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Converting existing septic system to constructed treatment wetland  RSS feed

 
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Posts: 1355
Location: Olympia, WA - Zone 8a/b
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My property currently has a very strange setup for the septic system. The septic tank discharge is pumped across part of my property, under a shared dirt driveway, under my neighbors backyard, and finally to a mound drain system in another neighbors field. I have an easement for this mound but really this is ridiculous - the mound is about 183 feet away from the septic tank. The backup area is located in another neighbors field that is located 1,500 feet away and on the other side of a stream. So ya I'm not thrilled with this current system. But I would love to replace it with a subsurface constructed treatment wetland.

The soil in my area is all clay which is why everyone has a mound drain system in my area. This is also why my current mound is on my neighbors area because of limited space to build one. This is another reason why a subsurface constructed treatment wetland appeals to me.

My question is how does one go about converting an existing septic system into a subsurface constructed treatment wetland? Also, what size would the wetland need to be? My house is just over 1,000 square feet with one bathroom, a kitchen and 3 bedrooms. Currently, there are three of us living in the house - myself, my wife, and my son.

I really like the idea of building a subsurface constructed treatment wetland and I love to replace my existing system with one. My understanding is that the septic tank would remain and I would build the wetland to deal with the discharge. But I'm really unsure about any details and what to watch out for and if this is really possible.

Thanks!
 
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Posts: 236
Location: Ireland
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Daron Williams wrote:My question is how does one go about converting an existing septic system into a subsurface constructed treatment wetland? Also, what size would the wetland need to be? My house is just over 1,000 square feet with one bathroom, a kitchen and 3 bedrooms. Currently, there are three of us living in the house - myself, my wife, and my son.



Hi Daron, Essentially you'd leave your septic tank as is, leave your drainage mound as is (you still need to get the final water into the ground, even though it will be cleaner) and build a new horizontal flow gravel reed bed system (sub surface system) to filter the septic tank effluent. Under Irish Codes you'd need a system capable of treating the septic tank waste from 5 persons (based on 3 bedrooms) x 5m2/p = 25m2 of reed bed area. Remember that any sloping embankments should be additional to this area; and I also regard this code sizing as being a tad small, so I usually recommend a size of c.40-50m2 at ground surface, with a sloping edge to a slightly smaller base area. The wetland layout should be c.5m x 10m in plan, or slightly narrower and longer if you have the space for it. The gravel depth should be 600mm at the inlet, and a slightly sloping base down to 700mm at the outlet, with a flow control pipe at the lower end to keep the water about 50-100mm below the level gravel surface.

What you really need to do is win one of my books and have a good read of it! :-)

Bear in mind that the book covers UK and Ireland, so you'll have to seek out a good engineer locally who can guide you through the local building codes and be sure that you are following the correct legal procedures.

Since your drainage mound is currently in place already, do you actually want to dig it up or remove it from your neighbours lands, or stop using it? You may have a reason, but from where I'm looking in, the easiest thing is to leave the existing pipe layout intact and just add a new reed bed to get the water cleaner prior to percolation. One advantage is that the heavy clay soils will take a lot longer to clog up if you have cleaner water going into them. If you want to remove the old system, then we can explore how to do that on your own land and perhaps add a willow planted infiltration area to mop up residual nutrients and grow some firewood.

 
Daron Williams
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I hope to win a copy The issue with the current system I have is it does not meet current codes but was grandfathered in since it existed before the codes were established. I would also love to get a system installed that could work just with gravity and not need a pump so if the power goes out I'm still good to go.

I have the space to do a willow area and would be very interested in that.
 
Feidhlim Harty
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So your existing system is pump-fed? I can see why you'd rather avoid that if you could - whatever about the codes.

Ok. Do you know the infiltration rate in the soil? Here in Ireland if it takes longer than 90 minutes for clean water to drop 25mm (after a presoak the day before the test and another presoak before the test to allow the soil to reach saturation) then your ground is considered unsuitable for building. You already have a house, so the solution is to find something - anything- that will work ok.

The soil depth above water table and bedrock are also important. Do you know how deep the dry subsoil is?
 
Daron Williams
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Feidhlim Harty wrote:So your existing system is pump-fed? I can see why you'd rather avoid that if you could - whatever about the codes.

