Microalgae-based Systems Research Articles

Integration of enzymatic pretreatment and sludge co-digestion in biogas production from microalgae

Integration of microalgae-based systems with conventional wastewater treatment plants provides an effective alternative to waste stream management. In this work, alkaline and enzymatic pretreatments of a microalgal culture mainly constituted by Chlorella sp. and Scenedesmus sp. and cultivated in wastewater from an industrial winery wastewater treatment plant were assessed. Microalgal enzymatic pretreatments were expected to overcome algal recalcitrancy before anaerobic digestion. pH-induced flocculation at pH 10 and 11 did not enhance microalgal harvesting and solubilisation, achieving a performance similar to that of natural sedimentation. Enzymatic hydrolysis of algal biomass was carried out using three commercial enzymatic cocktails (A, B and C) at two enzymatic doses (1% and 2% (v/v)) over 3 h of exposure time at 37 °C. Since pretreatments at a 1% dose for 0.5 h and 2% dose for 2 h achieved higher solubilisation, they were selected to evaluate the influence of the pretreatment on microalgal anaerobic digestibility. Biochemical methane potential tests showed that the pretreatments increased the methane production of the raw algal biomass 3.6- to 5.3-fold. The methane yield was 9–27% higher at the lower enzyme dose. Hence, microalgae pretreated with enzymes B and C at a 1% dose were co-digested with waste activated sludge (WAS). Even when the enzyme increased the methane yield of the inoculum and the WAS, the methane yield of the raw microalgae and WAS mixture was not significantly different from that obtained when algae were enzymatically pretreated. Nonetheless, co-digestion may achieve the goals of a waste recycled bio-circular economy.

Trends and progress in Microalgae-based wastewater treatment technologies: A review

A transformative change is underway in wastewater treatment as the world aims at meeting Sustainable Development Goal 6 in 2030, and the conventional wastewater treatment processes have high energy consumption and greenhouse emissions. Microalgae-based wastewater treatment process has emerged as an innovative technology that can reach the demand for lowering energy consumption, mitigating climate change, and recycling resources. This review provides an overview of the basic theories of microalgae-based wastewater treatment processes, microalgae species commonly used, impact factors of microalgae cultivation, the conventional and hybrid microalgae-based wastewater treatment systems. Moreover, suggestions are proposed for further research and development.

The role of microalgae-based systems in the dynamics of odorous compounds in the meat processing industry. Part II – olfactometry and sensory relevance

The role of microalgae-based systems in the dynamics of odorous compounds in the meat processing industry. Part II – olfactometry and sensory relevance

Removal of Nitrogen and Phosphorus from Thickening Effluent of an Urban Wastewater Treatment Plant by an Isolated Green Microalga.

Microalgae are photosynthetic microorganisms and are considered excellent candidates for a wide range of biotechnological applications, including the removal of nutrients from urban wastewaters, which they can recover and convert into biomass. Microalgae-based systems can be integrated into conventional urban wastewater treatment plants (WW-TP) to improve the water depuration process. However, microalgal strain selection represents a crucial step for effective phytoremediation. In this work, a microalga isolated from the effluent derived from the thickening stage of waste sludge of an urban WW-TP was selected and tested to highlight its potential for nutrient removal. Ammonium and phosphate abatements by microalgae were evaluated using both the effluent and a synthetic medium in a comparative approach. Parallelly, the isolate was characterized in terms of growth capability, morphology, photosynthetic pigment content and photosystem II maximum quantum yield. The isolated microalga showed surprisingly high biomass yield and removal efficiency of both ammonium and phosphate ions from the effluent but not from the synthetic medium. This suggests its clear preference to grow in the effluent, linked to the overall characteristics of this matrix. Moreover, biomass from microalgae cultivated in wastewater was enriched in photosynthetic pigments, polyphosphates, proteins and starch, but not lipids, suggesting its possible use as a biofertilizer.

