The ones that I work with are precast concrete units that get buried on your property so most likely they would come from a local supplier to you. I use these guys
Jensen Precast - Industrial Waste - Clarifiers here is the link if you want to read up a bit. Here is also a link to the clarifier itself
http://www.jensenprecast.com/products/pdf/industrialwaste/clarifiers/clarifiers.pdf
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