Ok. Do you know the infiltration rate in the soil? Here in Ireland if it takes longer than 90 minutes for clean water to drop 25mm (after a presoak the day before the test and another presoak before the test to allow the soil to reach saturation) then your ground is considered unsuitable for building. You already have a house, so the solution is to find something - anything- that will work ok.

The soil depth above water table and bedrock are also important. Do you know how deep the dry subsoil is?



Yup, pump-fed it would be great to switch to a gravity-fed system. The house I grew up in had a gravity-fed septic system.

I don't know the infiltration rate but it varies depending on the season. In later half of spring, summer, and first part of fall water seems to infiltrate quickly but during the rest of the year it is completely saturated and if I did down a foot or so I can watch water flow from all sides into the hole. This hole will remain filled with water for the whole wet season.

The soils in my area are the clay sediment resulting from the last ice age - there was a large lake over my area caused by the glaciers blocking the flow. Due to this the soils in my area are fairly young and due to poor management in the past there is very little organic material. They soils are basically just grey clay - I can take a chunk of it and easily mold it into a ball.



Here is the soil profile:



The above hole started filling with water from the surrounding soil soon after this picture - that was taken in I think late October.

This is an example of molding a chunk of the soil without anything being done to process the chunk other than taking it out of the ground.



That being said my crawlspace does remain dry for the most part though the lowest parts will collect some water but not enough to cause any problems. New houses are being built in the area and they soils are not classified as having a high ground water issue.

As far as the depth to water goes. What I have to go on is the well log for the shared well that I use. About 25 feet down there is a compacted fairly impermeable layer - this was due to the old lake. Above that layer is mostly clay but also a gravel layer above the impermeable layer. This area was classified as having seepage but not enough to be a perched aquifer. The impermeable layer is about 15 feet thick and below that is the aquifer that the houses in my area use.
 
Feidhlim Harty
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Hi Daron,

It's not an easy site by the looks of it! I don't know your soils as well as I know Irish conditions, but here's what I've done on some similar sites here, with heavy clay subsoils:

If you want to use the existing clay as a liner, to avoid plastic, then scratch the gravel reed bed idea and use a soil based constructed wetland instead. Again for 3 bedrooms, assume 5 people. Thus you'll need an area of 5m x 20m of shallow marsh-type constructed wetland. So, widen an area of your property that is down-gradient of the septic tank outlet, to those dimensions. Embankments are additional. These should be shallow sloping for safety (45' or shallower). Get the entire base of the system level, and roll it to compact the base. Backfill with 4" of weed-free topsoil and level this. Set a 4" pipe in the outlet end, and puddle the clay well around the pipe to seal around it. Place an elbow on the outer end, so that you can raise the elbow 4" above soil level in future to hold in water. Until plants are established, leave the elbow low, so that the water is a few inches below the level of the topsoil in the system. Plant with a selection of local wetland plants; something tall and vigorous. Typha (alternatively called cat-tail, bulrush or reed mace) is ideal, along with other plants with good tall growth, flat leaves and a liking for marshy soils.

This is a constructed wetland for sewage treatment. It will probably dispose of some liquid through percolation, but is not a percolation area. Here in Ireland what we've done where zero percolation to soil exists is to route the wetland effluent to a second wetland area of half the size of the first one - without compacting the base first, so that infiltration is enhanced. Anything exiting this area will be tertiary treated in quality. Exiting this (if needed) we go through a short (or long if you can) run of pipe, over-planted with willows. Note that the pipe must be either an open drain, or a drain covered with a plastic trench infiltrator unit. A cheaper alternative is to take a 1' pipe and cut in two to form two gutters. Use these face down on your trench base and cover with soil. Keep the trench base as shallow as possible to allow maximum treatment in the soil. Plant with willows to take up as much water as you can. The end of the pipe (by necessity on such sites) will go to a nearby watercourse. This is only legally permitted here in Ireland where no other option exists. Local regulations will apply, so you'll need to check those out yourself.

At this point, connect your septic tank outlet to your new wetland.

The book gives a longer description if you're interested. Feel free to ask via the forum though if you encounter any issues or questions.    
 
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