Nitrate removal rates in woodchip media of varying age

A variety of low-cost carbonaceous solids have been successfully tested in bioreactors designed for nitrate treatment. In many agricultural and wastewater settings, however, such reactors may be practical only if they are maintenance free for a number of years after installation. Although field installations have demonstrated consistent treatment over multi-year timeframes, the ability to accurately quantify slowly declining reaction rates in field settings is problematic because of variations in reactor flow rates, ambient temperatures and influent chemistry. In this study, laboratory column tests were undertaken on four samples of coarse wood particle media (woodchips), two that were fresh and two that had been in continuous operation in subsurface denitrifying bioreactors for periods of 2 and 7 years respectively. Four experimental runs were undertaken at increasing influent NO3-N concentrations of from 3.1 to 48.8 mg N L−1. Nitrate mass removal rates remained relatively constant and did not systematically increase in successive runs at higher NO3 concentrations indicating that NO3 was not the rate-limiting substrate at these concentrations. Thus, zero-order reaction kinetics were used to model the attenuation reaction (presumably denitrification). The 7-year-old media had a mean NO3-N removal rate of 9.1 mg N L−1 d−1 (6.4 g N m−3 media d−1), which remained within 75% of the rate for the 2-year-old media (12.1 mg N L−1 d−1 or 8.5 g N m−3 media d−11) and within 40–59% of the rate for the fresh chips (15.4–23.0 mg N L−1 d−1 or 10.8–16.1 g N m−3 media d−1). Results support field experience indicating that woodchips loose about 50% of their reactivity during their first year of operation as soluble organic compounds are leached out, but then relatively stable rates persist for a considerable number of years thereafter.