Key Takeaways
- An above-ground separator costs $500 to $1,500 to install, compared to $5,000 to $20,000 for an underground one (excavation and civil engineering included).
- Above-ground separators cover flow rates from 0.5 to 50 GPM. Underground units serve large multi-bay facilities and high-flow sites.
- In Quebec, the frost line reaches 1.2 to 1.8 m depending on the region. An underground separator installed below this depth is naturally protected against freezing.
- Access for pump truck emptying is simpler with a well-designed underground unit. An above-ground unit in a confined space can complicate technician access.
- For renovations of existing workshops with no planned excavation, above-ground is almost always the fastest and most economical solution.
What is the fundamental difference between an above-ground and an underground separator?
Both types rely on the same physical principle: the density difference between water and hydrocarbons causes oil to rise to the surface, where it is retained while the treated water continues to the sewage system. The difference is not technological. It is mechanical and logistical: where the unit is placed, how it is accessed, and what capacity it can achieve.
An above-ground separator (or surface mounted) is installed inside a building, fixed to the floor or placed on a slab. It remains visible, directly accessible by a technician, and connected to existing floor drains. Maintenance is performed without heavy lifting equipment.
An underground separator is installed in a buried tank, below the finished floor surface or outside the building. Access is via a manhole cover. Emptying requires a pump truck (vacuum truck) with a suction hose extending down to the collection chamber.
A flush-mounted separator occupies an intermediate position: the tank is housed in a concrete pit poured into the floor, but its top is flush with the finished surface. It does not clutter the workspace, but excavation is necessary during initial installation.
Above-ground separator: when is it the right choice?
For a workshop with 1 to 6 bays, the above-ground unit is the default answer in the vast majority of cases. Installation costs are contained between $500 and $1,500 CAD for installation, connection to existing drains, and commissioning, depending on the complexity of the existing piping network. For details on complete costs, consult our analysis of oil-water separator costs in Canada.
Available capacities
The above-ground OlioSep™ range covers flow rates from 0.5 to 50 GPM (approximately 2 to 190 L/min). This range satisfies almost all automotive workshops, light fleet garages, and moderate-flow industrial facilities. To find out what flow rate corresponds to your facility, consult our sizing guide.
Sediment and oil storage
OlioSep™ above-ground units feature a sediment collection chamber and an oil retention chamber. Emptying is done manually by a technician with an industrial vacuum cleaner or small portable pumping equipment. No pump truck is necessary for compact models.
Protection against freezing in Canada
This is the weak point of above-ground units in northern regions. A surface separator, in an unheated space, can freeze. Solidified oil blocks the collection chamber and renders the protection inoperative. Two solutions are available:
- Heated space: place the separator in a technical room maintained above 5°C in winter. This is the standard solution for indoor workshops in Canada.
- Thermal insulation: on OlioSep™ models, insulating sleeves and heating cables can be installed for applications in controlled cold environments.
When to choose an above-ground separator
- Workshop or garage with available heated indoor space
- 1 to 6 bay installation, peak flow rate less than 50 GPM
- Limited installation budget or renovation project with no planned excavation
- Need for direct and quick access for regular maintenance
- Tenant of a commercial space (avoids civil engineering work)
- Replacement of an old separator without modifying the floor
Underground separator: when should it be seriously considered?
An underground separator represents an investment of $5,000 to $20,000 CAD for installation, depending on the burial depth, soil type, site accessibility, and required backfill work. This figure includes the rental of the excavator, the pouring of the concrete foundation, and connection to existing piping networks. The equipment itself is in addition to these civil engineering costs.
Capacities and industrial applications
OlioSep™ underground separators serve high-flow facilities: multi-bay car washes, heavy truck depots, fuel transfer stations, oil terminals, busy gas stations. They are designed to receive multiple drains simultaneously and accumulate larger volumes of oil and sediment between empties.
Natural protection against freezing
In Quebec, the frost depth varies by region: from 1.2 m in the south of the province to 1.8 m or more in Abitibi or Saguenay. An underground separator installed below the frost line is thermally protected by the mass of the soil. Water and oil do not freeze, even during the harshest winters. This is the main argument in favor of underground units for outdoor installations or unheated garages.
