Evaluation of Onsite Preanoxic Recirculating Gravel Filter Wastewater Treatment Systems for Nitrogen Removal

Approximately 25 percent of the United States population is served by conventional onsite wastewater treatment systems (OWTS), which consist of a septic tank followed by a soil adsorption field with subsurface effluent dispersion. OWTS have limited nitrogen removal ability, and in some locations their nitrogen loading contributes to eutrophication and impairs groundwater for potable uses. Consequently, there is a need for OWTS with improved nitrogen removal efficiency. The design of onsite nitrogen removal systems should be simple with minimal mechanical equipment and chemical addition. These systems should also require very little operating attention and provide reliable nitrogen removal performance under varying household load conditions. This study evaluated the long-term performance of two new designs intended to meet these requirements while also producing an average total nitrogen effluent concentration of less than 20 mg/L. The designs tested were modifications of recirculating gravel filters (RGF), which have been used for many years. RGFs are easy to operate and are proven to be highly effective for the removal of organic pollutants such as biochemical oxygen demand (BOD) and total suspended solids (TSS), as well as biological oxidation of ammonia to nitrate. The modified RGF systems included an anoxic zone below the normal aerobic zone in which flow from the upper zone contacted septic tank effluent flow to promote biological denitrification for nitrogen removal. The systems were considered passive nitrogen removal systems because they do not require an exogenous carbon source. The differences between the two system designs included one having a vegetated aerobic zone (Vegetated RGF) and the other having a layer of oyster shells at the top of aerobic zone, and a different anoxic zone inlet design and upflow flow pattern (Enhanced RGF) versus a horizontal anoxic zone flow pattern for the Vegetated RGF. Each system treated 480 gallon/day of domestic wastewater with a typical diurnal flow pattern. Their performance was evaluated for twelve months, which also included five stress tests that simulated extreme wash loads conditions, low loading periods, vacation interruptions, and power failure that might occur for a single home. This research evaluated and compared the performance of these systems in regards to nitrogen, BOD, TSS, total phosphorus (TP), and fecal coliform removal. Their responses to the stress tests and the effect of temperature were also assessed. The Vegetated RGF achieved high average treatment efficiencies for BOD (98%), TSS (99%), and fecal coliform (1.4 log reduction). The Enhanced RGF was equally effective at removing BOD (97%), TSS (99%), and fecal coliform (1.1 log reduction). Both systems achieved a total phosphorus removal efficiency of about 40 percent. The effluent annual average total nitrogen concentrations were 15.2 and 8.6 mg/L with 95 th percentiles of 18.5 and 12.3 mg/L for the Vegetated RGF and Enhanced RGF systems, respectively. Both the Vegetated and Enhanced RGF systems had good nitrification efficiency, but some operational clogging in the feed distribution piping did cause higher effluent ammonia concentrations. The larger aerobic volume and lower nitrogen loading to the Vegetated RGF system provided a higher nitrification efficiency compared to the Enhanced RGF. A much greater denitrification efficiency was observed for the Enhanced RGF system compared to the Vegetated RGF system due to its improved method of contacting the nitrified flow from the upper aerobic zone and the septic tank effluent.