Energy recovery from waste printed circuit boards using microwave pyrolysis: product characteristics, reaction kinetics, and benefits

Energy recovery from waste printed circuit boards (PCBs) was carried out by using microwave pyrolysis. According to thermogravimetric analysis, the maximum weight loss rate of waste PCBs occurred at 323 °C. When waste PCBs was heated under microwave irradiation at 300 W, the temperature can be reached within 10 min. Compared with conventional pyrolysis, microwave pyrolysis can provide higher weight loss of waste PCBs by 3-5 wt%. Microwave pyrolysis is helpful for the delamination of waste PCBs. Almost 71% of the gaseous product can be directly used as a fuel or converted into other forms of energy. Microwave pyrolysis can produce more HBr than conventional pyrolysis by approximately 17%. The main components of liquid product were phenols and phenyls. The overall energy recovery from waste PCBs using microwave pyrolysis can be 62%. According to kinetic analysis, it would need 20 min of processing time to decompose the combustible fraction of waste PCBs at 300 W. The maximum processing capacity of the microwave pyrolysis system for waste PCBs can be 1.36 kg, with the energy production of 2710 kJ. Furthermore, the pyrolyzed PCBs can be further processed to recycle valuable metals. Therefore, microwave pyrolysis of waste PCBs can be a complete and effective circular economy system to create high energy and economic benefits.

A review on facilitating bio-wastes degradation and energy recovery efficiencies in anaerobic digestion systems with biochar amendment

In this review, progress in the potential mechanisms of biochar amendment for AD performance promotion was summarized. As adsorbents, biochar was beneficial for alleviating microbial toxicity, accelerating refractory substances degradation, and upgrading biogas quality. The buffering capacity of biochar balanced pH decreasing caused by volatile fatty acids accumulation. Moreover, biochar regulated microbial metabolism by boosting activities, mediating electron transfer between syntrophic partners, and enriching functional microbes. Recent studies also suggested biochar as potential useful additives for membrane fouling alleviation in anaerobic membrane bioreactors (AnMBR). By analyzing the reported performances based on different operation models or substrate types, debatable issues and associated research gaps of understanding the real role of biochar in AD were critically discussed. Accordingly, Future perspectives of developing biochar-amended AD technology for real-world applications were elucidated. Lastly, with biochar-amended AD as a core process, a novel integrated scheme was proposed towards high-efficient energy-resource recovery from various bio-wastes.

Design and construction of a double actuated mechanical speed breaker electricity generator

The economy runs on energy and quest for development means that more energy is needed. Statistics show that the majority of energy comes from burning fossil fuels, and they pollute our environment. This work is aimed at producing a mechanical speed breaker electricity generator that operates on two actuating strokes. It converts the linear motion of the rack gear that is depressed by the vehicular movement to rotary motion of the pinion gear keyed on the shaft and alternator. The machine gets actuated by compression and release of the springs, thus increasing the output of the machine. It consists of a stable frame, dome-shaped top, rack and pinion mechanism, chain and sprocket mechanism, freewheels, springs, flywheel, shafts, an alternator and a deep cycle battery. The design was carried out using engineering principles with due consideration to cost, serviceability, ease of operation, durability and performance. It is designed to be actuated by vehicles weighing 1,600kg (400kg per wheel) and a prototype rated 100kg was built. Tests were performed to determine the performance of the machine and the results showed that the speed breaker could produce a rotary speed of 1500 rpm at the alternator, thereby generating 204W of electrical power per cycle. The machine conversion efficiency was about 88.7%.

Municipal wastewater sludge as a renewable, cost-effective feedstock for transportation biofuels using hydrothermal liquefaction

U.S. municipal wastewater contains approximately 160 trillion Btu/y of influent chemical energy, but very little is recovered and utilized nationwide. Hydrothermal liquefaction (HTL) is a thermochemical process that converts biomass into a biocrude intermediate that can be upgraded to a variety of liquid fuels. HTL provides an opportunity to enhance energy recovery at wastewater treatment plants by transforming underutilized municipal wastewater solids into a renewable, cost-effective feedstock for transportation biofuels. In this study, we estimate total national economic sludge feedstock supply by performing discounted cash flow analyses at >15,000 U.S. wastewater treatment facilities to assess the net present value of 30-year HTL investments, with comparison to wider adoption of anaerobic digestion (AD). This analysis is the first to model HTL technology deployment across the real-world fleet of wastewater treatment plants. Analyses indicate treatment facilities ≥17 ML/d (4.6 million gal/d) could supply 9.77 Tg/y of dry solids feedstock to economically produce 3.67 GL/y of biocrude intermediate, thereby increasing energy, environmental, and financial sustainability of sludge treatment while reducing disposal costs and operational and environmental risk.

