Car Wash Sand/Oil Interceptor Sizing & Design Criteria
Both manual and automated drive thru car wash facilities in Austin Water’s jurisdiction must install a Sand/Oil Interceptor (SOI). SOIs are key to keeping oil, grit, and other pollutants out of Austin’s wastewater system.
For information on the state requirements for car washes, refer to TCEQ’s Environmental Rules for Car Washes (RG-434) (Adobe PDF).
Do not purchase a SOI without obtaining a plan approval including an Industrial Waste approval letter. Purchasing an interceptor prior to Industrial Waste approval exposes the purchaser to risk of rejection and replacement costs.
Disclaimer: Plan sets vary greatly from one project to another. The information on this page is applicable to most, but not all projects, especially unique or highly complicated ones.
- Manual Car Wash SOI Sizing Criteria
To determine the minimum liquid holding capacity (in gallons) for the interceptor, multiply the gallons per minute (gpm) flow capacity by the standard 12-minute retention time. Depending upon availability, a 15% ± variance between the calculated volume and the interceptor's actual volume is acceptable.
MANUAL CAR WASH
(Example: Handheld Spray - Brush Wand)CAR WASH SAND & OIL/GREASE INTERCEPTOR SIZING MINIMUMS:
Number of Bays GPM
Interceptor Volume - Gallons
(Minimum Liquid Holding Capacity)
Note: Proposed sizes may be larger.
1 Bay Only 20
250
2 Bay 32
350
3 Bay 44
500
4 Bay 56
750
5 Bay 68
750
6 Bay 80
1,000
7 Bay 92
1,000
8 Bay 104
1,250
9 Bay 116
1,250
10 Bay 128
1,250
Single bay handheld spray or brush wand type vehicle washes shall install a 20-gallon per minute (gpm) interceptor minimum.
Multiple bay vehicle washes with handheld spray and/or brush wands must increase interceptor flow capacity by 12-gpm for each additional wash bay at the facility.
- Automatic Drive Thru Car Wash SOI Sizing Criteria
To determine the minimum liquid holding capacity (in gallons) for the interceptor, multiply the gallons per minute (gpm) flow capacity by the standard 12-minute retention time. Depending upon availability, a 15% ± variance between the calculated volume and the interceptor's actual volume is acceptable.
AUTOMATED DRIVE THRU CAR WASH
SAND & OIL/GREASE INTERCEPTOR SIZING MINIMUMS:
Number of Bays GPM
Interceptor Volume - Gallons
(Minimum Liquid Holding Capacity)
Note: Proposed sizes may be larger
May Use
1 Bay only 40
480
500
2 Bays 80
960
1,000
3 Bays 120
1,440
1,500
4 Bays 160
1,920
2,000
Automated drive thru car wash facilities must install an interceptor(s) with a minimum flow capacity of 40-gpm for each wash bay.
- SOI Design Requirements
Proposed SOIs designs must meet the following criteria:
- Plan sets should show a SOI.
- Your plan set should include a SOI specification sheet.
- The SOI must be constructed of concrete or non-corrodible materials.
If Concrete Materials:
- Example of concrete design that meets Industrial Waste’s (IW) requirements:
Conceptual Drawing of COA approved GI design. - Despite being Grease Interceptors (GIs), models on the Manufacturers List of Grease Trap/Grease Interceptors can be used.
- There are also many similar designs on the market.
If Non-Corrodible Materials:
- Non-corrodible materials are those that cannot be degraded by the contents or surrounding environment and won’t be easily damaged by chemical actions such as water or acid.
- Non-corrodible is different from non-corrosive. Non-corrodible materials won’t rust, decay, or erode, and should have a 15-20+ years lifespan or lifetime guarantee. This minimizes regulatory oversight or intervention.
- Examples of non-corrodible materials include thermoplastic, fiberglass, or stainless steel.
Minimum Volume and Flow Capacity
Proposed SOI designs must meet minimum volume and flow capacity shown in “Manual Car Wash (Handheld Spray/ Brush Wand) SOI Sizes” and “Automated Drive Thru SOI Sizes” boxes on this webpage.Sample port requirements: A sample port must capture the combined wastewater flow from the common water meter. It must be installed downstream of where all plumbing fixtures from the common water meter have combined (known as “end of pipe”). This includes all grease interceptor or SOI effluent and sanitary waste lines. For additional details, visit the Sample Port Requirements section of the Pretreatment FAQs page.
If you have any questions, contact the Industrial Waste staff by email at IndustrialWaste@austintexas.gov or by phone at 512-974-7293. Alternatively, customers with specific questions regarding a Site Plan, Building Plan Review, or existing pretreatment devices may directly contact the Pretreatment Specialist assigned to the project’s zip code.