Production of microalgae using centrate from anaerobic digestion as the nutrient source

Abstract

In this paper freshwater microalgae production is studied using centrate from anaerobic digestion as the only nutrient source. The objective is not only to maximize biomass productivity but also to eliminate contaminants in order to release water which is clean enough for reuse in compliance with regulations. Experiments were performed indoors using two microalgae strains, Muriellopsis sp. and Pseudokirchneriella subcapitata, to establish the optimal centrate percentages within the culture medium. Results demonstrated that with 40–50% of centrate in the culture medium, productivity reached values of up to 1.13 and 1.02g·l−1·day−1 for Muriellopsis sp. and P. subcapitata, respectively. In addition, the removal of nitrogen and phosphorous contained in the culture medium exceeded 90%, the COD at the outlet being lower than 100mg·l−1. Above 50% of centrate, toxicity exists and culture performance decreases. No nitrification was observed as the ammonium was completely consumed at centrate percentages lower than 20%. Muriellopsis sp. demonstrated itself to be the more robust strain tolerating higher ammonium concentrations as well as achieving a higher quantum yield, of up to 0.50g·E−1, and a higher nitrogen removal rate, of up to 47.5mgN·l−1·day−1 and 3.8mgP·l−1·day−1. Consequently, it is possible to use centrate from anaerobic digestion as the sole nutrient source for the production of freshwater microalgae, thus reducing the biomass production cost by avoiding the use of expensive and non-sustainable fertilizers; not to mention the returns obtained from treating this type of residue. This combination can help to increase the possibilities of producing biofuels from microalgae by means of coupling their production to other treatment processes.