Energy balance and life cycle assessment of a microalgae-based wastewater treatment plant: A focus on alternative biogas uses

Assessing sustainability of microalgae-based wastewater treatment: Environmental considerations and impacts on human health

An integrated life cycle assessment (LCA) and quantitative microbial risk assessment (QMRA) were conducted to assess microalgae-mediated wastewater disinfection (M-WWD). M-WWD was achieved by replacing ultraviolet disinfection with a microalgal open raceway pond in an existing sewage treatment plant (STP) in India. Regarding impacts on human health, both M-WWD and STP yielded comparable life cycle impacts, around 0.01 disability-adjusted life years (DALYs) per person per year. However, QMRA impacts for M-WWD (0.053 DALYs per person per year) were slightly lower than that for STP while considering exposure to E. coli O157:H7 and adenovirus. Additionally, a comparative LCA resolved the dilemma about the appropriate utilization of microalgal biomass. Among biodiesel, biocrude, and biogas production, the lowest impacts of 0.015 DALYs per person per year were obtained for biocrude for 1 m3 water treated by M-WWD. Electricity consumption in microalgae cultivation was a major environmental hotspot. Overall, M-WWD, followed by production of microalgal biocrude, emerged as a sustainable alternative from environmental and public health perspectives. These findings set the foundation for pilot-scale M-WWD system development, testing, and economic evaluation. Such comprehensive investigations, encompassing LCA, QMRA, and resource recovery scenarios, offer crucial insights for stakeholders and decision-makers in wastewater treatment and environmental management.

The microalgae-based wastewater treatment system coupled with Cerium: A potential way for energy saving and microalgae boost

The microalgae-based system attracts more attention in wastewater treatment for high quality effluent, low carbon emission, and resource utilization. Light is the key factor for algae growth, but the light masking in sewage will cause low efficiency of the system. This study designed laboratory scale experiments with Chlorella to investigate the influence of cerium on the nutrient removal by algae wastewater treatment system under different light intensities. The best removal rates of NH₄-N, TP, and COD were 72.43%, 88.87%, and 68.08% under 50 µmol/(m ²·s) light intensity and 1 mg/L Ce. Low concentration of Ce could activate protein synthesis, electron transfer, and antioxidase, while excessive Ce might cause toxicity which could be relieved by strong light for energy supply and further activating superoxide dismutase (SOD) and catalase (CAT). Comparing to other similar experiences, this system reached an equal or greater performance on nutrients removal with better efficiency in light utilization. It might provide a new idea for microalgae-based system development.

Environmental Assessment of the Life Cycle of Electricity Generation from Biogas in Polish Conditions

Life cycle analysis allows for the assessment of the qualitative and quantitative relationship between selected areas of human activity and the consequences for the environment. One of the important areas is the production of electricity and heat, for which the main raw material in Poland is hard coal. An alternative may be to use biogas as a fuel for energy purposes. This article presents the assessment of environmental hazards caused by the production of energy from biogas. The analysis took into account the change of the substrate from maize silage, commonly used in Polish biogas plants, to waste from the domestic agri-food industry. The evaluation covered the acquisition of substrates, their transport to a biogas plant, generation of electricity from biogas, and management of the generated by-products. The analysis was done in terms of both the impact and sensitivity categories. It was found that the emission of pollutants related to the acquisition of the substrate plays a key role and the use of waste for the production of biogas used for energy production brings environmental benefits. The analysis has shown that replacing coal with biogas, regardless of the raw materials used in its production, results in a positive environmental effect, especially in the areas of human health and resources categories. The positive environmental effect of the production of electricity from biogas can be enhanced by switching raw materials from purpose-grown crops to waste from the agri-food industry and agriculture. An important factor influencing the environmental impact is the degree of heat utilization (the greater the percentage of heat utilization, the greater the environmental benefits) and management of all by-products.

