Biowaste management by hydrothermal carbonization and anaerobic co-digestion: Synergistic effects and comparative metagenomic analysis

The feasibility of anaerobic co-digestion in semicontinuous mode of two major urban biowaste, food waste (FW) and garden and park waste (GPW) (75 % FW and 25 % GPW) as well as the co-digestion of FW with the process water originated from the hydrothermal carbonization of GPW (95 % FW and 5 % process water), both on a COD basis, has been assessed. The effect of varying organic loading rate (OLR) from 1.5 to 3.5 g COD/L·d on methane yield, gross energy recovery, and microbiome population was evaluated. For comparison, anaerobic digestion of FW was also conducted to determine the best strategy for sustainable biowaste management. This study showed an optimal OLR of 2.5 g COD/L·d. Acetic and propionic acid content increased as OLR raised for each condition studied, while methane yield decreased at the highest OLR tested indicating overloading of the system. The anaerobic co-digestion of FW and process water showed a 10 % increase on methane production compared to anaerobic digestion of FW (324 vs. 294 mL CH4 STP/L·d). Moreover, it enhances the process due to a greater abundance and diversity of hydrolytic and acidogenic bacteria belonging to Bacterioidota, Firmicutes, and Chloroflexi phyla, as well as promotes the hydrogenotrophic pathway under higher propionic concentrations which is not usually favoured for methane production. The integration of hydrothermal carbonization of GPW with the anaerobic co-digestion of 95 % FW and 5 % of process water results in the highest potential energy recovery and could be a good strategy for sustainable management of urban biowaste.

Digitalization of phosphorous removal process in biological wastewater treatment systems: Challenges, and way forward

Phosphorus in wastewater poses a significant environmental threat, leading to water pollution and eutrophication. However, it plays a crucial role in the water-energy-resource recovery-environment (WERE) nexus. Recovering Phosphorus from wastewater can close the phosphorus loop, supporting circular economy principles by reusing it as fertilizer or in industrial applications. Despite the recognized importance of phosphorus recovery, there is a lack of analysis of the cyber-physical framework concerning the WERE nexus. Advanced methods like automatic control, optimal process technologies, artificial intelligence (AI), and life cycle assessment (LCA) have emerged to enhance wastewater treatment plants (WWTPs) operations focusing on improving effluent quality, energy efficiency, resource recovery, and reducing greenhouse gas (GHG) emissions. Providing insights into implementing modeling and simulation platforms, control, and optimization systems for Phosphorus recovery in WERE (P-WERE) in WWTPs is extremely important in WWTPs. This review highlights the valuable applications of AI algorithms, such as machine learning, deep learning, and explainable AI, for predicting phosphorus (P) dynamics in WWTPs. It emphasizes the importance of using AI to analyze microbial communities and optimize WWTPs for different various objectives. Additionally, it discusses the benefits of integrating mechanistic and data-driven models into plant-wide frameworks, which can enhance GHG simulation and enable simultaneous nitrogen (N) and Phosphorus (P) removal. The review underscores the significance of prioritizing recovery actions to redirect Phosphorus from effluent to reusable products for future considerations.

Ionic liquid catalyzed low-temperature hydrothermal carbonization of sewage sludge to produce hydrochar with low heavy metal content and positive energy recovery

An ionic liquid (IL, [DMAPA]HSO4) was prepared to facilitate the removal of heavy metals by hydrothermal carbonization (HTC) in sewage sludge (SS) and to obtain a positive energy recovery (ER, (Energyoutput/Energyinput - 1) > 0). The results found that the removal efficiencies of the Fe, Mn, Zn, Co, and Cd from SS exceeded 75 % with positive ER (6 %) at 20 wt% IL dosage (IL:SS). IL promoted the HTC reactions of proteins and polysaccharides to produce fixed carbon and small molecule polymers. The process mainly relies on IL to catalyze the dehydration and graphitization of SS and to destroy the heavy metal binding sites such as carboxyl and hydroxyl groups. Additionally, IL aids in constructing the macropore structures in hydrochar, thereby facilitating the release of heavy metals and water during the HTC process. This discovery holds promise for removing heavy metals from SS by one-pot HTC processes with positive energy recovery.

Domestic wastewater sludge valorization: Multi-criteria evaluation of anaerobic digestion vs. hydrothermal liquefaction

The emerging hydrothermal liquefaction (HTL) process is evaluated against the classical anaerobic digestion (AD) processes for stabilizing wastewater sludges and recovering their energy- and nutrient-contents. Although HTL affords faster stabilization, better process stability, and liquid fuel and sterile fertilizer recovery, it suffers from higher energy demand and lower technology readiness level. For a rational comparison of these pathways, a multi-criteria evaluation is conducted considering 21 technical, environmental, economic, and social criteria. Criteria values for the HTL-pathway were derived from laboratory tests while those for the AD-pathway were compiled from literature. Of the 16 process alternatives evaluated, the AD-pathway including nitrogen-recovery by air-stripping and phosphorus recovery by the MEPHREC® process ranked first followed by the HTL-pathway. This multi-criteria study suggests that the HTL-pathway could be engineered as a superior alternative for sludge stabilization and resource recovery if phosphorus recovery and its technology readiness level could be improved.

Utilizing a Fe3O4 Magnetite Nanoparticle for Anode Modification in a Microbial Desalination Cell to Treat Saltwater

The microbial desalination cell (MDC) is a bio-electrochemical system that exhibits the ability to oxidize organic compounds, produce energy, and decrease the saline concentrations within the desalination chamber. The selective removal of ions from the desalination chamber is significantly influenced by the anion and cation exchange membranes. In this study, a three-chamber microbial desalination cell was developed to treat seawater using a synthesize Fe3O4 magnetite nanoparticle (MNP)-modified anode. The impact of different performance parameters, such as temperature, pH, and concentrations of NPs, has been investigated in order to assess the performance of three-chamber MDCs in terms of energy recovery and salt removal. The evaluation criteria of the system included multiple factors such as chemical oxygen demand (COD), Coulombic efficiency (CE), desalination efficiency, as well as system aspects including voltage generation and power density. The highest COD% removal efficiency was 74% at 37 °C, pH = 7, and 30 g/L salt concentration with an optimized NPs concentration of 2.0 mg/cm2 impregnated on anode. The maximum Coulombic efficiency was 10.3% with the maximum power density of 4.3 W/m3. The effect of the nanoparticle concentration impregnated on the anode was clarified by the primary factor of analysis. This research has revealed consistent patterns in the enhancement of voltage generation, COD, and Coulombic efficiencies when incorporating higher concentrations of nanoparticles on the anode at a certain point.

Effects of food wastes based on different components on digestibility and energy recovery in hydrogen and methane co-production

This study was conducted for four organic fractions (carbohydrates, proteins, cellulose, lipids) at an inoculum concentration of 30 % and a total solid (TS) of 8 % to investigate the effect of the main components of food waste on the performance of the two-stage anaerobic digestion. The results showed that the gas phase products were closely related to the composition of the substrate, with the carbohydrate and lipid groups showing the best hydrogen (154.91 ± 2.39mL/gVS) and methane (381.83 ± 12.691mL/gVS) production performance, respectively. However, the increased protein content predisposes the system to inhibition of gas production, which is mutually supported by changes in the activity of dehydrogenase and coenzyme F420. Butyric acid (53.19 %) dominated the liquid phase products in both stages, indicating that all four organic fractions were butyric acid-based fermentation and that the final soluble chemical oxygen demand degradation reached 72.97 %-82.86 %. The carbohydrate and cellulose groups achieved the best energy recovery performance, with conversion rates exceeding 65 %. The above results can provide a useful reference for the resource utilization of food waste.

