Car Wash Sand/Oil Interceptor Sizing & Design Criteria

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 webpage.

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.