Modelling incineration for more accurate comparisons to recycling in PEF and LCA

When recycling is beneficial for the environment, results from a life cycle assessment (LCA) should give incentives to collection for recycling and also to the use of recycled material in new products. Many approaches for modeling recycling in LCA assign part of the environmental benefits of recycling to the product where the recycled material is used. For example, the Circular Footprint Formula in the framework for Product Environmental Footprints (PEF) assigns less than 45% of the benefits of recycling to a polymer product sent to recycling. Our calculations indicate that this creates an incorrect climate incentive for incineration of renewable LDPE, when the recovered energy substitutes energy sources with 100-300% more climate impact than the Swedish average district heat and electricity. The risk of incorrect incentives can be reduced through allocating part of the net benefits of energy recovery to the life cycle where the energy is used; we propose this part can be 60% for Sweden, but probably less in countries without a district-heating network. Alternatively, the LCA can include the alternative treatment of waste that is displaced at the incinerator by waste from the investigated product. These solutions both make the LCA more balanced and consistent. The allocation factor 0.6 at incineration almost eliminates the risk of incorrect incentives in a PEF of renewable polymers. However, the focus of LCA on one product at a time might still make it insufficient to guide recycling, which requires concerted actions between actors in different life cycles.

Hydrothermal liquefaction of municipal solid wastes for high quality bio-crude production using glycerol as co-solvent

This study is focused on the valorization of heterogeneous municipal solid waste collected from the landfill using hydrothermal liquefaction process using glycerol as a co-solvent. The effects of temperature (300-350 °C) and residence time (15-45 min) on the yields and quality of the product fractions were investigated at 8 wt% solid loading. The yield of bio-crude significantly increased from 15.2 wt% with water as the solvent, to 58 wt% with water-glycerol (1:1 v/v) as the solvent possessing an energy content of 35.6 MJ/kg at 350 °C, 30 min. The quality of the bio-crude obtained using glycerol was comparable to that using tetralin as a hydrogen donor co-solvent. Phenolic compounds and cyclooxygenates were the major compounds in the bio-crude, and aliphatic hydrocarbons increased with residence time. Maximum energy recovery of 95% was achieved in the products with an energy consumption ratio of 0.43 for the bio-crude signifying the energetic feasibility of the process.

Evaluation of bio-energy recovery from the anaerobic treatment of municipal wastewater by a pilot-scale submerged anaerobic membrane bioreactor (AnMBR) at ambient temperature

The potential of bio-energy recovery from real municipal wastewater was investigated using a one-stage pilot-scale submerged anaerobic membrane bioreactor (AnMBR) for a range of HRTs from 24 h to 6 h at ambient temperature around 25 °C. This pilot-scale AnMBR demonstrated a high COD removal efficiency of over 90% during an operation of 217 days for municipal wastewater treatment. The energy balance of the AnMBR was calculated from both theoretical and practical aspects. The theoretical net energy potential was calculated as 0.174 kWh/m³ by applying operational data to empirical equations, obtaining a bio-energy recovery efficiency of 69.4%. The practical net energy potential was estimated as -0.014 kWh/m³ using the powers of engines applied in a full-scale wastewater treatment plant. This is considerably lower than that of the conventional activated sludge process. These results are evidence of the potential of the AnMBR and feasibility in the treatment of municipal wastewater treatment.

Investigations on energy recovery capability of coconut oil for pre-cooling of apples from cold storage condensate

Assessment in a live cold storage plant revealed that the refrigeration process generated about 150-170 L of condensate per day at 10 °C-12 °C. In this study, phase change property of coconut oil was applied to recover and use the energy from condensate for pre-cooling apples. Charging studies of coconut oil was carried out experimentally and the charging time was found to be 234, 126, 95 and 93 min for condensate flow rates of 2.6 L/h, 4.25 L/h, 6.26 L/h and 8.33 L/h respectively. Discharging studies were conducted for natural and forced convection for pre-cooling of apples under two cases namely apples in direct contact (case i) and indirect contact (case ii) with the container. Under natural convection and direct contact, pre-cooling of 4 °C was achieved with a discharge duration of 260 min. In case (ii), pre-cooling of 3.5 °C was achieved with discharge duration of 304 min. Likewise, under forced convection, pre-cooling of 3.5 °C was achieved in 189 min for case (i) and 214 min for case (ii). Temperature difference between the top point and bottom point of the apple was 1.5 °C and 0.5 °C for case (i) and case (ii) respectively. Results confirmed that coconut oil as a PCM can be used for recovering energy from condensate for pre-cooling of apples before loading into a cold storage unit.

