Sludge Treatment Research Articles

Assessing the Amending and Fertilizing Properties of Digestates From Anaerobic Digestion of Faecal Sludge in Burkina Faso, West Africa

The aim of this work is to contribute to the agronomic utilization of digestate from the anaerobic treatment of faecal sludge at the Office National de l’Eau et de l’Assainissement (ONEA) Kossodo plant in Burkina Faso. Digestates were sampled and dried in the laboratory (25±2 °C). Physico-chemical and microbiological parameters were determined using standard methods. The degree of maturity of the digestate was determined using respirometry and humification tests. The effects of digestate on plants and on environment were studied using phytotoxicity, emergence, growth tests and microbiological analysis. The results revealed a pH of 8.54, electrical conductivity of 7300 µS/cm, 25.42% dry matter, 83.31% organic matter, and 489.8 g/kg total organic carbon. The C/N ratio was 9.7 and the NO3/NH4 ratio was 23.34, indicating mature compost despite other indicators suggesting that the digestate was only partially stabilized. Nutrient contents were 50.48 g/kg total nitrogen, 0.02 g/kg total phosphorus, and 4.72 g/kg total potassium. Water-soluble minerals included 1.97 g/kg ammonium, 45.99 g/kg nitrate, 0.02 g/kg nitrite, 0.35 g/kg phosphate, and 5.90 g/kg sulfate. Phytotoxicity tests showed that cabbage seeds had a germination rate of 89% with digestate filtrate compared to 100% in the control, while tomato seeds had a 100% germination rate with digestate filtrate. The germination indices were 137.58 for tomato and 82.10 for cabbage. Tests on tomato plant growth showed significant effects on biomass, height, leaf number, and branch number, with the 50% digestate proportion demonstrating the highest values. Microbiological analysis detected total aerobic flora at 4.75E+07 CFU/g, yeasts and molds at 7.77E+04 CFU/g, and coliforms at various levels, with potential pathogens such as S. aureus and Salmonella sp. reaching 6.14E+04 and 2.65E+06 CFU/g respectively. The presence of these pathogens suggests a need for further treatment of digestate to ensure safety before field application.

Behavior and flow of microplastics during sludge treatment in Japan.

Microplastic (MP) pollution is a growing public and scientific concern. In urban environments, wastewater treatment plants (WWTPs) are major sources of MPs. This study sampled sludge and separated water from each sludge treatment unit in two WWTPs in Osaka, Japan. Analyzing method for MPs in sewage sludge was optimized, ultrasonic pretreatment and double digestion were introduced into the analyzing method of MPs in sewage sludge, recovering test of standard MPs proved its high efficiency. Then MPs larger than 100 μm were extracted and analyzed, their size and type were recorded, the MP concentration was calculated, and the MP flow in the sludge treatment system was estimated. MPs were detected at every step of the sludge treatment process, and 13 types of MPs were identified. The MP concentration in sludge ranged from 81 ± 48 to 6470 ± 1490 particles/kg dry sludge (DS). In the separated water, MP concentrations were much lower, ranging from 0 to 1740 ± 794 particles/kg DS. During the thickening and dewatering processes, nearly all MPs were transferred into thickened or dewatered sludge; only 5-10 % of MPs returned to the primary sedimentation pond with the separated water. The most common types of MPs were PMMA, PE, and PS. No significant differences in MP type distributions were observed among sampling batches; however, significant differences in a few types of MPs were detected between treatment units, which requires further investigation. All detected MPs were smaller than 1000 μm; larger MPs might have been removed in the grit chamber before reaching the primary or secondary sedimentation ponds. Overall, the particle size distribution did not substantially change during sludge treatment.

Policy impact on economic and environmental sustainability of anaerobic digestion: Industrial case study Insights.

This paper thoroughly examines how policy incentives impact the economic and environmental sustainability of anaerobic digestion (AD) systems. It uses techno-economic and life cycle analyses, along with real industry data, to explore the entire AD process-from feedstock acceptance to digestate disposal. It evaluates the effects of various U.S. policy crediting programs on the economic viability of different AD pathways for treating sewage sludge and food waste. Tipping fees are identified as the primary driver of profitability, while policy credits play a crucial role in enhancing economic feasibility, particularly for renewable natural gas production. However, future regulatory changes could reshape this economic landscape. All AD pathways are found to significantly reduce greenhouse gas emissions, though economic outcomes are highly sensitive to digestate disposal costs and feedstock tipping fees. Co-digestion with food waste is proposed as a strategy to reduce dependence on policy credits and improve long-term economic stability.

