Digested Sewage Sludge Research Articles

The fate of Volatile Fatty Acids (VFAs) during the thermodynamic transition from hydrothermal carbonization to hydrothermal liquefaction: HtC-to-HtL

The valorization of biomass via hydrothermal treatment, a green and reliable thermochemical process, produces high-value chemicals. This study focuses on the transformation of Volatile Fatty Acids (VFAs) during the transition from Hydrothermal Carbonization (HTC) to Hydrothermal Liquefaction (HTL) and their impact on product quality. Conducted with anaerobically digested sewage sludge, the hydrothermal treatment varied temperatures from 280 °C to 340 °C. The primary objective was to investigate VFA concentrations and their link to hydrochar yield and gaseous by-product composition. A high-performance Parr 4577A hydrothermal reactor facilitated these experiments. For VFA assessment, a Shimadzu Nexis 2030 plasma GC-BID with an HP-FFAP column was utilized for the first time. The study revealed a U-shaped COD concentration curve (peaking at 10,510 mg/l at 340 °C) and an S-shaped Total Phenols profile (highest at 1314 mg/l at 325 °C). Notably, VFA concentrations dipped significantly at 295 °C and 325 °C but were higher at other temperatures, suggesting a complex underlying mechanism. This variability is partially attributed to the reduction in hydrochar mass yield with increasing temperature (46% at 280 °C to 31% at 340 °C) and a surprising spike in CO2 concentration (90% at 310 °C). These findings emphasize the critical role of carboxylation reactions in the HTC-to-HTL transition, enhancing the quality of liquid hydrothermal products.

Study on the fate of per- and polyfluoroalkyl substances during thermophilic anaerobic digestion of sewage sludge and the role of granular activated carbon addition

Limited information is available on the removal of per- and polyfluoroalkyl substances (PFAS) in anaerobic digestion (AD). Τhe fate of six PFAS was studied in thermophilic bioreactors in the presence of granular activated carbon (GAC) and voltage application. Reactors with GAC exhibited lower concentrations of volatile fatty acids and higher methane production compared to those with and without the application of voltage. Analysis of PFAS in dissolved and solid phase showed that their distribution was dependent on perfluorocarbon chain length and functional group. Mass balances showed that PFAS were not removed during conventional AD or after applying voltage; however, significant removal (up to 61 ± 8 %) was observed in bioreactors with GAC for perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), and perfluorooctane sulfonate (PFOS). Biomass characterization showed that in these bioreactors, the relative abundance of Acinetobacter and Pseudomonas was higher, indicating their potential role in PFAS biotransformation.

Carbon fixation via volatile fatty acids recovery from sewage sludge through electrochemical–pretreatment–based anaerobic digestion

Capturing the carbon in volatile fatty acids (VFA) produced from the anaerobic digestion (AD) of sewage sludge has the potential to not only provide economic benefits but also reduce greenhouse gas production. This study demonstrates a chemical–free method to collect VFA from an AD instead of methane that involves electrochemical pretreatment (EPT) of sludge. Experimental results show that applying 15 V EPT for 45 min enhances acidogenesis and selectively inhibits methanogenesis, leading to a substantial VFA accumulation (2563.1 ± 307.9 mg COD/L) and achieving 2.5 times more carbon fixation than via methane production. Interfacial thermodynamic analysis shows that EPT induces a decrease in both the repulsive electrostatic energy (from 152.9 kT to 12.2 kT) and the energy barrier (from 57.0 kT to 2.6 kT) in the sludge, leading to increased sludge aggregation and entrapment of microorganisms. Molecular docking sheds lights on how the methanogens interacts with the organic matter released from EPT (e.g., alanine–tRNA ligase), showing that these interactions potentially interfere with the proteins that are associated with the activities of the methanogens and the electron transfer pathways, thereby impeding methanogenesis. Integrating EPT into AD therefore facilitates the recovery of valuable VFA and the capture of carbon from freshwater sludge, providing notable economic and environmental benefits in sewage sludge treatment.

