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Sediment tank

NICK KEENAN | Posted in Green Products and Materials on

My city (Washington, DC) now requires all construction sites that are disturbing the earth to have sedimentation tanks, so that if an excavation needs to be pumped out the sediment can be removed from the water. I support this — we need to keep runoff out of our sewers and waterways — but it’s a pain. I priced a tank at $2000. It’s about eight feet long and needs two people to move it, it takes up space on the jobsite, and is a pain to store and transport.

I passed by a site today that had a homemade tank made of three blue plastic barrels connected with PVC pipe. The pipe was connected with unions so it could be disassembled. I have to assume that this has been inspected and approved by the city. I like this because it will be light and easy to transport, and best of all, cheap — I can get the barrels on Craigslist for $20 each.

I will try to post a picture later, but I’m wondering if anyone has experience with sediment tanks and thoughts about construction.

Thanks.

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Replies

  1. Expert Member
    NICK KEENAN | | #1

    Here's the tank I saw.

  2. Expert Member
    BILL WICHERS | | #2

    Looks like it would work to me, just a series of sediment traps. Septic tanks work in a very similar way. It would help to have a 90 point down on the inlet line, so that sedminent is directed to the bottom of the tank, and a tee on the outlet, with the top and bottom oriented vertically -- this prevents floating debris from getting out of the tank.

    The big question is: where did you find enough empty dirt to dig a big hole in DC? I've been there many times, and everything is built up...

    Bill

  3. Expert Member
    NICK KEENAN | | #3

    Thanks Bill, those are the kinds of tips I'm looking for.

    >The big question is: where did you find enough empty dirt to dig a big hole in DC? I've been there many times, and everything is built up...

    You knock down an existing building. Zoning restricts the floor area to land area ratio of every lot, and the economics right now are that every new house is built absolutely at the limit of the zoning ratio. This has only been true for about 15-20 years, so anything older is a candidate for a tear-down. In the residential neighborhoods there are lots of modest houses that were built between the wars, the small houses are all being torn down and replaced by bigger ones.

    What's a pain is I have to have a sediment tank on-site for any excavation, not just big holes. I could be digging a post hole and in theory I need a tank. It's a pain on small lots with limited access and storage.

  4. Expert Member
    BILL WICHERS | | #4

    >"What's a pain is I have to have a sediment tank on-site for any excavation, not just big holes. I could be digging a post hole and in theory I need a tank. It's a pain on small lots with limited access and storage."

    We run into this in the utility world with requirements for shoring deep holes near roadways to prevent collapse. Similar regulatory issue, but different application.

    If you have a very minor excavation, see if they'll let you get by with a surface barrier to debris runoff instead of a tank. Around here, straw bales are commonly used for this purpose. Basically you make a wall out of straw bales around your excavation, and anything trying to run off gets trapped in the straw bales that act like filters. This is commonly done here when digging near drainage ditches, for example -- straw bales are placed IN the ditch so that any debris getting washed down the ditch gets filtered out and doesn't contaminate whatever body of water the ditch ultimately drains into.

    Bill

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