Date of Award


Degree Type


Degree Name

Master of Science in Biological and Environmental Sciences (MSBES)


Interdepartmental Program

First Advisor

José Amador


Wastewater from onsite wastewater treatment systems (OWTS; also known as septic systems) can be a significant source of nitrogen (N) to coastal ecosystems. Because N limits primary production in coastal ecosystems, excessive inputs can cause eutrophication, which results in the loss of ecosystem services. To reduce N loading to marine waters, advanced N-removal OWTS are installed in N-sensitive areas. However, once installed, final effluent total nitrogen (TN) concentration from these systems is not always monitored, making it difficult to determine the extent to which they contribute to lowering N loads. To determine how well these systems reduce influent TN concentration, I monitored the performance of existing advanced N-removal OWTS located within N-sensitive areas of Rhode Island. Additionally, in an effort to provide information that could be used to improve the monitoring of these systems, I assessed the accuracy of rapid tests that can be used to evaluate system performance.

To evaluate the N-removal effectiveness of these systems, I measured a variety of wastewater properties from three of the most commonly-installed advanced N-removal OWTS within Rhode Island’s Greater Narragansett Bay Watershed: (i) Orenco Advantex AX20® (17 systems), (ii) Bio-Microbics MicroFAST® (14 systems), and (iii) SeptiTech D® Series (11 systems). Sampling was carried out monthly between March 2015 and August 2016. The compliance rate with state regulations (TN ≤ 19 mg N/L) was 70.6%, 64.3%, and 75.0% for Advantex, FAST, and SeptiTech systems, respectively. The median (range) final effluent TN concentration (mg N/L) for Advantex, FAST, and SeptiTech systems was 14.9 (0.6 - 61.6), 17.1 (0.6 - 104.9), 11.3 (0.1 - 41.6), respectively.

I investigated changes in effluent TN concentration at different time scales to determine how consistently the systems performed. Over the course of five, four-week sampling periods, SeptiTech systems had the highest median CV (56.0%), followed by Advantex (50.4%), and FAST (31.7%). In contrast, median coefficients of variation calculated at the month scale followed the order 62.8% (Advantex), 59.0% (SeptiTech), and 56.6% (FAST). Median final effluent TN concentrations for Advantex, FAST, and SeptiTech systems were lowest in fall and winter, which prompted examination of the relationship between temperature and TN concentration for each system type. Total N was generally not correlated with temperature: TN concentrations plotted against effluent temperature values resulted in R2 values of 0.007, 0.001, and 0.04 for Advantex, FAST, and SeptiTech systems, respectively.

Comparison of my findings to results of a similar study in Barnstable County, MA, where systems are monitored quarterly, and sampling and reporting of effluent TN is required, showed that the median final effluent TN concentration for Advantex, FAST, and SeptiTech systems were lower than ours, with values (mg N/L) of 13.5 for Advantex, 12.7 for FAST, and 20.2 for SeptiTech systems. Similarly, the Cape Cod, MA study data showed that 87% of Advantex, 79% of FAST, and 42% of SeptiTech systems had final effluent TN concentrations less than 19 mg N/L, which are higher percentages than reported in our study.

I identified the combination of wastewater properties that had the strongest correlation with TN to determine the properties that best predicted final effluent TN concentration. This information can be used to provide ranges of values of wastewater properties that can be expected to result in acceptable TN levels. Final effluent TN concentration was predicted by a different set of variables for each system type: ammonium, nitrate, and alkalinity for Advantex; ammonium, nitrate, average forward flow, and five-day biochemical oxygen demand (BOD5) for FAST; and, ammonium and effluent temperature for SeptiTech. Service providers were asked to make adjustments to seven underperforming systems to increase N-removal efficiency between December 2015 and March 2016. Total nitrogen was reduced to 19 mg N/L in only two out of seven systems, suggesting final effluent TN concentrations generally did not decrease in response to adjustments. My results suggest that advanced N-removal OWTS can reduce TN to meet regulatory standards, but N-removal effectiveness varies as a function of system type, time, and by individual system. Routine monitoring of advanced N-removal OWTS can enable service providers to proactively manage systems, which may affect their efficiency. However, improvement of performance after adjustment may require repeated visits and long-term monitoring.

In an effort to provide information that could translate into more effective maintenance visits/system adjustments, I evaluated the accuracy of a variety of rapid tests. Rapid tests provide an inexpensive, desirable alternative to standard laboratory analyses for testing advanced onsite wastewater treatment system (OWTS) effluent in the field. Despite their potential utility, their accuracy for analysis of effluent from advanced OWTS has not been assessed. I evaluated the accuracy of an initial suite of rapid tests commonly used to analyze wastewater (test strips for ammonium, pH, nitrate, and alkalinity; pH pocket meter; titration kit for dissolved oxygen (DO)) using final effluent from 42 study advanced N-removal systems. I compared values obtained using rapid tests to values obtained using standard laboratory methods. Significant differences between field and standard methods were found only for nitrate and pH test strips when the data were analyzed using ANOVA on ranks. However, regression analysis indicated that all test strip-based rapid methods and the DO titration kit produced values that deviated significantly from correspondence with standard analyses. When effluent samples were analyzed in the laboratory (to minimize sources of variability) using the same rapid tests, significant differences between rapid tests and standard analysis were not found, indicating that field conditions affected the accuracy of rapid tests. Evaluation of a suite of alternative rapid tests for ammonium, nitrate, pH, and alkalinity showed that test kits for ammonium and multi-analysis test strips for pH produced accurate results in the field. My results show that rapid tests may be used for field analysis of effluent, but their accuracy in the field needs to be considered before they are used to provide data to evaluate the function and treatment performance of advanced N-removal OWTS.

My findings show that advanced N-removal OWTS in Rhode Island can perform to standard, but their N-removal effectiveness may improve if routine monitoring and effluent TN analysis is required. Accurate rapid tests are available and can be used to quickly and cost-effectively evaluate advanced N-removal OWTS performance, which may result in more effective monitoring, and in turn increase N-removal efficiency.



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