Ammonia stripping by in situ biogas self-circulation to upgrade continuous thermophilic and mesophilic digestion of hydrothermal high-solid sludge

High-solid anaerobic digestion of hydrothermal sewage sludge has been developed. In order to upgrade the process by focusing on ammonia inhibition, a simply-equipped stripping system without additional alkali or heat supply was introduced by in situ biogas self-circulation. As the determined limit of total ammonia nitrogen at 1500 mg/L and 1000 mg/L for the mesophilic (MAD) and thermophilic anaerobic digestion (TAD) respectively and stripping rate at 5 L/min, continuous MAD and TAD was conducted in parallel. The stripping system successfully polished up the ammonia inhibition, and methanogenic capability of the TAD was promoted to approximately 90.0 % of the potential. Intermittent stripping mode proved usable. More frequent stripping was inevitable for the TAD as compared to the MAD. Hydraulic retention time below 20 d resulted in failure of the stripping mode due to rapid ammonia generation. Overall, this technology was practical in upgrading high-solid sludge digestion by effective ammonia control.

Continuous co-digestion of sewage sludge and highly concentrated waste bioplastic hydrolyzate without shortening hydraulic retention time

Bioplastics have garnered substantial interest as alternatives to conventional petroleum-based plastics. However, their management and conversion to biogas continues to be significantly challenging. In this study, we evaluated the suitability of various plastics for hydrolysis at 160 °C for 12 h using different solvents. The biogas yield (BGY) of the monomers constituting these plastics and the obtained plastic hydrolyzates was comprehensively evaluated. When water was used as a solvent, 100 % hydrolysis of polylactic acid (PLA) and polybutylene succinate (PBS) was observed. When polybutylene adipate co-terephthalate (PBAT) and polyhydroxybutyrate (PHB) were hydrolyzed with water, the degradation ratio was approximately 30 %; however, using an aqueous lactic acid solution as a solvent improved the degradation ratio to 78 % and 100 %, respectively. In the BGY test of the plastic hydrolyzates, the biogas volumes derived from the hydrolyzates were 563, 461, 337, and 573 mL/g COD-added for PLA, PBS, PBAT, and PHB, respectively. With 1,160 gCOD/L waste PLA hydrolyzate, continuous co-digestion of sewage sludge and the hydrolyzate was conducted. Organic loading rates of sewage sludge and the hydrolyzate were 2.3 and 2.4 gCOD/L/d, respectively. The operation was stable and the methane production volume from the PLA hydrolyzate was 414 L/kgCOD-added. Using highly concentrated PLA hydrolyzate, the hydraulic retention time was 19.3 days, which was only 0.7 days shorter than that of anaerobic digestion of sewage sludge only (20 days). Therefore, highly concentrated PLA hydrolyzate maintains the retention time of normal sewage sludge digestion. Conclusively, the present study has crucial practical implications for plastic waste management.

Auto-tuning data-driven model for biogas yield prediction from anaerobic digestion of sewage sludge at the south-tehran wastewater treatment plant: Feature selection and hyperparameter population-based optimization

In this study, two data-driven models, artificial neural networks and support vector regression (SVR), have been trained and optimized to predict the biogas yield from anaerobic digesters at the South-Tehran municipal wastewater treatment plant. The auto-tuning approach, including feature selection and hyperparameter population-based optimization, was applied through the genetic algorithm (GA) and particle swarm optimization to improve the training models' performance and help them obtain the best input parameters. The Shapley Additive Explanations (SHAP) analysis was also done to interpret models effectively and assign credit for a model's prediction to each feature. The findings demonstrated that biogas prediction using SVR-GA achieved the highest accuracy, with R2 values of 0.725 and 0.773, and RMSE values (regarding normalized datasets) of 0.477 and 0.492 for the train and test, respectively, while requiring the least computational time compared to other models. The auto-tuning technique, by removing the less important inputs, was able to show that temperature, pH, effluent and influent dry solids, effluent volatile solids (VS), and influent VS of waste sludge were the best input parameters for optimal biogas production modeling. The SHAP analysis revealed that VSeff and temperature were two of the most important features affecting biogas production, exhibiting an inverse impact.