Energy, Exergy, Exergoeconomic, and environmental (4E) analyses of thermal power plants for municipal solid waste to energy application in Bangladesh

With a population of 165 million, Bangladesh is undergoing rapid industrialization and urban development, and is well on track to move out from the group of least developed countries by 2024. This results in a significant increase in the urban energy needs and the amount of generated waste. Most of the municipal solid waste in Bangladesh is currently deposited in landfills, thereby contaminating nearby cultivable soils. It is desirable to have a system that recovers energy from the municipal solid waste in order to satisfy the increasing energy needs, while simultaneously addressing the land scarcity and pollution issues. This paper proposes using incineration to recover energy from municipal solid waste to produce electricity in the urban areas of Dhaka and Chattogram. A detailed technical analysis involving energy, exergy, exergoeconomic, and emission is presented. The power plants in these two cities show potential capacities of 169 MW and 83 MW respectively, with exergoeconomic factors of 61 %. The results indicate energy and exergy efficiencies of 32 % and 27 %, respectively, and a production cost in the range of 53.9-56.7 USD/MWh which is comparable to the production cost from the current power plants in Bangladesh. The proposed plants also result in a reduction in the greenhouse emissions and exhibit ecological efficiencies of over 87 %.

Comparative evaluation of simultaneous nitritation/denitritation and energy recovery in air-cathode microbial fuel cells (ACMFCs) treating low C/N ratio wastewater

Air-cathode microbial fuel cells (ACMFCs) can extract available electrons from the low C/N ratio wastewater (LCNW) for pollutant degradation and power generation. However, the multiple effects of operating parameters and their relationship between the performances and parameters are still lacking. In this study, several ACMFCs for simultaneous nitritation/denitritation (SND) and energy recovery were constructed and evaluated in terms of chemical oxygen demand (COD), NH₄⁺-N, C/N ratio, phosphate buffer solution (PBS), and external resistance (R_ext), and several derived parameters (e.g., organic loading rate (OLR), nitrogen loading rate (NLR)). Results indicated that ACMFCs could be used to treat LCNW successfully with high pollutant removal rates and sustainable current generation. Maximum removal efficiencies of 94% COD, 92% NH₄⁺-N, and 92% total nitrogen (TN) were achieved. A maximum power density of 1400 mW m^-2 and columbic efficiency of 69.2% were also obtained at a low C/N ratio of 1.7-2.6. Low C/N ratios promoted SND by balancing nitritation and denitritation. The microbial community and their predicated function results showed considerable nitrifiers and denitrificans were enriched in the ACMFCs, contributing to SND and power recovery. Further analyses showed that the NH₄⁺-N could inhibit SND, but PBS and R_ext had no obvious effects on this outcome. Co-occurrence network analysis demonstrated that power is positively correlated with COD and R_ext; strong correlations between organic removal and COD, and between nitrogen removal and ammonia, conductivity, and C/N ratio were also noted. Overall, the appropriate control of such parameters is necessary to achieve efficient SND in ACMFCs for LCNW treatment.

