In Situ Bioremediation

One of the most common types of in situ remediation is the bioremediation of organic COCs in aquifers by enhancing the metabolism of indigenous microorganisms present in the subsurface. Most in situ bioremediation is practiced in the aerobic mode. As introduced in Chapter 5, bioremediation is often practiced as the biostimulation of the metabolism of microbes. The biostimulation is carried out by the addition of oxygen or inorganic nutrients into the groundwater plume.

Addition of Oxygen to Enhance Biodegradation

Groundwater naturally contains low' levels of dissolved oxygen (DO). Even if it is fully saturated with air, the saturated dissolved oxygen (DOsat) concentration in groundwater would only be approximately 9 mg/L at 20°C. The biodegradation of organic COCs in the plume requires much more than that amount of DO.

The addition of oxygen to the groundwater can be done by air sparging (see Section 6.4.1) or pure oxygen sparging. The oxygen in the injected air can raise the DO to its saturation level of 8-10 mg/L. With pure oxygen injection, DO concentrations of up to 40 or 50 mg/L can be achieved.

Oxygen Addition from Air Sparging

Air sparging was discussed earlier as a mass transfer process, where air injected into the aquifer volatilizes COCs, transporting those COCs out of the w'ater and into the air stream that moves from the aquifer to the vadose zone. But air sparging also serves the function of adding oxygen into the aquifer to promote bioremediation. This process is called biosparging. Consequently, supplying oxygen to the plume to support aerobic biodegradation is one of the main functions of air sparging.

The oxygen transfer efficiency (OTE) is often used to evaluate the efficacy of aeration and is defined as:

The OTE should depend on factors such as injection pressure, the depth of the injection point in the aquifer, and characteristics of the geological formation, to name a few'. This is not a highly efficient process, around 10% efficient.

The rate of oxygen applied (Ra) can be calculated using the following steps.

Step 1. The oxygen concentration in the ambient air is approximately 21% by volume, which is equal to 210,000 ppmV. Convert this concentration to a mass concentration, G, using Eqs. 2.5 or 2.6.

Step 2. Calculate the rate of application, which is equal to a mass flow rate: Ra = GxQa,r.

Example 6.13: Determine the Rate of Oxygen Addition by Air Sparging

A biosparging well was installed into the plume of an aquifer impacted by hydrocarbons. The injection air flow rate into the well is 0.14 mVmin. Assuming the oxygen transfer efficiency (OTE) is 10%, determine the rate of oxygen addition to the aquifer through the sparging well. What would be the equivalent injection rate of water with a dissolved oxygen (DO) concentration of 9 mg/L?

SOLUTION:

(a) The oxygen concentration in the ambient air at 1 atm and T=20°C is approximately 21%, or 210,000 ppmV, which can be converted to a mass concentration using Eq. 2.5.

The molar volume (MV) of ambient air is determined using the Ideal Gas Law, Eq. 2.7:

Inserting this information back into Eq. 2.5:

The rate of oxygen injected in each well is

The rate of oxygen dissolved into the plume (Rd) through air injection in each well (using Eq. 6.42) = (56 kg/day)(10%) = 5.6 kg/day

(b) The DO concentration of the air-saturated reinjection water is approximately 9 mg/L at 20°C. The rate of oxygen transferred from water is also a mass flow rate:

If the required rate of oxygen supply by water is M = Rti = 5.6 kg/day, and the DO concentration in the water is C = 9 mg/L, then

Discussion:

  • 1. The oxygen transfer efficiency of 10% means that only 10% of the total oxygen sparged into the aquifer is dissolved into the aquifer. But, 90% of the oxygen injected can serve as the oxygen source for bioremediation in the vadose zone.
  • 2. Despite the relatively low oxygen transfer efficiency in this example, air sparging still adds a significant amount of oxygen to the aquifer. With regard to oxygen addition, an air injection rate of 0.14 m3/min with an oxygen transfer efficiency of 10% is equivalent to the injection of air- saturated water at 0.43 mVmin.

The DO level in the water can also be raised by the addition of chemicals, such as hydrogen peroxide (H202) and ozone (03). In this case, groundwater is extracted, mixed with H202 or O,, and reinjected into the aquifer. Each mole of hydrogen peroxide in water can dissociate into half a mole of oxygen and one mole of water, while one mole of ozone in water can dissociate into one and a half moles of oxygen as:

Ozone is ten times more soluble in water than pure oxygen and is a strong oxidant, along with hydrogen peroxide, as seen in Section 6.3.3. In addition to providing oxygen for biodegradation, ozone and hydrogen peroxide can also generate radicals to oxidize COCs and other inorganic and organic compounds present in the aquifer. At higher concentrations, though, they may become toxic to indigenous aerobic microorganisms and suppress their biological activities.

Other in situ biodegradation approaches rely on oxygen releasing compounds (ORCs). Common ORCs include calcium and magnesium peroxides that are introduced to the saturated zone in solid powder or a slurry through well. These peroxides release the oxygen to the aquifer when hydrated by groundwater passing through the wells. Magnesium peroxide has been more commonly used than calcium peroxide due to its lower solubility and prolonged release of oxygen. Oxygen amounting to ~10% of the mass of magnesium peroxide placed in the saturated zone is released to the aquifer over the active period (U.S. EPA 2017b).

