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WOOD-BASED FILTER FOR NITRATE REMOVAL IN SEPTIC SYSTEMS

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 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, 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) NO3-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.