WOOD-BASED FILTER FOR NITRATE REMOVAL IN SEPTIC SYSTEMS

摘要

The recognition that septic systems can generate groundwater plumes with nitrate concentrations exceeding the drinking water limit has led to a need for improved nitrogen removal in septic systems. Long-term (3 to 5 year) monitoring results 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 using a slowly soluble carbon source (wood byproduct material) incorporated into the filter. Results are presented for a house (sewage flow ~1 m 3 d -1 ), a trailer park (7 m 3 d -1 ), a communal residence (18 m 3 d -1 ), and an inn (73 m 3 d -1 ). In each case, the septic tank effluent was pretreated using a sand filter, then flowed passively through the denitrification filter, and finally was dispersed in a conventional tile bed. Influent (sand filter) NO ₃ -N concentrations, averaging 14.2 to 37.7 mg L -1 , were significantly 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 with system age at any of the sites. Results support previous mass balance calculations and pilot-scale field trials, suggesting that such filters have the potential to operate for years without the need for media replenishment. These filters offer a practical solution to nitrate control in small to medium sized on-site wastewater treatment systems where simplicity of operation and low maintenance are desirable