Influent and Effluent Assessment to Evaluate Nutrient Removal in Onsite Sewage Treatment Systems

This study sampled systems in Florida Keys as part of a project to develop an assessment method for onsite systems. Two permitting classes were included: onsite wastewater nutrient reducing systems (OWNRS), and aerobic treatment units or interim systems (ATU). Data presented here relate to 24-hour time composite samples, which were collected several times from 14 aerated onsite sewage treatment systems, including some from pretreatment tanks of four systems to characterize influent strength. While cBOD5 and TSS concentrations were similar to results obtained in previous Florida studies, and total phosphorus appeared slightly higher, total nitrogen concentration were more than as high twice, with an average of 85 mg/L TN. Effluent concentrations in samples varied widely, from well below standards to samples with concentrations similar to influent. Based on median concentrations of the four OWNR systems for which influent and effluent was monitored, the typical removal rate for total nitrogen was approximately 67% and the one for total phosphorus was 54%.