Performance evaluation of innovative and alternative on-site wastewater treatment systems in Craven County, NC

Innovative and alternative on-site wastewater treatment systems were installed at 18 sites. Monitoring over a two-year period was performed weekly for system operation and maintenance (O&M) performance and monthly for water quality. Systems were assessed to determine applicability and modification requirements for soils with moderate and severe limitations and to recommend long-term monitoring and maintenance requirements. The sand filters were effective at reducing most pollutants, particularly suspended solids and biological oxygen demand Enhancement to a recirculating sand filter allowed for greater reduction in all pollutants except total phosphorus, as compared to the intermittent, single-pass sand filter. The aerobic treatment unit (ATU) system worked well, but a filter is needed to minimize solid dispersal to the effluent distribution system. Constructed wetlands were effective in reducing most pollutants. A deeper 2-foot wetland cell had greater retention time and pollutant reduction as compared to a 1-foot depth cell for all variables except suspended solids. Unlike many wetland systems, phosphorus removal from septic tank effluent was substantial in wetlands which utilized a substrate with a high calcium-content (85% reduction to an average effluent concentration of 1 mg/L). An economic competitive alternative to chlorine tablet disinfection was not found. Bacterial control was poor with the ozone disinfection units due to inadequate mixing, Ultraviolet disinfection exhibited effective bacterial control, but was not low cost. The modified low pressure pipe (LPP) system in fill exhibited high rates of nitrification, but there was some potential of nitrate moving beyond the treatment field. Drip and spray irrigation treatments were effective ground absorption treatment technologies. Landscape and vegetation maintenance were the most critical O&M features during installation. Higher pressure levels of 4 to 8 feet in the LPP systems and corrugated sleeves around the LPP laterals were found to be necessary to reduce root penetration and system clogging. Water level control and cell depth of constructed wetlands were the most critical wetland design features to ensure healthy plants and water quality benefits. Unusual wastewater characteristics and increased complexity of the system increased the O&M.