Mass balance analysis of carbon and nitrogen in industrial scale mixotrophic microalgae cultures

Abstract

Large-scale cultivation of Chlorella vulgaris is of great interest given the extent of products and potential applications that can derive from its biomass. From an industrial point of view it is imperative to consistently obtain high productivities and high quality biomass at the lowest production costs. The mass balance of critical nutrients such as carbon and nitrogen is therefore necessary to quantify its recovery and consumption yields, the efficiency of the biomass production system and to identify operational optimization opportunities. The mass balance of C. vulgaris mixotrophic growth throughout scale-up from 10 m3 to 100 m3 on acetate and urea as carbon and nitrogen sources was calculated using a black-box model developed to illustrate the inputs and outputs of the system in quasi-real time and resulted on recovery factors of 0.99 ± 0.08 and 0.99 ± 0.25, respectively. Under these conditions C. vulgaris cultivation yielded a maximal productivity of 0.14 g L−1 d−1 and maximal growth rate of 0.38 d−1 . Both parameters decreased throughout scale up reaching an average productivity of 0.09 g L−1 d−1 with an average growth rate of 0.13 d−1 for the whole process. Global carbon and nitrogen yields measured were 0.76 molC-X molC −1 and 0.72 molN-X molN −1 . The mass balance determination indicates the incorporation of both acetate and urea carbon atoms into the biomass. Therefore, external inorganic carbon from CO2 was concluded to have little influence on microalgae growth in the conditions studied apart from pH control. Urea and ammonium were found to be effectively used by C. vulgaris cells. However, despite the satisfactory yield obtained for nitrogen, the metabolism of urea resulted in ammonium build-up in the culture medium. To our knowledge this is the first report of growth parameters and mass balance analysis of a Chlorella sp. culture in industrial scale closed tubular photobioreactors.