One tool for keeping nutrients out of surface streams is the Denitrifying Woodchip Bioreactor. These units are particularly good for reducing nitrogen nutrients in drainage that comes from an agricultural subsurface drain-tile system, or a channeled agricultural ditch, although they have also been used for urban stormwater runoff, aquaculture waste treatment, and rural residential septic systems as well.
Woodchip bioreactors are basically a big pit filled with coarse woodchips. The chips provide a “home” for a mix of naturally occurring soil bacteria, fungi and algae that can consume various nitrogen compounds. These bacteria occur naturally in soils and by producing key enzymes, play an important role in cycling nitrogen between the soil and the atmosphere.
For the geeks among us, Woodchip Bioreactors work on the “denitrification” side of the Nitrogen-cycle. This means that the organisms that colonize these units can take nitrogen in the form of nitrates and return it to the atmosphere as di-nitrogen gas. This process looks like:
The steps are, in order, Nitrate -> Nitrite -> Nitric Oxide (gas) -> Nitrous Oxide (gas) -> di-Nitrogen gas. Each step of this process is facilitated by a bacterial enzyme, the common codes for which are shown below the process steps. Different species produce different combinations of the enzymes, so it usually takes several different types of bacteria, fungi and algae to get the process to go all the way to di-Nitrogen gas. This optimal result is called “complete denitrification.”
If the conditions within the woodchip bed are balanced correctly, then a useful mix of the denitrifying organisms will be both present and “happy.” However, if conditions are not balanced correctly, the process can stop at any step between Nitrate and di-Nitrogen gas. Unfortunately, if this happens, Nitric and Nitrous Oxide are both powerful green-house gases and are (very!) undesirable by-products! It is also possible that if the process is allowed to go on too long, changes to the internal water chemistry will make other undesirable products possible, such as Hydrogen Sulfide gas (H2S) and Methane (CH4). Finding the correct balance is important!
The conceptual sketch below illustrates some of what is likely to be happening in a bioreactor unit. As the flow comes in (left side), any Oxygen in the water will be used up very quickly. As the oxygen is depleted, organisms that can use Nitrate as their energy source will kick in, and as they do, Nitrite will be produced as an intermediate “by-product” -- and can, in turn, be used by organisms that can “process” nitrite.Concentrations of Nitrate and Nitrite can be measured directly with water quality tests.For the remainder of the process, soil-gas tests must be conducted, OR the internal conditions can be inferred from the presence/absence of Hydrogen Sulfide or Methane, both of which have a distinctive odor.
The challenge for installing a Woodchip Bioreactor system is sizing – if it’s too small or runs water through too quickly, the likelihood of producing Nitric and Nitrous Oxide gas is increased, and if it’s too big or runs too slowly, it can produce Hydrogen Sulfide and/or Methane.
It's a “Goldilocks” problem… we have to get the sizing “just right”!
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