Smell in basement from drain tile pit (not septic smell)

Have a musty type odor (not from septic) just our drain tile system pit. I know it's just from stagnant water and humidity added to the air. So we could try using a humidifier I know. We have tried bailing it (we have never had enough water to warrant a pump, but it does collect some water, always some there). Have tried bleach in the water as well. Read about a sealed, vented cover. It came with a slotted cover which snaps on top of the pit. Don't know about venting it. Wondering if anyone has other solutions to the musty smell. It is in the basement of a tri-level house (tri-level with basement). Gilly