We moved into a new house about two months ago. As background, the house had not been lived in for several months before we moved in. We bought it from a contractor who flipped it. The previous owner was an elderly widow who wasn't able to keep up with the maintenance of the house and let a lot of things fall into disrepair.

There is sub-basement that is completely underground and which has a sump pump. This is our first time dealing with a sump pump, so we're not sure what's normal and what's not. The sump pump pit was completely dry when we moved in and possibly hadn't been used in years. We hooked up a dehumidifier to drain into the sump pump pit. It also fills when it rains.

About a month ago, we started noticing little flying bugs in our house, as well as a bad smell coming from the sump pump pit. Eventually, by discovering this forum and searching through old posts, we figured out we needed a new check valve and also that the bugs were drain flies that were breeding in the sump pump pit. We replaced the valve and (as recommended on this forum) I drained the pit, scrubbed all surfaces with dish detergent and rinsed it (twice), and then scrubbed everything out with bleach water. I also left bleach water in the pit above the water line to disinfect the pit. (The pump was unplugged so it didn't kick on and remove the water.) The walls of the pit seem to be permanently stained dark. No matter how much I scrubbed, I couldn't get the stain out, but I do believe the pit was completely clean.

The smell and the number of drain flies decreased initially. But about 10 days after I scrubbed out the pit, the smell was back (although not nearly as strong as before) and the number of flies seemed to be increasing. I took the lid off the pit to pour fresh water in, and about 20 flies flew out of the pit. (Clearly, they're still breeding in there.) The water also looks murky and has a film on the surface.

I'm really surprised it could get so filthy so quickly. We did have some hard rain for about four days straight after I cleaned out the pit and before we started noticing things were getting worse again. There was dirt in the bottom of the pit when I drained it, so I'm sure dirt is coming in with the groundwater. This may be what made the water murky, but I can't figure out the smell and the film.

Some other things which might be related: in the past couple of weeks we have also noticed some drain flies on the exterior walls and windows of our house. We only saw them inside before. Also, when we bought the house, we could tell the walls of the sub-basement had just been painted white. Now patches of it look yellow, as though moisture has seeped through the walls and stained them. But we have no water on the floor or walls.

I am going to scrub the pit out again and see what happens. Anybody have any other suggestions, or any idea what's going on here?

 

Is there a way you can cover the pit with a lid to help contain the odor?

 

I normally (once - sometimes twice per year) dump an entire gallon of Clorox into the pit at a time when the pump is not active and let it sit there for a few hours. I then come back and dump several buckets of hot water which makes the pump go off and pump out all the goo. The stuff that comes out is well.....amazing....

If the film on top of the water appears oily it could be your pump is going. You did not say what it smells like (rotten eggs ? - wet dirt ? - other ?), so it is hard to pinpoint what the odor could be. As for the flies, if the cleanout doesn't make them leave, why not put some fly paper in the area and catch them that way ?. Perhaps you can fit a strip inside the pit somehow.

 

If it fills when it rains then it must have been active.

I had fungus gnats which are very similar. Hot water, bleach and vinegar will kill the active flies but won't attack the eggs. Boric acid (Boraxo soap) is supposed to be effective and it will also help you clean the pit.

Otherwise gel type cleaners are formulated to remove organic matter and will help clean the pit walls. You could also use enzyme or bacterial drain cleaners to remove organic material.

 

Original poster here. We had plumber come out and send a camera through our lines. He found that the drain line from our kitchen sink and dishwasher had about 2.5 feet where the bottom of the pipe had completely rusted out. Since this house used to be on a septic tank, the pipe runs under the bottom of the slab. Apparently every drop from the sink and dishwasher has been discharging under the slab and working its way down into the sump pump. The plumber thinks this has been going on for years, maybe a decade, judging from the size of the rusted hole in the pipe.

I am confident that the plumber will be able to fix the pipe. My concern is making sure the drain flies will also go away. I have been pouring gallons of bleach down the drain to kill the flies, but I know this won't kill the eggs. His plan is to dig under the slab from outside the house until he reaches the hole in the pipe, make the repair, and then shovel regular gravel or shoot liquid gravel into any hole created by the water carrying the soil under the slab away for all these years. Will this be enough to kill any remaining flies and eggs? Or does more soil need to be removed? How do you tell what soil needs to be removed?

I'm concerned because I know we have some tiny cracks somewhere, because I have also found some slugs in our sub-basement. If they're getting in somehow, I'm sure the drain flies will continue to get in if we leave any alive under there.

 

Your slab should be sitting on gravel. I don't think a kitchen drain would have enough force to wash that away. That said, it can't hurt to check. If it were my house, and the slab wasn't finished, I'd spend the extra $ to open up the slab for the length of the pipe, and replace the entire pipe. This way you can be 100% sure there are no further leaks, and you can see any voids and fill them in.