How can we help

We can find a solution for your needs

  • By clicking send you agree to Privacy Policy

  • This field is for validation purposes and should be left unchanged.
1300 669 036 Get in touch

The Smart Water People

water | wastewater | sewage

Best Practices for Vehicle Washbay Pit Design

April 23rd, 2024


In industrial settings, particularly in facilities where vehicles are regularly washed, the management of wastewater is a critical concern. A vehicle washbay wastewater processing system typically comprises a sump or pit to collect the wastewater, an OWS to remove hydrocarbons, and, if water is to be recycled, a multi-media filtration system and disinfection system may also be installed.

An improperly designed wash bay system which lets too much solid material enter the Oil and Water Separators (OWS) can negatively affect the performance and efficacy of the treatment system and potentially damage hardware. Without proper treatment, contaminants such as oil, grease, and chemicals can enter waterways, posing environmental risks and potentially leading to regulatory violations.

Wash Bays or Wash Down Facilities include, but are not limited to facilities to wash down:

  • Mining and construction equipment including earthmovers, dump trucks, graders and other heavy vehicles.
  • Rail, aircraft and boat washes
  • Commercial car and truck wash facilities


The wastewater from the wash bay will contain different contaminants depending on the type of equipment that is being washed down, and where the equipment is being used. Typically vehicle washbays will be encountering a mix of suspended solids – grit, dust, metallic particles, etc., and hydrocarbon contaminants – petrol and diesel fuel residues, oils, grease, hydraulic fluid, etc.

Oil and Water Separators (OWS) are a major component of a vehicle or equipment wash bay system. The primary function of an OWS is to remove hydrocarbons from a wastewater stream. They can also remove some suspended solids from the water, however, this is not their primary function. Solids removal is the primary function of the sump or wastewater pit.

MAK Water OWS during factory testing

The pit or sump is a critical component of vehicle washbay infrastructure, serving as the primary collection point for wastewater generated during the washing process. Proper design is essential to ensure efficient capture and containment of contaminants, preventing their release into the environment.


Understanding the Importance of Washbay Pit Design

The design of the pit is a critical consideration in engineering a vehicle washbay system at it serves as the primary collection point for wastewater generated during the washing process. Proper design is essential to ensure efficient capture and containment of contaminants, preventing their release into the environment., as well as promoting optimised function of the OWS and other downstream water treatment hardware. When it comes to oil-water separator systems, the design of the pit plays a pivotal role in enhancing the effectiveness of the separation process.

Poor solids settling in the wash bay system can result in overloading the solids capacity of the Oily Water Separator, leading to ineffective water treatment and equipment malfunctions.

Why a wedge pit?

The shape of wedge pits promotes efficient settling of solids and sediments, minimising the debris and sediment making its way to the OWS, reducing the frequency of maintenance and enhancing performance.

Wedge pit designs are often more cost-effective compared to underground storage tanks or concrete chambers. The relatively simple construction requirements of wedge pits results in lower upfront costs, making them an attractive choice for facilities with ample space looking to implement efficient wastewater management solutions without breaking the budget.

Wedge pits allow easy access for inspection, cleaning, and routine maintenance tasks including sludge removal, debris clearance, and inspection of internal components, ensuring optimal performance of the oil-water separator system over time.


3-Stage Wedge Pit Washbay Sump Design

In an ideal setup, MAK Water recommends using a 3-stage Wedge Pit design.


Step 1: Drive in sump

The wedge pit / drive in sump catches the readily settleable fraction ( >2000µm particles) These are the large heavy particles, such as rocks & gravel, nuts, bolts, etc. This pit needs to be periodically de-sludged. For fast & easy maintenance, ideally the wedge pit should be sized and graded to suit driving in with a loader to regularly remove the build-up of solids.

The best designed wedge pits include inlet baffles or flow control mechanisms to ensure uniform distribution of wastewater across the width of the pit.

Baffled entry to pit helps evenly spread suspended solids across the pit’s area.

