Recently, our AC has started blowing sewer smelling air after we get home at night. During the day, it doesn't run much since we cut it up to 78 degrees while we're at work. It usually smells fine the first time it comes on in the evening, but it really stinks and smells like sewage the second time it comes on. We had an AC repairman come look. The AC drain is not plugged. There is not a dead animal in the unit. The ductwork does not appear to be overly dusty. We cleaned out the toilet vents and replaced a wax ring on the master bathroom toilet. The smell is worse in the master bathroom it seems. We were thinking maybe gas from our septic tank was backing up into the house, but there is no odor coming from the sinks, tubs, etc. It's definitely coming from the AC vents. Also, you can smell it coming from the unit when it's bad. We did recently have a sewer treatment plant installed which catches the runoff from our septic tank. The plumber says there is no way the sewer treatment plant could be making it smell, but it's quite a coincidence the smell happened shortly after the treatment plant was installed. I also notice it during the day on the weekends if we've been using a lot of water, like washing clothes or dishes. We don't know who else to call. Another AC person? Another plumber? We are at our wits end and tired of living in a stinky house. Any advice would be greatly appreciated!

Since no one has picked up on this so far I am going to try to help what I can.

I don't have all the details, so I am going to give a few suggestions base on my hunch and experience.

1)since I assume you have a compressor outside and the evaporator coil inside in either the furnace enclosure or an air handler enclosure with no furnace. Yes/no

If this is the case, you are recirculating the air in your home and not pulling in outside air, or at least only what comes in through opening doors etc. This would tell me that the smell is coming from inside your house and you are recirculating it through the source of the problem or re-distributing stinky air from something in the house.

Look at any unused floor drains that the P trap may have dried up in.

Check where the air conditioner is draining. It may be that someone tied it into your drain system without a P trap. Not likely, but I have seen worse. If this the case, then it was installed incorrectly. Since the smell seems to be stronger there, it is worth checking. The drain hose could be dumping into an open pipe instead of an drain with a P trap and the smell coming back up the drain for the evaporator and distributed with the cold air. The condensate hose or pipe goes right into the same cavity as the evaporator air does.

Did your HVAC tech open the evaporator coil cover and look to see if it was clean? Or did he just look to see if it was draining? Gunk on the coils can get wet and smelly

Duct work can collect moisture, lint etc. and mold. It can also be the home to a dead mouse.

Look for other sources in the home. A broken dryer vent can accumulate lint and moisture and smell really bad.

Your home plumbing if done correctly probably is keeping the sewer gas out, but any unused or little used drains can dry up the P trap and let gas into the home.

Sometimes the overflow section of a bathroom sink can collect some kind of bacteria and make a pretty good stink. This is the little hole towards the top rim of the sink that the overflow goes into and back down to the drain. We have tried all kinds of cleaners and disinfectants to clear it up once it gets in there.

Try these. Maybe one of my plumber friends here have another approach, but try this and get back to me.

Thanks so much for your helpful and thorough answer. Yes, you assume correctly. We do have the compressor outside and evaporator coils inside. And yes, we agree with you that the sewer smell is being recirculated from within the home. To address your suggestions:
1. We have checked the P trap in the one unused bathroom (on the opposite end of the house from the stink).
2. Our AC drains to the outside and not into the sewer system. The AC guy checked the drain pipe and said it was draining properly.
3. The AC tech did look at the coils but didn't say much about it other than that it was draining. Our next step is to clean the coils.
4. The reason I'm not thinking there's a dead mouse in the ductwork is that it does not smell all the time. We don't smell it all sometimes for a few days, and then WHAM! It's back again.
5. We will check the dryer vent. Haven't tried that yet.
6. Will also check the overflow section of the sinks.
I'll let you know what we find. Thanks again!