The potential impact of an implementation of microalgae-based wastewater treatment on the energy balance of a municipal wastewater treatment plant in Central europe

Integration of a photobioreactor for WWT by microalgae is calculated as a future alternative for cost-efficient and environmentally-friendly nutrient removal for municipal WWTPs. High growth rates and higher biogas yields (compared to conventional sewage sludge) of algal biomass can significantly improve WWTP energy balances. This study focuses on temperate climate zones with changing seasons and discusses energy potential of microalgae-enhanced wastewater treatment for an existing WWTP (32,000 PE) in Central Germany. For WWTP-dimensioning and determination of energy-rich biomasses for anaerobic digestion and CHP, actual influent load data was used and calculation was carried out according to valid regulations. Algae growth figures are based on pilot-scale test series from Germany and correspond to the relevant climatic and local process conditions. Computed results show a shift in the energy balance from a current energy demand of 662,173kWh a^-1 to an energy production of approx. 1,9MWh_el. a^-1 and 1 MWh_th. a^-1.

Sustainability evaluation of microalgae biodiesel production process integrated with nutrient close-loop pathway based on emergy analysis method

To make comprehensive assessment on sustainability of microalgae biofuel production process integrated with nutrient close-loop pathways, Emergy Analysis methodology was adopted based on case studies: microalgae biodiesel production integrated with Nutrient Recycling Pathway in Case A and microalgae biodiesel production integrated with Protein Production as By-Product Pathway in Case B. Emergy results show that microalgae biodiesel system integrated with Nutrient Recycling Pathway is more sustainable, and factor analysis shows that water source with higher unit emergy value and electricity with lower one are more favorable to improve sustainability performance of the integrated process. Besides, different generations of biofuel are also assessed by Emergy Analysis method, and the third-generation biodiesel shows the most sustainable potentials than the previous.

Life cycle and techno-economic assessment of source-separated wastewater-integrated microalgae biofuel production plant: A nutrient organization approach

Microalgae cultivating in wastewater enabling nutrient uptake for biomass and biofuel generation is regarded as a promising pathway to increase sustainability of microalgae biofuel production process. A source-separated nutrient organization approach for the process of wastewater-based microalgae biofuel is proposed in this study. Comparative life cycle assessment (LCA) method and Techno-economic analysis (TEA) approach are employed to assess sustainability and economic performance of the proposed nutrient approach. Two scenarios are used to validate our study. Scenario 1 is based on the source-separated nutrient delivery approach with microalgae integrated wastewater in tertiary treatment. Scenario 2 is based on non-separated-point nutrient delivery policy and microalgae integrated with secondary wastewater treatment. The results show that the source-separated nutrient approach is effective for reducing the environmental impacts and increasing commercial potential of microalgae biofuel.

Bioprocesses for the recovery of bioenergy and value-added products from wastewater: A review

Wastewater and activated sludge present a major challenge worldwide. Wastewater generated from large and small-scale industries, laundries, human residential areas and other sources is emerging as a main problem in sanitation and maintenance of smart/green cities. During the last decade, different technologies and processes have been developed to recycle and purify the wastewater. Currently, identification and fundamental consideration of development of more advanced microbial-based technologies that enable wastewater treatment and simultaneous resource recovery to produce bioenergy, biofuels and other value-added compounds (organic acids, fatty acids, bioplastics, bio-pesticides, bio-surfactants and bio-flocculants etc.) became an emerging topic. In the last several decades, significant development of bioprocesses and techniques for the extraction and recovery of mentioned valuable molecules and compounds from wastewater, waste biomass or sludge has been made. This review presents different microbial-based process routes related to resource recovery and wastewater application for the production of value-added products and bioenergy. Current process limitations and insights for future research to promote more efficient and sustainable routes for this under-utilized and continually growing waste stream are also discussed.

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.

