Increase DO in a Bioreactor with Hydrogen Peroxide & GPS-X Simulation
Updated: Nov 21, 2020
Hydrogen peroxide can be used to increase the dissolved oxygen (DO) concentration in an aerobic bioreactor, often referred to as “DO supplementation.” Using peroxide is rarely a first choice because chemically increasing DO is more expensive than using a mechanical method (diffused aeration, mechanical mixing, etc.). But if you are in a situation where, for example, you have aeration blowers out-of-service due to equipment failure and the DO has dropped below 2.0 mg/L, impacting nitrification and/or BOD/COD removal, hydrogen peroxide can be used to restore bioreactor oxygen levels, BOD removal, and nitrification.
As part of my job I am frequently involved with implementing hydrogen peroxide applications to provide industrial customers with a DO supplementation program. Of course, cost is always a concern but the key technical questions from those unfamiliar with the use of hydrogen peroxide revolve around how much oxygen is actually transferred to the mixed liquor suspended solids (MLSS), or the biomass more specifically, from a gallon or liter of hydrogen peroxide and how many gallons or liters will it take to increase the DO concentration in the bioreactor to >2.0 mg/L.
The oxygen contribution from a given concentration of hydrogen peroxide is straightforward and I will explain that first. The volume of hydrogen peroxide required to increase the DO concentration from a current value to a higher value can only be estimated and the best way I’ve found to do that is with an outstanding modeling and simulation program called GPS-X from Hydromantis (www.hydromantis.com) for which I provide an example later in this post.
Decomposition of Hydrogen Peroxide
Hydrogen peroxide decomposes to water and oxygen as shown in the following chemical equation.
In an activated sludge system, bacteria, in the presence of hydrogen peroxide, secrete catalase enzyme, which rapidly converts the hydrogen peroxide to water and oxygen, as depicted below.
Oxygen in Hydrogen Peroxide
So how do we determine how much oxygen is released when hydrogen peroxide decomposes? The table below shows the first step we have to take to determine the oxygen available from hydrogen peroxide as it decomposes. We need to convert the atomic weight of each element and compound to its molar mass equivalent, easily done with a periodic table. From the table we determine that oxygen comprises 47.04% of the molar mass.
For this example we are using a 50% hydrogen peroxide solution. 50% hydrogen peroxide has a density of 10.01 lb/gal with an oxygen content of 23.52%, as shown in the table below. Therefore, for every gallon of 50% hydrogen peroxide we add to a bioreactor, 2.4 pounds of oxygen have been added to the MLSS in the bioreactor.
Concentration of Hydrogen Peroxide
Hydrogen peroxide can be purchased in a wide range of concentrations. The more common concentrations are tabulated below.
For industrial applications, the higher the concentration of hydrogen peroxide, the more economical it becomes because less water is being transported. The downside is that as the concentration increases, handling and storage requirements become a greater concern because of the strong oxidizing potential of hydrogen peroxide.
The table below summarizes the oxygen contribution from hydrogen peroxide for a range of concentrations.
GPS-X: Hydrogen Peroxide Required to Increase the DO in a Bioreactor
Those needing to supplement (increase) the dissolved oxygen (DO) in their bioreactor want to know how much hydrogen peroxide will be required. It would be nice if there was a way to provide a simple, direct answer, but there isn't. From the analysis we did above, we can calculate how many pounds or kilograms of oxygen we are adding to a bioreactor per volume and concentration of hydrogen peroxide. But can't really use that information to calculate how much the dissolved oxygen concentration will increase in the bioreactor. Many factors come into play including the organic loading, wastewater temperature, pH, MLSS concentration, etc. So the best that can be done is to estimate the oxygen contribution using a sophisticated modeling program, and the one I use is GPS-X from a company called Hydromantis.
