Performance of aerobic treatment units: monitoring results from the Florida Keys.

Author:Roeder, Eberhard


The Florida Keys are an ecologically sensitive environment and a destination for growing numbers of permanent and transient visitors. To address the increasing negative ecological effects resulting from increased population, statutory onsite wastewater treatment requirements have increased over the last 15 years. Consequently, the use of aerobic treatment units (ATUs) has become widespread. From 1985 through April 2001, approximately 1,200 such systems were installed in the Florida Keys. In Florida, a biennial operating permit and a maintenance agreement between a maintenance entity and the system owner are required for ATUs. The county health department permits and inspects onsite systems, and until 2001 sampled ATUs yearly. The objective of the present study was to assess the performance of installed ATUs and to draw from these experiences to develop programs for monitoring and enforcing the performance of more advanced systems that will be required to address nutrient concerns.

Performance expectations for ATUs are based on the National Sanitation Foundation's (NSF's) Standard 40 for residential wastewater treatment systems. Florida requires testing to this standard for approval of aerobic treatment units (Standards for Onsite Sewage Treatment and Disposal Systems, 2004). Several studies have indicated that effluent from ATUs frequently exceeds these expected limits (Hutzler, Waldorf, & Fancy, 1978; Kellam, Boardman, Hagedorn, & Reneau, 1993: Maxfield, Daniell, Treser, & VanDerslice, 2003; Otis, Boyle, & Sauer, 1974; Sexstone et al., 2000).

In 2000, the second author analyzed the results of 584 grab samples that had been taken in 1999 from 580 ATUs by the Monroe County Health Department (MCHD). These samples were analyzed for five-day carbonaceous biochemical oxygen demand (CBO[D.sub.5]), total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP). For CBO[D.sub.5], one-sixth of the samples, and for TSS, close to half the samples, gave values that exceeded limits set by Florida secondary treatment standards (20 mg/L each). The effectiveness of ATUs in removing nitrogen was expectedly low, with nitrogen exceeding 10 mg/L in 72 percent of samples. The analysis suggested that the two most important factors influencing effluent sampling results were the method of discharge and the location of the sampling port. The type and manufacturer of ATU did not appear to influence the sampling results.

In the study reported here, the authors review results from the 2000 and 2001 sampling campaigns. Answers to the following questions were sought: How meaningful are grab samples? Are ATUs consistently meeting expectations for NSF-certified, secondary wastewater treatment units? Does it matter where and when a sample is taken? How variable are concentrations with respect to time?


MCHD designed the study to sample each system older than six months once during each year in accordance with statutory requirements. The systems were distributed between three field offices of the county health department. To maximize the effectiveness of staff time, the department used a convenience sampling scheme, sampling systems in close vicinity to each other on the same day The objective in sampling-point selection was to obtain a sample "as close to the end of the treatment stream as possible" (Florida Department of Health, 2000). For systems discharging into injection wells, a special feature of the Florida Keys, the required treatment includes a sand filter and chlorinator after the ATU. Sampling ports (mostly "Tee" or "Cross" connectors tied into the effluent line) have been required in systems installed after January 1995, but were frequently found dry. Therefore, sampling points included the treatment unit, sampling ports, pump chambers, and injection wells. Sampling ended April 2001, when legislative changes reduced permit fees and thus funding for sampling.

MCHD staff visited each system and took a grab sample from the sampling point. In some cases, access problems to the property and to the unit precluded sampling. Sampling-point location, time, and date were noted in the chain-of-custody information for each sample. The samples were then cooled on ice and sent overnight by express freight to analytical laboratories for analysis within 24 hours. The samples were analyzed for CBO[D.sub.5] (Standard Method [SM] 5210B), TSS (U.S. EPA Method 160.2), TN (U.S. EPA Method 300.0 and U.S. EPA Method 351.2), and TP (U.S. EPA Method 365.2). MCHD staff entered the results reported by laboratories onto the chain-of-custody document or into an Excel spreadsheet. The sampling time was not usually entered into the spreadsheet. In the fall of 2004, Florida Department of Health (DOH) staff completed data entry from paper records and aggregated all data into one spreadsheet.

For the purposes of this analysis the authors assumed that all ATUs...

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