Synergistic treatment of sewage sludge and food waste digestate residues for efficient energy recovery and biochar preparation by hydrothermal pretreatment, anaerobic digestion, and pyrolysis

The safe disposal of sewage sludge (SS) and food waste digestate residues (DR) is a tough issue considering the difficulty of dewatering and the environmental risks from heavy metals and pathogens. This study combined hydrothermal pretreatment (HP), anaerobic digestion (AD), and pyrolysis to synergistically dispose of SS and DR to enhance dewaterability, recover energy, and prepare biochar with heavy metal immobilization. The results showed that the solid contents of centrifuged cakes increased after HP at 180 °C of SS with the addition of 25% and 50% mass fractions of DR. The centrate from co-HP had a high chemical oxygen demand (COD) and was further treated by AD at 37 °C, producing 193.75 and 210.39 mL/g CODinput (25 °C and 1 atm) of cumulative CH4 under the addition of 50% and 75% mass fractions of DR, respectively. The methanogenic types in AD converted from hydrogen utilization to acetic acid utilization with increasing DR ratio. Zn, Cu, Ni, and Pb were primarily left in the biochar after 50% mass fraction of DR was mixed in the HP combined with pyrolysis at 700 °C; the chemical fractionation of Zn, Cu, Cr, and Pb in the biochar increased to over 85% of the residual fractions, resulting in the lowest potential ecological risk index (15.22, low risk). The CH4 produced from the AD of the co-HP centrate (50: 50, w/w) can supply heat for the HP process, reducing the energy input by 89%. This co-disposal strategy can effectively recover carbon resources from solid wastes to reduce carbon emissions.

Impact Evaluation of Wastewater Treatment Based on the Anaerobic Digestion of Sewage Sludge Using the Life Cycle Assessment Method

All the inputs and outputs of a technical system can be interpreted from an environmental point of view. Using the life cycle assessment (LCA) approach, some changes that are less harmful to the environment can be included in the system. This research aims to evaluate the environmental effects of the wastewater treatment plant (WWTP) in South Tehran, and the LCA method was used in this study. Based on the data of qualitative parameters obtained from the measurement of Tehran province’s water and sewage company, the environmental emissions were calculated and analyzed using SimaPro software (9.0.0) and the standards defined under the ReCiPe 2016-midpoint method. In the ReCiPe 2016 method, the results were expressed in two intermediate levels (including three classes of influence) and final (including 18). The results showed that the treated wastewater and chlorine factors had the most adverse environmental effects. Among the 18 effect classes, the treated wastewater in the class of marine environmental toxicity with the amount of 101.1531 kg 1,4-DCB had the most environmental impacts among other classes. The power consumed by the biogas-burning combined heat and power (CHP) unit in the wastewater treatment (WWT) process reduced the environmental effects in most impact classes. The most adverse environmental effects of the WWT process are related to damage to human health and the ecosystem. According to the findings, the use of CHP systems is suggested for energy saving and also for reducing harmful effects on the environment.

Enhancing nutrient and water recovery from liquid digestate: A comparative study of selective electrodialysis and conventional treatment methods

Ultrafiltration and reverse osmosis proved effective in treating the liquid fraction of anaerobically digested sewage sludge, both with and without an intermediate selective electrodialysis treatment step. The incorporation of selective electrodialysis significantly increased clean water recovery rates (from 38% to 88%) while concurrently reducing its dissolved solids load by almost two orders of magnitude, from 869.0 to 10.3 mg/L. Selective electrodialysis efficiently separated ions into stable fractions that could be post-processed to produce tailored solid fertilizers, such as struvite and ammonium sulfate, on demand. Throughout the system, heavy metals and pharmaceuticals were monitored for potential toxicity and harm to ecosystems. Heavy metals in the effluents of the treatment process both with and without the selective electrodialysis inclusion were below regulatory limits. Traces of ibuprofen, ciprofloxacin, oxytetracycline, and tetracycline were found in ultrafiltration and reverse osmosis concentrates. Currently, there are no established regulations for monitoring pharmaceuticals in digestate, but their presence could potentially complicate the direct use of these concentrates in agricultural applications.

