The Numbers That Drive Separator Sizing
Four figures every wash bay manager should know
x 60
Converts a pressure washer’s L/min rating to L/h. A 21 L/min unit delivers 1,260 L/h
1 L
Of water from every 1 mm of rain falling on 1 m² of unroofed wash bay
1,000 L/h
Minimum separator size for vehicle washing set by SA Water and Central Coast Council
700 to 10,000
L/h range of separators on Sydney Water’s listed pre-treatment for the auto industry
Sources: SA Water Vehicle Washing guideline; Central Coast Council; Sydney Water listed pre-treatment for the auto industry.
Key Takeaways
- Size to the highest flow in any one hour, never the daily average.
- Add every pressure washer and hose that can run at the same time.
- Add rainfall from any unroofed catchment that drains to the separator.
- With a sump and pump, the separator sees the pump rate, so the pump must not exceed the separator’s rating.
- Remove solids first, use quick-break detergents and a non-emulsifying pump.
- If you’re between sizes, go up.
Why Oil Water Separator Sizing Starts With Peak Flow
An oil water separator works by slowing water down long enough for oil droplets to rise and solids to sink. Push more water through than the unit is rated for, and that residence time disappears. Oil that should have been captured goes straight to the sewer.
A separator that handles your daily volume comfortably can still fail every morning. Ten vehicles washed over eight hours looks easy on paper. Ten vehicles washed in the first hour of a shift, with two operators, is a completely different load.
| Scenario | Wands running at once | Flow to separator (21 L/min washers) |
|---|---|---|
| 10 vehicles spread over 8 hours, one operator | 1 | 1,260 L/h |
| 10 vehicles in the first hour, two operators | 2 | 2,520 L/h |
The daily volume can be identical, yet the peak flow doubles. That’s why Sydney Water asks for listed pre-treatment equipment to be sized to your maximum hourly flow.
How to Work Out Your Wash Bay’s Maximum Flow Rate in Litres per Hour
Start with every source of water that can reach the separator at the same time. Then add them up. That total, in L/h, is your design flow.
1. List Sources
Every pressure washer, hose and drain that can feed the separator.
2. Convert to L/h
Multiply each rating in L/min by 60.
3. Add Rain
Add rainfall from any unroofed area that drains to the separator.
4. Choose Up
Pick a separator rated at or above the total peak.
Pressure Washer Output, Wash Duration and Multiple Bays Running at Once
Check the rated flow on each pressure washer’s plate, usually in litres per minute, and multiply by 60. A unit rated at 21 L/min delivers 1,260 L/h. Two running together in a double bay deliver 2,520 L/h.
| Rated flow (L/min) | One unit (L/h) | Two units (L/h) | Three units (L/h) |
|---|---|---|---|
| 10 | 600 | 1,200 | 1,800 |
| 15 | 900 | 1,800 | 2,700 |
| 21 | 1,260 | 2,520 | 3,780 |
| 25 | 1,500 | 3,000 | 4,500 |
| 30 | 1,800 | 3,600 | 5,400 |
Don’t Forget Hoses
An open hose can flow more than a pressure washer, so measure yours rather than guessing:
- Time how many seconds the hose takes to fill a 10 L bucket
- Divide 10 by the seconds, then multiply by 60 to get L/min
- Multiply by 60 again to get L/h
Example: 40 seconds gives 15 L/min, or 900 L/h.
Pumped From a Sump? Read This
If your wash water drains to a sump and is pumped to the separator, the separator sees the pump rate, not the washer rate. The sump absorbs the surge.
- The pump must be no larger than the separator’s rated flow
- The sump needs enough storage to hold peak wash water while the pump catches up
- SA Water, for example, asks for a settling and pump sump of at least 400 L where oil and grease levels are higher
Allowing for Rainfall on Unroofed Wash Bays
Rain is the load most people forget. The maths is simple: 1 mm of rain on 1 m² is 1 L of water.
Rain load (L/h) = rainfall intensity (mm/h) x unroofed catchment (m²)
A 50 mm per hour storm on a 60 m² unroofed catchment sends 3,000 L/h towards your separator, on top of any washing.
Rain Load by Catchment Size and Storm Intensity (L/h)
| Unroofed catchment | 25 mm/h | 50 mm/h | 100 mm/h |
|---|---|---|---|
| 20 m² | 500 | 1,000 | 2,000 |
| 40 m² | 1,000 | 2,000 | 4,000 |
| 60 m² | 1,500 | 3,000 | 6,000 |
| 100 m² | 2,500 | 5,000 | 10,000 |
Intensities shown are for illustration. Use local rainfall intensity data from the Bureau of Meteorology for your site.
Many authorities avoid the problem altogether. Water Corporation requires wash-down areas over 20 m² to be roofed, and SA Water requires all areas draining to sewer to be roofed. Where roofing isn’t practical, bunding and stormwater diversion keep clean rain out of the system.
Worked Example: Sizing a Two-Wand Depot
A depot runs two 21 L/min pressure washers and a hose measured at 15 L/min in a direct gravity-fed setup.
| Water source | Roofed bay (L/h) | Unroofed 60 m² bay in a 50 mm/h storm (L/h) |
|---|---|---|
| Two 21 L/min pressure washers | 2,520 | 2,520 |
| One hose at 15 L/min | 900 | 900 |
| Rainfall | 0 | 3,000 |
| Peak flow | 3,420 | 6,420 |
Peak Flow vs WashBay HQ Separator Ratings (L/h)
| OWS-1500 rating | 1,500 |
| OWS-3000 rating | 3,000 |
| Roofed bay peak | 3,420 |
| OWS-6000 rating | 6,000 |
| Unroofed bay peak in storm | 6,420 |
The Verdict
Roofed: 3,420 L/h means a 3,000 L/h separator is already too small. The 6,000 L/h model is the right call.
