Sewage Sludge Valorization Research Articles

Characterization, fractionation and untapped potential of phosphate-amended sewage sludge biochar in soil-plant systems

Sewage sludge, a byproduct of wastewater treatment, poses serious environmental and health risks due to its content of organic contaminants, heavy metals, and pathogenic microorganisms. With the growing global production of municipal wastewater, finding effective methods for managing and disposing of sewage sludge has become increasingly urgent. Traditional methods such as land disposal, dumping, and incineration have limitations and environmental drawbacks. However, recent advancements have shown promise in the valorization of sewage sludge, particularly through pyrolysis, which converts it into biochar for use in soil amendment and pollutant mitigation. This study aims to characterize and fractionate phosphate-amended sewage sludge biochar produced at 300 °C, 400 °C, and 500 °C, and to evaluate its potential use in soil-plant systems. It examines nutrient bioavailability in soil after the addition of this biochar and its effects on plant growth. The pyrolysis process resulted in biochar with high alkalinity (7.2–11.1), ash content ranging from 56.9% to 87.3%, and significant phosphorus retention, with phosphorus concentrations increasing with pyrolysis temperature (5.35%–9.38%). Phosphorus fractionation showed a shift toward more stable fractions particularly at 500 °C. Soil incubation experiments indicated increased phosphorus availability with HCl-extractable P showing a high extraction efficiency of up to 94.95%. In plant growth experiments, the amended biochar significantly enhanced growth, with corn showing an increase of up to 28.8% and wheat showing an increase of up to 86% compared to the control in the first four weeks after emergence. These findings indicate that phosphate-amended sewage sludge biochar enhances nutrient availability and supports plant growth, providing a sustainable solution for sewage sludge management, contributing to soil improvement and carbon sequestration, thereby addressing global environmental challenges.

Low-intensity magnetization pretreatment to enhance biomethane generation and the abundance of key microorganisms for anaerobic digestion of sewage sludge

This work explores the impact of static magnetic field (SMF) intensity on biomethane production from anaerobic digestion (AD) of sewage sludge. Two different SMF intensities (20 mT and 1.5 T) were applied to magnetize the sludge destined to the AD process. The magnetic pretreatment at 20 mT was particularly effective, as it increased biomethane production by 12.7 % compared to the control test. On the contrary, exposing the sludge to 1.5 T adversely affected biomethane production, resulting in a 15.1 % decrease. The positive correlation observed between low-intensity SMF exposure and enhanced biomethane yield, in contrast to the inhibitory effect of high-intensity SMF, suggests the existence of an optimal intensity threshold within the lower range for maximizing methane production. The impact of magnetic pretreatment on the anaerobic microbial community was investigated through high-throughput sequencing analysis of magnetized sludge samples. This approach enabled the identification of specific shifts in microbial populations associated with SMF exposure, thereby elucidating the role of SMF in modulating key microbial communities for the AD process. The findings of this study provide insights into the potential mechanisms underlying these responses and underscore the potential of SMF application for improving the anaerobic valorization of sewage sludge.

A comparative assessment of sewage sludge valorization via anaerobic digestion and supercritical water gasification: A techno-economic case study in Norway

This study compared the performance of supercritical water gasification (SCWG) to conventional anaerobic digestion (AD) for treating sewage sludge from a wastewater treatment plant (WWTP). Based on plant data from a WWTP in Norway, kinetic-based process models have been developed in Aspen Plus® to investigate the energy and economic feasibility of AD and SCWG for sewage sludge (SS) treatment. The findings showed that low ambient temperatures significantly reduced the energy efficiency of the AD, while their effect on the SCWG process was negligible. At the average ambient temperature of 7 °C and SS concentration of 7 wt%, AD and SCWG achieved thermal energy self-sufficiency (TES) of 0.76 and 0.63, respectively. Under these conditions, the AD and SCWG had energy conversion efficiencies (η) of 25.8% and 46.6%, respectively. However, increasing the SS content in the SCWG to 15 wt% improved the TES to 1.22 and η to 73.2%, making it a more attractive alternative to the AD.At 7 wt% SS concentration, the AD and SCWG processes required an investment cost of $ 4.08 million and $ 6.83 million, respectively. Under the study assumptions, neither AD nor SCWG proved profitable, revealing negative net present values (NPV) and operating costs of $0.96 million and $0.92 million, respectively.

