Multiple drivers and mechanisms of solid-water interfacial interactions in sludge dewatering: Roles of polarity and molecular structure of extracellular polymeric substances.

Water occurrence states in sewage sludge, influenced by sludge physicochemical properties, are crucial for sludge dewaterability and have recently been regarded as a research hotspot. Here, the multifold characteristics of sludge flocs during hydrothermal treatment, including rheological properties, solid-water interfacial interactions, and the polarity distribution and molecular structure of extracellular polymeric substances (EPS), were systematically investigated, and the impact of these characteristics on sludge dewaterability was explored in depth. Hydrothermal treatment at 80 °C and 100 °C induced the conversion of free water into bound water, while an increase in temperature to 180 °C resulted in a significant decrease in bound water content, approximately 4-fold lower than at 100 °C. In addition to the conventional view of decreased sludge surface hydrophilicity at high temperatures, the decline in bound water was associated with the reduction in sludge apparent viscosity. XAD resin fractionation identified the hydrophobic/hydrophilic EPS (HPO-/HPI) ratio as an important factor determining water occurrence states. Especially, hydrolysis of HPI-related hydrophilic proteins and subsequent increase in HPO-related tryptophan-like substances played a dominant role in reducing sludge viscosity and facilitating the release of bound water. Protein conformational analysis revealed that the disruption of α-helix structures and disulfide bonds significantly reduced EPS water-holding capacity, providing strong evidence for the potential of targeting these dense structure units to enhance sludge dewaterability. These findings provide a holistic understanding of multidimensional drivers of water occurrence states in sludge, and guide directions for optimizing sludge treatment efficiency through EPS modification.

Valorization of sewage sludge incineration ash as a novel soilless growing medium for urban agriculture and greenery.

Limited open areas for urban agriculture and greenery have led to the search for innovative, sustainable growing media to strengthen the food supply and improve atmospheric quality for a resilient city. Rampant land developments have caused soil to become increasingly scarce. Sewage sludge incineration ash (SSIA), the by-product of waste-to-energy (WtE) incineration of sewage sludge, is a major municipal waste containing phosphorus-fertilizing nutrients. For the first time, we investigated the novel application of SSIA as a soilless plant-growing medium with built-in fertilizer. SSIA outperformed topsoil in bulk density, water-holding capacity, porosity, and nutrient content. However, it was found that SSIA has a high salinity and should be treated first. Wheatgrass (Triticum aestivum L.), a fast-growing glycophyte, thrived in the desalinated SSIA, showing growth and nutrient content comparable to the topsoil case. Simultaneously, it demonstrated phytoremediation. The SSIA residue was then recycled into cementitious materials, using desalinating water for mixing. SSIA upcycle into a growing medium facilitates urban resource management by utilizing nutrients in sewage waste for eco-friendly plant cultivation, benefiting urban agriculture and greenery. It is also a prudent valorization step before further recycling SSIA to reduce landfill requirements.

Microbial Succession on Microplastics in Wastewater Treatment Plants: Exploring the Complexities of Microplastic-Microbiome Interactions.

Despite some effectiveness of wastewater treatment processes, microplastics accumulate in sewage sludge and their further use may contribute to the release of plastic microplastics into the environment. There is an urgent need to reduce the amount of microplastics in sewage sludge. Plastic particles serve as solid substrates for various microorganisms, promoting the formation of microbial biofilms with different metabolic activities. The biofilm environment associated with microplastics will determine the efficiency of treatment processes, especially biological methods, and the mechanisms of organic compound conversion. A significant source of microplastics is the land application of sewage sludge from wastewater treatment plants. The detrimental impact of microplastics affects soil enzymatic activity, soil microorganisms, flora, fauna, and plant production. This review article summarizes the development of research related to microplastics and discusses the issue of microplastic introduction from sewage sludge. Given that microplastics can contain complex composite polymers and form a plastisphere, further research is needed to understand their potential environmental impact, pathogenicity, and the characteristics of biofilms in wastewater treatment systems. The article also discusses the physicochemical properties of microplastics in wastewater treatment plants and their role in biofilm formation. Then, the article explained the impact of these properties on the possibility of the formation of biofilms on their surface due to the peculiar structure of microorganisms and also characterized what factors enable the formation of specific plastisphere in wastewater treatment plants. It highlights the urgent need to understand the basic information about microplastics to assess environmental toxicity more rationally, enabling better pollution control and the development of regulatory standards to manage microplastics entering the environment.