Current advances in microalgae-based treatment of high-strength wastewaters: challenges and opportunities to enhance wastewater treatment performance

Microalgae-based technologies, usually configured as high rate algal ponds (HRAP), are efficient, sustainable, and cost-effective alternatives for wastewater treatment due to their high removal efficiencies at low energy demand, ability to recover nutrients and ease of operation. HRAPs and other photobioreactors have been intensively studied in recent years for the treatment of high-strength wastewaters, which are mainly characterised by high and unbalanced (in terms of microalgae requirements) concentrations of organic carbon and nutrients. This review critically evaluated research papers that used microalgae-based systems for the removal of carbon and nitrogen from high-strength wastewaters. These systems can provide removal efficiencies up to 100% for organic matter and ammonium nitrogen. Relatively large area requirements, high evaporative losses, ammonia inhibition, poor light penetration and scattering, carbon dioxide limitation, and unbalanced nutrient ratios rank among the main current limitations of these technologies. Optimisation strategies, including modifications in bioreactor design and operation, can broaden their full-scale application for the treatment of high strength wastewaters.Graphical Abstract

Scaling-Up the Anaerobic Digestion of Pretreated Microalgal Biomass within a Water Resource Recovery Facility

Microalgae-based wastewater treatment plants are low-cost alternatives for recovering nutrients from contaminated effluents through microalgal biomass, which may be subsequently processed into valuable bioproducts and bioenergy. Anaerobic digestion for biogas and biomethane production is the most straightforward and applicable technology for bioenergy recovery. However, pretreatment techniques may be needed to enhance the anaerobic biodegradability of microalgae. To date, very few full-scale systems have been put through, due to acknowledged bottlenecks such as low biomass concentration after conventional harvesting and inefficient processing into valuable products. The aim of this study was to evaluate the anaerobic digestion of pretreated microalgal biomass in a demonstration-scale microalgae biorefinery, and to compare the results obtained with previous research conducted at lab-scale, in order to assess the scalability of this bioprocess. In the lab-scale experiments, real municipal wastewater was treated in high rate algal ponds (2 × 0.47 m3), and harvested microalgal biomass was thickened and digested to produce biogas. It was observed how the methane yield increased by 67% after implementing a thermal pretreatment step (at 75 °C for 10 h), and therefore the very same pretreatment was applied in the demonstration-scale study. In this case, agricultural runoff was treated in semi-closed tubular photobioreactors (3 × 11.7 m3), and harvested microalgal biomass was thickened and thermally pretreated before undergoing the anaerobic digestion to produce biogas. The results showed a VS removal of 70% in the reactor and a methane yield up to 0.24 L CH4/g VS, which were similar to the lab-scale results. Furthermore, photosynthetic biogas upgrading led to the production of biomethane, while the digestate was treated in a constructed wetland to obtain a biofertilizer. In this way, the demonstration-scale plant evidenced the feasibility of recovering resources (biomethane and biofertilizer) from agricultural runoff using microalgae-based systems coupled with anaerobic digestion of the microalgal biomass.

Characterisation of microalgae-based monocultures and mixed cultures for biomass production and wastewater treatment

Microalgae-based systems have been increasingly considered to remediate wastewater, while producing valued biomass. While microalgal monocultures are difficult to maintain in wastewater, mixed cultures have the potential to offer improved system stability and robustness. This research aims to provide a detailed comparison of microalgal monoculture and mixed culture photobioreactors (PBRs) in terms of microalgae growth, biomass harvesting potential, nutrient removal and organic characteristics. Some of the PBRs were further tested for their responses to the presence of wastewater-derived bacteria. Monocultures (containing species from Chlorophyta, Cyanophyta and Eustigmatophyta, respectively) and mixed cultures (mixing two or three species) were evaluated using advanced characterisation techniques including flow cytometry, liquid chromatography – organic carbon detection and excitation−emission matrix fluorescence spectroscopy. The impact of bacteria on system performance was explored by co-culturing some of the cultures with activated sludge. In general, mixed culture systems showed more stable biomass growth and improved nutrient removal uptake (by 20%), as compared to monocultures. However, overyielding (higher biomass production) was not observed in the mixed cultures. Mixing different microalgae tended to increase the concentrations of soluble extracellular materials (e.g. biopolymers) that were able to retain water within the biomass, leading to a lower biomass harvesting potential, as indicated by the capillary suction time. Adding activate sludge significantly increased reduction of measurable dissolved organic carbon concentration (particularly biopolymers and low molecular weight compounds) and nitrogen by >250%. However, wastewater-derived bacteria reduced the yield of all the cultures substantially (by 63–76%) and lowered the dewaterability of the cultures.