Application of acidic conditions and inert-gas sparging to achieve high-efficiency nitrous oxide recovery during nitrite denitrification

Nitrogen removal with energy recovery through denitrification dependent N₂O production is garnering recent attention due to its cost advantages. The most effective current method requires alternating COD and nitrite to achieve high N₂O production making it incompatible with typical wastewaters and consequently difficult to use in most settings. The work described here introduces a robust and highly efficient N₂O recovery approach which has the potential to work with wastewaters containing COD and nitrite simultaneously. This method relies on low pH incubation and inert gas sparging (IGS) to shift a community of mainly N₂ producing nitrite denitrifiers to a community that accumulates N₂O when incubated in the absence of IGS. Before experiencing IGS, samples from activated sludge incubated at a pH of 4.5 and 6.0 only achieved a maximum N₂O production efficiency (PE_N₂O) of ∼26%. After IGS the PE_N₂O values increased to ∼97.5% and ∼80.2% for samples from these same pH 4.5 and pH 6.0 reactors, respectively. IGS did not lead to N₂O production in a pH 7.5 bioreactor. Meta-omics analysis revealed that IGS resulted in an increase in bacteria utilizing the clade I nitrous oxide reductase (nosZ_I) relative to bacteria utilizing the clade II nitrous oxide reductase (nosZ_II). This likely results from IGS flushing out N₂O leaving nitrite as the principal nitrogen oxide available for respiration, favoring nosZ_I utilizing bacteria which are more likely to be complete denitrifiers. Metatranscriptomic analysis suggested that the high PE_N₂O values that occurred after stopping IGS result from the NO generated by chemodenitrification accumulating to levels that inactivate [4Fe:4S] clusters in the NosR protein essential for N₂O reduction in the nosZ_I denitrifiers. This study provides an efficient and straightforward method for N₂O recovery, widening the options for energy recovery from nitrogen-based wastes.

Process optimization and energy analysis of vacuum degasifier systems for the simultaneous removal of dissolved methane and hydrogen sulfide from anaerobically treated wastewater

The control of dissolved methane (CH₄) and hydrogen sulfide (H₂S) emissions in anaerobic effluents is essential for minimizing the environmental implications of greenhouse gases, odor, and carbon footprint, as well as for preventing energy loss in the form of unrecovered dissolved methane. This study assessed the feasibility of a vacuum degasifier for the removal of CH₄ and H₂S from staged anaerobic fluidized membrane bioreactor (SAF-MBR) effluent. The optimization results showed that the efficiency of the nozzle fitted degasifiers were superior to the media packed ones. In three-stage vacuum degasifiers at a -0.8 bar vacuum pressure, H₂S removal was mostly pH dependent and 88% removal efficiency was achieved with an initial concentration of 13.6 mg/L. Methane removal was dependent primarily on the number of degasifier units, and approximately 94% efficiency was achieved in a three-stage degasifier. Energy balance analysis showed that energy production exceeded the system energy requirements with 0.05-0.07 kWh/m³ of surplus energy. These results provide deep insights into this new technology for simultaneous removal of dissolved CH₄ and H₂S, which can be referred for potential future applications.

From waste to fuel: Energy recovery from household food waste via its bioconversion to energy carriers based on microbiological processes

In the present study the bioconversion of dried household food waste (FORBI) to energy carriers was investigated aiming to its sustainable management and valorization. FORBI was either directly fermented towards ethanol and hydrogen or was previously subjected to extraction with water resulting to a liquid fraction (extract) rich in sugars and a solid residue, which were then fermented separately. Subsequently, the effluents were assessed as substrates for methane production via anaerobic digestion (AD). Mono-cultures and co-cultures of C5 and C6 yeasts were used for the alcoholic fermentation whereas for the production of hydrogen, mixed acidogenic consortia were used. Taking into account the optimum yields of biofuels, the amount of recoverable energy was estimated based for each different approach. The maximum ethanol yield was 0.16 g ethanol per kg of FORBI and it was achieved for separate fermentation of liquid and solid fractions of the waste. The highest hydrogen yield that was observed was 210.44 L ± 4.02 H₂/kg TS FORBI for 1% solids loading and supplementation with cellulolytic enzymes. Direct AD of either the whole FORBI or its individual fractions led to lower overall energy recovery, compared to that obtained when fermentation and subsequent AD were applied. The recoverable energy was estimated for the different exploitation approaches of the waste. The maximum achieved recoverable energy was 21.49 ± 0.57 MJ/kg.

