Landfill leachate--a water and nutrient resource for algae-based biofuels

Enhancing nitrogen removal in mature landfill leachate by mixed microalgae through elimination of inhibiting factors

Nitrogen removal from landfill leachate (LL) using microalgae is a promising method and can realize CO₂ mitigation. But the performances are usually inhibited by high chromaticity, high free ammonia (FAN) and some complex macro molecular organic matter (MOM) in the LL. To achieve efficient nitrogen removal from LL, this study firstly pretreated the mature LL with ozone, decolorizer and activated sludge (AS) respectively, and then inoculated with mixed microalgae. The results showed that the synergistic effect of ozonation and microalgae was the best among the three, with 99.7% ammonia removal, 0.77 g/L (dry weight) microalgae biomass, and a maximum growth rate of 160 mg/L/d. Ozonation pretreatment significantly reduced the chromaticity and macromolecular organic matter of LL, with the chromaticity reduced from 2225 to 225 times and the 3D fluorescence intensity representing MOM reduced from 4089 a.u. to 986.1 a.u.. And it was found that the mixed microalgae grown after pretreatment by three different methods all were mostly Chlorella and very few Microcystis, and the density of microalgal populations (number of cells per unit volume) after ozonation was up to 10,650 cells/μL. This work provides a feasible and an economical way to remove ammonia nitrogen (NH+ 4-N) from landfill leachate.

Emerging microalgae-based technologies in biorefinery and risk assessment issues: Bioeconomy for sustainable development

Industrial wastewater treatment is of paramount importance considering the safety of the aquatic ecosystem and its associated health risk to humankind inhabiting near the water bodies. Microalgae-based technologies for remediation of environmental pollutants present avenues for bioenergy applications and production of value-added biochemicals having pharmaceutical, nutraceutical, antioxidants, carbohydrate, phenolics, long-chain multi-faceted fatty acids, enzymes, and proteins which are considered healthy supplements for human health. Such a wide range of products put up a good case for the biorefinery concept. Microalgae play a pivotal role in degrading complex pollutants, such as organic and inorganic contaminants thereby efficiently removing them from the environment. In addition, microalgal species, such as Botryococcus braunii, Tetraselmis suecica, Phaeodactylum tricornutum, Neochloris oleoabundans, Chlorella vulgaris, Arthrospira, Chlorella, and Tetraselmis sp., etc., are also reported for generation of value-added products. This review presents a holistic view of microalgae based biorefinery starting from cultivation and harvesting of microalgae, the potential for remediation of environmental pollutants, bioenergy application, and production of value-added biomolecules. Further, it summarizes the current understanding of microalgae-based technologies and discusses the risks involved, potential for bioeconomy, and outlines future research directions.

Leachate treatment using sequential microwave and algal photo-bioreactor: Effect of pretreatment on reactor performance and biomass productivity

The present study aims to design a lab-scale hybrid reactor, primarily focused on the removal of organics, nutrients, heavy metal and other toxic compounds, thereby, minimizing risk associated with the disposal of landfill leachate. The potential of a designed hybrid treatment system (i.e., sequential microwave (MW) with algal bioreactor) with and without pretreatment, i.e., coagulation-flocculation (CF), was evaluated based on several parameters. The CF pretreatment under optimized conditions has resulted in 90% turbidity and 76% COD removals from leachate; furthermore, the MW treatment achieved 91% ammonia removal from raw leachate. As a result, substantial algal growth was observed in the preliminary algal batch experiment conducted with MW and MW-CF treated samples. Subsequently, leachate treatment was carried out using sequencing batch reactor (SBR) systems, i.e., MW-algal SBR and CF-MW-algal SBR. Algal biomass growth and increment in DO level were observed in algal-SBR experiments. Under the optimized reactor conditions, TN and TP removal rates in the algal-SBR were found to be 1.67-20 mg/L/d and 0.6-9.6 mg/L/d, respectively. The majority of heavy metals present in the leachate were removed due to algal-uptake (mainly Zn2+) and bio-sorption (total-Fe, Cu2+ and Pb2+). Meanwhile, some amount of energy can be recovered from algal biomass as inferred from the cost benefit analysis. Overall, the hybrid treatment combining MW and algal-SBR has shown immense potential for sustainable leachate treatment.

