Chemical Oxidation of Organics Using Fenton’s Reagent
Fenton’s Reagent is also called Fenton’s Reaction or Fenton Chemistry. It is used to treat industrial wastewater streams that contain difficult or toxic organic compounds that can interfere with other common treatment systems such as biological treatment systems.
Equalization Tanks: Equalization tanks are required for most wastewater treatment systems to provide a consistent flow rate and consistent characteristics. Equalization tanks are used to collect wastewater over a period of time that is sufficient to “equalize” the flow so that the wastewater system receives a wastewater stream that is as consistent as possible.
As a rule of thumb. If the total flow occurs in less than 10 hours a day or if the characteristics of the wastewater varies from day to day an equalization tank is required. The actual size will of course be based on a case by case basis.
Fenton’s Reagent: is a solution of Hydrogen Peroxide with dissolved ferrous iron as a catalyst. It is used to oxidize organic contaminates found in industrial wastewaters. Fenton’s reagent is most commonly used to destroy organic compounds that are resistant to other wastewater treatment techniques such as biological treatment or carbon adsorption. Typical application is the destruction of organic solvents that are resistant to biological oxidation such as phenols, formaldehyde, methylene chloride and chlorinated solvents. It was developed in the 1890’s by Henry Fenton.
Hydrogen peroxide: is a chemical compound with the formula H2O2. In its pure form, it is a colorless liquid, similar to water, (H2O) only slightly more viscous. Hydrogen peroxide is the simplest peroxide (peroxides are compounds with an oxygen–oxygen single bond) the oxygen-oxygen bond is highly unstable making it a strong oxidizer, bleaching agent and disinfectant.
It is commonly available at 50 % strength. It is also available in higher strengths, but at strengths over 50 percent it can cause serious burns on skin, fire on contact and is a potential explosion hazard.
Industrial wastewater: Industrial wastewater may be composed of various chemicals, toxins, heavy metals, pharmaceuticals, petroleum based oils and greases. By weight industrial wastewater varies considerably and may have as much as 5 % solids.
Note: All industrial discharges to a public sewer system are subject to general and specific prohibitions identified in the Code of Federal Regulations identified in 40 (CFR) 403.6 which prohibits the discharge of any pollutant that may impair worker or public health and safety, or that might upset or pass through the wastewater treatment plant untreated. Always check with your local sewage treatment authority for permit requirements.
Ferrous (Fe2+): Ferrous iron is a divalent iron compound (+2 oxidation state), as opposed to ferric, which is a trivalent iron compound (+3 oxidation state). In the Fenton Reagent, Ferrous Sulfate FeSO4 in granular form is most commonly used. It is inexpensive and readily available in 50 pound bags.
pH: pH is a measure of how acidic or basic a solution is, The pH scale runs from 0 to 14. From 0 to 7 is acidic and from 7 to 14 is basic also called alkaline. A pH of 7 is neutral.
ORP-Oxidation Reduction Potential: Is a common measurement for the oxidizing potential of a wastewater stream. It is a millivolt (MV) reading. A positive reading indicates the wastewater has a positive oxidizing strength. The higher the mv reading the more the oxidizing power. A reading of +500 mv is required in the Fenton’s process. A negative reading indicates the wastewater has a reducing potential.
General Steps to Treat Wastewater Using Fenton’s Reagent
The procedure requires the following steps:
- pH adjustment: Wastewater is first drawn from an Equalization tank to the reactor tank where the pH is adjusted to 3.0 to 4.0. This is done by the addition of Sulfuric Acid (H2SO4). The lower pH will cause the iron to dissolve into the wastewater. At neutral or higher pH levels (>7 pH units) the iron granules of Ferrous Sulfate will not dissolve. Lowering the pH also brings the oxidation strength of the Fenton Reagent into the optimum range. Dosage is typically less than 5 gallons for 1,000 gallons of wastewater. Note: Sulfuric Acid can be purchased at 93% but it can dangerous at that concentration. A lower concentration is recommended. Commercially available concentrations vary. May be purchased in 55 gallon drums or in 290 gallon totes.
- Iron Catalyst: Adding the iron catalyst, Ferrous Sulfate, as dry granules or as a premixed solution in water and mixing for 2 to 4 hours. The amount required is determined by jar testing. Typical iron requirement is 8 to 10 pounds per 1,000 gallons of wastewater. Ferrous Sul;fate is purchased in 50 pound bags.
- Hydrogen Peroxide H2O2 addition: The H2O2 is added slowly. The pH is monitored continuously to maintain a pH of 3 to 5 during the oxidation step. H2O2 is added until an ORP reading of +500 is maintained. The temperature is also monitored to prevent overheating. 10 gallons per 1,000 gallons of wastewater is typical usage for wastewater. H2O2 is the cost driver for operational costs. Current cost is around $3.00/gallon. Purchased in 55 gallon drums or 290 gallon totes.
- Lime Flocculation and pH adjustment: The pH is now adjusted back to neutral (7 pH) using lime. If metals are present and metals removal is being conducted the pH will be adjusted in this step to optimal pH for the removal of the metal. The presence of iron in the reaction mixture makes it particularly suited to lime flocculation. 8 pounds is typical for 1,000 gallons. Purchased in 50 pound bags.Note: Metals removal is a separate subject but can be accomplished at this time in the process. Further pH adjustment is accomplished by using lime.
- Polymer Addition: A small amount of polymer is added to create a floc (flakes of precipitate material visible to the eye). Typical dosage of Polymer is 2 oz. per 1,000 gallons. Purchased in 5 gallon containers.
- Settling: The reactor tank is allowed to settle for 4 hours or more, no mixing.
- Decant: The clear water is now removed by decanting from the reactor vessel.
- Solids Removal: The settled solids are removed and sent to a filter press to remove excess water. The filter press will produce what is called filter cake. The solid residues, like a cake, can usually be sent to a landfill for disposal.Note: Jar testing should be conducted to determine dosage rates. Estimated dosage rates are based on wastewater produced at an aircraft paint striping and painting operation where the wastewater contains Phenols, and Methylene Chloride.
Fenton’s Reagent is typically applied in a batch process. However, it is being used in both continuous and batch processes. A typical batch operation would consist of an equalization tank, chemical storage and chemical metering pumps for H2O2, FeSO4, H2SO4 acid, and lime, a primary reactor, with low speed mixer, sludge holding tank, solids dewatering device (Filter Press or De-watering box), control panel with monitoring and controls for temperature, pH and ORP. The materials of construction for the reactor are typically Fiberglass or stainless steel, while those for the chemical storage systems are High Density Poly Ethylene (HDPE). Packaged Fenton’s systems are available but are costly. Wilson Environmental can provide you with engineering guidance on custom designs and retrofits.
A batch system can be fully automated to control the entire process. Only the amount of Iron addition must be calculated from jar testing or past experience with the same wastewater. The predetermined amount of Iron is then mixed in a slurry to be added in its entirety, and automatically, to the process.
Understanding the Science
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