Initial Validation of a Replicated Field-scale Denitrifying Bioreactor Facility in a Boreal Environment

Abstract

Denitrifying bioreactor technology, where a solid carbon source (woodchips) acts as a reactive medium to intercept agricultural tile drainage water, has been successfully used to convert N (NO3-) to di-nitrogen (N2) gas. Four replicated field-scale (24 m long x 3 m wide x 1 m deep), bioreactors were built and operated at the St. John’s Research and Development Centre and were successful at removing a notable amount of nitrate (N) from agricultural subsurface drainage water. The objective of this study was to investigate the internal flow dynamics of one of these field-scale bioreactors as a proxy for the others. The hydraulic conditions in the bioreactor system developed differently than expected; asymmetric flow rates led to long average hydraulic retention time (HRT) and a highly dispersed residence time distribution, which was revealed by a sodium chloride tracer test. To measure the internal flow a known amount of sodium chloride (salt) was added to water before it entered the bioreactor and samples were collected in 30 minutes intervals. The temperature of water samples taken from the inlet, outlet, and sample ports ranged from 14.5 to 18.4°C With a N removal of 62 to 66% the bioreactor proved at the same time to be very effective under the boreal environment of Newfoundland and Labrador (NL). Mass removal rate (MRR) was calculated to evaluate the performance of woodchip bioreactor. The average MRR was 3.87 gm-3day-1 and the highest was 7.19 gm-3day-1 respectively. The theoretical retention time was calculated to be approximately 10.64 h based on the active flow volume, the length and depth of the system. In comparison the observed retention was 18.18 h