Assessment of pilot-scale sewage sludge pelletization for non-food crop fertilization: nutrient content, pathogenicity, and growth performance.

Application of sewage sludge as fertilizer can be beneficial for sustainable agriculture as it could largely account for nitrogen and phosphorus demand for crops and has lower costs compared to other disposal routes, e.g., incineration, and sanitary landfills. This study evaluates the feasibility of pilot-scale pelletization of sewage sludge for non-food crops (e.g., ornamental plants). The co-pelletization method was designed by mixing sewage sludge and binder (tapioca starch) at a 9:1 sludge-to-starch weight ratio. The amount of nitrogen (N), phosphorus (P), and potassium (K) of the resultant pellets were determined at 5.7%, 4.9%, and 0.2%, respectively. Following Malaysian and US Standards, non-essential elements and pathogenicity of the pelletized sewage sludge were measured below the predetermined limits and hence safe for agricultural application. The planting trial using 50% inorganic fertilizer + 50% sewage sludge pellets exhibited a promising result on the growth of the flowering plant Celosia plumosa, with having better dimension and color, 20% higher in height, 4% more chlorophyll content, 54% more leaf, 43% greater stem growth, and 27% more flowers compared to control. Likewise, the planting trial on Tagetes erecta resulted in 10.5% wider leaf, 10.6% heavier leaf dry weight, and 12.5% more chlorophyll content compared to control with full usage of inorganic fertilizer. By considering liquidities to operate the production facility, the economic analysis estimated that the production cost per ton of pelletized sewage sludge produced was USD 0.98.

Utilization of industrial wastes in non-sintered bricks: microstructure and environmental impacts.

Recycling industrial solid wastes as building materials in the construction field exhibits great environmental benefits. This study designed an eco-friendly non-sintered brick by combining multiple industrial solid wastes, including sewage sludge, fly ash, and phosphorus gypsum. The mechanical properties, microstructure, and environmental impacts of waste-based non-sintered bricks (WNBs) were investigated comprehensively. The results revealed that WNB exhibited excellent mechanical properties. In addition, steam curing could further promote the strength development of WNB. The compressive strength of WNB with 10 wt% of sewage sludge reached 13.5 MPa. Phase assemblage results indicated that the incorporation of sewage sludge promoted the generation of ettringite. Mercury intrusion porosimetry results demonstrated that the pore structure of WNB varies with the dosage of sewage sludge. Life-cycle assessment results revealed that the energy consumption and CO2 emission of WNB were 45% and 17% lower than those of traditional clay bricks. Overall, the development of WNB in this study provided insights into the co-disposal of industrial solid wastes.

Biochar from Co-Pyrolyzed Municipal Sewage Sludge (MSS): Part 2: Biochar Characterization and Application in the Remediation of Heavy Metal-Contaminated Soils.

The disposal of municipal sewage sludge (MSS) from wastewater treatment plants poses a major environmental challenge due to the presence of inorganic and organic pollutants. Co-pyrolysis, in which MSS is thermally decomposed in combination with biomass feedstocks, has proven to be a promising method to immobilize inorganic pollutants, reduce the content of organic pollutants, reduce the toxicity of biochar and improve biochar's physical and chemical properties. This part of the review systematically examines the effects of various co-substrates on the physical and chemical properties of MSS biochar. This review also addresses the effects of the pyrolysis conditions (temperature and mixing ratio) on the content and stability of the emerging pollutants in biochar. Finally, this review summarizes the results of recent studies to provide an overview of the current status of the application of MSS biochar from pyrolysis and co-pyrolysis for the remediation of HM-contaminated soils. This includes consideration of the soil and heavy metal types, experimental conditions, and the efficiency of HM immobilization. This review provides a comprehensive analysis of the potential of MSS biochar for environmental sustainability and offers insights into future research directions for optimizing biochar applications in soil remediation.

Hydrodynamic disintegration effects assessment by CFD modelling integrated with bench tests.

