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Environmental Engineer:
Applied Research and Practice
Modular Nitrogen Removal in Distributed Sanitation Water Treatment Systems
Author: Daniel P. Smith, Ph.D., P.E., DEE
Abstract
Removal of nitrogen from onsite sanitation water was demonstrated using three experimental two-stage biofiltration systems treating septic tank effluent (STE). Each system consisted of a vertical flow unsaturated trickle filter followed by a horizontal saturated denitrification filter with elemental sulfur as electron donor. Unsaturated filter media were clinoptilolite, expanded clay, and recycled granular rubber. The sole mechanical component of each filter was a single liquid pump that provided pressure dosing at a 30 min. interval and a hydraulic loading rate of 3 gallons per square foot per day (gal/ft2-day) (0.12 m/day). The systems were operated for over eight months on STE with an average total nitrogen (TN) concentration of 72.2 milligrams per liter (mg/L). The two-stage filters employing clinoptilolite, expanded clay and granular rubber media produced average final effluent TN of 2.2, 2.6 and 27 mg/L, respectively, with average TN removal efficiencies of 97, 96 and 65%. Dissolved organic nitrogen was the predominant form of nitrogen in treated effluent. Reduction of carbonaceous biochemical oxygen demand (CBOD) exceeded 94% for all three systems. The results demonstrated that the two-stage biofiltration design using a single pump was highly effective in reducing nitrogen in onsite sanitation water, and produced an effluent of comparable quality to centralized wastewater systems that employ external carbon dosing for tertiary denitrification.
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