Above Ground vs. Underground Oil Water Separators: Which Do You Need?

April 10, 2026 ERE Inc. 10 min read

In This Article

An above-ground oil water separator costs $500–$3,000 installed and can be running the same day. An underground unit costs $15,000–$60,000 or more once excavation, concrete, and backfill are factored in. That cost gap alone shapes most installation decisions, but it's not the only factor that matters.

This article breaks down both installation types across every decision variable: footprint, capacity, maintenance access, freeze protection in Canadian winters, and typical applications. If you want a broader foundation first, the complete OWS guide covers how separators work, what regulations apply, and when you need one. For this article, we're focused on one specific question: which installation type is right for your facility?

How Does Each Installation Type Actually Work?

The core separation chemistry is the same across all types. Oil rises, sediment sinks, and clean effluent exits through a standpipe or weir. The installation type determines where the vessel sits relative to your floor, how wastewater gets in, and how you access it for cleanout.

Surface mount (above-ground): The separator sits on the floor, typically near a floor drain, wash bay, or sump. Inlet piping connects from below or the side. The unit is fully visible, accessible from all sides, and requires no concrete cutting or excavation. Most surface mount units are self-contained and compact enough to fit in a corner of a service bay.

Underground (below-grade): The separator is buried in a concrete vault or direct-burial tank, installed below the finished floor. Wastewater flows in by gravity from floor drains across a larger area. A manhole or access riser sits at floor level. Excavation depth in Canadian climates typically reaches 1.2–1.8m to place the vessel below the frost line, deeper in northern Ontario, Manitoba, or Quebec interior regions.

Flush mount (in-floor): A third option that bridges the two. The vessel body is recessed into the floor slab, but it does not require deep excavation. The top surface is flush with the finished floor, and a removable lid provides cleanout access. It occupies no above-floor footprint and suits facilities where driving or walking over the unit is a daily reality.

What's the Real Footprint Difference?

Surface mount units take up floor space but nothing underground. Underground units consume no floor space but require a larger excavation footprint during installation, and they affect the structural slab permanently.

For a small surface mount unit rated at 0.5–5 GPM, the vessel is typically 24"–36" in diameter and 24"–48" tall. It can sit against a wall, under a workbench, or in a utility area without disrupting operations. Larger surface mount units in the 25–50 GPM range are bigger but still manageable inside a commercial bay.

Underground systems for commercial car washes, truck wash facilities, or multi-bay operations are sized for higher sustained flows. Their footprint underground can span several square meters of floor slab, which matters if you're retrofitting an existing building. Cutting and replacing concrete, managing rebar, and waterproofing the vault adds weeks to the project timeline.

How Do Flow Capacity and Storage Compare?

Surface mount oil water separators typically handle 0.5 to 50 GPM, which covers most single-bay auto shops, small fleet maintenance facilities, and equipment wash pads. Underground systems are specified for higher sustained flows, often 50 GPM and above, and their larger vessel volume provides greater sediment and oil storage capacity between cleanouts.

Sediment storage is a practical concern for high-grit operations: truck washing, mining equipment cleandown, and heavy construction yard drainage. If you're generating a lot of coarse sediment, a larger underground sump gives you more time between vacuum truck visits. Surface mount units need more frequent cleanouts in high-sediment applications, sometimes monthly vs. quarterly or semi-annually for a properly sized underground unit.

Which Installation Type Is Easier to Maintain?

Surface mount units win on maintenance access. Every surface, lid, drain plug, and oil outlet is at eye level or within arm's reach. You don't need a confined space entry permit to inspect or pump out a surface mount OWS. That matters for small operators who self-service their equipment with a shop vac and a bucket.

Underground units require a vacuum truck for cleanout in virtually all cases. Access is through a manhole or riser, which may require confined space procedures depending on vessel depth and jurisdiction. In Canada, most provincial OH&S codes trigger confined space entry requirements for vessels deeper than 1.2m. That's exactly where underground OWS units typically sit. Budget for vacuum truck service every 3–6 months depending on discharge volume and sediment load.

How Does Canadian Winter Affect Each Type?

Freeze protection is a real engineering concern for surface mount units in Canada. An above-ground separator sitting on an unheated floor in a northern Ontario shop can freeze solid if ambient temperatures drop below -10°C for extended periods. That's not a theoretical risk. It happens, and it can crack the vessel or block the outlet standpipe entirely.

Mitigation options for surface mount units include:

Keeping the unit inside a heated space (most common solution)
Wrapping the vessel and inlet/outlet piping with heat trace cable
Insulating the vessel with foam wrap rated for the local design temperature
Draining the unit seasonally if the wash bay closes for winter

Underground units below the frost line, typically 1.2m in southern Ontario and Quebec and 1.5–1.8m in Manitoba, Saskatchewan, and northern regions, are naturally protected. Soil insulation keeps liquid from freezing even when surface temperatures drop to -30°C. This is a genuine operational advantage for outdoor applications: equipment wash pads, fuel depots, and vehicle staging areas that can't be enclosed.

What Does Installation Cost?

Surface mount OWS installation runs $500–$3,000 depending on unit size, local labor rates, and whether inlet piping needs modification. That includes the separator, inlet/outlet connections, and a simple floor drain tie-in. No permits beyond standard plumbing in most jurisdictions.