The hydrodynamic disintegration process depends, among others, on operational parameters like rotational speed or introduced energy. The study presents an interdisciplinary approach to the hydrodynamic disintegration parameters impact assessment on the internal processes and disintegration effects on the example of sewage sludge treatment. Three rotational speeds were considered, including fluid properties change at selected disintegration stages. Disintegration effects were measured in the bench tests. Soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFA) were measured before and after disintegration process. The assessment of the effects of disintegration employed the disintegration degree and the assessment of the course of methane production employed biochemical methane potential (BMP) tests. Fluid properties change during the disintegration stages does not cause a significant change in the flow structure. Due to the mathematical modelling results, at 1500 rpm no cavitation phenomenon was observed. Although, the bench tests results indicates, for the rotational speed 1500 rpm, organic compounds released to the liquid were characterised by higher susceptibility to biological decomposition than those released for 2500 and 3000 rpm (as suggested by the low SCOD/VFA values for 1500 rpm). Obtained results have confirmed, that the main phenomenon responsible for the disintegration effect is mechanical shredding not cavitation.

Biochar from Co-Pyrolyzed Municipal Sewage Sludge (MSS): Part 1: Evaluating Types of Co-Substrates and Co-Pyrolysis Conditions.

With the increasing production of municipal sewage sludge (MSS) worldwide, the development of efficient and sustainable strategies for its management is crucial. Pyrolysis of MSS offers several benefits, including volume reduction, pathogen elimination, and energy recovery through the production of biochar, syngas, and bio-oil. However, the process can be limited by the composition of the MSS, which can affect the quality of the biochar. Co-pyrolysis has emerged as a promising solution for the sustainable management of MSS, reducing the toxicity of biochar and improving its physical and chemical properties to expand its potential applications. This review discusses the status of MSS as a feedstock for biochar production. It describes the types and properties of various co-substrates grouped according to European biochar certification requirements, including those from forestry and wood processing, agriculture, food processing residues, recycling, anaerobic digestion, and other sources. In addition, the review addresses the optimization of co-pyrolysis conditions, including the type of furnace, mixing ratio of MSS and co-substrate, co-pyrolysis temperature, residence time, heating rate, type of inert gas, and flow rate. This overview shows the potential of different biomass types for the upgrading of MSS biochar and provides a basis for research into new co-substrates. This approach not only mitigates the environmental impact of MSS but also contributes to the wider goal of achieving a circular economy in MSS management.

Investigating the Dewatering Efficiency of Sewage Sludge with Optimized Ratios of Electrolytic Manganese Residue Components.

As an industrial waste residue, Electrolytic Manganese Residue (EMR) can greatly promote sludge dewatering and further particle-size optimization can significantly strengthen sludge dewaterability. In this study, the effects of ammonium sulfate, calcium sulphate dihydrate, and manganese carbonate in EMR on sludge dewatering performance were investigated using the response surface optimization method. It was found that the optimized ratio of three components in EMR was 1.0:1.6:2.2 based on capillary suction time (CST), specific resistance of filtration (SRF), and zeta potential of dewatered sludge. The composition ratio of particle-size optimized EMR was modified based on the above optimization, resulting in a significant increase in sludge dewatering performance (CST and SRF reduced by 8.7% and 11.2%, respectively). Compared with those in original sludge, the content of bound extracellular polymeric substances in the conditioned sludge with optimized ratio was drastically reduced while that of soluble extracellular polymeric substances was slightly increased, which was in accordance with the decline of fluorescence intensity. These findings indicated the disintegration of extracellular polymeric substances, the enhancement of hydrophobicity, and dewatering properties of the sludge. In summary, optimized EMR can effectively intensify the dewaterability of sludge, providing a competitive solution for dewatering and further disposal of sludge.

Selective adsorption of antibiotics from human urine using biochar modified by dimethyl sulfoxide, deep eutectic solvent, and ionic liquid.

