Choosing an oil water separator isn’t just an equipment purchase; it’s a critical business decision that directly impacts your budget, long-term liability, and regulatory compliance. As a facility manager, you face the challenge of selecting a system that not only works but also aligns perfectly with your site’s specific requirements and legal obligations. The uncertainty can be significant, but the right choice ensures efficiency and protects your company from risk.
This article provides a clear, head-to-head comparison of above grade and below grade oil water separators. We will break down the key differences in application, maintenance, installation, and total cost of ownership. By understanding these factors, you can make a confident and cost-effective decision that guarantees compliance with regulations like the Clean Water Act, preventing costly fines and environmental hazards.
Key Takeaways
- Application is Key: Above grade separators are ideal for processing wastewater from specific industrial activities (pump feed), while below grade units are typically used for stormwater runoff and gravity flow applications.
- Maintenance & Accessibility: Above grade systems offer easy, direct access for inspection and maintenance, reducing long-term service costs and preventing hidden issues from escalating.
- Liability & Risk: Below grade separators carry a higher risk of undetected leaks, which can lead to expensive soil and groundwater contamination cleanup costs and significant environmental liability.
- Compliance: The right separator choice is essential for meeting EPA Effluent Guidelines and the specific discharge limits set by your facility’s National Pollutant Discharge Elimination System (NPDES) permit.
The Regulatory Stakes: Why This Choice Matters
Managing industrial wastewater isn’t just good practice; it’s a legal requirement. Federal laws mandate that facilities control their pollutant discharges to protect the nation’s waterways. This regulatory framework is the primary reason why selecting the right oil water separator is so crucial for your operations.
Under the Clean Water Act, the EPA issues Effluent Guidelines which are national standards for industrial wastewater discharges to surface waters and publicly owned treatment works. These are not mere suggestions—they are enforceable regulations that set specific limits on the concentration of pollutants, like oil and grease, that your facility can release.
An oil water separator is your frontline tool for achieving and maintaining compliance with these standards. The choice between an above grade or below grade system will significantly influence your ability to meet these legal obligations consistently and cost-effectively, safeguarding your business from steep fines and operational disruptions.
Defining the Contenders: Above Grade vs. Below Grade
Before comparing them, it’s important to understand the fundamental design and purpose of each system.
What is an Above Grade Oil Water Separator?
An above grade oil water separator is a system installed on the surface, typically on a concrete pad. Its defining feature is visibility and accessibility. Every component, from the inlet and outlet to the coalescing media and oil collection chamber, is in plain sight and within easy reach.
This design makes above grade units ideal for processing wastewater from specific industrial activities where water is pumped to the separator. They excel at removing non-emulsified hydrocarbons from sources like vehicle wash bays, maintenance shops, and manufacturing processes. The unparalleled access simplifies routine inspections and maintenance, making it easy to identify and address potential issues before they become serious problems. For facilities needing to ensure compliance with a reliable, easy-to-service unit, an oil water separator provides an effective solution for separating oils and other contaminants, protecting plumbing systems, meeting environmental regulations, and reducing the risk of costly downtime. By selecting a unit designed for durability and efficiency, operators gain peace of mind knowing their wastewater treatment process is consistent, safe, and optimized for daily operations.
What is a Below Grade Oil Water Separator?
A below grade, or underground, oil water separator is installed entirely beneath the surface. These systems are often chosen for managing large, variable flows like stormwater runoff from parking lots or entire industrial sites. They are also used in applications that rely on gravity to move water or in areas where surface space is limited or needed for high traffic, such as loading docks or driveways.
Because they are buried, below grade separators must be constructed with robust materials like concrete or thick-gauge steel to withstand the immense pressure of soil and vehicle loads. While this design preserves valuable surface real estate, it inherently creates significant challenges for inspection, maintenance, and leak detection, which we’ll explore next.