Co-pyrolysis of sewage sludge and biomass waste into biofuels and biochar: A comprehensive feasibility study using a circular economy approach

Enormous annual sewage sludge (SS) volumes pose global environmental challenges owing to contamination and significant greenhouse gas emissions. Here, we investigated the economic viability of co-pyrolyzing SS and biomass waste to produce biofuels (bio-oil and gas) and biochar. Net present worth (NPW) analysis, the sale product break-even price, and sludge handling price (SHP) were used to determine the profitability of co-pyrolysis compared with SS pyrolysis alone and conventional treatment methods. In this study, the sale prices of biochar based on quality (i.e., stability, carbon sequestration effectiveness, and heavy metal content) were estimated to be 2.24, 1.44, and 0.98 CAD/kg for high-, medium-, and low-grade biochar. The bio-oil prices, estimated based on the higher heating values of bio-oil and diesel, ranged from 0.80 to 1.22 CAD/kg. Sawdust (SD) and wheat straw (WS) were the chosen co-pyrolysis feedstocks, with four mixing ratios (20, 40, 60, and 80 wt%). Economically, SD (40 wt% mixing ratio) co-pyrolysis achieved the best performance, with a maximum NPW of 8.71 million CAD. SD single and co-pyrolysis were the only profitable scenarios. Moreover, SS single pyrolysis and WS co-pyrolysis exhibited higher profitability than conventional SS treatment methods, with SHPs of 65 and 40 CAD/1000 kg dry sludge, respectively. Sensitivity analysis highlighted the dependence of economic performance on biochar and bio-oil market value. This study offers the first economic analysis of this approach and enhances our understanding of the potential of co-pyrolysis for biofuel and biochar production, providing innovative solutions for the environmental challenges of SS disposal.

The effect of total ankle arthroplasty on mechanical energy exchange

Total ankle arthroplasty (TAA) is a common surgical solution for patients with debilitating arthritis of the ankle. Prior to surgery patients experience high levels of pain and fatigue and low mechanical energy recovery. It is not known if TAA restores healthy levels of mechanical energy recovery in this patient population. This study was designed to determine whether mechanical energy recovery was restored following TAA. Ground reaction forces during self-selected speed walking were collected from patients with symptomatic, unilateral ankle arthritis (N = 29) before and one and two years after primary, unilateral TAA. The exchange of potential (PE) and kinetic (KE) energy was examined, and direction of change (%congruity) and energy exchange (%recovery) between the two curves was calculated, with those subjects with low congruity experiencing high energy recovery. Linear regressions were used to examine the impact of walking speed, congruity, and amplitude of the center of mass (COM) displacement on %recovery, while ANOVA and ANCOVA models were used to compare energy recovery and congruity across the three time points. Gender, BMI, and age at surgery had no effect in this study. TAA improved walking speed (p = 0.001), increased energy recovery (p = 0.020), and decreased congruity (p = 0.002), and these levels were maintained over at least two years. Differences in congruity were independent of walking speed. In some patients, especially those who are severely debilitated by ankle arthritis, TAA is effective in restoring mechanical energy recovery to levels similar to an asymptomatic population of a similar age recorded by other studies.

Strategies of managing solid waste and energy recovery for a developing country - A review

Solid waste is considered one of the major pollutants of both water and surface worldwide. The growing global population, urban expansion, and industrial growth are the main reasons for solid waste generation. This has become a major challenge with both regional and worldwide consequences. The yearly generation of municipal solid wastes around the world is 2.01 BT (billion tons) among which about 33 % are not ecologically handled. To address this, proper solid waste management, especially recycling waste products, is crucial to achieving sustainability. High-income countries are able to recycle 51 % of their waste, while low-income countries only recycle 16 % of their waste. Inadequate solid waste management practices can only compound environmental and social problems. To handle these issues thermochemical and biochemical methods are used to convert solid waste to energy. Thermochemical method is suitable for developing countries though it is energy extensive. This review provides a detailed analysis of developing countries' solid waste management and energy recovery. It explores energy recovery technologies, including thermochemical and biochemical waste conversion processes.

Two-Stage and One-Stage Anaerobic Co-digestion of Vinasse and Spent Brewer Yeast Cells for Biohydrogen and Methane Production

This study aimed to evaluate the two-stage and one-stage anaerobic co-digestion of vinasse and spent brewer yeast cells (SBY) for biohydrogen and methane production. Optimization of the vinasse-to-SBY ratio and fly ash concentration of the two-stage and one-stage production processes was investigated. In the two-stage process, the vinasse-to-SBY ratio and fly ash concentration were optimized, and the leftover effluent was used for methane production. The optimum conditions for biohydrogen production were a vinasse-to-SBY ratio of 7:3% v/w and fly ash concentration of 0.4% w/v, in which the maximum hydrogen yield was 43.7 ml-H2/g-VSadded. In contrast, a vinasse-to-SBY ratio of 10:0% v/w and fly ash concentration of 0.2% w/v were considered optimal for methane production, and resulted in a maximum methane yield of 214.6 ml-CH4/g-VSadded. For the one-stage process, a vinasse-to-SBY ratio of 10:0% v/w and fly ash concentration of 0.1% w/v were considered optimal, and resulted in a maximum methane yield of 243.6 ml-CH4/g-VSadded. In the two-stage process, the energy yield from hydrogen (0.05-0.47 kJ/g-VSadded) was 0.62%-11.78%, and the major fraction was approximately 88.22%-99.38% gain from methane (3.19-7.73 kJ/g-VSadded). For the one-stage process, the total energy yield distribution ranged from 4.20 to 8.77 kJ/g-VSadded.

Global trends of waste-to-energy (WtE) technologies in carbon neutral perspective: Bibliometric analysis

Waste-to-energy (WtE) technology is at the forefront of low-carbon municipal solid waste (MSW) treatment. MSW has been favoured by researchers in recent years due to its high potential to dispose of resources with WtE technology, which contributes to the carbon neutrality goal. However, there is a lack of research that integrates MSW WtE treatment from a global perspective and explores its future direction. Bibliometric methods are widely used because of their advantages in qualitative and quantitative literature information analysis. A comprehensive search was conducted in the Web of Science (WOS) Core Collection database, covering the period from 1990-2022, resulting in the collection of 702 articles. Subsequently, bibliometric software such as CiteSpace, VOSviewer, and Bibliometrix, were jointly employed for co-occurrence, co-citation, and cluster analyses, providing an in-depth qualitative and quantitative analysis of the research hotspots and development trends of WtE technology for MSW treatment. The research findings indicate a rapid growth in studies on carbon emission reduction through WtE technology for MSW treatment since 2015, with these related articles accounting for 50% of articles. Globally, China, the United States, Italy, and other countries were active research regions, with Chinese research institutions making the largest contributions. However, contributions from developing countries are limited. Furthermore, this study systematically elaborates on the research hotspots in this field. Finally, this study identified some frontier research hotspots and directions. Research on WtE technology primarily focuses on technological methods and policy management, particularly from the carbon neutrality perspective, emphasis WtE technology sustainability in reducing carbon emissions and achieving carbon neutrality goals. Promoting the use of assisted decision-making models in the MSW management process, and focusing on the conversion of food waste into valuable energy. It is hoped that these research directions will provide new ideas for the balanced and rapid development of WtE technology.

Three dimensional nickel foam carried sea urchin-like copper-cobalt-cerium cathode for enhanced tetracycline wastewater purification in photocatalytic fuel cell

Photocatalytic fuel cells (PFCs) regarded as a potential sustainable technique, have been broadly reported. In this work, the carbon quantum dot-loaded TiO2 photoanode and sea urchin-like CuCoCe ternary metal oxide cathode materials are successfully synthesized and used to construct PFC systems for efficient tetracycline (TC) degradation (45 mg/L) and simultaneous electricity generation. The results demonstrate that the CQDs-modified TiO2 photoanode has improved absorption intensity in both the UV and visible regions, and the photocurrent density at 1.23 V vs RHE reached 1.31 mA cm-2, which is 1.3 times higher than that of the original TiO2 photoanode. The established PFC system achieves the highest removal ratio of 96.9 % for TC in 60 min with a maximum power density of 0.77 mW cm-2. The PFC system can operate efficiently over a wide pH range (3.0-9.0). Furthermore, quenching experiments and ESR spectra show that the main reactive oxygen species in the degradation process are •O2-, 1O2 and •OH. This study provides meaningful way to develop multiple metal oxides as cathode of PFC system for efficient organic pollutant degradation and energy recovery.

Towards the implementation of hydrothermal carbonization for nutrients, carbon, and energy recovery in centralized biogas plant treating sewage sludge

In recent years, extensive experimental research on hydrothermal carbonization (HTC) of sewage sludge has been performed, to study the effects of process conditions on hydrochar characteristics and nutrient, carbon, and energy recovery from sewage sludge. To promote the implementation of HTC, this study assessed HTC (230 °C, 30 min) integration into an advanced centralized biogas plant by analyzing its theoretical effects on the fates of sewage sludge solids, nitrogen, phosphorus, and carbon. The study used the mass and nutrient flows and concentrations obtained from laboratory studies, and the studied biogas plant had an original layout that employed hygienization. HTC integration decreased the solid product volume by up to 56 % and, increased the recovery of ammonium in ammonia water by 33 % and methane by 1.4 %, while increasing the biogas plant energy demand by 4 %. The changes in the nutrient and solids flows and their recovery potentials show the need to consider the rearrangements of the liquid and gas flows in the biogas plant and the re-dimensioning of stripping process.