Antibiotics present in human urine pose significant challenges for the use of urine-based fertilizers in agriculture. This study introduces a novel two-stage approach utilizing distinct biochar types to mitigate this concern. Initially, a modified biochar selectively adsorbed azithromycin (AZ), ciprofloxacin (CPX), sulfamethoxazole (SMX), trimethoprim (TMP), and tetracycline (TC) from human urine. Subsequently, a separate pristine biochar was employed to capture nutrients. Biochar, derived from sewage sludge and pyrolyzed at 550 and 700 °C, was modified using dimethyl sulfoxide, deep eutectic solvent, and ionic liquid to enhance antibiotic removal in the first stage. The modifications introduced hydrophilic functional groups (-OH/-COOH), which favor antibiotic adsorption. Adsorption kinetics followed the pseudo-second-order model, with the Langmuir isotherm model best describing the adsorption data. The maximum adsorption capacities for AZ, CPX, SMX, TMP, and TC after the modification were 196.08, 263.16, 81.30, 370.37, and 833.33 µg/g, respectively. Pristine biochar exhibited a superior ammonia adsorption capacity compared to the modified biochar. Hydrogen bonding, electrostatic attraction, and chemisorption drove antibiotic adsorption on the modified biochar. Regeneration efficiency declined due to solvent accumulation and potential byproduct formation on the biochar surface (<30% removal capacity after three cycles). This study presents innovative biochar modification strategies for selective antibiotic adsorption, laying the groundwork for environmentally friendly urine-based fertilizers in agriculture.

Energy recovery from syngas and pyrolysis wastewaters with anaerobic mixed cultures.

The anaerobic digestion of aqueous condensate from fast pyrolysis is a promising technology for enhancing carbon and energy recovery from waste. Syngas, another pyrolysis product, could be integrated as a co-substrate to improve process efficiency. However, limited knowledge exists on the co-fermentation of pyrolysis syngas and aqueous condensate by anaerobic cultures and the effects of substrate toxicity. This work investigates the ability of mesophilic and thermophilic anaerobic mixed cultures to co-ferment syngas and the aqueous condensate from either sewage sludge or polyethylene plastics pyrolysis in semi-batch bottle fermentations. It identifies inhibitory concentrations for carboxydotrophic and methanogenic reactions, examines specific component removal and assesses energy recovery potential. The results show successful co-fermentation of syngas and aqueous condensate components like phenols and N-heterocycles. However, the characteristics and load of the aqueous condensates affected process performance and product formation. The toxicity, likely resulting from the synergistic effect of multiple toxicants, depended on the PACs' composition. At 37 °C, concentrations of 15.6 gCOD/gVSS and 7.8 gCOD/gVSS of sewage sludge-derived aqueous condensate inhibited by 50% carboxydotrophic and methanogenic activity, respectively. At 55 °C, loads between 3.9 and 6.8 gCOD/gVSS inhibited by 50% both reactions. Polyethylene plastics condensate showed higher toxicity, with 2.8 gCOD/gVSS and 0.3 gCOD/gVSS at 37 °C decreasing carboxydotrophic and methanogenic rates by 50%. At 55 °C, 0.3 gCOD/gVSS inhibited by 50% CO uptake rates and methanogenesis. Increasing PAC loads reduced methane production and promoted short-chain carboxylates formation. The recalcitrant components in sewage sludge condensate hindered e-mol recovery, while plastics condensate showed high e-mol recoveries despite the stronger toxicity. Even with challenges posed by substrate toxicity and composition variations, the successful conversion of syngas and aqueous condensates highlights the potential of this technology in advancing carbon and energy recovery from anthropogenic waste streams.

Guidelines for efficient nitrogen preservation in sewage sludge-based fertilizers.

This study explores sustainable methods to mitigate nitrogen (N) loss in agriculture amid rising food demands and limited arable land. It examines sewage sludge (SS) as an alternative to synthetic N fertilizers. SS is rich in nitrogen (4.21 ± 0.42 %) and phosphorus (3.60 ± 0.72 %), making it suitable for nutrient recovery and soil enhancement. Unfavorable sludge management methods result in the loss of 950,000 tons of nitrogen, meeting almost 10 % of the EU's nitrogen fertilization demand. This research evaluates SS treatment methods, including chemical conversion, thermal treatment, and biological composting, focusing on nitrogen conservation efficiency. Results show nitrogen loss during hydrolysis is minimized at pH 4 to 8 but increases significantly as ammonia (NH3) at pH 9 to 11, ranging from 4.2 % to 9 %. Neutralizing the hydrolysate is crucial; using solid KOH resulted in 13.5 % nitrogen loss, 11 times more than using slightly alkaline ash (1.22 %). Adding ash during drying reduced nitrogen emissions by 30 % compared to traditional drying at 105 °C. Improving the C/N ratio with food residues reduced nitrogen losses by 46.3 % during composting. These findings highlight the importance of pH control in chemical processes and temperature regulation in thermal treatments. Adding residues from other processes, such as biomass combustion waste, enhances SS processing conditions. Understanding nitrogen retention mechanisms is crucial for the environmental sustainability of SS usage. Efficient nitrogen retention strategies improve the fertilization value of SS and reduce its environmental footprint by lowering greenhouse gas emissions, particularly ammonia. Reducing nitrogen loss during SS treatment significantly lowers ammonia emissions, a major contributor to greenhouse gas emissions. These results help determine optimal methods for managing and processing SS to minimize emissions and increase agricultural usability.