Comparative life cycle assessment of on-site sanitation systems using lagoons or drying beds for fecal sludge treatment in low-income tropical countries

ABSTRACT Environmental challenges in low-income countries, such as Haiti, persist due to inadequate sanitation infrastructure. This study assesses the environmental impacts of nine on-site sanitation systems to identify those with the least environmental impacts and explore improvement options. Nine scenarios were developed, each representing different systems for managing 1 ton of fecal sludge over 1 year. The ‘Impact World + ’ and ‘IPCC 2013 GWP 100a’ methods evaluated impacts on ecosystems, human health, and climate change. Data sources included interviews, weighing records, and scientific publications. Results show that Scenario 8 (Flush Toilet – Evacuation – Planted Drying Beds) is most impactful on health (1.17 × 10−2 DALY), while Scenario 1 (Composting Toilet – Evacuation – Unplanted Drying Beds) is least impactful (1.77 × 10−3 DALY). For ecosystem impacts, Scenario 2 (Container-based Toilet – Evacuation – Planted Drying Beds) is most impactful (3.81 × 103 PDF·m2·year), while Scenario 6 (VIP latrine – Evacuation – Lagoons) is least impactful (3.52 × 103 PDF·m2·year). Key hotspots include toilet paper, wood shavings, GHG emissions, and water use. The study recommends an integrated approach combining environmental life cycle assessment (LCA) with life cycle cost assessment and social LCA for sustainable decision-making on sanitation systems in low-income countries.

Study on the synergism mechanism of composite surfactants for oily sludge treatment using microemulsion method

The nonionic surfactant was considered to be the most effective surfactant for the treatment of oily sludge. In this paper, Tween20 and Span80 (T/S) composite surfactant was chosen as emulsifier for the preparation of microemulsion, and their synergistic effect was explored. Clint equation and molecular dynamics simulation was used to interpret the formation of mixed micelles and the results showed that T/S composite surfactant with the mass ratio of 11:1 exhibited the good synergetic effect. Then, the microemulsion prepared using composite surfactants was applied for the treatment of oily sludge. Under the optimal conditions of solid-liquid ratio of 1:1(g: mL), stirring temperature of 40 °C, stirring time of 30 min and stirring speed of 400 rpm, the residual oil rate was reduced to 0.25%, which was much lower than other reported results using chemical methods. Interface tensiometer and quartz crystal microbalance measurement showed that the interfacial tension decreased significantly after the addition of composite surfactant. This work provides a feasible strategy for oily sludge treatment using microemulsion, which promoted the development of oily sludge treatment processes based on microemulsion in practical industry.

Migration and Transformation of Heavy Metals During the CO2-Assistant Thermal Treatment of Oily Sludge

Thermal treatment has significant advantages in resource recovery for oily sludge (OS). However, the instability of heavy metals (HMs) within the residue poses a considerable risk of secondary pollution. This study explored the migration and transformation of HMs from OS under varying conditions (i.e., temperature, constant-temperature duration time, and different ratios of O2 and CO2). The elevation of the pyrolysis temperature augmented the decomposition of organic matter and total petroleum hydrocarbons (TPHs). However, the increased temperature also diminished the stabilization of HMs, and facilitating the HM’s transfer to oil and gas, particularly for HMs (i.e., As and Pb) with low boiling points. The constant-temperature duration time exhibited a weak impact on HM transformation, but the internal heating mechanism of microwave pyrolysis promoted the stabilization of HMs through vitrification. The existing O2 with oxidizing properties facilitated the oxidation of organic matter and TPHs to CO2 and H2O, which also promoted the transformation of HMs into oxidized states for stabilization. Comparatively, CO2 promoted the thermal cracking and disrupted the stability of HMs to a certain extent. Above all, this work revealed the migration and transformation of HMs in OS varied with the thermochemical methods and possessed an important significance for the immobilization and stabilization of HMs.

