Woodchip-amended Rapid Infiltration Basins for enhanced removal of nitrate from secondary effluent

Abstract

In order to provide additional discharge capacity to the rapidly expanding Town of Middletown, DE, Rapid Infiltration Basins (RIBs) are being considered for the disposal and further nitrogen treatment of secondary lagoon effluent. A pilot-scale RIB system consisting of four test basins (25 by 60 feet) was constructed in the buffer areas of an existing spray irrigation field. The test basins contained 0, 10, 20, or 30% by volume of finely graded woodchips in the top 1-foot of soil, which acted as an external carbon source. Three woodchip-amended laboratory columns were also operated to simulate the RIBs and act as a model for predicting carbon depletion in the field. Between May 2014 and March 2015, lagoon effluent was loaded into the basins in volumes of 7,100-41,000 gallons per day for 1-9 days, with resting periods (no loading) of 6-13 days. Suction lysimeters were installed at 3 feet and 6 feet below the surface for the collection of samples following infiltration. After some initial flushing effects, the system was shown to remove significant amounts of nitrate, with the highest removal (≥50% under some conditions) observed in the basin amended with 30% woodchips. Improved performance was observed for increased sampling depth and increased woodchip volume. The short (1-2 day) loading plan was shown to be superior to the extended (9 day) loading. During the winter months the 6-foot samples continued to show substantial treatment, while some operational concerns were raised from excessive ice buildup. The laboratory column study predicted a woodchip lifespan of 2-3 years. Recommendations for the full-scale operation of the RIB system for enhanced denitrification were suggested based on the results of this study.