Microalgae-bacteria consortia dynamics in a long term operated membrane-coupled high-rate algal pond (MHRAP)

Traditional activated sludge-based technologies have significant drawbacks, including high energy requirements and greenhouse gas emissions. Microalgae-based processes offer a promising, low-cost, and environmentally friendly alternative. However, the knowledge of treatment systems based on microalgae-bacteria consortia is limited, and even more so is their microbial composition and its relationship with operational parameters. Thus, this study explores the dynamics of microalgae-bacteria consortia in a long-term operated membrane-coupled high-rate algal pond (MHRAP) for wastewater treatment. For this, a pilot-scale MHRAP plant, located in a wastewater treatment plant in Valencia (Spain), was monitored under various hydraulic retention times (HRT) and wastewater influents: i) effluent from a primary settler and ii) effluent form pre-treatment. The biomass retention time was kept constant at 6 days. The composition of the bacterial community was studied through 16S rDNA sequencing, while 18S rDNA sequencing was used to study the microalgae. The results indicate that shorter HRTs significantly increased bacterial diversity, but not eukarya. Principal Co-ordinates Analysis (PCoA) revealed that the HRT and the incoming wastewater quality control the type of the bacterial populations. However, this effect was not observed in eukaryotic organisms. The dominant microalgae genera identified were Desmodesmus and Coelastrella, with Coelastrella becoming more prevalent at shorter HRTs. For bacteria, Verrumicrobiota dominated (18-56%) at high HRT while Proteobacteria was dominant (28-44%) at HRTs below 6 days. The changes observed in the bacterial composition, including the ammonia oxidizing bacteria (AOB) community (mainly Nitrosomonas), suggest that photo-inhibition could be taking place. The nitrite oxidizing bacteria (NOB) community was dominated by Nitrospira and Candidatus Nitrotoga. Operational parameters such as light intensity, pH, and nitrite concentration were found to significantly influence the microbial community structure. Higher light intensity and alkaline pH favored the growth of Desmodesmus, while Coelastrella thrived under lower HRTs. Bacterial diversity plays a crucial role in the treatment process, while microalgae primarily support aerobic bacterial processes by providing oxygen. These findings contribute to a deeper understanding of the complex biological processes in microalgae-bacteria consortia and offer insights into improving wastewater treatment technologies.