InterNACHI Septic cheat sheet

A conventional gravity system has four functional parts: a building sewer, a septic tank, a distribution box or drop box, and an absorption area (drainfield) in soil.

The septic tank provides two functions only - settling of solids and flotation of fats, oils, and grease (FOG) into a scum layer; digestion is partial and anaerobic.

Three tank zones: scum layer on top, clear (supernatant) liquid in the middle, sludge layer on the bottom; the outlet baffle draws from the clear zone.

A properly functioning tank retains the scum layer and sludge while allowing only relatively clear effluent to exit; baffles (inlet and outlet tees) are the devices that make this separation work.

A distribution box (D-box) divides effluent equally among drainfield laterals by gravity; it must be level, and equal splitting depends on all outlets being at the same elevation.

A pump chamber with a float- or timer-controlled pump is used where the drainfield is above or remote from the tank, or where timed dosing is required.

Gravity flow requires roughly 1/4 inch per foot (about 2 percent) slope on the building sewer and laterals; excessive slope causes liquid to outrun solids.

Alternative/secondary treatment units - aerobic treatment units (ATUs), media filters, and drip systems - add aeration or filtration before dispersal and require more frequent servicing than a septic tank alone.

Soil texture governs permeability; the three mineral fractions are sand (largest particles, fastest percolation), silt, and clay (smallest, slowest, most restrictive).

A percolation test measures the rate at which water drops in a test hole; a slow rate (high minutes per inch) means a larger absorption area is required, and a very slow rate may make the site unsuitable.

Percolation rate is expressed in minutes per inch (mpi) of water-level drop; loading rate is expressed in gallons per day per square foot (gpd/ft²) of trench bottom.

Required absorption area = daily design flow ÷ soil loading rate; daily design flow is commonly estimated from bedroom count (e.g., a typical two-bedroom house at roughly 150 gallons per bedroom per day).

Setbacks and separation distances are locally regulated; typical concerns include wells, surface water, property lines, foundations, and depth to a limiting condition such as bedrock or seasonal high groundwater.

A vertical separation distance (unsaturated soil between the trench bottom and a limiting layer) is required so effluent is treated before reaching groundwater or bedrock.

High groundwater, bedrock, fragipan, or a perched water table are limiting conditions; a site with insufficient separation is unsuitable for a conventional drainfield.

Slope matters: steeply sloping sites need special designs (serial/step loading, drop boxes, or mounds) because effluent migrates downslope in a level trench.

Groundwater flow direction and nearby wells or surface water determine whether the site can protect them; a drainfield should not be downslope and close to a drinking-water source.

A septic inspection is a limited, non-invasive visual inspection unless the tank is opened and pumped; the report must state exactly what was and was not examined.

Confined-space rules apply to tank interiors: a septic tank can accumulate hydrogen sulfide, methane, and oxygen-deficient atmospheres, so entry requires monitoring and is outside a standard visual inspection.

Before opening a tank, locate the lids, keep bystanders and vehicles off the drainfield, and never lean over an open tank; a fall into a tank is a fatality risk.

The inspection sequence: review records and homeowner history, check fixtures and drains for slow flow and gurgling, locate and inspect the tank, then inspect the D-box and absorption area for surfacing effluent.

Tank condition items to document: lid and riser condition, baffle/tee presence and integrity, cracks or deteriorated concrete, evidence of high water level, and scum/sludge thickness.

Effluent level relative to the outlet invert is diagnostic: a level at or above the outlet means the drainfield is not accepting flow (backup or saturated soil).

Drainfield inspection is surface-level: look for ponded or odorous effluent, unusually lush or dead vegetation in strips, and soft ground along lateral lines; probing is limited and must be described.

Report writing should record the date, weather, occupants, water use, method used, limitations, and photographic evidence; findings must distinguish observed conditions from inferences.

A septic inspection does not certify the system's future performance; the report should state that hidden components (buried laterals, tank interior if not opened) were not evaluated.

Recommended tank pumping interval: generally every 3–5 years for a typical household, adjusted by tank size, occupancy, and solids accumulation; measuring scum and sludge depth is the reliable trigger.

A common rule of thumb is to pump when the combined scum and sludge layers occupy roughly 25–30 percent of the tank's liquid depth.

Tank pumping must remove both the scum and the sludge, not just the liquid; leaving solids behind shortens the interval before the next pump-out.