Circular economy in waste management - Socio-economic effect of changes in waste management system structure

Due to the fast development, the EU economy has grown over its own raw material production. To enable future economic development, the EU is trying to develop a sustainable and resource-efficient economy. This path is emphasized through the idea of "Closing the Loop" which is integrated into EU legislation by a Circular Economy Package and emphasizes avoidance of waste production and its recovery. New waste management goals require significant changes in the waste management system structure which introduces new problems and one of them is an increase in the costs for the system users (citizens). To assess the impact of these changes, the time-dependent Life Cycle Assessment based waste, material, and energy flow tracking framework is adapted and used to calculate material and energy production which can be monetised. As waste management plants/facilities are built with public money, to provide public service, in economic calculations annual cash flow of the system is equalized with zero, taking into account all incomes (incomes from products like energy vectors, secondary materials, and compost) and expenses (like an investment and operating costs). From these calculations, variable (volatile, time-dependent) and average system gate fee (operating cost per tonne of input waste), which is charged to the system users, are calculated. A possible increase in system cost can cause the issue of social unacceptance, which decision-makers, elected by the citizens, want to avoid. Results show that energy recovery of waste generates higher income then material recovery while overall lower system costs, and lower sensitivity of the system cost, is observed in the material recovery based scenarios. The lowest system costs are calculated for the scenario which combines material and energy recovery and avoids investments in final disposal/recovery facilities by outsourcing this service. The main problem with outsourcing the final disposal/recovery stage is the uncertainty of the cost of such service. It is found that this kind of approach has not been previously used for the assessment of the socioeconomic sustainability of the whole waste management systems and represents a good tool for decision-makers.

Performance evaluation of algae assisted microbial fuel cell under outdoor conditions

This study reports for the first time an operation of an outdoor algae assisted Microbial Fuel Cell (MFC). The MFC (10 L) comprised of low-cost materials like rock phosphate blended clayware & low-density polyethylene bags as anodic & cathodic chamber respectively. Algae biomass after lipid extraction at 2 g/l served as electron donor at the anode. Chlorella vulgaris at cathode provided oxygen as electron acceptor and served as lipid source. The MFCs performed well in all aspects namely energy recovery, algae productivity, and cost of operation. The 5% RP-MFCs gave 0.307 kg/m³d algal productivity, 0.09 kg/m³d lipid productivity, and 11.5318 kWh/m³ of net energy recovery (NER). Rock phosphate served as a slow and constant source of phosphorus supporting algae growth. Proteobacteria (45.14%) were the dominant phyla while Alicyliphilus (5.46%) and Dechloromonas (4.74%) were the dominant genera at the anode. The estimated cost of the system was $11.225 only.

Inferring cost of transport from whole-body kinematics in three sympatric turtle species with different locomotor habits

Chelonians are mechanically unusual vertebrates as an exoskeleton limits their body wall mobility. They generally move slowly on land and have aquatic or semi-aquatic lifestyles. Somewhat surprisingly, the limited experimental work that has been done suggests that their energetic cost of transport (CoT) are relatively low. This study examines the mechanical evidence for CoT in three turtle species that have differing degrees of terrestrial activity. Our results show that Apolone travels faster than the other two species, and that Chelydra has higher levels of yaw. All the species show poor mean levels of energy recovery, and, whilst there is considerable variation, never show the high levels of energy recovery seen in cursorial quadrupeds. The mean mechanical CoT is 2 to 4 times higher than is generally seen in terrestrial animals. We therefore find no mechanical support for a low CoT in these species. This study illustrates the need for research on a wider range of chelonians to discover whether there are indeed general trends in mechanical and metabolic energy costs.

