Outdoor Raceway Ponds Research Articles

Development of large-scale microalgae production in the Middle East

Microalgae in the Middle East can theoretically address food security without competing for arable land, but concerns exist around scalability and durability of production systems under the extreme heat. Large-scale Chlorella sorokiniana production was developed in outdoor raceway ponds in Oman and monitored for 2 years to gather data for commercial production. Biological and technical challenges included construction, indoor/outdoor preculturing, upscaling, relating productivity to water temperature and meteorological conditions, harvesting, drying, and quality control. Small cultivation systems required cooling for initial scale-up, but, despite maximum temperatures of 49.7 °C, water temperatures were at acceptable levels by evaporative cooling in larger raceway ponds. Contamination with Vampirovibrio chlorellavorus was identified by 16S rDNA amplicon sequencing and addressed by culture replacement. Productivities ranged from 8 to 30 g-dry weight m-2d-1, with estimated annual productivity of 16 g-dry weight m-2d-1 as functions of solar intensity and water temperature, confirming that the region is suitable for commercial microalgae production.

Monochromatic light filters to enhance biomass and carotenoid productivities of Dunaliella salina in raceway ponds

Monochromatic blue and red wavelengths are more efficient for light to algal biomass conversion than full-spectrum sunlight. In this study, monochromatic light filters were used to down-regulate natural sunlight to blue (400–520 nm) and red (600–700 nm) wavelengths to enhance biomass productivity of Dunaliella salina in outdoor raceway ponds. Growth indices such as cell size, pigment concentrations, biomass yield, photosynthetic efficiency, and major nutritional compositions were determined and compared against a control receiving unfiltered sunlight. Results showed that red light increased biomass productivity, lipid, and carotenoid contents but decreased cell volume, chlorophyll production, and cell weight. Conversely, blue light increased cell volume by 200%, cell weight by 68%, and enhanced chlorophyll a and protein contents by 35% and 51%, respectively, over red light. Compared to the control treatment, photoinhibition of D. salina cells at noon sunshine was decreased 60% by utilizing optical filters on the pond’s surface.

Evaluation of microalgal strains and microalgal consortium for higher lipid productivity and rich fatty acid profile towards sustainable biodiesel production

This present study has attempted to screen four microalgal strainsChlorella vulgaris, Coelastrellasp.Scenedesmus dimorphusandChlorococcumsp. and consortium for biodiesel application.Coelastrella sp. was found to show marginally higher optical density of 1.13 on 19th day whereas C. vulgaris and S. dimorphus consortium showed 1.59 OD. Regarding the dry cell weight, S. dimorphus and Chlorella vulgaris was found to yield higher DCW at about 0.544 and 0.508 g/L, respectively. In outdoor pond raceway pond, C. vulgaris and S. dimorphus yielded 0.76–0.80 g/L while consortium showed 0.87 g/L biomass. In the case of lipid content, S. dimorphus, C. vulgaris and consortium accumulated 36.4, 35.5 and 39.2% lipids, respectively in lab, whereas in outdoor raceway pond it was 26.4, 32.3 and 34.5%, respectively. The fatty profile of Chlorella and Scenedesmus sp. showed linolelaidic acid, and cis-8,11,14-eicosatrienoic acid as prevalent fatty acids whereas the consortium has 53.5% oleic acid than other fatty acids.

Annotated Genome Sequence of the High-Biomass-Producing Yellow-Green Alga Tribonema minus.

ABSTRACTHere, we report the annotated genome sequence for a heterokont alga from the class Xanthophyceae. This high-biomass-producing strain, Tribonema minus UTEX B 3156, was isolated from a wastewater treatment plant in California. It is stable in outdoor raceway ponds and is a promising industrial feedstock for biofuels and bioproducts.