The application of microalgae in removing organic micropollutants in wastewater

Micropollutants have become a serious environmental problem with several negative outcomes for human health and ecosystems. Many efforts have been made to remove micropollutants using a variety of physical, chemical and biological methods. By far, the most attention has been paid to microalgae-based technologies for wastewater treatment in order to obtain high-quality effluents, recover algal biomass for fertilizers, protein-rich feed, biofuel, and put them to other practical use. This paper reviews the potential of microalgae-based systems for the removal of organic micropollutants from open ponds to closed photobioreactors coupled by suspended microalgal cells, immobilized cells, or microalgae-microbial consortia. The inhibition of micropollutants on microalgae growth as well as micropollutant removal mechanisms performed by microalgae-based systems are also discussed. Other treatment methods for the removal of micropollutants are analyzed to show the advantages and limitations of microalgae-based treatment strategies, from which some possible combined systems can be suggested. Finally, some recommendations for future studies on this topic are proposed.

Charting a development path to deliver cost competitive microalgae-based fuels

Access to affordable and clean energy is a United Nations' Sustainable Development Goal. The prospects for cheap, reliable, renewable stationary energy and electric vehicles have rapidly improved, driven by de‑carbonisation and sustainability agendas, enhanced policy interventions and maturing technology options. In contrast, low-carbon renewable liquid fuel production systems remain critically underdeveloped amid market-distorting subsidies to fossil fuels. Advanced microalgae-based systems can deliver renewable fuels essential to meet United Nations Framework Convention on Climate Change COP 21 commitments. Here we use TELCA, an integrated triple-bottom-line Techno-Economic and Life-Cycle Analysis platform (economic - $·L−1 diesel, social – ERoEI MJ·MJ−1, environmental - GHG emissions g CO2·MJ−1), to define the role that key technology advances and policy settings can play in driving down the price of renewable microalgae-based diesel from US$11.11 gal−1 (US$2.93 L−1) to a point of cost competitiveness with fossil-based diesel US$3.00 gal−1 (US$0.79 L−1). This price reduction could be attained while simultaneously achieving an ERoEI of 4.1 and limiting GHG emissions to 21 g CO2-e MJ−1. Importantly, TELCA allows different jurisdictions to competitively adjust their policy settings to attract an expanding microalgae-based renewable fuel industry (case-study provided).

Attenuation of nitrates, antibiotics and pesticides from groundwater using immobilised microalgae-based systems

Groundwater pollution by nitrates and organic microcontaminants (OMCs) such as pesticides and antibiotics has increased in recent years due to the intensification of agriculture and livestock activities. Here we demonstrate, for the first time, the suitability of using microalgae immobilised in different materials (luffa sponge and polyurethane foam) to attenuate nitrates, antibiotics (sulfacetamide, sulfamethazine, and sulfamethoxazole), pesticides (bromacil, atrazine, diuron, bentazone, and mecoprop) from groundwater in two operational modes (batch and continuous-feeding). The results from the batch experiments show that OMC kinetic removal rates ranged from 0.01 to 0.18 d−1, with half-lives from 4 to 69 days. Immobilised microalgae in luffa and foam materials in the batch study was found to enhance the attenuation of selected OMCs from 36% to 51%, on average, after 10 operational days. Microalgae reactors in continuous-feeding operational mode out performed batch mode in terms of OMC removal (65% vs. 50%, on average) at a hydraulic residence time (HRT) of 8 days, whereas nitrate removal was greater in the batch experiments (81 vs. 48%, on average). OMC attenuation showed a high HRT dependence, but immobilised reactors were more resilient to the decrease in HRT. Further studies are needed, including the assessment of transformation products as well as the scale-up of the system to check the feasibility of the technology. Nevertheless, we expect our assay to be the starting point for the applicability of immobilised-microalgae-based systems for the treatment of polluted groundwater.

