Transport Depot Wash Down: Sizing the Oil Water Separator for Heavy Vehicles
A truck wash oil water separator for an Australian transport depot is a 3000L coalescing plate unit in most cases, with a 2000L unit only where one bay washes a handful of rigids a day on cold water. The deciding number is peak flow reaching the separator from the pit pump, not the size of the slab or the number of trucks on the books. Silt load is the second deciding number, and on a depot it usually causes more trouble than the oil.
Most depot separators we see failing were sized for a car wash. Prime movers are not cars.
What size separator a transport depot wash bay needs
A transport depot wash bay needs a separator sized to the flow of every lance that can run at once, stepped up for hot water, detergent and heavy silt. In the Kwikflo KCPS range that puts almost every depot on the 2000L or 3000L model. The 1000L and 1500L units belong on light vehicle bays and workshops, not on a bay where B doubles get washed.
Depot wash setup versus separator size
| Depot wash setup | Typical load | Model | Price (NSW, VIC, QLD, ACT) |
|---|---|---|---|
| Single bay, one lance, rigid trucks and vans, cold water, a few washes a day | Moderate flow, moderate silt | KCPS-2000 | $8,626 |
| Single bay, prime movers and trailers, hot water pressure washer, detergent | High flow, heavy silt | KCPS-3000 | $9,699 |
| Two or more bays, or one bay washing most of the day | Near continuous flow | KCPS-3000, and talk to us about splitting the load | From $9,699 |
| Depot workshop floor drains only, no vehicle washing | Low flow, drips and spills | KCPS-1500 | $6,720 |
Prices current as of October 2026, AUD including GST, delivered to NSW, VIC, QLD or ACT. The KCPS-3000 is $10,199 delivered to SA and $10,599 delivered to WA. Confirm the flow rating with your hydraulic consultant and water authority before ordering.
On the fence between two sizes? Go up. A depot never washes fewer trucks next year.
How to work out peak flow for a truck wash
Peak flow for a truck wash is the combined flow of every lance or wash gun that can run at the same time, capped by the pump that lifts water from the collection pit into the separator. Add up the rated flow on the pressure washer plates, not the brochure for the wash chemical. Then look at the pit pump, because the separator only ever sees what that pump delivers.
A worked example, using typical numbers. One hot water commercial pressure washer is commonly rated somewhere around 15 to 21 litres a minute. Two of them on a two bay pad, both running on a Friday afternoon, is roughly 30 to 42 litres a minute arriving at the pit. Add a low pressure rinse hose and you are past 50. Your consultant will use the actual plate ratings, but that order of magnitude is why depots end up on the big unit.
The collection pit is the buffer between the lances and the separator. A KCPS unit is fed through a 32mm inlet by a Kwikflo diaphragm pump switched by a float in the pit, so the pit needs a minimum 500L working volume or the pump short cycles while the bay is busy. On a two bay depot, more pit volume is cheap insurance.
The general sizing logic for every site type sits in our guide to which size oil water separator your site needs.
Why heavy vehicles are harder than cars
Heavy vehicles are harder on a separator than cars because they bring more water, more silt and heavier oils in every wash. A prime mover and trailer has many times the surface area of a sedan, so each wash runs longer at full flow. The chassis carries clay, gravel and road grime off unsealed yards and quarry roads. And the oil is not just a few sump drips: it is diesel, hydraulic oil off tipper rams and cranes, and fifth wheel grease that packs onto the plates.
Fifth wheel grease deserves a special mention. It is thick, sticky and arrives in lumps once a degreaser hits it. Some floats to the top in the inlet chamber as it should. Some sticks to the plate pack and stays there until somebody washes it off. Depots that degrease fifth wheels on the wash pad every week should expect to clean plates more often than the manual suggests.
Hot water and detergent add another layer. Both emulsify oil into fine droplets that take longer to reach a plate and merge, which is why a heated wash bay sits one size up from a cold one with the same flow.
Silt is the real depot problem
Silt, not oil, is what stops a depot separator working. Mud off a truck chassis lands in the sludge hopper far faster than oil reaches the drum, and once silt coats the coalescing plates the water short circuits past them. The separator then sends oily water to sewer while looking perfectly normal from the outside.
Every depot washing trucks that leave sealed roads needs a silt trap or settling pit upstream of the pit pump. Not optional. Size it generously and get it pumped out before it is full, because a silt trap that has filled up is just a pipe.
On the separator itself, the sludge hopper drains through a 50mm PVC ball valve. Drain it on a schedule set by what comes off the trucks: a quarry or earthmoving fleet may need it weekly, a linehaul fleet running on bitumen goes much longer. Our oil water separator maintenance guide covers the full routine.
What the KCPS-3000 depot package includes
The KCPS-3000 ships as a floor mounted package with the separator tank, galvanised floor mount frame, waste oil drum, Kwikflo diaphragm pump and an oil proof MAC3 float switch. Treated water leaves through a 50mm outlet to sewer, collected oil goes through a separate 50mm outlet into the waste oil drum, and sludge drains from the hopper through its ball valve. Oil skimmers can be added for manual or automatic oil removal.