Here is an example of an industrial facility that has a very high chemical oxygen demand (COD) combined with out-of-service aeration blowers. For this treatment system I've entered basic information such as the influent flow rate, COD concentration, total suspended solids (TSS) concentration, wastewater pH, air and water temperature, aeration blower capacity, bioreactor volume, etc. The more plant-specific operating data you can enter into the model, the more accurate will be your simulation results. A simple process flow diagram of this wastewater plant, created using GPS-X, is shown below.
To be noted from the diagram are the two sources of flow to the wastewater system: 1) the relatively constant influent flow and 2) a periodic batch treatment of high-strength or off-spec wastewater that creates a significant and variable additional oxygen demand. In the first simulation run I set the off-spec wastewater flow and the hydrogen peroxide flow to zero to establish baseline operating conditions. GPS-X produces the following detailed output for the bioreactor from this simulation.
Note the DO concentration is 1.2 mg/L, low in itself and certainly insufficient to sustain nitrification. Also note the F:M ratio is greater than 1.0, too high for a complete mix bioreactor, where the F:M ratio should be in the range of 0.2 to 0.6 kg BOD/kg MLVSS (mixed liquor volatile suspended solids). One final note is the output showing flow rates in and out of the bioreactor as US customary units and all other parameter as SI units. I did this on purpose to show how the program automatically calculates all parameters in both systems, making it easy for me being in the United States, and easy for the rest of the world who uses the international system of units.
As a modeling and simulation program, GPS-X allows you to run a simulation, adjusting the inputs you are interested in while monitoring outputs of concern. In this model our goal is very specific: We want to determine the hydrogen peroxide dosing required to increase the DO from 1.2 mg/L to >2.0 mg/L and we also want to know how the hydrogen peroxide dose will change (increase) when the off-spec wastewater is introduced.
The graph below shows the change in the DO concentration in response to changes in the off-spec wastewater flow rate and the hydrogen peroxide dosing. As the simulation runs you have complete control over all of the systems inputs, allowing you to adjust them as you need while observing how the system responds. This simulation tells us (and the customer!) we will need to feed approximately 800 L/d of hydrogen peroxide when the off-spec wastewater flow is zero. The hydrogen peroxide feed rate will need to be increased to approximately 1,200 L/d during the time off-spec wastewater flow is entering the bioreactor. This allows us to answer a very important question for the customer and provides the data needed to develop a cost estimate for the DO supplementation program. Once the DO supplementation program is running, data generated from the actual operation can then be used to further calibrate the GPS-X model.
I think it is easier to track changes when analyzing one variable at a time when you need to explain the data in the form of, for example, a PowerPoint presentation, in contrast to the actual running of the simulation. The data generated in GPS-X can also be exported to Excel and that output is shown below. The first Excel graph below shows the impact to dissolved oxygen in the bioreactor in response to changes in the off-spec wastewater flow rate when there is no addition of hydrogen peroxide.
The starting DO was 1.2 with no off-spec wastewater and no hydrogen peroxide. With the simulation running, as the off-spec wastewater flow is introduced, the DO drops to below 1.0 mg/L. The oxidation of organics (COD/BOD) is going to slow and nitrification will come to a stop. You can see from the graph the direct impact the high-strength off-spec wastewater has on the DO concentration in the bioreactor.
The second Excel graph below shows the impact to the dissolved oxygen concentration in the bioreactor in response to changes in hydrogen peroxide dosing when the off-spec wastewater flow is off. You can see how directly the bioreactor DO concentration responds to changes in the addition of oxygen from the hydrogen peroxide.
I wanted to show specifically how the oxygen content in a given concentration of hydrogen peroxide is calculated, something easily done. Computer modeling of wastewater treatment systems is far more complicated, requiring numerous inputs to improve the accuracy of the model. GPS-X greatly facilitates the modeling effort, providing guidance on properly characterizing the wastewater influent. You then add the unit processes that define the wastewater system you are modeling and enter the data to size each unit process. What I've shown here is a quick overview that barely touches upon the amazing capability of the GPS-X program. To learn more I encourage you to go to the Hydromantis website.
If you want a better view of the graphics in this post you can download a PDF file here.