Anaerobic valorization of sewage sludge pretreated through hydrothermal carbonization: Volatile fatty acids and biomethane production

Hydrothermal carbonization (HTC) emerged as an effective technology for the treatment of various types of wet biomass and organic residues, including sewage sludge, offering the potential for sludge reduction and resource recovery. HTC pretreatment impact on downstream sludge fermentation is investigated. Results obtained at optimal conditions for HTC pretreatment (170 °C for 30 min) indicated that soluble carbon was significantly increased in the liquid fraction, enhancing feedstock availability for fermentation. Semi-continuous fermentation of HTC-treated sludge resulted in a stable process in which a mixed microbial community produced volatile fatty acids (VFAs) with longer chain acids content, acidification yield of 0.59 ± 0.05 g COD-VFA g−1 CODin and volumetric productivity of 1.6 ± 0.5 g COD-VFA L−1 d−1. Biomethane Potential tests evidenced high values for hydrochar. Overall, the HTC pretreatment enables improved conversion efficiencies, in the view of valorizing the liquid for VFA synthesis and the hydrochar for biomethane production.

Impact of key parameters on the hydrothermal carbonization process of sewage sludge: A systematic review

The hydrothermal carbonization (HTC) process is a promising green technology for reducing and reusing sewage sludge and can be a powerful tool against the current problems of energy scarcity, climate imbalance, and environmental pollution. Despite its admirable robustness, HTC can be easily tuned by controlling temperature and reaction time, as well as by biomass:water mass ratio and pH. The systematized knowledge of the relationship between these parameters is fundamental to successful innovative research aiming at scaling up the process. In this review, a bibliometric analysis based on ten years of scientific publications was carried out. The PRISMA guidelines were adopted to select forty-seven works as a source of information and critical discussion on key findings of reactions performed in high-pressure reactors. The global scientific community is mainly concerned with solid products (hydrochar) and the study of mechanisms of removal/adsorption of nutrients, minerals, and pollutants. The gaseous phase is in the background, opening up numerous research possibilities. Future outlooks are focused on coupling HTC with biorefineries to strengthen the valorization of sewage sludge, contribute to the production of biofuels and biofertilizers, and benefit communities currently without access to environmental sanitation.

Bio-adhesives derived from sewage sludge via hydrothermal carbonization: Influence of aqueous phase recycling

This paper describes the preparation of formaldehyde-free plywood bio-adhesive using hydrochar from sewage sludge in the presence (and absence) of aqueous phase containing a high content of inorganic matter. Aqueous phase recycling was observed to enhance the removal of low molecular weight proteins and the retention of large molecular weight proteins in hydrochar. The hydrochar-based bio-adhesive, incorporating the aqueous phase, exhibits excellent wet shear strength, meeting the Chinese national standard GB/T 9846-2015 (≥0.7 MPa). The wet shear strength of the bio-adhesive with tap water surpasses that of the bio-adhesive with the aqueous phase. During hot-pressing, both low molecular weight proteins in the aqueous phase and large molecular weight proteins in hydrochar participate in existing reactions including those between inter-components in bio-adhesives and bio-adhesives/plywood. The presence of low molecular weight proteins in the aqueous phase negatively affects wet shear strength and water resistance. The formation of covalent bonds, such as C–N and C–O–C, emerges as the key factor to enhance the wet shear strength of bio-adhesives. This study presents a novel approach for the valorization of sewage sludge, particularly the aqueous phase.

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.

Valorization of sewage sludge for facile and green wood bio-adhesives production

A method is presented herein for the design of wood bio-adhesives using sewage sludge extracts (SSE). SSE was extracted from SS using deep eutectic solvents and processed with glycerol triglycidyl ether (GTE) to disrupt the secondary structure of proteins. An additive was also used to improve mechanical performance. The resulting bio-adhesive (SSE/GTE@TA) had a wet shear strength of 0.93 MPa, meeting the Chinese national standard GB/T 9846–2015 (≥0.7 MPa). However, the high polysaccharide content in SSE would weaken the mechanical properties of wood bio-adhesives. The key to improve bio-adhesive quality was the formation of a strong chemical bond via Maillard reaction as well as higher temperatures (140 °C) to reduce polysaccharide content via dehydration. This approach has lower environmental impact and higher economic efficiency compared to incineration and anaerobic digestion of sewage sludge. This work provides a new perspective on the high-value utilization of SS and offers a novel approach to developing bio-adhesives for the wood industry.

