14.1 Onsite Sewage Treatment & Disposal: Septic Tanks & Drainfields
Key Takeaways
- The Florida onsite sewage rule is on the Plumbing Contractor reference list as Chapter 64E-6, F.A.C., July 16, 2018, and it may be brought into the examination as a bound PDF; the program has since transferred from the Department of Health to the Department of Environmental Protection and the rule is now numbered Chapter 62-6.
- Table I sets estimated residential sewage flow at 100 gpd for a 1-bedroom unit of 750 square feet or less, 200 gpd for 2 bedrooms at 751 to 1,200 square feet, 300 gpd for 3 bedrooms at 1,201 to 2,250 square feet and 400 gpd for 4 bedrooms at 2,251 to 3,300 square feet, adding 60 gpd for each additional bedroom or each additional 750 square feet.
- Table II sets minimum effective septic tank capacity from estimated daily flow, beginning at 900 gallons for flows through 300 gpd, then 1,050 gallons at 301 to 400 gpd, 1,200 gallons at 401 to 500 gpd, and 1,350 gallons at 501 to 600 gpd.
- Table III sets the maximum sewage loading rate to the absorption surface at 0.80 gallons per square foot per day for trenches and 0.60 for beds in slightly limited sandy soils, dropping to 0.65 and 0.35 in moderately limited soils.
- Minimum setbacks are 75 feet from a private potable well, 100 feet from a public drinking water well serving a facility at 2,000 gpd or less, 200 feet from a public well above 2,000 gpd, 50 feet from a non-potable well, 75 feet from surface water bodies, and 5 feet from building foundations and property lines.
Onsite Sewage Treatment & Disposal: Septic Tanks & Drainfields
Content Area E sub-topic 3 of the Florida blueprint is "Sewage Disposal Systems (Septic Tanks, Pumps, Lift Stations, and Interceptors)." The DBPR reference list makes plain how seriously the department treats it: reference R44 is Chapter 64E-6, Florida Administrative Code, Standards for Onsite Sewage Treatment and Disposal Systems, July 16, 2018, and it is one of only two references the department lets you bring in as a bound PDF. If you walk into the plumbing exam without that rule tabbed, you will lose points you could have looked up.
[!NOTE] A naming change worth understanding. Florida's Clean Waterways Act (Chapter 2020-150, Laws of Florida) transferred the Onsite Sewage Program from the Department of Health to the Department of Environmental Protection, and the rule chapter was renumbered from 64E-6 to 62-6, with the individual rules renumbered in parallel (64E-6.008 became 62-6.008, and so on). The DBPR reference list still names 64E-6, July 16, 2018, and that is the edition the examination answers are keyed to. Bring the named edition; know that a current permit application in the field will cite 62-6.
1. Step One: Estimate the Sewage Flow (Table I)
Everything downstream is sized from the estimated daily flow, and the rule supplies the number rather than letting you calculate one.
Residential
| Dwelling Unit | Estimated Flow |
|---|---|
| 1 bedroom with 750 sq ft or less of building area | 100 gpd |
| 2 bedrooms with 751 to 1,200 sq ft | 200 gpd |
| 3 bedrooms with 1,201 to 2,250 sq ft | 300 gpd |
| 4 bedrooms with 2,251 to 3,300 sq ft | 400 gpd |
| Each additional bedroom, or each additional 750 sq ft or fraction thereof | add 60 gpd |
| Other, per occupant | 50 gpd |
Four footnotes carry real weight:
- Footnote 4: where the bedroom count and the building area do not coincide, the criterion producing the greatest estimated flow applies. A 3-bedroom house of 2,600 square feet is sized as a 4-bedroom.
- Footnote 3: residential wastewater is calculated as 50 percent blackwater and 50 percent graywater.
- Footnote 6: residential estimates assume a maximum of two persons per bedroom; residential care facilities housing more add 50 gpd per additional occupant.
- Footnote 1: for food operations, kitchen wastewater is normally calculated as 66 percent of the total establishment wastewater flow.
