Open Raceway Research Articles

Oxygen transfer and evolution in microalgal culture in open raceways

The mass transfer characteristics of all sections of a 100 m(2) raceway were evaluated. The efficiency of different diffusers was determined dynamically and the most effective was used for steady state system characterisation at water depth 0.2 m and velocity 0.22 m s(-1). Mass transfer coefficients at a gas flow rate of 6 m(3) h(-1) were 164.50, 63.66, 0.87 and 0.94 h(-1) for the paddlewheel, sump, straight and curved channel sections, with associated oxygen transfer rates of 106, 172, 27 and 39 g h(-1). Oxygen supersaturation during algal cultivation led to a reduction in biomass productivity, which was more severe with pure CO2 than flue gas. Simulations showed the energy required to increase mass transfer and reduce oxygen concentrations was more than compensated for by increased biomass and potential energy yields. Oxygen removal is likely to be a critical criterion, and maintenance of mass transfer by sparging may be necessary even when CO2 is not required.

Efficiency of CO2 fixation by microalgae in a closed raceway pond

Microalgae contain about 50% of carbon, which means that a total of 1.83ton of CO2 is needed to produce 1ton of microalgae. The cost of CO2 supply for microalgal large scale cultivation should be considered and the low CO2 fixation efficiency by microalgae will lead to much more expenditure of CO2. In this study, a closed raceway pond was constructed by covering a normal open raceway pond with a specially designed transparent cover, which directly touched the surface of microalgal culture media. This cover prevented supplied CO2 escaping into atmosphere and thus increased the retention time of CO2. The CO2 gas–liquid mass transfer and CO2 fixation efficiency by microalgae in the closed raceway pond were investigated, and the model of CO2 fixation by microalgae was developed. Through the model, the CO2 fixation efficiency increased to 95% under intermittent gas sparging.

Biotechnological Potentials of Microalgae: Past and Present Scenario

Increased population, shortage of fossil fuels and climatic changes constitute global challenges demanding actions and strategic planning for securing access of food, feed, biochemicals and other biotechnological potentials in future. Therefore, alternative sources demand innovative thinking and microalgal biomass offers a great potential for meeting future challenges. These do not need agricultural soil for growth and their cultivation requires 99% less fresh water than conventional agriculture. They exhibit high biomass yields per unit area and there is no CO2 emissions compared to fossil fuels. Microalgae can be used to enhance the nutritional value of food and animal feed owing to their chemical composition. They play a crucial role in aquaculture and fish feeding and can be incorporated in cosmetics. Although these offer the potential to produce high yields of proteins, however, only few like Spirulina have been exploited commercially. Potential for the pigments namely chlorophyll, carotenoids and phycobilins and enzymes like lactamase, protease and lipase are being assessed by the researchers. Additionally, utilization of algal biomass for vitamins, toxins, enzymes and biochemicals of therapeutic significance is being studied. Many are used in nutritional supplements and cosmetics utilizing Dunaliella, Spirulina, Chlorella, Haematococcus etc. The production of beta carotene from Dunaliella in open raceway ponds was possible by the organism's ability to withstand high salinity. The fresh water alga Haematococcus produces astaxanthin which has found use in fish aquaculture (red colour of Salmon fish) as well as innovative anti-inflammatory and antioxidant applications in human nutrition. The lipids of Nannochloropsis are used in cosmoceutical and skin care applications and efforts are underway to move into nutrition applications. The more specific components of some algae known as omega-3 fatty acids are very important in products directed at maintaining heart health. Researches in such directions can bring about potential economic benefits in the form of commercial opportunities, start up of new companies and jobs as well as environmental benefits, reduced used of agricultural land, carbon dioxide reduction and increase of oxygen release to the atmosphere.

Microalgal biomass production pathways: evaluation of life cycle environmental impacts.