Access for pump truck emptying
Maintenance of an underground separator requires a vacuum truck. The technician lowers a suction hose into the access manhole to pump out sediment sludge and accumulated oil. This access must be planned from the design stage: the manhole cover must be of sufficient size (minimum 600 mm inner diameter), located away from heavy traffic lanes or protected by a cast iron frame adapted to the traffic load.
A poorly positioned manhole under a parking ramp or behind fixed equipment can turn a routine emptying into costly engineering work.
When to choose an underground separator
- Unheated outdoor site or open installation exposed to freezing
- Peak flow rate greater than 50 GPM or busy multi-bay installation
- Floor space constraint in the building (no room for an above-ground unit)
- New construction with floor not yet poured (integration during civil engineering phase)
- Commercial car wash, heavy truck depot, oil terminal
- Local regulations requiring an underground unit (check with the municipality's water department)
The flush-mounted separator: the intermediate solution
The flush mount addresses a specific case: you need the capacity of a larger model, but you cannot have equipment protruding from your work floor. This is the case for workshops with solid floors without a dedicated technical area, quick wash stations where vehicles move over the entire surface, and facilities where insurance standards or owner rules prohibit any visible equipment.
The flush-mounted OlioSep™ tank is installed in a pit poured into the concrete slab. The top of the unit is flush with the finished surface. Access for maintenance is via a floor cover at ground level. Excavation is necessary for initial installation, but it is shallower and less costly than a complete underground unit.
This type of installation is well suited for planned renovations where floor work is already scheduled. It is not justified for a quick installation in an existing workshop without major work.
Direct comparison: above-ground vs. flush-mounted vs. underground
| Criterion | Above-ground | Flush-mounted | Underground |
|---|---|---|---|
| Installation Cost | $500 - $1,500 | $2,000 - $6,000 | $5,000 - $20,000 |
| Flow Rate Covered | 0.5 - 50 GPM | 2 - 50 GPM | 10 - 200+ GPM |
| Frost Protection | Requires heated space | Partial (insulating slab) | Complete (below frost line) |
| Maintenance Access | Direct (technician alone) | Floor cover | Pump truck required |
| Floor Space Required | Yes (protruding) | No (flush) | No (outside building possible) |
| Ideal for | Heated 1-6 bay workshops | Renovations with floor work | Outdoor sites, high flows |
All installation values are in CAD and exclude equipment cost.
FAQ — Frequently Asked Questions about Installation Type Selection
Is an above-ground oil-water separator as efficient as an underground separator?
Yes, for the same flow rate and under correct installation conditions, both types achieve the same level of separation. Efficiency depends on sizing, technology (coalescing or gravity), and maintenance, not the tank's location. A correctly sized above-ground OlioSep™ produces an effluent compliant with the 15 mg/L required by most Quebec municipalities.
Can an above-ground separator be installed outdoors in Canada?
Technically yes, but practically not recommended in most Canadian regions without thermal protection measures. In Quebec winters, an above-ground unit exposed to outdoor temperatures will freeze. If outdoor installation is unavoidable, an insulated and heated shed must be planned, or an underground model installed below the frost line should be chosen.
What is the cost difference between an above-ground and an underground separator?
Installing an above-ground separator costs between $500 and $1,500 CAD for labor and connections. Underground installation, including excavation, formwork, concrete, and backfill, costs from $5,000 to $20,000 CAD depending on depth and accessibility. This difference often offsets the purchase of a higher capacity above-ground equipment for suitable applications.
Does a commercial car wash need an underground separator?
Not necessarily, but often yes. A busy multi-bay car wash generates peak flows that easily exceed 50 GPM, and the work surface does not allow for protruding equipment. A flush-mounted or underground model is generally the chosen solution. A single-bay car wash with moderate flow can use an above-ground model if heated space is available. The first step remains the calculation of the peak flow rate.
Can an above-ground separator be installed in an existing garage?
Yes, and this is precisely the main advantage of above-ground units in renovation projects. The unit connects to existing floor drains without pouring concrete. Installation is generally done in one day. This is why most Canadian automotive workshops upgrading an existing space initially opt for above-ground, unless there is a specific technical constraint.
Not sure which type of installation is right for your facility?
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Read this article in English: Above Ground vs. Underground Oil Water Separators
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- Oil-water separators: complete guide for industry
- Oil-water separator tank: sizing and selection
- Cost of an oil-water separator in Canada
- Oil-water separator maintenance