Enhancing Nutrient Removal in High-Strength Synthetic Wastewater: The Impact of Phototrophic Purple Bacteria on a Microalgal–Bacterial Consortium

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This study evaluated the performance of a non-aerated 22 L column photobioreactor treating synthetic wastewater (SWW) under increasing organic loading rates (SWW x1, x2, and x3) at a hydraulic retention time of 2 days to investigate how purple phototrophic bacteria (PPB) inoculation timing affects operational stability and nutrient removal. Two strategies were compared: supplementing a microalgae–bacteria consortium with PPB after 15 days (Phase I) versus co-inoculation from startup (Phase II). Phase I maintained active photosynthesis, reaching dissolved oxygen (DO) levels of 14.7 ± 3.8 mg L−1 in SWW x2. Conversely, Phase II sustained persistently low DO (<1.5 mg L−1), which correlated with reduced inorganic carbon fixation and raised effluent inorganic carbon to 205.2 mg L−1 under SWW x3. The highest load (SWW x3) also induced orthophosphate accumulation and elevated ammonium levels up to 150 mg L−1 due to nitrogenase suppression. Phase II also exhibited a higher nitrite accumulation (approx. 10 mg L−1) with minimal nitrate (<3 mg L−1), suggesting impaired nitrifying activity. In conclusion, the early PPB co-inoculation strategy established a distinct microbial community trajectory that favored a sub-optimized, dissipative metabolic regime that impaired carbon fixation and resource recovery under high loading rates.