Comprehensive Genome Engineering Toolbox for Microalgae Nannochloropsis oceanica Based on CRISPR-Cas Systems

Microalgae can produce industrially relevant metabolites using atmospheric CO₂ and sunlight as carbon and energy sources, respectively. Developing molecular tools for high-throughput genome engineering could accelerate the generation of tailored strains with improved traits. To this end, we developed a genome editing strategy based on Cas12a ribonucleoproteins (RNPs) and homology-directed repair (HDR) to generate scarless and markerless mutants of the microalga Nannochloropsis oceanica. We also developed an episomal plasmid-based Cas12a system for efficiently introducing indels at the target site. Additionally, we exploited the ability of Cas12a to process an associated CRISPR array to perform multiplexed genome engineering. We efficiently targeted three sites in the host genome in a single transformation, thereby making a major step toward high-throughput genome engineering in microalgae. Furthermore, a CRISPR interference (CRISPRi) tool based on Cas9 and Cas12a was developed for effective downregulation of target genes. We observed up to 85% reduction in the transcript levels upon performing CRISPRi with dCas9 in N. oceanica. Overall, these developments substantially accelerate genome engineering efforts in N. oceanica and potentially provide a general toolbox for improving other microalgal strains.

Adapting the algal microbiome for growth on domestic landfill leachate

We aimed to improve algal growth rate on leachate by optimising the algal microbiome. An algal-bacterial consortium was enriched from landfill leachate and subjected to 24 months of adaptive laboratory evolution, increasing the growth rate of the dominant algal strain, Chlorella vulgaris, almost three-fold to 0.2 d^-1. A dramatic reduction in nitrate production suggested a shift in biological utilisation of ammoniacal-N, supported by molecular 16S rRNA taxonomic analyses, where Nitrosomonas numbers were not detected in the adapted consortium. A PICRUSt approach predicted metagenomic functional content and revealed a high number of sequences belonging to bioremediation pathways, including degradation of aromatic compounds, benzoate and naphthalene, as well as pathways known to be involved in algal-bacterial symbiosis. This study enhances our understanding of beneficial mechanisms in algal-bacterial associations in complex effluents, and ultimately enables the bottom-up design of optimised algal microbiomes for exploitation within industry.

Removal of Pollutants from an AMD from a Coal Mine by Neutralization/Precipitation Followed by “In Vivo” Biosorption Step with the Microalgae Scenedesmus sp

This work evaluates the benefits of a complementary treatment step of acid mine drainage (AMD) using the algae Scenedesmus sp. in terms of algae biomass production, residual metal removal, and the toxicity of the discharged water. Conventional treatment by neutralization/precipitation of an AMD from a coal mine in Brazil was conducted with Ca(OH)2 at pH 8.7. Algal growth studies were performed in the treated AMD, with and without a nutrient supply. The raw effluent and treatments were compared in terms of residual concentration of metals and sulfate, conductivity, and toxicity with the Allium cepa and Daphnia magna test organisms. The results show that the conventional treatment allowed a major metal removal, reduction in the conductivity, and good indices in the toxicological parameters evaluated. The biosorption with in vivo microalgae improved the quality of the effluent for residual metals. No significant toxicity was observed to Allium cepa in all treatments performed, while the Daphnia magna test indicated a reduction in toxicity after the biosorption step. It was concluded that algae growth can be carried out in treated mine waters, providing algae biomass and helping to achieve the standards for water discharge.