The role of microalgae-based systems in the dynamics of odors compounds in the meat processing industry

The role of microalgae-based systems in the dynamics of odors compounds in the meat processing industry

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

The techno-environmental performance of a medium-scale wastewater treatment system using high-rate algal ponds was evaluated through mass and energy balances and life cycle assessment. The system involves wastewater primary treatment, microalgae-based secondary treatment, thermal hydrolysis with steam explosion of microalgae, anaerobic co-digestion of pre-treated microalgal biomass and primary sludge, and biogas cogeneration. Furthermore, two scenarios based on alternative biogas uses were considered: (i) biogas for heat and electricity, and (ii) biogas for heat, electricity, and biomethane. Pumping wastewater to the primary settler arose as the main source of electricity consumption. When compared to conventional activated sludge plants, a large decrease in the energy consumption was observed for the secondary treatment. Moreover, a favourable life-cycle performance was generally found for the microalgae-based systems when displacing conventional energy products. Finally, the preference for a specific scenario on biogas use was found to be highly conditioned by the techno-environmental aspects prioritised by decision-makers.

Treatment of food waste digestate using microalgae-based systems with low-intensity light-emitting diodes.

Anaerobic digestion of food wastes coupled with digestate post-treatment using microalgae-based systems could recover large amounts of energy and nutrients worldwide. However, the development of full-scale implementations requires overcoming microalgae inhibition by high ammonia concentrations and low light transmittances affecting photosynthesis. This study evaluated the potential of microalgae-based reactors supplied with red light-emitting diodes (LEDs) at low intensity (660 nm and 15 µmol·m-2·s-1) to treat food waste digestate. LED reactors were compared with control reactors exposed to solar radiation. From a range of species in the inoculum, Chlorella vulgaris showed high adaptation to both lighting regimes and digestate environmental conditions, characterized by a C:N:P ratio of 74:74:1. Removal efficiencies for control and LED reactors were 84.0% and 95.8% for soluble chemical oxygen demand (COD) and 89.4% and 53.0% for ammonia, respectively. Approximately 50% of ammonia in control reactor and 15% in LED reactor was lost from the systems, whereas 17% and 36% of ammonia was transformed to organic nitrogen in control and LED reactors, respectively. Low-intensity LEDs maintained microalgae growth in levels similar to solar radiation and supported efficient digestate treatment, showing a potential for further application in optimization of full scale reactors at a relatively low energy cost.

Nursery of young Litopenaeus vannamei post-larvae reared in biofloc- and microalgae-based systems

A 13-day nursery trial was conducted to evaluate the performance of young Litopenaeus vannamei post-larvae (from PL6 to PL18) reared in both biofloc and microalgae-based systems at a stocking density of 67 PLs L−1. The effects of different concentrations of total suspended solids (TSS) on PL performance were also evaluated. One experimental group was reared in a conventional microalgae-based system with daily water exchange and daily addition of microalgae (herein called microalgae treatment). The other two experimental groups were reared using biofloc technology (BFT) with daily dextrose addition and no water exchange, but in the “Biofloc-500” treatment, TSS were maintained at around 500mgL−1, while in the “Biofloc-700” treatment, TSS were maintained at around 700mgL−1. Water quality variables remained within the appropriate range for larval culture. In microalgae treatment, ammonia control was likely associated with its assimilation into microalgae biomass and daily water exchange. In biofloc tanks, however, the addition of dextrose stimulated the production of bacterial biomass from ammonia. This system required only 12.9% of the water used by the microalgae treatment since water was not exchanged during the culture. The nursery of young PLs resulted in similar (P>0.05) performance in all treatments: survival >94%, PL length ∼ 11.5mm, and PL dry weight ∼ 1.2mg. In addition, the salinity stress test (>90.0%) was not significantly different among treatments. Our results indicate that BFT can be as effective as the microalgae-based system for the nursery of young L. vannamei post-larvae. We also found that post-larvae performance was similar (P>0.05) between biofloc treatments, indicating that organisms can tolerate environments with large quantities of solids.

