Above, Flush, or Below Grade: Selecting the Ideal Oil Water Separator System
✨ Key Points
- Oil water separators protect waterways and help facilities stay compliant by removing oils and fuels from wastewater.
- Choosing above, flush, or below grade systems affects cost, maintenance, and long-term performance.
- The right OWS setup depends on your site layout, regulations, and how easy access needs to be.
Industrial facilities are the backbone of the modern economy, but their operations can place significant stress on the environment, particularly our water resources.
For sites that handle hydrocarbons, preventing contaminated runoff from entering municipal sewer systems or natural waterways is a top priority.
This is where oil water separator (OWS) systems become an essential technology.
These systems are engineered to remove oils, fuels, and other hydrocarbons from wastewater, ensuring that your facility meets strict environmental compliance standards and avoids costly fines.
However, the effectiveness of a separator depends entirely on choosing the right installation type for your specific site.
The decision between an above grade, flush with grade, or below grade system dictates everything from installation cost to maintenance routines and long-term performance.
This article provides a clear framework for comparing these three core designs to help you make an informed, defensible decision for your facility.
The Three Core Designs of Oil Water Separator Systems
Oil water separators are primarily categorized by their installation profile relative to the ground or floor surface.
This “grade” dictates how the system is installed, how it receives contaminated water, and what applications it is best suited for.
Understanding these fundamental differences is the first step in selecting a system that aligns with your operational needs.
The three main categories are:
- Above Grade: Installed on a solid surface and fed by a pump.
- Flush With Grade: Installed level with the surrounding surface and fed by gravity.
- Below Grade: Buried deep underground for high-traffic areas, also fed by gravity.
Each design is engineered to solve specific site challenges. Choosing the right system starts with understanding these designs.
Examining performance-proven oil water separator systems categorized this way can provide a clear picture of how these designs translate to real-world equipment.
At a Glance: Comparing Above, Flush, and Below Grade Separators
For a quick overview, this table compares the three system types across the key criteria that facility and compliance managers need to consider.
| Feature | Above Grade | Flush With Grade | Below Grade |
|---|---|---|---|
| Installation | Sits on a concrete pad | Installed level with the surface | Buried deep underground |
| Required Feed | Pump Feed | Gravity Flow | Gravity Flow |
| Accessibility | Easiest | Moderate | Most difficult |
| Footprint | Requires dedicated floor space | Minimal surface footprint | Zero surface footprint |
| Best For | Indoor facilities, pump-fed systems | Wash bays, maintenance shops | Parking lots, high-traffic areas |
Above Grade Oil Water Separators
Above grade oil water separators are the most straightforward design, characterized by their accessibility and reliance on a mechanical feed system.
How They Work & Key Features
These units are installed on a solid, level surface, such as a concrete pad inside a building.
Their defining operational feature is that they exclusively handle a pump feed, meaning contaminated water is actively pumped from a collection sump or source into the separator inlet.
Constructed from durable materials, above grade systems are built to withstand demanding industrial environments.
Their greatest advantage is direct, unimpeded access to all components.
Coalescing media, oil sumps, and sludge baffles are all easily reachable, simplifying inspection, cleaning, and service procedures significantly.
Pros, Cons, and Ideal Applications
Evaluating an above grade system requires weighing its ease of maintenance against its physical and operational requirements.
- Pros: Easiest to inspect, maintain, and service, which can lower long-term labor costs. The installation process is less complex and costly compared to options requiring excavation.
- Cons: Requires a dedicated pump system and the associated energy and maintenance costs. The unit consumes valuable floor space, which can be a major drawback in constrained facilities. It is not suitable for outdoor areas or any location subject to vehicle traffic.
- Ideal Applications: Perfect for manufacturing plant interiors, industrial workshops, equipment rooms, and any location where wastewater is already collected in a sump and requires a pump for transfer and treatment.
Flush With Grade Oil Water Separators
Flush with grade systems offer a practical compromise between the accessibility of an above grade unit and the space-saving benefits of a buried one.
How They Work & Key Features
As the name implies, these systems are installed so that their top surface is level with the surrounding floor or ground.
They are specifically designed for gravity flow, where wastewater from the surrounding area naturally drains into the unit through its inlet pipe.
Typically constructed from robust, corrosion-resistant materials like fiberglass, these separators are built for longevity in wet environments.
Access for maintenance is provided through heavy-duty manholes or hatches that sit flush with the surface, keeping the system safely out of the way of daily foot traffic and light-duty vehicle movement.
Pros, Cons, and Ideal Applications
This design is highly effective in environments where gravity flow is available and a low profile is essential.