Enhancing microalgae growth and product accumulation with carbon source regulation: New perspective for the coordination between photosynthesis and aerobic respiration

The coordination between photosynthesis and aerobic respiration under mixotrophic cultivation can make a difference to the growth and biochemical composition of microalgae. However, the response of carbon metabolism to carbon source composition under mixotrophic microalgae cultivation has not been well studied. In this study, the synergistic effects of inorganic carbon (IC) and organic carbon (OC) supply on the growth and carbon metabolism of Chlorella vulgaris under mixotrophic cultivation were investigated. The increase of the proportion of HCO₃^- had a positive effect on the expression of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which promoted the biomass production and carbon fixing. The activity of citrate synthase was attenuated with the increase of IC/OC ratio, indicating that the energy needed for the biomass production in groups with high IC/OC ratio was contributed by photoreaction. Biochemical analysis showed that CO₃^2- was more efficient than HCO₃^- for carbohydrate and lipid accumulation of Chlorella vulgaris, and the highest amount of carbohydrate (30.2%) and lipid (35.8%) was recorded with the combined use of CO₃^2- and glucose. The results could provide a new perspective on carbon metabolism and enzyme regulation in mixotrophic microalgae cultivation.

Environmental impacts and greenhouse gas emissions assessment for energy recovery and material recycle of the wastewater treatment plant

This study investigated the environmental burdens concerning the recycling/recovery process of a wastewater treatment plant's construction material waste and biogas. Detailed data inventories of case studies were employed in several scenarios to explore the role of end-of-life treatment methods. The ReCiPe 2016 and the Greenhouse gas Protocol life cycle impact methods were conducted to measure the impact categories. The construction and demolition phases were considered for recycling potential assessment, while the operational phase was examined for assessing the advantages of energy recovery. Metal and concrete recycling show environmental benefits. Increasing the reprocessing rate requires more water consumption but results in: firstly, a decrease of 18.8% in total damage; secondly, reduces problematic mineral scarcity by 3.9%; and thirdly, a shortfall in fossil fuels amounting to 11.6%. Recycling concrete helps to reduce the amount of GHG emissions 1.4-fold. Different biogas treatment methods contribute to various outcomes. Biogas utilization for on-site energy purposes has more advantages than flaring and offsite consumption. Electricity and heat generation originating from biogas can provide 70% of the energy requirement and replace 100% natural gas usage. Biomethane production from biogas requires extreme power and more resources. Meanwhile, producing heat and electricity can offset 102.9 g of fossil CO₂, and manufacturing biomethane contributes the equivalent of 101.2 g of fossil fuel-derived CO₂. Reducing 10% of recovered electricity creation could rise 19.19% global warming indicator of the wastewater treatment plant.

Onset Investigation on Dynamic Change of Biohythane Generation and Microbial Structure in Dual-chamber versus Single-chamber Microbial Electrolysis Cells

Biohythane is alternative fuel to replace fossil fuel for car combustion, and biohythane generation could be potential pathway for energy recovery from wastewater treatment. Microbial electrolysis cell (MEC) is electrochemical technique to convert waste to methane and hydrogen gas for biohythane generation, but the feasibility and stability of MEC needs further investigation to assure sustainable energy recovery. System configuration is paramount factor for electrochemical reaction and mass transfer, and this study was to investigate the configuration impact (single vs dual chamber) of MEC for biohythane generation rate and stability. This study showed that dual-chamber MEC could separate methane and hydrogen gas production in the anode and cathode, and combined both together to produce biohythane. To reduce ohmic resistance for higher current, cation exchange membrane (CEM) was removed from dual-chamber to single-chamber MEC. However, free hydrogen diffusion was allowed in the single chamber since CEM was removed. The diffused hydrogen and substrate towards the cathode would favor the methanogen growth, and thus the hydrogen was consumed to reduce the biohythane generation and energy recovery efficiency (i.e., 7.5 × 10^-3 reduced to 5.7 × 10^-3 kWh kg^-1 degraded COD day^-1 after converting dual-chamber to single-chamber MEC). Absolute abundance of methanogen in single-chamber MEC was greatly boosted, as Methanosarcina and Methanobacteriale on the anode surface, increased by 132% and 243%, respectively, while the original dual-chamber MEC could maintain Geobacter growth for high current generation. This is the keystone study to demonstrate the importance of dual-chamber MEC for the feasibility and stability for the biohythane generation, building up the foundation to use electrochemical device to convert the organic waste to the alternative biohythane.