BackgroundMicroalgae are touted as an attractive alternative to traditional forms of biomass for biofuel production, due to high productivity, ability to be cultivated on marginal lands, and potential to utilize carbon dioxide (CO2) from industrial flue gas. This work examines the fossil energy return on investment (EROIfossil), greenhouse gas (GHG) emissions, and direct Water Demands (WD) of producing dried algal biomass through the cultivation of microalgae in Open Raceway Ponds (ORP) for 21 geographic locations in the contiguous United States (U.S.). For each location, comprehensive life cycle assessment (LCA) is performed for multiple microalgal biomass production pathways, consisting of a combination of cultivation and harvesting options.ResultsResults indicate that the EROIfossil for microalgae biomass vary from 0.38 to 1.08 with life cycle GHG emissions of −46.2 to 48.9 (g CO2 eq/MJ-biomass) and direct WDs of 20.8 to 38.8 (Liters/MJ-biomass) over the range of scenarios analyzed. Further anaylsis reveals that the EROIfossil for production pathways is relatively location invariant, and that algae’s life cycle energy balance and GHG impacts are highly dependent on cultivation and harvesting parameters. Contrarily, algae’s direct water demands were found to be highly sensitive to geographic location, and thus may be a constraining factor in sustainable algal-derived biofuel production. Additionally, scenarios with promising EROIfossil and GHG emissions profiles are plagued with high technological uncertainty.ConclusionsGiven the high variability in microalgae’s energy and environmental performance, careful evaluation of the algae-to-fuel supply chain is necessary to ensure the long-term sustainability of emerging algal biofuel systems. Alternative production scenarios and technologies may have the potential to reduce the critical demands of biomass production, and should be considered to make algae a viable and more efficient biofuel alternative.

미세조류 옥외 배양시스템을 이용한 도시하수 정화 및 미생물 군집다양성 분석

미세조류는 광합성을 통하여 바이오디젤과 같은 부가가치상품을 생산할 수 있으며, 미세조류를 이용한 생명공학 기술이 주목 받고 있다. 그러나 질소원과 탄소원은 미세조류 배양 비용을 높여 충분한 바이오매스 생산에 제한요소가 되고 있다. 미세조류를 배양하는데 도시하수를 이용하는 것은 생산단가를 낮추는 좋은 대안이 될 수 있으며, 본 연구에서는 옥외 수질정화 배양 시스템(mini raceway open pond)을 이용하여 적용했다. 실험에 사용한 도시하수는 하수종말처리장의 1차 침전지를 거친 유입수를 이용하였으며, 토착 미세조류를 mini raceway open pond에서 배양하였다. 체류시간 6일의 운전 후 TN, TP, COD-<TEX>$_{Mn}$</TEX>, <TEX>$NH_3$</TEX>-N의 평균 제거 효율은 80.18%, 63.56%, 76.34%, 96.74%로 각각 나타났다. 18S rRNA gene 분석결과 녹조류인 Chlorella, Scenedesmus가 우점하였으며, 16S rRNA gene 분석결과 Rhodobacter, Luteimonas, Agrobacterium, Thauera, Porphyrobacte의 5종의 bacteria가 동정되었다. 이러한 결과를 통하여 미세조류를 이용한 호기성 처리나 과도한 발전비용 없이 효과적인 하수처리를 할 수 있는 가능성을 확인하였다. 그리고 도시하수는 미세조류 배양에 필요한 탄소원과 질소원을 제공할 수 있으며 미세조류 바이오매스는 상업적 목적으로 이용될 수 있는 가능성을 확인할 수 있었다. Microalgal biotechnology has gained prominence because of the ability of microalgae to produce value-added products including biodiesel through photosynthesis. However, carbon and nutrient source is often a limiting factor for microalgal growth leading to higher input costs for sufficient biomass production. Use of municipal wastewater as a low cost alternative to grow microalgae as well as to treat the same has been demonstrated in this study using mini raceway open ponds. Municipal wastewater was collected after primary treatment and microalgae indigenous in the wastewater were encouraged to grow in open raceways under optimum conditions. The mean removal efficiencies of TN, TP, COD-<TEX>$_{Mn}$</TEX>, <TEX>$NH_3$</TEX>-N after 6 days of retention time was 80.18%, 63.56%, 76.34%, and 96.74% respectively. The 18S rRNA gene analysis of the community revealed the presence of Chlorella vulgaris and Scenedesmus obliquus as the dominant microalgae. In addition, 16S rRNA gene analysis demonstrated that Rhodobacter, Luteimonas, Porphyrobacter, Agrobacterium, and Thauera were present along with the microalgae. From these results, it is concluded that microalgae could be used to effectively treat municipal wastewater without aerobic treatment, which incurs additional energy costs. In addition, municipal wastewater shall also serve as an excellent carbon and nitrogen source for microalgal growth. Moreover, the microalgal biomass shall be utilized for commercial purposes.

