Many still view oil water separators as simple concrete boxes, but evolving regulations and technology demand far more precision. Proper classification, correct sizing, and awareness of site-specific risks are essential. Without this, operators risk non-compliance, flooding, or costly retrofits. A new design mindset is urgently required.

The design and specification of oil/water separators often falls into a ‘no man’s land’ wherein there is a vast lack of understanding and appreciation towards different type and size systems along with the intricacies of incorporating these into specific site requirements. Indeed, while some sites might be similar, no two sites are alike and what might have been the solution on one project cannot be blindly applied to another.

On new design-and-build projects, design of the oil separator is usually included in the engineering scope and addressed accordingly and correctly, typically after consultation with suppliers in the industry to discuss and confirm product specification, design and availability. However, retro-fit installations fall through this net and are driven by either the property owner, tenant or a small contractor, looking to “do the right thing” where we are asked to “quote on an oil separator” without any thought or indication as to the type and size required. Unfortunately this is often followed by “just your smallest and cheapest unit will be fine.”

In South Africa, many engineers and municipalities still view oil water separators as old concrete box, gravity-fed systems. Until recently, some municipalities even issued 1970s hand-drawn sketches as standard designs, with no reference to site-specific sizing or modern requirements. Technology has advanced, and with rising contaminant volumes from industry, all stakeholders must adapt and raise their standards.

Standards and bylaws

SANS 50858-1-2002 [Part 1: Principles of product design, performance and testing, marking and quality control] defines CLASS II separators as having a “Maximum permissible content of residual oil” of 100 ppm whilst the newer CLASS I separators are restricted to a maximum of only 5 ppm contamination passing through the system. This code allows a bit of relaxation when physically testing separator systems by stating that “No individual sample shall have a higher value than 10 mg/ℓ for CLASS I or 120 mg/ℓ for CLASS II”.

Municipal bylaws, however, typically allow up to 10 ppm separation which is a factor of 10x lower than the expected separation achieved from a CLASS II system. So, even though TABLE B.1 of ANNEX B of SANS 50858-2-2003 [Part 2: Selection of nominal size, installation, operation and maintenance] provides for configurations that include CLASS II separators with TABLE B.2 stipulating which type separator can discharge from what type installation and into what type municipal network, public sewer or stormwater, both regulations and bylaws must be cross referenced with the stricter requirement being addressed.

As per same TABLE B.2, in some instances and due to their vastly superior filtration capacities, discharge from a CLASS I separator can be fed directly into stormwater networks whilst CLASS II separators, when acceptable for use, may only discharge to the public sewer network.

Once the design team have established which CLASS separator is relevant, based on available discharge network Vs contaminant and if CLASS II will suffice then the unit can be sized based on flow rate, required retention time and resulting overall volume. For a full retention system, ie one where the collection of silt and oil are combined into the same unit, available storage volumes of each of these must also be considered in the calculations.

In comparison, CLASS I separators are easier to size as the suppliers list the flow rate capacity (ℓ/s) of each unit. All other calculations have already been taken into account such that the designer simply needs to scroll down the list to find the relevant system and spec accordingly.

Sizing formulae

Before finalising flow rate, six key factors must be addressed:

1. Application – Workshop, wash-bay, parking runoff. Municipal bylaws and SANS 50858-2-2003 provide guidance on which CLASS is permitted and where discharge can go.
2. Spill risk – Every separator has a maximum oil storage capacity. A small unit may handle daily traces but fail catastrophically during a major spill, such as a ruptured transformer releasing 2 000 ℓ of oil. Closure devices will shut outlets when capacity is reached, risking flooding. In high-risk areas, upstream bunding or flow-restricted closure valves are recommended.
3. Indoor vs outdoor – Stormwater can create far higher flows than actual wastewater, overwhelming the system. For example, two pressure cleaners might produce <2 ℓ/s, but a 1:20 year storm could exceed 10 ℓ/s.
4. Stormwater catchments – Designers must not ignore stormwater from other site areas. “Clean” water should be diverted away, preventing unnecessary upsizing of separators.
5. What can be separated – These systems treat light liquids below 0.95 SG (petrol, diesel, mineral oils). Emulsified oils, fats, grease, or solids need specialised treatment.
6. Flow rate formula –
   NS = (Qr + fx × Qs) fd

• NS = nominal size (ℓ/s)
• Qr = stormwater flow rate (ℓ/s)
• Qs = wastewater flow (ℓ/s)
• fx = impediment factor (e.g. 2 for industrial effluent)
• fd = density factor for the liquid.

NS is the nominal size of the separator. This will obviously be the final size of the oil separator to match against a supplier’s catalogue.

Qr is the maximum flow rate of rainwater, in ℓ/s. If relevant, this will be the stormwater flow for the catchment area and the type and intensity storm considered in the design. As previously mentioned, this is an area often overlooked or incorrectly calculated. Note that this formula assumes that stormwater and wastewater will flow at the same time which is very often not the case. An open air industrial car wash will not be washing trucks during a rain storm. However, an indoor workshop connected to an outdoor washbay could be subject to both a waste and rain water flow simultaneously. Hence why it is sometimes prudent to prepare two (or more) scenario calculations and work on the worst case (highest flow rate) rather than both/all combined.

