Environmental sustainability of integrating the organic Rankin cycle with anaerobic digestion and combined heat and power generation

Two-stage anaerobic membrane bioreactor for co-treatment of food waste and kitchen wastewater for biogas production and nutrients recovery

Co-digestion of organic waste and wastewater is receiving increased attention as a plausible waste management approach toward energy recovery. However, traditional anaerobic processes for co-digestion are particularly susceptible to severe organic loading rates (OLRs) under long-term treatment. To enhance technological feasibility, this work presented a two-stage Anaerobic Membrane Bioreactor (2 S-AnMBR) composed of a hydrolysis reactor (HR) followed by an anaerobic membrane bioreactor (AnMBR) for long-term co-digestion of food waste and kitchen wastewater. The OLRs were expanded from 4.5, 5.6, and 6.9 kg COD m^-3 d^-1 to optimize biogas yield, nitrogen recovery, and membrane fouling at ambient temperatures of 25-32 °C. Results showed that specific methane production of UASB was 249 ± 7 L CH₄ kg^-1 COD_removed at the OLR of 6.9 kg TCOD m^-3 d^-1. Total Chemical Oxygen Demand (TCOD) loss by hydrolysis was 21.6% of the input TCOD load at the hydraulic retention time (HRT) of 2 days. However, low total volatile fatty acid concentrations were found in the AnMBR, indicating that a sufficiently high hydrolysis efficiency could be accomplished with a short HRT. Furthermore, using AnMBR structure consisting of an Upflow Anaerobic Sludge Blanket Reactor (UASB) followed by a side-stream ultrafiltration membrane alleviated cake membrane fouling. The wasted digestate from the AnMBR comprised 42-47% Total Kjeldahl Nitrogen (TKN) and 57-68% total phosphorous loading, making it suitable for use in soil amendments or fertilizers. Finally, the predominance of fine particles (D10 = 0.8 μm) in the ultrafiltration membrane housing (UFMH) could lead to a faster increase in trans-membrane pressure during the filtration process.

A review of conventional and exergetic life cycle assessments of organic Rankine cycle plants exploiting various low-temperature energy resources

The importance of organic Rankine cycle (ORC) plants to the development of future energy infrastructure is widely acknowledged, due largely to their ability to exploit low-temperature thermal energy sources such as industrial waste heat and renewable energy resources. In this regard, different schemes are being proposed in the literature for the technical and economic developments of ORC plants. Also, the environmental feasibility assessments of ORC-based energy systems have been gaining gradual attention recently, but relative to the technical and economic aspects, the life cycle assessment (LCA) studies on ORC are at an infancy stage. It is therefore aimed in this study to systematically review and collate in a single document, the conventional and exergy-based life cycle assessment studies applied to ORC plants. Doing so, it was found that less than 3% of the over 7000 documents available on ORC in the SCOPUS database analyzed the environmental impact. Also, the ecoinvent was observed as the LCA inventory database most frequently in use, usually in the SimaPro software. Additionally, literature data revealed that the choice of the organic working fluid and the consideration of its leakage over the plant's lifetime have significant effects on the environmental impacts of ORC plants. Moreover, the common methods of conducting the exergy-based LCA (exergoenvironmental analysis) of ORC plants are succinctly reported in this manuscript, including the definitions of the most relevant exergoenvironmental performance metrics. It is hoped that this effort would spur the inclusion of LCA in future analyses of ORC plants, towards the achievement of a more sustainable energy conversion technology.

Long-term operation of the pilot scale two-stage anaerobic digestion of municipal biowaste in Ho Chi Minh City

A pilot-scale two-stage anaerobic digestion system, which includes a feed tank (0.4 m³), a hydrolysis reactor (1.2 m³) followed by a methane fermenter (4.0 m³) was set up and run at the municipal solid waste landfill located in Ho Chi Minh City (HCMC), Vietnam. The feed that was separated from urban organic solid waste was collected at households and restaurants in District 1, HCMC. This study aimed to investigate the resource recovery performance of the pilot two-stage anaerobic digestion system, in terms of carbon recovery via biogas production and nutrient recovery from digestate. The average organic loading rate (OLR) of the system was step increased from 1.6 kg volatile solids (VS)·m^-3·d^-1, 2.5 kg VS·m^-3·d^-1 and 3.8 kg VS·m^-3·d^-1 during 400 days of operation. During the long-term operation at three OLRs, pH values and alkalinity were stable at both hydrolysis and methanogenesis stages without any addition of alkalinity for the methanogenesis phase. High buildup of propanoic acid and total volatile fatty acid concentrations in the fermenter did not drop pH values and inhibit the methanogenic process at high OLRs (2.5-3.8 kg VS m^-3·d^-1). The obtained total chemical oxygen demand (tCOD) removal performance was 83-87% at the OLRs ranging from 2.5 kg VS·m^-3·d^-1 and 3.8 kg VS·m^-3·d^-1, respectively. The highest biogas yield of 263 ± 64 L·kg^-1 tCOD removed obtained at OLR of 2.5 kg VS·m^-3·d^-1. It is expected that a full scale 2S-AD plant with capacity of 5200 tons day^-1 of biowaste collected currently from municipal solid waste in HCMC may create daily electricity of 552 MWh, thermal energy of 630 MWh, and recovery of 16.1 tons of NH₄⁺-N, 11.4 tons of organic-N, and 2.1 tons of TP as both organic liquid and solid fertilizers.