Predicting cost growth and performance of first-generation algal production systems

Estimates for algal production cost vary widely due to differing assumptions. Differences in assumptions make comparisons between proposed algal production systems difficult. Existing economic analyses have ignored potential capital cost growth and under performance of early generation algal production plants, which impact the preliminary unit cost of algal biofuels, which could affect investment decisions. Therefore the goal of this work was to compare the capital cost growth (ratio of actual to estimated cost), plant performance (ratio of actual performance to design), and unit cost growth factor (the ratio of cost growth to plant performance), of potential algal production pathways. Three production technologies were investigated: (1) open raceway ponds (ORP), (2) tubular photobioreactors (PBR), and (3) systems coupling photobioreactors to open raceway ponds. The greatest cost growth (1.5–1.8) was estimated for PBR systems, while the lowest cost growth (1.2–1.4) was estimated for the ORP systems and coupled systems. Plant performance was estimated to range from 13% to 40% of nameplate capacity. These results imply that unit cost growth for algal biofuels could range from 3 to 14 times current predictions, and illustrates large hurdles facing algal biofuels technologies that have yet to be implemented at scale.

Effect of cell rupturing methods on the drying characteristics and lipid compositions of microalgae

This paper investigated the effect of cell rupturing methods on the drying characteristics and the lipid compositions of a green algae consortium grown in an open raceway pond. The ruptured microalgae samples obtained from French press, autoclave and sonication methods were used for conducting thin layer drying experiment at four drying temperatures (30, 50, 70 and 90°C). The rate of moisture removal at each drying condition was recorded until no change in moisture loss. A typical drying curve for a microalgae consortium indicated that the rate of drying was limited by diffusion. Among three drying models (Newton, Page and Henderson–Pabis) used to fit the drying data, Page model fitted well on the experimental drying data with a coefficient of determination (R2) of 0.99. Solvent extraction of French press ruptured cells produced the highest total lipid yield with no significant change in lipid compositions.

Air–Water Fluxes of N2O and CH4 during Microalgae (Staurosira sp.) Cultivation in an Open Raceway Pond

The industrial-scale production of biofuels from cultivated microalgae has gained considerable interest in the last several decades. While the climate benefits of microalgae cultivation that result from the capture of atmospheric CO(2) are known, the counteracting effect from the potential emission of other greenhouse gases has not been well quantified. Here, we report the results of a study conducted at an industrial pilot facility in Hawaii to determine the air-water fluxes of N(2)O and CH(4) from open raceway ponds used to grow the marine diatom Staurosira sp. as a feedstock for biofuel. Dissolved O(2), CH(4), and N(2)O concentrations were measured over a 24 h cycle. During this time, four SF(6) tracer release experiments were conducted to quantify gas transfer velocities in the ponds, and these were then used to calculate air-water fluxes. Our results show that pond waters were consistently supersaturated with CH(4) (up to 725%) resulting in an average emission of 19.9 ± 5.6 μmol CH(4) m(-2) d(-1). Upon NO(3)(-) depletion, the pond shifted from being a source to being a sink of N(2)O, with an overall net uptake during the experimental period of 3.4 ± 3.5 μmol N(2)O m(-2) d(-1). The air-water fluxes of N(2)O and CH(4) expressed as CO(2) equivalents of global warming potential were 2 orders of magnitude smaller than the overall CO(2) uptake by the microalgae.