Efficient organic pollutants conversion and electricity generation for carbonate-containing wastewater based on carbonate radical reactions initiated by BiVO4-Au/PVC system

In this paper, we propose an efficient simultaneous refractory organics degradation and electricity generation method for carbonate-containing wastewater based on carbonate radical reactions initiated by a BiVO₄-Au/PVC (PVC: photovoltaic cell) system. In the system, nanoporous BiVO₄ film and Au modified PVC were used as photoanode and photocathode, respectively. HCO₃^- was used as the electrolyte. Carbonate radicals, which have lower recombination rates than hydroxyl radicals and strong oxidation abilities, can be generated easily by the capture reaction of hydroxyl radicals with HCO₃^-, which is one of the most abundant anions in the aquatic environment. The results show that the removal ratios of rhodamine b, methyl orange and methylene blue in the system increased sharply to 77.98 %, 89.15 % and 93.2 % from 18.23 %, 21 % and 23.14 % (BiVO₄-Pt/ SO₄^2-), respectively, after 120 min. Meanwhile, the short-circuit current density is up to 2.19-2.41 times larger than the traditional system. Other common ions in natural water minimally affected the properties of the new system. The excellent performance could be ascribed to large amounts of carbonate radicals in the system, which have great potential for efficient carbonate-containing wastewater treatment and energy recovery.

Energy recovery and nutrients recycling from municipal sewage sludge

Hydrothermal Liquefaction (HTL) could be a promising and better alternative to other techniques for energy recovery from municipal sewage sludge (MSS). However, the nutrients (i.e., N, and P) recovery potential from the byproducts, generated in the HTL of MSS, needs to be studied so that a comprehensive sludge management practice could be adopted. In this study, HTL process temperature (275-400 °C), and reaction time (30-120 min) were first investigated for biocrude yield and release of the nutrients to the aqueous phase liquid (APL) and biochar. The maximum energy recovery (i.e., 59%) and maximum energy return on investment (i.e., 3.5) were obtained at 350 °C and 60 min of holding time. With the increase in HTL reaction time, the concentration of nitrogen in the APL increased (5.1 to 6.8 mg/L) while the concentration of phosphorus decreased (0.89 to 0.22 mg/L); the opposite was observed for the biochar. The nutrient recycling efficiency from the APL using microalgae was found to be strain-specific; nitrogen recycling efficiency by Picochlorum sp. and Chlorella sp. were 95.4 and 58.6%, respectively. The APL, derived from 1 kg MSS, could potentially produce 0.49 kg microalgal biomass. Since the concentrations of various metals in the biochar samples were substantially lower compared to their concentrations in raw MSS, the application of biochar as a soil conditioner could be very promising. Overall, net positive energy could be recovered from MSS using the HTL process, while the nutrients in the APL could be used to cultivate specific microalgae, and biochar could be applied to enhance the soil quality.

Performance and microbial community of a novel combined anaerobic bioreactor integrating anaerobic baffling and anaerobic filtration process for low-strength rural wastewater treatment

A novel combined bioreactor integrating anaerobic baffling and anaerobic filtration process was developed and operated for 210 days to treat low-strength rural wastewater. The effects of hydraulic residence time (HRT) and organic loading rate (OLR) on chemical oxygen demand (COD) removal and methane (CH₄) production of the combined bioreactor were investigated. The combined bioreactor can start up successfully in 25 days and achieve enhanced performance. The COD removal rate and CH₄ yield were influenced significantly by HRT and OLR. The influent COD was removed effectively through the synergistic effects of the anaerobic baffling and anaerobic filtration. The baffle zone played the main role in the degradation of the pollutants, and the filter zone mainly contributed to improve the resistance to shock loading. High-throughput sequencing technology was used to analyze the bacterial and archaeal community structure and diversity. Clostridium_sensu_stricto, Longilinea, Acetoanaerobium, Arcobacter, and Acinetobacter were found to be the dominant bacteria. While Methanothrix and Methanoregula were the dominant archaea, which were responsible for methane generation. This study not only highlights the good energy recovery and resource utilization potential of the combined bioreactor but also presents significant guidance for the application of the combined anaerobic process for low-strength rural wastewater treatment.

