Algal biomass valorization for biofuel production and carbon sequestration: a review

The world is experiencing an energy crisis and environmental issues due to the depletion of fossil fuels and the continuous increase in carbon dioxide concentrations. Microalgal biofuels are produced using sunlight, water, and simple salt minerals. Their high growth rate, photosynthesis, and carbon dioxide sequestration capacity make them one of the most important biorefinery platforms. Furthermore, microalgae's ability to alter their metabolism in response to environmental stresses to produce relatively high levels of high-value compounds makes them a promising alternative to fossil fuels. As a result, microalgae can significantly contribute to long-term solutions to critical global issues such as the energy crisis and climate change. The environmental benefits of algal biofuel have been demonstrated by significant reductions in carbon dioxide, nitrogen oxide, and sulfur oxide emissions. Microalgae-derived biomass has the potential to generate a wide range of commercially important high-value compounds, novel materials, and feedstock for a variety of industries, including cosmetics, food, and feed. This review evaluates the potential of using microalgal biomass to produce a variety of bioenergy carriers, including biodiesel from stored lipids, alcohols from reserved carbohydrate fermentation, and hydrogen, syngas, methane, biochar and bio-oils via anaerobic digestion, pyrolysis, and gasification. Furthermore, the potential use of microalgal biomass in carbon sequestration routes as an atmospheric carbon removal approach is being evaluated. The cost of algal biofuel production is primarily determined by culturing (77%), harvesting (12%), and lipid extraction (7.9%). As a result, the choice of microalgal species and cultivation mode (autotrophic, heterotrophic, and mixotrophic) are important factors in controlling biomass and bioenergy production, as well as fuel properties. The simultaneous production of microalgal biomass in agricultural, municipal, or industrial wastewater is a low-cost option that could significantly reduce economic and environmental costs while also providing a valuable remediation service. Microalgae have also been proposed as a viable candidate for carbon dioxide capture from the atmosphere or an industrial point source. Microalgae can sequester 1.3 kg of carbon dioxide to produce 1 kg of biomass. Using potent microalgal strains in efficient design bioreactors for carbon dioxide sequestration is thus a challenge. Microalgae can theoretically use up to 9% of light energy to capture and convert 513 tons of carbon dioxide into 280 tons of dry biomass per hectare per year in open and closed cultures. Using an integrated microalgal bio-refinery to recover high-value-added products could reduce waste and create efficient biomass processing into bioenergy. To design an efficient atmospheric carbon removal system, algal biomass cultivation should be coupled with thermochemical technologies, such as pyrolysis.

     

Flow around surface-mounted permeable cubes on solid and deformable surfaces

Environmental Fluid Mechanics - A wind tunnel experiment was carried out to characterize the flow surrounding rectangular prisms of varying permeability, each set mounted on a stationary plane-bed...     

The Effect of Shoreline Recreational Angling Activities on Aquatic and Riparian Habitat Within an Urban Environment: Implications for Conservation and Management

There is growing concern that recreational shoreline angling activity may negatively impact littoral and riparian habitats independent of any direct or indirect influences of fish harvest or fishing mortality through mechanisms such as disturbance (e.g., trampling, erosion) and pollution (e.g., littering). We sampled a suite of aquatic and terrestrial variables (i.e., water quality, aquatic and terrestrial macrophytes, soil compaction, anthropogenic refuse) at 14 high shoreline angling-activity sites (identified by way of interviews with conservation officers and angling clubs) within an urban area (Ottawa, Canada). For each high angling-activity site, a nearby corresponding low angling-activity site was sampled for comparison. We found that the percentage of barren area and soil compaction were greater in areas of high angling activity compared with areas that experienced relatively low angling activity. In addition, terrestrial and aquatic macrophyte density, height, and diversity were lower at high angling-activity sites. Angling- and non-angling-related litter was present in large quantities at each of the high angling-activity sites, and comparatively little litter was found at low angling-activity sites. Collectively, these findings indicate that shoreline angling does alter the riparian environment, contributing to pollution and environmental degradation in areas of high angling intensity. With growing interest in providing urban angling opportunities and in response to increasing interest in developing protected areas and parks, a better understanding of the ecologic impacts of shoreline angling is necessary to address multiuser conflicts, to develop angler outreach and educational materials, and to optimize management of angling effort to maintain ecologic integrity of riparian and aquatic ecosystems.     