Economic comparison of open pond raceways to photo bio-reactors for profitable production of algae for transportation fuels in the Southwest

As energy prices climb there is an increasing interest in alternative, renewable energy sources. One possible source of renewable bio-fuel is algae. This research uses a multi-year, Monte Carlo financial feasibility model to estimate the costs of production and chance of economic success for commercial size algal biofuel facilities in the Southwest. Capital and operating costs and productivity information from Davis et al. were used to develop parameters to define and simulate two types of algae production systems; open pond and photo-bioreactor (PBR).The financial feasibility of PBRs is substantially lower than for open ponds. In the base case, average total costs of production for lipids, including financial costs, were $12.73/gal and $31.61/gal for open ponds and PBRs, respectively. The chance of economic success for the base situation was zero for both open ponds and PBRs. The financial feasibility analysis showed that the only way to achieve a 95% probability of economic success in the PBR system was to reduce CAPEX by 80% or more and OPEX by 90% or more. For the open pond system there were several options that could return a 95% or greater chance of economic success, for example, reducing CAPEX by 60% and OPEX by 90%.

Hectare-scale demonstration of high rate algal ponds for enhanced wastewater treatment and biofuel production

High rate algal ponds (HRAPs) are shallow, paddlewheel-mixed open raceway ponds that are an efficient and cost-effective upgrade for the conventional wastewater treatment ponds used by communities and farms the world over. HRAPs provide improved natural disinfection and nutrient removal and can be further enhanced by carbon dioxide (CO2) addition to promote algal growth which is often carbon limited. This paper discusses the construction and operation of a 5-ha demonstration HRAP system treating primary settled wastewater at the Christchurch wastewater treatment plant, New Zealand. The system consisted of four 1.25-ha HRAPs that were constructed from an existing conventional pond. Algae were harvested from the HRAP effluent in specially designed settlers, which concentrated the algal/bacterial biomass to 1–2% organic solids for conversion to bio-crude oil following dewatering. Performance data from the first 15 months of HRAP operation (without CO2 addition) are presented. The four demonstration HRAPs had reasonable replication of both treatment performance and algal/bacterial productivity with similar annual average wastewater treatment efficiency (~50% removal of BOD5, ~87% removal of fBOD5, ~65% removal of ammoniacal-N, ~19% removal of dissolved reactive phosphorus and ~2 log removal of Escherichia coli), algal species composition and algal/bacterial biomass production (~8 g m−2 day −1 volatile suspended solids). These results were in good agreement with the results for pilot-scale HRAP without CO2 addition in New Zealand. This study provides further indication of the potential for energy efficient and effective wastewater treatment using HRAP, while biofuel conversion of the harvested algal bacterial biomass could provide a valuable niche distributed energy source for local communities.

In situ carbon supplementation in large-scale cultivations of Spirulina platensis in open raceway pond

The objective of this study was to estimate the CO2 absorptivity provided by an in situ carbon supply system using a photosynthetic culture of the cyanobacterium Spirulina platensis in an open raceway pond. The effects of initial total carbon concentrations (ranging from 0 to 0.1 mol/L), suspension depths (ranging from 5 to 20 cm) and pH values (ranging from 8.9 to 11.0) on the CO2 absorptivity were studied. The results indicated that CO2 absorptivity was positively correlated with pH value, negatively correlated with total carbon concentration, and only negligibly affected by the suspension depth. The optimum total carbon concentration range and pH range were 0.03 similar to 0.09 mol/L and 9.7 similar to 10.0, respectively. An average CO2 absorptivity of 86.16% and average CO2 utilization efficiency of 79.18% were achieved using this in situ carbon-supply system in large-scale cultivation of Spirulina platensis, with an initial total carbon concentration of 0.06 mol/L and pH value of 9.8. Our results demonstrated that this system could obtain a favorable CO2 utilization efficiency in outdoor, large-scale cultivation of Spirulina platensis in open raceway ponds.