Exploring the pyrolysis process of simulated oily sludge: Kinetics, mechanism, product distribution, and S/N elements migration

Pyrolysis is an important method for energy recovery and harmless treatment of oily sludge, definite reaction mechanism and the transformation of S and N elements is a key to improve the pyrolysis products. In this paper, a simulated oil sludge (SOS) was pyrolyzed at various temperatures of 400–700 °C in a tube furnace focusing on pyrolysis process, kinetic parameters, reaction mechanisms, S and N element migration patterns and product distributions. Kinetic parameters were deducted by FWO, KAS, Friedman and Starink methods, and the activation energy were 186.23–231.20 kJ/mol (Avg. 210.71 kJ/mol), 190.18–233.80 kJ/mol (Avg. 212.02 kJ/mol), 196.93–242.01 kJ/mol (Avg. 209.88 kJ/mol) and 184.82–230.95 kJ/mol (Avg. 218.67 kJ/mol), respectively, showed high similarity. All the pre-exponential factors were higher than 109 s−1, which indicated high reactivity of SOS during pyrolysis, and the pyrolysis process followed the nucleation growth model (A3). Pyrolysis temperatures had a significant influence on products distribution. The maximum yields of pyrolysis tar and gas were observed at 550 °C and 700 °C, respectively. Pyrolysis tar was dominated by aromatics and acids, while pyrolysis gas was mainly composed of H2 and CH4. Additionally, high temperatures could facilitate the transfer of more S and N into tar or gas products, and S and N compounds were mainly thiophene-S, sulfoxide-S, pyridine-N and pyrrole-N in char and CS2, CH3SH, COS, SO2, H2S, NH3, HCN and NOx in gas.

Strategic carbon emission assessment in sludge treatment: A dynamic tool for low-carbon transformation

The carbon–neutral target presents a significant challenge for the sewage sludge treatment and disposal (SSTD) industry, necessitating strategic planning for a low-carbon transition. However, flexible and comprehensive carbon emission analysis tools to support this goal remain lacking. This study presents a carbon emission analysis tool to evaluate the carbon emission characteristics and future mitigation potentials of SSTD. The tool integrates life cycle inventory (LCI) modeling-based analysis, sensitivity analysis, regression analysis, and scenario analysis. Carbon emissions are dynamically calculated based on sludge properties, technological level, and industry external parameters, providing a foundation for adaptable evaluation tailored to local conditions. The framework considers the potential effects of multi-parameter and multi-aspect changes in scene design, both within and outside the industry, to achieve dynamic and comprehensive simulations. A case study conducted in Wuhan, China, demonstrated the usability and application processes of the framework. The results indicated that carbon emissions from SSTD are projected to more than double from 2021 to 2060 without interventions. Among the mitigation measures, energy and chemical savings would yield the largest reduction potential, followed by the technical layout adjustment and the promotion of energy efficiency. Operational optimization in the sludge industry and outside the industry would contribute the least. With all mitigation measures applied, emissions could decrease to –82.91 kt CO2-eq in 2060, equivalent to 13.03% compensation for emissions from the sewage treatment line. Among all the processes, incineration routes are recommended due to their current and future low carbon emissions. The cooperative resource route of anaerobic digestion and land use also shows promise as it progressively demonstrates superior performance with increasing organic matter and nutrient content of sludge. Critical factors, sub-processes, and emission types for different routes were identified and can be optimized accordingly. The developed method demonstrates sufficient flexibility to be applied to other cities and larger-scale regions, thereby offering technical and strategic support for SSTD towards carbon–neutral operation.

A novel strategy for waste activated sludge treatment: Recovery of structural extracellular polymeric substances and fermentative production of volatile fatty acids

Structural extracellular polymeric substances (SEPS) as valuable biopolymers, can be extracted from waste activated sludge (WAS). However, the extraction yield is typically low, and detailed information on SEPS characterizations, as well as proper treatment of the sludge after SEPS extraction, remains limited. This study aimed to optimize the conditions of heating-Na2CO3 extraction process to increase the yield of SEPS extracted from WAS. Subsequently, SEPS were characterized, and, for the first time, insights into their protein composition were uncovered by using proteomics. A maximum SEPS yield of 209 mg/g volatile solid (VS) was obtained under optimal conditions: temperature of 90°C, heating time of 60 min, Na+ dosage of 8.0 mmol/g VS, and pH required to precipitation of 4.0, which was comparable to that from the aerobic granular sludge reported in literature. Proteomics analysis unveiled that the proteins in SEPS primarily originated from microorganisms involved in nitrogen fixation and organic matter degradation, including their intracellular and membrane-associated regions. These proteins exhibited various catalytic activities and played crucial roles in aggregation processes. Besides, the process of SEPS extraction significantly enhanced volatile fatty acid (VFA) production during the anaerobic fermentation of residual WAS after SEPS extraction. A maximum VFA yield of 420 ± 14 mg COD/g VSadded was observed in anaerobic fermentation of 10 d, which was 77.2 ± 0.1% higher than that from raw sludge. Mechanism analysis revealed that SEPS extraction not only improved WAS disintegration and solubilization but also reduced the relative activity of methanogens during anaerobic fermentation. Moreover, SEPS extraction shifted the microbial population during anaerobic fermentation in the direction towards hydrolysis and acidification such as Fermentimonas sp. and Soehngenia sp. This study proposed a novel strategy based on SEPS extraction and VFA production for sludge treatment, offering potential benefits for resource recovery and improved process efficiency.