Hydrothermal carbonisation of mechanically dewatered digested sewage sludge-Energy and nutrient recovery in centralised biogas plant

This study aimed to assess the role of hydrothermal carbonisation (HTC) in digestate processing in centralised biogas plants receiving dewatered sludge from regional wastewater treatment plants and producing biomethane and fertilisers. Chemically conditioned and mechanically dewatered sludge was used as such (total solids (TS) 25%) or as diluted (15% TS) with reject water in 30 min or 120 min HTC treatments at 210 °C, 230 °C or 250 °C, and the produced slurry was filtered to produce hydrochars and filtrates. The different hydrochars contributed to 20-55% of the original mass, 72-88% of the TS, 74-87% of the energy content, 71-92% of the carbon, above 86% of phosphorous and 38-64% of the nitrogen present in the original digestates. The hydrochars' energy content (higher heating values were 11.3-12.2 MJ/kg-TS) were similar to that of the digestates, while the ash contents increased (from 43% up to 57%). HTC treatments produced filtrates in volumes of 42-76% of the dewatered digestate, having a soluble chemical oxygen demand (SCOD) of 28-44 g/L, of which volatile fatty acids (VFAs) contributed 10-34%, and methane potentials of 182-206 mL-CH₄/g-SCOD without any major indication of inhibition. All 32 pharmaceuticals detected in the digestates were below the detection limit in hydrochars and filtrates, save for ibuprofen and benzotriazole in filtrate, while heavy metals were concentrated in the hydrochars but below the national limits for fertiliser use, save for mercury. The integration of HTC to a centralised biogas plant was extrapolated to enhance the annual biogas production by 5% and ammonium recovery by 25%, and the hydrochar was estimated to produce 83 GJ upon combustion or to direct 350 t phosphorous to agriculture annually.

Enhanced recovery of nitrous oxide from incineration leachate in a microbial electrolysis cell inoculated with a nosZ-deficient strain of Pseudomonas aeruginosa

High concentrations of nitrous oxide were recovered from partial nitrification treated leachate in a microbial electrolysis cell (MEC) inoculated with a nosZ-deficient strain of Pseudomonas aeruginosa. N₂O conversion efficiencies > 90% were achieved when a potential of 0.8 V was applied to the MEC. The ΔnosZ strain was enriched in the 0.8 V MEC, but Achromobacter dominated the non-current control. Nitric oxide reductase genes were highly expressed by ΔnosZ cells growing in the 0.8 V MEC, consistent with enhanced nitrous oxide production rates. Concentrations of phenazine derivatives and transcripts from phenazine biosynthesis genes were also high in the 0.8 V MEC. Phenazine derivatives are known to act as electron shuttles, enhance biofilm formation, and help ward off competitors, thereby increasing the survivability of the ΔnosZ strain in the MEC. These results show that applied current stabilized growth of the ΔnosZ strain in the reactor and allowed it to sustainably generate high concentrations of nitrous oxide.

High-quality syngas production: The green and efficient utilization of waste tire and waste heat from the steelmaking converter process

This study proposes a new technology in which waste tire powders are injected into a converter vaporization cooling flue for gas recovery via pyrolysis using high-temperature waste heat. The higher temperature pyrolysis behavior of waste tire powder under different heating rates was investigated using a TG-MS technique. A drop tube furnace was used to simulate the converter vaporization cooling flue to investigate the effect of high temperatures on waste tire powder pyrolysis. The results indicated that secondary pyrolysis occurred above 900 °C with low weight and weight loss rates, which were considerably lower than those observed in the thermal degradation stage. The main gaseous products formed were CO, CO₂, H₂, CH₄, and H₂O. The drop tube furnace experimental results indicated that high temperatures can facilitate the degradation of waste tire powder to generate more H₂ and CO and improve the low heating values. At 1200 °C, the H₂ and CO contents were approximately 19.60% and 4.90%, respectively. The low heating value was 29.64 MJ/Nm³. The char yield was in the range of 32.67%-37.33%; the fixed carbon content increased from 79.63% to 84.75%. The results provide preliminary verification of the feasibility of injecting waste tire powders into a converter vaporization cooling flue for gas recovery.