Bioaccumulation of Cadmium in Blue Green Algae Spirulina (Arthrospira) Indica

In the present paper, toxic effects and bioaccumulation of cadmium (II) ions by blue-green algae Spirulina (Arthrospira) indica were discussed. Lab scale open race way ponds (0.5 m long, 0.4 m wide and 0.075 m deep) with Zarrouk medium and S. (Arthrospira) indica culture was loaded with various initial cadmium (II) ion concentrations (1-10 mg/L). The growth of S. (Arthrospira) indica was monitored for eight days. The growth of S. (Arthrospira) indica was adversely affected by Cd (II) at high concentrations (>1 mg/L), however at low concentrations upto 1 mg/L, Cd (II) could stimulate its growth slightly. The toxic effect of cadmium (II) ions was modeled by assuming Monod growth kinetics with competitive inhibition. The percentage decrease in the S. (Arthrospira) indica biomass was found to be 54.32% at 4 mg/L Cd (II) after 7 days. The lethal concentration and time (LCt50) of Cd (II) ions for seven days was measured as 3.61 mg/L. The organism was found tolerant up to 1 mg/L of Cd (II) and can have potential applications in removal of low concentration of Cd (II) ions from contaminated waters. The biosorption experiment was conducted at various initial Cd (II) concentrations (1-4 mg/L) with live S. (Arthrospira) indica . The biosorption equilibrium time was found to be 6 min. It had been observed that at the initial stage (0-5 min) the adsorption rate was so rapid that 80% of the metal had been biologically adsorbed. The experimental equilibrium data was verified using Langmuir and Freundlich adsorption isotherms. The maximum biosorption capacity of live S. (Arthrospira) indica was estimated to be 66.97 mg Cd (II) per gdw of S. (Arthrospira) indica . The study provides a new insight into the impact of cadmium (II) ions on S. (Arthrospira) indica , especially with regard to its toxicity, bioaccumulation and bioremediation potential.

Mass culture of Botryococcus braunii Kutz. under open raceway pond for biofuel production

Three different strains of colonial green alga Botryococcus bruanii Kutz. AP103, AP104 and AP105 were isolated from the freshwater lake Kolleru, Andhra Pradesh, India. The morphological features of these three isolates were studied under laboratory conditions. Molecular identification of the strains was carried out using 18S rRNA analysis and their systematic position was confirmed to species level as B. braunii. Among these isolates, B. braunii AP103 recorded highest biomass concentration of 1.7±0.12gL−1, 17% lipids, 17% proteins, 32% carbohydrates and 13% hydrocarbons under laboratory condition. Under open raceway pond AP103 produced a biomass concentration of 1.8±0.13gL−1, 19% lipids, 33% carbohydrates, 18% proteins and 11% hydrocarbons. The hydrocarbons profile showed the presence of heptadecane (34%) and hexadecane (12.5%). Oleic (25.7%), linolenic (34.26%) and palmitic (9.42%) acids were the major fatty acids present in the lipids extracted from AP103.

