Date Published: September 29, 2015
Publisher: Public Library of Science
Author(s): Lili Yao, Jianye Shi, Xiaoling Miao, Yiguo Hong.
Biomass, nutrient removal capacity, lipid productivity and morphological changes of Chlorella sorokiniana and Desmodesmus communis were investigated in mixed wastewaters with different CO2 concentrations. Under optimal condition, which was 1:3 ratio of swine wastewater to second treated municipal wastewater with 5% CO2, the maximum biomass concentrations were 1.22 g L-1 and 0.84 g L-1 for C. sorokiniana and D. communis, respectively. Almost all of the ammonia and phosphorus were removed, the removal rates of total nitrogen were 88.05% for C. sorokiniana and 83.18% for D. communis. Lipid content reached 17.04% for C. sorokiniana and 20.37% for D. communis after 10 days culture. CO2 aeration increased intracellular particle numbers of both microalgae and made D. communis tend to be solitary. The research suggested the aeration of CO2 improve the tolerance of microalgae to high concentration of NH4-N, and nutrient excess stress could induce lipid accumulation of microalgae.
Concerns about the depletion of petroleum resources reserves  coupled with the rise of the global energy demand, and an increasing awareness of the environmental impact of associated CO2 emissions, have made the development of renewable and environmentally friendly energy sources necessary . In this sense, biodiesel which has properties similar to fossil-fuels production from photosynthetic microorganisms has been recognized as reliable and renewable energy sources for the steady supply of energy. Many studies have demonstrated that microalgae were superior to other raw materials for the production of biodiesel [3–4]. However, the development of microalgae-based biodiesel still faces many challenges. One of the most critical challenges is to establish economical means of supplying water and nutrients for cultivation since microalgae require a huge volume of medium for mass scale growth, which leading to major operating costs associated with the nutritional supply . Meanwhile, the reclamation of wastewater is of pivotal importance to achieving sustainability in our society at the global level. Fortunately, in addition to their high biomass and lipid productivities, some microalgae strains also have potential environmental benefits, such as mitigation of CO2 through photosynthesis  and bioremediating wastewater by removing large amounts of nutrients and heavy metals . Therefore, an algae-based wastewater and CO2 treatment system may be the key to solve both problems.
In summary, the present study showed that it is feasible to increase biomass and total lipid productivity by mixing SW and STMW coupled with a proper CO2 concentration. Both Chlorella sorokiniana and Desmodesmus communis cultivated in 1:3 (SW:STMW) medium achieved the highest nutrient removal rate with or without extra CO2 aeration. C. sorokiniana obtained the maximum biomass concentration (1.31g L-1) and maximum lipid productivity (0.023g L-1 d-1) in medium 2:2 (SW:STMW) under 5% CO2 concentration. The results suggested that SW and STMW have great potential to become sources of nutrition for microalgae by mixing them at a suitable ratio.