THE FUTURE OF CONSTRUCTED TREATMENT WETLANDS FOR THE ON-SITE DISPOSAL OF DOMESTIC WASTEWATER

Constructed treatment wetlands for the on-site disposal of domestic wastewater fail to meet acceptable effluent guidelines in the United States and show significant differences in treatment efficiencies despite similar design. Twenty-one wetlands serving from 1-7 people were analyzed. These systems were divided into two groups based on location and available monitoring data. Effluent removal efficiency is consistent with previous investigations. Biological oxygen demand was reduced 80-90 percent, total suspended solids 64-78 percent, fecal coliform 87-97 percent, total phosphorous 49-87 percent and total nitrogen 66-70 percent. The National Pollutant Discharge Elimination System program proposes decreasing discharge standards for biological oxygen demand, total suspended solids and total nitrogen, as well as implementing zero “off lot discharge”. Discharge standards were exceeded in the wetland systems with biological oxygen demand below 30 mg/l in 72-97 percent of the samples and below the proposed 15 mg/l standard in 45-86 percent of the samples. Total suspended solids were met at the 30 mg/l standard in 77-85 percent of the samples and the 18 mg/l standard in 59-76 percent of the samples. Total nitrogen is poorly treated with 16-48 percent meeting the 10 mg/l standard and 4-37 percent meeting the proposed 4.5 mg/l standard. Fecal coliform and total phosphorous standards remain unchanged under proposed guidelines but are only met in 75-87 percent and 25-76 percent of the samples (for fecal and P respectively). Only two systems were found to consistently meet the more stringent criteria, and all systems have “off lot discharge”, which makes treatment wetlands a non-viable option for the on-site treatment of domestic wastewater.