Qs is the maximum flow rate of wastewater, in ℓ/s. This will be the flow from the cleaning equipment and should be easy enough to calculate. High pressure water jets are rated according to a maximum flow rate and this is the figure that should be considered, however, the code makes an allowance for a minimum of 2l/s for the first high pressure water cleaner and additional 1 l/s per unit thereafter.

fd = density factor for the liquid

For liquids >0.85 SG, CLASS II units must be oversized compared with CLASS I. For example, a calculation producing 6 ℓ/s might require 12 ℓ/s in CLASS II but only 9 ℓ/s in CLASS I. This often makes CLASS I smaller and cheaper overall.

Example: A workshop and wash-bay using three cleaners (<1 ℓ/s each) plus a 400 m² outdoor area subject to 50 mm/hr rainfall. With effluent density 0.85–0.9, the design results in ~12 ℓ/s capacity required.

Accordingly, you will often find that the CLASS I separators can be smaller than the CLASS II unit for the same site.

ROCLA’s new range ESK-Z Lamella Oil Separators from ecol Unicon can treat 100 ℓ/s flow by means of 2 x 50ℓ/s systems each requiring an oil separation tank of only 1500 mm internal diameter x 2.7m deep. So, at the outset, when a CLASS II unit might be deemed the most cost effective option, on closer investigation, the better CLASS I system might work out cheaper overall due to space, materials and labour savings.

Additionally, CLASS II separators have minimum requirements for surface area (Amin), total volume (Vmin) and light liquid storage volume (V1 min), all defined as a ratio to NS (overall volume).

Generally to treat waste water (trade effluent) from industrial processes, vehicle washing, cleansing of oil covered parts or other sources, e.g. petrol station forecourts take note that we apply a factor of 2 for (f x) to what is considered contaminated flow – so flow directly from the operation as opposed to storm-water which is not subject to this loading factor.

So to summarise this into an example.

Let’s consider an operation that has an undercover and enclosed workshop alongside same layout wash-bay. Coupled to this and directly in front of these two work stations is a “holding area” where vehicles can be washed / worked on. It is assumed that there will be a division of stormwater flow between this holding area and the balance of the site with separate catchpit channels / slopes collecting the “clean” rainwater from elsewhere, discharging it accordingly and the potentially contaminated stormwater from the vehicle holding area.

The washbay will have 2 x high pressure water cleaners and the workshop, one. These same cleaners will be used to wash the outside holding area as and when required.
So the worst case scenario here would be that both the washbays are operational together with the workshop hosing down their space, all during a rain-storm.
The water jets will be assumed to be less than 1ℓ/s each such that all three must be considered at 4 ℓs in total. As per the formula, this contaminated flow must be subject to a safety factor of 2, resulting in 8 ℓ/s for this part of the site.

Let’s not get into specifics for the storm-water but using a rainfall event of 50mm/hour or 139mm/s.ha and an outdoor holding area of 400m2 give us 6 ℓ/s stormwater flow to be considered.

We will assume light liquid density between 0.85 and 0.9 with loading factor 1.5 for CLASS I separator (or 2 for a CLASS II separator).

Accordingly we get (6 + (2 x 4)) x 1.5 = 21ℓ/s. but if we issue instructions that the washbay and workshop are not to operate cleaning during a rainstorm then we can split this into 9 ℓ/s or 12 ℓ/s and hence design for the higher being 12 ℓ/s.

Additional equipment

SANS 50858 also requires the inclusion of a grit/silt chamber placed upstream of the filter unit to remove as much floatables and solids as possible and avoid these entering the oil separator chamber and impeding the operation.

The code has a very simple design guide for three different sizes of these grit tanks, rated at 100*Ns, 200*Ns or 300*Ns depending on the type installation (Ns being the flow rate in ℓ/s). Typically a site where small concentrations of silt and sand are to be expected, the smaller unit will suffice. But washbays for muddy mine vehicles would require the largest unit. So by example, an oil separator sized at 3 ℓ/s would have either 300 ℓ, 600 ℓ or 900 ℓ overall size grit chamber.

Additional optional equipment can include :

• High pressure shut off valve to close the inlet of the separation chamber
• Automatic draw off [ADD] devices sit inside and at the top of the oil chamber, opening and closing via a complex system of floats, constantly draining the accumulated oil from the surface, through the tank wall and into a separate chamber that will only consist of separated oil.
• Oil and grit depth sensors are also relatively freely available in the market to warn of a system reaching maximum levels of either.

Once a system has been installed, commissioned and operational, the end user must make contact with an oil recycler who will then visit the site periodically and monitor/measure grit, sludge and oil levels and then make recommendations as to when the system should be maintained.

Maintenance would include shutting an upstream valve, draining the grit, sludge and oil, then draining the remaining water. The filter media will be removed and cleaned and the system filled with clean water and flushed with this water also be removed by the recycler. The filter will be replaced, the tanks filled with clean water and the valve opened to allow free flow of effluent. The recycler will then continue to monitor and eventually recommend a routine relevant to the site.

By Justin Kretzmar, sales engineer, Rocla