Never flush wipes (even 'flushable' ones), feminine products, dental floss, or heavy FOG; these accumulate as solids or mat in the tank and drainfield.

Water conservation reduces hydraulic load: fix leaking toilets and faucets, spread laundry loads across the week, and avoid simultaneous heavy water use that surges the drainfield.

Protect the drainfield: keep vehicles, heavy equipment, and structures off the absorption area; do not plant trees or deep-rooted shrubs over laterals.

Divert roof drains, sump pumps, and surface runoff away from the tank and drainfield; added clear water overloads the absorption area and can saturate soils.

Do not use septic additives marketed to 'eliminate pumping'; biological additives do not remove accumulated solids, and some products harm the biological balance.

Surfacing effluent or ponded, odorous water over the drainfield is the classic sign of hydraulic failure of the absorption area.

Slow drains throughout the house, gurgling fixtures, and sewage backup into the lowest fixture indicate a full tank, blocked line, or saturated drainfield.

A high effluent level in the tank (above the outlet invert) points to a drainfield that is not accepting flow; a low level with backup points to a blockage or broken line.

Odors: hydrogen sulfide (rotten egg) near the tank is normal in small amounts; strong odor in the house suggests a dry trap, a broken vent, or a failing system.

Excessive sludge and scum, a missing or broken outlet baffle, and cracked tank walls are defects that let solids reach the drainfield and cause premature failure.

Root intrusion, crushed or offset laterals, and a settled D-box are common physical defects; a level D-box is required for equal distribution.

Remediation options range from pumping and baffle repair through drainfield restoration (resting, aeration, or replacement) to a new absorption area or alternative system.

A failed drainfield cannot be 'fixed' by pumping alone; pumping relieves the tank but does not restore the soil's acceptance capacity.

The most common cause of premature failure is hydraulic overload from excessive water use or solids carryover, followed by physical damage and root intrusion.

A septic inspection is a limited, non-invasive visual inspection unless the tank is opened and pumped; the report must state exactly what was and was not examined.

Confined-space rules apply to tank interiors: a septic tank can accumulate hydrogen sulfide, methane, and oxygen-deficient atmospheres, so entry requires monitoring and is outside a standard visual inspection.

Before opening a tank, locate the lids, keep bystanders and vehicles off the drainfield, and never lean over an open tank; a fall into a tank is a fatality risk.

The inspection sequence: review records and homeowner history, check fixtures and drains for slow flow and gurgling, locate and inspect the tank, then inspect the D-box and absorption area for surfacing effluent.

Tank condition items to document: lid and riser condition, baffle/tee presence and integrity, cracks or deteriorated concrete, evidence of high water level, and scum/sludge thickness.

Effluent level relative to the outlet invert is diagnostic: a level at or above the outlet means the drainfield is not accepting flow (backup or saturated soil).

Drainfield inspection is surface-level: look for ponded or odorous effluent, unusually lush or dead vegetation in strips, and soft ground along lateral lines; probing is limited and must be described.

Report writing should record the date, weather, occupants, water use, method used, limitations, and photographic evidence; findings must distinguish observed conditions from inferences.

A septic inspection does not certify the system's future performance; the report should state that hidden components (buried laterals, tank interior if not opened) were not evaluated.

Surfacing effluent or ponded, odorous water over the drainfield is the classic sign of hydraulic failure of the absorption area.

Slow drains throughout the house, gurgling fixtures, and sewage backup into the lowest fixture indicate a full tank, blocked line, or saturated drainfield.

A high effluent level in the tank (above the outlet invert) points to a drainfield that is not accepting flow; a low level with backup points to a blockage or broken line.

Odors: hydrogen sulfide (rotten egg) near the tank is normal in small amounts; strong odor in the house suggests a dry trap, a broken vent, or a failing system.

Excessive sludge and scum, a missing or broken outlet baffle, and cracked tank walls are defects that let solids reach the drainfield and cause premature failure.

Root intrusion, crushed or offset laterals, and a settled D-box are common physical defects; a level D-box is required for equal distribution.

Remediation options range from pumping and baffle repair through drainfield restoration (resting, aeration, or replacement) to a new absorption area or alternative system.

A failed drainfield cannot be 'fixed' by pumping alone; pumping relieves the tank but does not restore the soil's acceptance capacity.

The most common cause of premature failure is hydraulic overload from excessive water use or solids carryover, followed by physical damage and root intrusion.