Abstract
This study aimed at examining and comparing the nutrient removal efficiency, biomass productivity and microbial community structure of two outdoor pilot-scale photobioreactors, namely a bubble column and a raceway pond, treating the liquid fraction of an agricultural digestate. Bacterial and eukaryotic communities were characterized using a metabarcoding approach and quantitative PCR. The abundance, composition, diversity, and dynamics of the main microbes were then correlated to the environmental conditions and operational parameters of the reactors. Both photobioreactors were dominated either by Chlorella sp. or Scenedesmus sp. in function of temperature, irradiance and the nitrogen compounds derived by nitrification. Other species, such as Chlamydomonas and Planktochlorella, were sporadically present, demonstrating that they have more specific niche requirement. Pseudomonas sp. always dominated the bacterial community in both reactors, except in summertime, when a bloom of Calothrix occurred in the raceway pond. In autumn, the worsening of the climate conditions decreased the microalgal growth, promoting predation by Vorticella sp. The study highlights the factors influencing the structure and dynamics of the microbial consortia and which ecological mechanisms are driving the microbial shifts and the consequent reactor performance. On these bases, control strategies could be defined to optimize the management of the microalgal-based technologies.
Highlights
The water industry is facing a massive transition to transform wastewater treatment plants (WWTPs) into resource recovering factories [1]
Pseudomonas sp. always dominated the bacterial community in both reactors, except in summertime, when a bloom of Calothrix occurred in the raceway pond
Understanding the biotic and abiotic mechanisms which favor the growth of desired microbial species or consortia might help to manipulate the community in a way to enhance treatment efficiency and productivity of valuable biomass
Summary
The water industry is facing a massive transition to transform wastewater treatment plants (WWTPs) into resource recovering factories [1] In this light, the integration of microalgae-based technologies into WWTPs has gained attention because of their cost-effective bioremediation capability, the relatively low energy requirement, and the possibility to recover valuable biomass [2]. Outdoor open systems (i.e., raceway ponds, bubble columns, flat panels) are considered so far as the most viable method of microalgal cultivation on wastewaters, though the treatment process is less controllable. Such systems are very susceptible to unavoidable environmental changes and contamination from the surrounding environments [10]. Understanding the biotic and abiotic mechanisms which favor the growth of desired microbial species or consortia might help to manipulate the community in a way to enhance treatment efficiency and productivity of valuable biomass
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