Underground installation is a different category entirely. Excavation, concrete cutting, waterproofed vault construction, backfill, and slab restoration bring the total to $15,000–$60,000 before the separator unit itself. Larger commercial systems for truck washes or industrial yards can exceed $100,000 once civil work, engineering drawings, and municipal permits are included. Ontario, Quebec, and BC each have specific requirements for below-grade OWS installations in commercial applications. Confirm with your local authority having jurisdiction (AHJ) before specifying.

For a full cost breakdown by installation type, province, and facility size, see the Oil Water Separator Cost in Canada guide.

When Should You Choose Above-Ground?

Choose a surface mount oil water separator when:

Floor space is available and the unit can sit in a corner, utility room, or service bay without disrupting workflow
Budget is constrained: $500–$3,000 installed vs. five-figure civil work
Retrofit installation: no appetite for concrete cutting or structural work
Flow rates are under 50 GPM: one or two wash bays, a lube shop, a small fleet yard
The space is heated: freeze protection is a non-issue in a climate-controlled environment
Self-service maintenance is preferred: no confined space, no vacuum truck required for small units
Temporary or relocatable installation: surface mount units can be moved if your operation changes

When Should You Choose Underground?

Choose an underground oil water separator when:

Floor space is at a premium and you can't spare any above-grade footprint
High flow rates are required: commercial car washes, truck wash bays, large industrial wash pads
Outdoor installation in an unenclosed area where above-grade freeze protection isn't practical
Long intervals between cleanouts are operationally necessary, because larger sump volume means less frequent service
A permanent, municipal-compliant installation is required by the AHJ or building permit
Multiple floor drains feed a central collection point, and underground gravity systems handle this cleanly
New construction where excavation is already planned and marginal cost is lower

What About Flush Mount — The In-Floor Option?

Flush mount separators are the practical middle path that most specification guides skip. The vessel is partially recessed into the floor slab, typically 150–300mm below finished floor level, with a removable lid that sits flush with the surrounding concrete. You can drive a forklift over it. You can walk over it. It takes up zero functional floor space.

Flush mount units don't require the deep excavation of an underground system. Most installations involve cutting a defined opening in the existing slab, setting the vessel, and pouring back around it: a one-day civil task vs. a week-long underground project. Maintenance access is straightforward. Lift the lid, vacuum out sediment, inspect, replace. No confined space entry is required for standard flush mount depths.

The trade-off is capacity. Flush mount units sit between surface mount and underground in terms of vessel volume and flow rate. They're well-matched to single-bay and two-bay operations where above-floor profile is the primary objection to a surface mount unit.

OlioSep™: ERE's Line Covers All Three

ERE's OlioSep™ line is built around these three installation types. The Surface Mount series handles 0.5–50 GPM and is designed for quick installation in service bays, equipment yards, and fleet maintenance facilities. The Flush Mount option gives facilities the low-profile look without excavation costs. Underground systems are available for larger commercial applications where capacity, freeze protection, or site requirements push you below grade.

All OlioSep™ units are sized for Canadian applications: materials rated for cold-climate performance, standard inlet/outlet configurations for Canadian drain systems, and documentation packages that support AHJ permit submissions.

Browse OlioSep™ Oil Water Separators

If you're not sure which capacity you need, the sizing article walks through the calculation by bay count, wash type, and peak flow rate: Oil Water Separator Tank: How to Size It Right.

Need help with oil water separators?

ERE Inc. has been Canada's environmental equipment specialist for 30+ years.

→ Request a Quote | 1-888-287-EREC | Browse Oil Water Separators | sales@ereinc.com

Frequently Asked Questions

Is an above-ground oil water separator as effective as an underground unit?

Yes. Separation efficiency depends on vessel design, retention time, and proper sizing, not burial depth. A correctly sized surface mount OWS will meet the same effluent standards as an underground unit. Underground units hold more volume between cleanouts, but they don't separate oil more effectively.

Can I install a surface mount OWS outdoors in Canada?

You can, but freeze protection is required. Heat trace cable, foam insulation rated to your local design temperature, or a heated enclosure will prevent the unit from freezing in winter. In most of Canada, design temperatures range from -20°C to -40°C depending on province and region. An unprotected above-ground unit outdoors in a Manitoba winter will not survive the season.

What's the typical installation cost difference between surface mount and underground?

Surface mount units install for $500–$3,000 including labor and basic piping. Underground systems typically cost $15,000–$60,000 once excavation, concrete, vault construction, and restoration are included. The cost gap reflects civil work, not the separator unit itself. For many small facilities, the surface mount pays back its lower capital cost within the first year compared to a financed underground installation.

Do I need an underground unit for a commercial car wash?

In most Canadian jurisdictions, yes. Commercial car washes generate sustained flows of 50 GPM or more across multiple simultaneous bays, and municipal sewer bylaws typically require a permitted, below-grade interceptor. Single-bay or self-serve bays with lower peak flows may qualify for a surface mount or flush mount unit. Confirm with your local municipality.

Can I retrofit an above-ground unit into an existing facility?

Yes, and this is one of the strongest arguments for surface mount. You need a floor drain tie-in and an available outlet to the sewer or holding tank. No structural work required. Most retrofits are completed in a single day. Underground retrofits in existing buildings require concrete cutting, excavation inside the structure, and slab restoration, which adds cost and downtime that many operating facilities can't absorb.

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