Anaerobic MBR technology for treating municipal wastewater at ambient temperatures

An innovative way to treat municipal wastewater and produce energy at the same time is anaerobic treatment. Anaerobic processes are traditionally used for high-strength wastewater or municipal sludge treatment and only recently have been applied for the treatment of low strength municipal wastewater To investigate the performance of anaerobic wastewater treatment through the incorporation of membrane technology, a 40 L laboratory scale Anaerobic Membrane Bioreactor (AnMBR) with a flat sheet submerged membrane along with a 40 L reservoir for trapping and measuring the biogas produced have been installed and set in operation. The scope of this study is to examine, through long term bench scale experiments, the impact that different temperatures and also different operating conditions have on the efficiency of AnMBR in order to identify the possibility of integrating this technology into Wastewater Treatment Plants (WWTPs). This paper evaluates the efficiency of AnMBR in the temperature range 14-26 °C, operating at three different hydraulic retention times (HRTs). The three different HRTs examined were 2 d, 1 d and 12 h. Each HRT is divided into two different temperature ranges. As the HRT decreased the effluent quality decreased and the membrane fouled more rapidly. AnMBR was able to produce permeate water with an average COD of 51 ± 8 mg L^-1 at an HRT of 2 d during the summer period with an average temperature of 24 °C. The effluent COD increased to 67 ± 10 mg L^-1 and reached 91 ± 5 mg L^-1 for HRT 1 d and 12 h respectively for the same temperature range.

Mechanical pretreatment of municipal biowaste to produce an aqueous slurry dedicated to anaerobic digestion

The present study investigated a wet mechanical pretreatment to improve methane production by anaerobic digestion from biowaste material by separating a biodegradable aqueous slurry fraction (ASF) from a more recalcitrant particulate fraction (PF). Four source-sorted municipal biowastes were studied, namely household (HBW), supermarket (SBW), restaurant (RBW), and green biowaste (GBW). The treatment consisted in soaking the waste in water and then pressing the slurry through a grid with 3-mm openings to separate the two fractions. Methane production of ASF and PF obtained from the four biowastes were measured using the BMP protocol and compared to the potential of the respective untreated biowaste. Results were very different for GBW as compared to the other three BWs. With GBW, which was the most lignocellulosic of the BW studied, only 17% of the initial methane potential was recovered in the ASF. The extraction was much better on the other biowastes and increased in the following order: HBW (58%) ≃ RBW (57%) < SBW (67%). The ASF from these biowastes exhibited low total solid contents and high BMPs (416, 408, and 423 NL_CH4.g^-1_vs for HBW, RBW, and SBW respectively). The experimental results obtained in this study therefore showed that wet pressing separation was an efficient pretreatment to improve and facilitate methane production by anaerobic digestion of biowaste such as HBW, RBW, and SBW.

Management of off-specification compost by using co-hydrothermal carbonization with olive tree pruning. Assessing energy potential of hydrochar

In this work the management of a waste called off-specification compost (OSC) was proposed via hydrothermal carbonization (HTC). The composition of this residue makes it not suitable for agronomic purposes because of the Spanish regulation requirements. Therefore, a way of management and/or valorisation needs to be found. The energy recovery through co-HTC with olive tree pruning (OTP) was evaluated. Blending of OSC with lignocellulosic biomass allows to obtain a coal-like product with physicochemical properties similar to those of a lignite, characterised by its high carbon content. Blends of 25, 50 and 75% of OSC with OTP were analysed. The individual OSC does not present good parameters for being used as solid fuel based on its chemical composition, however, the blend of 75% of biomass with 25% of OSC does. With a higher heating value of 26.19 MJ/kg, this blend shows the best energy yield and energy densification ratio. Thermogravimetric and kinetic analysis reveal that as biomass content in the blend increases, the more the hydrochar behaves as a solid fuel, therefore OSC can be used for energy purposes while its current use of landfill disposal can be reduced.