Predicting Opportunities for Greening and Patterns of Vegetation on Private Urban Lands

This paper examines predictors of vegetative cover on private lands in Baltimore, Maryland. Using high-resolution spatial data, we generated two measures: “possible stewardship,” which is the proportion of private land that does not have built structures on it and hence has the possibility of supporting vegetation, and “realized stewardship,” which is the proportion of possible stewardship land upon which vegetation is growing. These measures were calculated at the parcel level and averaged by US Census block group. Realized stewardship was further defined by proportion of tree canopy and grass. Expenditures on yard supplies and services, available by block group, were used to help understand where vegetation condition appears to be the result of current activity, past legacies, or abandonment. PRIZM™ market segmentation data were tested as categorical predictors of possible and realized stewardship and yard expenditures. PRIZM™ segmentations are hierarchically clustered into 5, 15, and 62 categories, which correspond to population density, social stratification (income and education), and lifestyle clusters, respectively. We found that PRIZM 15 best predicted variation in possible stewardship and PRIZM 62 best predicted variation in realized stewardship. These results were further analyzed by regressing each dependent variable against a set of continuous variables reflective of each of the three PRIZM groupings. Housing age, vacancy, and population density were found to be critical determinants of both stewardship metrics. A number of lifestyle factors, such as average family size, marriage rates, and percentage of single-family detached homes, were strongly related to realized stewardship. The percentage of African Americans by block group was positively related to realized stewardship but negatively related to yard expenditures.     

Wetland Flowpaths Mediate Nitrogen and Phosphorus Concentrations across the Upper Mississippi River Basin

Eutrophication, harmful algal blooms, and human health impacts are critical environmental challenges resulting from excess nitrogen and phosphorus in surface waters. Yet we have limited information regarding how wetland characteristics mediate water quality across watershed scales. We developed a large, novel set of spatial variables characterizing hydrological flowpaths from wetlands to streams, that is, “wetland hydrological transport variables,” to explore how wetlands statistically explain the variability in total nitrogen (TN) and total phosphorus (TP) concentrations across the Upper Mississippi River Basin (UMRB) in the United States. We found that wetland flowpath variables improved landscape-to-aquatic nutrient multilinear regression models (from R² = 0.89 to 0.91 for TN; R² = 0.53 to 0.84 for TP) and provided insights into potential processes governing how wetlands influence watershed-scale TN and TP concentrations. Specifically, flowpath variables describing flow-attenuating environments, for example, subsurface transport compared to overland flowpaths, were related to lower TN and TP concentrations. Frequent hydrological connections from wetlands to streams were also linked to low TP concentrations, which likely suggests a nutrient source limitation in some areas of the UMRB. Consideration of wetland flowpaths could inform management and conservation activities designed to reduce nutrient export to downstream waters.     

Quantifying the effects of trampling and habitat edges on forest understory vegetation – A field experiment

We investigated the effects of human trampling on boreal forest understory vegetation on, and off paths from suburban forest edges towards the interiors and on the likelihood of trampling-aided dispersal into the forests for three years by carrying out a trampling experiment. We showed that the vegetation was highly sensitive to trampling. Even low levels of trampling considerably decreased covers of the most abundant species on the paths. Cover decreased between 10 and 30% on paths which had been trampled 35 times, and at least by 50% on those trampled 70–270 times. On-path vegetation cover decreased similarly at forest edges and in the interiors. However, some open habitat plant species that occurred outside the forest patches and at forest edges dispersed into the forests, possibly through the action of trampling. A higher cover percentage of an open habitat species at the forest edge line increased its probability to disperse into the forest interior. The vegetation community on, next to, and away from lightly trampled paths remained the same throughout the trampling experiment. For heavily trampled paths, the community changed drastically on the paths, but stayed relatively similar next to and away from the paths. As boreal vegetation is highly sensitive to the effects of trampling, overall ease of access throughout the forest floor should be restricted to avoid the excessive creation of spontaneous paths. To minimize the effects of trampling, recreational use could be guided to the maintained path network in heavily used areas.     

Quantitative evaluation of the chilled-ammonia process for CO2 capture using thermodynamic analysis and process simulation

There is strong world-wide interest in developing new and improved processes for post-combustion capture of CO₂, often using chemical absorption. Developers of new processes make positive claims for their proposals in terms of low energy consumption, but these are usually difficult to validate. This paper demonstrates that rigorous application of thermodynamic analysis and process simulation provides a powerful way to quantitatively estimate the energy requirements of CO₂-capture processes by applying the methodology to the analysis and evaluation of the chilled-ammonia process.     

