Removing oil from wastewater depends on the type and concentration of oil present. Several methods exist, each suited to different situations.
Types of Oil and Removal Methods
Wastewater oil can be categorized into free oil (easily separated), emulsified oil (tiny droplets dispersed in water), and dissolved oil (molecules mixed within the water).
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Free Oil: This is the simplest to remove, often using methods like:
- Gravity separation: Oil, being less dense than water, floats to the surface and can be skimmed off. This is a primary treatment often used in conjunction with other methods. [Reference: Gravity separators accomplish removal of free oil as a primary treatment. (sciencedirect.com)]
- API oil-water separators: These utilize gravity and sometimes coalescing media to separate oil and water effectively. [Reference: Removal of free oil and grease from a wastewater stream reduces the potential for equipment problems to occur further downstream. (yokogawa.com)]
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Emulsified Oil: This requires more advanced techniques:
- Chemical Treatment: Lowering the pH (e.g., using sulfuric acid) breaks the emulsion, allowing the oil to separate. This is often followed by dissolved oxygen or nitrogen addition. [Reference: Removal requires chemical addition to lower the pH followed by addition of dissolved oxygen or nitrogen to remove the emulsified oils as they break free from the wastewater. (Provided Reference)] [Reference: One of the most common options is pH adjustment with oil skimming. This involves lowering the pH of the water using sulfuric acid. (northindustrial.net)]
- Coagulation and Flocculation: Chemicals are added to clump the oil droplets together, making them easier to remove through sedimentation or flotation. [Reference: Coagulation with alum and ferric chloride ... (fluencecorp.com)] [Reference: Oil can be removed by the addition of blended products that bind the oil for easy removal. Combined with the proper flocculant, the oil can be completely removed. (hubbardhall.com)]
- Organoclays: These special clays adsorb oil and grease, removing them from the water. [Reference: This article defines what organoclays are and details how they are being used to remove oil and grease from wastewater. (wwdmag.com)]
- Air Flotation: Air bubbles are introduced to attach to the oil droplets, causing them to float to the surface for skimming. [Reference: Sedimentation, flotation and chemical treatments: some of the processes, also exploited by MITA technologies, for the removal of oils and greases. (mitawatertechnologies.com)]
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Dissolved Oil: This is the most challenging to remove and often requires biological treatment:
- Biological Treatment: Microorganisms consume the dissolved oil, breaking it down. However, excessively acidic conditions from oil decomposition can inhibit this process. A balanced pH is crucial. [Reference: FOG decomposes into acids and too acidic an environ will kill the bugs. If you balance out with alkaline buffer but you have a lot of fat- boom-(reddit.com)] [Reference: Dissolved Oil is a true molecular solution within the water and can only be removed with biological treatment. (Provided Reference)]
- Advanced Oxidation Processes (AOPs): These methods use strong oxidants to break down dissolved oil molecules.
Practical Considerations
The best approach depends on factors including:
- Type and concentration of oil: Free oil is easier to remove than emulsified or dissolved oil.
- Wastewater volume and flow rate: This affects the choice of equipment and treatment methods.
- Regulations and environmental considerations: Local regulations dictate acceptable oil limits in discharged wastewater.
