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 ofpretreated, 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{sup}3 d{sup}(-1)), a trailer park (7 m{sup}3 d{sup}(-1)), a communal residence (18 m{sup}3 d{sup}(-1)), and an inn (73 m{sup}3 d{sup}(-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{sub}3-N concentrations, averaging 14.2 to 37.7 mg L{sup}(-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{sup}(-1) d{sup}(-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.