The correlations among wastewater internal energy, energy consumption and energy recovery/production potentials in wastewater treatment plant: An assessment of the energy balance

Given the economic and environmental importance of energy use in wastewater treatment plants (WWTPs), the need to assess the energy balance of WWTPs has become a growing concern. Previous studies have suggested that energy balance or even net energy production may be achieved in WWTPs under specific conditions. However, information regarding the energy consumption and the energy recovery/production potential in WWTPs as a function of the influent characteristics is still very limited. In this paper, by exploring the correlations among wastewater internal energy, energy consumption and energy recovery in WWTPs, a novel net energy consumption (NEC) model was developed for predicting the energy self-sufficiency level of WWTPs. From our results, exponential regression showed a high accuracy in predicting the annual energy consumption, the annual excess sludge production and the bioreactor footprints in WWTPs. Wastewater with more internal energy which is determined by influent chemical oxygen demand (COD) concentration and flow rate, not only leads to higher energy consumption in WWTPs, but also results in an increase in the excess sludge production, bioreactor footprints and wastewater volume. This means that the WWTPs could achieve energy saving or even net energy production by incorporating sludge incineration, photovoltaic (PV) generation and thermal energy recovery. By combing regression analysis with theoretical formula, the annual net energy demand of WWTPs reached -0.187-0.466 kWh·m^-3 in the range of wastewater condition studied (the influent COD concentration range of 60-800 mg·L^-1 and the flow rate range of 1296-100,000 m³·d^-1). The NEC model reveals that the net zero energy consumption may be achieved by integrating the better understandings of wastewater internal energy, energy conversion methods and environmental media energy, which is of value to policy makers for the planning of new WWTPs and provides theoretical support for the selection of available energy recovery methods.

Nitrous oxide production from wastewater treatment: The potential as energy resource rather than potent greenhouse gas

Nitrous oxide (N₂O), produced from wastewater treatment, is a potent greenhouse gas and has become a global concern in recent years. However, N₂O has also been commonly used as a powerful oxidant for energy generation. As such, an increasing effort has been devoted to explore the energy potential of N₂O from wastewater treatment processes recently. Nevertheless, the holistic knowledge on energy recovery from nitrogen in wastewater is still lacking for facilitating its further development. Striving for sustainable wastewater treatment, this review paper aimed to give the up-to-date status on several essential aspects regarding the N₂O recovery as an energy resource rather than emission as a greenhouse gas, including energy production via N₂O decomposition, main biotic N₂O production sources, the potential bioprocesses used for N₂O recovery, and the possible N₂O harvesting strategies. We then put forward perspectives for N₂O recovery and future challenges to improve our understanding of the energy generation, microbial processes involved and harvesting approaches in order to potentially achieve sustainable wastewater treatment via N₂O recovery.

Biocomponent-based microalgal transformations into biofuels during the pretreatment and fermentation process

Microalgal cell wall integrity and composition have a significant impact on the fermentation process and biofuel recovery. In this study, various biofuels (bioethanol, higher alcohols (C3-C5), and biodiesel) were produced by the fermentation of carbohydrates and proteins, and transesterification of lipids from three different microalgal strains (Pseudochlorella sp., Chlamydomonas mexicana, and Chlamydomonas pitschmannii), each possessing different proportions of bioconstituents (carbohydrates, proteins, and lipids). Changes in the cell wall structure and thickness were observed before and after fermentation using transmission electron microscopy. Pseudochlorella sp. showed the highest yields of bioethanol (0.45 g-ethanol/g-carbohydrates), higher alcohols (0.44 g-higher alcohols/g-proteins), and biodiesel (0.55 g-biodiesel/g-lipids), which consequently revealed a maximum energy recovery (42%) from whole constituents. This study suggests that different physiological properties, including cell wall thickness and the proportion of bioconstituents in microalgae, could have a significant impact on the pretreatment and fermentation efficiencies for biofuels production.

A review on mainstream deammonification of municipal wastewater: Novel dual step process

The conventional biological nitrogen removal process is receiving increasing pressure partially due to its energy-negative operation. To address this challenge, various mainstream deammonification processes have been explored for energy-neutral municipal wastewater treatment, whereas these processes appear challenging to be sustainably and stably achieved in conventional process configurations. Therefore, this review aimed to provide a comprehensive analysis of the state-of-the-art of mainstream deammonification, while highlighting the major technical challenges. It appeared that recently developed novel dual step process, i.e. A-B processes, could provide a feasible engineering option for mainstream deammonification, where A-stage is designed for COD capture with the aim to enhance energy recovery, and B-stage is tailored for nutrient removal/recovery. This indeed may lead to a promising integrated mainstream deammonification process towards energy-efficient and environmentally sustainable nitrogen removal. Meanwhile, this review also offered an opinion on future municipal wastewater treatment, aiming for concurrent water reclamation and energy recovery.