Acid mine drainage (AMD) treatment by neutralization: Evaluation of physical-chemical performance and ecotoxicological effects on zebrafish (Danio rerio) development

Acid mine drainage (AMD) represents a major problem in the mining industry worldwide due to the risk of water and soil pollution. Its active treatment involves the addition of alkaline reagents such as NaOH or Ca(OH)₂ to increase the pH and precipitate the dissolved metals, although substantial amounts of dissolved ions might persists. Under a remediation approach, the aim of this work was to assess the chemical and physical characteristics of treated effluent and to evaluate its ecotoxicological effects on zebrafish (Danio rerio) embryonic and larval stages, through developmental, functional, morphological, and behavioral end-points. The studied AMD sample, highly associated with pyrite, presented high sulfate and dissolved metal ions content and was submitted to the following treatment conditions: NaOH - pH 7.0 and 8.7, and Ca(OH)₂ - pH 7.0 and 8.7. All neutralizing treatments resulted in a satisfactory reduction of the metals concentration, with best results achieved using Ca(OH)₂ at pH 8.7; although Mn and As still remained above or very near the discharge maximum limits according to Brazilian legislation. Therefore, an additional step was employed to Mn and As adsorption by algal biomass. Regarding in-vivo toxicological assays, no significant lethality was recorded in all treated AMD groups, although adverse effects were observed in all endpoints analyzed. Ca(OH)₂ groups performed closer to control than NaOH-treated groups. The additional polishing stage treatment with the algae Scenesmus sp. allowed tenuous improvements in terms of removal of residual amounts of As and Mn but not in the toxicological characteristics of treated AMD.

A Review of Landfill Leachate Treatment by Microalgae: Current Status and Future Directions

Solid waste generation has been projected to increase worldwide. Presently, the most applied methodology to dispose of solid waste is landfilling. However, these landfill sites, over time release a significant quantity of leachate, which can pose serious environmental issues, including contamination of water resources. There exist many physicochemical and biological landfill leachate treatment schemes with varying degrees of success. With an increasing focus on sustainability, there has been a demand for developing eco-friendly, green treatment schemes for landfill leachates with viable resource recovery and minimum environmental footprints. Microalgae-based techniques can be a potential candidate for such a treatment scenario. In this article, research on microalgae-based landfill leachate treatments reported in the last 15 years have been summarized and critically reviewed. The scale-up aspect of microalgae technology has been discussed, and the related critical factors have been elucidated. The article also analyzes the resource recovery potential for microalgal techniques with respect to leachate treatment and explores possible methodologies to minimize the environmental footprints of the microalgae-based treatment process. The future research potential in the area has been identified and discussed.

Prospects of integrating algae technologies into landfill leachate treatment

Landfilling of municipal waste, an environmental challenge worldwide, results in the continuous formation of significant amounts of leachate, which poses a severe contamination threat to ground and surface water resources. Landfill leachate (LL) is generated by rainwater percolating through disposed waste materials and must be treated effectively before safe discharge into the environment. LL contains numerous pollutants and toxic substances, such as dissolved organic matter, inorganic chemicals, heavy metals, and anthropogenic organic compounds. Currently, LL treatment is carried out by a combination of physical, chemical, and microbial technologies. Microalgae are now viewed as a promising sustainable addition to the repertoire of technologies for treating LL. Photosynthetic algae have been shown to grow in LL under laboratory conditions, while some species have also been employed in larger-scale LL treatments. Treating leachate with algae can contribute to sustainable waste management at existing landfills by remediating low-quality water for recycling and reuse and generating large amounts of algal biomass for cost-effective manufacturing of biofuels and bioproducts. In this review, we will examine LL composition, traditional leachate treatment technologies, LL toxicity to algae, and the potential of employing algae at LL treatment facilities. Emphasis is placed on how algae can be integrated with existing technologies for biological treatment of LL, turning leachate from an environmental liability to an asset that can produce value-added biofuels and bioproducts for the bioeconomy.

