The Slow Negative Effect of Drought on Denitrifying Bacteria Bioreactors’ Rate of Removal

Abstract

High nitrate levels in water can cause health issues and eutrophication. While most focus on treating nitrates in stormwater runoff, base-flow from groundwater can also contain nitrates. Bioreactors using denitrifying bacteria could remove nitrates perpetually rather than only after rainfall, and one bioreactor treating base-flow could be a substitute for dozens of bioreactors treating stormwater (Easton et al.). However, drought can decrease bioreactor efficiency as the bacteria die off. This study was conducted to determine how drought may impair bioreactor efficiency. In this experiment, two bioreactors were made in November with nitrate-laced distilled water, rocks, and woodchips. By creating the same conditions in both tanks, the rate of removal in each could be compared to check accuracy. Nitrate concentration readings were taken using nitrate probes and HACH TNT 835 tests, the latter being most accurate. The November average rate of removal was -0.02754 mg/L per hour. The tanks were then left alone without any additional nitrates, lowering the bacteria population to simulate drought. In December, the rate was tested again, and it had barely decreased to -0.02745 mg/L per hour. Then, in July, the bioreactors were drained and refilled with nitrate-laced water, causing the now smaller population to begin denitrification again. The new average rate, found using ANCOVA analysis, dropped to -0.00617 mg/L per hour. As such, drought can decrease bioreactor efficiency drastically, but the decrease in rate of removal occurs slowly enough for bioreactors to function during short droughts, increasing the potential applications of base-flow bioreactors in drier regions.