Texas OSSF Design Flow (Chapter 285)
In Texas, wastewater design flow is determined using standard flow tables based on the number of bedrooms, dwelling size, and whether water-saving devices are used. These tables are defined in Chapter 285 and are used to size on-site sewage facilities (OSSF).
Under 30 TAC Chapter 285, the residential table includes separate rates for homes with and without water-saving devices. Typical single-family residential values are:
- 1 or 2 bedrooms: 225 gallons/day without water-saving devices, or 180 gallons/day with water-saving devices
- 3 bedrooms: 300 gallons/day without water-saving devices, or 240 gallons/day with water-saving devices
- 4 bedrooms: 375 gallons/day without water-saving devices, or 300 gallons/day with water-saving devices
- 5 bedrooms: 450 gallons/day without water-saving devices, or 360 gallons/day with water-saving devices
- 6 bedrooms: 525 gallons/day without water-saving devices, or 420 gallons/day with water-saving devices
- For larger dwellings, add 75 gallons/day without water-saving devices or 60 gallons/day with water-saving devices for each additional 1,500 square feet or increment thereof
The calculator defaults to the higher, more conservative column. Select the water-saving option only where that assumption is appropriate for the project.
Absorptive area calculation
Drainfield absorptive area is estimated using: Area = Design Flow / Soil Application Rate.
Soil classification significantly affects system size. Slower clay soils require larger areas, while sandy soils require less. This calculation is only the absorptive-area arithmetic; it does not determine site suitability, groundwater separation, restrictive horizon separation, flood hazard, slope suitability, reserve area, or local design requirements.