Synergy of biofuel production with waste remediation along with value-added co-products recovery through microalgae cultivation: A review of membrane-integrated green approach

Development of advanced biofuels such as bioethanol and biodiesel from renewable resources is critical for the earth's sustainable management and to slow down the global climate change by partial replacement of gasoline and diesel in the transport sector. Being a diverse group of aquatic micro-organisms, algae are the most prominent resources on the planet, distributed in an aquatic system, a potential source of bioenergy, biomass and secondary metabolites. Microalgae-based biofuel production is widely accepted as non-food fuel sources and better choice for achieving goals of incorporation of a clean fuel source into the transportation sector. The present review article provides a comprehensive literature survey as well as a novel approach on the application of microalgae for their simultaneous cultivation and bioremediation of high nutrient containing wastewater. In addition to that, merits and demerits of different existing conventional techniques for microalgae culture reactors, harvesting of algal biomass, oil recovery, use of different catalysts for transesterification reactions and other by-products recovery have been discussed and compared with the membrane-based system to find out the best optimal conditions for higher biomass as well as lipid yield. This article also deals with the use of a tailor-made membrane in an appropriate module that can be used in upstream and downstream processes during algal-based biofuels production. Such membrane-integrated system has the potential of low-cost and eco-friendly separation, purification and concentration enrichment of biodiesel as well as other valuable algal by-products which can bring the high degree of process intensification for scale-up at the industrial stage.

Study of landfill leachate as a sustainable source of water and nutrients for algal biofuels and bioproducts using the microalga Picochlorum oculatum in a novel scalable bioreactor

High water demand is a major challenge for the algae industry, so cultivating algae in wastewater can have the double benefit of biomass production and water remediation. The use of landfill leachate (LL), which is wastewater generated in landfills, was investigated to grow the microalga Picochlorum oculatum in a novel horizontal bioreactor (HBR), a low-cost modular cultivation system that reduces water evaporation and contamination risk thanks to its enclosed design. Pilot-scale (150 L) and commercial-scale (2000 L) HBRs that were operated outdoors in Florida using LL in batch and semi-continuous modes generated high cell density cultures (1.7·10⁹ cells mL^-1) and reached up to 1.9 g L^-1 of dry biomass suitable for biofuel production. Demonstrating the ability of ample non-potable water sources, such as LL, to support algae cultivation is essential for improving the sustainability and cost-effectiveness of commercial algal biofuels and bioproducts, as freshwater resources become increasingly scarce.

Avoiding Conflicts between Future Freshwater Algae Production and Water Scarcity in the United States at the Energy-Water Nexus

Sustainable production of algae will depend on understanding trade-offs at the energy-water nexus. Algal biofuels promise to improve the environmental sustainability profile of renewable energy along most dimensions. In this assessment of potential US freshwater production, we assumed sustainable production along the carbon dimension by simulating placement of open ponds away from high-carbon-stock lands (forest, grassland, and wetland) and near sources of waste CO 2 . Along the water dimension, we quantified trade-offs between water scarcity and production for an ‘upstream’ indicator (measuring minimum water supply) and a ‘downstream’ indicator (measuring impacts on rivers). For the upstream indicator, we developed a visualization tool to evaluate algae production for different thresholds for water surplus. We hypothesized that maintaining a minimum seasonal water surplus would also protect river habitat for aquatic biota. Our study confirmed that ensuring surplus water also reduced the duration of low-flow events, but only above a threshold. We also observed a trade-off between algal production and the duration of low-flow events in streams. These results can help to guide the choice of basin-specific sustainability targets to avoid conflicts with competing water users at this energy-water nexus. Where conflicts emerge, alternative water sources or enclosed photobioreactors may be needed for algae cultivation.

Microalgal lipids production and nutrients recovery from landfill leachate using membrane photobioreactor

The aim of this work was to realize high-efficiency nutrients recovery from landfill leachate (LL) for microalgal lipids production. Negative effects of LL on microalgal lipid synthesis was revealed and a scalable membrane-based tubular photobioreactor (SM-PBR) was proposed to offset these negative effects. Microalgal biomass concentration was improved from 0 g/L in the traditional PBR to 2.13 g/L in the SM-PBR. Major operating conditions were optimized to enhance nutrients recovery and lipid productivity. The maximum N recovery efficiency of 74.31% and the maximum daily lipid production of 404.98 mg/d were obtained under the volume ratio of 5:3 (microalgae culture/LL stream) and phosphate feeding concentration of 50 mg/L. The obtained lipid was convinced to have a good combustion and anti-degradation property, with high cetane number (>52%) and low linolenic acid content (<12%). The SM-PBR provided a feasible approach for large-scale microalgal lipid production with LL.