The tank is available in polyethylene or stainless steel. Polyethylene is chemical resistant, UV stabilised and 5mm thick on both tank and lid, and it suits the large majority of depots. Stainless is for aggressive chemistry or a specification that demands it.
One thing the KCPS-3000 does not include, on purpose: a junction box or control panel. Those get specified separately to suit the site electrical design and the hazardous area rating. Budget for them and for the electrician.
Plate packs full of sludge can weigh up to 30kg. On a depot servicing its own separator, that means two people or a lifting aid, not the apprentice on their own at 5pm.
Multi bay depots and splitting the load
A depot with two or more truck bays washing at the same time should be designed around the combined peak flow, and that may mean more than one separator rather than one bigger tank. The KCPS-3000 is the largest unit in the Kwikflo coalescing plate range. If your consultant's peak flow figure is beyond what one unit is rated for, the usual answer is separate pits and separators per bay or pair of bays, which also means one unit can be serviced while the other keeps the depot washing.
Ring us with the numbers before you commit to a slab layout. Moving a pit after the concrete is poured is the most expensive conversation in this whole process.
Site rules that decide whether a depot install passes
Four site requirements decide whether a depot separator gets signed off, and they matter as much as the separator model.
Bund for the full separator volume and keep rain out. A depot wash pad open to the sky collects stormwater, and a summer storm will push oily water straight through any separator. Roof the pad, or grade it so rain off the yard never reaches the wash drain. Stormwater belongs in the stormwater system, not your trade waste line.
Treat the separator as a Class 1, Zone 2 hazardous area. The zone extends 500mm above the unit and sets the rating of every electrical item near it. The detail is in our oil water separator hazardous area guide.
Use oil proof float switches. The MAC3 float supplied with the KCPS-3000 is oil proof. A cheap standard float swapped in later will fail and lie about the level.
Give the discharge pit at least 500L of working volume. On a busy depot, more.
Water authorities set the oil and grease limit in your trade waste consent, and the conditions vary between authorities. Your hydraulic consultant or licensed plumber will confirm what applies to your site. If you need help with the whole drainage package, we offer engineering design support, and the pit pump side is covered in our water pumps range.
Frequently asked questions
What size oil water separator does a truck wash need in Australia?
Most Australian truck washes need a 3000L coalescing plate oil water separator such as the Kwikflo KCPS-3000, with a 2000L unit suitable only for a single cold water bay washing a few rigid trucks a day. The size is set by peak flow from the lances that can run at once and the pit pump feeding the separator, then stepped up for hot water, detergent and heavy silt.
Does a transport depot wash bay need an oil water separator?
Yes, a transport depot wash bay discharging to sewer needs pre-treatment, and an oil water separator is the standard way to meet the oil and grease limit in the trade waste consent. The local water authority sets the limit and approval conditions, and a hydraulic consultant or licensed plumber confirms the flow rating and configuration for the site.
How much does a truck wash oil water separator cost?
A Kwikflo KCPS-3000 truck wash oil water separator costs $9,699 delivered to NSW, VIC, QLD or ACT, $10,199 to SA and $10,599 to WA, in AUD including GST as of October 2026, and the KCPS-2000 costs $8,626. The KCPS-3000 price includes the tank, frame, plate pack, diaphragm pump, MAC3 float and waste oil drum, but not the junction box, control panel, pit, bund or site works.
Do I need a silt trap before a depot oil water separator?
Yes, any depot washing trucks that drive on unsealed yards, quarry roads or construction sites needs a silt trap or settling pit upstream of the separator. Silt from heavy vehicles fills the sludge hopper and coats the coalescing plates far faster than oil does, and coated plates let oily water pass straight to sewer.
Can one separator handle two truck wash bays?
One KCPS-3000 can handle two truck wash bays only if the combined peak flow of both bays, as calculated by the hydraulic consultant, sits within the separator's rated flow. Where the combined flow is higher, the usual design is a separate pit and separator per bay or pair of bays, which also lets one unit be serviced while the other keeps working.
Why does the KCPS-3000 not include a control panel?
The KCPS-3000 is supplied without a junction box or control panel because both must be specified to suit the site electrical design and the Class 1, Zone 2 hazardous area rating around the separator. A licensed electrician specifies and installs them as part of the site works.
Send us your depot details
Tell us how many bays you run, what you wash (rigids, prime movers, tippers, buses), whether you use hot water or detergent, how many lances can run at once, whether the trucks leave the bitumen, and which water authority you deal with. We will come back with a model, configuration and delivered price.
Talk to us about your depot wash bay
For the broader picture on car and light vehicle bays, see our wash bay oil separator guide. Fleet operators and contractors fitting out more than one site should ask about trade pricing.
This guide is general information for Australian commercial and industrial sites and is not a substitute for advice on your specific installation. Discharge limits, separator sizing and approval conditions are set by your local water authority through your trade waste consent. Hazardous area classification, electrical work and connection must be carried out by suitably qualified people. Confirm the required flow rating and configuration with your hydraulic consultant or licensed plumber before you order. Prices and specifications are current at the date of publication and change without notice.