There used to be a sewage smell coming from the ejector pump.
To solve this, I put a vacuum on reverse on the vent pipe and then sprayed a soapy solution onto the top of the pump.
I watched some air bubbles come out of certain areas where the lid has either cracked or broken over the years.
That seemed to solve the problem in the short term although if you leave it for a week or so, the smell would come back but usage on a day to day basis was fine.

Now that the colder weather has appeared again with temps down to near zero overnight, the smell has come back on a daily basis in the morning, ie the water sits in the tank overnight and as soon as the pump goes in the morning, some air comes out of the leaks.

I cannot find any more air seepage holes with the vacuum method but it seems odd that the symptom has come back with the colder weather.
Could there be a negative pressure problem with the vent? Is there anyway to solve this?

I have attached a picture of the pump:
(a) lots of silicone as you can see my attempts to seal it.
(b) the vent pipe runs about 6ft the right of the pump, then 10ft to the outside, then you'll see an outside picture, where it runs 25ft up to the roof.
(c) The vent pipe looks like a 2inch pipe (6cm diameter)
(d) There is no central stack venting in this house as all wastewater runs into the ejector pump in the basement and the only venting is this pipe from the ejector pump

Pictures:
https://drive.google.com/folder/d/0B...RjaXctX2c/edit

 

Pressurizing the chamber and using bubble soap can work but it only works where you put the soap and if the soap can bridge the gap and form a bubble. Then if there was a bubble and you didn't see it often the soap is gone so it makes no more bubbles. Another problem is that unless you plug the inlets to the tank it's possible that the air you blow in is bubbling out the past the traps in your fixtures.

I find smoke more reliable as it will leak out anywhere it can. The problem is getting smoke inside the chamber. If you could easily remove the lid you could ventilate it to make sure there are no flammable gasses. Then put a smoke bomb inside and close the lid.

Since you are getting a smell there is probably a leak somewhere. It might not be where you can see. Maybe the sump is cracked below the slab where you can't see or one of the lines draining into it has a leak.

 

I tried blowing in a smoke grenade once but the vacuum filter absorbed everything. Can't find any normal/white smoke round here. If I open the lid and put one in, it takes a long time to put the 8 bolts back on so I would be smoked out before I could test.

Any thoughts on the winter vs summer venting? It's clearly worse in winter.
What about an inline fan? I guess a fan is only one way and it needs to draw air in when pumped.

 

The house is closed up tight in cold weather so it could be related.

Air needs pressure to move to much extent. Changing air pressure from wind/weather or running a furnace can pump air in and out but I would suspect a leak for a more continual problem. And, by leak I don't mean a simple crack in the lid. The stuff on top you can see is easy but consider any pipes entering the sump and their fittings into the tank. If the lid has cracked with age what about the sides of the tank and connections to the incoming pipes? So, I'm wondering if it's not a gas leak but possibly a liquid.

 

Can I install a 2" fan?
Don't think it's liquid as the smell only happens when the pump turns on and when it's been left overnight.
All pipes feed into the tank from sides under the lid.
If it's cracked down there then it's all in concrete.

 

Should venting pipe be straight for easy air flow?
This one has 4 bends and travels 40ft

 

Found the problem doing a DIY smoke test.
There is smoke coming out from between the basin and the concrete - perhaps there is a crack just below the surface.
How can I seal this off? Presumably, running sealant at the join of basin and concrete will not seal it off properly?

 

You can excavate around the outside and see if there is a leaking fitting or if it's one of the pipes leading into the tank. Repairing a plastic sewage tank may be Band-Aid fix but there are lots of wonder products out there you might try depending on what you find. There are plastic epoxies, spray on waterproofers, old fashioned tar or roofing cement and caulk. Also, you might consider replacing the tank/chamber if you have the area dug out.

 

These tanks look pretty deep, wouldn't I need a pneumatic drill for that?