Pellet Production from Miscanthus: Energy and Environmental Assessment

The production of wood pellets has grown considerably in the last decades. Besides woody biomass, other feedstocks can be used for pellet production. Among these, miscanthus presents some advantages because, even if specifically cultivated, it requires low inputs such as fertilisers and pesticides and shows high biomass yield (up to 28 tons of dry matter ha−1 in Europe). Even if in the last years some studies evaluated the environmental impact of woody pellet production, there is no information about the environmental performances of miscanthus pellet production. In this study, the environmental impact of miscanthus pellet was evaluated using the Life Cycle Assessment approach with a cradle-to plant gate perspective. Primary data were collected in a small-medium size pelletizing plant located in Northern Italy where miscanthus is cultivated to be directly processed. The results highlight how the miscanthus pellet shows lower environmental impact compared to woody pellet, mainly due to the lower energy consumption during pelletizing. The possibility to pelletize the miscanthus biomass without any drying offsets the environmental impact related to the miscanthus cultivation for all the evaluated impact categories (except for Marine eutrophication). In detail, for global warming potential, 1 ton of miscanthus pellet shows an impact of 121.6 kg CO2 eq. (about 8% lower respect to woody pellet) while for the other evaluated impact categories the impact reduction ranges from 4 to 59%. Harvesting, which unlike the other field operations is carried out every year, is by far the main contributor to the impacts of the cultivation phase while electricity is the main contributor to the pelletizing phase.

Economic and Environmental Impact Assessment of Renewable Energy from Biomass

For a holistic evaluation of sustainability, the economic and environmental aspects should be considered jointly to avoid trade-offs between the two dimensions. In this manuscript, the themes addressed, and the approaches used in this Special Issue “Economic and Environmental Impact Assessment of Renewable Energy from Biomass” to investigate the sustainability are summarized. Different approaches such as Energy Analysis, Life Cycle Assessment, technical and economic evaluation of key processes are applied to different renewable energy pathways (biogas, wood biomass, by-product valorization, etc.). The different manuscripts accepted in this Special Issue increases our comprehension and understanding of the relation between economic and environmental performances of renewable energy from biomass.

Combining Biomass Gasification and Solid Oxid Fuel Cell for Heat and Power Generation: An Early-Stage Life Cycle Assessment

Biomass-fueled combined heat and power systems (CHPs) can potentially offer environmental benefits compared to conventional separate production technologies. This study presents the first environmental life cycle assessment (LCA) of a novel high-efficiency bio-based power (HBP) technology, which combines biomass gasification with a 199 kW solid oxide fuel cell (SOFC) to produce heat and electricity. The aim is to identify the main sources of environmental impacts and to assess the potential environmental performance compared to benchmark technologies. The use of various biomass fuels and alternative allocation methods were scrutinized. The LCA results reveal that most of the environmental impacts of the energy supplied with the HBP technology are caused by the production of the biomass fuel. This contribution is higher for pelletized than for chipped biomass. Overall, HBP technology shows better environmental performance than heat from natural gas and electricity from the German/European grid. When comparing the HBP technology with the biomass-fueled ORC technology, the former offers significant benefits in terms of particulate matter (about 22 times lower), photochemical ozone formation (11 times lower), acidification (8 times lower) and terrestrial eutrophication (about 26 times lower). The environmental performance was not affected by the allocation parameter (exergy or economic) used. However, the tested substitution approaches showed to be inadequate to model multiple environmental impacts of CHP plants under the investigated context and goal.