A life cycle assessment of energy recovery using briquette from wastewater grown microalgae biomass

Microalgae biomass (MB) is a promising source of renewable energy, especially when the cultivation is associated with wastewater treatment. However, microalgae wastewater technologies still have much to improve. Additionally, microalgae biomass valorization routes need to be optimized to be a sustainable and feasible source of green bioenergy. Thus, this paper aimed to evaluate the environmental impacts of the production of briquettes from MB, cultivated during domestic wastewater treatment. Also, it was evaluated how much the drying of the MB affected the life cycle and the environment. Improvements in the life cycle to mitigate the environmental impacts of this energy route were proposed. Cradle-to-gate modeling was applied to obtain a life cycle assessment (LCA) from cultivation to the valorization of MB, through its transformation into a solid biofuel. With LCA, it was possible to identify which technical aspect of the process needs to be optimized so that environmental sustainability can be achieved. Two scenarios were compared, one with the microalgae growth in a high-rate algal pond (HRAP) (scenario 1) and the other in a hybrid reactor, formed by a HRAP and a biofilm reactor (BR) (scenario 2). LCA highlighted the electric power mix, representing, on average, 60% of the total environmental impacts in both scenarios. The valorization of MB in briquettes needs to consume less energy to offset its yield. The environment suffered pressure in freshwater eutrophication, due to the release of 3.1E-05 and 3.9E-05 kg of phosphorus equivalent; in fossil resources scarcity, with the extraction of 1.4E-02 and 4.5E-02 kg of oil equivalent; and in climate change, by the emission of 1.0E-01 and 1.9E-01 kg of carbon dioxide (CO₂) equivalent, in scenarios 1 and 2, respectively. Scenario 1 was highly damaging to terrestrial ecotoxicity, with the release of 3.5E-01 kg of 1,4 Dichlorobenzene, coming from the CO₂ used in MB growth. This category was the one that most negatively pressured the environment, differing from scenario 2, in which this input was not required. This was the only impact category in which scenario 2 had a better environmental performance when compared to scenario 1. Cotton, required in scenario 2, represented up to 87% of emissions in some of the evaluated categories. Despite the impacts that occurred in the two modeled scenarios, the environmental gains due to the use of wastewater for microalgae growth, replacing the synthetic cultivation medium, stood out. In the sensitivity analysis, two alternative scenarios were proposed: (i) electricity consumption for drying has been reduced, due to the natural decrease of MB humidity, and (ii) MB briquettes were considered a substitute for coal briquettes. Results indicated that pressures on climate change and fossil resource scarcity were eliminated in both scenarios and this also occurred for freshwater eutrophication in scenario 2. This paper contributes to the improvement and development of converting MB routes into more sustainable products, causing less pressure on the environment. Also, the study contributes to filling a gap in the literature, discussing methods and technologies to be improved, and consequently making microalgae biotechnology environmentally feasible and a potential renewable energy alternative.

Life cycle assessment of volatile fatty acids production from protein- and carbohydrate-rich organic wastes

Volatile fatty acids (VFAs) are platform molecules with numerous applications. They can be obtained by adjusting the operational conditions of anaerobic digestion to avoid methanogenesis while focusing on fermentative stages. There are gaps in the knowledge of how, from a life-cycle perspective, the fermentative process performs in VFAs production from waste, including environmental consequences of substituting common commodities in the current market. Mass and energy balances of VFAs production from protein-rich microalgal and carbohydrate-rich agro-industrial wastes were used herein as a key source of inventory data for life cycle assessment. Two waste treatment options were considered: (i) VFAs production (anaerobic fermentation) plus anaerobic digestion of the resulting waste after VFAs separation, and (ii) anaerobic digestion of the original waste for bioenergy. Several scenarios were formulated to evaluate their life-cycle performance. VFAs production generally shows a better environmental behaviour than conventional anaerobic digestion, principally due to the substitution of conventional chemicals.