Enzymatic hydrolysis of waste streams originating from wastewater treatment plants.

BACKGROUND: Achieving climate neutrality is a goal that calls for action in all sectors. The requirements for improving waste management and reducing carbon emissions from the energy sector present an opportunity for wastewater treatment plants (WWTPs) to introduce sustainable waste treatment practices. A common biotechnological approach for waste valorization is the production of sugars from lignocellulosic waste biomass via biological hydrolysis. WWTPs produce waste streams such as sewage sludge and screenings which have not yet been fully explored as feedstocks for sugar production yet are promising because of their carbohydrate content and the lack of lignin structures. This study aims to explore the enzymatic hydrolysis of various waste streams originating from WWTPs by using a laboratory-made and a commercial cellulolytic enzyme cocktail for the production of sugars. Additionally, the impact of lipid and protein recovery from sewage sludge prior to the hydrolysis was assessed. RESULTS: Treatment with a laboratory-made enzyme cocktail produced by Irpex lacteus (IL) produced 31.2 mg sugar per g dry wastewater screenings. A commercial enzyme formulation released 101 mg sugar per g dry screenings, corresponding to 90% degree of saccharification. There was an increase in sugar levels for all sewage substrates during the hydrolysis with IL enzyme. Lipid and protein recovery from primary and secondary sludge prior to the hydrolysis with IL enzyme was not advantageous in terms of sugar production. CONCLUSIONS: The laboratory-made fungal IL enzyme showed its versatility and possible application beyond the typical lignocellulosic biomass. Wastewater screenings are well suited for valorization through sugar production by enzymatic hydrolysis. Saccharification of screenings represents a viable strategy to divert this waste stream from landfill and achieve the waste treatment and renewable energy targets set by the European Union. The investigation of lipid and protein recovery from sewage sludge showed the challenges of integrating resource recovery and saccharification processes.

Effect of polyethylene terephthalate plastics on nitrogen, sulphur and chlorine contaminants via sludge pyrolysis and Pb/Cu adsorption properties of biochar.

Pyrolysis is an effective process for disposing of municipal sewage sludge (SS). Plastics can affect the SS pyrolysis behaviour and pyrolysis products due to their low ash and high hydrocarbon ratio. The secondary pollutants from the pyrolysis process may also be affected. Therefore, polyethylene terephthalate (PET), a typical plastic, was chosen to investigate the release characteristics of pollutants containing nitrogen, sulphur, and chlorine via SS pyrolysis, and the changes of biochar to adsorb two typical heavy metals, Pb and Cu. The pyrolysis of PET plastics facilitates the migration of N toward solid and liquid-phase products, S and Cl to the gas-phase products via pyrolysis. Oxygenated compounds of pyrolytic volatiles decreased from 38.18% to 28.43%, concurrently promoting the formation of phenolic compounds. The co-pyrolysis improved the quality of biochar and the ability to adsorb Pb and Cu. This systematic study can provide some support for the further improvement of SS pyrolysis technology, and will also be beneficial for subsequent applications.

Hazards of toxic metal(loid)s: Exploring the ecological and health risk in soil-crops systems with long-term sewage sludge application.