Energy recovery from sewage sludge: Product characteristics, heating value prediction and reaction kinetics

Energy recovery from sewage sludge was carried out by using microwave and conventional torrefaction. The microwave torrefaction was carried out by using a laboratory-scale microwave oven that provides single-mode microwave irradiation at 2.45 GHz, and the amount of sewage sludge for each experiment was approximately 20 g. The efficiency of microwave heating can be substantially promoted at higher power level, resulting in higher heating rate and maximum temperature. According to higher energy yield and heating value of torrefied sewage sludge, the optimum power level for bioenergy produced by microwave torrefaction of sewage sludge should be 200 W. Because of lower mass yield and temperature required to obtain the same yield, microwave heating can be more effective than conventional heating for sewage sludge torrefaction. The elemental composition of torrefied sewage sludge at 400 W was similar to that of anthracite, and its low hydrogen and oxygen contents could prevent excessive formation of smoke. Two correlations were obtained to predict the HHV of SS based on proximate and elemental compositions. With the recovery of liquid and gas products as bioenergy, the energy return on investment for microwave torrefaction of sewage sludge can be up to 16.4, much higher than the minimum value required for a sustainable society. Because of lower activation energy but higher pre-exponential factor, microwave heating can be approximately five times faster than conventional heating.

End-of-life and waste management of disposable beverage cups

Different human activities have caused and currently cause catastrophic environmental phenomena, and unfortunately, a significant negative contribution to these catastrophic phenomena can be attributed to uncontrolled plastic production, use and release everywhere. On the other hand, the plastics offer numerous comforts and advantages, and for this reason, the modern life is unthinkable without plastic. Currently, numerous scientific papers and large audience advertisings, related to the production and use of polymers made by natural sources, i.e. bio-based polymers, as a valid alternative to the petroleum-based counterparts, have been published. Therefore, for production of daily disposables and usages, the choice of petroleum-based polymers, coming from fossil-based resources, or bio-based polymers, coming from renewable resources, can be correctly understood and evaluated taking into account different issues concerning resources supplying, production technology and costs, application properties and performance, and finally, waste management. Current paper is focused on a reflection point related to waste management through burning/incineration (i.e. oxidation) of disposable beverage cups (volume 200 ml). The simple calculations of oxidation process of petrol-based or bio-based materials, which is the theoretical basis of waste management through burning/incineration, highlights that none of cup materials, can be considered better than the others to produce daily disposables and usages.

A critical review on energy recovery and non-hazardous disposal of oily sludge from petroleum industry by pyrolysis

This review systematically reports the pyrolysis of oily sludge (OS) from petroleum industry in regards to its dual features of the energy recovery potential and the environmental risks. The petroleum hydrocarbons are the nonbiodegradable fractions in OS that possess hazardous properties, i.e. ignitability and toxicity. Besides, complicated hazardous elements (i.e. N, S and Cl) and heavy metals inherently existing in OS further aggravate the environmental risks. However, the high oil content and heating value of OS contribute to its huge energy resource potential. Considering the energy demand and the environmental pressure, the ultimate purposes of the OS management are to enhance the oil recovery efficiency to minimize the oil content as well as to stabilize the hazardous elements and heavy metals into the solid residue. Among various OS management technologies, pyrolysis is the most suitable approach to reach both targets. In this review paper, the pyrolysis principle, the kinetics and the product distribution in three-phases are discussed firstly. Then the effects of operating parameters of the pyrolysis process on the quality and the application potential of the three-phase products, as well as the hazardous element distribution are discussed. To further solve the dominant concerns, such as the oil content in the solid residue, the pyrolytic oil quality and the migration of hazardous elements and heavy metals, the potentials of the catalytic pyrolysis and the co-pyrolysis with additives are also summarized. Also, the typical pyrolysis reactors are then presented. From the perspective of the energy efficiency and the non-hazardous disposal, the integrated technology combining the pyrolysis and the combustion for the OS management is recommended. Finally, the remaining challenges of OS pyrolysis encountered in the research and the industrial application are discussed and the related outlooks are itemized.

