* The Florida Keys are an ecologically sensitive environment.
* They are also a destination for growing numbers of permanent and transient visitors.
* To address the increasing negative ecological effects, 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.
* Florida requires a biennial operating permit for ATUs.
* It also requires a maintenance agreement between a maintenance entity and the system owner.
* The county health department permits and inspects onsite systems, and until 2001 sampled ATUs yearly.
* The study reported here assessed the performance of ATUs under real-life conditions.
* A county health department sampled two-thirds of the ATU systems on record during 2000 and 2001.
* Grab 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).
* 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?
* Samples taken from tee ports and cross ports had higher concentrations of CBO[D.sub.5], TSS, and TN than did samples taken from the treatment unit itself.
* This result suggests that samples taken from such ports are not representative of treatment unit effectiveness.
* Alternative sampling-port designs should be evaluated.
* The effect of additional treatment (tank, sand filter, chlorination) between the treatment unit and pump chambers and injection wells was significant for all parameters.
* Such effects should be considered in defining the performance of systems.
* Between 8 a.m. and 2 p.m., time of day had no significant effect on sampling results.
* Concentrations for all parameters and all samples were higher on Wednesdays.
* Concentrations appeared also to vary significantly by month.
* Future studies of individual systems should clarify the relationship between effluent...