Sewage sludge (SS) is commonly used as agricultural fertilizer worldwide. However, the toxic metal(loid)s in SS raises concerns about soil contamination and the potential risks to human health. This study, conducted since 2007 on the North China Plain, examines the impact of SS use on crops. An experiment was designed with five treatments: conventional fertilization (CK) and four levels of SS application (W1, W2, W3, and W4: 4.5, 9.0, 18.0, and 36.0 t ha-1, respectively). Soil concentrations of eight toxic metal(loid)s (Zn, Cu, Cr, Cd, Ni, Pb, As, and Hg) were analyzed to assess pollution risk using various indices. Health risks associated with maize and wheat grains were also evaluated. Additionally, the impact of long-term SS application on crop yield, soil quality, and human health within a wheat-maize rotation system was examined. SS application increased wheat and maize yields by 5.37 to 19.08 % and 6.97 to 17.94 %, respectively, across treatments W2 to W4. Despite the toxic metal(loid)s in the grains remaining within safe limits, their concentrations showed an upward trend, especially under the W4 treatment. Moreover, SS application significantly increased the soil Zn, Cu, Cr, Cd, Pb, and Hg levels (P < 0.05) without exceeding the national standards. The geo-accumulation index values revealed rising pollution levels for Zn, Cu, Cd, and Hg, which shifted from no contamination to moderate contamination and then to moderate-to-high contamination, yet the overall pollution level remained safe. Soil ecological risks increased from moderate to serious, with Hg posing the greatest risk, particularly under the W4 treatment. Long-term crop intake from the area significantly exposed children and adults to As, contributing 42.12 % and 34.62 % to hazard index (HI), respectively. The HI values for toxic metal(loid)s in these grains surpassed one in both age groups, suggesting health risks from long-term SS cultivated crops.

Sewage sludge pyrolysis 'kills two birds with one stone': Biochar synergies with persulfate for pollutants removal and energy recovery.

The disposal and resource utilization of sewage sludge (SS) have always been significant challenges for environmental protection. This study employed straightforward pyrolysis to prepare iron-containing sludge biochar (SBC) used as a catalyst and to recover bio-oil used as fuel energy. The results indicated that SBC-700 could effectively activate persulfate (PS) to remove 97.2% of 2,4-dichlorophenol (2,4-DCP) within 60 min. Benefiting from the appropriate iron content, oxygen-containing functional groups and defective structures provide abundant active sites. Meanwhile, SBC-700 exhibits good stability and reusability in cyclic tests and can be easily recovered by magnetic separation. The role of non-radicals is emphasized in the SBC-700/PS system, and in particular, single linear oxygen (1O2) is proposed to be the dominant reactive oxygen. The bio-oil, a byproduct of pyrolysis, exhibits a higher heating value (HHV) of about 30 MJ/kg, with H/C and O/C ratios comparable to those of biodiesel. The energy recovery rate of the SS pyrolysis system was calculated at 80.5% with a lower input cost. In conclusion, this investigation offers a low-energy consumption and sustainable strategy for the resource utilization of SS while simultaneously degrading contaminants.

Investigation of intermittent aeration and oxic settling anaerobic process combination for nitrogen removal and sewage sludge reduction.

A pilot plant with a conventional activated sludge (CAS) system with intermittent aeration (IA) was monitored. The system was configured as an Oxic Settling Anaerobic (OSA) process with the insertion of one anaerobic side-stream reactor (ASSR). The pilot plant was fed with real wastewater and an intensive experimental campaign was carried out including sludge minimization, nitrogen and carbon removal, GHG emissions and biokinetic parameters. The experimental campaign was divided into periods: Period I, II, and III. In Periods I and II, the ASSR reactor was operated with two different hydraulic retention times (HRT), 4 and 6 h, with an aeration/non-aeration ratio of 30 min/30 min. In Period III, the HRT in the anaerobic reactor was the same as in Period II. In contrast, the biological reactor's aerated/non-aerated ratio was increased to 40 min/20 min. Results demonstrated that combining IA and OSA might be effective in the reduction of excess sludge production. The yield coefficient decreased from Period I to Period II (Yobs from 0.41 to 0.25 gTSS gCOD-1, in Period I and II, respectively). Nevertheless, the HRT increase in the ASSR compromised the system performance regarding nitrification and greenhouse gas emissions and worsened the sludge settleability. However, the increase in the aeration duration was beneficial in restoring the system's nitrification and denitrification ability and carbon footprint. The lowest carbon footprint was obtained during Period III (6.8 kgCO2/d).

Extracellular-proton-transfer driving high energy-conserving methanogenesis in anaerobic digestion.

