We recently replaced all of our outdoor sewer lines after a pre-christmas sewer back-up disaster. When they dug the lines we found we still had an outdoor catchbasin in use for our basement drains and kitchen sink drains. This is consistent with the age of our home (built around 1915) and location (just outside Chicago). While newer houses don't use catchbasins as old-fashioned "grease traps" anymore, ours does. Now that we have new sewer lines (plastic replaced clay) our sewer is running great. But when I lift the cap off the catchbasin, is smells like sewer. Further, when I run my dishwasher it seems to send sewer smell back up towards a new toilet I set in the basement. Since the sewage backed up through the catchbasin into our basement, do I need to have this cleaned out? A plumber set the toilet for me, and has already come out to caulk the base of it for the smell, and it doesn't seem to be leaking. But when we do dishes or laundry that send water into the catchbasin, it seems to be generating gas back towards the house. Help?

Ask your plumber if the catch basin has a trap associated with it. It should. Even if it does, the discharge from the dishwasher or washer may be drawing the water out of the trap. This will happen if the trap isn't properly vented. Once the water is gone from the trap, the catch basin is open to the sewer, and therefore the odor. Another possibility, if there is a trap, is that the water in the trap evaporates and opens the catch basin to the sewer. There are two possible cures for this...the expensive way is to have a small water supply to the trap to keep it from evaporating away..the inexpensive way is to dump a couple of glasses of water into the catch basin any time anyone goes to the basement. You might try the cheap way and see if it solves the problem.

It should not be expensive or troublesome to have the trap pumped out. That is the first thing I would do, especially since it apparently received some sewage from the earlier problem.
Is the catch basin/grease trap still in use; still receiving the gray water from the kitchen and basement drains? If it is, then the dishwasher outflow could easily be forcing out fumes if it is not properly vented; see Home Health Care's response. It does not make sense that the fumes should be forced to the basement toilet though, because the gray water and blackwater systems should be totally isolated from each other. I have seen some odd configurations though; perhaps they share a vent?? Or perhaps the plumber didn't know that the basement drain ran to the trap and tied the new toilet into it?? That particular case would need to be fixed immediately.

This may require a little investigation, but the first things I would do is get as much of the sewage out of the trap as possible, and make sure that the new toilet isn't emptying into it.

Thanks for the comments! Here's what I know. If I'm standing outside above the "manhole cover" of the catch basin, and look inside, water from the basement and kitchen flows in at around 9 o'clock. In the basin, there is standing water, which I guess is technically a "trap" right? It looks mostly clean, but there is some floaty stuff that is around walls at about 7 o'clock. Water flows out at around 2 o'clock and eventually ties into the sewer line. The toilet that is in question definitely does not flow into the catch basin. When the sewer first backed up (from years of roots and old offset short clay pipe runs), it came up through our basement drain, technically the lowest point in the house. For it to have come up there meant sewage had backed up through the system, including the catch basin and back through the basement drains. Now that we have all new sewer lines outside, we also have new cleanouts. I can see that there is nothing but smooth flow through the new sewer pipes. The new toilet that has the smell around it was put onto the floor by a plumber, over a floor I laid myself. After talking to the plumber again today, he seems to think it might just not be sealed well enough over the drain. There wasn't a toilet flange in the floor, it was merely a toilet put over a homemade rig of bolts cemented into the floor. I added ceramic tile, added an extension flange into the clay and did my best (I'm not a plumber) to seal it. Then the plumber came to set the toilet. He used a large amount of plumber's putty ("we feel it's better than a wax ring") and left. After a week, the sewer smell began. He came back, added caulk around the base of the toilet and it seemed to do the trick. But now it's back, and it seemed to coincide with the use of water that emptied into the catch basin. So that's my dilema. Do I just have a toilet seal problem (probably from my floor work and/or plumber setting the toilet) OR do I have a catch basin issue. Or both. Thanks for the help, if the above LONG explanation sheds any light, please let me know!