Characterization of a novel strain of Tribonema minus demonstrating high biomass productivity in outdoor raceway ponds

Photosynthetic algae represent a large, diverse bioresource potential. Yellow-green algae of the genus Tribonema are candidates for production of biofuels and other bioproducts. We report on a filamentous isolate from an outdoor raceway polyculture growing on municipal reclaimed wastewater which we classified as T. minus. Over one year of cultivation in 3.5 m2 raceway ponds fed by reclaimed municipal wastewater, T. minus cultures were more productive than the native algal polycultures, with annual average productivities of 15.9 ± 0.3 and 13.4 ± 0.4 g/m2/day, respectively. The biochemical composition of T. minus biomass grown outdoors was constant year-round, with 28.3 ± 0.4% carbohydrates, 37.6 ± 0.7% proteins, and 6.1 ± 0.3% fatty acids (measured as methyl esters), with up to 4.0% of the valuable omega-3 eicosapentaenoic acid, on an ash-free dry-weight basis. In summary, T. minus was more productive, easier to harvest and produced higher quality biomass than the native polycultures.

Outdoor cultivation of Chlorella pyrenoidosa in paddy-soaked wastewater and a feasibility study on biodiesel production from wet algal biomass through in-situ transesterification

Sustainable resources management, incorporating energy markets and resources such as electricity, fossil fuels, renewable and sustainable energy capital is essential for society to understand production and conversion of various forms of energy, their current as well as future supply. Waste-to-energy (WTE) or energy-from-waste (EFW) is a well-identified transitional technology which could prevent complete depletion of renewable resources. In our present study, the selected microalgae (Chlorella pyrenoidosa) was cultured in paddy-soaked wastewater (PWW) using outdoor raceway ponds of 50 L capacity where biotransformation of nutrients (NH3–N removal: 75.89 ± 0.69%; PO4–P removal: 73.71 ± 0.75%; yield co-efficient YN: 6.12 mg biomass/mg of N; YP: 7.77 mg biomass/mg P) has occurred with better growth and biochemical composition (dry biomass weight: 1.56 ± 0.11 g/L; chlorophyll: 15.57 ± 0.14 mg/L; specific growth rate (SGR): 0.42/d; lipids: 27.47 ± 1.41% biomass; carbohydrates: 23.77 ± 1.00% and protein: 46.12 ± 3.55%). Further, the obtained algal lipid was identified for a wide range of fatty acid methyl esters (FAME) and consequently brought forward to in-situ single-step transesterification by optimizing reaction conditions. Central composite design (CCD) of response surface methodology (RSM) has given optimized conditions of sample amount: 2 g (wet); methanol sulphuric acid volume: 3 mL; and hexane volume: 4 mL, under the reaction temperature of 90 °C for maximum biodiesel conversion (46.54% of algal lipids). The outcome of our current research may add value to the application and development of WTE technology for sustainable energy conservation.

The Feasibility of Using Inland Desalination Concentrate (DC) as an Alternative Substrate for Spirulina platensis Mass Cultivation

The chemical composition of inland desalination concentrate (DC) contains several microelements necessary for metabolism in microalgae. This study investigated the biomass concentration and biochemical composition of Spirulina platensis cultivated in DC under indoor (laboratory) and outdoor (raceway ponds) units. Initially, S. platensis was cultivated indoor testing different concentrations 0-, 20-, 40-, 60-, 80-, 100-% of DC, and results showed that mixing 60% DC with 40% Paoletti Synthetic Medium (PSM) is a suitable medium for S. platensis. After that, the optimum 60% DC to cultivate S. platensis under outdoor raceway pond was utilized. In parallel, a control assay using 100% PSM constituents was performed. S. platensis cultured in optimum DC, under outdoor conditions, exhibited biomass productivity (~ 10 g m−2 day−1) and protein content (45.0%) comparable to literature data. In addition, S. platensis cultivated in optimum DC was also able to synthesize carbohydrates, amino acids, fatty acids, and phycocyanin, which could be potentially used as biofertilizer and supplement in animal and aquaculture feed. Our results demonstrated the valorization of brine management through S. platensis cultivation, which can be considered as a step forward of reducing freshwater and conventional nutrients for algal mass cultivation.