The application of chitosan quaternary ammonium salt to replace polymeric aluminum ferric chloride for sewage sludge dewatering

Inorganic coagulants such as poly aluminum ferric chloride (Al/Fe) are applied conventionally to sewage sludge dewatering and can be retained in the sludge cake, causing its conductivity to increase and generate secondary pollution. To reduce these disadvantages, there is a need to develop alternative, more sustainable chemicals as substitutes for conventional inorganic coagulants. In the present investigation, the application of a polymeric chitosan quaternary ammonium salt (CQAS) is explored as a complete, or partial, replacement for Al/Fe in the context of sludge dewatering processes. Laboratory experiments using digested sewage sludge showed that CQAS could effectively substitute for over 80 % of the Al/Fe inorganic coagulant in the sludge dewatering process. This substitution resulted in a reduction of sludge cake conductivity by more than 50 %. Simulation of sludge dewatering curves and imaging of the sludge surface indicated that the addition of CQAS led to an increase in nanosized pores, and a decrease in the specific resistance of the sludge filter cake as the dosage of Al/Fe decreased to around 30 %. The variations of fluorescence emission, quantum yield and carboxylic and amino groups, suggested that the chelating of Al/Fe decreased due to the bridging effects of CQAS. The CQAS had different flocculation bridging effects on various EPS fractions, which varied the amount of protein chelated with Al/Fe in each fraction. This study provides new information about the benefits of replacing conventional inorganic coagulants with natural organic polymers for sewage sludge dewatering, in terms of reduced sludge cake conductivity and greater dry solids content.

Biotransformation of Pb and As from sewage sludge and food waste by black soldier fly larvae: Migration mechanism of bacterial community and metalloregulatory protein scales

The accumulation and transformation of lead (Pb) and arsenic (As) during the digestion of sewage sludge (SS) by black soldier fly larvae (BSFL) remain unclear. In this study, we used 16 s rRNA and metagenomic sequencing techniques to investigate the correlation between the microbial community, metalloregulatory proteins (MRPs), and Pb and As migration and transformation. During the 15-day test period, BSFL were able to absorb 34–48 % of Pb and 32–45 % of As into their body. Changes in bacterial community abundance, upregulation of MRPs, and redundancy analysis (RDA) results confirmed that ZntA, EfeO, CadC, ArsR, ArsB, ArsD, and ArsA play major roles in the adsorption and stabilization of Pb and As, which is mainly due to the high contribution rates of Lactobacillus (48–59 %) and Enterococcus (21–23 %). Owing to the redox reaction, the regulation of the MRPs, and the change in pH, the Pb and As in the BSFL residue were mainly the residual fraction (F4). The RDA results showed that Lactobacillus and L.koreensis could significantly (P < 0.01) reduce the reducible fraction (F2) and F4 of Pb, whereas Firmicutes and L.fermentum can significantly (P < 0.05) promote the transformation of As to F4, thus realizing the passivation Pb and As. This study contributes to the understanding of Pb and As in SS adsorbed by BSFL and provides important insights into the factors that arise during the BSFL-mediated migration of Pb and As.

Magnetic porous microspheres enhancing the anaerobic digestion of sewage sludge: Synergistic free and attached methanogenic consortia