Energy neutrality versus carbon footprint minimization in municipal wastewater treatment plants

This work aimed to compare the carbon footprint (CF) of six full-scale wastewater treatment plants (WWTPs). The CF was estimated in the range of 23-100 kg CO_2e per population equivalent. In the total CF, the direct emissions held the highest share (62-74%) for the plants with energy recovery from biogas. In the plants depending entirely on the power grid, the indirect emissions due to energy consumption dominated the total CF (69-72%). The estimated CF was found highly sensitive towards the choice of N₂O emission factors. A dual effect of external substrates co-digestion on the CF has been presented. After co-digestion, the overall CF decreased by 7% while increasing the biogas production by 17%. While applying the empirical model, the level of energy neutrality was strongly related to the ratio of the indirect to direct emissions.

Laboratory Conversion of Cultivated Oleaginous Organisms into Biocrude for Biofuel Applications

Hydrothermal liquefaction (HTL) is a thermochemical process for the wet conversion of oleaginous microorganisms and other biomass and carbon rich feedstocks to biofuels under subcritical conditions. It is a novel green process that produces biocrude as a primary product along with other by-products which include gases, aqueous phase coproduct (ACP) and solid residues. Here we describe in detail the protocols for the conversion of biomass to biocrude through HTL and separation, quantification and analyses of HTL products.

Biomethane recovery from olive mill residues through anaerobic digestion: A review of the state of the art technology

Residues from production of olive oil are generated yearly in great amounts, both in liquid and solid forms. Different waste treatment systems were proposed in literature, to minimize environmental pollution while raising the energy recovery. Anaerobic digestion is one of the available routes to recover energy from waste via production of biogas while reducing organic load and pollutants to the environment. The use of farming and agro industrial wastes as co-substrate in anaerobic digestion can induce benefits related to the simultaneous treatment of different wastes. In particular, co-digestion can significantly enhance the process stability as well as the bio-methane generation. This work aims at reviewing the latest achievements in anaerobic digestion of olive mill residues, focusing on the aspects that can mostly favor the process, principally from a technical but also from an economical point of view. For the mono-digestion processes, methane yields up to 419 L_CH4 kg_VS^-1 were reported for olive mill wastewaters (Calabrò et al. 2018), while a production of 740 L_CH4 kg_VS^-1 was achieved when digesting olive mill solid waste together with olive mill wastewater and milk whey (Battista et al. 2015). An increase up to 143% in the methane yield was also reported when the feedstock was subjected to a 5 days aeration before digesting it in a semi-continuous stirred tank reactor (González-González and Cuadros 2015).

Efficient nitrous oxide recovery from incineration leachate by a nosZ-deficient strain of Pseudomonas aeruginosa

In this study, nitrous oxide was recovered from a lab-scale moving-bed biofilm reactor (MBBR) treating partial nitrification-treated leachate supplemented with a nosZ-deficient strain of Pseudomonas aeruginosa. Batch culture tests with the nosZ-deficient strain determined that the threshold for free nitrous acid (FNA) inhibition was 0.016 mg/L and that FNA concentrations above this threshold severely inhibited denitrification and transcription of genes from the dissimilatory nitrate reduction pathway (narG, nirS, and norB). High nitrite removal and N₂O conversion efficiencies (>95%) were achieved with long-term operation of this MBBR. N₂O accounted for the majority of biogas (80%) produced when the MBBR was fed partial nitrification-treated leachate with high nitrite concentrations and the drainage ratio was adjusted to 30%. Bacterial community analysis revealed that the nosZ-deficient Pseudomonas strain remained metabolically active and was primarily responsible for denitrification processes in the reactor. This study presents a promising method for N₂O recovery from incineration leachate.