Metabolic engineering of phosphite metabolism in Synechococcus elongatus PCC 7942 as an effective measure to control biological contaminants in outdoor raceway ponds

BackgroundThe use of cyanobacteria and microalgae as cell factories to produce biofuels and added-value bioproducts has received great attention during the last two decades. Important investments have been made by public and private sectors to develop this field. However, it has been a challenge to develop a viable and cost-effective platform for cultivation of cyanobacteria and microalgae under outdoor conditions. Dealing with contamination caused by bacteria, weedy algae/cyanobacteria and other organisms is a major constraint to establish effective cultivation processes.ResultsHere, we describe the implementation in the cyanobacterium Synechococcus elongatus PCC 7942 of a phosphorus selective nutrition system to control biological contamination during cultivation. The system is based on metabolic engineering of S. elongatus to metabolize phosphite, a phosphorus source not normally metabolized by most organisms, by expressing a bacterial phosphite oxidoreductase (PtxD). Engineered S. elongatus strains expressing PtxD grow at a similar rate on media supplemented with phosphite as the non-transformed control supplemented with phosphate. We show that when grown in media containing phosphite as the sole phosphorus source in glass flasks, the engineered strain was able to grow and outcompete biological contaminants even when the system was intentionally inoculated with natural competitors isolated from an irrigation canal. The PtxD/phosphite system was successfully used for outdoor cultivation of engineered S. elongatus in 100-L cylindrical reactors and 1000-L raceway ponds, under non-axenic conditions and without the need of sterilizing containers and media. Finally, we also show that the PtxD/phosphite system can be used as selectable marker for S. elongatus PCC 7942 transgenic strains selection, eliminating the need of antibiotic resistance genes.ConclusionsOur results suggest that the PtxD/phosphite system is a stable and sufficiently robust strategy to control biological contaminants without the need of sterilization or other complex aseptic procedures. Our data show that the PtxD/phosphite system can be used as selectable marker and allows production of the cyanobacterium S. elongatus PCC 7942 in non-axenic outdoor reactors at lower cost, which in principle should be applicable to other cyanobacteria and microalgae engineered to metabolize phosphite.

Anaerobic digestion effluent treatment using microalgae and nitrifiers in an outdoor raceway pond with fluidized carriers.

Combining microalgae and nitrifiers in a single photobioreactor has attracted attention as an alternative approach for conventional nitrogen removal from wastewater. However, nitrifiers are known to be sensitive to light exposure. This study demonstrated the effectiveness of using fluidized carriers to mitigate light stress in nitrifiers. An outdoor raceway pond containing microalgae and nitrifiers with fluidized carriers was used to treat two-fold diluted anaerobic digestion effluent (785 mg-N L-1 as a form of dissolved total Kjeldahl nitrogen: TKN) over 50 days. The average daily sunlight intensity reached the inhibition level of nitrifiers (423 μmol photons m-2 s-1); however, stable nitrification with a specific ammonium oxidation rate of 55 mg-N g-total suspended solid-1 day-1 was observed. TKN was mostly removed via nitrifier metabolism (ammonium oxidation and uptake: 40.1%) and partially via microalgae uptake (5.7%). Different microalgae-based processes including that of this study were compared in terms of tolerances to a high dissolved TKN concentration and strong light. Our system showed a relatively higher resistance to not only light exposure but also TKN because the nitrification process decreased the free ammonia level to less than 0.25 mg L-1, which allowed microalgae to grow despite the high ammonium concentration.