Production of Chlorella protothecoides biomass, chlorophyll and carotenoids using the dairy industry by-product scotta as a substrate

Microalgae-based systems for the production of high value molecules are an emergent area, representing a great promise for industrial applications. The main challenge, however, is the development of high efficiency strategies for the large-scale production at low costs. The aim of this study was to evaluate the potential of ricotta cheese whey (scotta) to be used as a low-cost alternative substrate to grow the microalga Chlorella protothecoides. Furthermore, a salt and light stress condition was imposed in order to improve the carotenogenesis process. A significant reduction in lactose concentration was observed along the cultivation in the culture mediums containing scotta, indicating that the tested C. protothecoides shifted to mixotrophic growth, using the organic carbon source provided. Mixotrophic cultures presented a higher amount of biomass than the autotrophic one, however, the cellular accumulation of chlorophyll and carotenoids was higher in the latter culture. Despite this, the stress strategy that we applied enhanced carotenogenesis, allowing the cellular accumulation of well quoted carotenoids, namely astaxanthin and lutein/zeaxanthin. The results suggest that scotta has a great potential to be used as a culture medium to grow C. protothecoides. Moreover, through an adequate stress strategy it is possible to control carotenogenesis, allowing the production of high amounts of the desirable high value molecules.

Impact of Natural Cationic Polymers on Charge and Clarification of Microalgae Suspensions

Microalgae-based systems for wastewater treatment, biofuel production, and nutrient recovery require effective solid/liquid separation processes that frequently include coagulation and flocculation steps where discrete particle (i.e., microalgae) suspensions are destabilized to initiate particle agglomeration, followed by conglomeration into larger flocs, respectively. The choice of coagulant affects process economics and sustainability. This study investigated the use of natural cationic polymers (chitosan and cationic starches with varying degrees of substitution) and the synthetic cationic polymer (polydiallyldimethylammonium chloride [pDADMAC]) to flocculate Chlorella protothecoides and Nannochloropsis salina. Algae surface charge and flocculation efficiency were examined in response to variable pH, coagulant dosage, cell concentration, and salinity. Overall effectiveness of coagulation, flocculation, and sedimentation is referred to as flocculation efficiency and is determined by measuring the percentage decrease in optical density of the microalgae suspensions. Cationic starch with a degree of substitution (DS) of 0.5 and chitosan neutralized the charge on C. protothecoides at lower dosages compared with pDADMAC. C. protothecoides flocculation efficiencies >95% were achieved with dense cultures (1 g/L algae dry weight) at cationic starch (DS 0.5) and chitosan dosages of 0.02 g/g algae dry weight. Zeta potential measurements indicated that complete charge neutralization was not necessary for maximum flocculation. Of the polymers tested, zeta potential analysis indicated that only chitosan was sensitive to pH. Maximum flocculation efficiency of N. salina was achieved through chitosan precipitation and subsequent sweep floc at pH 8.0. However, cationic starch (DS 0.5) and pDADMAC optimum doses were lower than that required for N. salina flocculation with chitosan.