Are algae a promising ecofriendly approach to micro/nanoplastic remediation?

How to reduce microplastic pollution in aquatic ecosystem has become the focus of the global attention. The re-removal of microplastics of wastewater treatment plant (WWTP) effluent is gradually being put on the agenda. Recently, algae have been used as an ecofriendly remediation strategy for microplastic removal. Microplastics in sewage can be removed by algae through interception, capture, and entanglement, and can also form heterogeneous aggregates with algae, thereby reducing their free suspensions. Algae can recover nitrogen and carbon from wastewater and can be made into biochar, biofertilizers, and biofuels. However, problematically, this technology has been in the laboratory research stage, and existing research results cannot provide effective basis for its application. Microplastic removal via algae is influenced by wastewater flow rate, microplastic types, and pollutants. Microplastics are only physically fixed by algae, and ensuring that microplastics do not re-enter the environment during resource and capacity recovery is also a key factor limiting the implementation of this technology. The topic of this paper is to discuss the performance of the current tertiary wastewater treatment process - algae process to remove microplastics. Algae can remove nitrogen and phosphorus pollutants in sewage and remove microplastics at the same time, which can realize energy recovery and reduce ecological risks of the effluent. Although algae combined tertiary sewage treatment is a green technology for microplastic removal, its application still needs to be explored. The key challenges that need to be addressed, from single laboratory conditions to complex conditions, from small-scale testing to large-scale simulations, lie ahead of the application of this friendly technology.

Assessing the energy recovery potential at district metered areas inlets of water supply systems: A Spanish case study

The energy required for various processes in the water cycle can have significant economic and environmental impacts. Therefore, efficient energy management in urban water supply systems is crucial for a sustainable operation. By installing energy recovery technologies in these facilities, it is possible to reap the benefits of the infrastructure design by saving energy. In this study, a new methodology to assess the energy recovery at the inlets of district metered areas is presented, considering the city of Murcia (Spain) as case study. This methodology is based on creating a detailed model of city water supply system and calibrating such model with an experimental campaign of measurements. Then, the assessment of the hydraulic potential recovery is analysed through two different energy estimators, one considering the minimum available net head and the other assuming a variable net head. Results show that there are several points where turbines could be installed, most of them recovering in between 1000-5000 kWh, which could be used to cover the yearly energy consumption of about 24-120 m2 of a school or 10-50 traffic lights of such area. Moreover, in some points it could be recovered up to 14500 kWh. Even though these values are not high, the energy recovered could be used for self-consumption of nearby electrical loads, at the time that reduces the pressure in the system, thus leading to leak reductions. Moreover, this kind of energy recovery does not reduce the potential of other proposals for upstream energy recovery, such as replacing pressure reduction valves with turbines instead. The scripts developed to apply the proposed methodology are available in EPANET-Octave file exchange for the researcher community.

Life cycle assessment and cost-benefit analysis of small-scale anaerobic digestion system treating food waste onsite under different operational conditions

This study employed life cycle assessment and cost-benefit analysis to evaluate the environmental and economic profile of a real decentralized small-scale anaerobic digestion (AD) system treating food waste (FW). Different operational conditions, including temperature, biochar addition, biogas engine efficiency, and FW loading, were compared via scenario analysis. Biochar addition could potentially obtain carbon reduction and save fossil fuel. Moreover, at high FW loading and biogas engine efficiency, biochar addition achieved 1-3190% better performance than the system without biochar in all the nine impact categories. The system under mesophilic conditions performed worse than ambient conditions due to high energy demand. All the current scenarios resulted in a monetary loss at US$ 480 k-681 k, while profit was possible if the capital cost and operator salary decreased significantly. Overall, operating the small-scale AD system under ambient temperature with biochar addition was preferred due to its potential environmental benefits and economic profits.

Analysing the mechanism of food waste anaerobic digestion enhanced by iron oxide in a continuous two-stage process

The food waste (FW) digestion performance can be enhanced by introducing iron oxide (IO) into digesters. However, the role of IO in continuous two-stage digesters in enhancing the FW anaerobic digestion remains unclear. In this study, the effect of IO on the bioenergy recovery from a two-stage digestion process was investigated. The bioenergy recovery was significantly increased by up to 208.43 % with IO addition. The activities of dehydrogenase, α-amylase, and protease increase by 0.82-1.44, 7.24-14.56 and 7.97-20.45 times, respectively, as compared with that of the blank. With IO addition, the metabolic pathway in hydrolytic-acidogenic (HA) reactor shifted from lactic acid fermentation to butyric fermentation, which promoted stable methane production in methanogenic (MG) reactor. The activity of coenzyme F420 increased by 19.19-39.01 times, indicating that IO facilitated FW digestion by promoting hydrogenotrophic methanogenesis. The enhancement in the enzyme activity was attributable to the Fe2+ generated by dissimilatory iron reduction. According to the microbial analysis, IO enhanced interspecies hydrogen transfer between Methanobacterium and Syntrophomonas. Furthermore, IO improved direct interspecies electron transfer between Geobacter sulfurreducens and Methanosarcina. The effluent recirculation strategy greatly facilitated the hydrolysis and acidification of FW, which was critical for improving the two-stage process performance.

Combination of partial nitrification and microbial fuel cell for simultaneous ammonia reduction, organic removal, and energy recovery

The chemical oxygen demand (COD) in municipal wastewater has become an obstacle for anammox in mainstream applications. In this study, the single chamber microbial fuel cell (MFC) was installed as an influent device for a partial nitrification-sequencing batch reactor (PN-SBR) to realize integrating COD removal and partial nitrification. After 80 days of operation, the nitrite accumulation rate reached 93%, while the COD removal efficiency was 56%. The output voltage and the power density of MFC were 66.62 mV and 2.40 W/m3, respectively. The content of EPS, especially polysaccharides in the stable phase, has increased compared with the seed sludge. The most dominant genus in MFC anode biofilm and SBR granular sludge was Thauera, which has organic compounds degradation capacity and could degrade nitrate. This study revealed the microbial interaction between MFC and partial nitrification and provided a new strategy for stable ammonia and nitrite supply for mainstream anammox plants.

Valorization of slow pyrolysis vapor from biomass waste: Comparative study on pyrolysis characteristics, evolved gas evaluation, and adsorption effects

Pyrolysis vapor is an important byproduct in the production of biochar from biomass waste, and its emission may pose potential environmental risks. To achieve green production of biochar and efficient utilization of pyrolysis vapors, a novel strategy is proposed in this study to use pristine biochar as an adsorbent to adsorb the pyrolysis vapors. According to thermogravimetry-Fourier infrared spectroscopy-mass spectrometry evaluation, the evolved vapors mainly consisted of oxygenated compounds, hydrocarbons, CO2, CO, and H2O. With pyrolysis temperature increasing, ethers, phenols, hydrocarbons, acids/ketones, and CO2 were changed in the same direction based on two-dimensional correlation spectroscopy analysis. Moreover, butene, propargyl alcohol, and butane were the most abundant ionic fragments. After adsorbing pyrolysis vapors, the heating value of the biochar increased by a maximum of 3.2 MJ kg-1 with changes of physicochemical properties. This strategy provides a theoretical basis for green preparation of biochar while recovering energy from pyrolysis vapors.

Co-hydrothermal carbonization of swine manure and lignocellulosic waste: A new strategy for the integral valorization of biomass wastes

Co-hydrothermal carbonization (co-HTC) is a promising strategy to improve hydrothermal carbonization (HTC) of low-quality wastes. HTC of swine manure (SM), with high N (2.9 wt%), S (0.7 wt%) and ash (22.6 wt%) contents, as well as low C (35.6 wt%) and higher heating value (HHV; 14.3 MJ kg-1), resulted in a hydrochar with unsuitable characteristics as a solid fuel. Co-HTC of SM and garden and park waste (GPW) improved hydrochar properties (C content (43 - 48 wt%) and HHV (18 - 20 MJ kg-1), and decreased N (∼2 wt%), S (<0.3 wt%) and ash (<15 wt%) content. A high GPW ratio (>50 wt%) during co-HTC resulted in a hydrochar similar to that obtained from GPW. The co-HTC increased nutrient migration to the process water, which allowed the precipitation of salt with high P (7.8 wt%) and negligible heavy metal content. Anaerobic digestion of co-HTC process water allowed high organic matter removal (up to 65%), and methane production (315 - 325 mL CH4 g-1CODadded). Gross energy recovery by HTC and anaerobic digestion was 5 - 6-fold higher than anaerobic treatment of feedstocks. Therefore, co-HTC of SM and GPW with a ratio > 50% GPW proved to be a suitable approach to valorize and manage SM and obtain value-added products (hydrochar, mineral fertilizer and methane).