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A comparative life cycle assessment of microalgae production by CO2 sequestration from flue gas in outdoor raceway ponds under batch and semi-continuous regime

To be a sustainable energy source, a microalgal cultivation model should address important environmental challenges for its large-scale implementation. In this work, the performance of an outdoor raceway pond system was evaluated for biomass production and sequestration of flue gas carbon-dioxide under different conditions. The microalgae growth was influenced by several physio-chemical factors such as temperature, solar irradiance, carbon-dioxide in the incoming feed, cross-contamination etc. Batch microalgal cultivation, without any external carbon supplementation (0.04% CO2, v/v), exhibited lower areal biomass productivity, whereas flue gas (10 ± 2% CO2, v/v) feeding increased dissolved medium carbon concentrations, which improved carbon-dioxide fixation rate. To further improve the system’s performance, semi-continuous regime was adopted that resulted in maximum biomass density of 0.42 g/L and 3.5-fold increase in areal productivity of 11.488 g/m2.s. Next, a “cradle-to-gate” Life Cycle Assessment (LCA) was performed using SimaPro 8.3.0.14 software to identify the potential environmental impacts associated with the production of Chlorella vulgaris biomass (functional unit: 1 kg). Results showed cultivation step to have significant environmental impact (>75%) in terms of GHG emission, energy requirement and other impact categories as estimated by GWP 100 IPCC 2013, CED and ReCiPe Endpoint method. Further, sensitivity analysis assisted in identifying the effect of variables with greatest impact on systems performance in the proposed scenarios; especially with the co-location of cultivation systems and flue gas emitting point sources. The results of attributional LCA revealed that flue gas utilization by microalgae in outdoor raceway ponds has less carbon footprint when a semi-continuous cultivation is carried out and will be more suitable when raceway ponds are co-located with thermal power plants.

Red luminescent solar concentrators to enhance Scenedesmus sp. biomass productivity

Increasing biomass productivity of an outdoor mass microalgal culture is the main aim of any large-scale algal producer. Under nutrient enriched conditions, light is the primary limits to growth of any microalgae. This normally results in a high level of photolimitation when paddle wheel driven raceway ponds are used as no light can penetrate beyond the top few centimetres of the culture. To achieve high biomass productivity, there is a need to develop an efficient system to deliver light into the depth of microalgal cultures in raceway ponds. We investigated red luminescent solar concentrators (LSCs) in outdoor raceway ponds to downgrade the sunlight, re-emit and, deliver it into the depth of Scenedesmus sp. cultures operated at 21 cm depth. Biomass productivity of Scenedesmus sp. significantly increased by 18.5% when red LSCs were used (9.4 g m−2 d−1). Scenedesmus sp. protein (52.9% of biomass) and lipid (16.5% of biomass) contents with red LSCs were also 15% and 10% higher than those in control with no LSCs. Protein, lipid and carbohydrate productivity of Scenedesmus sp. were also improved by 35%, 20% and 16% when red LSCs used. Therefore, if the aim is biomass production for animal feed, there would be 18.5% less cultivation area for generating the same biomass productivity using red LSCs. This can lead to a significant reduction in the cost of biomass production.

Enhancing the production of astaxanthin by mixotrophic cultivation of Haematococcus pluvialis in open raceway ponds

Mixotrophic cultivation of Haematococcus is efficient to increase astaxanthin productivity. However, application of this technique in open bioreactors is restricted because of bacterial contamination. In this study, mixotrophic cultivation of Haematococcus was carried out successfully in an outdoor raceway pond. The Haematococcus cells grew on light first, and then acetate/acetic acid was supplemented when nitrate was depleted from the culture media. Under such conditions, Haematococcus cells grew on intracellular nitrogen pool, while bacterial reproduction was limited. The average astaxanthin productivity reached 140 mg m−2day−1, which is about 1.2 times that of the simple phototrophic cultivation. This study explores a potential way to enhance astaxanthin productivity, and the results are helpful to the development of the Haematococcus industries.

