Choosing between an above-grade and below-grade oil water separator is a critical decision for any facility manager, engineer, or project specifier. This choice goes far beyond a simple preference for location; it directly impacts your project’s upfront costs, long-term maintenance budgets, site logistics, and, most importantly, your ability to maintain regulatory compliance. Get it right, and you have an efficient, reliable system. Get it wrong, and you could face operational headaches and serious financial and legal risks.
The challenge lies in balancing competing priorities. You need a solution that fits your site’s physical constraints while also minimizing the total cost of ownership and reducing environmental liability. This decision is governed by stringent regulations. Under the EPA’s Spill Prevention, Control, and Countermeasure (SPCC) rule, many facilities are required to have mandated containment systems to prevent discharges into navigable waters. Non-compliance is not an option.
This article provides a clear, data-driven framework for selecting the right oil water separator. We will dissect the key differences between above-grade and below-grade designs to help you specify a solution that ensures both compliance and long-term operational efficiency for your facility.
Defining the Designs: What Are Above-Grade and Below-Grade Separators?
An above-grade oil water separator is a unit installed on a concrete pad or a similar prepared surface. These systems are typically used in pump-feed applications and are valued for their easy access. All piping, controls, and access points are visible and within reach, simplifying routine checks and service.
A below-grade oil water separator is a unit that is buried underground. These are often used for gravity flow applications that collect water from large areas like parking lots, airports, or fueling stations. They are also specified for deep burial situations or sites that require drive-over conditions where surface space is at a premium.
While the location is the primary distinction, the true performance of any system comes down to its design and build quality. The use of high-efficiency coalescing media and robust, corrosion-resistant materials is critical for ensuring long-term structural integrity and reliable separation. For specifiers evaluating their options, seeing how these principles are applied in a range of high-performance separators is a crucial step in making an informed decision.
Head-to-Head Comparison: Key Differences for Specifiers
To make the best choice, you need to understand the practical trade-offs between these two designs. The initial purchase price is only one part of a much larger equation that includes installation complexity, ongoing maintenance, and potential liability.
Installation & Site Requirements
The installation process for each separator type is fundamentally different, carrying distinct implications for project timelines and budgets.
Above-Grade Separators
- Process: Requires a prepared, level concrete pad for placement. Hookup to inlet and outlet piping is straightforward.
- Costs: No major excavation is needed, which typically results in lower initial labor costs and less site disruption.
- Constraints: The primary requirement is an adequate surface footprint. This can be a challenge on space-constrained sites.
Below-Grade Separators
- Process: Requires significant excavation, which may also involve shoring to prevent collapses, dewatering if the water table is high, and specialized heavy equipment. The process concludes with careful backfilling and compaction around the tank.
- Costs: The complexity of excavation and associated site work leads to significantly higher upfront installation costs.
- Constraints: Soil conditions, the presence of bedrock, and high water tables can further complicate installation and drive up costs. However, they are the go-to solution when surface space is unavailable.
Maintenance, Inspection & Accessibility
Long-term serviceability is where the differences between above-grade and below-grade systems become most apparent. This factor directly impacts operational costs, efficiency, and risk.
Above-Grade Separators
- Accessibility: All components—including hatches, valves, coalescing media, and the tank exterior—are easily accessible for visual inspection, cleaning, and parts replacement.
- Visibility: Any potential issues, such as a small leak from a fitting or signs of corrosion, are immediately visible and can be addressed proactively.
- Maintenance: Servicing is simpler, faster, and does not require confined space entry protocols, reducing labor costs and safety risks.
Below-Grade Separators
- Accessibility: The “out of sight, out of mind” nature of a buried tank can become a major liability. Internal maintenance and media cleaning require confined space entry, which involves specialized training, equipment, and safety procedures.
- Visibility: It is impossible to visually inspect the external integrity of the tank. A small leak could go undetected for years, slowly contaminating the surrounding soil and groundwater.
- Maintenance: Difficult access can lead to deferred or inadequate maintenance. This neglect reduces the separator’s efficiency and dramatically increases the potential for an undetected environmental release.
Long-Term Costs & Liability
When evaluating the total cost of ownership, specifiers must look beyond the initial purchase and installation price to consider the financial implications over the system’s entire lifecycle.
Above-Grade Separators
- Costs: Lower long-term maintenance and service costs due to easy access and simpler procedures.
- Liability: Environmental liability is significantly reduced. Any leak from the tank or piping is immediately contained on the concrete pad, preventing soil and groundwater contamination.
Below-Grade Separators
- Costs: Higher maintenance labor costs due to the need for confined space entry and specialized pumping equipment.