High-efficiency nutrients reclamation from landfill leachate by microalgae Chlorella vulgaris in membrane photobioreactor for bio-lipid production

Using microalgae to treat landfill leachate is a promising approach due to the effective nutrients reclamation ability and additional profit of bio-lipid production. To offset the negative effect of landfill leachate on microalgae cells, a membrane photobioreactor (m-PBR) was adopted in the study, in which microalgae biomass concentration was improved from 0.66 in traditional photobioreactor (T-PBR) to 0.95 g/L. Nutrients reclamation efficiencies of leachate were analyzed according to elemental balance, and the results showed that nitrogen reclamation efficiency was generally lower than 50% while phosphorus reclamation efficiency was higher than 70% due to elemental availability. The nitrogen and phosphorus reclamation efficiencies in the m-PBR were much higher than that in the T-PBR. Besides, lipid produced from the m-PBR had a high cetane number of 60.96% and low linolenic acid content of 8.32%, which demonstrated good combustion properties of the microalgae-based lipid when using landfill leachate as nutrients source.

Phycoremediation of landfill leachate with the chlorophyte Chlamydomonas sp. SW15aRL and evaluation of toxicity pre and post treatment

Landfill leachate treatment is an ongoing challenge in the wastewater management of existing sanitary landfill sites due to the complex nature of leachates and their heavy pollutant load. There is a continuous interest in treatment biotechnologies with expected added benefits for resource recovery; microalgal bioremediation is seen as promising in this regard. Toxicity reduction of landfill leachate subsequent to phycoremediation was investigated in this study. The treatment eventuated from the growth of the ammonia tolerant microalgal strain Chlamydomonas sp. SW15aRL using a N:P ratio adjustment in diluted leachate for facilitating the process. Toxicity tests ranging over a number of trophic levels were applied, including bacterial-yeast (MARA), protistean (microalgae growth inhibition test), crustacean (daphnia, rotifer) and higher plant (monocot, dicot) assays. Ammonia nitrogen in the diluted landfill leachate containing up to 158mgl^-1 NH₄⁺-N (60% dilution of the original) was reduced by 83% during the microalgal treatment. Testing prior to remediation indicated the highest toxicity in the crustacean assays Daphnia magna and Brachionus calyciflorus with EC50s at 24h of ~ 35% and 40% leachate dilution, respectively. A major reduction in toxicity was achieved with both bioassays post microalgal treatment with effects well below the EC20s. The microalgae inhibition test on the other hand indicated increased stimulation of growth after treatment as a result of toxicity reduction but also the presence of residual nutrients. Several concurrent processes of both biotic and abiotic natures contributed to pollutant reduction during the treatment. Modifying phosphate dosage especially seems to require further attention. As a by-product of the remediation process, up to 1.2gl^-1 of microalgal biomass was obtained with ~ 18% DW lipid content.

Influence of Scale on Biomass Growth and Nutrient Removal in an Algal-Bacterial Leachate Treatment System

Data collected from experiments conducted at a flask scale are regularly used as input data for life cycle assessments and techno-economic analyses for predicting the potential productivities of large-scale commercial facilities. This study measures and compares nitrogen removal and biomass growth rates in treatment systems that utilize an algae-bacteria consortium to remediate landfill leachate at three scales: small (0.25 L), medium (100 L), and large (1000 L). The medium- and large-scale vessels were run for 52 consecutive weeks as semibatch reactors under variable environmental conditions. The small-scale experiments were conducted in flasks as batch experiments under controlled environmental conditions. Kolomogov-Smirnov statistical tests, which compare the distributions of entire data sets, were used to determine if the ammonia removal, total nitrogen removal, and biomass growth rates at each scale were statistically different. Results from the Kolmogov-Smirnov comparison indicate that there is a significant difference between all rates determined in the large-scale vessels compared to those in the small-scale vessels. These results suggest that small-scale experiments may not be appropriate as input data in predictive analyses of full scale algal processes. The accumulation of nitrite and nitrate within the reactor, observed midway through the experimental process, is attributed to high relative abundances of ammonia- and nitrite-oxidizing bacteria, identified via metagenomic analysis.