Commercial (selected entries from Table I)
| Establishment | Estimated Flow |
|---|---|
| Restaurant operating 16 hours or less per day | 40 gpd per seat |
| Restaurant operating more than 16 hours per day | 60 gpd per seat |
| Restaurant using single-service articles only, 16 hours or less | 20 gpd per seat |
| Bar and cocktail lounge | 20 gpd per seat, add 15 gpd per pool table or video game |
| Office building | 15 gpd per employee per 8-hour shift or 15 gpd per 100 sq ft, whichever is greater |
| Hotel or motel, regular | 100 gpd per room |
| Resort hotel, camp, cottage | 200 gpd per room |
| Self-service laundry facilities | add 750 gpd per machine |
| Barber and beauty shops | 75 gpd per service chair |
| Doctor and dentist offices | 250 gpd per practitioner, plus 15 gpd per employee per 8-hour shift |
| Schools, day type | 10 gpd per student, plus 4 for showers, 4 for cafeteria, 15 per day school worker |
Footnote 2 is the interstate trap: systems serving high-volume establishments such as restaurants, convenience stores and service stations located near interstate-type highways require 3.0 times the Table I volumes.
2. Step Two: Size the Septic Tank (Table II)
| Average Sewage Flow (gpd) | Minimum Effective Septic Tank Capacity | Minimum Total Pump Tank Capacity (Residential / Commercial) |
|---|---|---|
| 0 - 200 | 900 gal | 150 / 225 |
| 201 - 300 | 900 gal | 225 / 375 |
| 301 - 400 | 1,050 gal | 300 / 450 |
| 401 - 500 | 1,200 gal | 375 / 600 |
| 501 - 600 | 1,350 gal | 450 / 600 |
| 601 - 700 | 1,500 gal | 525 / 750 |
| 701 - 800 | 1,650 gal | 600 / 900 |
| 801 - 1,000 | 1,900 gal | 750 / 1,050 |
| 1,001 - 1,250 | 2,200 gal | 900 / 1,200 |
| 1,251 - 1,750 | 2,700 gal | 1,350 / 1,900 |
| 1,751 - 2,500 | 3,200 gal | 1,650 / 2,700 |
Note the word effective — it is the liquid capacity below the outlet invert, not the gross shell volume.
Tank Construction Rules That Go With Table II
- Where multiple family dwelling units are jointly connected to one septic tank, the Table II capacity is increased by 75 gallons for each dwelling unit connected.
- All septic tanks shall be multiple chambered or shall be placed in series to achieve the required effective capacity.
- An approved outlet filter device is required, installed within or following the last septic tank or compartment before distribution to the drainfield, and placed for accessible routine maintenance. The requirement includes blackwater tanks but does not include graywater tanks, grease interceptors or laundry tanks.
- Graywater tanks: where a separate graywater tank and drainfield are used, minimum effective capacity is 250 gallons where the system receives not more than 75 gpd; above 75 gpd the capacity is the average daily flow plus 200 gallons for sludge storage.
- Where separate graywater and blackwater systems are used, the blackwater system may be reduced, but by no more than 25 percent, and the minimum capacity for a septic tank disposing of blackwater is 900 gallons.
- Tanks are permitted under a driveway only with traffic lids, and a concrete structure placed over a tank needs a soil or plastic barrier so it does not bond to the tank.
3. Step Three: Size the Drainfield (Table III)
The drainfield is sized from the estimated flow and the soil texture, using the maximum sewage loading rate to the absorption surface:
| USDA Soil Texture | Limitation (Percolation Rate) | Trench (gal/sq ft/day) | Bed (gal/sq ft/day) |
|---|---|---|---|
| Sand; coarse sand not associated with a seasonal water table less than 48 inches; loamy coarse sand | Slightly limited (less than 2 min/inch) | 0.80 | 0.60 |
| Loamy sand; sandy loam; coarse sandy loam; fine sand | Slightly limited (2-4 min/inch) | 0.80 | 0.60 |
| Loam; fine sandy loam; silt loam; very fine sand; very fine sandy loam; loamy fine sand; loamy very fine sand; sandy clay loam | Moderately limited (5-10 min/inch) | 0.65 | 0.35 |
| Clay loam; silty clay loam; sandy clay; silty clay; silt | Moderately limited (over 15 but not over 30 min/inch) | 0.35 | 0.20 |
| Clay; organic soils; hardpan; bedrock | Severely limited (over 30 min/inch) | Unsatisfactory for a standard subsurface system | Unsatisfactory |
| Coarse sand with estimated wet season high water table within 48 inches of the bottom of the proposed drainfield; gravel; fractured rock; oolitic limestone | Severely limited (less than 1 min/inch with a water table less than 4 feet below the drainfield) | Unsatisfactory for a standard subsurface system | Unsatisfactory |
Two rules attach to the table:
- Minimum absorption area is based on estimated sewage flows and Table III so long as estimated flows are 200 gpd or higher. Where estimated flows are less than 200 gpd, system size shall be based on a minimum of 200 gpd.
