Impact of Phage Cocktail Application on the Microbiota of Saline Produced Water During Ammonia Removal

Bacteriophages offer a targeted approach to industrial biocontrol, with the potential to control undesirable bacteria while limiting direct effects on nonsusceptible microorganisms. However, their compatibility with microbial functions essential to saline produced-water treatment remains poorly understood. This study evaluated produced-water treatment microcosms after a single application of a four-phage cocktail at 107 PFU mL−1. Sludge that acclimated to 55 or 125 g L−1 NaCl was monitored for ammonia removal, infective phage persistence, cell membrane integrity, and prokaryotic community composition. Control microcosms maintained ammonia removal above 94%, whereas treated systems showed a transient reduction after cocktail application. Ammonia removal recovered to above 90% by day 16 at 55 g L−1 NaCl and by day 55 at 125 g L−1 NaCl, indicating a more persistent response at higher salinity. Infective phage titers declined by day 6 and were undetectable from day 8 onward, with no evidence of detectable phage amplification. Fluorescence microscopy indicated greater membrane permeabilization in phage-treated microcosms at higher salinity. The microbial community remained dominated by Proteobacteria, particularly Halomonas. Overall, phage cocktail application coincided with a temporary and reversible decline in ammonia removal, particularly at higher salinity, emphasizing the need to evaluate its compatibility with essential biological treatment functions before practical implementation.