This is a 2 inch external standpipe for a row of 5x 275 gallon pallet tanks (see: Adding Capacity). The tanks are plumbed out their bottom drain to a 2 inch main then to this stand pipe to set water level and on to filtration via a 50 foot 3 inch line. With the valve closed the water flows up and around setting the water level. Open the valve and the tanks drain. Assembled with repurposed parts (hence the extra couples). Installed to the line with fernco couples.
