NITRITE MAY ACCUMULATE IN DENITRIFYING WALLS WHEN PHOSPHATE IS LIMITING

摘要

Permeable in situ denitrifying walls or barriers have been investigated as a means of removing nitrate from flowing groundwater. Denitrifying walls function by providing a microbial energy source in a highly permeable matrix. They usually are constructed by back-filling a trench or excavated area with a mixture of sand and fine gravel, which provide a porous matrix, and a substrate that serves as an energy source or electron donor for microbial denitrifiers. The present laboratory scale study demonstrates a need for adding phosphate when remediating some natural waters and shows that nitrite may accumulate in large amounts when phosphate is limiting. Denitrifying columns, 2.5 x 30 cm sand filled glass tubes infused with a vegetable oil substrate, were used as a scale model of a denitrifying wall. The water used was a natural groundwater that contained 16 to 18 mg/L nitrate-N and ～ 0.010 mg/L phosphate-P. When this low phosphate groundwater was pumped through the denitrifying columns little denitrification was observed and nitrite accumulated. When supplemental phosphate at 0.040 and 0.080 mg/L-P was used denitrification was still retarded and nitrite still accumulated. Increasing phosphate to 0.160 mg/L-P (N/P = 100) resulted in a rapid decrease in nitrate and only a brief accumulation of nitrite in the column effluents. The addition of solid rock phosphate or Biofos~(TM) to the denitrifying columns at the time of packing provided adequate phosphate for denitrification over an extended period of time. These results illustrate the importance of assuring that adequate phosphate is available in denitrifying walls.