Unroofed: 6,420 L/h is beyond the largest model. Roofing the bay, or pumping from a sump at a controlled rate, is cheaper than chasing the rain with a bigger separator.
Common Separator Sizes and the Sites They Suit
Sydney Water’s listed pre-treatment for the auto industry includes separators from 700 L/h up to 10,000 L/h, with the largest listed for car washing only. Some authorities, including SA Water and Central Coast Council, set 1,000 L/h as the minimum for vehicle washing.
WashBay HQ above-ground units come in three models, each with a compact 1.93 m by 0.45 m footprint:
| Model | Rated flow | Choose it when your peak is | Typical sites |
|---|---|---|---|
| OWS-1500 | 1,500 L/h | Up to 1,500 L/h | Single-bay workshop or small fleet with one wand running |
| OWS-3000 | 3,000 L/h | 1,501 to 3,000 L/h | Mid-size depots, council yards and regional transport with one or two wands |
| OWS-6000 | 6,000 L/h | 3,001 to 6,000 L/h | Mining, quarry, Defence and large transport hubs with multiple bays and surge loads |
| Above 6,000 L/h | Talk to us | Over 6,000 L/h | Reduce the peak by roofing or controlled pumping, or add a second above-ground unit |
If you’re between sizes, go up.
Coalescing Plate, Vertical Gravity and Vertical Tube Separators Compared
| Separator type | How it separates oil | Strengths | Watch out for |
|---|---|---|---|
| Coalescing plate (CPS) | Inclined plates help small droplets merge and rise | Compact for its flow rate, widely accepted | Plates can clog under high solids and need regular clean-down |
| Vertical gravity (VGS) | Vertical chamber design and gravity separation | Accepted by many authorities, simple internals | Footprint and performance vary by design |
| Vertical tube coalescing (WashBay HQ) | Water flows down through vertical polymer tubes where droplets merge and rise | Flow path designed to resist blockage, no filter bag consumables | Still needs pre-settling and scheduled servicing |
Water Corporation accepts vertical gravity separators, coalescing plate separators and hydrocyclones, while single, double or triple interceptors aren’t accepted as final pre-treatment.
How Solids Load and Pre-Settling Affect Separator Performance
Solids are the silent killer of separator performance. Mud and grit settle on plates and in chambers, shrink the working volume and cut residence time, so a correctly sized unit behaves like an undersized one.
Put solids removal first. SA Water’s guideline calls for a silt trap with a 2 mm bucket, then a settling sump before any separator. Mining, quarry and construction sites usually need more settling capacity than a city workshop, and WashBay HQ offers optional settling tanks for sites with heavy grit loads.
Detergent Choice and Emulsified Oil
Some degreasers are designed to hold oil in suspension. That’s great for cleaning, but an emulsion won’t separate by gravity, so it passes straight through.
Water Corporation, SA Water and Central Coast Council all limit detergents to quick-break products.
Pump Choice Matters Too
A centrifugal pump can shear oil into tiny droplets, which is why authorities ask for non-emulsifying pumps.
Sydney Water’s listed equipment pairs separators with specific pumps, mostly diaphragm types.
Matching Separator Output to Trade Waste Discharge Limits
Your separator only has to meet one number: your authority’s limit.
| Benchmark | Parameter | Limit |
|---|---|---|
| Sydney Water (2026-27) | Petroleum hydrocarbons (flammable) | 10 mg/L |
| SA Water vehicle washing guideline | Oil and grease | Up to 100 mg/L |
| WashBay HQ separator design | Total petroleum hydrocarbons | Under 10 mg/L, no visible oil sheen |
That design target gives you margin under most limits. Discharge to stormwater is a different question entirely, so check with your state EPA before planning it.
Pump Selection and Listed Equipment Requirements
- Use listed pairings. Sydney Water’s listed equipment pairs separators with specific pumps, mostly diaphragm types.
- Plan for compressed air. Air-operated diaphragm pumps need a reliable compressed air supply, so include the compressor in your design.
- Control with level sensors. Pumps should be switched by level sensors in the sump.
- Match the pump to the separator. The pump’s maximum flow must not exceed the separator’s rating.
Warning Signs Your Separator Is Undersized
Watch for these signs, especially on your busiest days.
- Visible oil sheen at the outlet or sampling point
- Failed trade waste samples, particularly after peak periods
- Sump high-level alarms or overflows during washing
- Sludge building up faster than your service interval allows
- Oily water backing up onto the wash bay floor after rain
If any of these show up, check your peak flow before blaming the separator.
Planning for Fleet Growth Without Oversizing
| Approach | What it costs you |
|---|---|
| Oversizing | Capital, footprint and servicing |
| Undersizing | Compliance |
| The middle path | Design the sump, pump and plumbing for where your fleet will be in a few years, then choose a separator for today’s peak plus a sensible margin |
Because WashBay HQ separators sit above ground, a second unit can be added later without breaking concrete.
Talk to Your Authority Before You Change Anything
Adding wash bays, extending hours or switching detergents can all affect your trade waste approval. Most authorities expect to hear about changes before they happen, not after a failed sample.
Sizing Your Separator in Three Steps
1. Discovery Call
Send us your pressure washer plates, bay dimensions, roof details and fleet numbers.
2. Proposal
We run the peak flow numbers and send a recommended model and quote within 24 hours.
3. Design & Approval
You get specifications and documentation ready for your trade waste approval.