Impact of sandwich-type composite anodic membrane on membrane fouling and methane recovery from sewage sludge and food waste via electrochemical anaerobic membrane bioreactor

Membrane fouling presents a big challenge for the real-world implementation of anaerobic membrane bioreactors (AnMBRs) in digesting high-solid biowastes. In this study, an electrochemical anaerobic membrane bioreactor (EC-AnMBR) with a novel sandwich-type composite anodic membrane was designed and constructed for controlling membrane fouling whilst improving the energy recovery. The results showed that EC-AnMBR produced a higher methane yield of 358.5 ± 74.8 mL/d, rising by 12.8% compared to the AnMBR without applied voltage. Integration of composite anodic membrane induced a stable membrane flux and low transmembrane pressure through forming an anodic biofilm while total coliforms removal reached 97.9%. The microbial community analysis further provided compelling evidence that EC-AnMBR enriched the relative abundance of hydrolyzing (Chryseobacterium 2.6%) bacteria and methane-producing (Methanobacterium 32.8%) archaea. These findings offered new insights into anti-biofouling performance and provided significant implications for municipal organic waste treatment and energy recovery in the new EC-AnMBR.

Energy recovery in a commercial building using pico-hydropower turbines: An Australian case study

Optimising energy use in systems and buildings is crucial to reduce climate change. This paper aims to address the gap in knowledge for pico-hydropower (<5 kW) that has been identified as an area of untapped potential in the water industries. A literature review and multivariate analysis are used to find a suitable pico-hydro turbine to install into a coral reef aquarium system in a government owned facility. Key findings from the literature review are untapped potential, gaps in knowledge and global quantification of small hydropower for energy recovery, and lack of enabling data contributing to slow uptake of small hydropower. The study showed a propeller pico-hydropower turbine could be used to recover approximately 10% of the energy used for pumping water through a filtration system. At 2.3 m available head, and 90 L/s water flow, power output up to 1.124 kW was achieved. The project was economically viable with financial and non-financial benefits for the life cycle of the product. There remain sparse case studies for energy recovery using small hydropower in the scientific literature. A growing number of authors see the potential of this renewable energy technology to reduce global greenhouse gas emissions and contribute to the UN Sustainable Development Goals to provide affordable clean energy and address climate change. This study helps to shine a light on opportunities to find value from waste using a novel application of hydropower in a water industry.

Nitrogen-containing wastewater fuel cells for total nitrogen removal and energy recovery based on Cl•/ClO• oxidation of ammonia nitrogen

The excess nitrogen discharge into water bodies has resulted in extensive water pollution and human health risks, which has become a critical global issue. Moreover, nitrogenous wastewater contains considerable chemical energy contributed by organic pollutants and nitrogenous compounds. Therefore, the treatment of various kinds of nitrogen-containing wastewater for nitrogen removal and energy recovery is of significance. Biological methode and advanced oxidation processes (AOPs) are the main methods for nitrogen removal. However, biological treatment is easily inhibited by high-salinity, high ammonia nitrogen (NH₃-N/NH₄⁺-N), nitrite and toxic organics in wastewater, which limits its application. AOPs mainly induce in situ generation of highly reactive species, such as hydroxyl radical (HO•), sulfate radical (SO₄•^-) and chlorine radicals (Cl•, ClO•, Cl₂•^-), for nitrogen removal. Nevertheless, HO• shows low reactivity and N₂ selectivity towards NH₃-N/NH₄⁺-N oxidation, and SO₄•^- also demonstrates unsatisfactory NH₃-N/NH₄⁺-N removal. It has been shown that Cl•/ClO• can efficiently remove NH₃-N/NH₄⁺-N with high N₂ selectivity. The generation of Cl•/ClO• can be triggered by various techniques, among which the PEC technique shows great potential due to its higher efficiency for Cl•/ClO• generation and eco-friendly approach for pollutants degradation and energy recovery by utilizing solar energy. Cl•/ClO• oxidation of NH₃-N/NH₄⁺-N and nitrate nitrogen (NO₃^--N) reduction can be strengthened through the design of photoanode and cathode materials, respectively. Coupling with this two pathways, an exhaustive total nitrogen (TN) removal system is designed for complete TN removal. When introducing the mechanism into photocatalytic fuel cells (PFCs), the concept of nitrogen-containing wastewater fuel cells (NFCs) is proposed to treat several typical types of nitrogen-containing wastewater, achieving high-efficiency TN removal, organics degradation, toxic chlorate control, and energy recovery simultaneously. Recent research progress in this field is reviewed, summarized and discussed, and in-depth perspectives are proposed, providing new ideas for the resource treatment of nitrogen-containing wastewater.

Transformation from biofiltration unit to hybrid constructed wetland-microbial fuel cell: Improvement of wastewater treatment performance and energy recovery

This study aimed to compare the performance of biofiltration, constructed wetland, and constructed wetland microbial fuel cell (CW-MFC). The transformation from a biofiltration unit to a hybrid CW-MFC was demonstrated with the advantages of improvement of wastewater treatment while generating electricity simultaneously. The introduction of plants to the upper region of the bioreactor enhanced the DO level by 0.8 mg/L, ammonium removal by 5 %, and COD removal by 1 %. The integration of electrodes and external circuits stimulated the degradation rate of organic matter in the anodic region (1 % without aeration and 3 % with aeration) and produced 5.13 mW/m³ of maximum power density. Artificial aeration improved the nitrification efficiency by 38 % and further removed the residual COD to an efficiency of 99 %. The maximum power density was also increased by 3.2 times (16.71 mW/m³) with the aid of aeration. In treating higher organic loading wastewater (3M), the maximum power density showed a significant increment to 78.01 mW/m³ (4.6-fold) and the COD removal efficiency was 98 %. The ohmic overpotential dominated the proportion of total loss (67-91 %), which could be ascribed to the low ionic conductivity. The reduction in activation and concentration loss contributed to the lower internal resistance with the additional aeration and higher organic loading. Overall, the transformation from biofiltration to a hybrid CW-MFC system is worthwhile since the systems quite resemble while CW-MFC could improve the wastewater treatment as well as recover energy from the treated wastewater.

Hydrochar mediated anaerobic digestion of bio-wastes: Advances, mechanisms and perspectives

Bio-wastes treatment and disposal has become a challenge because of their increasing output. Given the abundant organic matter in bio-wastes, its related resource treatment methods have received more and more attention. As a promising strategy, anaerobic digestion (AD) has been widely used in the treatment of bio-wastes, during which not only methane as energy can be recovered but also their reduction can be achieved. However, AD process is generally disturbed by some internal factors (e.g., low hydrolysis efficiency and accumulated ammonia) and external factors (e.g., input pollutants), resulting in unstable AD operation performance. Recently, hydrochar was wildly found to improve AD performance when added to AD systems. This review comprehensively summarizes the research progress on the performance of hydrochar-mediated AD, such as increased methane yield, improved operation efficiency and digestate dewatering, and reduced heavy metals in digestate. Subsequently, the underlying mechanisms of hydrochar promoting AD were systematically elucidated and discussed, including regulation of electron transfer (ET) mode, microbial community structure, bio-processes involved in AD, and reaction conditions. Moreover, the effects of properties of hydrochar (e.g., feedstock, hydrothermal carbonization (HTC) temperature, HTC time, modification and dosage) on the improvement of AD performance are systematically concluded. Finally, the relevant knowledge gaps and opportunities to be studied are presented to improve the progress and application of the hydrochar-mediated AD technology. This review aims to offer some references and directions for the hydrochar-mediated AD technology in improving bio-wastes resource recovery.

Towards powerful magnetocaloric devices with static electro-permanent magnets

INTRODUCTION

Magnetocaloric energy conversion represents an alternative to existing refrigeration, heat pump and energy harvesting technologies. A crucial part of a magnetocaloric device concerns the magnetic field source. It uses mainly rare-earth materials and consists of moving parts and a drive system while displaying a limited energy efficiency and unavailability of fast and variable control of the magnetic field. Recent advances in efficient heat transfer for high-frequency magnetic cooling call for new developments of magnetic field sources that can operate with high efficiency at high frequencies.