The addition of exogenous materials is a commonly reported method for promoting the anaerobic digestion (AD) of sludge. However, most exogenous materials are nano-sized and their use encounters problems relating to a need for continuous replenishment, uncontrollability and non-recyclability. Here, magnetic porous microspheres (MPMs), which can be controlled by magnetic forces, were prepared and used to enhance the methanogenesis of sludge. It was observed that the MPMs were spherical particles with diameters of approximately 100 µm and had a stable macroporous hybrid structure of magnetic cores and polymeric shells. Furthermore, the MPMs had good magnetic properties and a strong solid–liquid interfacial electron transfer ability, suggesting that MPMs are excellent carriers for methanogenic consortia. Experimental results showed that the addition of MPMs increased methane production and the proportion of methane in biogas from AD by 100.0 % and 21.2 %, respectively, indicating the MPMs notably enhanced the methanogenesis of sludge. Analyses of variations in key enzyme activities and electron transfer in sludge samples with and without MPMs in AD revealed that the MPMs significantly enhanced the activities of key enzymes involved in hydrolysis, acidification and methanation. This was achieved mainly by enhancing the extracellular electron transfer to strengthen the proton motive force on the cell membrane, which provides more energy generation for methanogenic metabolism. A careful examination of the variations in the morphology, pore structure and magnetism of the MPMs before and after AD revealed that the MPMs increased the prevalence of many highly active anaerobes, and that this did not weaken the magnetic performance. The microbial community structure and metatranscriptomic analysis further indicated that the acetotrophic methanogens (i.e., Methanosaeta) were mainly in a free state and that CO2-reducing methanogens (i.e., Methanolinea and Methanobacterium) mainly adhered to the MPMs. The above synergistic metabolism led to efficient methanogenesis, which indicates that the MPMs optimised the spatial ecological niche of methanogenic consortia. These findings provide an important reference for the development of magnetic porous materials promoting AD.

Effects of Anaerobic Digestates and Biochar Amendments on Soil Health, Greenhouse Gas Emissions, and Microbial Communities: A Mesocosm Study

This study addresses the need for a comprehensive understanding of digestate and biochar in mitigating climate change and improving soil health, crucial for sustainable agriculture within the circular bioeconomy framework. Through a mesocosm experiment, soil was amended with digestates from pilot-scale reactors and two concentrations of biochar produced by pyrolysis of digested sewage sludge and waste wood. The Germination Index (GI) assay assessed phytotoxicity on Lactuca sativa and Triticum aestivum seeds. Greenhouse gas emissions (CO2, CH4, N2O) measurements, soil characteristics analyses, and the study of microbial community structure enriched the study’s depth. The GI assay revealed diverse responses among by-products, dilution rates, and plant types, highlighting the potential phyto-stimulatory effects of digestate and biochar water-extracts. While digestate proved to be effective as fertilizer, concerns arose regarding microbial contamination. Biochar application reduced Clostridiaceae presence in soil but unexpectedly increased N2O emissions at higher concentrations, emphasizing the need for further research on biochar’s role in mitigating microbial impacts. CO2 emissions increased with digestate application but decreased with a 10% biochar concentration, aligning with control levels. CH4 uptake decreased with digestate and high biochar concentrations. The study underscores the importance of tailored approaches considering biochar composition and dosage to optimize soil greenhouse gas fluxes and microbial communities.

Stabilization of PFAS-contaminated soil with sewage sludge- and wood-based biochar sorbents

Sustainable and effective remediation technologies for the treatment of soil contaminated with per- and polyfluoroalkyl substances (PFAS) are greatly needed. This study investigated the effects of waste-based biochars on the leaching of PFAS from a sandy soil with a low total organic carbon content (TOC) of 0.57 ± 0.04 % impacted by PFAS from aqueous film forming foam (AFFF) dispersed at a former fire-fighting facility. Six different biochars (pyrolyzed at 700–900 °C) were tested, made from clean wood chips (CWC), waste timber (WT), activated waste timber (aWT), two digested sewage sludges (DSS-1 and DSS-2) and de-watered raw sewage sludge (DWSS). Up-flow column percolation tests (15 days and 16 pore volume replacements) with 1 % biochar indicated that the dominant congener in the soil, perfluorooctane sulphonic acid (PFOS) was retained best by the aWT biochar with a 99.9 % reduction in the leachate concentration, followed by sludge-based DWSS (98.9 %) and DSS-2 and DSS-1 (97.8 % and 91.6 %, respectively). The non-activated wood-based biochars (CWC and WT) on the other hand, reduced leaching by <42.4 %. Extrapolating this to field conditions, 90 % leaching of PFOS would occur after 15 y for unamended soil, and after 1200 y and 12,000 y, respectively, for soil amended with 1 % DWSS-amended and aWT biochar. The high effectiveness of aWT and the three sludge-based biochars in reducing PFAS leaching from the soil was attributed largely to high porosity in a pore size range (>1.5 nm) that can accommodate the large PFAS molecules (>1.02–2.20 nm) combined with a high affinity to the biochar matrix. Other factors like anionic exchange capacity could play a contributing role. Sorbent effectiveness was better for long-chain than for short-chain PFAS, due to weaker, apolar interactions between the biochar and the latter's shorter hydrophobic CF2-tails. The findings were the first to demonstrate that locally sourced activated wood-waste biochars and non-activated sewage sludge biochars could be suitable sorbents for the ex situ stabilization and in situ remediation of PFAS-contaminated soil, bringing this technology one step closer to full-scale field testing.

