Abstract
Microalgae cultivation for biomass production and nutrient removal implies the use of natural light and minimal control of the temperature for obtaining a low cost production. The aim of this study was to quantify the effect of temperature control at natural light on biomass productivity and removal of NH4-N and PO4-P of a mesophilic strain of Chlorella. Chlorella sp. was grown in reject water of anaerobically digested municipal sludge, sludge liquor, inside a greenhouse compartment (Ås, Norway, 59°N) using batch cultures (300 mL). Five experiments were conducted from May to September, and effects of different levels of temperature control and diurnal variations were investigated. The highest biomass productivities (0.45 g L−1 day−1) in the linear growth phase were obtained at daily light integrals ≥12 mol day−1 m−2. Results showed that the average temperature was of more importance than the night or day temperature range. At average temperatures <22 °C for cultures with no temperature control, the productivity decreased by 23 and 39 % compared to cultures with full temperature control (24–25 °C). In one experiment, the productivity was reduced at no temperature control due to prolonged high daytime temperatures (>32 °C) and were followed by a lower NH4-N removal rate. Otherwise, temperature had little effect on NH4-N removal. The level of temperature control did not affect removal of PO4-P. Cellular starch content varied from ~15–38 % in the evening and was generally lower at no temperature control. In the morning the starch content was reduced to ~4–12 % with no difference between the different levels of temperature control. (~4–12 %).
Highlights
Microalgae cultivation for biomass production has a higher environmental impact when considering energy use, greenhouse gas emissions and water usage, compared to conventional crops (Clarens et al 2010)
The results show that high biomass productivities and nutrient removal rates can be obtained at a wide range in temperature and irradiance
Chlorella sp. showed a wide plateau in optimal temperature and within this range, the average temperature was more important for productivity than the day or night temperature range
Summary
Microalgae cultivation for biomass production has a higher environmental impact when considering energy use, greenhouse gas emissions and water usage, compared to conventional crops (Clarens et al 2010). The environmental impact can only be reduced by using wastewater and flue gas as a nutrient and carbon source (Clarens et al 2010). Microalgae can grow in raw SL, the growth rate is significantly increased when lowering the content of SS, which can be carried out by the simple process of dilution (Åkerström et al 2014). At high latitudes such as Norway, the growth season spans from April to September. Apart from the length of the dark period, the biomass loss is affected by temperature, growth phase and biochemical composition (Doucha and Livansky 2006; Han et al 2013; Ogbonna and Tanaka 1996; Torzillo et al 1991)
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