Groundwater quality effects and operational considerations of an unlined constructed wetland treating raw sewage

Abstract

An unlined constructed wetland (CW) and control, each designed to treat 900 gallons per day (gpd) of raw sewage were built to evaluate operational requirements and groundwater quality effects near Tucson, Arizona. Bulrush (Scirpus olneyi) delayed clogging of coarse sand and thrived under greater hydraulic and organic loading rates than previously reported. Plants appeared to contribute to improved hydraulic conductivity: During 8 months of operation infiltration (v) averaged 0.28 ft day-1 (9.9 x 10-7 m sec-1) and 0.1 ft day-1 (3.5 x 10-7 m sec-1) in the wetland and control cells, respectively. While bacteria levels in leachate from both wetland and control underwent 5-to-6 log reduction, the regulatory limit ofO.O cfu/100 ml was not achieved. Analysis of other water quality parameters suggests that while turbidity and organic material are successfully removed in both cells, ammonia levels are problematical. After nearly one year of operation, equivalent protection of a clay liner as required by ADEQ (vi= 0.0028 ft day-1, or 10 -s m sec-1 ) was not achieved, suggesting that a sand based CW treating raw wastewater would require lining to prevent exceeding regulatory standards for nitrogen and total coliform bacteria.