Tying into Existing Septic System
I’m trying to figure out a way to tie an accessory building into the septic system of the house. The problem is the path from the new building to the septic tank is about 60 feet, and is uphill. Building up the grade where the new building is going is impractical for a number of reasons.
Is a sewage pump an option? I tried searching for an example of it, but all I see are applications of a toilet or laundry drain being pumped from a basement up to a sloped drain. In my case, the drain itself is lower than it needs to be. Can the sewage pump be placed underground, near the tank? If it is inside the building, that means it will be pumping up a long, gradual slope, which I’m guessing it’s not intended to do.
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You can put in sewage pumps in underground basins, but they have to be sealed -- these are different from the more common "sump pump" setups. Once you put in a pump/basin assembly, you can just drain everything to that in the usual way, and it will automatically pump out when it fills. The pump can push uphill to deal with your remote septic system. You're probably looking at a few thousand dollars for the pump and basin.
Check that your existing septic system has enough capacity to handle the new building. There are various codes about this, and the codes state how many gallons of tank and the size of the drain field needed per bedroom (usually). Check with your municipality first to make sure your existing system will still be OK with the extra structure tied in.
Bill
Ugh. Hopefully we sized the tank generously, I will have to look it up. This is a case where the codes don't really reflect reality. Regardless of how many bedrooms or fixtured put into this building, there will be the same number of people living there. I think the number of people actually determines the usage rather than the building.
Can the basin be on either end? Like can it be just outside the building, and it's pumping up a gentle slope pipe to the septic tank?
The basin has to go at a low point at the structure it serves, so that things can drain into it. The septic tank can be anywhere as long as the sewage pump can handle the head (resistance to flow) presented by the combination of the frictional loss from the length of the pipe and the elevation difference between the pump and the place it's pumping to.
I usually see the basin installed in the basement of the structure it serves, but I suppose it could be put outside too -- I've just never seen that myself, and most of the basins have a removeable lid the pipes go through, so I don't know how well that would work outdoors.
Bill
This is a slab on grade building, but I guess the concept would be the same as a basement installation. Complicates the slab a bit, but probably no more work than dealing with it in a hole outside.
If you have the option to put it outside, I would do that. You really don't want one of these things in something like a closet inside your living space. I myself haven't seen an outdoor unit, but I'm quite sure they exist (and Malcolm seems to indicate that they do too), so I'd get one designed for outdoor installation and go that route.
I'd put an overflow switch in the tank too (typically just a float switch mounted on the side), and wire it to activate a light or horn inside the structure. If the pump fails -- and they do fail, ocassionally, and you don't realize it, nasty things can ooze out of the basin and make a reeking nasty mess. An overflow switch will give you some advance warning of problems and let you fix things before they get really nasty.
Bill
Bill,
Both putting it outside, and having a small overflow tank with an alarm, are great suggestions. Clearing a blockage or fixing a pump failure when the drains are backed-up when the pump is inside is a nightmare.
tplb,
The pumps differ in that they have to macerate the sewage before sending it to the septic tank. They can be located in the house, or outside.
https://www.amazon.ca/s?k=macerator+sewer+pump&hvadid=328032166494&hvdev=c&hvlocphy=9001623&hvnetw=g&hvqmt=e&hvrand=16939276248410598134&hvtargid=kwd-565609655514&hydadcr=26275_9479306&tag=googcana-20&ref=pd_sl_3gpyt7xv3v_e
Codes base septic sizing on the number of bedrooms and bathrooms, because it's a more accurate predictor of use than using the present number of occupants, which will no doubt change over time. The sizing affects not only the tank, but the length of the field.
Having seen and dealt with failures of macerating pumps both indoors and out, I heartily agree with the suggestion to put the pump outside if possible.
The biggest obstacle is likely to be your local government and its rules. When most people are building, they put the septic out for bid and the low bid tends to win. The low bid was likely the smallest system the local rules allowed at the time. Over time the rules change and require bigger systems. Often if you want or need to change any part of a system you will be required to make it comply with the current code. If the old system is undersized by the current rules a new system could be required.
Note I think it is a mistake to build at the lowest point on your property. One day the 100-year rainfall event will accrue and your new building could be at the bottom of what looks like a lake. The water needs a clear path to drain away from the building locate you new septic pump in this path several feet lower than the building.
Note the fresh water line to the new building should be in separated from the sewage by at least 10 feet in a separate trench.
Note if you attempt to evade the local sewage rules any contractor willing to preform such work is by definition unscrupulous and untrust worthy.
Walta
We built the house, so I know we didn't select the septic contractor based on price. I hope we sized the system with some headroom, I just can't remember off the top of my head. It's only 6 years old, and I don't think requirements have changed since then.
Not sure where you got the idea, but the build site is not at the lowest point on the property. Aside from where the house is, it's the probably the highest spot on the property.