Low-Temperature Rapid Drying of Viscous Sludge Via Non-Phase Change Based on Particle High-Speed Self-Rotation and Medium Dispersion in Cyclone

Drying operations are of grave importance to realize the reduction and utilization of sewage sludge resources, but the conventional thermal evaporation drying (TED) technology faces challenges due to the need for a large amount of thermal energy to conquer the phase-change latent heat of moisture. Herein, we report a non-phase change technology based on particle high-speed self-rotation in a cyclone for fast, low-temperature drying of viscous sludge with high moisture contents. Dispersed phase medium (DPM) is introduced into the cyclone self-rotation drying (CSRD) reactor to enhance the dispersion of the viscous sludge. The effects of carrier gas temperature, feeding rate, size, and proportion of DPM particles in the drying process are systematically examined. Under optimal operating conditions, the weighted content of moisture in the viscous sludge could be reduced from 80% to 15.01% in less than 5 s, achieving a high drying efficiency of 95.79%. Theoretical calculations also reveal that 89.26% of the moisture is removed through a phase change pathway, contributing to a 522-fold increase in the drying rate of CSRD compared to TED technology. This investigation presents a sustainable effective approach for high moisture viscous sludge treatment with low energy consumption and carbon emissions.

A pilot-scale evaluation of residual sludge quality in a worm-sludge treatment reed bed in the Mediterranean region

A pilot-scale study on sludge treatment reed beds investigated the combined effects of earthworms and Arundo donax on sewage sludge dewatering and residual sludge quality. Four units were tested: one planted with earthworms, one planted without earthworms, one unplanted with earthworms, and one control, each unit replicated. Over a year, 24 cycles of sludge (dry and volatile solid contents of 24.71 g.L−1, and 19.14 g.L−1) were fed onto the units at a sludge loading rate: 43.59 kg.DS.m−2.year−1. Afterward, the units experienced 132 days of resting period, increasing dry solids from 21 to 70 % and decreasing volatile solids from 81 to 69 % on average (40 % sludge volume reduction). The bottom layers of the planted unit with earthworms showed a 30 % reduction in volatile solids, indicating improved sludge stabilization. Macronutrient abundance in the residual sludge followed the sequence N > Ca > P > K > S > Mg. The planted unit with earthworms reduced micronutrient concentrations by 22 % compared to the control unit (Fe > Na > Mn > B > Mo). Earthworms also played a key role in reducing heavy metal concentrations by 11 % compared to the planted unit without earthworms (Zn > Cr > Pb > Ni > Cd). Heavy metal levels in the residual sludge met EU and Portugal standards, with a 99.9 % reduction in Escherichia coli and fecal coliforms. Cost estimation showed centrifugation and W-STRB scenarios cost 167 and 183 €.PE−1 for a ten-year operation, with O&M costs of 7 and 3 €.PE−1.year−1, respectively.

The Efficacy of a Combination of Milk Thistle, Artichoke, and Green Tea in the Treatment of Biliary Sludge: An Interventional Prospective Open Study

