Evaluation of Advanced Owts Performance within the Greater Narragansett Bay Watershed.

Advanced nitrogen removal OWTS are installed in coastal areas throughout the United States to reduce excess nitrogen loading to marine waters. Rhode Island’s Narragansett Bay has experienced the negative effects of nitrogen (N) overloading, resulting in a loss of ecosystem services. This has prompted their installation within the Greater Narragansett Bay watershed for the past 30 years. However, final effluent N concentrations are not monitored after the system installation. To evaluate the N-removal effectiveness of these systems, we monitored the performance of the three most commonly installed OWTS within the greater Narragansett Bay watershed: (i) Orenco Advantex AX20^® (17 systems), (ii) Bio-Microbics MicroFAST^® (14 systems), and (iii) SeptiTech D^® Series (11 systems), collecting data monthly from March 2015 to August 2016. Temporal variations in N-removal efficiency were found, but were generally not driven by temperature: on average, only 13.4% of systems had statistically significant negative correlations between final effluent TN concentrations and sample temperature. Our results indicate that performance of N-removal OWTS varies according to month and system type with TN compliance rates ranging from less than 65% to 75% over the sampling period. FAST systems had the highest median TN concentration in mg N/L (17.1), followed by Advantex (14.9), and SeptiTech (11.4). Performance data were compared to results of a similar study in Massachusetts, where systems are monitored more frequently, which revealed that the median effluent TN concentration of Narragansett Bay systems (16.7) was greater than Massachusetts systems (13.3). Adjustments were made to underperforming systems between December 2015 and April 2016 intended to increase N-removal efficiency, but performance generally did not improve. The wastewater properties that best predicted TN levels were NH₄⁺, NO₃^-, pH, biological oxygen demand (BOD), and forward flow. Our results suggest that the differences in the monitoring of advanced N-removal OWTS may have an effect on their efficiency and that these systems are capable of reducing TN concentration to acceptable concentrations. Our data also indicate that  additional site visits and adjustments to underperforming systems in Rhode Island are necessary for N- removal efficiency to increase.