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Topic Closedleach field vs dry well

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nashva View Drop Down
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Location: Nashua, NH USA
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Direct Link To This Post Topic: leach field vs dry well
    Posted: 09-Jun-2004 at 8:13am
When I bought my house approx 5 years ago. I was told it had a septic tank with a leach field. I recently had my septic pumped (as part of the agreement on the sale of the home) and the new owners were told it has a dry well. I called the NH State Offices to ask for information about the installation of my system. The person I spoke to at the State level told me that a dry well is actually a vertical leach field. The City Health Officer said the same. Is this true? I have had no issues with the septic system since I purchased the home.
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DUNBAR View Drop Down
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Direct Link To This Post Posted: 09-Jun-2004 at 9:30am
John Aldrich on this website will be able to answer your question. Very familiar with these.

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"Your best interest is secured by making the right decisions the first time."

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John Aldrich View Drop Down
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Direct Link To This Post Posted: 09-Jun-2004 at 10:29am
Hey Dunbar, you are putting quite a bit of pressure on me to perform. :>O

nashva, a leach field is a leach field, a dry well is a dry well, and they are both physical structures for the dispersal of septic tank effluent into the soil. A dry well is sometimes referred to as a seepage pit.

Typical Leach Field:
The typical leach field consists of an excavated trench or bed that is relatively shallow, and contains 12 to 18 inches of gravel. Typically the effluent is applied over the entire surface of the gravel through the use of perforated leach field pipes. The septic tank effluent trickles down through the gravel and is applied to the surface of the soil on the bottom of the trench or bed. As this nutrient rich effluent, which also contains suspended solids that have escaped the septic tank, is applied to the soil surface, a clogging mat begins to grow. This clogging mat consists of biological slimes, colonies of anaerobic microorganisms, fungi, and an iron compound called Ferric Sulfide.

The thickness of the clogging mat is the mechanism that controls the percolation rate of the effluent into the soil below. At some point in time the thickness of the clogging mat grows to the point where the percolation rate of the effluent through it is less than the application rate of the septic tank effluent. When that point is reached, the leach field fails by either surfacing or by backing up into the septic tank or into the house.

So, the key to a successful leach field is to have the ability to control the thickness of the clogging mat. The best way to achieve this control is to discontinue application of septic tank effluent. Typically, most septic tank/soil absorption systems consist of only one leach field, so when that single leach field fails, construction of a new leach field is required. If the soil absorption system is initially designed and installed with two leach fields, with fifty percent of the required leach field capacity in each field, and a diversion valve, then the effluent flow can be alternated between the two fields on an annual basis merely by turning the valve. Alternation of flow allows half of the leach field to rest while the other half is performing its intended function.

The resting field becomes unsaturated and air replaces the saturated anaerobic condition in the clogging mat. The oxygen in that air oxidizes the Ferric Sulfide into another iron compound called Ferrous Sulfate. Ferric Sulfide is that black slimy material one finds at the bottom of the leach field. When it is oxidized, it changes state and becomes a permeable granular material similar in appearance to rust. This new compound becomes part of the soil.

The aerobic condition in the clogging mat attracts obligate, aerobic, micro and macro organisms (worms, grubs, and other soil critters) which then consume the organic material in the clogging mat.

Typical Dry Well (Seepage Pit)
The typical dry well is merely a deep hole excavated in deep soils and then filled with clean gravel. Some dry wells have a perforated pipe placed in the center of the excavation and extends to the bottom of the pit. The septic tank effluent is applied to the dry well through this pipe. Dry wells perform the same function as a leach field, but typically contain much less infiltrative soil surface area. The hydraulic, chemical, and biological processes occurring in a dry well are the same as those that occur in a leach field. The service life of a typical dry well is much shorter than the service life of a typical single leach field. Both of these septic tank effluent dispersal systems are inferior to a properly maintained alternating leach field system. Dry wells are usually installed on sites that are limited in area and on sites that have very deep soils.

So nashva, I hope that this very short explanation of these two septic tank effluent dispersal technologies adequately answers your question. :>)


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nashva View Drop Down
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Direct Link To This Post Posted: 09-Jun-2004 at 11:23am
yes it helps thank you
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