Background: In Western countries, 10–20% of adults have gallstones, of which about 1–3% of patients are symptomatic for biliary colic. The treatment with ursodeoxycholic acid is debated. Silymarin is emerging nowadays as a natural substance with choleretic and beneficial properties, useful in the case of gallbladder sludge. Aim: The aim of our study was to evaluate the efficacy of supplementation with a mix of natural compounds (milk thistle 150 mg, artichoke 150 mg, and green tea 150 mg) in patients affected by biliary sludge in reducing biliary colic and biliary sludge and improving lipid profiles after 3 months of treatment compared to a control group. Patients and Methods: This was an interventional open study on 65 consecutive adult patients (23 M/42 F; mean age 61.0 ± 18.7 years) affected by biliary sludge without indication for surgery, admitted to the Internal Medicine Department of San Carlo di Nancy Hospital Rome. Forty patients were treated with milk thistle 150 mg, artichoke 150 mg, and green tea 150 mg, 2 capsules/day for three months, and followed up for abdominal ultrasound, the frequency of occurrence of biliary colic, and blood tests. Twenty-five control group patients were followed up for abdominal ultrasound, the frequency of occurrence of biliary colic, and blood tests after 3 months. Results: In Group A, we observed a disappearance of biliary sludge in 32.4% (12/37) of patients compared to 8.7% in the control group (p < 0.05). In 32.4% of treated patients, we report a reduction in biliary sludge. There were no side effects during treatment. In both groups, we registered a significant reduction in transaminases and gamma-glutamyl transferase (γ-GT) (p < 0.05). No significant modification of the lipid profile was observed. Conclusions: The treatment with a mix of natural compounds (milk thistle, artichoke, and green tea) in patients affected by symptomatic biliary colic resulted in efficacy and safety in reducing biliary sludge, biliary colic, and transaminases levels after three months of treatment. The limitations of this study include a small number of patients and the absence of long-term follow up after the end of treatment.

An innovative sludge-derived capsule for self-healing cementitious materials

An innovative eco-efficient capsule utilizing drinking water treatment sludge (DWTS) as a healing agent for concrete crack sealing was developed. In this study, the core materials comprised a mix of DWTS and calcium hydroxide, granulated by poly (ethylene glycol). A non-toxic polymer, Ethyl cellulose (EC), is applied as the protective shell material. The morphology, internal structure, and leakage mechanism of capsules, self-healing performance of cracked mortars and the composition of resulting healing products were assessed. Obtained results indicated that EC uniformly covered the spherical core material following the designed coating procedures. The main elements released from the capsules were Ca, Al and Si after dissolution of polyethylene glycol (PEG), which would contribute to pozzolanic reactions inside the matrix. Cracks with an initial width of 400 μm were completely healed after 7 days of curing. Such a healing process also led to an 73% enhancement in compressive strength at a healing age of 28 days. The water tightness of capsule-based samples improved by more than 90% in the first 7 days, compared to only 10% in control samples. The predominant healing products were calcium carbonate in the form of calcite, and some content of aluminium-bearing phases derived from the pozzolanic reaction of DWTS.

Study on the strength characteristics and microscopic mechanism of sewage sludge ash modified lime soil

The large amount of sewage sludge ash (SSA) generated by incineration treatment of sewage sludge each year can seriously pollute the environment, and there is an urgent need to seek an effective resource utilization method for SSA treatment. SSA has the significant pozzolanic activity, and can be used as an auxiliary cementitious material. For that, SSA was adopted to modify lime soil in this work. The strength characteristics of SSA modified lime soil (SLS) were investigated by conducting unconfined compressive strength test and unconsolidated-undrained triaxial compression test, and X-ray diffraction (XRD) test and scanning electron microscopy (SEM) test were conducted to investigate the mineral composition, microstructure and porosity of SLS. It is observed that the unconfined compressive strength (UCS) increases first and then decreases with increasing SSA content in the range of 0–20 %, and reaches the maximum value while SSA content is 15 %, which increases by 25 % at the curing age of 28 days comparing to the specimen without SSA. Additionally, the triaxial strength and cohesive force both have the same change law as UCS with SSA increasing, and 15 % can be used as the optimal content of SSA to modify lime soil. The added SSA can promote the pozzolanic reaction, and more hydration products fill the specimen porosity so that the specimen strength can be improved. Furthermore, the porosity of SLS decreases first and then increases with SSA content increasing, and there is a linear relationship between the porosity and strength of SLS. Finally, a simple function is proposed, which can simulate the quantitative relationship between UCS and the triaxial strength with satisfying accuracy. Generally, the research results can be used as a reference for the application of SSA in soft soil foundation treatment.