OBJECTIVES

We report the concept of an electro-permanent magnetic (EPM) field source that efficiently recovers magnetic energy. In contrast to existing magnets, it allows very well-controlled operation without any moving parts. The main objective of this paper is to present a numerical and experimental study in which such an EPM was designed, built and tested.

METHODS

An extensive numerical investigation of the proposed design was carried out in terms of various geometrical and operating parameters. One of the design variations was built and experimentally evaluated for its energy efficiency and temperature increase at various operating frequencies.

RESULTS

We demonstrate an energy efficiency of these magnets of over 80% and operation with frequencies up to 50 Hz, which is crucial for future high-power-density and high-frequency magnetocaloric devices.

CONCLUSIONS

Considering high energy efficiency at high operating frequencies, such EPMs would allow for miniaturization, making them a viable option for future compact magnetocaloric devices.

A novel strategy for efficiently transforming waste activated sludge into medium-chain fatty acid using free nitrous acid

The global energy crisis is approaching due to rapid population growth and overexploitation of fossil fuels. Therefore, the development and use of new and renewable energy sources is already in the extreme urgency. This work developed a novel technology to efficiently produce renewable liquid bioenergy from discarded wastes, by effectively transforming sewage sludge into high-value medium chain fatty acids (MCFA). The maximum MCFA yield in the anaerobic sludge fermentation was revealed to be 10.6 times of control when utilizing sewage sludge with 1.78 mg-N/L free nitrous acid (FNA) pretreatment. The carbon flow from sewage sludge into MCFA in the fermentation system was significantly enhanced with appropriate levels (0.71-1.78 mg-N/L) of FNA pretreatment. Compared to FNA pretreatment, however, its direct addition severely inhibited total products (i.e., carboxylates and complex alcohols) generation because of the toxicity on live cells (decreasing to 8.3 %-13.9 %) in sludge. Kinetic models (one-substrate and two-substrate) were utilized to investigate the mechanism of MCFA promotion by FNA pretreatment on anaerobic sludge fermentation, in which linear relationship analysis between FNA-derived organic release and the fitted parameters were also performed. The results indicated that the conversion of refractory materials into rapidly bioavailable substrates for MCFA production contributed to increasing MCFA production rate and potential. Moreover, the relative abundances of functional microorganisms related to hydrolysis-acidification and chain elongation process increased under FNA pretreatment, further favoring the MCFA production. This study provides a novel and effective technology of sludge energy recovery that can achieve the next-generation sustainable sewage sludge management.

Performance evaluation and energy potential analysis of anaerobic membrane bioreactor (AnMBR) in the treatment of simulated milk wastewater

An anaerobic membrane bioreactor (AnMBR) was employed as primary treatment unit for anaerobic treatment of simulated wastewater to produce high effluent quality. A lab scale hollow fiber membrane was used to scrutinize the performance of AnMBR as a potential treatment system for simulated milk wastewater and analyze its energy recovery potential. The 15 L bioreactor was operated continuously at mesophilic conditions (35 °C) with a pH constant of 7.0. The membrane flux was in the range of 9.6-12.6 L/m². h. The different organic loading rates (OLRs) of 1.61, 3.28, 5.01, and 8.38 g-COD/L/d, of simulated milk wastewater, were fed to the reactor and the biogas production rate was analyzed, respectively. The results revealed that the COD removal efficiencies of 99.54 ± 0.001% were achieved at the OLR of 5.01 gCOD/L/d. The highest methane yield was found to be at OLR of 1.61 gCOD/L/d at HRT of 30 d with the value of 0.33 ± 0.01 L-CH₄/gCOD. Moreover, based on the analysis of energy balance in the AnMBR system, it was found that energy is positive at all the given HRTs. The net energy production (NEP) ranged from 2.594 to 3.268 kJ/gCOD, with a maximum NEP value of 3.268 kJ/gCOD at HRT 10 d HRT. Bioenergy recovery with the maximum energy ratio, of 4.237, was achieved with an HRT of 5 d. The study suggests a sizable energy saving with the anaerobic membrane process.

Insights into the decolorization of mono and diazo dyes in single and binary dyes containing wastewater and electricity generation in up-flow constructed wetland coupled microbial fuel cell

The treatment of single and binary azo dyes, as well as the effect of the circuit connection, aeration, and plant on the performance of UFCW-MFC, were explored in this study. The decolorization efficiency of Remazol Yellow FG (RY) (single dye: 98.2 %; binary dye: 92.3 %) was higher than Reactive Black 5 (RB5) (single: 92.3 %; binary: 86.7 %), which could be due to monoazo dye (RY) requiring fewer electrons to break the azo bond compared to the diazo dye (RB5). In contrast, the higher decolorization rate of RB5 in binary dye indicated the removal rate was affected by the electron-withdrawing groups in the dye structure. The closed circuit enhanced about 2% of color and 4% of COD removal. Aeration improved the COD removal by 6%, which could be contributed by the mineralization of intermediates. The toxicity of azo dyes was reduced by 11-26% and the degradation pathways were proposed. The dye removal by the plants was increased with a higher contact time. RB5 was more favorable to be uptook by the plant as RB5 holds a higher partial positive charge. 127.39 (RY), 125.82 (RB5), and 58.66 mW/m³ (binary) of maximum power density were generated. The lower power production in treating the binary dye could be due to more electrons being utilized for the degradation of higher dye concentration. Overall, the UFCW-MFC operated in a closed circuit, aerated, and planted conditions achieved the optimum performance in treating binary azo dyes containing wastewater (dye: 87-92%; COD: 91%) compared to the other conditions (dye: 83-92%; COD: 78-87%).

Face mask and medical waste generation in the City of Baguio, Philippines: its current management and GHG footprint

The daily use of facemask to prevent virus transmission increases the negative effect on the environment because of improper waste disposal. Due to the absence of baseline data, the impact of facemask and medical waste generation, as well as the community's management practice, should be studied to avoid further environmental degradation. In this study, we surveyed 384 respondents and conducted computational analysis to provide an overview of the household's facemask usage and ecological footprint in combating Covid-19. Results showed that most respondents (48.7%) use two facemasks per day. Thus, an estimated 417,834 facemasks are disposed daily, generating 3,585 kg/day of additional waste. The average medical waste of Covid-infected individuals is 3.29 kg per day per capita. This yields 22,438 kg. of CO₂ eq., which could contribute to the global warming potential; however, there is also a potential recovery of 61.572 gigajoules of energy for power generation. Most respondents are aware of proper facemask waste management practices, but some lacks application regarding responsible waste disposal. Despite the contribution of facemask to the overall solid waste generation, the city's current management remains a challenge since disposable facemasks are potentially mixed with other types of waste from its storage, collection, and disposal.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10163-023-01601-2.

Swine manure treatment technologies as drivers for circular economy in agribusiness: A techno-economic and life cycle assessment approach

Anaerobic digestion has been employed as a technology capable of adding value to waste coupled with environmental impact mitigation. However, many issues need to be elucidated to ensure the systems viability based on this technology. In this sense, the present study evaluated technically, environmentally, and economically, four configurations of swine waste treatment systems focused on the promotion of decarbonization and circularity of the swine chain. For this, a reference plant, based on a compact treatment process named SISTRATES® (Portuguese acronym for swine effluent treatment system) was adopted to serve as a model for comparison and validation. The results showed the importance of prioritization of the energy recuperation routes through anaerobic digestion, providing increased economic benefits and minimizing environmental damage. Thus, the SISTRATES® configuration was the one that presented the best designs in a circular context, maximizing the recovery of energy and nutrients, along with the reduction of greenhouse gas emissions, ensuring the sustainability of the pig production chain.