- Liability: The liability risk is substantial. An undetected leak from a corroded or damaged underground tank can lead to extensive soil and groundwater contamination, triggering a costly and complex environmental remediation project. Proactive maintenance is a crucial part of a facility’s compliance strategy under regulations like the Clean Water Act (CWA), making the inaccessible nature of buried tanks a serious concern.
Choosing the Right Separator for Your Application
The technical comparison provides the data, but the final decision must be based on your specific application and operational priorities.
When to Specify an Above-Grade Separator
An above-grade system is the superior choice in several common scenarios:
- Industrial Process Wastewater: Ideal for manufacturing plants, workshops, and other facilities where process water can be collected and pumped to the separator. Frequent monitoring is often beneficial in these settings.
- Vehicle & Equipment Wash Bays: The effluent from wash bays is easily collected in a sump and pumped to an above-grade unit for treatment.
- Maintenance Facilities: For vehicle and aircraft maintenance shops, an above-grade separator provides the best combination of performance and easy serviceability.
- Priority on Low Liability: When minimizing long-term environmental liability and ensuring simple, low-cost maintenance are the top priorities.
- Retrofit Projects: A strong option for existing facilities or sites where excavation is impractical, cost-prohibitive, or prohibited.
When a Below-Grade Separator is the Necessary Choice
Despite the maintenance and liability challenges, a below-grade unit is sometimes the most practical or only feasible option:
- Large Drainage Areas: Necessary for applications that rely on gravity flow to collect stormwater runoff from large, impervious surfaces like parking lots, airports, and fueling terminals.
- Extreme Space Constraints: Required for urban sites or facilities with no available surface footprint for equipment.
- Aesthetic Requirements: Specified when zoning codes or facility aesthetics prohibit visible industrial equipment.
- Drive-Over Conditions: The only option when the separator must be installed beneath roadways, parking areas, or other high-traffic zones.
When a below-grade separator is required, it is absolutely critical to choose a unit built with exceptional structural integrity and high-quality, corrosion-resistant materials to mitigate the inherent risks.
Beyond the Basics: Flush-With-Grade and Other Considerations
The choice isn’t always limited to a fully visible or fully buried unit. A third category, the Flush-With-Grade Oil Water Separator, offers a hybrid solution. These systems are installed below ground but feature large manway covers that are flush with the surrounding surface. This design is often used for designated drainage areas and provides easier top-side access for maintenance than a fully buried tank, blending accessibility with the benefits of a below-ground installation.
Furthermore, factors like the anticipated solids loading and the presence of emulsified oils can influence the final system design. Heavy solids may require larger sludge chambers, while certain chemical emulsifiers may necessitate different coalescing media or additional treatment steps.
Making the Final Decision: A Specifier’s Checklist
- Flow Type & Rate: Is your application pump-fed or does it require gravity flow? What is the maximum flow rate (GPM) the separator must handle?
- Site Footprint: Do you have adequate space and a suitable location for an above-grade unit, or is burial necessary due to space, traffic, or aesthetic constraints?
- Maintenance Protocol: Does your facility have the resources, training, and safety protocols for regular confined space entry? Or is easy, frequent visual access a higher priority?
- Regulatory & Risk Profile: How critical is minimizing the risk of soil and groundwater contamination at your site? Regulations like the Resource Conservation and Recovery Act (RCRA) mandate liquid-tight containment for certain materials, making leak prevention paramount.
- Total Cost of Ownership: Have you looked past the initial equipment price to factor in the complete costs of installation, long-term maintenance, and potential environmental liability?
The Foundation of Performance: Why Materials and Integrity Matter Most
Ultimately, whether you choose an above-grade or below-grade separator, its long-term effectiveness and lifespan depend entirely on the quality of its construction. A poorly built unit will fail regardless of its location.
Investing in a system constructed from the highest quality materials is non-negotiable. This ensures the structural integrity needed to prevent corrosion, cracks, and leaks over decades of service. Performance-proven separators are those engineered to meet and exceed exacting standards, providing reliable compliance year after year. A well-engineered system is not an expense; it is a critical investment in protecting your facility’s operations, your budget, and the environment.
Conclusion
The choice between an above-grade and a below-grade oil water separator presents a clear trade-off. Above-grade systems prioritize long-term accessibility, lower maintenance costs, and minimized environmental liability. In contrast, below-grade units are powerful problem-solvers for sites with specific constraints related to gravity flow, limited space, or traffic requirements.
The “best” oil water separator is always the one that best aligns with your specific site requirements, operational protocols, and risk tolerance. A thorough evaluation of the total cost of ownership—not just the initial purchase price—is essential for making a sound, defensible investment. By focusing on quality construction and a design matched to your application, you can ensure you have a critical asset that delivers environmental compliance and operational peace of mind for years to come.
