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Septic tanks for flood-prone areas

Hello. In a small city with public toilets, the proposed design by local engineers is to have a septic tank with a soakaway for the effluent. The problem is that this area has a relatively high water table, and floods occasionally during the year, so the soakaway would not work well. Are there any better ideas in this circumstance? Thank you, Remi

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Can you lookup some manuals on landfill construction? The mentality is the same: Keeping leachate away from groundwater by having an underdrain system to move the groundwater away from the multiple layers that are used as barriers between the leachate and groundwater. After constructing the underdrain system with rocks, etc., covering it with geotxtile and then adding a clay barrier (2 feet or 0.5 meters) to the top of the geotextile. Then, if possible, adding an HDPE liner. Basically, if your are stuck with a certain location, can the choice of geometry, e.g., mildly sloped sidewalls for placing clay liners, etc., and materials like clay, bentonite, and HDPE liners make a difference?

mohavi gravatar imagemohavi ( 2019-10-27 16:05:17 +0100 )edit

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Hi Remi, can you share a little more information? What sewerage treatment is available in the city? Is there a way of getting waste to any existing treatment location? Is there a particular need for decentralized management of waste? How many users are anticipated per day and are they pour flush toilets with water for washing or something different? Is it a hilly area or quite flat? Do people use groundwater from wells, boreholes or springs for drinking water or other household uses close to where the toilets will be? What is the flooding like? Is it coastal or from a river? How long does the flooding last and are people displaced during that time? Would the toilets themselves flood or would just the planned infiltration area be affected? Any details you can share about the situation can help determine what the options are for sanitation.

This guide might be of interest to you; https://www.eawag.ch/en/department/sa...

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Hi Tess, thanks. It's a small city with no sewerage, no real FS treatment either. I don't know the rest of the details (asking for a colleague there), I was just sounding out various options...

Rémi Kaupp gravatar imageRémi Kaupp ( 2019-10-17 13:00:20 +0100 )edit
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In high water table areas, you can have a sealed tank and regularly de-sludge latrines, effluent should be taken to a treatment plant. Requires a maintenance plan and budget...

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Thanks Liz. This is a small city with no treatment plant (yet...) so everything is pretty much decentralised.

Rémi Kaupp gravatar imageRémi Kaupp ( 2019-10-17 12:59:43 +0100 )edit
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What Liz proposes is a cess pit - basically a large bucket latrine. Cheaper to build but higher maintenance with very regular emptying. A sealed tank may float if (partially) empty during floods. Raising the whole thing may help emptying and avoid the floatation.

The septic tank option (assuming you have a sufficient water supply) may work with tweaks. The floatation thing will still be around, but as the tanks are only emptied infrequently (longer than a year), this can timed for the dry season. A bigger problem is the soakaway. High groundwater tables will mean there is nowhere for the effluent to infiltrate to. One way to cope with this is to use trenches rather than deep soakaway pits. These maximise the wall area of the soakaway trenches and they can be shallow, above the groundwater if possible. This can be arranged as a “herring bone” pattern to save space. Planting large leaved plants in the area may help increase evapotranspiration.

Another alternative is to go for a constructed wetland. This can cope with flooding! A granular soil is planted up with local wetland plants and the effluent flows through this. The final effluent could be discharged into a pond or stream, as long as it’s not directly used as a water supply. Having this at the end of a soakaway trench means that the effluent can infiltrate if possible but the wetland acts as an additional stage.

The septic tanks will still need emptying periodically.

On a wider scale, you have to ask the question why people are living in an area that floods. This really isn’t suitable as combatting the floods is very very very expensive. Improving transport links may help people to live elsewhere but still get to work and this may be more sustainable than trying to improve an area that floods.

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Thank you Brian, all very useful, I have passed on to the team!

Rémi Kaupp gravatar imageRémi Kaupp ( 2019-10-21 15:17:59 +0100 )edit
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What Liz proposes is a cess pit - basically a large bucket latrine. Cheaper to build but higher maintenance with very regular emptying. A sealed tank may float if (partially) empty during floods. Raising the whole thing may help emptying and avoid the floatation.

The septic tank option (assuming you have a sufficient water supply) may work with tweaks. The floatation thing will still be around, but as the tanks are only emptied infrequently (longer than a year), this can timed for the dry season. A bigger problem is the soakaway. High groundwater tables will mean there is nowhere for the effluent to infiltrate to. One way to cope with this is to use trenches rather than deep soakaway pits. These maximise the wall area of the soakaway trenches and they can be shallow, above the groundwater if possible. This can be arranged as a “herring bone” pattern to save space. Planting large leaved plants in the area may help increase evapotranspiration.

Another alternative is to go for a constructed wetland. This can cope with flooding! A granular soil is planted up with local wetland plants and the effluent flows through this. The final effluent could be discharged into a pond or stream, as long as it’s not directly used as a water supply. Having this at the end of a soakaway trench means that the effluent can infiltrate if possible but the wetland acts as an additional stage.

The septic tanks will still need emptying periodically.

On a wider scale, you have to ask the question why people are living in an area that floods. This really isn’t suitable as combatting the floods is very very very expensive. Improving transport links may help people to live elsewhere but still get to work and this may be more sustainable than trying to improve an area that floods.

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And...

On a longer time scale, small bore/ sustainable sewerage may be an option if water consumption at a household level is high.

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And...

On a longer time scale, small bore/ sustainable sewerage may be an option if water consumption at a household level is high.

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And to add to Tess’s questions, what happens to grey water/ sullage. Diverting it away from the black water can reduce the load on the septic tank but it will still need a soakaway, perhaps after a simple rock filter.

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