Development of waste-to-energy through integrated sustainable waste management: the case of ABREN WtERT Brazil towards changing status quo in Brazil

In the context of circular economy, it is known that once waste is generated, it should be subject to proper treatment for recovering material or energy before being disposed. Many countries worldwide, especially developing countries such as Brazil, have been struggling to effectively apply sustainable waste management in municipalities and still rely on dumpsites and unsuitable landfills. Misinformation, a weak legal framework, lack of financial resources and poor infra-structure as well as pressure from organizations profiting from the expansion of landfills are some factors contributing to the preservation of the negative status quo: the "landfill culture". Material recovery, i.e., recycling and composting, is applied to less than 5% of Brazilian municipal waste, while 95% is disposed of in landfills or dumpsites. In this context, ABREN WtERT (Waste-to-Energy Research and Technology Council) Brazil was created in 2019 as the first permanent organization formed to promote the development of energy and material recovery from waste focused on the waste-to-energy (WTE) market. In this paper, the strategy proposed and implemented by the organization towards changing the status quo in Brazil through an integrated sustainable waste management approach is described. The proposed strategy integrates the concepts of Sustainability and Circular Economy for minimizing landfill disposal (avoiding methane emissions) and maximizing material/energy recovery. Among others, the approach focuses on changing the public opinion regarding thermal treatment facilities, mainly incinerators, which has been wrongly linked to pollution, excessive public expenditures and considered a harm to the recycling industry. The activities performed by ABREN include engaging public and private institutions, enhancing education, leading the publication of research and business studies, gathering industry members and academy experts, as well as creating strategic alliances with players around the globe. As a result, within a few years, major outcomes were achieved in Brazil, such as: (i) changes in the legal framework, (ii) launching of a specific public auction category for sponsoring electricity production from WTE facilities, and (iii) establishment of official targets for municipalities to decrease landfill disposal and increase recycling/biological treatment and energy recovery from thermal treatment. Among the national goals, it should be highlighted the target regarding the increase from zero to 994 MW of electricity production from municipal solid waste, which will require building dozens of new WTE facilities. Global outcomes are expected as well since Brazil is the seventh largest country of the globe and the most influential in Latin America. International and national business deals should thrive due to the need of operational skills and technology imports, and the avoidance of carbon emissions will positively reflect the world climate. In parallel, there is also potential for the academy to benefit from research projects and investments if the WTE national industry is to be developed in the long term.

Application of concatenated stepped solar still system (CS4) for RO-waste-water purification: an experimental study

This experimental study investigates the capability of a concatenated stepped solar still system (CS₄) for RO-waste-water purification by assessing its thermo-enviro-economic aspects. In order to confirm the supremacy of CS₄, a simple stepped solar still system (S5) of equivalent basin area (0.75 m²) is fabricated and tested. The experiments are conducted on CS₄ and S5 simultaneously at different flow rates for comparison. The productivity of CS₄ is observed to be 24.7 % higher than that of S5; thus, its pollutant removal efficiency is better. As compared to S5, CS₄ is found to be more sustainable, economical, and better in terms of distillate production and thermal and exergy efficiencies which are optimum at 50 ml/min. The distillate production, thermal efficiency, and exergy efficiency of CS₄ at 50 ml/min flow of RO-waste-water are 2.8 kg/day, 33.80%, and 1.93%, respectively. The second and third units in CS₄ work in active mode that increases the solar energy utilization by 34.19% at optimal flow rate. The CO₂ mitigation capability of CS₄ is 8.89 tons and its distillate production cost is $0.021. At optimum flow rate, its energy and economic payback periods are evaluated as 199 and 483 days, respectively.

Enhanced anaerobic digestion of waste activated sludge with periodate-based pretreatment

The potential of periodate (PI) in sludge anaerobic digestion is not tapped, although it has recently attracted great research interest in organic contaminants removal and pathogens inactivation in wastewater treatment. This is the first work to demonstrate significant improvement in methane generation from waste activated sludge (WAS) with PI pretreatment and to provide underlying mechanisms. Biochemical methane potential tests indicated that methane yield enhanced from 100.2 to 146.3 L per kg VS (VS, volatile solids) with PI dosages from 0 to 100 mg per g TS (TS, total solids). Electron spin resonance showed PI could be activated without extra activator addition, which might be attributed to the native transition metals (e.g., Fe²⁺) in WAS, thereby generating hydroxyl radical (•OH), superoxide radicals (•O₂ ^-), and singlet oxygen (¹O₂). Further scavenging tests demonstrated all of them synergistically promoted WAS disintegration, and their contributions were in the order of •O₂ ^- > •OH > ¹O₂, leading to the release of substantial biodegradable substances (i.e., proteins and polysaccharides) into the liquid phase for subsequent biotransformation. Moreover, fluorescence and ultraviolet spectroscopy analyses indicated the recalcitrant organics (especially lignocellulose and humus) could be degraded by reducing their aromaticity under oxidative stress of PI, thus readily for methanogenesis. Microbial community analysis revealed some microorganisms participating in hydrolysis, acidogenesis, and acetoclastic methanogenesis were enriched after PI pretreatment. The improved key enzyme activities and up-regulated metabolic pathways further provided direct evidence for enhanced methane production. This research was expected to broaden the application scope of PI and provide more diverse pretreatment choices for energy recovery through anaerobic digestion.

Characterisation and perspectives of energetic use of dissolved gas recovered from anaerobic effluent with membrane contactor

Biogas is a source of renewable energy, and its production and use has been validated in anaerobic-based sewage treatment plants (STPs). However, in these systems, a large amount of methane is lost as dissolved methane (D-CH₄) in the liquid effluent. In this study, the characteristics and potential energetic uses of the gas recovered during the desorption of D-CH₄ from anaerobic effluents with hollow fibre membrane contactors were investigated. A pilot-scale experiment was performed using sewage and two types of membrane contactors. The recovered gas contained considerable amounts of CH₄, CO₂, H₂S, N₂, and O₂; therefore, a gas upgrade is required prior to its use as a biofuel. The recovery process should be energetically self-sustainable, and induce a considerable decrease in the STP carbon footprint. Recovering D-CH₄ with membrane contactors could increase the energetic potential of anaerobic-based STPs up to 50 % and allow for more sustainable systems.

Fifty years of sewage sludge management research: Mapping researchers' motivations and concerns

Sewage sludge management is torn between a desire for pollution prevention and reuse of a valuable resource. Reconciling these interests in sustainable management is a challenge for researchers. This study focuses on how research on sewage sludge management practices has evolved and scrutinizes how this research is interlinked with concerns and societal issues such as contaminants, economic efficiency, and legislation. Based on published academic papers on sewage sludge management between 1971 and 2019, this study found four trends in research focused on sewage sludge management: a decreasing interest in disposal (landfilling and sea dumping), a dominant interest in land application, a growing interest in sewage sludge as product, and a stable interest in energy recovery. Research on disposal focuses on increasing sludge volumes, legislative changes, and economic challenges with an interest in waste co-treatment. Research on land application concerns nutrient use and contaminants, mainly heavy metals. Research on sewage sludge as a product focuses on the extraction of certain resources and less on use of sewage sludge specifically. Research on energy recovery of sewage sludge focuses on volume reduction rather than contaminants. Two-thirds of the papers are detailed studies aiming to improve single technologies and assessing single risks or benefits. As management of sewage sludge is multifaceted, the narrow focus resulting from detailed studies promotes some concerns while excluding others. Therefore, this study highlights potential gaps such as the combination of nutrient use and disposal and energy recovery and nutrient use.

Coupling high-rate activated sludge process with aerobic granular sludge process for sustainable municipal wastewater treatment

Achieving a neutral/positive energy balance without compromising discharge standards is one of the main goals of wastewater treatment plants (WWTPs) in terms of sustainability. Aerobic granular sludge (AGS) technology promises high treatment performance with low energy and footprint requirement. In this study, high-rate activated sludge (HRAS) process was coupled to AGS process as an energy-efficient pre-treatment option in order to increase energy recovery from municipal wastewater and decrease the particulate matter load of AGS process. Three different feeding strategies were applied throughout the study. AGS system was fed with raw municipal wastewater, with the effluent of HRAS process, and with the mixture of the effluent of HRAS process and raw municipal wastewater at Stage 1, Stage 2 and Stage 3, respectively. Total suspended solids (TSS), chemical oxygen demand (COD), ammonia nitrogen (NH₄⁺-N), and total phosphorus (TP) concentrations in the effluent were less than 10 mg/L, 60 mg/L, 0.4 mg/L, and 1.3 mg/L respectively at all stages. Fluctuations were observed in the denitrification performance due to changes in the influent COD/total nitrogen (TN) ratio. This study showed that coupling HRAS process with AGS process by feeding the AGS process with the mixture of HRAS process effluent and raw municipal wastewater could be an appropriate option for both increasing the energy recovery potential of WWTPs and enabling high effluent quality.