Anaerobic digestion (AD) is a promising technology to realize the conversion from organic matters to methane, which is highly mediated by syntrophic microbial community via mutualistic interactions. However, small energy available in methanogenic conversion usually limits the metabolic activity. To adapt such energy-limited environment, efficient energy conservation is critical to support active physiological functions of anaerobic consortia for methanogenic metabolism. In this study, the contribution of extracellular proton transfer (EPT) enhancement to achieving energy-conserving methanogenesis in AD was explored. Proton-conductive medium (PCM) was applied to construct efficient proton transport pathway, and a large number of protons from extracellular water were found available to upregulate methanogenesis in AD, as indicated by the increase in the content of 2H (D) in methane molecules (over 40.7%), among which CO2-reduction-to-CH4 was effectively enhanced. The increases of adenosine triphosphate (ATP) concentration (+54.1%) and gene expression activities related to ATPase (+100.0%) and proton pump (+580.1%) revealed that enhanced EPT by PCM promoted transmembrane proton motive force generation to facilitate ATP synthesis. Based on genome-centric metatranscriptomic analyses, MAG14, MAG63 and MAG61 with high energy conservation activity displayed most pronounced positive response to the EPT enhancement. In these core MAGs, the metabolic pathway reconstruction and the key genes activity identification further proved that EPT enhancement-driven efficient ATP synthesis stimulated the cross-feeding of carbon and proton/electron to facilitate microbial mutualism, thereby resulting in the high energy-conserving methanogenesis. Overall, our work provides new insights into how EPT enhancement drives high energy-conserving methanogenesis, expanding our understanding of the ecological role of EPT in AD.

Carbide slag and sodium metasilicate synergistic activation of sludge ash-based alkali-activated materials: Towards a cleaner activation approach.

Traditional activators such as sodium hydroxide and sodium silicate are commonly used in the preparation of alkali-activated materials; however, their significant environmental impact, high cost, and operational risks limit their sustainable use in treating solid waste. This study explores the innovative use of carbide slag (CS) and sodium metasilicate (NS) as alternative activators in the production of sewage sludge ash-based alkali-activated materials (SSAM) with the aim of reducing the carbon footprint of the preparation process. The results demonstrate that CS effectively activates the sewage sludge ash, enhancing the compressive strength of the SSAM to 40 MPa after curing for 28 d. When used in conjunction with NS, it synergistically improves the mechanical properties. Furthermore, the microstructure and phase composition of the SSAM are characterized. Increasing the quantities of CS and NS accelerates the dissolution of the precursor materials, promoting the formation of a higher quantity of hydration products. This significantly reduces the number of voids and defects within the samples, further enhancing the densification of the microstructure. Environmental assessments reveal that CS and NS offer substantial sustainability benefits, confirming the feasibility of activating SSAM using these materials. This approach provides a less energy-intensive and more environmentally friendly alternative to conventional activation methods and presents an effective strategy for managing large volumes of sewage sludge ash and CS.

Characteristics of nutrients and heavy metals release from sewage sludge biochar produced by industrial-scale pyrolysis in the aquatic environment and its potential as a slow-release fertilizer and adsorbent.

To assess the application potential of sewage sludge biochar produced by industrial-scale pyrolysis (ISB), the release characteristics of nutrients (NH4+, PO43-, K, Ca, Mg and Fe) and heavy metals (Mn, Cu, Zn, Pb, Ni and Cr) were investigated. Their release amounts increased with decreasing initial pH and increasing solid-liquid ratios (RS-L) and temperature. The release types of NH4+, K, Mg, and Mn were diffusion/dissolution, while those of Cu, Zn, Pb, Ni, and Cr were diffusion/resorption. The release types of PO43- and Ca varied with initial pH and RS-L, respectively. The chemical actions played dominant roles in their release, while particle surface diffusion and liquid film diffusion determined the rates of diffusion and resorption phases, respectively. The release of NH4+, PO43-, K, Ca, Mg, Mn and Zn was a non-interfering, spontaneous (except PO43-), endothermic, and elevated randomness process. The release efficiency of NH4+, PO43- and K met the Chinese standard for slow-release fertilizers, while the total risk of ISB was low. The eutrophication and potential ecological risks of ISB were acceptable when the dose was less than 3 g L-1 and the initial pH was no lower than 3. In conclusion, ISB had potential as a slow-release fertilizer and adsorbent.