Here in New Jersey, a modification like this would require an engineer’s design and I’m not an engineer. But as I think about how this might be done, I would probably add a new septic tank and pump tank for the accessory building. The septic tank receives all of the waste from the accessory building and then the water leaves the new septic tank by gravity and goes into the pump tank. The dosing pump inside the pump tank then pumps the water up to the absorption area, bypassing the existing septic tank. This setup will “dose” the absorption area when the pump tank fills up, as opposed to a gravity system which sends water to the absorption area whenever waste enters the septic tank. If you have a distribution box, you can possibly convey the effluent right into the d-box. There is an alarm inside the house and/or accessory building to alert you if the dosing pump fails. With this setup, you are only pumping water instead of raw sewage. You will need to have 2 septic tanks emptied periodically instead of just one, although the second tank may need less emptying depending on its use. And you have to make sure your absorption area can handle the additional effluent, but your old septic tank will not need to be changed to a larger one since it is being bypassed. And I agree that it doesn’t matter how many people are living on the property, it matters how many people *could* be living there, and here in NJ, that is determined by bedroom count. Both the treatment tank (aka septic tank) and absorption area (aka leach field) must both be sized correctly. If you find that your absorption area is not sized correctly, you could add a complete small gravity septic system for the accessory building provided that it is not too close to any wells, property lines, etc. Septic systems are very expensive here in NJ and most of that expense is in the absorption area, but we also have costs that you may not have there, such as engineering fees, the outlawing of seepage pits for new absorption areas, etc.
I have lived with a sump septic pump at my workplace for almost 20 years. Once someone flushed some paper towels and it clogged. Otherwise never a problem, about 24k gallons have gone through it per the water meter, so low use really
I wonder if a 60 foot long rise will be problematic. My shop setup has a 10 foot rise then a drop to eventual grade., check valve at chest height.
So 2 inch or so pipe maybe 6 foot for total volume sitting on the check valve
It seems like 60 feet of pipe would be quite a bit of effluent sitting against the check valve
I think you need a septic engineer
gusfhb,
It's only the head that affects pressure. The length of the pipe just increases friction losses. He may need a septic designer, but not for that pipe.
Wasn't thinking about head but simply gallons of effluent thinking there being problematic
“Not sure where you got the idea, but the build site is not at the lowest point on the property. Aside from where the house is, it's the probably the highest spot on the property.”
That great news. The only possible reason to consider locating the pump inside the house is because no lower point exists. Sooner or later the pump will fail and when it does life is much better when the mess is outdoors and flows away from the building.
Most governments will require any change to the system be designed by someone they have approved of as qualified.
Walta
I’ll add my 2 cents from a legality perspective. Here in Montana, unless your parcel is over 20 acres, you will most likely have a DEQ approval for on site sanitation. 99.99% of approvals specifically call out ONE single family residence. Given Montana is among the most lenient states in the country, I imagine this is very common across the country. Here, if you’re over 20 acres, you will only have a septic permit from the county sanitation department. That will also say “one single family dwelling” unless you specifically had them permit it for two.
That being said, practically the septic will handle the load. However when you go to sell some day, it will be a major issue. If you want to do it legit, you have two options, both involve hiring and engineer or consultant and rewriting the DEQ approval. The new approval will allow you to hook two residences to one septic or they will make you install another septic system on your lot.
I'm guessing "DEQ" means Department of Environmental Quality? Similar organizations go by different names in different states. Here in Maine it's the DEP--Department of Environmental Protection. We are required to have a licensed site evaluator design all waste disposal systems or changes to existing systems.
Correct
As a Master Plumber, Adrienne has the best answer. Add a second septic tank located near the accessory dwelling, and then you’ll be pumping effluent “water” and won’t need the grinder pump. You typically don’t install a check valve after the pump. In cold climates we install the pump on a block at the bottom of the tank, make a pvc goose neck, and then leave the tank horizontally. The lowest 90 bend in the good neck has a drain hole drilled into the bottom so that the effluent can drain back into the pump chamber. With a check valve, the force main pipe would stay full after the pump shuts off and would potentially freeze before the pump kicked back on.
If there are trees or other obstacles you can directionally bore the pipe for the forcemain. Before you tie into your drain field pipe you will transition back into a 4” gravity pipe 10’-20’ away from the connection point.
In my jurisdiction an accessory building (workshop, personal art studio, etc) would not require a larger system. An accessory dwelling would. Most drain fields can be enlarged if there is room, but it’s kind of a pain…
The building is basically a workshop, so no additional bedrooms. I just wanted a toilet and utility sink in there. There is some thought of pre-installing some underfloor plumbing for possible conversion to ADU at some point in the future, like if one of us ends up not being able to go up stairs, our kids can take over the house and we'd live in this building. Given those possible use cases and the info collected here, it probably makes the most sense to 1)install a sewer drain in a place where a new septic tank and field could go 2)install a greywater system for a utility sink and skip the toilet for now.
If your plans look to the building department like the building could be used as living space the more likely they are to require the septic to be upgraded. If the plans show the ruff ins for a kitchen and bath then you will need to prove you have the septic capacity before you will get a permit.
Walta
I put a system like this in my first house because of elevation differences between our house and the municipal sewer main. A spherical concrete tank was buried outside the house at a location where the building sewer could drain by gravity into the tank. We put a grinder (macerating) pump into the tank along with a high level alarm. Worked great until the mother-in-law flushed a bunch of fabric wipes down the toilet and burned out the pump.
These types of setups should always be installed outside of the house to protect against potential failures of the pump, backwater valves, or other critical components.
plumb_bob,
Many of the lots around me are zoned for a main residence and small ADU. Owners often have their tank and field sized to support the future ADU, and use the set up you did when they get round to building it. If the main field and tank are sufficient to support the new use, I don't see the advantage of a new tank and pump chamber to serve just the ADU.
Agreed, extra cost and complexity for no gain, as long as the existing system is sized for the additional volume.