Red and blue luminescent solar concentrators for increasing Arthrospira platensis biomass and phycocyanin productivity in outdoor raceway ponds

Achieving high biomass productivity is critical for establishing a successful large-scale algal facility. Microalgae cultures in raceway ponds are normally light limited. To achieve high biomass productivity, there is a need to develop a system to deliver light into the depth of microalgal cultures in raceway ponds. We investigated red and blue luminescent solar concentrators (LSCs) in outdoor raceway ponds to downgrade the sunlight, re-emit and, deliver it into the depth of Arthrospira platensis culture operated at 21 cm depth. When red LSCs were used, the biomass productivity (12.2 g m−2 d−1) and phycocyanin productivity (8.5 mg L−1 d−1) of A. platensis increased 26% and 44%, respectively. However, using blue LSCs resulted in no significant increase in A. platensis biomass productivity. Therefore, for generating same phycocyanin productivity using red LSCs, 44% less cultivation area would be required. This can lead to a significant reduction in the cost of phycocyanin production.

Continuous cultivation of Arthrospira platensis for phycocyanin production in large-scale outdoor raceway ponds using microfiltered culture medium

In the present study, the effect of filtrating algal culture medium for reuse by using microfiltration membranes on microalgal growth, microbiological contamination, and phycocyanin production of Arthrospira platensis was investigated. Results showed that microfiltered culture medium affected microalgal growth, microbiological contamination, and phycocyanin production of A. platensis significantly. Microfiltered culture medium could enhance biomass production, photosynthesis, and phycocyanin accumulation and decrease microbiological contamination during continuous cultivation of A. platensis compared to the control. The profile of microbial communities, which contained the 10 phyla of microorganisms including bacteria and microzooplanktons, was identified for the first time for industrial algae systems of A. platensis with extreme conditions (salt-alkaline stress conditions). The application of the established strategy can enhance phycocyanin production of A. platensis while mitigating microbiological contamination.

Long-term reliable culture of a halophilic diatom, Amphora sp. MUR258, in outdoor raceway ponds

The microalgal industry as a source of high-value products (i.e. β-carotene, astaxanthin) was established over 50 years ago. However, only a very small number of species have been commercialised. There is a need for new species and new products to expand this industry. The objective of this study was to examine the reliability and productivities of long-term outdoor culture of a newly isolated halophilic diatom, Amphora sp. MUR258 (Bacillariophyceae), in raceway ponds in Perth, Western Australia. The Amphora sp. was grown in outdoor raceway ponds as a semi-continuous culture for about 13 months at a culture salinity between 8.6 and 14.9% (w/v) NaCl. The highest cell density (167 × 104 cells mL−1), specific growth rate (0.29 day−1) and biomass and lipid productivities (24 and 6.8 g m−2 day−1, respectively) were achieved in summer. The annual average of biomass (ash-free dry weight) and lipid productivities was 7 and 2.2 g AFDW m−2 day−1, respectively. Minor contamination by a Navicula sp. was seen during winter, but was not a significant problem. No major protozoan contamination was seen. These results indicate the potential of reliable large-scale cultivation of Amphora sp. MUR258 as a potential source of diatom lipid and/or fucoxanthin.

Microalgae grow on source separated human urine in Nordic climate: Outdoor pilot-scale cultivation

Human urine contributes approximately 80% of nitrogen and 50% of phosphorous in urban wastewaters while having a volume of only 1–1.5 L/d per capita compared to 150–200 L/d per capita of wastewater generated. There is interest to study source separation of urine and search methods to recover the nutrients form the urine. In this study, the objective was to use the nutrients in source separated urine for outdoor cultivation of microalgae in Nordic climate. A freshwater green microalga Scenedesmus acuminatus was grown in different dilutions (1:20 and 1:15) of source separated human urine, in a semi-continuously operated outdoor raceway pond with a liquid volume of 2000 L, at hydraulic retention time of 15 d. The microalgae could remove 52% nitrogen and 38% phosphorus even at culture temperatures as low as 5 °C, while obtaining a biomass density of 0.34 g VSS/L. Harvested microalgal biomass could be used to produce methane with a yield of 285 L CH4/kg volatile solids.

