The term ‘Electron Acceptor Demand Threshold’ (EADT) may sound new, but it simply formalizes a concept used since the earliest days of bioremediation. The concept is that for a given site impacted with hydrocarbons, a certain amount of electron acceptors will need to be applied to fully break down the mass present. There are a couple of components to this idea that are broken down below.
- Accurate contaminant mass – To calculate the EADT, the mass of contaminants in the groundwater and soil must be estimated. For groundwater, it is simply the groundwater concentration multiplied by the groundwater volume. Soil contaminant mass is best calculated with recent soil data but can be inferred from groundwater using Csoil = Cgw * Foc * Koc.
- Electron Acceptor Demand – Using defined ratios of electron acceptors to hydrocarbon contaminants, the amount of electron acceptors needed to fully break down the contamination can be calculated.
Electron Acceptor Ratios (per 1 lb of hydrocarbon, based on octane):
· Oxygen (O₂): 3.5 lb O₂
· Nitrate (NO₃⁻): 4.5 lb NO₃⁻
· Sulfate (SO₄²⁻): 4.5 lb SO₄²⁻
After completing the steps above, the Electron Acceptor Demand Threshold can be calculated. For example, a 140-gallon gasoline (1,000 lb.) release will require the following:
- ~3,500 lb of oxygen (O₂)
- ~4,500 lb of nitrate (NO₃⁻)
- ~4,500 lb of sulfate (SO₄²⁻)
With the EADT calculated, to find the amount of product needed to be applied, the EADT is divided by the electron acceptor concentration in the bioremediation product. Examples are found below.
- 3,500 lb. O2 / 18% O2 (calcium peroxide) = 19,444 lb. calcium peroxide
- 4,500 lb. NO3- / 54% NO3- (ETEC’s CBN nutrient) = 8,333 lb. CBN nutrient
- 4,500 lb. SO42- / 13% SO42- (liquid sulfate product) = 34,615 lb. liquid sulfate product
The EADT is the most important concept to consider when attempting bioremediation of petroleum hydrocarbons. Appropriate masses of electron acceptors need to be applied to achieve success. There are always other factors to consider (see our article about the top 5 roadblocks in bioremediation), but failure to overcome the EADT is a major reason why bioremediation has failed to achieve cleanups to regulatory goals on petroleum release sites.