Substitution modelling in life cycle assessment of municipal solid waste management

Life cycle assessment (LCA) is gaining importance worldwide in guiding waste management policies. The capacity of co-products such as recycled materials and recovered energy to avoid primary production of equivalent products largely determines the environmental performance of waste treatment technologies. Estimating the reductions in resource use, emissions, and impacts enabled by this substitution of primary production is often the most influential and controversial factor in quantifying the overall environmental performance of a waste management strategy. This study aims to critically evaluate the modelling of substitution in LCAs of recovered material from municipal solid waste management systems (MSWMS) by answering two questions. First, to what extent is substitution modelling transparently documented in the literature? Second, are the substitution ratios justified to represent physically realistic replacement of one product by another? To address these questions, we performed a systematic analysis of 51 LCA studies on MSWMS published in the peer-reviewed literature. We found that 22% of the substitution ratios are only implicitly expressed. A significant proportion of substitution ratios is not justified (65%), while for the remaining 35%, justifications do not represent physically realistic substitutions. We call for more rigor and transparency, and we propose guidance for the documentation of substitution ratios, with the aim of reaching more credible and robust analyses. For the justification of a substitution ratio to be considered physically realistic, information should notably be provided concerning loss of quality, the function performed by substitutable materials, and the sector of use.

Substitution modelling in life cycle assessment of municipal solid waste management

Life cycle assessment (LCA) is gaining importance worldwide in guiding waste management policies. The capacity of co-products such as recycled materials and recovered energy to avoid primary production of equivalent products largely determines the environmental performance of waste treatment technologies. Estimating the reductions in resource use, emissions, and impacts enabled by this substitution of primary production is often the most influential and controversial factor in quantifying the overall environmental performance of a waste management strategy. This study aims to critically evaluate the modelling of substitution in LCAs of recovered material from municipal solid waste management systems (MSWMS) by answering two questions. First, to what extent is substitution modelling transparently documented in the literature? Second, are the substitution ratios justified to represent physically realistic replacement of one product by another? To address these questions, we performed a systematic analysis of 51 LCA studies on MSWMS published in the peer-reviewed literature. We found that 22% of the substitution ratios are only implicitly expressed. A significant proportion of substitution ratios is not justified (65%), while for the remaining 35%, justifications do not represent physically realistic substitutions. We call for more rigor and transparency, and we propose guidance for the documentation of substitution ratios, with the aim of reaching more credible and robust analyses. For the justification of a substitution ratio to be considered physically realistic, information should notably be provided concerning loss of quality, the function performed by substitutable materials, and the sector of use.

Performance and microbial community variations of a upflow anaerobic sludge blanket (UASB) reactor for treating monosodium glutamate wastewater: Effects of organic loading rate

To investigate the effect of the organic loading rate (OLR) on anaerobic treatment of monosodium glutamate (MSG) wastewater, a lab-scale up-flow anaerobic blanket (UASB) reactor was continuously operated over a 222-day period. The overall performances of COD removal and methane recovery initially exhibited an increase and subsequently decreased when the OLR was increased from 1 g-COD/L/d to 24 g-COD/L/d. At the optimal OLR of 8 g-COD/L/d, superior performance was obtained with a maximum COD removal efficiency of 97%, a methane production rate of 2.3 L/L/d, and specific methanogenic activity of 86 mg-CH₄/g-VSS/d (feeding on glutamate), with superior characteristics of sludge in VSS concentration, average diameter of granules, and settling velocity. According to the results of the specific methanogenic activity, the methanogenic pathway was more inclined to pass through acetate than through hydrogen. Methanosarcina instead of Methanosaeta, with Methanobacterium and greatly increased Firmicutes, dominated in the UASB reactor after long term operation. These results support that the OLR had a substantial effect on both the treatment and energy recovery efficiency of MSG wastewater as well as on microbial community variations in the UASB reactor.

Organics transformation and energy production potential in a high rate A-stage system: A demo-scale study

Current high-rate activated sludge (HRAS) process is an aerobic A-stage process that would cause significant organic loss resulted from the mineralization. In this study, the feasibility of operating a high rate A-stage without aeration (HRNS) was carried out in a demo-scale plant (275 m³/h). The organics transformation and energy production potential in A-stage were explored. The developed A-stage process was demonstrated to be more effective for organics recovery compared to that operated with aeration (53.82% versus 40.94%), despite its relatively low total COD removal efficiency (54.3% versus 63.5% with aeration). Minor organics (accounted for 1.75% of incoming COD) was found to be lost in HRNS process. Moreover, sludge generated from HRNS had higher degradability and higher methane compared to that from HRAS. Overall, this study documented the feasibility of high rate A-stage without aeration, and acted as a guide in achieving energy neutrality or even energy-positive wastewater treatment.