Analysis of the Physical and Chemical Composition of Sludge from the Water Treatment Plant

The properties and composition of the sludge generated in water treatment systems depend primarily on the type and composition of the water to be treated, the treatment methods, and the type and doses of chemical reactants. The sludge produced in the water treatment plant (WTP) under study follows the technological processes of coagulation, flocculation, sedimentation, and filtration. The analyses aimed to characterize the sludge in terms of its physico-chemical properties and classify it in terms of its potential discharge into the river and management. Four series of sediment tests were conducted over a calendar year (March, June, September and December), analysing selected parameters using various test methods, including the X-ray crystallography (XRF) method. The publication's authors showed that the sediment consists mainly of sand, clay, and silt particles with grain sizes ranging from 0.001 mm to 1 mm. Silica (53.78%), alumina (23.58%), calcium oxide (8.28%), iron (III) oxide (5.61%), and potassium oxide (2.36%) represent the main chemical constituents present in the sediment. The authors characterized the sediment in terms of the content of biogenic compounds: various forms of phosphorus and nitrogen, organic compounds – determined as total organic carbon (TOC), selected metals, and the content of individual elements (carbon, hydrogen, oxygen, nitrogen, sulfur). In addition, the sludge samples were also characterized in terms of calorific value, ash content, water content, and heat of combustion. Discharging WTP sludge into rivers, ponds, and lakes or storing dewatered sludge is an environmentally unfriendly form of disposal for this type of waste. The authors see the possibility of conducting further research on using WTP sludge in wastewater treatment, removing heavy metals from aqueous solutions, producing cement and construction materials, and recovering or recycling.

Evaluation of Biochemical Methane Potential and Kinetics of Organic Waste Streams for Enhanced Biogas Production

Organic waste has the potential to produce methane gas as a substitute for petrol-based fuels, while reducing landfilling and possible environmental pollution. Generally, anaerobic digestion (AD) is used only in wastewater treatment plants as a tertiary stage of sewage sludge treatment, generating a fraction of the energy that such process plants require. In this study, four different wastes—food waste (FW), dairy industry waste (DIW), brewery waste (BW), and cardboard waste (CBW)—were tested for biogas production. The biochemical methane potential (BMP) of each sample was evaluated using an automatic methane potential system (AMPTS). Operating parameters such as pH, temperature, total solids, and volatile solids were measured. Experiments on the anaerobic digestion of the samples were monitored under mesophilic conditions (temperature 37 °C, retention time 30 days). Specific methane yields (SMYs), as well as the theoretical methane potential (BMPth), were used to calculate the biodegradability of the substrates, obtaining the highest biodegradability for BW at 95.1% and producing 462.3 ± 1.25 NmL CH4/g volatile solids (VS), followed by FW at an inoculum-to-substrate ratio (ISR) of 2 at 84% generating 391.3 NmLCH4/g VS. The BMP test of the dairy industry waste at an inoculum-to-substrate ratio of 1 was heavily inhibited by bacteria overloading of the easily degradable organic matter, obtaining a total methane production of 106.3 NmL CH4/g VS and a biodegradability index of 24.8%. The kinetic modeling study demonstrated that the best-fitting model was the modified Gompertz model, presenting the highest coefficient of determination (R2) values, the lowest root means square error (RMSE) values for five of the substrates, and the best specific biogas yield estimation with a percentage difference ranging from 0.3 to 3.6%.

Coupling Iron Coagulation and Microalgal–Bacterial Granular Sludge for Efficient Treatment of Municipal Wastewater: A Proof–of–Concept Study

The rapid expansion of global urbanization and industrialization has significantly increased the discharge of municipal wastewater, leading to issues of carbon emissions and energy consumption when using traditional biological treatment processes. This study proposes an innovative process that couples iron coagulation with microalgal–bacterial granular sludge (MBGS), with optimization and regulation based on operational conditions. The study found that the coagulation performance achieved optimal levels at an iron concentration of 25 mg/L and an anionic polyacrylamide concentration of 1 mg/L, which could remove approximately 61% of the organics and over 90% of phosphorus from raw wastewater. By relying on heterotrophic microorganisms, such as Proteobacteria, Bacteroidota, and Chloroflexi, along with the synergistic interaction between algae and bacteria, the subsequent MBGS process could further effectively remove organics over the day-night cycles. Moreover, the addition of inorganic carbon sources of NaHCO3 increased the abundance of denitrification-related genes, reduced the accumulation of nitrite within MBGS, and led to effective total nitrogen removal. These results indicate that the iron coagulation–MBGS coupling process can efficiently treat municipal wastewater, offering potential for environment-sustainable pollutant removal with reduced energy consumption. These findings provide valuable insights for the practical engineering application of MBGS in wastewater treatment systems aiming for carbon-neutral wastewater treatment.