From oil refinery to microalgal biorefinery

The objective of this study was to develop an integrated system of biotransformation of carbon dioxide in oil refineries. The liquid and gaseous wastes from oil refining were considered for the cultivation of Aphanothece microscopica Nägeli in a bubble column photobioreactor. Growth kinetics, carbon dioxide removal and oxygen release rates, carbon balance, lipid production and biofuel quality were determined. The results showed the potential use of oil refinery wastes in microalgae-based systems under optimized culture conditions. The maximum specific growth rate was 1.4day−1 and the maximum carbon dioxide elimination capacity was 22.9mg/Lmin. Each CO2 mass unit bioconverted resulted in approximately 0.75 O2 of mass units released. Carbon balance analyses indicated that a small fraction (3.64%) of the carbon dioxide is fixed into a biomass form. Volatile organic compounds (92.0%) were the main products of carbon dioxide transformation in the photobioreactor, in such conditions. For the production of biodiesel, it is possible to obtain 0.08glipid/Lday with this process. The quality properties of the biodiesel were an ester content of 99.7%, a cetane number of 51.3, an iodine value of 79.9gI2/100g, a degree of insaturation of 65.3% and a cold filter plugging point of 24.9°C. Based on these results, the process developed could be considered a promising emerging biorefinery platform of waste-material-utilization type.

Beyond the Fossil Fuel Era: On the Feasibility of Sustainable Electricity Generation Using Biogas from Microalgae

In this century, biofuels will play an important role as fossil fuel alternatives. However, the long-term sustainability of large-scale biomass-based energy production chains is largely unknown. The current study evaluates the effect of nitrogen and phosphorus nutrient recovery on net energy ratio (NER) and land use, comparing a prospective industrial-scale microalgae production system with established maize agriculture. The functional unit was the delivery of 1.0 TWh of electrical energy using biomethane firing. Nutrient recovery was modeled by embedding anaerobic digestion (AD) as downstream processing step in the biomass production chains. The main finding was that maize-based biomethane electricity provision outperforms a prospective microalgae system in terms of NER, estimated at 4.9 and 3.2, respectively, when utilizing cogenerated heat. In the absence of external fossil fuel input, the renewable maize- and microalgae-based systems would require surface areas of 6.2 × 104 and 3.9 × 104 ha, respectively. Sustainability of microalgae-based biofuel production is not set by areal productivity or microalgal lipid content but rather by nutrient recovery, which is an important finding that requires prioritization in microalgae research.

Does participation on concurrent clinical trials compromise the quality of the data or its timely submission

A 13-day nursery trial was conducted to evaluate the performance of young Litopenaeus vannamei post-larvae (from PL6 to PL18) reared in both biofloc and microalgae-based systems at a stocking density of 67 PLs L−1. The effects of different concentrations of total suspended solids (TSS) on PL performance were also evaluated. One experimental group was reared in a conventional microalgae-based system with daily water exchange and daily addition of microalgae (herein called microalgae treatment). The other two experimental groups were reared using biofloc technology (BFT) with daily dextrose addition and no water exchange, but in the “Biofloc-500” treatment, TSS were maintained at around 500 mg L−1, while in the “Biofloc-700” treatment, TSS were maintained at around 700 mg L−1. Water quality variables remained within the appropriate range for larval culture. In microalgae treatment, ammonia control was likely associated with its assimilation into microalgae biomass and daily water exchange. In biofloc tanks, however, the addition of dextrose stimulated the production of bacterial biomass from ammonia. This system required only 12.9% of the water used by the microalgae treatment since water was not exchanged during the culture. The nursery of young PLs resulted in similar (P > 0.05) performance in all treatments: survival >94%, PL length ∼ 11.5 mm, and PL dry weight ∼ 1.2 mg. In addition, the salinity stress test (>90.0%) was not significantly different among treatments. Our results indicate that BFT can be as effective as the microalgae-based system for the nursery of young L. vannamei post-larvae. We also found that post-larvae performance was similar (P > 0.05) between biofloc treatments, indicating that organisms can tolerate environments with large quantities of solids.