Nitrogen Loading from Onsite Wastewater Treatment Systems in the Greater Narragansett Bay (Rhode Island, USA) Watershed: Magnitude and Reduction Strategies
Onsite wastewater treatment systems (OWTS) are an important part of the water infrastructure in the USA. Advanced OWTS are used instead of conventional OWTS to lower nitrogen (N) inputs to coastal ecosystems and groundwater sources used for drinking. Knowledge of the N load from OWTS helps identify drivers of excess N and develop strategies to lower N inputs. We used wastewater flow and effluent total N (TN) concentration to determine the mass N load from 42 advanced N-removal OWTS technologies (Orenco Advantex AX-20®, BioMicrobics MicroFAST®, SeptiTech D® series) and 5 conventional OWTS within the Rhode Island, USA, part of the Greater Narragansett Bay watershed. The median N load (g N/system/day) followed the order: conventional systems (31.1) > AX-20 (10.8) > FAST (10.1) > SeptiTech (9.6), and was positively correlated with flow. Results of a Monte Carlo simulation estimated the N load from the current distribution of conventional and advanced systems (105,833 systems total; Current scenario) to the watershed at 1,217,539 kg N/year. Compared to the Worse Case scenario (100% conventional OWTS), advanced OWTS currently prevent 53,898 kg N/year from entering the watershed. The per capita N load (kg N/capita/year) from OWTS under the current scenario is 4.68, and 1.47 for a local wastewater treatment plant (WTP) with biological N removal (BNR). Replacing 5150 conventional OWTS yearly with the most effective OWTS technology would result in a per capita N load from OWTS equivalent to that for a WTP with BNR after ~15 years, with a yearly cost of $174.24 per additional kilogram of N removed. Increasing the proportion of advanced OWTS that achieve the final effluent standard of 19 mg TN/L—through monitoring and recursive adjustment—would reduce the time and cost necessary to achieve parity with the WTP. Advanced N-removal OWTS are an important part of the water infrastructure that can lower N load to the Narragansett Bay watershed.
KeywordsOnsite wastewater treatment systems Nitrogen Wastewater treatment plants Biological nitrogen removal
This project was funded by the USDA National Institute of Food and Agriculture, Hatch Multi-State NE 1545 Project [accession number 1007770] and under agreement CE96184201 awarded by the US Environmental Protection Agency (USEPA) to the New England Interstate Water Pollution Control Commission (NEIWPCC), in partnership with the Narragansett Bay Estuary Program (NBEP), to J. A. Amador and G. W. Loomis. Although the information in this doument has been funded wholly or in part by the USEPA, it has not undergone the Agency’s publications review process and therefore, may not necessarily reflect the views of the Agency and no official endorsement should be inferred. The viewpoints expressed here do not necessarily represent those of the NBEP, NEIWPCC, or EPA nor does mention of trade names commercial products, or causes constitute endorsement or recommendation for use.
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