- The Department may require additional drainfield area for moderately limited soils and site conditions, but not more than twice the amount required for a slightly limited soil.
[!IMPORTANT] Look closely at that last severely-limited row. Coarse sand and oolitic limestone are rejected not because water moves too slowly but because it moves too fast — a percolation rate under 1 minute per inch with a water table less than 4 feet below the drainfield means effluent reaches groundwater before the soil can treat it. Much of coastal and south Florida sits on exactly that material, which is why so many Florida sites need a mound or filled system rather than a standard subsurface drainfield.
4. Setbacks — The List to Memorize
Systems established after the effective date of the rule shall be placed no closer than:
| Feature | Minimum Distance |
|---|---|
| Private potable well or multi-family water well | 75 feet |
| Public drinking water well serving a facility with estimated sewage flow of 2,000 gpd or less | 100 feet |
| Public drinking water well serving a facility with estimated flow more than 2,000 gpd | 200 feet |
| Non-potable water well | 50 feet |
| Storm sewer pipe | 10 feet to the maximum extent possible, never less than 5 feet |
| Design high-water line of a retention or detention area or swale holding water less than 72 hours, or a normally dry ditch | 15 feet |
| Surface water bodies (laterally) | 75 feet |
| Groundwater interceptor drains | 15 feet |
| Building foundations (including pilings for elevated structures), mobile home walls, swimming pool walls, property lines | 5 feet — and systems shall not be located under buildings |
| Water storage tanks in contact with the ground, or potable water lines | 10 feet, unless the lines are sealed within a sleeve for at least 10 feet or are Schedule 40 PVC or stronger; in no case within 24 inches |
Additional siting rules:
- Sidewalks, decks and patios are not subject to the 5-foot setback, but drainfields shall not be installed beneath them.
- Potable water lines within 5 feet of the drainfield shall not be at an elevation lower than the drainfield absorption surface.
- Unobstructed area: suitable, unobstructed land shall be available at not less than 1.5 times the required drainfield absorption area, contiguous to the drainfield, and in addition to the setbacks. The rule's own example: a 200-square-foot drainfield requires a 300-square-foot total unobstructed area.
- Fill material must be allowed to settle at least 6 months or be compacted to a density comparable to surrounding natural soil, and must be suitable slightly limited soil.
- Lot minimum under FS 381.0065(4)(a) is one-half acre (21,780 square feet) exclusive of paved areas, rights-of-way and surface water bodies, with flow allowances computed at 2,500 gallons per acre per day where a public water supply serves the lot.
5. Worked Sizing Example
Project: a new single-family home in a Florida subdivision on well water. Four bedrooms, 2,400 square feet of building area. Soil borings show fine sand, slightly limited, with a wet season high water table well below 48 inches. The drainfield will be a trench system.
Step 1 — Estimated sewage flow (Table I). Four bedrooms at 2,251 to 3,300 square feet: Check footnote 4: the bedroom row and the area row agree, so no upward adjustment.
Step 2 — Minimum effective septic tank capacity (Table II). 400 gpd falls in the 301-400 gpd row: The tank must be multiple chambered or two tanks in series, and an approved outlet filter is installed in or after the last compartment.
Step 3 — Drainfield absorption area (Table III). Fine sand is slightly limited; the trench loading rate is 0.80 gal/sq ft/day: Had the designer chosen an absorption bed instead of trenches, the rate drops to 0.60 and the area rises: Trenches win on area because effluent moves through the trench sidewalls as well as the bottom.
Step 4 — Unobstructed area. contiguous to the drainfield and outside all setbacks.
Step 5 — Check the setbacks. The system must sit at least 75 feet from the private potable well, 5 feet from the house foundation and property lines, 10 feet from any storm sewer pipe, and 75 feet from any surface water body. Any one of those failing sends the drainfield somewhere else on the lot — which is why the setback check belongs on the site plan before the tank is ever ordered.
A new Florida home has 3 bedrooms shown on the floor plan but 2,600 square feet of building area. What estimated sewage flow governs under Table I?
Under Table II of the Florida onsite sewage rule, what is the minimum effective septic tank capacity for a residence with an estimated flow of 500 gallons per day?
A drainfield is proposed in loamy sand, a slightly limited soil, for a home with an estimated flow of 320 gallons per day using trenches. What minimum absorption area does Table III require?
What is the minimum setback from an onsite sewage treatment and disposal system to a private potable well, and to a public drinking water well serving a facility with an estimated sewage flow of more than 2,000 gallons per day?