Effect of Emerging Micropollutants on the Anaerobic Digestion of Sewage Sludge

The recovery of valuable resources from wastewater treatment plants (WWTPs) has received a great deal of attention as part of the concept of a circular economy. Anaerobic digestion for stabilizing sewage sludge in WWTPs, which produces biogas and stabilized biosolids, is a mature technology used worldwide. However, despite the necessity of achieving safe and reliable organic recycling, studies on the effect of some emerging micropollutants on this process are rare. This knowledge gap is of growing relevance because of the increasing use of some endocrine-disrupting compounds (EDCs), microplastics (MPs), and engineered nanoparticles (NPs) in industry and human life. These compounds are ubiquitous in wastewater streams and, therefore, may have serious effects on the course of the anaerobic digestion of sewage sludge, raising concerns about their effects on the environment. This article provides a comprehensive overview of the mechanisms by which selected EDCs, MPs, and NPs affect the valorization of sewage sludge, with a focus on the production of CH4, H2, and volatile fatty acids. This study takes into consideration the performance during all stages of anaerobic digestion, the shifts in microbial abundance and diversity, and the activity of key enzymes during the treatment process.

Hydrothermal Coupled Pyrolytic Treatment of Sewage Sludge and Food Waste Digestate for Heavy Metal Immobilization and Biochar Properties Enhancement

Hydrothermal Coupled Pyrolytic Treatment of Sewage Sludge and Food Waste Digestate for Heavy Metal Immobilization and Biochar Properties Enhancement

Effects of Short Retention Times and Ultrasound Pretreatment on Ammonium Concentration and Organic Matter Transformation in Anaerobic Digesters Treating Sewage Sludge

Anaerobic digestion of sewage sludge is limited at the hydrolysis stage of the process. The goal of this study was to assess the effects of sludge retention times and ultrasound pretreatment on the ammonium concentration and organic matter transformation in anaerobic digesters treating sewage sludge. To achieve this, two laboratory-scale semicontinuous anaerobic digesters were operated for a period of over 70 d, including a control reactor and another fed by pretreated sludge. Both anaerobic systems were fed with mixed sludge (50%/50% primary/secondary treatment) in mesophilic conditions (37 °C), with solid retention times (SRT) of 7.5 d (Phase I) and 3 d (Phase II). The performance of the anaerobic digestion process was assessed in terms of the methane yield and the total and soluble chemical organic demand, total solids, and volatile solids removal. The results showed that the ultrasound pretreatment caused an increase of around 22.2% in CODt removal for an SRT of 7.5 d. Meanwhile, an SRT of 3 d resulted in a decrease of up to 92.4% in CODt removal. The performance in terms of biogas production and organic matter removal was significantly affected by the SRT reduction to 3 d, showing that the process is not viable in these conditions.