Digestate treatment with algae-bacteria consortia: A field pilot-scale experimentation in a sub-optimal climate area

This paper addresses the efficiency of a microalgae-based agricultural digestate treatment at pilot-scale in an outdoor raceway pond (880 L, pH-dependent CO2 dosage) and in a bubble column (74.5 L, air-bubbling). Specifically, nitrogen removal, evolution of the algae-bacteria consortium, and the actual process applicability in the Po Valley climate are discussed. The performance of the two reactors varied seasonally. The average algal productivity in the raceway was 32.4 ± 33.1 mg TSS·L−1·d−1 (8.2 ± 8.5 g TSS·m−2·d−1) while in the PBR it was 25.6 ± 26.8 mg TSS·L−1·d−1; the average nitrogen removal was 20 ± 29% (maximum 78%) and 22 ± 29% (maximum 71%) in the raceway and in the column, respectively. Nevertheless, nitrification had a key role as 61 ± 24% and 52 ± 32% of the nitrogen load was oxidized in the raceway and in the column, respectively.

Full-scale validation of an algal productivity model including nitrogen limitation

Accurate predictions of algal productivity under nutrient-limiting conditions are needed to assess the economics of full-scale algal cultivation for the developing markets of food, feed, and at longer term, green chemistry and biofuel. In particular, predicting pigments production from micro-algae is a critical milestone in the assessments of high-value chemicals production from micro-algae. This study validates a mathematical model predicting algal biomass productivity in outdoor raceway ponds under nitrogen-limiting conditions. The model was first validated from experimental data collected during Dunaliella salina cultivation in indoor photobioreactors and accounts for the impact of light, temperature, and nitrogen concentration on algal productivity (overall accuracy on algal concentration of ±2.7 mg L−1, N = 48). The model was then validated against data collected in outdoor raceway ponds over a period of 2 years, representing a total of 111 days of cultivation. Biomass and extracellular nitrogen concentrations predictions were accurate within ±0.055 g L−1 (N = 69) and ±0.0024 g L−1 (N = 26), respectively. Model inaccuracies were mostly due to measurement errors and uncertainties on model inputs. Measured carotenoids concentrations were found proportional to the biomass concentrations in the outdoor raceway ponds. By coupling this linear correlation to the productivity model, predicted carotenoids concentrations were in good agreement with experimental data (accuracy within ±0.0046 g L−1, N = 55). The mathematical model developed in this study has therefore the potential to refine previous assessments of algal cultivation for biofuels and pigments production.

Identification of harmful protozoa in outdoor cultivation of Chlorella and the use of ultrasonication to control contamination

Contaminating organisms in mass cultivation present one of the major challenges that must be overcome for successful commercialization of algal biofuels. The present study identified a range of contaminating organisms in Chlorella cultures cultivated in outdoor raceway ponds at the Arizona Center for Algae Technology and Innovation (Mesa, AZ). Nineteen organisms or Operational Taxonomic Units (OTU) in the Chlorella culture were identified by a combination of microscopic observation and 18S rRNA denaturing gradient gel electrophoresis (DGGE). More detailed analyses identified these contaminating organisms as 2 fungi, 7 flagellates, 3 amoebae, 4 ciliates, 1 rotifer, and 2 large insects. Among them Poterioochromonas sp., a small flagellate, appeared to be one of the most harmful causing culture collapse. In order to control Poterioochromonas sp., various operational parameters and application strategies of ultrasonic treatment were investigated. During sixteen-day consecutive Chlorella cultivation in batch mode, the ultrasonication conditions of 6 L min−1 flow rate with the power of 495 W at 100% amplitude and a treatment frequency of once for 1 h every day, was proven to be the most effective in preventing Poterioochromonas outbreak in Chlorella culture with volume of 60 L. The above ultra-sonication method was also effective at destroying an unknown fungus, an amoeba (Acanthocystis sp.), and ciliates (a member of the family Orchitophryidae). Our findings can serve as a technical foundation for the application of ultrasonication to control some of the contaminating microorganisms in mass cultivation of microalgae including Chlorella.