i have had nearly 40 years to think about how i want to attempt a larger scale digester than my original 55 gal drum the last time i made methane, when i recently and unexpectedly came across palletized liquid shipping containers known as "totes" that are available from certain firms "used" and basically empty but often needing a wash still.
surplus tanks at http://www.gottagosurplus.com/containers.htm
i bought 4 from a friend. each holds 330 gallons, has a screw on top with rubber o-ring gasket, and an emptying faucet with valve at the base. i'm hoping to set them up in a greenhouse and get them up to 95 degrees. i'm planning on buying a macerating pump to move the mixed manure/water slurry into each batch digester as necessary. i'll probably connect a flexible, transparent plastic hose near the top of each tote and run it through a limewater filter and then to a floating gas cap (actually, 30 gal plastic barrels from a dairy that receives iodine and soap in them and then throws them away).
just thought i would pass the idea along. my cost was about $5o each, but depending on who i bought them from (and whether they had been clean or not) the cost could have been a bit higher.
i also plan on painting them or at least tarping them to protect the plastic container from UV rays.
these particular ones had technical grade mineral oil in them, used as a release agent for a plastics injecting fabrication plant.