Sludge Thermal Hydrolysis Research Articles

The interaction between sludge and microplastics during thermal hydrolysis of sludge

In municipal wastewater treatment plants (WWTPs), large number of microplastics (MPs) accumulated in wastewater migrated into sludge. Thermal hydrolysis of sludge (THS) was one of the most promising processes for promoting changes in molecular structure of MPs. The physicochemical properties and degradative pathways of polyethylene (PE) and polyethylene terephthalate (PET) in THS under different temperatures were studied in this paper. It was found that there was a mutual promotion relationship between sludge degradation and MPs aging. The presence of PE and PET MPs not only increased organics and nitrogen concentrations of sludge filtrate, but also enhanced the transformation of organics like proteins. Sludge accelerated the aging of PE and PET MPs. The friability of PE and PET MPs was increased with more surface fragmentation and breakage under the temperature of 120 ℃−180 ℃. Moreover, PE and PET MPs occurred thermal oxidation and reduction reactions with significant chemical structure changes at 160 °C and 140 °C, respectively. Pristine PE and PET had multiple carbon and oxygen active sites. During THS reaction, not only PE and PET reacted hydrolysis/decomposition to produce short-chain hydroxyl-terminated compounds, but also hydrothermal shear broke the polymer molecules and formed carboxyl-terminated and olefin-terminated low-carbon chains. This study provided some promising sign for in situ microplastic removal during sludge treatments.

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.

Assisting role of carbonaceous skeleton in sludge thermal hydrolysis and press filtration

As a key step in disposal and reutilization, sludge dewatering is very difficult, since extracellular polymers substances (EPS) binds the water, and compressible organic matter deforms and causes water filtration channels to collapse. Sludge dewaterability was demonstrated to enhance by carbonaceous skeleton (CSkel)-assisted thermal hydrolysis in our previously study. This work further investigated the assisting role of different types of CSkel in EPS decomposition during sludge thermal hydrolysis stage and channels reformation during press filtration stage. Two major types of CSkel, lignocellulosic waste (waste sawdust, waste straw, processing by-product) and protein-rich waste (shrimp shells, jatropha oil cake), were selected. The experimental results showed that in the thermal hydrolysis stage, the decomposition of lignocellulosic waste would increase fatty acids production by 28%, resulting in an acidic environment that reduced the total amount of three hydrophilic amino acids, i.e., glycine, serine and threonine. These promoted the release of water from the sludge. In the press filtration stage, average pore size of sludge was reduced by approximately 87% and nanoscale holes began to appear and increase. Assisting of CSkel rebuilt the filtration channels which brought good connectivity between the pores in sludge cake. Lignocellulosic waste proved significantly more effective than protein-rich waste in achieving a water removal rate of 88.63% under 1 MPa. This study provided a basis for selecting suitable CSkel to optimize sludge dewatering for subsequent utilization.

Review of Melanoidins as By-Product from Thermal Hydrolysis of Sludge: Properties, Hazards, and Removal

Melanoidins, as macromolecular heterogeneous organic polymers, are produced from the Maillard reaction between amino and carbonyl groups during the thermal hydrolysis pretreatment (THP) of sludge. The brown color and recalcitrance of melanoidins pose a serious threat to wastewater treatment systems, such as invalidating UV disinfection and decreasing the efficiency of anaerobic digestion; thus, they have gradually received much concern in recent years. However, currently the study on THP-origin melanoidins is limited by a lack of reliable extraction and quantification methods. This paper presents a comprehensive review of the physical, chemical, and biological properties of melanoidins from different sources to fill the research gap on THP-origin melanoidins. The adverse effects of melanoidins on the management of wastewater and sludge are discussed, and for the first time, special attention is paid to the potential environmental hazards of THP-origin melanoidins to natural ecosystems. The removal technologies of melanoidins are summarized and compared as well. Finally, the suggested areas that future studies should focus on are provided. This review is dedicated to providing guidance on melanoidin research and management for the better development of the THP industry.

