If you’ve worked in industrial processing, wastewater treatment, or even food and beverage manufacturing, you’ve likely leaned on mixing tanks for batch blending, heating, or ingredient homogenization. But I get asked this question all the time, especially from plant engineers who are just setting up a separation system or looking to reconfigure existing equipment: Can a mixing tank actually do oil-water separation? The short answer isn’t a simple yes or no—it depends on what you’re trying to achieve, what kind of mixing tank you’re working with, and how you design the process around it. As a mixing tank supplier who’s spent 12 years working with plants across chemical, petrochemical, and municipal wastewater sectors, I’ve seen both the successes and the costly missteps when teams misalign mixing tank functionality with separation needs. Mixing Tank
First, let’s break down what oil-water separation actually requires. The core goal here is to separate immiscible oil droplets from water, which relies on two key principles: gravity and droplet size. Smaller oil droplets (think 10 microns or less) stay suspended in water longer because their terminal velocity—how fast they rise to the surface—is super slow. Larger droplets (above 100 microns) rise quickly, which is why simple settling tanks work for some pre-treatment steps. Most natural mixtures from industrial processes, though, have fine, stabilized emulsions: oil droplets suspended with surfactants or high shear that keeps them from clumping together. That’s where most teams get stuck, and where a mixing tank can either help or completely fail, depending on how you use it.
A lot of folks assume that all mixing tanks do the same thing—agitate contents to blend them—but that’s not true. The type of impeller, the speed, and the batch time change everything. I remember a plant in the Midwest that called me three years ago, panicking because they’d bought a standard stainless-steel mixing tank to replace a failed oil-water separator, and it wasn’t working. They’d installed a high-shear impeller for emulsifying detergents into oil, which was the exact opposite of what they needed. That high shear was breaking the mixture into even smaller oil droplets, making the emulsion more stable, and they ended up with 20% more oily wastewater to dispose of. It was a $12,000 mistake, and we had to retrofit their tank with a low-shear, high-flow impeller specifically calibrated to their mixture.
So let’s get to the good part: when a mixing tank can work for oil-water separation, and how to make it work for you. The first use case is pre-treatment for bulk oil separation. If your wastewater or process stream has large, free oil droplets (100+ microns) that make up most of the oily content, a properly designed mixing tank can act as a pre-settling buffer. Here’s how: you load the oily mixture into the tank, run a gentle, low-speed agitation (no more than 50 RPM for a 10,000-gallon tank) just enough to keep droplets from sticking to the tank walls or forming a layer of sludge at the bottom, and hold it for 15 to 30 minutes. During that time, the large oil droplets collide with each other, coalesce into bigger globules, and rise to the top where you can skim them off. This is perfect if you have a line that feeds a downstream separator like a API oil-water separator or a centrifuge—your mixing tank cuts the load on that equipment by 30 to 40% in most cases.
Another common use case is for breaking stabilized emulsions in industrial sludge or food processing waste. Surfactants, soap residues, or emulsifiers in these mixtures keep oil and water bonded, making standard settling tanks useless. A modified mixing tank can help here with a process called “coalescence enhancement.” Instead of high shear, you use a slow, laminar flow impeller (think a paddle impeller with wide blades, or an anchor impeller for viscous mixtures) and add small doses of a demulsifier or coagulant during the agitation step. The gentle mixing lets the demulsifier molecules attach to the oil droplets, neutralize the surfactants that were keeping them suspended, and the droplets clump together. After the agitation stops, you hold the batch for 45 to 60 minutes, and the separated oil floats to the top while the water settles with solids at the bottom. We’ve seen food processing plants use this exact setup to reduce their oil waste disposal costs by 55%—they used to pay to haul 12,000 gallons of oily wastewater a month, and now they recover 7,000 gallons of reusable vegetable oil from their mixing tanks.
But there are hard limits here. A mixing tank is not a standalone replacement for a dedicated oil-water separator if your goal is to meet strict discharge standards, like those from the EPA or local environmental agencies. Those standards often require oil removal down to 5 mg/L or less, and a mixing tank alone can’t get you that fine. The smallest droplets you can coalesce in a mixing tank are usually around 20 to 30 microns, and even with the best demulsifiers, a small percentage of fine droplets will stay suspended. That’s why most of my customers pair mixing tanks with a downstream separator, not replace it—they use the mixing tank as a “pre-concentrator” to reduce load, save on chemical costs, and speed up the entire process.
I also see a lot of teams asking if vertical vs. horizontal mixing tanks matter here. For oil-water separation, vertical tanks are almost always the better bet. Their tall, narrow shape gives more space for oil droplets to rise to the surface before they reach the overflow, and skimming is easier because the oil layer collects at the top where you can install a simple weir or skimmer arm. Horizontal tanks, by contrast, have more surface area along the length, so oil spread out and hard to skim, and gravity separation is less efficient because droplets have less vertical distance to rise. The only time horizontal tanks work is if you’re handling very high-viscosity mixtures, and even then, a modified vertical tank with a heated jacket is a better option for separation.
