Investigating temperature dependence in denitrifying woodchip beds used in onsite septic treatment systems

Abstract

A 2012 study that tested three pilot onsite denitrifying treatment systems found a Vegetated Denitrifying Woodchip Bed achieved the highest total nitrogen removal. However, denitrification in this system was also much more temperature dependent than the other systems tested. This study examined the nitrogen removal performance of two current Denitrifying Woodchip Bed systems that were recently installed in the Hood Canal watershed. Data from the Hood Canal systems, and the 2012 pilot systems, was analyzed to establish the casual basis for denitrification temperature dependence. The results of this analysis suggest three things. First, the Hood Canal systems had incomplete nitrification, but still had 50% greater input nitrate concentration than the woodchip bed reactor tested in 2012. Second, in all three wood chip reactors tested the available carbon from the woodchips decreased with temperature, reducing the electron donor availability. Third, carbon released from the woodchips was a lower quality electron donor and therefore more sensitive to temperature fluctuations. A benchtop experiment was carried out to test denitrification rates using synthetic wastewater media and woodchip-based media as electron donors in typical summer and winter temperatures for western Washington State. This experiment showed nitrate removal in the high temperature synthetic wastewater treatment was extremely rapid with 98% removal after only 2 days. Conversely, nitrate removal in the high temperature woodchip-based was 89% after 12 days. In the cold synthetic wastewater treatment nitrate removal averaged 79% after 12 days, whereas in the cold woodchip media nitrate removal averaged 39% after 12 days. This study indicates that woodchip based carbon is a much lower quality electron donor, and the combination of low temperature, low quality organic substrates and low woodchip substrate concentrations may greatly slow nitrate removal. To alleviate these temperature constraints on cold weather nitrate removal in woodchip-based reactors I recommend supplementing the reactors with additional carbon. This can be done effectively by dosing the systems with methanol or another simple form carbon substrate. The amount of methanol required to provide additional carbon sufficient enough for complete denitrification was calculated to cost $5.25 a month.