Two-Stage anaerobic digestion in agroindustrial waste treatment: A review

The anaerobic digestion is a process widely recognized as an interesting alternative for the treatment and stabilization of residual organic substrates. However, several technical limitations were observed based on the characteristics of the organic matter submitted to the process, such as the presence of high concentrations of soluble sugars or fats. The technology of anaerobic digestion in multiple stages is described as a viable option in the control of variables, optimizing the environmental conditions of the main microorganisms involved in the process, assuring high solid removal and methane production, besides allowing a higher energy yield through the generation of molecular fuel hydrogen. Several studies reviewed the process of anaerobic digestion in multiple stages in the treatment of food waste, although few report its use applied directly to agroindustrial residues. Thus, the present work aims to review the literature evaluating the scenario and viability of the multi-stage anaerobic digestion process applied to agroindustrial effluents. Effluents such as manipueira, vinasse, and dairy wastewater are substrates that present high yields when treated by AD processes with stage separation. The high concentration of easily fermentable sugars results in a high production of molecular hydrogen (co-product of the production of volatile acids in the acid phase) and methane (methanogenic phase). The great challenges related to the development of the sector are focused on the stability of the composition and yield of hydrogen in the acid phase, besides the problems resulting from the treatment of complex residues. Thus, the present study suggests that future works should focus on the technologies of new microorganisms and optimization of process parameters, providing maturation and scale-up of the two-stage anaerobic digestion technique.

Impacts of COVID-19 on energy demand and consumption: Challenges, lessons and emerging opportunities

COVID-19 has caused great challenges to the energy industry. Potential new practices and social forms being facilitated by the pandemics are having impacts on energy demand and consumption. Spatial and temporal heterogeneities of impacts appear gradually due to the dynamics of pandemics and mitigation measures. This paper overviews the impacts and challenges of COVID-19 pandemics on energy demand and consumption and highlights energy-related lessons and emerging opportunities. The discussion on energy-related issues is divided into four main sections: emergency situation and its impacts, environmental impacts and stabilising energy demand, recovering energy demand, and lessons and emerging opportunities. The changes in energy requirements are compared and analysed from multiple perspectives according to available data and information. In general, although the overall energy demand declines, the spatial and temporal variations are complicated. The energy intensity has presented apparent changes, the extra energy for COVID-19 fighting is non-negligible for stabilising energy demand, and the energy recovery in different regions presents significant differences. A crucial issue has been to allocate and find energy-related emerging opportunities for the post pandemics. This study could offer a direction in opening new avenues for increasing energy efficiency and promoting energy saving.

Evaluation of biogas production potential of trace element-contaminated plants via anaerobic digestion

Within the domain of phytoremediation research, the proper disposal of harvestable plant parts, that remove pollutants from contaminated soil, has been attracted extensive attention. Here, the bioenergy generation capability of trace metals (Cu, Pb, Zn, Cd, Mn, and As) polluted plants was assessed. The biogas production potential of accumulators or hyperaccumulator plants, Elsholtzia haichowensis, Sedum alfredii, Solanum nigrum, Phytolacca americana and Pteris vittata were 259.2 ± 1.9, 238.7 ± 4.2, 135.9 ± 0.9, 129.5 ± 2.9 and 106.8 ± 2.1 mL/g, respectively. The presence of Cu (at approximately 1000 mg/kg) increased the cumulative biogas production, the daily methane production and the methane yield of E. haichowensis. For S. alfredii, the presence of Zn (≥500 mg/kg) showed a significant negative impact on the methane content in biogas, and the daily methane production, which decreased the biogas and methane yield. The biogas production potential increased when the content of Mn was at 5 000-10,000 mg/kg, subsequently, decreased when the value of Mn at 20,000 mg/kg. However, Cd (1-200 mg/kg), Pb (125-2000 mg/kg) and As (1250-10,000 mg/kg) showed no distinctive change in the cumulative biogas production of S. nigrum, S. alfredii and P. vittata, respectively. The methane yield showed a strong positive correlation (R² =0.9704) with cumulative biogas production, and the energy potential of the plant residues were at 415-985 kWh/ton. Thus, the anaerobic digestion has bright potential for the disposal of trace metal contaminated plants, and has promising prospects for the use in energy production.

