Understanding Your Septic System

Subsurface Sewage Disposal Systems in New Hampshire

Septic System Terms

ISDS – Individual Sewage Disposal System – The components of the onsite septic system from the pipe leaving the structure thru to and including the leach field components. The new state term for onsite septic system.

EDA – Effluent Disposal Area (new state term for leach field) – The septic grade crushed stone or septic sand, dispersal components, vents and distribution box of the onsite septic system, which facilitates the dispersal of treated effluent into the natural receiving soil layer.

Dispersal Component – Disperses effluent fed from distribution box throughout septic grade crushed stone or septic sand. They are usually arranged in rows that are fed in parallel or series and spaced apart a distance that is specified by the manufacturer’s design manual. Some of the approved products are:

  • Perforated Pipe with 2 lines of holes at 5 & 7 o’clock position (Traditional system)
  • Leaching Tubes: 12” diameter perforated tube wrapped in media & filter fabric
    • 2 brands: Presby Environmental & GeoFlow
  • Chambers: Hollow cavity, open on the bottom, created out of concrete or plastic
    • Concrete that’s rated @ H20 can be driven over with cars
    • Plastic can not be driven over

Distribution Box (d-box) – Evenly distributes effluent from tank(s) to disbursal components via solid pipe. Usually made of concrete. Concrete needs to be rated H20 in order to be able to be driven over with cars.

Forcemain – 1 ½” or 2” pipe that delivers effluent from pump chamber to d-box. Pipe needs to be rated SDR 26 or 21 in order to be able to be driven over with cars.

Sewage Pipe – 4” or 6” pipe that delivers by gravity effluent from structure to septic tank and/or from septic tank to d-box. Pipe needs to maintain a 2% slope from structure to septic tank and a 1% slope from tank to d-box. Pipe needs to be rated SDR 26 or 21 in order to be able to be driven over with cars.

Pump Chamber (or Pump Station) – A unit housing an effluent pump and regulating floats that delivers effluent from the septic tank to the d-box. A visual/audio alarm should be mounted, in an easy to notice place, to alert owner when pump has a problem.

Septic Tank – A concrete or plastic watertight unit designed to allow solids to settle out of sewage from house (and be stored till pumped by sewage hauler) before effluent is sent to EDA.

“Aeration” or “Pretreatment” Tank – A tank in which effluent is brought into contact with air for the purpose of facilitating biological degradation so a cleaner effluent is delivered to EDA for dispersal. Usually used on lots where proposed replacement septic system design can not meet all current setbacks or near environmentally sensitive areas.

Baffles – Usually a “T” shaped PVC pipe placed at the inlet and outlet of the septic tank. At the inlet, it helps keep the incoming sewage from stirring up the solids that have already settled out in the septic tank. At the outlet, the “T” baffle should extend down 40% of the liquid depth. It prevents solids from going out to the leach field and clogging the soil interface which causes field failure. Baffles are probably the most important part of your system. You should have them checked every time your tank is pumped.

Vents – They are needed when a field is pumped to or if a field has more than 18” of cover. It’s very important for the bacteria in the field to get plenty of air so they can thrive and break down the effluent as it is dispersed into the soil. The more air the better the bacteria thrive and the less likely your field will fail prematurely. Vents can be run remotely up the side of your house, fence, pole, etc. Decorative vents are also available that look like granite posts, etc.

“Ejector pump” or “Grinder pump” – Usually used to deliver sewage from a basement bathroom to a gravity sewer to the septic tank.

Test Pit – A hole dug by a backhoe or excavator that is large enough for a septic designer to get into and log the color, structure, texture, consistency, seasonal high water table, observed water, restrictive soils and/or ledge.

Perc Test – A hole dug to a depth below the topsoil but above the seasonal high water table to determine how many minutes it takes for an inch of water to percolate into the soil. (Measured in Minutes/Inch)

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