HYDROTHERMAL CARBONIZATION OF SEWAGE SLUDGE – AN EFFECTIVE APPROACH TO TREAT AND MANAGE SEWAGE SLUDGE IN RURAL AREAS OF GERMANY?

As the result of new regulation from the German Sewage Sludge Ordinance (AbfKlärV 2017) and the future obligation to recover phosphorus, thermal treatment (mono-incineration) has become increasingly popular, whereas land-based utilization has decreased. Germany has produced 1.71 million metric tons (DM) of sewage sludge in the year 2021. Sewage sludge contains important nutrients such as phosphorus but also heavy metals and organic pollutants making the direct utilization of sewage sludge in agriculture controversial. Rural areas in particular have benefited from land-based sewage sludge utilization however the future ban on direct land-based utilization is forcing them to find alternative solutions for sewage sludge treatment and management. Hydrothermal carbonization (HTC) has developed considerably over the last 15 years and offers a viable alternative for the utilization of municipal and industrial organic waste such as sewage sludge. The process takes place in an aqueous environment without the need for pre-drying sewage sludge and thereby facilitating direct processing. HTC is especially suitable in combination with the recovery of nutrients like phosphorus. Technologies to recover this essential resource are important because phosphorus is an element that cannot be substituted and is therefore essential. HTC could make a significant contribution to sewage sludge management in combination with phosphorus recovery. However, the technology has yet to establish itself as a sewage sludge valorization process (2023) and is not yet a recognized state-of-the-art. Nevertheless, the HTC technology could gain greater relevance in the future, especially as an alternative valorization pathway for sewage sludge in rural areas of Germany.

Valorization of Sewage Sludge with Pine Sawdust as Biochar: Optimization of the Torrefaction Conditions and Biochar Quality

Valorization of Sewage Sludge with Pine Sawdust as Biochar: Optimization of the Torrefaction Conditions and Biochar Quality

Improved pyrolysis for sewage sludge valorization via enhanced microcosm vermicomposting

Improved pyrolysis for sewage sludge valorization via enhanced microcosm vermicomposting

Valorization of sewage sludge via air/steam gasification using activated carbon and biochar as catalysts

The catalytic gasification of sewage sludge was explored using different catalyst particle sizes, bed temperatures, and gasifying agents. Activated carbon and sawdust biochar were used as catalysts in this study. The effects of the porosity of activated carbon on tar adsorption and cracking, and biochar containing alkali and alkaline earth metallic species (AAEMs), which can enhance gasification reactions, were investigated under different conditions. A catalyst bed temperature of 800 °C and particle size of 0.5–1.7 mm showed optimum conditions for increasing H2 and CO content and decrease CO2 fraction; however, a higher solid residue (∼38 wt%) was also observed due to the high ash content in sewage sludge. Furthermore, the introduction of steam had a positive effect on the H2 increment, owing to the presence of inherent AAEMs in the biochar, promoting water gas shift and hydrocarbon steam reforming reactions. The results of this study provide effective measures for using activated carbon and affordable biochar to enhance H2 production with a parametric effect on the catalyst bed.

Urban sewage sludge valorization to biodiesel production: Solvent-free lipid recovery through adsorption on used 3 Ply Safety Face Masks

The use of urban sewage sludge as the feedstock of lipids to produce biodiesel is a topic of great interest and potential. However, the extractive step, conventionally operated with organic solvents, makes this use unfeasible and with a high environmental impact. In this work, for the first time, the solvent-free recovery of lipids from sewage sludge was studied through adsorption onto non-polar polymeric materials (Spunbonded nonwoven Polypropylene (s-nwPP)). After being immersed in sedimented sewage sludge (TS: 6.9 wt%), s-nwPP sheets were easily separated, dried and analyzed: sewage sludge’s lipids were preferentially adsorbed onto the polymeric surface. Free fatty acids (FFAs) were the dominant species and were recovered from s-nwPP sheets through acidic methanol in a separated reactive washing step as fatty acid methyl esters (FAMEs) A crude product was obtained through this sequence of adsorption and reactive desorption operations, with a FAMEs content of over 55 wt%. After preliminary investigations on commercial s-nwPP sheets, 3 Ply Safety Face Masks were also positively tested. pH, weight ratio polymer/wet sludge (R) and temperature were the parameters optimized: operating at 343 K under acidic conditions (pH of sludge<2), with an R equal to 6%, a total FFAs recovery yield of 70% as FAMEs was already achieved after two adsorption cycles. A preliminary evaluation of the economic feasibility demonstrated that used PP surgical facial masks allow the recovery and generation of crude biodiesel from sewage sludge at a very reasonable price with minimal environmental impacts.