Societal implications of sustainable energy action plans: from energy modelling to stakeholder learning

This article investigates the potential impact of sustainable energy action plans (SEAPs) on local development through a two-step methodology involving participatory planning and quantitative analysis. The first phase relies on a participatory system mapping (PSM) approach and generates a causal structure at the basis of the urban model. In the second phase, we transform the qualitative map into a system dynamic model which evaluates the effect of the SEAP on social, economic and environmental indicators. This methodology was applied to the case of Cascina Municipality (Italy). Through scenario analysis, we show that some indirect feedback can harm the achievement of the 20% emission reduction target. This process allows the local authority and stakeholders to evaluate the impact of emission reduction policies on CO₂ emissions and local development, thereby generating collective learning on the systemic implications of the plan. We show that this method can enhance the ambition of emission mitigation efforts by small towns.     

Microbial biomass yield and turnover in soil biodegradation tests: carbon substrate effects

Most of the standardized biodegradation tests used to assess the ultimate biodegradation of environmentally degradable polymers are based solely on the determination of net evolved carbon dioxide. However, under aerobic conditions, it has to be considered that heterotrophic microbial consortia metabolize carbon substrates both to carbon dioxide and in the production of new cell biomass. It is generally accepted that in the relatively short term, 50% of the carbon content of most organic substrates is converted to CO₂, with the remaining carbon being assimilated as biomass or incorporated into humus. The latter is particularly important when the metabolism of the organic matter occurs in a soil environment. A straightforward relationship between the free-energy content of a carbon substrate (expressed as the standard free-energy of combustion) and its propensity for conversion to new microbial biomass rather than mineralization to CO₂ has been established. This can potentially lead to underestimation of biodegradation levels of test compounds, especially when they consist of carbon in a fairly low formal oxidation state and relatively high free-energy content. In the present work, the metabolism of different kind of carbon substrates, especially in soil, is reviewed and compared with our own experimental results from respirometric tests. The results show that conversion of highly oxidized materials, such as the commonly used reference materials, cellulose or starch, to CO₂ may be significantly overestimated. The addition of glucosidic material to soil leads to greatly increased respiration and is accompanied by a very low conversion to biomass or humic substances. In contrast, relatively less oxidized substrates metabolize more slowly to give both CO₂ and biomass to an extent which may be significantly underestimated if glucosidic materials are used as the reference. The need for an overall carbon balance taking into account both the carbon immobilized as biomass and that volatized as CO₂ must be considered in standard respirometric procedures for assessing the biodegradability of slowly degrading macromolecules.     

Impacts of Mauritia flexuosa degradation on the carbon stocks of freshwater peatlands in the Pastaza-Marañón river basin of the Peruvian Amazon

Tropical peat swamp forests (PSF) are characterized by high quantities of carbon (C) stored as organic soil deposits due to waterlogged conditions which slows down decomposition. Globally, Peru has one of the largest expanse of tropical peatlands, located primarily within the Pastaza-Marañón river basin in the Northwestern Peru. Peatland forests in Peru are dominated by a palm species—Mauritia flexuosa, and M. flexuosa-dominated forests cover ~?80% of total peatland area and store ~?2.3 Pg C. However, hydrologic alterations, land cover change, and anthropogenic disturbances could lead to PSF’s degradation and loss of valuable ecosystem services. Therefore, evaluation of degradation impacts on PSF’s structure, biomass, and overall C stocks could provide an estimate of potential C losses into the atmosphere as greenhouse gases (GHG) emissions. This study was carried out in three regions within Pastaza-Marañón river basin to quantify PSF’s floristic composition and degradation status and total ecosystem C stocks. There was a tremendous range in C stocks (Mg C ha^?1) in various ecosystem pools—vegetation (45.6–122.5), down woody debris (2.1–23.1), litter (2.3–7.8), and soil (top 1 m; 109–594). Mean ecosystem C stocks accounting for the top 1 m soil were 400, 570, and 330 Mg C ha^?1 in Itaya, Tigre, and Samiria river basins, respectively. Considering the entire soil depth, mean ecosystem C stocks were 670, 1160, and 330 Mg C ha^?1 in Itaya, Tigre, and Samiria river basins, respectively. Floristic composition and calcium to Magnesium (Ca/Mg) ratio of soil profile offered evidence of a site undergoing vegetational succession and transitioning from minerotrophic to ombrotrophic system. Degradation ranged from low to high levels of disturbance with no significant difference between regions. Increased degradation tended to decrease vegetation and forest floor C stocks and was significantly correlated to reduced M. flexuosa biomass C stocks. Long-term studies are needed to understand the linkages between M. flexuosa harvest and palm swamp forest C stocks; however, river dynamics are important natural drivers influencing forest succession and transition in this landscape.