Building a Pumpable Septic Tank in Shallow Groundwater

[javry]

Hi Guys - I'm managing a light civil project in Souteastern Afghanistan. We're building a clinic here. The design calls for a septic tank that would be pumped out every few months as opposed to having a leach field. So it's really kind like a holding tank - of sorts. It's supposed to be constructed with reinforced concrete. It's about 10 meters long, 5 meters wide, and 3.5 meters deep.

As with most things in Afghanistan, the design is generally done some place far away from the build location and in this case, we have a septic tank that, if built according to desgn, will have parts of it sitting below the water table. We started encountering GW at a little over 2 meters and the bottom of the excavation stays wet most of the time [and BTW - we're not in the rainy season yet].

Can you guys give me some pointers on what to do here? We're pretty much stuck with the location but we can do a change order with the design via RFI. I can probably reduce the depth by a foot or two and increase the length and width to maintain volume but that's about it. Any other ideas like moisture barriers and such? Keep in mind where we're located. Thanks guys.

 

[jgailla]

This is a situation we encounter often in Florida, where we have high groundwater.
I have only designed septic systems with leach fields, not holding tanks only.

If possible, you should probably get the tank out of the groundwater by elevating the area and using a grinder pump to lift the sewage from your wetwell to your holding tank.
If you leave the tank below groundwater, you are running the risk of contaminating the water supply.

How much this contamination matters depends on where your water supply is, the population density, etc.

I don't think this would be allowed in the US, but circumstances are different in remote areas.

If you absolutely must keep the tank below groundwater, which may be better if power is intermittent, keep it as far away from the water supply as possible and see if you can find some clay to use as a liner around the tank.

 

[bimr]

You might look into evaluating the size of the tank. 175 cubic meters is a substantial volume. Not sure what size transport truck that you have access to, but the tank may hold a volume of as many as 6 tank trucks.

The volume of waste generated at a clinic would not typically be so large that it would require a tank of that size.

Of course, there is not much information presented, so this is conjecture. You need to know the number of patients, climate, etc.

 

[kieran1]

done a few of these recently in Africa and used shipping containers surrounded in concrete so to water proof them. It simplified the formwork and we welded steel waffle plates inside. Had another overflow tank as a back up as the sucker truck was a bit unrealiable

I originally designed the tank as a reinforced waterproof concrete tank and based upon 400 litres per person per day although this was conserative and you could probably reduce this by a minimum 50% for Afghanistan

Kieran

 

[Ron]

Since you obviously have access to concrete, then consider constructing a work mat at the bottom of your excavation, then building your tank on top of that.

Can you dewater at all? If not, at least dig a sump or two and pump from those...that should allow you to get a work mat constructed. A work mat is just a rough concrete slab or mat that impedes the groundwater so that you can build the tank. You can even waterproof a work slab with coal tar, asphalt or a membrane.

Also, you might consider initially depositing sewage into a wet well then pumping to the holding tank...then you can put it anywhere you want it.

 

[dicksewerrat]

How about bringing in some HDPE manholes and use them as your interior and place concrete around them to resist flotation. The manhole sections can be heat fused to make a watertight structure.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[javry]

These are all great ideas guys. I'm due in the field in a few minutes but will post some design details when I get back.

 

[javry]

Here are some files that hopefully will give you guys an idea of what the tank is supposed to look like. I know it's capacity is way over designed, as some have remarked. But in this part of the world there's just no telling when the sucker truck will come around. There is a dry latrine just down the road from the clinic that is literally over flowing - and people are still using it - if that awful picture helps. Well - anyway...

Please note the wall thickness [300mm or 11.8"]. We started encountering increased moisture at about 2 meters. By 3 meters we were in wet soil. At this point, we're looking seriously at 1]maintaining the design depth but including a clay liner with HDPE inside and outside as moisture barriers or 2] raising the tank to where it will extend about a meter above grade and going with a pump station. Of course, for those of us coming out of the DOE RI/FS days, there is always the "no action alternative" which, in this case, means "build it as designed and fugitaboutit". To be honest I don't think it will come to that -but then you never know. I sure hope the drawings come through alright.

 

[javry]

Here's another file. Guess it didn't get through before.

 

[TGLG]

There are a few things I see in your initial post that are a bit worrisome.

You say "it's supposed to be constructed of reinforced concrete." Does this mean it will definitely be reinforced concrete, or is it possible that other materials might be used? Are there detailed structural drawings for the reinforcement? What measures are in place for quality control during construction?

Given the size of the tank, how heavy it will be when full (and even when empty), and the fact that it will be built in groundwater, it seems there is a good chance of differential settlement, which could cause the tank to crack.

What are the local soils like? Will they be capable of supporting the tank (even during the wet season) or will the tank need to be supported on piles?

Have you performed buoyancy computations? The tank certainly isn't going to float when full, but what happens if it's pumped empty during the wet season?

Is there a local precast concrete industry, and if so, might you be better off installing several smaller precast tanks rather than one big one?
Using smaller precast tanks could improve your chances of getting a completely watertight end product, because you would be able to inspect the every tank before installation and refuse any units that are less than perfect, which is something you can't do with cast-in-place. Having multiple tanks also reduces the risk of cracking caused by differential settlement (although you'd need to build some flexibility into the piping system).

 

[chicopee]

Make sure that the effect of buoyancy is considered in your analysis.

 

[javry]

I'll try to answer each of these as best as I can.

"You say "it's supposed to be constructed of reinforced concrete." Does this mean it will definitely be reinforced concrete, or is it possible that other materials might be used? Are there detailed structural drawings for the reinforcement?"

The original design definitely calls for RCC and I do have a RCC profile that I can send"

"What measures are in place for quality control during construction?" We have a QC Mgr for that but guys please keep in mind that we are in a remote village in southeastern Afghanistan. That's why I need your input.

Given the size of the tank, how heavy it will be when full (and even when empty), and the fact that it will be built in groundwater, it seems there is a good chance of differential settlement, which could cause the tank to crack.

"What are the local soils like? Will they be capable of supporting the tank (even during the wet season) or will the tank need to be supported on piles?" There is no no USGS soil type data collected here. To give you an idea of how remote this area is, when we dug the water well, the driller showed up with a cable tool rig. He dug the well but didn't collect any gw or soil data as we would have done in the states. The best I can say about that is there are other structures built in this same area that seem to be doing fine w/o piles.

"Have you performed buoyancy computations?"
If we go with the clay liner option, we will certainly do bouyancy calcs. Good point!

"The tank certainly isn't going to float when full, but what happens if it's pumped empty during the wet season?"
Another good point. Thank you

"Is there a local precast concrete industry, and if so, might you be better off installing several smaller precast tanks rather than one big one?"
There is nothing remotely close to that here. The 3 options mentioned earlier are probably as far as we're going to be able to go w/o doing a complete change order - and you know how that is on a gov't job.

I'll try to get some more dwgs and a couple of photos for you guys.

 

[Ron]

javry...another thought is just to build a small extended aeration sewage treatment plant or buy a package plant. Used to be very popular in the US in the 70's and 80's, but as municipalities expanded, less so.

Simple concept, not much equipment required and you won't have to pump anything but a sludge tank on rare occasions. Effluent can be used to promote vegetation.

 

[beej67]

I don't like the wet well idea in Afghanistan. I would call your power supply there particularly stable.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com