14.1 Onsite Sewage Treatment & Disposal: Septic Tanks & Drainfields

Onsite Sewage Treatment & Disposal: Septic Tanks & Drainfields

Onsite sewage systems require a separate flow, tank, soil and siting analysis. This section retains worked table exercises for learning; the current permit rules and any approved local-exam reference edition must be checked separately. A state contractor reference list does not authorize a local journeyman candidate to bring a bound rule PDF.

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1. Supplied Design Exhibit A: Flow Selection

The numerical tables in Exhibits A–C are provided training-design criteria. They are not represented as the current adopted Tables I–III of Rules 62-6.008 and 62-6.013. Actual design needs the current permit-specific flow, tank, treatment and soil requirements. Here the learning task is to choose the controlling row, preserve units and apply the given conditions consistently.

For this exercise, choose the greater residential flow produced by the bedroom count and building-area band. Do not average conflicting rows.

Additional exercise assumptions are: residential flow is split 50% blackwater/50% graywater; the base residence allowance assumes up to two occupants per bedroom and the provided care-facility case adds 50 gpd per additional occupant; a food-operation calculation assigns 66% of the total to kitchen flow. These values are supplied only when that case asks for the corresponding split; they are not current-law assertions.

The following commercial schedule is also a provided exercise schedule, not a permit table. Use the listed units and conditions before multiplying.

For a designated high-volume interstate exercise only, apply the provided 3.0 multiplier after selecting the base establishment rate. For example, a 20-seat restaurant operating 12 hours uses 20 × 40 = 800 gpd before that multiplier and 2,400 gpd with it. Do not apply this exercise modifier to an actual establishment without its governing approved criteria.

2. Supplied Design Exhibit B: Effective Tank Capacity

For this training design, use the following effective-capacity and pump-tank schedule. A boundary value belongs to its inclusive row: 500 gpd is in 401–500, not 501–600.

Effective capacity is the specified operating liquid volume, not the tank's gross exterior volume. The provided construction specification uses multiple chambers or tanks in series and an accessible approved outlet filter before the drainfield. Product approval, structural loads, access and actual permit provisions remain separate checks.

Optional exercise conditions, when stated: jointly connected dwelling units add 75 gallons per unit to the scheduled capacity; a separate graywater case uses 250 gallons through 75 gpd and daily flow + 200 gallons above that; a blackwater reduction is capped at 25% and cannot reduce the exercise's 900-gallon minimum. These are supplied problem rules, not claims about the currently adopted tank rule. Do not apply one optional condition to a case that has not specified it.

Traffic support is a separate issue. Current Rule 62-6.005 addresses traffic lids and preventing bonding where concrete structures are placed over tanks. A traffic-rated lid does not automatically authorize an otherwise unsuitable tank, drainfield or access arrangement.

3. Supplied Design Exhibit C: Absorption-Area Analysis

The training site's approved soil report supplies the following illustrative loading schedules. They do not establish a statewide soil classification from appearance or a single percolation observation.

For these exercises, use a provided minimum sizing flow of 200 gpd where estimated flow is lower. Required area equals sizing flow divided by the applicable loading rate. Round upward where the design selects whole square feet. A lower loading rate increases required area; using a bed rate for a trench or multiplying flow by rate gives the wrong result.

Soil evaluation also considers the seasonal water table, limiting layers, groundwater movement and treatment opportunity. Very rapid infiltration is not automatic approval: insufficient separation or treatment can make a site unsuitable. A rejected site has no valid conventional-system area merely by choosing the smallest number in the table. Obtain an approved alternative design; a mound or fill is not automatically a cure for every limitation.

4. Current Siting Provisions and Their Conditions

The following summarizes the ordinary siting provisions in Rule 62-6.005, effective June 8, 2026. It is not an exhaustive exception analysis. Existing-lot, statutory surface-water and other expressly authorized provisions require their own eligibility review; the worked new-lot example below assumes none of those exceptions.

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, inclusive of the drainfield itself, contiguous and outside the setback areas. 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.
• Keep three area concepts distinct. Rule 62-6.005(7)(a)'s specified lot route starts at one-half acre (21,780 ft²), with dated-lot alternatives and conditions. Subdivision net density and authorized flow based on net usable lot area have their own exclusions; do not subtract every paved surface from the lot-size test by assumption.
• Under the applicable net-usable-area flow provision, the allowance is 2,500 gallons/acre/day for the specified public-water category and 1,500 gallons/acre/day for private-well, limited-public, cistern and other stated categories. The rule excludes paved/prepared roadbeds within public rights-of-way or easements and surface-water bodies from that flow area, with its specified optional treatment of qualifying contiguous unpaved/uncompacted rights-of-way. Calculate the correct area and water-supply category before comparing design flow.
• Existing lots and alternative treatment routes have additional conditions. Do not transfer a historical-lot well-distance exception to an ordinary new-lot case, or treat a flow-area allowance as a substitute for drainfield sizing and setbacks.

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5. Worked Sizing Example

Training project: a new single-family home on private well water, with the supplied Exhibits A–C approved for this hypothetical design and no siting exception invoked. Four bedrooms, 2,400 square feet of building area. The supplied site report approves Schedule A and independently confirms the required seasonal-water-table and limiting-layer conditions. The drainfield will be a trench system.

Step 1 — Estimated sewage flow (supplied Exhibit A). Four bedrooms at 2,251 to 3,300 square feet:

$$Q = \mathbf{400\text{ gallons per day}}$$

Apply the supplied greater-flow condition: the bedroom row and the area row agree, so no upward adjustment.

Step 2 — Effective septic tank capacity (supplied Exhibit B). 400 gpd falls in the 301-400 gpd row:

$$V_{tank} = \mathbf{1,050\text{ gallons minimum effective capacity}}$$

The supplied construction specification requires the stated chamber/series arrangement and outlet filter; actual permits require current rule and product review.

Step 3 — Drainfield absorption area (supplied Exhibit C). Schedule A supplies a trench loading rate of 0.80 gal/sq ft/day:

$$A = \frac{400\text{ gpd}}{0.80\text{ gal/sq ft/day}} = \mathbf{500\text{ square feet of trench absorption area}}$$

Had the designer chosen an absorption bed instead of trenches, the rate drops to 0.60 and the area rises:

$$A_{bed} = \frac{400}{0.60} = \mathbf{667\text{ square feet}}$$

The difference here comes from the two supplied design loading rates. Do not independently credit extra sidewall area or assume a trench is always the appropriate site design.

Step 4 — Unobstructed area.

$$A_{unobstructed} = 1.5 \times 500 = \mathbf{750\text{ square feet}}$$

inclusive of the 500-ft² drainfield, contiguous and outside all setbacks; this is 750 ft² total, not 500 + 750.

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 storm sewer piping to the maximum extent possible and never less than 5 feet, and the ordinary 75-foot surface-water setback where no stated exception applies. A storm sewer distance of 8 feet does not by itself prove a mandatory relocation: document whether the maximum feasible separation and the absolute minimum are satisfied. Resolve every applicable siting condition before ordering the system.