To find a solution, you need to determine the cause. Is the smell that of musty "dirty sock" odors or of rotting garbage, or of rotting eggs, or of stagnant water? If it has just started out of the blue, is there a chance you are washing a larger volume of clothes? or is there a chance the kids were playing indoors all winter and danced upon the sump pit cover? First try pouring a couple cups of bleach on the floor right at and around the sump pit lid cover. dirty sock odor is indicative of mold spores or mildew, or bacteria colonies, which the bleach will remove adequately. If that doesn't do the trick, unscrew the bolts on the sump pit lid and carefully lift the lid off, to examine its gasket for rips and tears. It is a gasket that maintains a gas tight seal and may have been ruptured. While you have the lid off, lift the float to see if the sump pump evacuates all the liquid, and then considerable liquid returns. If this is the case, replace the check valve.
: We recently have been getting a bad smell out of a sump pit when washing clothes. We have a septic
: system and this pit is used just for ejecting washing machine output into the main system. This has
: started out of the blue!