Side-By-Side Comparison of Denitrification Performance Between a Woodchip Bioreactor and a Wetland System.

A woodchip denitrifying bioreactor and a wetland have been monitored concurrently since fall 2011 in Central Minnesota.  The tile-drained areas flowing into the bioreactor (west site) and a 4.0-ha wetland (east site) cover 51 and 41 ha, respectively.  At the west site, water is pumped from a collection sump to either the bioreactor or a neighboring wetland.  At the east site, water is pumped to the 4.0-ha wetland, which is hydraulically connected to an adjacent lake.  The cropped area is in a corn (Zea mays L.)-corn-soybean [Glycine max (L.) Merr.] rotation; field management includes application of commercial fertilizer (west) and a sequence of turkey manure and commercial fertilizer (east) during corn years.  No fertilizer or manure is applied during soybean years. 

Water temperature, pH, redox potential, and dissolved oxygen are measured at 30-m intervals along the length of the 1.8 x 1.8 x 107 m bioreactor.  Inflow and outflow are measured and sampled automatically.  In 2012, precipitation was 523 mm, most of which was received by mid-June, well below the long-term average of 749 mm.  Due to the low precipitation and installation issues, only 16% of drain flow was treated through the bioreactor.  Load reductions for nitrate-N + nitrite-N, dissolved reactive phosphorus (DRP), and total suspended solids (TSS) were 100, 93, and 61%, respectively.

The east site wetland volume is 18,500 m³.  Bi-directional flow to and from the lake was observed.  Wetland inflow from tile drainage and the lake were 42,000 and 15,000 m³, respectively.  Outflow from the wetland to the lake was 64,000 m³.  Nitrate-N concentration comparisons at the east pump station and at the wetland outlet showed a decrease of 98%.  Total phosphorus and TSS loads were relatively high at the wetland outlet.  DRP concentrations increased over the observation period possibly due to internal loading.