Abstract: The inhibitory effects of free ammonia (FA) on microalgae/cyanobacteria in wastewater-treating photobioreactors (PBR) can strongly reduce their treatment efficiency, increasing the operational costs and undermining the stability of the system. Although FA-promoting conditions (high pH, temperature and ammoniacal nitrogen concentration) are commonly met in outdoor PBRs, photosynthesis inhibition from FA has been scarcely explored and is rarely considered in microalgae-bacteria growth models. Two pilot systems and a series of lab-scale monocultures were tested using a photo-respirometry approach, to evaluate the effects of FA (8.5?136 mg NH3 L?1) on photosynthesis. Two mathematical inhibition models were compared, with the aim of selecting best-fitting equations to describe photo-respirometric experiments. A set of calibrated inhibition parameters was obtained for microalgae and cyanobacteria, growing in monocultures or in mixed algae-bacteria consortia. Cyanobacteria were more sensitive to FA than green microalgae and mixed phototrophs-bacteria consortia showed a higher resistance compared to monocultures. Estimated inhibition parameters were used to describe common operational/environmental conditions in algae-bacteria systems, demonstrating the potential drop in photosynthetic activity under those relevant operational conditions.

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