Integration of a side-stream microalgae process into a municipal wastewater treatment plant: A life cycle analysis

This study investigates the environmental improvements associated to the integration of a microalgae unit as a side-stream process within an existing municipal wastewater treatment facility in northern Italy. Microalgae are fed on the centrate from sludge dewatering, rich in nutrients, and on the CO₂ in the flue-gas of the combined heat and power unit. The produced biomass is recirculated upflow the water line where it settles and undergoes anaerobic digestion generating extra biogas. A life cycle assessment was performed collecting primary data from an algal pilot-scale plant installed at the facility. Fifteen environmental indicators were evaluated. Compared to the baseline wastewater treatment, the new algal configuration allows an improvement for 7 out of 15 indicators mainly thanks to the electricity savings in the facility. Some recommendations are provided to improve the performance of the algal system in the scaling up.

Wastewater treatment using oxygenic photogranule-based process has lower environmental impact than conventional activated sludge process

The Life Cycle Assessment (LCA) methodology was applied to assess the environmental feasibility of a novel wastewater treatment technology based on oxygenic photogranules (OPG) biomass in comparison to a conventional activated sludge (CAS) system. LCA using laboratory scale experimental data allowed for eco-design of the process during the early stage of process development at laboratory scale. Electricity consumption related to artificial lighting, the fate of the generated biomass (renewable energy and replacement of mineral fertilizer), and the nitrogen flows in the OPG system were identified as major contributors to the potential environmental impact of the OPG treatment system. These factors require optimization in order to reduce the environmental impact of the overall OPG system. Nonetheless, the environmental impact of a non-optimized OPG scenario was generally lower than for a CAS reference system. With an optimization of the artificial lighting system, an energy neutral treatment system may be within reach.

Life-cycle assessment of sewage sludge-based large-scale biogas plant

The aim of this work was to study the life-cycle assessment (LCA) and impact of a biogas plant based on the municipal sewage sludge (6000 m³ capacity biogas plant at Wastewater Treatment Plant (WWTP), Delawas, Jaipur - Rajasthan, India), analyzing the environmental effects instigated due to basic systems of biogas production and also to examine the impact of using biogas as an alternate fuel using ReCiPe and midpoint methods. The results indicatedthat the construction of plant was insignificant to the whole life cycle impacts. Biogas plant showed negative GHG emissions (-0.2385 kg CO₂ eq/m³) compared to coal-based electricity plants and digestate produced could be a good option to replace chemical fertilizer. Biogas production and agricultural spreading contributed -3.059E-08 kg CFC-11 eq/m³ towards beneficial effects which may be attributed to the avoidance of electricity and artificial fertilizers. The results indicated that sewage sludge-based biogas plant showed beneficial impact on the environment.

A review of biochemical and thermochemical energy conversion routes of wastewater grown algal biomass

Microalgae are recognized as a potential source of biomass for obtaining bioenergy. However, the lack of studies towards economic viability and environmental sustainability of the entire production chain limits its large-scale application. The use of wastewaters economizes natural resources used for algal biomass cultivation. However, desirable biomass characteristics for a good fuel may be impaired when wastewaters are used, namely low lipid content and high ash and protein contents. Thus, the choice of wastewaters with more favorable characteristics may be one way of obtaining a more balanced macromolecular composition of the algal biomass and therefore, a more suitable feedstock for the desired energetic route. The exploration of biorefinery concept and the use of wastewaters as culture medium are considered as the main strategic tools in the search of this viability. Considering the economics of overall process, direct utilization of wet biomass using hydrothermal liquefaction or hydrothermal carbonization and anaerobic digestion is recommended. Among the explored routes, anaerobic digestion is the most studied process. However, some main challenges remain as little explored, such as a low energy pretreatment and suitable and large-scale reactors for algal biomass digestion. On the other hand, thermochemical conversion routes offer better valorization of the algal biomass but have higher costs. A biorefinery combining anaerobic digestion, hydrothermal carbonization and hydrothermal liquefaction processes would provide the maximum possible output from the biomass depending on its characteristics. Therefore, the choice must be made in an integrated way, aiming at optimizing the quality of the final product to be obtained. Life cycle assessment studies are critical for scaling up of any algal biomass valorization technique for sustainability. Although there are limitations, suitable integrations of these processes would enable to make an economically feasible process which require further study.

