In this study, 11 reverse osmosis (RO) concentrate samples from six full-scale potable reuse facilities in the southwestern United States were treated by a new photobiological treatment process using brackish water diatoms at a laboratory scale. While eight out of the 11 RO concentrate samples were successfully treated by the photobiological treatment, the other three samples obtained from the facilities where non-nitrified effluent was used as a source water were unsuitable for the treatment due to high levels of ammonia-N in these samples. As low as 16 mg · L−1 of ammonia-N was found to be inhibitory. Lower pH was found to be undesirable because of lower calcium removal efficiency. Ozone pre-treatment and the presence of antiscalant appeared to have no impact on the photobiological process. This study demonstrated a wider applicability of this photobiological process for the treatment of RO concentrate from potable reuse facilities with different process schemes.