Growth and bioactivity of two chlorophyte (Chlorella and Scenedesmus) strains co-cultured outdoors in two different thin-layer units using municipal wastewater as a nutrient source

The application of microalgae in wastewater treatment has recently been at the forefront of interest due to the increasing concern about environmental protection and economic sustainability. This work aimed to study two chlorophyte species, Chlorella vulgaris and Scenedesmus acutus, co-cultured outdoors in centrate of municipal wastewater as a nutrient source. Two different thin-layer units were used in these trials — thin-layer cascade (TLC) and thin-layer raceway pond (TL-RWP), suitable for this purpose due to their high biomass productivity and better culture transparency when using muddy wastewater. The units were operated in batch, and subsequently in semi-continuous growth regime — and monitored in terms of photosynthetic performance, growth, nutrient removal rate, and bioactivity. The results showed that the co-cultures grew well in the centrate, achieving the maximum biomass densities of 1.3 and 2.1 g DW L−1 in TLC and TL-RWP, respectively, by the end of the batch regime and 1.9 and 2.0 g DW L−1 by the end of the semi-continuous regime. Although TL-RWP grown cultures showed faster growth, the TLC-one revealed better nutrient removal efficiencies batch wise than the culture grown in TL-RWP — removing up to 48% of total nitrogen and 43% of total phosphorus. Conversely, the latter was more efficient under the semi-continuous regime (54% and 42% consumption of total nitrogen and phosphorus, respectively). In the harvested biomass, an important antimicrobial activity (specifically antifungal) was detected. In this sense, the in-vitro growth of the oomycete Pythium ultimum was inhibited by up to 45% with regard to the control. However, no biostimulating activity was observed. The present findings confirm the possibility of using these two species for biomass production in municipal wastewater centrate using highly productive thin-layer systems. This technology can be a valuable contribution to circular economy since the produced biomass can be re-applied for agricultural purposes.