Influences of humic acids released during sludge thermal hydrolysis on anaerobic digestion: New insights from enzymatic perspectives

Influences of humic acids released during sludge thermal hydrolysis on anaerobic digestion: New insights from enzymatic perspectives

A neglected contributor of thermal hydrolysis to sludge anaerobic digestion: Fulvic acids release and their influences

Fulvic acids (FAs) belong to inert organic matters in sewage sludge and their influences are often overlooked during convectional anaerobic digestion (AD). Currently, thermal hydrolysis (TH) has been widely applied on sludge pretreatment before AD processes, which makes FAs undergo drastic evolutions and aggressive to sludge AD. Results in the present study indicated that FAs concentration in the liquid was elevated by over incredible 150 folds during sludge TH, from 3.4 mg/L in raw sludge to 590.0 mg/L in hydrolyzed sludge at 180 °C. Moreover, during sludge TH, the chemical structures of FAs, including aromatic condensation degree, elemental composition and functional group, also underwent significant changes, which enhanced FAs electron transfer capability, reduced their biodegradability and promoted their roles on sludge AD. Furthermore, fortunately, the evolutions of FAs were favorable to sludge AD in general. Methane production could be promoted by about 20% under FAs concentration of 0.6 g/L, and the FAs extracted from hydrolyzed sludge presented higher promotion performances than that of the raw FAs, in which 180 °C FAs were particularly conspicuous. Furthermore, FAs evolutions would present differential influences on each phase of sludge AD, promotional to acidogenesis and methanogenesis but inhibitory to hydrolysis. Pearson correlation analysis indicated FAs influences on sludge AD, particularly the hydrolysis phase, were not only related to their concentration, but also chemical structure. The findings of this study demonstrated that FAs influences should not be negligible anymore during sludge AD with TH pretreatment. Meanwhile, since FAs promotion on sludge AD was closely related to their concentration and chemical structure, it would be significant to take FAs evolutions as auxiliary indexes for the regulations of sludge TH.

Sludge Thermal Hydrolysis for Mitigating Oxidative Stress of Polystyrene Nanoplastics in Anaerobic Digestion: Significance of the Solids Content

Sludge Thermal Hydrolysis for Mitigating Oxidative Stress of Polystyrene Nanoplastics in Anaerobic Digestion: Significance of the Solids Content

Humic acids promotion or inhibition of sludge anaerobic digestion depends on their redox potentials

Thermal hydrolysis pretreatment has been widely employed to improve sludge anaerobic digestion (AD), but also results in excessive releases of humic acids (HAs) from sludge. The influences of HAs on sludge AD recently have resulted in ever-increasing concerns, but those reported results are not consistent and sometimes even contradictory. The results in the present study indicated whether HAs promote or inhibit sludge AD mainly depends on their redox potentials. Generally, the HAs derived from sludge with low redox potentials were seemingly facilitative to sludge AD, whereas commercial HAs with high redox potentials presented inhibitory effects. In fact, the influences of HAs on each stage of sludge AD, namely hydrolysis, acidification and methanogenesis, were also quite different, and mostly the differences were related to HAs redox potentials. Mechanism analysis indicated HAs with different redox potentials did not evidently influence microbial community profile, but resulted in noticeable changes in microbial activities. Protease and α-glucosidase activities were substantially inhibited by HAs, which became more pronounced as HAs redox potentials decreased from 154 to −238 mV. The relative activity of F420 was shifted to 89.1 %, 104.1 %, 121.3 %, 92.4 % and 73.2 % with the additions of HAs at redox potentials of 154, −63, −392, −419 and −458 mV, respectively. Furthermore, the presence of HAs accelerated interspecies electron transfer and assisted ferredoxin (Fd) in intracellular electron transfer of methanogens. HAs with too high (≫ −398 mV) or too low (≪ −398 mV) redox potentials were not conducive to methanogenesis. The findings of this study could forcefully clarify those contradictory results reported recently about the influences of HAs on sludge AD. Overall, all the findings in this present study indicate that it is of great necessity to regulate sludge thermal hydrolysis based on the redox potentials of released HAs.

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.

Fate of intracellular, extracellular polymeric substances-associated, and cell-free antibiotic resistance genes in anaerobic digestion of thermally hydrolyzed sludge

Thermal hydrolysis of sludge is a promising approach to mitigate antibiotic resistance genes (ARGs) propagation in anaerobic digestion (AD). Although ARGs in sludge may be fractioned into intracellular, extracellular polymeric substance (EPS)-associated, and cell-free ARGs, the fate of these different fractions in AD has never been investigated. This study presents a detailed characterization of intracellular and extracellular ARGs in AD of sludge thermally hydrolyzed at 90 °C and 140 °C. EPS-associated ARGs represented the major fraction of the total extracellular ARGs in all samples, while its lowest abundance was observed for thermal hydrolysis at 140 °C along with the lowest EPS levels. The results suggested a positive correlation between EPS-associated ARGs with intracellular and cell-free ARGs. Furthermore, various EPS components, such as proteins and e-DNA, were positively correlated with β-lactam resistance genes. sul1 dominated all samples as an EPS-associated resistance gene. These results provide new insights into the significance of different ARGs fractions in their overall dissemination in AD integrated with thermal hydrolysis.

