Year-long assessment of a pilot-scale thin-layer reactor for microalgae wastewater treatment. Variation in the microalgae-bacteria consortium and the impact of environmental conditions

Abstract

This work studies microalgae production using urban primary wastewater as the only nutrient source in a pilot-scale thin-layer cascade photobioreactor (TLPBR). The experiments were carried out by operating the photobioreactor in semi-continuous mode at a constant dilution rate (D = 0.3 days−1) uninterrupted over ten months. The TLPBR was effective at removing the main nutrients (nitrogen, phosphorus and chemical oxygen demand, COD) with biomass productivities ranging from 28.3 g·m−2·day−1 to 47.3 g·m−2·day−1 over the different seasons of the year. In addition, the evolution of the cells' photosynthetic efficiency (photosystem II) was also analysed. The removal of the main nutrients was correlated with the changes in the microbial-bacteria consortium existing in the culture system. These microbial fluctuations were correlated with the temperature and the solar irradiance impinging on the culture surface. The most abundant bacterial phyla were Proteobacteria and Bacteroidetes with the most dominant order being Rhodobacterales and Sphingomonadales, belonging to the Alphaproteobacteria class. Functional analysis of the relationship between bacterial populations, removal of major nutrients and the environmental parameters showed that the three main bacteria orders together (41% of the total) represent the main bacteria groups, which remained over the months tested; however, their abundance was affected by fluctuations, which might be explained by environmental variations and major nutrients such as the chemical oxygen demand (COD).