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

Aerobic treatment units (ATUs) are used to enhance or replace traditional septic-tank/drainfield onsite sewage treatment and disposal systems (OSTDS). The objective of the study reported here was to assess the performance of ATUs under real-life conditions. Monroe County Health Department personnel in the Florida Keys sampled two-thirds of approximately 1,200 ATU systems on record during 2000 and 2001. Grab samples were analyzed for five-day carbonaceous biochemical oxygen demand (CBOD5), total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP). Results showed a non-normally distributed wide range of concentrations from less than 1 to thousands of milligrams per liter. In about half of the grab samples, TSS exceeded 30 mg/L, and in a quarter, CBOD5 exceeded 25 mg/L. Concentrations differed significantly (p < .05) by sampling point location, with CBOD5 and TSS more affected than TN and TP These results reflected significantly higher concentrations at sampling ports and additional treatment occurring in system components after treatment by the ATU. Differences by day of week and by month were also significant (p < .05), but time of day was not associated with significant differences. No significant improvement was found with newer ATUs over older ones. The high variability of ATUs and the low frequency of sampling pose a challenge to monitoring and enforcement programs.