Activated sludge thermal hydrolysis for liquid fermentation to produce VFAs: Exploring the balance of carbon release between quantity, quality and recovery

Thermal hydrolysis has been widely applied to improve organics bioconversion during sludge anaerobic treatment currently, based on which, liquid fermentation to produce volatile fatty acids (VFAs) with high concentration and good purity has been successfully developed by only using hydrolysate as the substrate to avoid the interference of “useless” residual solids. Therefore, obtaining high-quality hydrolysate is the prerequisite for VFAs production via liquid fermentation. However, previous studies on sludge thermal hydrolysis either only focused on organics release or only on sludge dewatering. Actually, the quantity, quality and recovery of the carbon released from sludge are equally important, and a balance between them should be established. Results in the present study indicated that organics concentration in sludge hydrolysate could not be arbitrarily enhanced by increasing thermal hydrolysis intensity or sludge concentration, and interestingly there seemed a threshold of around 32 g/L that the highest concentration the sludge hydrolysate could reach. Moreover, with the increase of hydrolysis intensity, the bioavailability of sludge could be promoted but reached the maximum with BOD5/COD of around 0.44 at 180 °C, while sludge dewaterability could be improved but also trended to stable after 160 °C. The findings of this study demonstrate that excessively high hydrolysis intensity would not only waste energy but also induce forms of non-biodegradable organics. The performances of sludge liquid fermentation, including VFAs production and sludge reduce, were closely related to hydrolysis intensity, the choice of which should be based on the balance of the quantity, quality and recovery of the released carbon.

Influence of thermal hydrolysis on sludge anaerobic digestion: Release of humic acid promotes electron transport of methanogen

Thermal hydrolysis pretreatment (THP) has been widely applied to promote anaerobic digestion (AD). Humic acids (HAs) are a group of insoluble organics and often present in the conventional AD of sludge, but are reportedly substantially released during THP. Thus, the influences of the released HAs on AD shall be investigated, in particular on methanogenesis. Our results indicated that the presence of HAs extracted from thermally hydrolyzed sludge improved methanogenesis by 15.8–80.8%. However, HAs harvested at different temperatures presented distinctively different capabilities in enhancing methane production. Seemingly, the promotion efficiency of HAs to methanogenesis did not significantly respond with any individual characteristics of HAs in aspects of molecular weight, electrical activity and electroactive group compositions. However, the efficiency presented an evident correlation with the superimposed effects of the characteristics aforementioned. Therefore, a hypothesis on the mechanism involved in the phenomenon observed above was proposed that only the HAs with low molecular weight, allowing the molecules to penetrate cells could affect methanogenesis, and not all of the electroactive groups of HAs were involved in microbial electron transfer. Moreover, it was found that only the group of quinones (Q1) with side chain of electron withdrawing group played key roles in accelerating methyl acetyl-CoA degradation and promoting methanogenesis. Directional regulation of methanogenesis promotion assisted with HAs was realized by selectively removing or retaining the Q1 groups in HAs, which also evidently sustained the proposed mechanism aforementioned. The findings of this study demonstrate that high temperature was conducive to the release of organics during sludge thermal hydrolysis, whereas the temperature above 160℃ led to changes of HAs structure and then subsequently undermined methane production from sludge.

Integrated two-phase acidogenic-methanogenic treatment of municipal sludge with thermal hydrolysis

The purpose of this research was to investigate the impact of two process configurations integrating two-phase anaerobic digestion (AD) of municipal sludge with thermal hydrolysis (TH). The TH was positioned either before or after the acidogenic fermentation phase. The fermentation process was carried out under the semi-continuous flow regime with a retention time of three days. The TH was done at a temperature of 170 °C and for 30 min. Among all the tested scenarios, the TH of sludge followed by the acidogenic fermentation resulted in the highest COD solubilization ratio (39.5%) and volatile fatty acids production (6,420 ± 400 mg/L), which was 630% and 500% more than that of the raw sludge, respectively. The sequential TH/fermentation process achieved 40% higher ultimate methane yield (240 mL/g COD) than the non-pretreated (raw) sludge. Positioning TH after the fermentation process reduced the ultimate methane yield to 231 240 mL/g COD, although it was still 32% higher than that of the raw sludge. The analysis of methane production rate and biodegradation kinetics data suggested the formation of refractory intermediates during the thermal process of sludge, which reduced the overall performance rate during the first week of the AD process. It was also revealed that acidogenic fermentation of thermally-processed sludge could diminish the adverse effect of the recalcitrant compounds formed during the thermal hydrolysis on the subsequent AD process.