Revealing the microbial mechanism of Fe0 and MnO2 mediated microbial fuel cell-anaerobic digestion coupling system and its energy flow distribution

Microbial fuel cell-anaerobic digestion (MFC-AD) is a new sludge treatment technology with multi-path energy recovery. In this study, Fe⁰ and MnO₂ with gradient concentration were added to investigate its effects on the sludge reduction, electrochemical performance, extracellular polymeric substances (EPS) of sludge, microbial community, electron distribution and energy flow of the MFC-AD system. Results showed that the highest sludge reduction 59% (49%), was obtained at 10 g/L Fe⁰ (5 g/L MnO₂) adding and its total energy recovery efficiency increased by 100% (71%) compare to the control. Different Fe⁰ and MnO₂ concentrations lead to different microbial mechanisms: at 10 g/L Fe⁰ or 5 g/L MnO₂, it prefers to promote extracellular electrons transfer, favoring the Geobacter, Shewanella and Acinetobacter enrichment, while at 5 g/L Fe⁰ or 0.5 g/L MnO₂ it plays a more important role in substrate metabolism of anaerobic digestion, with Clostridium, Roseomonas lacus, and Methylocystis enriched. Correspondingly, the electron quantity distribution from biomass to recovered energy ends (Current, CH₄ and VFAs), was influenced by Fe⁰ and MnO₂ concentration, indicating the controllability of the energy flow.

A review on biological methodologies in municipal solid waste management and landfilling: Resource and energy recovery

Rapid industrialization and urbanization growth combined with increased population has aggravated the issue of municipal solid waste generation. MSW has been accounted for contributing tremendously to the improvement of sustainable sources and safe environment. Biological processing of MSW followed by biogas and biomethane generation is one of the innumerable sustainable energy source choices. In the treatment of MSW, biological treatment has some attractive benefits such as reduced volume in the waste material, adjustment of the waste, economic aspects, obliteration of microorganisms in the waste material, and creation of biogas for energy use. In the anaerobic process the utilizable product is energy recovery. The current review discusses about the system for approaching conversion of MSW to energy and waste derived circular bioeconomy to address the zero waste society and sustainable development goals. Biological treatment process adopted with aerobic and anaerobic processes. In the aerobic process the utilizable product is compost. These techniques are used to convert MSW into a reasonable hotspot for resource and energy recovery that produces biogas, biofuel and bioelectricity and different results in without risk and harmless to the ecosystem. This review examines the suitability of biological treatment technologies for energy production, giving modern data about it. It likewise covers difficulties and points of view in this field of exploration.

Energy recovery from high ash-containing sewage sludge: Focusing on performance evaluation of bio-fuel production

Hydrothermal liquefaction (HTL) has shown great potential to convert sewage sludge (SS) with high moisture into bio-crude. However, the disposal and reutilization of hydrothermal liquefaction wastewater (HTLWW) is a critical issue. Anaerobic digestion (AD) is proven to be an alternative to treat organic wastewater. Therefore, energy recovery from high ash-containing SS was studied by integrating AD with HTL. The effect of temperature on HTL efficiency was investigated and then methane production from HTLWW was conducted by AD with organic loading increasing from 2 g COD/L to 6 g COD/L. Results showed that the maximum bio-crude yield of 23.5 % was obtained at 350 °C. Methane yield of 309.4 mL CH₄/g COD_removed was achieved at 2 g COD/L with COD removal rate of 72.5 %. Meanwhile, the microbial structure and abundance showed great shifts resulting from the adaptation to complex compounds. JGI-000079-D21, Aminicenantales, and Bacteroidetes_ vadinHA17 predominated in the bacterial community. Due to the presence of the toxic substances in HTLWW, such as phenolic and nitrogenous heterocyclic compounds, there was a decrease in methane yield when the organic loading was higher than 4 g COD/L. The organic matters in extracellular polymeric substances (EPS) were rich in fulvic acid-like and humic acid-like substances due to the attack and stimulation of toxicants. Under the condition of unstable fermentation, Advenella and Bacillus first appeared as phenol and pyridine degrading bacteria, respectively. The microbial diversity declined sharply to demonstrate the toxic effect of the refractory organics existing at high organic loading. The enrichment of Methanosaeta in methanogens meant that acetotrophic metabolism is the dominant pathway in methanogenesis. In this study, the profile of bio-fuel production from high ash-containing SS would provide an integrated reference to treat wet biomass and recover energy simultaneously.

The potential of animal manure management pathways toward a circular economy: a bibliometric analysis

Improper disposal of animal waste is responsible for several environmental problems, causing eutrophication of lakes and rivers, nutrient overload in the soil, and the spread of pathogenic organisms. Despite the potential to cause adverse ecological damage, animal waste can be a valuable source of resources if incorporated into a circular concept. In this sense, new approaches focused on recovery and reuse as substitutes for traditional processes based on removing contaminants in animal manure have gained attention from the scientific community. Based on this, the present work reviewed the literature on the subject, performing a bibliometric and scientometric analysis of articles published in peer-reviewed journals between 1991 and 2021. Of the articles analyzed, the main issues addressed were nitrogen and phosphorus recovery, energy generation, high-value-added products, and water reuse. The energy use of livestock waste stands out since it is characterized as a consolidated solution, unlike other routes still being developed, presenting the economic barrier as the main limiting factor. Analyzing the trend of technological development through the S curve, it was possible to verify that the circular economy in the management of animal waste will enter the maturation phase as of 2036 and decline in 2056, which demonstrates opportunities for the sector's development, where animal waste can be an economic agent, promoting a cleaner and more viable product for a sustainable future.

Tracking the diversity and interaction of methanogens in the energy recovery process of a full-scale wastewater treatment plant

Methanogens have been significant for the achievement of carbon neutrality in wastewater treatment plants due to their crucial roles in the anaerobic digestion of sludge. Nevertheless, the phylogenetic diversity of methanogens and their versatile metabolism have been continuously investigated, the current scientific knowledge regarding these microbes appears inadequate and requires more evaluations. This study is considered an endeavor in which functional genes sequencing was used to reveal the diversity of methanogens in the sludge process of the wastewater treatment plant. The information obtained was substantially more than that employing 16s sequencing. The methanogenic microbial resources were appropriate to sustain a self-inoculated energy recovery with a potential ability to boost methane production. A constancy was observed in 16 S rRNA gene and mcrA gene sequencing results, where the bacterial or Methanosaeta concilii dominated community of DS (digest sludge) was distinct from the inoculum sources TS (total sludge), CTS (concentrated total sludge), and HTS (hydrolysis total sludge), indicating the independent development of DS. A quantitative cross-network was constructed by coupling the absolute quantify of 16 S rRNA and mcrA sequences. The Methanobacterium petrolearium actively interacted with bacteria in the DS community rather than the dominant species (Methanosaeta concilii). Moreover, the unclassified methanogens were identified to be significantly prevalent in all communities, suggesting that unknown methanogenic taxa might be imperative in accomplishing community functions. Collectively, the findings of this research study will shed light on the comprehensive knowledge of microbial communities, especially the methanogenic microbiota. This will further enhance the exploration of the phylogenetic diversity of methanogens and their corresponding impacts in energy recovery from wastewater treatment plants.

Hydrothermal carbonization of pulp and paper industry wastewater treatment sludges - characterization and potential use of hydrochars and filtrates

The pulp and paper industry's mixed sludge represents waste streams with few other means of disposal than incineration. Hydrothermal carbonization (HTC) could be advantageous for the sludge refinement into value-added products, thus complementing the concept of pulp and paper mills as biorefineries. Laboratory HTC was performed on mixed sludge (at 32% and 15% total solids) at temperatures of 210-250 °C for 30 or 120 min, and the characteristics of the HTC products were evaluated for their potential for energy, carbon, and nutrient recovery. The energy content increased from 14.9 MJ/kg in the mixed sludge up to 20.5 MJ/kg in the hydrochars. The produced filtrates had 12-15-fold higher COD and 3-5-fold higher volumetric methane production than untreated sludge filtrates, even though the methane yield against g-COD was lower. The increased value of the hydrochars in terms of energy content and carbon sequestration potential promote HTC deployment in sludge treatment and upgrading.

Energy generation and revenue potential from municipal solid waste using system dynamic approach

The Municipal Solid Waste (MSW) generation per capita in developing countries is generally said to grow in proportion to the gross national product. Composting and waste to energy have a brief history as management strategies for MSW in India and as alternatives to landfilling. Analysis of Energy generation and compost potential from waste can minimize the impact of MSW on the environment with the added advantage of providing a local source of energy. The study has been carried out to develop a system dynamic (SD) model to predict the energy generation, treatment, and cost analysis for MSW up to 2030. The predictive model developed in this study showed the generation rate of electrical energy potential augmented from 0 in 2001 to 58,380 MWh in 2007 and 319,875 MWh in 2030. Whereas, the production rate of compost reduced from 77,000 tonnes in 2001 to 45,000 tonnes in 2006 and then improved to 390,000 tonnes in 2030. In addition, the predicted revenue generated from different treatment facilities increased from 0 in 2001 to Rs.335 million (4.36 million USD) in 2007 and Rs.2569 million (33.4 million USD) in 2030. As a result, revenue generated could cover the budgets required for MSW treatment and disposal services in 2030, where the required budget is negative because revenue exceeds expenditures. The developed SD model can improve a municipal solid waste management system for any City.

