Production of microalgae using pilot-scale thin-layer cascade photobioreactors : effect of water type on biomass composition

The microalga Tetradesmus almeriensis was produced in a 63 m(2 )thin-layer cascade photobioreactor using either freshwater supplemented with fertilisers, primary urban wastewater, secondary urban wastewater, or diluted fresh pig slurry. The reactors were operated in semi-continuous mode at dilution rate of 0.3 day(-1) for over a month. Overall, the biomass productivity was highly influenced by the type of water used, the highest being that produced using freshwater (30.3 g m(-2).day(- 1)) given its greater nutrient content. The composition of the pro-duced biomass was also significantly influenced by the type of water used. The protein and lipid contents were higher when the biomass was produced using wastewater. These differences were not only attributed to the microalgal cells' response to the different nutrient availabilities but also to the microbial diversity of the bio-masses themselves. Indeed, Illumina sequencing revealed that the inoculated strain was only the most abundant in the culture produced using freshwater (44.6%) and diluted pig slurry (52.9%). When produced using primary or secondary wastewater the genus Tetradesmus represented only 1.6 and 4.9% of the total eukaryotic pop-ulations, respectively. Several algal predators were identified in the cultures including parasites of the genus Amoeboaphelidium and rotifers from the genus Adineta. The results highlighted the importance of characterising microalgal cultures to identify potential pathogens and predators, and to implement contingency actions.