Thermal Hydrolysis of Sludge Counteracts Polystyrene Nanoplastics-Induced Stress during Anaerobic Digestion

Thermal Hydrolysis of Sludge Counteracts Polystyrene Nanoplastics-Induced Stress during Anaerobic Digestion

Phosphorus transformation during the carbonaceous skeleton assisted thermal hydrolysis of sludge

On the basis of the carbonaceous skeleton assisted thermal hydrolysis that we proposed to achieve efficient sludge dewatering, this work further explored phosphorus (P) transformation in the process. The results showed that during independent thermal hydrolysis in the temperature range of 120–240 °C, organic-P was first decomposed into soluble-P and particulate-P in liquid, and then combined with Ca, Fe, and Al to form more apatite-P (AP) and less non-apatite inorganic-P (NAIP). When the skeleton assisted the sludge thermal hydrolysis, the turning point of the hydrolysis temperature would reduce from 180 °C to 150 °C, at which the liquid-P began to decrease and the organic-P generally decomposed. Moreover, the increment in the content of AP halved while that of NAIP doubled compared to that in the process without the carbonaceous skeleton. These effects come from the exogenous components introduced by adding the skeleton, which were different from the sludge. Compared with the P-rich compound and metal elements that tend to bond with phosphate introduced by the skeleton, hemicellulose as a main organic component played a leading role in the different P transformations of AP and NAIP. The hemicellulose slightly increased the acidity of sludge products, thereby inhibiting AP production and promoting the production of recyclable NAIP. Overall, the carbonaceous skeleton assisted thermal hydrolysis was beneficial for P recovery with a very low filtrate loss rate.

Alkaline Thermal Hydrolysis of Sludge: Browning Reactions and Their Influence on Anaerobic Digestion and Protein Recovery

Alkaline Thermal Hydrolysis of Sludge: Browning Reactions and Their Influence on Anaerobic Digestion and Protein Recovery

Analysis of organic matter conversion behavior and kinetics during thermal hydrolysis of sludge and its anaerobic digestion performance

In thermal hydrolysis (TH) of waste activated sludge (WAS), the material transformation of a specific temperature heating for a set duration is generally examined. However, this study looked at the material changes of TH as the temperature rose (90–210 °C) and the kinetic derivation of soluble chemical oxygen demand (SCOD), protein, and carbohydrate using the Coats-Redfern model. It was found that the proportion of soluble protein and soluble carbohydrate in the organic components and their contents reached the maximum (17.39 and 8.10 g L−1 respectively) at 180 °C. Differently, volatile fatty acid (VFA), amino acids, and ammonia nitrogen increased with the TH temperature and reached a maximum at 210 °C. The fitting equations of non-isothermal dynamics at the medium- and low-temperature stages (90–180 °C) at n = 1, 0.5, and 2 were studied. When n = 1, the activation energies of COD, protein, and carbohydrate were 33.32, 23.34, and 36.15 kJ mol−1, respectively. And the kinetic analysis results were in good agreement with the experimental results (the maximum rate of increase in protein and carbohydrate was at 135–150 °C and 150–180 °C, respectively). Moreover, the pattern of anaerobic digestion performance of WAS was comparable to the trend of protein and carbohydrate in TH, the highest cumulative methane production was 159.68 mL·g−1VS for the TH sludge at 180 °C. This study provided a theoretical foundation for the use of thermal hydrolysis in engineering.

