WOOD-BASED FILTER FOR NITRATE REMOVAL IN SEPTIC SYSTEMS

The recognition that septic systems can generate groundwater plumes with nitrate concentrations exceeding thedrinking water limit has led to a need for improved nitrogen removal in septic systems. Long-term (3 to 5 year) monitoringresults are presented for four full-scale, on-site wastewater treatment systems using a novel porous media filter (Nitrex filter)for enhanced nitrogen removal. The filter removes nitrogen by denitrification of pretreated, nitrified, septic tank effluent usinga slowly soluble carbon source (wood byproduct material) incorporated into the filter. Results are presented for a house (sewageflow ~1 m3 d-1), a trailer park (7 m3 d-1), a communal residence (18 m3 d-1), and an inn (73 m3 d-1). In each case, theseptic tank effluent was pretreated using a sand filter, then flowed passively through the denitrification filter, and finally wasdispersed in a conventional tile bed. Influent (sand filter) NO3-N concentrations, averaging 14.2 to 37.7 mg L-1, weresignificantly attenuated in the denitrification filters at each site (p < 0.05) by amounts ranging between 87% and 98%.Reaction rates were temperature dependent, ranging from 7 to >10 mg N L-1 d-1, and showed no sign of deteriorating withsystem age at any of the sites. Results support previous mass balance calculations and pilot-scale field trials, suggesting thatsuch filters have the potential to operate for years without the need for media replenishment. These filters offer a practicalsolution to nitrate control in small to medium sized on-site wastewater treatment systems where simplicity of operation andlow maintenance are desirable.