Environmental Impact of the High Concentrator Photovoltaic Thermal 2000x System

High Concentrator Photovoltaic Thermal (HCPV/T) systems produce both electrical and thermal energy and they are efficient in areas with high Direct Normal Irradiance (DNI). This paper estimates the lifecycle environmental impact of the HCPV/T 2000x system for both electrical and thermal functionalities. Process-based attributional method following the guidelines and framework of ISO 14044/40 was used to conduct the Life Cycle Assessment (LCA). The midpoint and endpoint impact categories were studied. It was found that the main hotspots are the production of the thermal energy system contributing with 50% and 55%, respectively, followed by the production of the tracking system with 29% and 32% and the operation and maintenance with 13% and 7%. The main contributor to the lifecycle environmental impact category indicators was found to be the raw materials acquisition/production and manufacturing of the thermal energy and tracking systems. The results indicate that the lifecycle environmental impact of the HCPV/T 2000x system is lower compared to fuel-based Combined Heat and Power (CHP) and non-Renewable Energy Sources (non-RES) systems.

Enhanced hydrolysis of waste activated sludge for methane production via anaerobic digestion under N2-nanobubble water addition

Anaerobic digestion (AD) is a relatively safe and economically feasible disposal technique for waste activated sludge (WAS), in which hydrolysis of complex organic matters is the rate-limiting step. The aim of this study is to explore the efficiency of applying nitrogen gas nanobubble water (N₂-NBW) to AD of WAS and reveal the possible mechanisms. The possible effects of N₂-NBW on different processes during AD of WAS were investigated and N₂-NBW was expected to enhance the hydrolysis step. Results showed that after N₂-NBW addition, sludge particles possessed more negative charges (indicated by zeta potential) than the control with deionized water (DW) addition. The total methane production of NBW group was 402 mL/g-VS_reduced, 29% higher than the control group. Moreover, mechanism investigations revealed that N₂-NBW addition not only improved the disintegration of high molecular weight compounds (proteins and polysaccharides), but also enhanced the activities of four extracellular hydrolases by 14-17%. Results from the present work showed that the enhancement of N₂-NBW addition on methane production from AD of WAS was mainly through the augmentation of hydrolysis of WAS, as little effect on methanogenesis and VS reduction was discerned. The promotion effect of N₂-NBW on hydrolysis suggests that N₂-NBW addition is a promising pretreatment strategy for AD of WAS with no chemical addition at low energy consumption, thus, increasing the economic feasibility of WAS disposal.

Improving the Sustainability of Dairy Slurry by A Commercial Additive Treatment

Ammonia (NH3), methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2) emissions from livestock farms contribute to negative environmental impacts such as acidification and climate change. A significant part of these emissions is produced from the decomposition of slurry in livestock facilities, during storage and treatment phases. This research aimed at evaluating the effectiveness of the additive “SOP LAGOON” (made of agricultural gypsum processed with proprietary technology) on (i) NH3 and Greenhouse Gas (GHG) emissions, (ii) slurry properties and N loss. Moreover, the Life Cycle Assessment (LCA) method was applied to assess the potential environmental impact associated with stored slurry treated with the additive. Six barrels were filled with 65 L of cattle slurry, of which three were used as a control while the additive was used in the other three. The results indicated that the use of the additive led to a reduction of total nitrogen, nitrates, and GHG emissions. LCA confirmed the higher environmental sustainability of the scenario with the additive for some environmental impact categories among which climate change. In conclusion, the additive has beneficial effects on both emissions and the environment, and the nitrogen present in the treated slurry could partially displace a mineral fertilizer, which can be considered an environmental credit.

Economic and Global Warming Potential Assessment of Flexible Power Generation with Biogas Plants

Demand-oriented power generation by power plants is becoming increasingly important due to the rising share of intermittent power sources in the energy system. Biogas plants can contribute to electricity grid stability through flexible power generation. This work involved conducting an economic and global warming potential (GWP) assessment of power generation with biogas plants that focused on the Austrian biogas sector. Twelve biogas plant configurations with electric rated outputs ranging from 150–750 kW and different input material compositions were investigated. The results from the economic assessment reveal that the required additional payment (premium) to make power generation economically viable ranges from 158.1–217.3 € MWh−1. Further, the GWP of biogas plant setups was analyzed using life cycle assessment. The results range from −0.42 to 0.06 t CO2 eq. MWh−1 and show that the 150 kW plant configurations yield the best outcome regarding GWP. Electricity from biogas in all scenarios outperformed the compared conventional electricity sources within the GWP. Greenhouse gas (GHG) mitigation costs were calculated by relating the needed premium to the CO2 eq. saving potential and range from 149.5–674.1 € (t CO2 eq.)−1.