Effect of Organic Solvents on Microalgae Growth, Metabolism and Industrial Bioproduct Extraction: A Review

In this review, the effect of organic solvents on microalgae cultures from molecular to industrial scale is presented. Traditional organic solvents and solvents of new generation-ionic liquids (ILs), are considered. Alterations in microalgal cell metabolism and synthesis of target products (pigments, proteins, lipids), as a result of exposure to organic solvents, are summarized. Applications of organic solvents as a carbon source for microalgal growth and production of target molecules are discussed. Possible implementation of various industrial effluents containing organic solvents into microalgal cultivation media, is evaluated. The effect of organic solvents on extraction of target compounds from microalgae is also considered. Techniques for lipid and carotenoid extraction from viable microalgal biomass (milking methods) and dead microalgal biomass (classical methods) are depicted. Moreover, the economic survey of lipid and carotenoid extraction from microalgae biomass, by means of different techniques and solvents, is conducted.

Metabolic regulation of triacylglycerol accumulation in the green algae: identification of potential targets for engineering to improve oil yield

The great need for more sustainable alternatives to fossil fuels has increased our research interests in algal biofuels. Microalgal cells, characterized by high photosynthetic efficiency and rapid cell division, are an excellent source of neutral lipids as potential fuel stocks. Various stress factors, especially nutrient-starvation conditions, induce an increased formation of lipid bodies filled with triacylglycerol in these cells. Here we review our knowledge base on glycerolipid synthesis in the green algae with an emphasis on recent studies on carbon flux, redistribution of lipids under nutrient-limiting conditions and its regulation. We discuss the contributions and limitations of classical and novel approaches used to elucidate the algal triacylglycerol biosynthetic pathway and its regulatory network in green algae. Also discussed are gaps in knowledge and suggestions for much needed research both on the biology of triacylglycerol accumulation and possible avenues to engineer improved algal strains.

Phycoremediation of landfill leachate with chlorophytes: Phosphate a limiting factor on ammonia nitrogen removal

The potential of microalgae to bioremediate wastewater has been reported in numerous studies but has not been investigated as extensively for landfill leachate, which may be attributed to its complex nature and toxicity. In this study we explored if microalgal phycoremediation could constitute an alternative biological treatment option for landfill leachate management in regions with temperate climatic conditions. The aim of this study was to assess the performance of microalgae species at relatively low temperature (15 °C) and light intensity (14:10 h, light: dark, 22 μmol m(-2) s(-1)) for reduction in energy inputs. Four chlorophyte strains originating from the North-West of Ireland were selected and used in batch experiments in order to evaluate their ability to reduce total ammonia nitrogen, oxidised nitrogen and orthophosphate in landfill leachate. The Chlamydomonas sp. strain SW15aRL isolated from raw leachate achieved the highest level of pollutant reduction whereby a decrease of 51.7% of ammonia nitrogen was observed in 10% raw leachate (∼100 mg l(-1) NH4(+)-N) by day 24 in experiments without culture agitation. However, in the experiment conducted with 10% raw leachate supplemented with phosphate, a decrease of 90.7% of ammonia nitrogen was obtained by day 24 while also achieving higher biomass production. This series of experiments pointed to phosphorus being a limiting factor in the microalgae based phycoremediation of the landfill leachate. The effective reduction of ammonia nitrogen in landfill leachate can be achieved at lower temperature and light conditions. This was attained by employing native species adapted to such conditions and by improving nutrient balance.