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

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* 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...

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