Microalgal-bacterial granular sludge process: A game changer of future municipal wastewater treatment?

This article is in the hope to open a fundamental discussion on what should future municipal wastewater treatment process be. A paradigm shift of treatment technology from present single functionality of removing to multiple-functionality of synergetic water-resource-energy recovery and carbon neutral for maximizing both environmental and economic sustainability. However, the current treatment technologies could hardly meet such requirements. It is elucidated in this article that a microalgal-bacterial granular sludge process could offer a promising option for achieving the multiple goals of municipal wastewater reclamation including energy generation, resource recovery and carbon reduction.

Development of a dynamic optimization framework for waste management systems

Waste to energy (WTE) technologies have emerged as an alternative solution to municipal solid waste management. WTE systems provide major environmental and economic benefits by converting waste into accessible energy, as part of an integrated solid waste management (ISWM) strategy. However, previous studies showed that establishing an ISWM strategy based on a single type of WTE systems does not necessarily realize maximum benefits. Hence, optimizing the selection of WTE systems as part of a hybrid waste management strategy can potentially achieve maximum benefits and minimize negative impacts. However, such task is challenging due to the various alternatives and objectives, particularly those related to the material and energy recovery systems. This article presents the methods used to develop a systematic optimization framework that identifies the most beneficial set of ISWM systems through mathematical modelling. The methods include the procedures of the established framework, including base model computations, as well as the comprehensive modelling and optimization methods.•The energy recovery, carbon footprint, and financial profitability are computed for selected WTE facilities.•The multi-objective mathematical programming is solved using the weighted comprehensive criterion method (WCCM).•The model is implemented in CPLEX software using mathematical programming language (OPL).

A comprehensive review of nutrient-energy-water-solute recovery by hybrid osmotic membrane bioreactors

Hybrid osmotic membrane bioreactor (OMBR) takes advantage of the cooperation of varying biological or desalination processes and can achieve NEWS (nutrient-energy-water-solute) recovery from wastewater. However, a lack of universal parameters hinders our understanding. Herein, system configurations and new parameters are systematically investigated to help better evaluate recovery performance. High-quality water can be produced in reverse osmosis/membrane distillation-based OMBRs, but high operation cost limits their application. Although bioelectrochemical system (BES)/electrodialysis-based OMBRs can effectively achieve solute recovery, operation parameters should be optimized. Nutrients can be recovered from various wastewater by porous membrane-based OMBRs, but additional processes increase operation cost. Electricity recovery can be achieved in BES-based OMBRs, but energy balances are negative. Although anaerobic OMBRs are energy-efficient, salinity accumulation limits methane productions. Additional efforts must be made to alleviate membrane fouling, control salinity accumulation, optimize recovery efficiency, and reduce operation cost. This review will accelerate hybrid OMBR development for real-world applications.

Bridging the gap to hydrochar production and its application into frameworks of bioenergy, environmental and biocatalysis areas

Hydrothermal carbonization (HTC) is a facile, low-cost and eco-friendly thermal conversion process that has recently gained attention with a growing number of publications (lower 50 in 2000 to over 1500 in 2020). Despite being a promising technology, problems such as operational barriers, complex reaction mechanisms and scaling have to be solved to make it a commercial technology. To bridge this current gap, this review elaborates on the chemistry of the conversion of lignocellulosic biomass. Besides, a comprehensive overview of the influence of the HTC operational conditions (pH, temperature, water:biomass ratio, residence time and water recirculation) are discussed to better understand how hydrochar with desired properties can be efficiently produced. Large-scale examples of the application of HTC are also presented. Current applications of hydrochar in the fields of energy, biocatalysis and environment are reviewed. Finally, economic cost and future prospects are analyzed.