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- Outdoor large scale microalgae consortium culture for biofuel production in south africa
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- 1. Journal of Energy Technologies and Policy www.iiste.org ISSN 2224-3232 (Paper) ISSN 2225-0573 (Online) Vol.3, No.11, 2013 – Special Issue for International Conference on Energy, Environment and Sustainable Economy (EESE 2013) Outdoor Large scale Microalgae consortium culture for biofuel production in South Africa: Overcoming adverse environmental effects on microalgal growth Maranda Esterhuizen-Londt Ben Zeelie InnoVenton/DCTS; Nelson Mandela Metropolitan University; Summerstrand Campus (North); University Way; Summerstrand; Port Elizabeth; 6001; SOUTH AFRICA *Email address of corresponding author: firstname.lastname@example.org Abstract In nature, microalgal blooms occur regularly and in contrast with laboratory cultivation procedures, these blooms are not axenic and this seems to add to the longevity and intensity of the bloom. For this reason a consortium of microalgae and bacteria is used at InnoVenton for large scale biomass cultivation. The biomass produced, has successfully been used to produce biocrude. Laboratory cultivation procedures also require a large energy input in terms of artificial lighting, heating and aeration making it a costly endeavour. However, this stringent control of the culture to avoid contamination and ensure optimal growth conditions is essential when cultivating the microalgae for medical and pharmaceutical applications. When culturing for a chemical application such as biofuel production, this is not the case, therefore allowing for the economical outdoor consortium cultivation approach employed at InnoVenton. In this study we investigated whether morning heating of the media will overcome low consortium growth rates experienced in winter and whether the use of glucose, ethanol and acetate will overcome biomass loss exhibited at night. The results showed that heating increased the growth rate relative to those measured in summer and that heating all day in winter did not induce better growth rates in comparison to only heating for an hour at sunrise. We show that all three carbon sources are efficient at overcoming biomass loss at night with glucose being the most effective. In conclusion, employing these two techniques, the same growth rate theoretically can be achieved year round with large scale outdoor cultivation. Keywords: consortium, microalgae, biocrude 1. Introduction Large scale microalgal cultivation is constantly critiqued due to its high costs and is therefore not a front contender for use in biofuel production (Christi, 2007; Wijffels and Barbosa, 2010; Wu et al., 2012; Greenwell et al., 2012). Current cultivation methods are based on laboratory techniques essential for ensuring contaminant free biomass for medical and pharmaceutical applications (Christi, 2007). However, for a chemical process application such as biofuel production, this is not necessary and the cultivation costs can therefore be reduced with outdoor cultivation employing ambient light and heating. This can be very efficient when cultivation takes place in a country such as South Africa with such temperate seasons. In nature, blooms occur naturally and are in no way axenic. The symbiotic relationship of the algae with the bacteria seems to prolong bloom events and increase the bloom density (Fukami et al., 1997). InnoVenton therefore employs such a large scale cultivation system consisting of a consortium of microalgae and bacteria with demonstrated ability to produce biocrude from the biomass (Esterhuizen-Londt and Zeelie, in press). However, even though the seasons experienced in South Africa are mild, it still influence the growth rates of the consortium and together with night biomass loss, this allows opportunity for enhancement of the annual biomass turnover. 1.1 Heating Daily circadian rhythms coordinate and monitor physiological events in microalgae so that metabolism, physiology and behavioural patterns occur at optimal phases of the diurnal cycle. Circadian rhythms are temperature compensated (Suzuki and Johnson, 2001) and thus the environmental temperature will have a direct impact on microalgae growth. In general, temperature is the major regulating factor in cell regulation, morphological and physiological responses of microalgae (Zeng et al., 2011). Cell growth rate or division is directly proportional to temperature. This stems from optimal temperatures required for enzymes involved in metabolism and physiological responses. Optimal temperatures for microalgae growth range from 25 to 35oC with a maximum cell growth rate generally around 30oC with the minimal temperature for supporting microalgal growth recorded at 16oC (Converti et al., 2009). However, optimal temperature is also affected by other factors such as light intensity and humidity. Therefore, microalgae growth rate in an environmental setting will be 39 EESE-2013 is organised by International Society for Commerce, Industry & Engineering.
