So I have a cartridge filter but the city required the filter drain plumbed to a cleanout regardless of whether it was a DE or cartridge filter. So I recently noticed that the drain valve was in the open position a couple months ago and closed it. No water was exiting at the clean out as the pipe was capped. I saw the water in the window after the valve looked grey and thought isn't the water in that pipe between the filter drain valve and clean out going to get really bad? Plus I can't drain the water out of the filter body when I clean the filter elements. So I cut the cap off above the clean out and opened the drain valve, wow that water smelled bad. So should I open this drain valve ever month or so to keep the water from turning black, lol?
Picture of the drain valve connected to filter drain, this photo was taken in when they installed plumbing, so I can see they had the blue handle in the open position:
The line from the filter drain is the 1.5" line going over the top and into the clean out funnel.