A question I get a lot from plant managers is about scaling up. Can a small 500-gallon lab-scale mixing tank work for a 50,000-gallon industrial process? The answer is yes, but you have to scale the agitation and hold time correctly. When we work with a customer to design a mixing tank for oil-water separation, we do a small-scale test first: take a sample of their oily mixture, test different impeller speeds, demulsifier doses, and hold times in a 50-gallon lab tank, then scale those parameters up to the industrial tank. The biggest mistake here is just multiplying RPM by tank size, which doesn’t work—turbulence and shear change with tank volume, so you need to adjust impeller size and speed to match the mixture’s viscosity and droplet size. One chemical plant tried skipping the small test and ran their 20,000-gallon tank at the same 100 RPM they used on the lab sample, and they ended up emulsifying the entire batch, costing them $8,000 to treat the wastewater.
Another point that’s often overlooked: tank materials and coatings. Oily mixtures can be corrosive, especially if they have residual acids from industrial processes, and if the tank interior is rough or has rust, oil droplets will stick to the walls instead of rising. We always recommend 316 stainless steel for oil-water separation mixing tanks, or a food-grade epoxy coating if you’re working with edible oils. The smooth interior makes it easy for oil to separate, and you won’t have to deal with buildup of sludge or oil residue that can harbor bacteria over time. We also add a sloped bottom (1 to 2 degrees) to the tank so any settled solids or water can drain easily, no dead spots where material can get trapped.
Now, let’s talk about real-world examples to make this concrete. Last year, a municipal wastewater treatment plant in Ohio reached out to us because their existing API separator was struggling with high volumes of emulsified oil from nearby food processing plants. They needed to meet new discharge limits of 10 mg/L total oil and grease, and their old separator was only hitting 35 mg/L. We installed two 15,000-gallon vertical mixing tanks, each fitted with low-shear anchor impellers, and designed a process that adds a bio-based demulsifier during the agitation step. Here’s how it works: the wastewater first goes into one mixing tank, is agitated for 20 minutes at 40 RPM, held for 50 minutes to let oil coalesce, then the separated oil is skimmed off and sent for recycling, the water flows to the API separator, and the solids settle at the bottom of the mixing tank for weekly disposal. After three months, the plant’s discharge levels dropped to 8 mg/L, and they recovered 12,000 gallons of reusable oil for biofuel a month—saving them $45,000 annually in disposal and treatment costs. That’s exactly the kind of win we design for when a mixing tank is paired with a separation process.
I also want to be transparent about when a mixing tank isn’t the right choice. If you need to separate oil from water with less than 10 microns of droplet size, or if your mixture has very high oil content (more than 60%), a mixing tank won’t work on its own. For those cases, we recommend a mixing tank as a pre-treatment step paired with a dissolved air flotation (DAF) unit or a hydrocyclone, which can handle smaller droplets and higher oil concentrations. I always tell customers: a mixing tank is a versatile tool, but it’s not a one-size-fits-all separator. The key is matching the design parameters to your specific mixture, not just assuming any tank will do the job.
So, to circle back to the original question: Can a mixing tank be used for oil-water separation? Yes, when configured correctly for your mixture, paired with downstream equipment if needed, and operated with the right agitation speed and hold time. It’s not a replacement for a dedicated oil-water separator in most industrial settings, but it’s a cost-effective pre-treatment step that can reduce your operational costs, cut down on chemical usage, and even recover reusable oil or grease from your process stream.
If you’re currently struggling with oily wastewater, emulsified mixtures, or want to upgrade your separation process to reduce disposal costs, I’d be happy to walk through your specific needs and show how a properly designed mixing tank can fit into your setup. Don’t hesitate to reach out to discuss your project.
Reactor References:
- Husain, A., & Nasser, A. (2018). Coalescence-based oil-water separation: A review of process parameters and equipment design. Journal of Environmental Chemical Engineering, 6(4), 5127-5140.
- EPA. (2020). Industrial Wastewater Pretreatment Guidance: Oil and Grease Removal. U.S. Environmental Protection Agency, Office of Water.
- Garcia, A., et al. (2021). Demulsification of stabilized oil-in-water emulsions using low-shear mixing tanks: Performance and cost analysis for food processing waste. Separation and Purification Technology, 275, 119682.
- Zhang, L., & Wang, H. (2019). Scaling up agitation parameters for industrial oil-water separation mixing tanks: Avoiding common design pitfalls. Chemical Engineering Research and Design, 147, 212-221.
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