Head-to-Head Comparison: Key Differences for Your Facility
To make an informed decision, you need to see how these systems stack up across the factors that matter most to your facility’s operations, budget, and risk management strategy.
| Feature | Above Grade Oil Water Separator | Below Grade Oil Water Separator |
| Application | Best for controlled process wastewater streams (e.g., wash bays, equipment cleaning). | Best for large, variable flows like stormwater runoff or general site drainage. |
| Flow Type | Designed for pump feed applications where water is actively sent to the unit. | Designed for gravity flow systems where water drains naturally into the unit. |
| Installation | Simpler and less costly. Requires a level concrete pad but no excavation. | Complex and expensive. Requires significant excavation, shoring, and backfilling. |
| Site Footprint | Occupies visible surface space. | Preserves surface space, allowing for drive-over conditions. |
| Maintenance | Easy and direct. All components are accessible for quick inspections and manual cleanouts. | Difficult and costly. Access is limited to manholes, often requiring confined-space entry and vacuum trucks for service. |
| Leak Detection | Immediate. Any leak is instantly visible, preventing environmental contamination. | Delayed or impossible. Leaks can go undetected for years, leading to severe soil and groundwater contamination. |
| Long-Term Liability | Very low. The risk of hidden environmental damage is virtually eliminated. | Very high. The potential for undetected leaks creates a significant and ongoing financial and environmental liability. |
Application & Flow Type
The source and movement of your water are the first considerations. Above grade separators are engineered for controlled process wastewater. If you have a specific area, like a pressure washing station, that generates oily water, you would pump that stream directly to an above grade unit for treatment.
Below grade separators, conversely, are designed for gravity-fed systems. They are placed at a low point on a site to collect stormwater runoff or drainage from a large area. Trying to use one type for the other’s application leads to inefficiency and poor performance.
Installation & Site Footprint
Installation is a major point of difference. An above grade system requires a simple concrete slab, making the process faster, more predictable, and significantly less expensive. The trade-off is that it occupies a permanent, visible footprint on your property.
A below grade system requires a major construction project. The process involves excavation, potentially shoring the pit to prevent collapse, setting the heavy unit, and carefully backfilling. This complexity is compounded by potential obstacles like poor soil conditions or existing underground utilities, driving up costs and project timelines. The primary benefit is that once installed, the surface area remains usable.
Maintenance & Accessibility
This is where the long-term operational costs diverge dramatically. With an above grade separator, your maintenance team can visually inspect the entire unit in minutes. Cleaning is straightforward and can often be done manually without specialized equipment. Problems are spotted and fixed immediately.
Maintaining a below grade separator is a far more involved process. Inspections are limited to what you can see by looking down a manhole. A thorough assessment or cleanout requires confined-space entry permits and procedures, along with expensive vacuum trucks to pump out the contents. This difficulty means maintenance is often deferred, increasing the risk of system failure and clogs.
Making the Right Choice: A Decision-Making Checklist
Use these questions to evaluate your site’s needs and determine the best path forward:
- What is my primary water source? Is it controlled process wastewater from a specific activity, or is it variable stormwater runoff from a large area?
- How will water get to the separator? Will you be pumping the water (pump feed), or will it flow naturally downhill (gravity flow)?
- How much space do I have? Is a visible surface footprint acceptable, or is preserving ground-level space for traffic or other uses a top priority?
- What is my budget priority? Am I focused on the lowest initial installation cost, or am I prioritizing lower long-term maintenance costs and reduced risk?
- How much risk am I willing to accept? Is the potential for hidden leaks and multi-million-dollar environmental cleanup an acceptable risk for my company?
Ensuring Compliance and Peace of Mind
Ultimately, your choice must circle back to regulatory compliance. The EPA guidelines are enforced at the local level through a permitting system. As an industrial facility, your discharge is governed by a specific permit that sets enforceable limits on pollutants.
The NPDES (National Pollutant Discharge Elimination System) permitting program establishes these discharge limits and conditions for industrial sources. Your facility’s NPDES permit contains the exact numbers you must hit. A performance-proven, properly installed, and regularly maintained oil water separator is the key to consistently meeting those numbers.
Choosing the right system isn’t just about treating water—it’s about protecting your business. It’s about building a reliable and defensible compliance strategy that prevents fines, avoids shutdowns, and preserves your company’s reputation as a responsible environmental steward.
Conclusion
The decision between an above grade and a below grade oil water separator is not one-size-fits-all. It depends entirely on your site’s application, operational priorities, and tolerance for risk.
Below grade systems offer a hidden footprint but come with the burden of high installation costs, difficult and expensive maintenance, and the ever-present risk of a catastrophic, undetected leak. For many industrial process applications, the potential liability is simply too great.
Above grade systems provide unmatched accessibility, which translates into simple maintenance, lower long-term costs, and the virtual elimination of environmental liability from hidden leaks. By making issues visible and easy to correct, they offer a more reliable and lower-risk solution for managing process wastewater. Investing in the right system upfront is the most effective way to ensure long-term operational efficiency, regulatory compliance, and peace of mind.