- 2. Journal of Energy Technologies and Policy www.iiste.org ISSN 2224-3232 (Paper) ISSN 2225-0573 (Online) Vol.3, No.11, 2013 – Special Issue for International Conference on Energy, Environment and Sustainable Economy (EESE 2013) affected by daily and season temperatures and thus the length of the day with temperatures above 16oC. It has been shown that the eukaryotic green microalgae Dunaliella sp. displays biomass accumulation proportional to the length of the photoperiod (Foy et al., 1976). It is important to sustain microalgae cultures with a high photosynthetic capacity since this will result in a higher biomass yield. Respiration and photorespiration are wasteful processes and will lead to a loss in biomass yield and should thus be avoided or essentially kept to a minimum. Temperature has a strong effect on these processes when CO2 and/or light is limiting. High temperatures will lead to increased respiration (Ogbanna and Tanaka, 1996); thus decreasing the efficiency of photosynthesis as more energy is expended then produced (Pulz, 2001). 1.3 Night biomass loss Microalgae photosynthesize to produce carbohydrates and adenosine triphosphate (ATP) needed for cell growth and reproduction. When the light intensity is too low to support photosynthesis, cells do not grow but respire to remain viable and thus leading to reduced biomass. It has been reported that as much as 35% of the biomass produced during the day can be lost at night (Grobelaar and Soeder, 1985; Torzilla et al., 1991; Ogbanna and Tanaka, 1996). Ogbanna and Tanaka (1996) reported that during the night the carbohydrate concentration decreased, however, the protein content increased thus intracellular carbohydrate stores are utilized. Nightly biomass loss depends on the daily light level, light period temperature and the temperature at night. Ogbanna and Tanaka (1996) reported that the biomass lost at night decreased with higher daytime temperatures but increased with high light intensity. In their attempts to overcome biomass loss Ogbanna and Tanaka (1996) reported that it could not be completely circumvented but only reduced by decreasing the temperature and avoiding mixing at night. Ogbanna and Tanaka (1996) successfully overcame biomass loss at laboratory scale by the addition of glucose, acetate or ethanol to their cultures thereby achieving continuous growth. They reported that acetate and ethanol was preferred in order to avoid the risk of contamination. 1.4 Research Aim The aim of this study was to determine whether supplying exogenous heat at daybreak to raise the temperature above 16oC is effective to increase the effective photogenic period during winter months. We also investigate the effect of the three organic carbon sources glucose, sodium acetate and ethanol on the microalgae consortium grown outdoors in a greenhouse at large scale in order to avoid biomass loss at night. Seeing as harvesting does not take place on a daily basis at InnoVenton, this will lead to increased biomass yield. 2. Method 2.1 Cultivation details The microalgae consortium dominated by eukaryotic Scenedesmus sp., Chlamydomonas sp. and Chlorella sp., and cyanobacteria including Limnothirix sp. were obtained from various sources, combined with naturally occurring symbiotic bacterial were cultured non-sterilely over a year period for the communities to establish and stabilize. The consortium was cultured in vertical enclosed photobioreactors (PBR, Figure 1) with airlift sparging supplemented with 5% CO2 in the InnoVenton greenhouse. Figure 1: Image showing the InnoVenton greenhouse with the PBR setup and placement. 40 EESE-2013 is organised by International Society for Commerce, Industry & Engineering.
- 3. Journal of Energy Technologies and Policy www.iiste.org ISSN 2224-3232 (Paper) ISSN 2225-0573 (Online) Vol.3, No.11, 2013 – Special Issue for International Conference on Energy, Environment and Sustainable Economy (EESE 2013) The PBRs were solely a function of the environmental temperature and season unless stated otherwise. The mixed consortium of microalgae, cyanobacteria and heterotrophic bacteria were fed with a feed mixture based on the carbon, hydrogen, nitrogen, and sulphur (CHNS) analysis of the consortium (Esterhuizen-Londt and Zeelie, in press). 2.2 Heating The standard consortium was used to determine whether increasing the temperature of the culture media at the start of the photoactive period (early morning) affected the biomass accumulation on a daily basis. The heterogeneous culture was inoculated into tap water and fed once daily. The pH of the culture was controlled to start at 7.5. The treatment consisted of a PBR (in triplicate) partially suspended in heated water (30oC) and the control consisted of a PBR partially suspended in water at ambient temperature. In one set of experiments, the treatment PBRs were heated at 30oC for the entire photoperiod and for the other set, the treatment PBRs were only heated at the start of the photoperiod for one hour (to ensure that the culture medium’s temperature was raised to above 16oC). The experiment was replicated over three days to account for environmental variation. The biomass growth was measured as increase in turbidity with a TB 200 turbidity meter (Orbeco) and the pH and temperature was monitored with a Hanna pH and temperature meter at the start, mid-point and at the end of the photoperiod. It should be noted that the relationship between turbidity and biomass as dryweight is not linear. Statistical analysis was conducted using Statistica 2010TM. The analysis of variance (ANOVA) test was used to determine significant differences between each treatment set and its corresponding control. 2.3 Night biomass loss For the mixotrophic growth experiment, six PBRs were used with random placement in the InnoVenton Greenhouse. The microalgae consortium was operated in fed-batch mode with an approximate starting density of 0.5 g DW L-1.Each treatment (glucose, sodium acetate and ethanol) was conducted in a separate PBR with its own corresponding control. Each PBR was sparged with air supplemented with 5% CO2 during the entire photoperiod. The cultures were fed with the normal feed stock daily; however, the pH was altered only on day 1 to approx. 7.5 using acetic acid to avoid any unnecessary carbon addition. The microalgae were left to grow during daylight hours and before sunset the glucose, acetate and ethanol was added according to the uptake rate and cell density per respective PBR (Ogbanna and Tanaka, 1996). The microalgae growth was measured as a function of turbidity (TB 200 turbidity meter, Orbeco) and pH and temperature was measured (Hanna meter) over the period of 84 hours (3 dark periods included). This was repeated three times to be able to account for the variation in environmental conditions. Statistical analysis was conducted in Statisica 2010TM. Regression analysis with indicator variables was used to differentiate between the indicators and the control treatments. 3. Results and Discussion 3.1 Heating Environmental culture compared to laboratory culture in a closed controlled system, will usually have more losses in biomass due to sedimentation, grazing, parasitism and general loss. It is nearly impossible to determine these losses and thus the growth rate or biomass yield should be seen as a minimum value. The biomass accumulation per day is thus used as an indication of growth. Figure 2A and B shows there is a significant difference between the growth of the heated PBRs versus the controls for each day 1 (pView More