Step 2: Sedimentation sump

From the Drive-in Sump, the wastewater overflows into the subsequent sedimentation sump. As hydrocarbons float on water, they move freely over the barrier throughout the system. Another advantage of the wedge pit system is that pumps that would emulsify the hydrocarbons and impact their removal are eliminated.

The sedimentation sump will catch and settle the smaller ⪆200µm fraction of particles which take longer to settle to the bottom. This sump also needs to be periodically de-sludged, though typically much less frequently than the Drive-in Sump.

Area is the key component for this sump’s dimensions as its goal is to maximise the settling of finer particles. Primary sizing parameters are Surface Overflow Rate (SOR), Specific Gravity (SG) and Settling Velocity(SV). Assumptions are usually made as the solids particle size distribution and SG’s are rarely known.

Depth of the pit is somewhat important as it relates to how much sludge can be stored in this sedimentation tank before it becomes an issue and also to provide a workable volume for the subsequent OWS Feed sump.

Step 3: Skimmer sump

Last step is the skimmer sump where depth is the key factor to suit a sensible pump out frequency. For any given OWS, the minimum recommendation is for 30-minute cycles.

The Skimmer Sump is the final sump, where water is then pumped to the oily-water treatment system for the separation of oil and water.

Again, it is critical to let the water overflow from the sedimentation pit into the skimmer sump as the solids settles down and the hydrocarbons float.

You can see the overflow baffle during the installation of this washbay recycling system.

By this stage, the waste water should be very low in solids and the skimmer would then pump the oily water to the OWS treatment system, maximising the separation.


Common 2-Stage Setup:

Most sites have existing wash bay pits/sump setups which are a 2 stage process. Overflow is still crucial for the hydrocarbons to flow freely into the skimmer while settling as much of the solids as possible.


In this case, the sedimentation pit isn’t present, thus more fine particles will make their way to the skimmer sump and to the OWS and other treatment hardware.

This is a common setup that is still manageable, however;

  1. The drive-in sump will need more frequent cleans.
  2. The skimmer sump will also act as the sedimentation sump and will require more frequent cleaning.
  3. MAK Water recommends setting the low-water level sensor at a higher level to provide some space between the greater sludge build up and the lower skimming point.
  4. Finally ,the hoppers at the bottom of the OWS should be drained frequently to avoid build up of solids in the system.


Understanding Oil-Water Separators:

MAK Water Oil-Water Separators are designed to remove free floating hydrocarbons such as petrol, diesel & other fuel residues, hydraulic fluid, oil, grease as well as heavier suspended solids from a wastewater stream. The OWS utilises the principle of gravity separation, allowing lighter substances like oil to rise to the surface while heavier solids settle at the bottom. The separated oil & grease is then skimmed off, and the treated water is discharged or further processed as necessary, solids sludge can be drained off via a valve in the bottom of the unit body.

Follow this link for further information on our OWS.


The OWS can deal with suspended solids, however, this is not the primary function of the unit. Well designed and maintained pits will limit the amount of solids going through the system and enhance the overall efficiency of the OWS.



Effective management of vehicle washbay wastewater is crucial for environmental stewardship and regulatory compliance in industrial settings. By incorporating best practices in washbay wedge-pit design, facilities can enhance the performance and efficiency of oil-water separator systems, ensuring the effective removal of hydrocarbons before discharge. Proper sizing and consideration of gravity separation requirements, maintenance are key factors to consider when designing washbay wedge-pits for optimal integration with oil-water separator systems. By adhering to these best practices, facilities can mitigate environmental risks and uphold their commitment to sustainable wastewater management.

Case studies

There have been over 4,000 MAK Water Clearmake Oil-water separator systems manufactured in Australia and delivered to our customers. Follow this link to find a selection of case studies of notable MAK Water OWS projects.


About the Author

Ludo Fol

Ludo Fol is Technical Sales Manager – Western Australia for MAK Water. Ludo has gained broad experience across sales, customer service, general management and finance gained across the water, manufacturing and banking industries in Australia and France. Email him at, or connect with Ludo on LinkedIn here.