Sustainability assessment of alternative waste-to-energy technologies for the management of sewage sludge

The management of sewage sludge still represents a challenge in the EU sustainability plan for biowaste. Although there are consolidated alternatives for the valorization of sewage sludge (incineration, pyrolysis and gasification), technical issues related to heavy metals and other pollutants are not sufficiently understood considering the whole waste-to-energy process. In addition, societal-economic and environmental aspects are usually not included in the evaluation of these conversion technologies. In this study, we propose an integrated assessment from a sustainability perspective to evaluate the valorization of sewage sludge by thermal conversion, comparing different alternatives based on existing Waste-to-energy (WtE) technologies. The results provide an insightful vision on the challenges to manage the sewage sludge disposal and to transform the obtained waste into new raw materials with added value. In addition, the evaluation of the WtE alternatives shows that they face important challenges preventing their application, being the gasification the best performing technology according to the sustainability assessment. Finally, a decentralized scheme based on sewage sludge gasification is further evaluated using real data from wastewater treatment plants in Andalusia.

Sewage Sludge Activated Carbon as an Adsorbent and Catalytic Support: Evaluation of Nitrate Removal and Suzuki‐Miyaura Cross‐Coupling

AbstractThis manuscript reports the valorization of sewage sludge with the production of activated carbon. Thus, a series of 11 adsorbents are produced, activated with different substances (salt, acid, and base), and their adsorptive properties are carefully evaluated. The C6 series of activated carbons prove to be very efficient for removing nitrate anions from aqueous medium and, due to this, it is evaluated as a catalytic support for palladium nanoparticles. The generated solid, named Pd@CA6F, is the first report of a catalyst containing activated carbon from sewage sludge that is efficient in the Suzuki‐Miyaura cross‐coupling. The catalytic system developed is eco‐friendly, phosphine‐free, and of low cost. Good yields are obtained in the coupling of aryl iodides with phenylboronic acid in a short time. The same catalyst load could be recycled for up to ten times.

Sewage sludge as an object of vermicomposting

This article focuses on the analysis of the properties of sewage sludge before and after vermicomposting. The aim of the study is to give detailed characteristics of changes in the properties of sewage sludge after vermicomposting. This will help to obtain a more detailed picture of the processes that take place during vermicomposting of sewage sludge, and a more comprehensive assessment of the possibility of using vermicomposts from sewage sludge as fertilizers. In this article, a large number of parameters for vermicomposts derived from this type of waste have been outlined for the first time. According to the data obtained, vermicomposting resulted in an increase of almost all nutrients, acidification of the substrate, and a decrease in basal respiration. Summing up all the data obtained makes is possible to discuss the potential prospects for the use of vermicomposting aimed at valorization of sewage sludge.

Improved valorization of sewage sludge in the circular economy by anaerobic digestion: Impact of an innovative pretreatment technology

Anaerobic digestion (AD) of sewage sludge shows low carbon conversion efficiency (CCE) due to the poor biodegradability of sewage sludge. The lack of digestibility is specifically linked to the waste-activated sludge (WAS) making up the majority of sewage sludge along with a smaller portion of primary sludge, depending on the wastewater treatment plant configuration. In this study, we examine the Advanced Wet Oxidation & Steam Explosion process (AWOEx) for improving the CCE of digested sewage sludge (DSS) by thermophilic AD. The effect of the pretreatment temperature in the range between 160 and 185 °C at a fixed residence time of 20 min with and without oxygen added at a dosage of 5 % of the organics present was tested. Methane yield improved by 97.92 % to 183.91 ± 4.93 mL/g vS over the untreated DSS (control), whose methane yield was 92.92 ± 9.07 mL/g vS We have demonstrated for the first time that 84 % of the organics in sewage sludge can successfully be transformed into biogas following AWOEx pretreatment, which can contribute significantly to the circular economy instead of greenhouse gas emissions from landfilling.