A review on mycelial pellets as biological carriers: Wastewater treatment and recovery for resource and energy

Mycelial pellets, a new environment friendly biological carrier, have received wide attention from researchers due to porosity, stability and unique biocompatibility. In this article, the theoretical basis and mechanism of mycelial pellets as a biological carrier were analyzed from the properties of mycelial pellets and the interaction between mycelial pellets and other microorganisms. This article aims to collate and present the current application and development trend of mycelial pellets as biological carriers in wastewater treatment, resource and energy recovery, especially the symbiotic particle system formed by mycelial pellets and microalgae is an important way to break through the technical bottleneck of biodiesel recovery from wastewater. This review also analyzes the research hotspots and trends of mycelial pellets as carriers in recent years, discusses the challenges faced by this technology, and puts forward corresponding solutions.

Engineering pressure retarded osmosis membrane bioreactor (PRO-MBR) for simultaneous water and energy recovery from municipal wastewater

Osmotic membrane bioreactors (OMBR) have gained increasing interest in wastewater treatment and reclamation due to their high product water quality and fouling resistance. However, high energy consumption (mostly by draw solution recovery) restricted the wider application of OMBR. Herein, we propose a novel pressure retarded osmosis membrane bioreactor (PRO-MBR) for improving the economic feasibility. In comparison with conventional FO-MBR, PRO-MBR exhibited similar excellent contaminants removal performance and comparable water flux. More importantly, a considerable amount of energy can be recovered by PRO-MBR (4.1 kWh/100 m²·d), as a result of which, 10.02% of the specific energy consumption (SEC) for water recovery was reduced as compared with FO-MBR (from 1.42 kWh/m³ to 1.28 kWh/m³). Membrane orientation largely determined the performance of PRO-MBR, higher power density was achieved in AL-DS orientation (peak value of 3.4 W/m²) than that in AL-FS orientation (peak value of 1.4 W/m²). However, PRO-MBR suffered more severe and complex membrane fouling when operated in AL-DS orientation, because the porous support layer was facing sludge mixed liquor. Further investigation revealed fouling was mostly reversible for PRO-MBR, it exhibited similar flux recoverability (92.4%) to that in FO-MBR (95.1%) after osmotic backwash. Nevertheless, flux decline due to membrane fouling is still a restricting factor to power generation of PRO-MBR, its power density was decreased by 38.2% in the first 60 min due to the formation of fouling. Overall, in perspective of technoeconomic feasibility, the PRO-MBR demonstrates better potential than FO-MBR in wastewater treatment and reclamation and deserves more research attention in the future.

Waste heat recovery research - a systematic bibliometric analysis (1991 to 2020)

Human usage of non-renewable energy resources has caused many environmental issues, which include air pollution, global warming, and climate irregularities. To counter these issues, researchers have been seeking after alternative renewable energy sources and ways to manage energy more efficiently. This is where energy recovery technologies such as waste heat recovery (WHR) come into play. WHR is a form of waste to energy conversion. Waste heat can be captured and converted into usable energy instead of dumping it into the environment. In the more recent years, the WHR research field has gained great attention in the scientific community as well as in some energy-intensive industries. This article presents a bibliometric overview of the academic research on WHR over the span of 30 years from 1991 to 2020. A total of 5682 documents from Web of Science (WoS) have been retrieved and analyzed using various bibliometric methods, including performance analysis and network analysis. The analyses were performed on different actors in the field, i.e., funding agencies, journals, authors, organizations, and countries. In addition, several network mappings were done based on co-citation, co-authorship, and co-occurrences of keywords analyses. The research identified the most productive and influential actors in the field, established and emergent research topics, as well as the interrelations and collaboration patterns between different actors. The findings can be a robust roadmap for further research in this field.

Enhanced organic degradation and biogas production of domestic wastewater at psychrophilic temperature through submerged granular anaerobic membrane bioreactor for energy-positive treatment

This study deals with the conversion of organic matter into methane at ambient temperature, during anaerobic digestion of domestic wastewater combined with a submerged ultrafiltration membrane with no gas-sparging. A one-stage submerged granular anaerobic membrane bioreactor (G-AnMBR) and a control anaerobic digester (UASB type) were operated during four months, after 500 days of biomass acclimatization to psychrophilic and low loading rate conditions. Membrane barrier led to the retention of biomass, suspended solids and dissolved and colloidal organic matter which greatly enhanced total COD (tCOD) removal (92.3%) and COD to methane conversion (84.7% of tCOD converted into dissolved and gaseous CH4). G-AnMBR overcame the usual long start-up period and led to a higher sludge heterogeneity, without altering the granular biomass activity. The feasibility of the G-AnMBR without gas-sparging was also assessed and the net positive energy balance was estimated around + 0.58 kWh.m^-3.

Optimal use of glycerol co-solvent to enhance product yield and its quality from hydrothermal liquefaction of refuse-derived fuel

Refuse-derived fuels (RDF) are rich in resources that make them an attractive feedstock for the production of energy and biofuels. Hydrothermal liquefaction (HTL) is a promising thermochemical conversion technology to handle wet feedstocks and convert them to valuable bio-crude, bio-char and aqueous products. This study highlights the advantages of using glycerol as the co-solvent along with water in different proportions to produce bio-crude from RDF via HTL. The ratio of water:glycerol (vol.%:vol.%) was varied for each experiment (100:0, 90:10, 80:20, 70:30, 60:40, 50:50), and the product yields and their quality were studied. The results demonstrate that increasing the proportion of glycerol until 50 vol.% in the solvent enhances the bio-crude yield (36.2 wt.%) and its higher heating value (HHV) (30.9 MJ kg-1). Deoxygenation achieved in the bio-crude was 42%. The production of bio-char was minimum (9.5 wt.%) at 50 vol.% glycerol with HHV of 31.9 MJ kg-1. The selectivity to phenolic compounds in the bio-crude increased, while that of cyclic oxygenates decreased when the glycerol content was more than 20 vol.%. The gas-phase analysis revealed that the major deoxygenation pathway was decarboxylation. The yield of aqueous products drastically increased with the addition of glycerol. The minimum amount of glycerol in the co-solvent that favours an energetically feasible process with low carbon footprint is 30 vol.%. Using 50 vol.% glycerol resulted in the highest energy recovery in the bio-crude and bio-char (80%), the lowest energy consumption ratio (0.43) and lowest environmental factor (0.1). The mass-based process mass intensity factor, calculated based on only bio-crude and bio-char as the valuable products, decreased with an increase in addition of glycerol, while it was close to unity when the aqueous phase is also considered as a valuable product.

Comparative study of the environmental impacts of used cooking oil valorization options in Thailand

Used cooking oil (UCO) is a valuable resource that can be utilized in different ways. Appropriate management of UCO waste can provide environmental and economic benefits, compared to improper disposal practices. This study assessed the environmental impacts of potential UCO valorization options in Thailand. Altogether, 14 scenarios, including 10 for alternative energy recovering processes (S1-10) and other options such as soap production (S11), use in dry pig feed (DPF) production (S12), synthesis of plastics (S13) and polyol (S14), were considered. The defined system boundaries for each scenario include pretreatment, material and energy consumption, and waste treatment stages for the treatment of 1000 kg UCO. Environmental impacts in terms of global warming potential (GWP), freshwater eutrophication potential (FEP), fossil resource scarcity (FRS), and freshwater, terrestrial, and marine eco-toxicity (FE, TE, and ME, respectively) were analyzed using the ReCiPe Midpoint (H) method. The results revealed that all the current waste valorization options create an environmental burden and contribute towards GWP. Scenarios 7 and 10 showed environmental credits for FEP, FE, and ME indicators while scenario 9 did so for FRS. The processes direct energy consumption resulted in the highest contribution to GWP in Scenarios 1, 5-8, 10, 12, and 13. Environmental effects of material consumption and waste treatments were found to be the highest in bio-oil and DPF production, respectively. However, co-products produced could not offset the burden created by energy and material consumption. Overall, the results showed better environmental performance from energy recovery-based UCO management options compared to alternative processes.