Carbon dioxide fixation by microalgae cultivated in open bioreactors

The biofixation of carbon dioxide (CO 2) by microalgae has been proven to be an efficient and economical method, mainly due to the photosynthetic ability of these microorganisms to use this gas as a source of nutrients for their development. The aim of this work was to study the growth of Spirulina LEB18 and Chlorella kessleri microalgae, exposed to controlled and non-controlled conditions, with the injection of different concentrations of CO 2. The cultures was carried out in 6 L open raceway ponds, under controlled conditions at 30 °C and 39 μE m −2 s −1 and under non-controlled conditions, protected by a tunnel of transparent film. The experiments were subjected to CO 2 injections at concentrations of 0.038, 6, 12 and 18% (v/v). The highest concentration of biomass (4.95 g L −1) and maximum daily fixation (0.21 g g −1 d −1) were obtained for Spirulina LEB18 in culture that was prepared in non-controlled conditions with an injection of 6% (v/v) of CO 2. C. kessleri had maximum ( p < 0.0008) specific growth rate (0.84 d −1) when grown with 18% (v/v) of CO 2 in non-controlled conditions of cultivation.

Two phase microalgae growth in the open system for enhanced lipid productivity

Enhancement of the intracellular lipid content of Nannochloropsis sp. was first studied in a two phase culture system i.e. 7 days in photoautotrophic mode followed by 3 days in mixotrophic mode in the presence of either glycerol, glucose and sucrose in small scale closed system. During the first phase of growth, average biomass productivity was approximately 111 mg/l/day, and after 7 days the biomass contained 9.5% lipid as fatty acid methyl esters (FAME). Addition of fixed organic carbon sources to the cultures in the second phase resulted in higher biomass productivity. During the second growth phase (day 8–10), maximum volumetric FAME productivity was 51.7 mg/l/day when grown in the presence of 2 g/l glycerol in mixotrophic culture, an increase of over 72% compared to photoautotrophic culture alone. The strain, Nannochloropsis sp. was able to grow in hyper-saline water; however the biomass yield decreased when the salinity of the culture was over 50 ppt. Taking advantage of the strains ability to grow in hyper-saline water, the lipid productivity of dual culture system, i.e. phototrophic for 8 days followed by next 4 days in presence of glycerol, was then studied in open raceway pond.

Increased CO2 and the effect of pH on growth and calcification of Pleurochrysis carterae and Emiliania huxleyi (Haptophyta) in semicontinuous cultures

The effects of changes in CO(2) and pH on biomass productivity and carbon uptake of Pleurochrysis carterae and Emiliania huxleyi in open raceway ponds and a plate photobioreactor were studied. The pH of P. carterae cultures increased during day and decreased at night, whereas the pH of E. huxleyi cultures showed no significant diurnal changes. P. carterae coccolith production occurs during the dark period, whereas in E. huxleyi, coccolith production is mainly during the day. Addition of CO(2) at constant pH (pH-stat) resulted in an increase in P. carterae biomass and coccolith productivity, while CO(2) addition lowered E. huxleyi biomass and coccolith production. Neither of these algae could grow at less than pH 7.5. Species-specific diurnal pH and pCO(2) variations could be indicative of significant differences in carbon uptake between these two species. While E. huxleyi has been suggested to be predominantly a bicarbonate user, our results indicate that P. carterae may be using CO(2) as the main C source for photosynthesis and calcification.

Drying Characteristics of a Microalgae Consortium Developed for Biofuels Production

Microalgae are a promising renewable biomass feedstock that can be used to produce a wide range of bioproducts and biofuels. Drying is a critical step in algal biomass production as it improves shelf life, facilitates easier transport and storage, and enhances the extraction efficiency of lipids and other value-added compounds. This article investigated the thin-layer drying characteristics of a robust consortium of green algae consisting of Scenedesmus bijuga, Chlamydomonas globosa, and Chlorella minutissima grown in an open raceway pond. The microalgae consortium samples were harvested with 20% solids content using a centrifuge and were used for conducting thin-layer drying experiments in a convective oven. The change in moisture with time at each selected drying temperature (30°C, 50°C, 70°C, and 90°C) was recorded until the sample reached equilibrium moisture content. A typical drying curve for a microalgae consortium indicated that microalgae drying occurred at a falling rate, and the rate of drying was limited by diffusion. Three thin-layer drying models (Newton, Page, and Henderson-Pabis) were used to fit the microalgae drying data. The Page model fitted well with the drying data with high R2 (0.99) and low RMSE (0.02) values. Effective diffusivity of moisture from the microalgae was determined using the Henderson-Pabis model and increased from 1.94 × 10-10 to 1.74 × 10-9 m2 s-1 with increase in drying temperature from 30°C to 90°C. The low value of moisture diffusion rate from the microalgae samples may be attributed to shrinkage and case hardening of algae cells at the top drying surface. Both the effective diffusivity and the drying constant had significant dependence on drying temperature and were related with Arrhenius-type relationships. Biochemical compositions and fuel characteristics of the dried microalgae consortium were determined, and the dried microalgae can be an attractive feedstock for biofuel production.