Effect of operating conditions on the hydrothermal valorization of sewage sludge

AbstractThis study followed a quantitative approach, with the purpose of converting municipal sewage to valuable bio‐oils. The sewage sludge was subjected to hydrothermal liquefaction in 60 mL stainless steel batch reactors, where the effects of temperature, solvent composition and solvent content were investigated, and all the other process parameters were maintained at constant values. Temperatures were explored between 220 and 370°C. The two solvents investigated were de‐ionized water (H2O) and ethanol (E) which were applied in the following compositions: 1:0, 1:1 and 0:1 (H2O:E). The five solvent contents (moisture contents) investigated were 75, 80, 85, 90 and 95 wt%. The temperature of 310°C emerged categorically as the optimal temperature across the various solvent contents. The best yield was achieved when H2O/E (1:1) performed as a co‐solvent at 85% solvent content and 310°C, which amounted to 40.69 wt%. Bio‐oils derived by the use of pure H2O solvent were overall the least effective in yielding high bio‐oil quantities. The lowest bio‐oil yield obtained was 6.42 wt% where pure H2O was the solvent at extreme conditions of 370°C and 95% solvent content. The order of efficiency of solvent content was as follows: 85% &gt; 80% &gt; 75% &gt; 90% &gt; 95%. Among the six temperatures investigated, the intermediate temperature, 310°C, gave the highest conversions. The highest conversion across all of the runs was 85.65% at 310°C with 90% solvent content using the co‐solvent H2O/E. The order of superiority of solvent was H2O/E &gt; pure ethanol &gt; pure water. © 2022 The Authors. Biofuels, Bioproducts and Biorefining published by Society of Industrial Chemistry and John Wiley &amp; Sons Ltd.

Accompanying effects of sewage sludge and pine needle biochar with selected organic additives on the soil and plant variables

The effects of synthetic fertilizer and nutrient leaching are causing serious problems impacting soil function and its fertility. Mitigation of nutrient leaching and use of chemical fertilizer is crucial as fertile land adds up sustainability to climate changes. Biochar produced from agricultural bio-waste and municipal solid waste has been used for crop production and when applied in combination with organic nutrients may support mitigation of nutrient loss and adverse effects of chemical fertilizers. Different types of biochar and their application for soil enhancement have been observed, pine needle and sewage sludge derived low-temperature biochar along with compost, organic fertilizer in the form of manure and microalgal biomass may interact with soil chemistry and plant growth to impact nutrient loss and compensate the hazardous effect of chemical fertilizer, but it has not been investigated yet. This present study elaborates application of sewage sludge and pine needle biochar produced at 400 °C in an application rate of 5 % w/w and 10 t h−1 in combination with compost, manure and microalgal biomasses of Closteriopsis acicularis (BM1) and Tetradesmus nygaardi (BM2) on the growth of Chickpea (Cicer arietinum) and Fenugreek (Trigonella foenum-graecum) crop assessed in a pot experiment over a two crop (Chickpea - Fenugreek) cycle in Pakistan. Results depict that the pine needle biochar with additives has increased plant height by 104.1 ± 2.76 cm and fresh biomass by 49.9 ± 1.02 g, buffered the soil pH to 6.5 for optimum growth of crops and enhance carbon retention by 36 %. This study highlights the valorization of sewage sludge and pine needle into biochar and the effect of biochar augmentation, its impact on soil nutrients and plant biomass enhancement. The greener approach also mitigates and helps in the sustainable management of solid wastes.

Hydrothermal liquefaction of sewage sludge for biofuel application: A review on fundamentals, current challenges and strategies

Scientific studies have demonstrated that hydrothermal liquefaction (HTL) can be suggested as a cost-effective and eco-friendly solution for the valorization of sewage sludge in the fields of waste management and renewable energy production. On-going efforts are continued extensively to improve conversion efficiency in order to transform this technology into industrial application. The present review focuses on state-of-the-art of HTL of sewage sludge, systematically, the HTL fundamentals, the influences of the essential operation parameters, main existing challenges and integrated upgrading are evaluated in detail. Opportunities and strategies including pre-treatment, co-liquefaction, post-application of aqueous and solid phases are addressed. Additionally, some available results regarding the continuous-flow tests of sewage sludge are summarized. Finally, further considerations and recommendations are made for improving or completing the economic feasibility of HTL process.