Car Wash Sand/Oil Interceptor Sizing & Design Criteria

Both manual and automated drive-through car wash facilities in Austin Water's jurisdiction must install a sand and oil interceptor (SOI). SOIs keep oil, grit and other pollutants out of Austin's wastewater system.

For state requirements, see TCEQ's Environmental Rules for Car Washes — RG-434 (PDF).

Note: Do not purchase a SOI before obtaining plan approval and an Industrial Waste approval letter. Purchasing an interceptor before approval creates risk of rejection and replacement costs.

Plan sets vary from project to project. The information on this page applies to most projects but may not cover unique or highly complex situations. For questions, email IndustrialWaste@austintexas.gov or call 512-974-7293. For questions about a specific site plan, building plan review or existing pretreatment device, contact the Pretreatment Specialist assigned to your zip code (PDF).

Manual Car Wash SOI Sizing

To determine the minimum liquid holding capacity in gallons, multiply the flow capacity in gallons per minute (gpm) by the standard 12-minute retention time. A 15% variance (±) between the calculated volume and the interceptor's actual volume is acceptable depending on availability. 

 

Single-bay handheld spray or brush wand car washes must install a minimum 20-gpm interceptor. Multi-bay facilities must increase flow capacity by 12 gpm for each additional wash bay.

 

Manual car wash — minimum interceptor sizes (handheld spray or brush wand)

Number of Bays	Flow (GPM)	Minimum Liquid Holding Capacity (gallons)
1	20	250
2	32	350
3	44	500
4	56	750
5	68	750
6	80	1,000
7	92	1,000
8	104	1,250
9	116	1,250
10	128	1,250

Proposed sizes may be larger than the minimums shown.

Automated Drive-Through Car Wash SOI Sizing

The same 12-minute retention time formula applies. Automated drive-through facilities must install an interceptor with a minimum flow capacity of 40 gpm per wash bay.

Automated drive-through car wash — minimum interceptor sizes

Number of Bays	Flow (GPM)	Minimum Liquid Holding Capacity (gallons)	Acceptable Size (gallons)
1	40	480	500
2	80	960	1,000
3	120	1,440	1,500
4	160	1,920	2,000

Proposed sizes may be larger than the minimums shown.

SOI Design Requirements

Proposed SOI designs must meet the following criteria:
 

• Plan sets must show the SOI and include a SOI specification sheet
• The SOI must be constructed of concrete or non-corrodible materials

Concrete Materials

A conceptual drawing of a City of Austin-approved grease interceptor design (PDF) is available for reference. Models on the Manufacturers List of Grease Traps and Grease Interceptors (PDF) may also be used despite being classified as grease interceptors rather than SOIs. Many similar designs are available on the market.

Non-Corrodible Materials

Non-corrodible materials cannot be degraded by their contents or the surrounding environment and will not be easily damaged by chemical action such as water or acid. Note: Non-corrodible is not the same as non-corrosive. Non-corrodible materials will not rust, decay or erode and should have a lifespan of 15–20 years or a lifetime guarantee. Examples include thermoplastic, fiberglass and stainless steel.

 

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 — the "end of pipe" location. This includes all grease interceptor or SOI effluent and sanitary waste lines. See the Pretreatment Requirements and Resources page for additional details.