FireWater (New in 2023)
Biological denitrification is a biochemical reaction where specialized bacteria convert nitrate into nitrogen gas. The gas escapes our reactor tanks via vents in the top. Assuming you have the correct bacteria, you only need to create the right conditions inside the “bioreactor” for denitrification to occur. The most important factor is the ratio of the nutrients in the water (called the Redfield ratio). This is the ratio of carbon:nitrate:phosphorus in the drainage water. Most drainage or tailwater already has nitrate and phosphorus. For the denitrification process to proceed normally, we often need to add biologically available carbon to achieve the correct carbon:nitrogen ratio.
Key Criteria 1
Cost per Kilogram of Nitrate Removed
The US EPA maintains an excellent document on external sources of carbon for wastewater treatment (the link opens in a new window). A quick glance at the document and you can see that two simple alcohols, methanol and ethanol, have the lowest dose requirement. But the actual cost of removing nitrate is the unit cost of carbon multiplied by the dose. In other words, a less effective solution such as glycerin might be a lower cost solution is the price per gallon of glycerin is extremely low. Assuming that we have prices for every carbon source, it should be pretty easy to figure out which one is the most effective.
Key Criteria 2
Transportation and Logistics
Carbon logistics can be challenging. Glycerin is quite heavy at 10 lbs per gallon and is shipped as a liquid without any major Hazmat concerns. Sodium acetate can be shipped as a solid (granular) which lowers transport costs. Methanol is extremely hazardous to use and transport. Large wastewater treatment plants have the infrastructure to do this but farmers and growers do not. Acetic acid is already being used on the farm by some growers as an herbicide but you must wear specialized equipment during handling (extreme irritant). Ethanol, like methanol, is a fire and explosion risk at high concentrations. But ethanol is also the most effective carbon source due to its low dosage requirement.
Key Criteria 3
Tunneling Through the Cost Barrier
At Tailwater, we applied Amory Lovins’ concept of tunneling through the cost barrier to develop an external carbon solution. Two of these carbon sources, ethanol and acetate, can be synthesized using bacteria. Of the two, ethanol at 15% can be produced via fermentation of sugar using off-the-shelf yeast. While fermentation of vinegar (acetate) can produce only very low levels (6%) or so. At 15%, ethanol does not require (in the US) special hazmat handling (similar to wine). This suggests a two step process. Ferment the ethanol to 15% at a central location, ship the 15% to your site, then concentrate on-demand to 50% or higher.
We can produce the required sugars from existing agricultural waste streams that are high in dissolved carbon. Thanks to the multi-year effort to develop 2nd generation ethanol for fuel, we can take advantage of these commercial enzymes to potentially lower the cost of ethanol dramatically over time and recycle this organic waste.