Developing a robust indicator to evaluate circular economy through reuse strategy: A case study of using water treatment sludge as a coagulant for dewatering of iron ore tailings slurry

Due to the lack of tools, which are simple enough to assess the reuse strategy in the concept of circular economy (CE), in the present paper, reuse efficiency index (REI) was introduced as a new indicator, inspired by previously developed indexes, namely circular economy index (CEI) and value-based resource efficiency (VRE). The REI, as a measure of economic efficiency of reuse strategy, is the ratio of the net value obtained from the reuse of end of life (EOL) product instead of using a new product over the value of materials and energy inputs required to reproduce the EOL product. To calculate REI for any specific EOL product, information regarding mass, physicochemical properties and operational data on the consumption of energy, which are mainly available in the financial archives of companies, is crucial and the market price of EOL product must also be predicted. In another part of the study, the reuse of ferric chloride-based water treatment sludge (FCS) as a coagulant, instead of using high cost polymeric coagulant, for iron ore tailings slurry dewatering was evaluated through REI. The results showed that depending on the market prices of energy and materials as well as the iron (FeCl3) content in the FCS, the REI could vary between 0.51 and 1.04, corresponding to 51–104% added value. Overall, REI is based on accessible information and its simplicity and comprehensibility makes it applicable for managers and policymakers to invest and create technologies for reaching closed-loop cycles in the concept of CE through reuse action.

Sustainable utilization of phosphorus-rich sludge in cement composites: Multiscale evaluation and mechanistic analysis

The treatment of sludge poses challenges for resource utilization and environmental protection, and utilizing sludge as supplementary cementitious materials (SCMs) presents a viable solution. However, the phosphorus content significantly influences the cement hydration process. This study primarily investigates the feasibility of phosphorus-rich sludge (PRS) as SCMs and examines the effects of combining PRS with calcined clay on cement hydration. The research focuses on hydration heat, mechanical properties, microstructure, and durability of composites comprised of PRS, limestone, calcined clay, and ordinary Portland cement. The results indicate that PRS delays early hydration reactions, leading to a decrease in early compressive strength; after 7 days, the compressive strength is approximately 14.3 MPa (sample S20C10L15). However, over time, the strength increases to 34.1 MPa at 28 days, reflecting a 138.5 % increase compared to the 7-day compressive strength (with sample OPC exhibiting only 69.1 %). This correlates with the hydration exothermic peak occurring in the later stages and the transition of phosphorus's effect on cement hydration from inhibitory to non-inhibitory. The phosphorus in PRS partially replaces silicon atoms in C-(A)-S-H gel, forming C-S(A,P)-H gel. This process refines pore sizes within PRS cementitious materials, enhancing compressive strength over time. Early hydration inhibition and the subsequent pozzolanic reaction of PRS reduce portlandite content, which lowers carbonation durability and thermal expansion coefficient. However, the quaternary blend of PRS, limestone, calcined clay, and ordinary Portland cement demonstrates excellent mechanical properties. This study provides experimental evidence for reducing the environmental impact of PRS and exploring carbon reduction strategies in the cement industry.

Comparative Study on the Applicability of Solid Waste Treatment by Fixed Grate and Fluidized Bed Incinerators: A Bibliometric Review

Using thermal treatment in the waste management process has advantages such as reducing volume and the possibility of energy recovery. There are several incineration technologies with different characteristics and potential. Incineration, however, requires efficient environmental control to reduce associated risks, such as the emission of harmful compounds. The paper aims to compare solid waste treatment processes in grate and fluidized bed incinerators based on technical and environmental characteristics. For this purpose, a bibliometric review was conducted, and consulting works available in the scientific literature describe the waste treatment process with these two technologies. The searches were carried out in the Web of Science and Web of Knowledge databases using the following search engines: “fixed grade incinerator,” “grate incinerator” and “fluidized bed incinerator”. Grate incinerators present a range of elective waste for treatment compared to fluidized bed incinerators, as they do not require homogeneous waste. The study compared solid waste incineration in fixed grate and fluidized bed furnaces. While promising for sludge treatment, the fluidized bed technology may require pre-treatment, increasing costs and limiting its use to larger facilities. Fixed grate furnaces can handle a wider variety of waste without this additional step but generate more halogenated compounds when plastic waste is present. These technologies were primarily studied in Asia and Europe, where landfill space is limited and more significant potential for energy recovery exists. Adopting these technologies in other regions depends on waste characteristics, economic conditions, and environmental impacts.