Underestimated humic acids release and influence on anaerobic digestion during sludge thermal hydrolysis

Humic-like acids (HAs) are abundant in sewage sludge but mainly bonded with solids. Thus, their influences are often neglected in conventional digestion processes. Currently thermal hydrolysis pretreatment (THP) has been widely adopted in sludge anaerobic digestion (AD) to enhance hydrolysis of complex matters and further to improve methane production. However, the impacts of enhanced release of HAs and the mechanisms involved are not well understood and need to be further investigated because the substantial amounts of HAs present in AD could severely threaten the sludge AD processes. Results in the present study indicated that the concentration of soluble HAs in sludge was elevated by 90 times due to the THP, from 8 mg/L in raw sludge to 727 mg/L in the pretreated sludge hydrolyzed at 180 °C. Moreover, the structural characteristics of soluble HAs, including aromatic condensation degree, elemental composition and functional group, also showed substantial differences with the increased temperature of the THP. Furthermore, the release of HAs presented significant influences on sludge digestion. Acidification rate was inhibited by over 50% with 0.4 g/L of HAs, whereas methanogenesis was improved by nearly 200% with 0.8 g/L HAs and inhibited about 50% with 2.0 g/L. The activities of proteinase and co-enzyme F420 were decreased by 20% and increased by 19%, respectively, under HAs stress at 0.6 g/L for 5 days. Moreover, molecular structural changes of soluble HAs also contributed to the influences. Especially, the E4/E6 value representing the degree of HAs aromatic condensation and C/N ratio of soluble HAs were closely correlated with their inhibition degree to sludge hydrolysis. The findings of this study demonstrate that the influences of HAs are evident and also vary to the different steps of anaerobic digestion processes, which shall not be negligible during the sludge digestion that is with THP. Due to the rate-limiting step was methanogenesis in the AD process of pretreated sludge by thermal hydrolysis, HAs concentration was recommended at low level, for example around 1.0 g/L, to accelerate or not limit methanogenesis.

Significant effect of pH on tetracycline resistance genes reduction during sludge thermal hydrolysis treatment

Thermal hydrolysis (TH) treatment has been verified to effectively reduce antibiotic resistance genes (ARGs) in waste activated sludge. This study focused on the effect of TH parameters on ARGs reduction, and the optimal conditions basing on tetracycline (tet) resistance genes reduction rate in sludge phase were pH 3, temperature 160 °C and reaction time 2 h. The pH was found to play a critical role in tet genes reduction behavior. Additionally, the tet genes distributions in TH supernatant are considered as well because genes will release from solid to liquid phase as sludge cell breaking. The lowest genes content in TH supernatant was also observed at pH 3. Further investigation revealed that contents of DNA and organics released from sludge phase to liquid phase under pH 3 were less than that in neutral and alkaline condition; moreover, the organics in TH supernatant was found to protect DNA. Therefore, under the acidic condition, the genes had less releasing from the sludge phase and the DNA protection effect of organics was weaker, resulting in the lowest tet genes content in TH supernatant. Moreover, bacterial community structure after TH was closely related with the ARGs content. The bacterial on phylum and genus level showed various responses to TH pH values. Although the Firmicutes phylum and Escherichia-Shigella genus exhibited the stronger resistance and had higher accumulation in TH acidic condition, more possible total tet genes hosts were destroyed in acidic condition, causing less ARGs remaining in the sludge phase under pH 3.

Removal and generation of microplastics in wastewater treatment plants: A review

Wastewater treatment plants (WWTPs) are the primary recipients of microplastics (MPs), prior to their discharge into natural waterbodies. The aim of this article is to summarize the generation process of MPs and the efficiency of their removal by treatment technologies currently adopted by WWTPs, as well as the influence of sludge treatment on the fragmentation of MPs. WWTPs mainly remove MPs by means of adhesion, sedimentation, and filtration. The average removal efficiency of MPs is less than 90% and is affected by the choice of wastewater treatment process and the properties of MPs (such as the size, density and morphology). The secretion of biological enzymes by microbes and the metabolites of biofilm may promote the hydrolysis of microbial carrier materials, resulting in the release of endogenous MPs under shear force. Furthermore, during wastewater pumping, disinfection and sludge thermal hydrolysis treatment, MPs may form sub-micron-scale secondary MPs through mechanical, ultraviolet, ozone and thermal degradation processes. Processes such as thermal drying of sludge and lime stabilization have been found to lead to the fragmentation of MPs increase the specific surface area and number of MPs, leading to an increase in the potential ecological threat posed by MPs in sludge. Currently, the level of MP pollution throughout the environment is continually increasing. This review will help improve wastewater and sludge treatment processes, effectively reducing the risk of MPs entering the natural water bodies and soil through WWTPs.