction (MR) were identified as potential precursors to humic acid (HA). In terms of biological effects, DOM_12h showed the highest rice germination and growth activity, whereas that produced by AHT for a longer period (> 12 h) displayed phytotoxicity owing to the accumulation of toxic substances. Plant biostimulants (such as amino acids and HAs) in DOM improved energy metabolism and carbohydrate storage in rice seedlings by upregulating the "starch and sucrose metabolism" pathways. Toxic substances (such as pyrrole, pyridine, and melanoidin) in DOM can activate cell walls formation to inhibit abiotic stimuli in rice seedlings through the biosynthesis of phenylpropanoid pathway. These findings provide a theoretical basis for optimizing sludge hydrothermal humification and recovering high-quality liquid fertilizers.

Catalytic hydrothermal carbonization of wet organic solid waste: A review

Hydrothermal carbonization has gained attention in converting wet organic solid waste into hydrochar with many applications such as solid fuel, energy storage material precursor, fertilizer or soil conditioner. Recently, various catalysts such as organic and inorganic catalysts are employed to guide the properties of the hydrochar. This review presents a summarize and a critical discussion on types of catalysts, process parameters and catalytic mechanisms. The catalytic impact of carboxylic acids is related to their acidity level and the number of carboxylic groups. The catalysis level with strong mineral acids is likely related to the number of hydronium ions liberated from their hydrolysis. The impact of inorganic salts is determined by the Lewis acidity of the cation. The metallic ions in metallic salts may incorporate into the hydrochar and increase the ash of the hydrochar. The selection of catalysts for various applications of hydrochars and the environmental and the techno-economic aspects of the process are also presented. Although some catalysts might enhance the characteristics of hydrochar for various applications, these catalysts may also result in considerable carbon loss, particularly in the case of organic acid catalysts, which may potentially ruin the overall advantage of the process. Overall, depending on the expected application of the hydrochar, the type of catalyst and the amount of catalyst loading requires careful consideration. Some recommendations are made for future investigations to improve laboratory-scale process comprehension and understanding of pathways as well as to encourage widespread industrial adoption.

Method development for the identification, extraction and characterization of melanoidins in thermal hydrolyzed sludge

Melanoidins, the brown late-stage Maillard reaction products, are responsible for color development and refractoriness in thermal hydrolyzed sludge (THS), causing negative effects on wastewater treatment. This study aimed to develop a methodology for the identification, isolation and preliminary characterization of the THS melanoidins. After thermal hydrolysis, the formation of melanoidins were confirmed by physicochemical indicators and excitation-emission matrix fluorescence analysis. The macroporous resin adsorption method was adopted to successfully extract melanoidins from THS with high recovery and selectivity. The main chemical components of the extracted melanoidins were carbohydrate (23.1 %), protein (43.8 %) and phenol (13.7 %), and the C/N was 4.5. In addition, furans, alcohols and sulfur-containing volatile substances were detected by pyrolysis-gas chromatography-mass spectrometry. Fourier transform infrared spectroscopy determined that functional groups such as CO, CN, NH, C-O-C, amide I and phenyl were present in the structure of THS melanoidins, and nuclear magnetic resonance spectroscopy indicated the formation of heterocyclic macromolecular structures. Their formation pathways were speculated to involve the cross-linkage of low-molecular-weight components (e.g. proteins, Amadori and Schiff base compounds) and the polymerization of heterocyclic units (e.g. furans, pyroles and pyrazines). The above results clarify the fundamental characteristics of the melanoidins formed during sludge thermal hydrolysis and will help improve subsequent research on melanoidins control.

Untangling the effect of solids content on thermal-alkali pre-treatment and anaerobic digestion of sludge

Total solids (TS) content is critical for thermal hydrolysis and anaerobic digestion (AD) performance, but its role in thermal-alkaline pre-treatment (TAP) is unclear. Therefore, this study aimed to reveal the key role of TS content in TAP and AD of waste activated sludge. The results showed that the optimum TS content of TAP (at 90 °C for 1 h, pH = 10) was 8 %. Sludge disintegration and methane production increased from 19.7 ± 2.2 % to 34.3 ± 2.9 % and from 167.4 ± 4.2 to 246.0 ± 6.2 mL/g volatile solids, respectively, when TS content were increased from 2 % to 8 %. A high TS content will likely promote sludge disintegration since it will reduce heat loss and improve heating efficiency. Additionally, increasing TS content from 2 % to 10 % minimized the production of intracellular reactive oxygen species by 30.4 ± 0.7 % and increased cell viability by 11.5 ± 2.6 %. In contrast, excessive TS content (i.e., ≥10 %) deteriorated the fluidity of sludge, which prevents it from disintegration. Once TS reached 10 %, the accumulation of ammonia nitrogen and volatile fatty acids reached 812.7 ± 27.4 and 1932.0 ± 5.3 mg/L, respectively, which reduced the activity of acidulase and coenzyme F420 and shifted the archaeal community from acetylotrophic to hydrogenotrophic methanogens. This article provides new insights into the TS content in TAP and AD technology.

Elucidating the role of solids content in low-temperature thermal hydrolysis and anaerobic digestion of sewage sludge

The role of total solids content in low-temperature thermal hydrolysis and anaerobic digestion of sewage sludge was investigated. Increasing total solids from 2% to 6% improved thermal hydrolysis and anaerobic digestion performance, while increasing it further to 12% decreased methane production. Maximum sludge solubility (22.9% ± 0.6%) and methane production (320 ± 7 mL/g volatile solids) were achieved at 6% solids. The increase in solids content from 2% to 6% improved heating efficiency and volatile fraction content, which facilitated sludge solubilization and enrichment of methanogens. However, further increases in solids content resulted in a stable floc structure with excess ammonia nitrogen and volatile fatty acids, which limited the release of substrates and reduced the abundance of acidifying bacteria and methanogens, ultimately leading to reduced methane production. An in-depth understanding of the role of solids content opens up new avenues for improved low-temperature thermal hydrolysis of sludge.