A critical review on recent methods used for economically viable and eco-friendly development of microalgae as a potential feedstock for synthesis of biodiesel

Microalgae are being considered as a viable feedstock for large-scale production of biodiesel. However, though it may look simpler to obtain lipids from microalgae, the overall process of choosing an algal strain, cultivation, harvesting, dewatering, and extraction of oil is quite complicated and not economically prudent at this time. A thorough understanding of algae and the overall biodiesel production process discussed in this paper is vital so that focused research might lower the costs involved. Various diverse species of microalgae are currently being used as feedstocks for biofuel. Heterotrophic culture may be preferred over photoautotrophic cultivation. For cultivation, specially fabricated photobioreactors (PBRs) have the capability to overcome the constraints and limitations of the open raceway ponds, although the former are cost intensive as compared to the latter. Exergy analysis of algal-biodiesel-carbon dioxide cycle shows the overall process to be renewable and hence should attain global attention.

Microalgae biomass and lipid production potential, and mechanical stress compliance

Microalgae bioreactors that capture industrial carbon dioxide (CO2) emissions to produce lipids for biodiesel are of significant interest. Sun-lit open raceways are generally considered the most economic method for mass cultivation, but the large physical footprint of these shallow systems can limit both industrial site availability and CO2 transfer into the culture medium. To address these issues, a deep top-lit only, gas-lift bioreactor to culture microalgae and capture CO2 was designed and investigated. The results show a three times increase in areal biomass and lipid productivities when compared to traditional raceways used in large-scale microalgae production. Operational factors exerting significant effects on areal biomass and lipid productivities were identified through Plackett-Burman experimental design as gas flow rate, feed gas CO2 content, and dispersion height. By employing response surface methodology, models to predict areal biomass and lipid productivities were derived. The resulting desirability function was then applied to obtain the optimal combination of operational parameters that maximize lipid production per unit area occupied by the bioreactor, whilst keeping biomass production low to reduce downstream processing costs. The optimum operational parameters that fulfill the requirements of the optimization function resulted in areal biomass productivity of 32.1 gdwm−2d−1 and areal lipid production of 198.4 gLipidm−2.

Microalgal biomass production: challenges and realities

The maximum quantum yield (Φ (max)), calculated from the maximum chlorophyll a specific photosynthetic rate divided by the quantum absorption per unit chlorophyll a, is 8 photons or 0.125 mol C per mol Quanta light energy. For the average solar radiation that reaches the earth's surface this relates to a photosynthetic yield of 1.79 g(dw) m(-2) day(-1) per percentage photosynthetic efficiency and it could be doubled for sunny, dry and hot areas. Many factors determine volumetric yields of mass algal cultures and it is not simply a question of extrapolating controlled laboratory rates to large scale outdoor production systems. This is an obvious mistake many algal biotechnology start-up companies make. Closed photobioreactors should be able to outperform open raceway pond cultures because of the synergistic enhancement of a reduced boundary layer and short light/dark fluctuations at high turbulences. However, this has not been shown on any large scale and to date the industrial norm for very large production systems is open raceway production ponds. Microalgal biomass production offers real opportunities for addressing issues such as CO(2) sequestration, biofuel production and wastewater treatment, and it should be the preferred research emphasis.