The Role of Advanced Water Treatment Processes on Dissolved Organic Nitrogen Removal and Fate of Nitrogen in Residual Waste Streams

Most wastewater treatment plants that follow stricter discharge restrictions for nutrients (nitrogen, phosphorus) effectively remove the inorganic nitrogen in their treatment processes, leaving dissolved organic nitrogen (DON) to be the dominant portion of their effluent total nitrogen. DON present in reclaimed water can deteriorate the water quality in receiving water bodies, promoting algae growth and eutrophication. Moreover, DON poses adverse effects in water reuse practices, serving as a precursor of carcinogenic nitrogenous disinfection by-products (N-DBPs) and promoting membrane fouling. Enhanced coagulation, ozonation, and biological activated carbon (BAC) are well-established processes in advanced water treatment trains for further effluent polishing and potable reuse practices; however, no information is available on their effectiveness for DON removal under real operational conditions. This study aims to address DON removal efficiency and process implications by enhanced coagulation, ozonation, and BAC from reclaimed water at the pilot scale. An enhanced coagulation/flocculation/filtration pilot system was installed at the Truckee Meadows Water Reclamation Facility (TMWRF) for a year, and three different Al- and Fe-based coagulants (aluminum sulfate (alum), poly-aluminum chloride (PACl), and ferric chloride (FC)), along with a cationic polymer coagulant aid (Clarifloc) were tested at various conditions. Optimum coagulant doses and pH values were determined through jar tests and subsequently applied at the pilot-scale. The sludge production rate by each coagulant, the fate of DON in generated solids, and the extent of the possible release of captured DON from coagulation sludge were also studied. Pilot-scale enhanced coagulation with alum (24 mg/L) exhibited variable DON removal efficiency (6–40%; 21% on average), which was enhanced by adding the polymer coagulant aid, resulting in an average DON removal of 32%. PACl (40 mg/L) and FC (100 mg/L) demonstrated more consistent DON removal (average 45% and 57%, respectively), with minimal improvement observed with the addition of the polymer. Overall, enhanced coagulation effectively reduced DON in the reclaimed water at the pilot scale while showcasing additional benefits such as dissolved organic carbon (DOC) and orthophosphate removal. FC had a lower sludge production rate at the pilot, and Fe- based solids showed a lower release of captured DON compared to the Al-based solids in simulated lab experiments. Furthermore, the impact of enhanced coagulation and ozonation combined with BAC treatment on DON removal in reclaimed water was investigated using a pilot-scale advanced treatment train at the Reno-Stead Water Reclamation Facility (RSWRF). The results demonstrated that enhanced coagulation by a combination of PACl (56 mg/L) and aluminum chlorohydrate (4 mg/L) resulted in 36 ± 8% and 27 ± 9% DON removal from the RSWRF's secondary and filter effluent, respectively. Ozonation alone exhibited minimal DON removal; however, ozonation combined with BAC treatment effectively removed DON from the RSWRF's secondary and filter effluent by 40 ± 6% and 42 ± 5%, respectively. The findings from this study indicated enhanced coagulation and ozonation/BAC to be viable treatments for partial DON removal from reclaimed water