Aerobic residential onsite sewage systems: an evaluation of treated-effluent quality.

* This retrospective cohort study used existing data to evaluate the quality of effluent from residential aerobic treatment sewage systems.

* The study looked at the systems of the types most commonly installed in Kitsap County, Washington:

--Multi-Flo[TM],

--Norweco[TM], and

--Whitewater[TM].

* The following parameters were used to determine performance:

--five-day biochemical oxygen demand (BODs),

--total suspended solids (TSS), and

--fecal coliform bacteria.

* Most (77 percent) of the systems were less than one year old at the time of sampling.

* Nevertheless, effluent from over a third of the systems failed to meet NSF certification standards for BO[D.sub.5] and TSS.

* For both parameters, the NSF standard is <30 milligrams per liter (<30 mg/L).

* Over two-thirds of systems failed to meet Washington State Board of Health Treatment Standard 2 criteria for BO[D.sub.5] and TSS.

* For both parameters, Treatment Standard 2 is <10 mg/L.

* An average of 59 percent of the systems failed to meet state standards for fecal coliform.

* The state standard is <800 fecal coliform bacteria per 100 milliliters.

* The authors recommend the following measures:

--more in-the-field evaluations of influent waste strength and how this factor affects the performance of the aerobic treatment systems,

--improvement of homeowners' knowledge about their systems,

--operation and maintenance agreements that would require periodic effluent sampling and testing (as well as allow maintenance and operating personnel to troubleshoot performance problems), and

--more frequent operation and maintenance inspections.