IN-SITU DENITRIFICATION OF SEPTIC-SYSTEM NITRATE USING REACTIVE POROUS-MEDIA BARRIERS - FIELD TRIALS

A new alternative septic-system design is presented utilizing reactive porous media barriers for passive in situ attentuation of NO3-. The reactive material consists of solid organic carbon (sawdust) which promotes NO3 attentuation by heterotrophic denitrification. Four field trials are discussed demonstrating two barrier configurations: as a horizontal layer positioned in the vadose zone below a conventional septic-system infiltration bed and as a vertical wall intercepting a horizontally flowing downgradient plume. During one year of operation both barrier configurations have been successful in substantial attentuation (60 to 100%) of input NO3- levels of up to 125 mg/l as N. The horizontal layer configuration can be readily installed during the construction of new filtration beds, whereas the vertical wall configuration may be more appropriate for retrofitting existing septic systems where NO3- contamination has already occurred. The layer configuration allows the flexibility of constructing the barrier in the vadose zone by using coarse silt or fine sand matrix material that has the ability to remain tension-saturated, and thus anaerobic, even when positioned above the water table. Advantages of the barrier system are that it is simple to construct, no surface structures or additional plumbing are necessary, and treatment is passive requiring no energy consumption and little or no maintenance. Mass balance calculations and preliminary results suggest that conventiently sized barriers have the potential to last for decades without replenishment of the reactive material.