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.

Unveiling microbial structures during raw microalgae digestion and co-digestion with primary sludge to produce biogas using semi-continuous AnMBR systems

Methane production from microalgae can be enhanced through anaerobic co-digestion with carbon-rich substrates and thus mitigate the inhibition risk associated with its low C:N ratio. Acclimated microbial communities for microalgae disruption can be used as a source of natural enzymes in bioenergy production. However, co-substrates with a certain microbial diversity such as primary sludge might shift the microbial structure. Substrates were generated in a Water Resource Recovery Facility (WRRF) and combined as follows: Scenedesmus or Chlorella digestion and microalgae co-digestion with primary sludge. The study was performed using two lab-scale Anaerobic Membrane Bioreactors (AnMBR). During three years, different feedstocks scenarios for methane production were evaluated with a special focus on the microbial diversity of the AnMBR. 57% of the population was shared between the different feedstock scenarios, revealing the importance of Anaerolineaceae members besides Smithella and Methanosaeta genera. The addition of primary sludge enhanced the microbial diversity of the system during both Chlorella and Scenedesmus co-digestion and promoted different microbial structures. Aceticlastic methanogen Methanosaeta was dominant in all the feedstock scenarios. A more remarkable role of syntrophic fatty acid degraders (Smithella, Syntrophobacteraceae) was observed during co-digestion when only microalgae were digested. However, no significant changes were observed in the microbial composition during anaerobic microalgae digestion when feeding only Chlorella or Scenedesmus. This is the first work revealing the composition of complex communities for semi-continuous bioenergy production from WRRF streams. The stability and maintenance of a microbial core over-time in semi-continuous AnMBRs is here shown supporting their future application in full-scale systems for raw microalgae digestion or co-digestion.

Mexican biomasses valorization through pyrolysis process: Environmental and costs analysis

Biomasses valorization by pyrolysis is a good option for reducing environmental problems. In this study, the environmental performance of three Mexican biomass valorizations (castor husk, coffee pulp and Pinus sawdust) by the pyrolysis was compared. The environmental impacts of all equipment involved in pyrolysis were evaluated. In addition, the financial viability of pyrolysis technology of coffee pulp was studied. The biomass with the lowest impact for all the selected categories was the Pinus sawdust, followed by castor husk and coffee pulp. The GWP category had values greater than 700 kg CO₂eq for all the biomass studied. GWP category is caused by the emissions, mainly due to the high amounts of CH₄ and CO₂ released for all the studied biomasses. Furthermore, the equipment with the greatest impact are the separator, the pyrolyzer and the cyclone. Finally, it was observed that even the least favorable biomass with the environment is viable from a financial point of view.

Integration of Green Energy and Advanced Energy-Efficient Technologies for Municipal Wastewater Treatment Plants

Wastewater treatment can consume a large amount of energy to meet discharge standards. However, wastewater also contains resources which could be recovered for secondary uses under proper treatment. Hence, the goal of this paper is to review the available green energy and biomass energy that can be utilized in wastewater treatment plants. Comprehensive elucidation of energy-efficient technologies for wastewater treatment plants are revealed. For these energy-efficient technologies, this review provides an introduction and current application status of these technologies as well as key performance indicators for the integration of green energy and energy-efficient technologies. There are several assessment perspectives summarized in the evaluation of the integration of green energy and energy-efficient technologies in wastewater treatment plants. To overcome the challenges in wastewater treatment plants, the Internet of Things (IoT) and green chemistry technologies for the water and energy nexus are proposed. The findings of this review are highly beneficial for the development of green energy and energy-efficient wastewater treatment plants. Future research should investigate the integration of green infrastructure and ecologically advanced treatment technologies to explore the potential benefits and advantages.