- Pros: Extremely space-efficient, as there is no surface obstruction to interfere with operations. They are the go-to solution for gravity-fed applications and offer easier maintenance access than fully buried systems.
- Cons: Installation requires excavation, adding to the initial project cost and complexity. While accessible, maintenance is more involved than with an above grade model, as it requires opening hatches and working from the surface.
- Ideal Applications: The standard choice for vehicle maintenance and wash bays, service garages, and any indoor or protected outdoor area where spills and runoff are cleaned by hosing them toward a central floor drain.
Below Grade Oil Water Separators
Below grade oil water separators are the most heavy-duty and robustly engineered option, designed for the most demanding site conditions.
How They Work & Key Features
These units are engineered for deep burial and drive-over conditions.
Their construction is focused on extreme structural integrity, allowing them to withstand immense soil and traffic loads without compromising performance.
They are the only viable option for installation in areas like parking lots and roadways.
Like flush with grade models, below grade separators operate on gravity flow, collecting stormwater runoff or other contaminated flows from a large surface area.
They represent the ultimate space-saving solution, as the entire system is completely hidden underground, leaving the surface fully available for its intended use.
Pros, Cons, and Ideal Applications
The benefits of a below grade system are unmatched in certain scenarios, but they come with significant installation and maintenance trade-offs.
- Pros: Provides a zero operational footprint on the surface. It can handle heavy and continuous vehicle traffic, making it ideal for transportation hubs. The buried installation protects the system from surface activities, weather, and potential impact damage.
- Cons: The highest installation cost due to the need for extensive excavation, potential shoring, and backfilling. Access for maintenance and inspection is the most complex, typically requiring specialized equipment to lift heavy manhole covers and access deep internal components.
- Ideal Applications: Essential for high-traffic parking lots, fueling stations, airports, logistics centers, and any site requiring a fully buried wastewater treatment solution that cannot interfere with surface traffic.
A Framework for Selecting the Right System
Making the right choice requires a systematic evaluation of your facility’s unique conditions.
Follow these three steps to identify the ideal oil water separator for your application.
Step 1: Assess Your Site Conditions and Feed Type
Your physical location and water flow mechanics are the first and most important factors.
- Is the application indoors or outdoors? Indoor applications often favor above grade systems for easy maintenance, while outdoor sites typically require flush with or below grade models.
- Will the area need to support vehicle traffic? If the answer is yes, a below grade unit is your only safe and durable option. Flush with grade models may handle light traffic, but below grade systems are specifically engineered for heavy, continuous loads.
- Is the flow driven by gravity or a pump? This is the fundamental question. If water drains naturally to the separator’s location, you need a flush or below grade system. If water collects in a sump and must be lifted for treatment, an above grade system is the correct choice.
Step 2: Determine Your Required Flow Rate (GPM)
The flow rate, measured in Gallons Per Minute (GPM), is the maximum volume of water the separator can process effectively.
Sizing the unit correctly is crucial; an undersized separator will be overwhelmed, leading to poor treatment performance and compliance violations.
Factors that influence your GPM calculation include the surface area draining to the unit, local rainfall intensity for outdoor applications, and the output of hoses or pumps for indoor systems. Because this calculation is so critical, it is always best to consult with an expert.
They can help you accurately determine the GPM needed for your specific application, whether it’s a low-flow system (0-50 GPM) or a large-scale, high-flow solution (up to 5000 GPM).
Step 3: Understand Regulatory & Compliance Demands
The ultimate goal of an oil water separator is to ensure your facility’s discharged water meets environmental standards.
Regulations can vary based on your location and the specific use of the separator.
For example, under the EPA’s Spill Prevention, Control, and Countermeasure (SPCC) rule, how a separator is used dictates the specific compliance requirements.
A unit used for general wastewater treatment may have different obligations than one used as part of a secondary containment plan for bulk oil storage.
Always verify local and federal discharge limits to ensure the system you select is capable of achieving the required effluent quality.
Conclusion: Making the Right Choice for Your Facility
The choice between an above grade, flush with grade, and below grade oil water separator is not about which system is “best” overall, but which is perfectly suited to your specific application.
By systematically evaluating your site, you can confidently select a system that delivers reliable performance and ensures regulatory compliance.
Remember the three primary decision drivers: feed type (pump vs. gravity), site location (indoor vs. high-traffic outdoor), and maintenance accessibility.
By using the framework provided, you can conduct a thorough site analysis that leads to a clear and logical conclusion.
Investing in the correct, performance-proven separator is a critical step toward achieving operational efficiency, protecting the environment, and upholding your commitment to corporate stewardship.