Enhancing Methane Yield in Anaerobic Co-Digestion of Primary Sewage Sludge: A Comprehensive Review on Potential Additives and Strategies

Traditionally, anaerobic digestion has been applied to mixed sludge, combining primary sludge (PS) with secondary sludge. However, recent research has unveiled the advantages of dedicated PS digestion due to its higher energy content. Anaerobic digestion (AD) of primary sewage sludge can offer a sustainable solution for managing sewage sludge while generating renewable energy. The present study provides a comprehensive examination of the current state of knowledge regarding the anaerobic digestion of PS. Co-digestion of PS with organic substrates, including food waste and agro-industrial residues, emerges as a promising approach to boost biogas production. Additionally, the utilization of additives such as glucose and clay minerals has shown potential in improving methane yield. Critical factors affecting AD, such as pretreatment methods, carbon-to-nitrogen (C/N) ratio, temperature, pH, volatile fatty acids (VFAs) levels, organic loading rates (OLR), inoculum-to-substrate ratio (ISR), and the role of additives, have been meticulously studied. Finally, this review consolidates existing knowledge to advance our understanding of primary sewage sludge anaerobic digestion, fostering more efficient and sustainable practices in sludge management and renewable energy generation.

Optimizing Formic Acid-Assisted Co-HTL of Digested Sewage Sludge and Lignocellulosic Waste for Enhanced Bio-Crude Yield and Energy Recovery

In recent years, hydrothermal liquefaction (HTL) has gained attention as a means of enhancing and increasing the production of biofuels from biomass. Co-HTL involves the simultaneous processing of two or more feedstocks, with the potential for interactions that can affect the overall yield and quality of the resulting biofuels. This study investigates the bio-crude yield, chemical composition, and energy content of bio-crudes obtained through formic acid-assisted hydrothermal liquefaction of combined digested sewage sludge (DSS) and lignocellulose (LC). The bio-crude yields are in the range of 26.8–58.9 wt%, with a higher heating value (HHV) of approximately 32 MJ/kg. The best experiment shows that mixtures with more DSS and high levels of process condition variables (350 °C, formic acid present, and 50 wt% EtOH) give high bio-crude yields with a maximum value of 58.9 wt%. For comparison, pure DSS and LC run at these process conditions resulted in a bio-crude yield of 52.5 wt% and 48.3 wt%, respectively. Partial least squares (PLS) regression reveals a synergistic effect from mixing the feedstocks, as the quadratic term of the regression equation for mixture ratio shows a negative coefficient. GC–MS data show that combining feedstocks results in the formation of new compounds, mostly phenols, that are not present in the bio-crudes from the separate feedstocks. Thus, combining feedstocks will not only increase the resource availability for hydrothermal liquefaction and streamline the process but will also increase the overall production of bio-crude with its synergistic effect.

Low permeability sealing materials based on sewage, digestate and incineration industrial by-products in the final landfill cover system

This study explores repurposing municipal solid waste from the sewage, digestate, and incineration industries as landfill-sealing materials, aligning with circular economy principles. The main materials include digested sewage sludge (DSS), pretreated digested sewage sludge (PDSS), and digestate sludge (DS), with biomass bottom ash (BBA) and Al-anodizing waste (AAW) introduced as additives, complemented by 1.5 wt% water glass to enhance the sealing performance. The research evaluates the characteristics of the various treated and sourced sludges and additives, assessing their influence on water permeability, reaction products and the environmental consequences. Results demonstrate the achievement of low permeability in the sludge-based sealing materials, with optimal performance observed in the specimens prepared with DSS and AAW (k value = 3.78 ×10−12 m/s). Thermal Pressure Hydrolysis pre-treatment in sewage plants reduces the organic content in PDSS, resulting in a slight increase in permeability. DS-based specimens exhibit higher permeability due to their relatively lower organic content in DS. Gypsum is the primary reaction product attributed to leachable sulphate in BBA and AAW. Water glass addition in BBA-modified specimens promotes silica gel formation, while AAW effectively reduces matrix permeability as an externally added gel-like substance. Additionally, heavy metals (As, Pb and Cr) derived from the by-products are effectively immobilized in the sealing materials owning to the coagulation effect of organic matter in the sludge and sulphates in the products. Overall, this novel approach to landfill sealing materials exhibits promising applications in the Netherlands, offering cost savings and reduced environmental impact by recycling industrial by-products.