Example 6.14: Determine the Effectiveness of Hydrogen Peroxide Addition as an Oxygen Source for Bioremediation

For bioremediation, a typical maximum concentration of hydrogen peroxide in the injected groundwater is 1,000 mg/L. Determine the mass concentration of oxygen that 1,000 mg/L of hydrogen peroxide can provide. Compare this concentration with the typical dissolved oxygen concentration found in groundwater.

SOLUTION:

(a) From Eq. 6.43, one mole of hydrogen peroxide can yield one-half mole of oxygen:

Molecular weight of hydrogen peroxide (H202) = (1 x2) + (16x2) =

34 g/mol

Molecular weight of oxygen (02)=16x2=32 g/mol

(b) Molar concentration of 1,000 mg/L hydrogen peroxide

Molar concentration of oxygen (assume 100% dissociation of hydrogen peroxide), according to Eq. 6.43:

Mass concentration of oxygen in water from hydrogen peroxide addition

The DOsal in groundwater is typically 9 mg/L. Therefore adding H202 contributes 470/9 = 52 times more oxygen for bioremediation. However, note that a typical DO is much lower than the DOsaI.

Addition of Nutrients to Enhance Biodegradation

In the subsurface, nutrients that enhance microbial activity usually already exist. However, with the presence of organic COCs, additional nutrients are often needed to support bioremediation. The nutrients to enhance microbial growth are assessed primarily on the nitrogen and phosphorus requirements of the microorganisms. The suggested C:N:P molar ratio is 120:10:1, as shown in Table 5.1 in Chapter 5. The nutrients are typically added at concentrations ranging from 0.005 to 0.02% by weight (U.S. EPA 1991). Much of the discussion for in situ groundwater bioremediation is similar to in situ vadose zone remediation, as described in Section 5.4. The difference is that now we are adding nutrients to stimulate the biodegradation of COCs in the soil matrix that is saturated with water.

For procedures to calculate the nutrient requirements for groundwater bioremediation, follow the steps below:

Step 1: Determine the moles of COC in a unit volume of aquifer.

Step 2: Determine the moles of C in the COC in the same unit volume of aquifer.

Step 3: Use the given C:N:P molar ratio to determine the moles of N and P needed.

Step 4: Given the moles of N and P, calculate the mass of N and P needed.

Step 5: Given the mass of N and P required, calculate the mass of the N and P compounds needed.

Example 6.15: Determine the Nutrient Requirement for In Situ Groundwater Bioremediation

A groundwater aquifer is impacted by gasoline. The average dissolved gasoline concentration of the groundwater samples is 20 mg/L. At this site, on average, 1 m3 of aquifer contains 225 g of gasoline. The porosity is 0.35. Assume gasoline has the same formula as heptane, C7H16. In situ bioremediation is being considered for aquifer restoration, using ammonium sulfate (NH4)2S04 and trisodium phosphate Na,P04-12H20 as the added N and P compounds.

Assuming no nutrients are available in the groundwater for bioremediation and that the optimal molar C:N:P ratio has been determined to be 100:10:1, determine the mass of nutrients needed to support the biodegradation of gasoline, on a basis of 1 m3 of aquifer. If the plume is to be flushed with 100 pore volumes of oxygen- and nutrient-enriched water, what would be the required nutrient concentration in the water for reinjection?

SOLUTION:

Basis: 1 m3 of aquifer

(a) Determine the moles of gasoline in 1 m3 of aquifer:

MW of gasoline (C7HI6) = 7x12 + 1 x 16 = 100 g/mol

(b) Determine the number of moles of C:

Since there are 7 carbon atoms in each gasoline molecule, as indicated by its formula C7HI6, then:

(c) Determine the mass of N and mass of (NH4)2S04 needed (using the C:N:P ratio of 100:10:1):

According to the formula (NH4)2S04, 2 moles of N are present for each mole of (NH4)2S04. So the number of moles of (NH4)2S04 needed are = 1.58 4- 2 = 0.79 mol/m3

(d) Determine the mass of P and mass of Na3P04-12H20 needed (using the C:N:P ratio of 100:10:1):

According to the formula Na3P04-12H20, 1 mole of P is present for each mole of Na3P04-12H,0. So the number of moles of Na3P04-12H20 needed is = 0.158 mol/m3

  • (e) The total nutrient requirement is 104 g of(NH4)2S04 and 60 g of Na3P04-12H,0 per m3 of aquifer.
  • (f) Determine the volume of water in 100 pore volumes

(g) Determine the nutrient concentration for 100 pore volumes:

The minimum required concentration for (NH4)2S04 = 104 g 4- 35,000 L = 0.00297 g/L = 2.97 mg/L и 0.0003% by weight The minimum required concentration for Na3P04-12H20 = 60 g 4 35,000 L = 0.00171 g/L = 1.71 mg/L * 0.0002% by weight

Discussion: The concentration, totaling about 0.0005% by weight is the theoretical amount. In real applications, one may want to add more to compensate for the loss due to adsorption to the aquifer material before reaching the plume. This makes the nutrient concentration fall in the typical range of 0.005-0.02% by weight.

In Situ Chemical Oxidation

While advanced oxidation process (AOP) is an ex situ groundwater treatment technique, chemical oxidation can also be practiced in situ (in situ chemical oxidation, ISCO). In groundwater ISCO, the chemical oxidation process is the same as the vadose zone process described in Section 5.5. An oxidizing agent, typically hydrogen peroxide, ozone, or a combination of these two, is injected into the groundwater, as shown in Figure 6.14. The resulting radicals destroy the organic COCs in the impacted groundwater.

 
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