Our home is 30+ years old and was built before the city sewer line was put it and unfortunately was laid above the level of our basement floor... consequently our house (and all the ones built at the same time) have sumps for basement wastewater. (in our case it is used only for the laundry) We returned home several weeks ago to find the odor of raw sewage coming from the area where the pipes come into the house and where the sump pump is located. We had been away for the week and so I thought it might have been stagnant water in the sump and so I ran three washer loads of water through the sump. The pump ran well but the smell persisted for another hour. My drains all run freely and there is no odor upstairs in the bathrooms or kitchen.

However, now almost every evening around the same time we get the horrible raw sewage smell that lasts for about an hour. There is no smell in the daytime or when we first come home from work.

Any ideas as to what may be causing the smell?

Don't know what is causing the smell but the only remedy would seem to be to redo the lines to the sewer making them drain properly or you'll never get rid of the smell.

So does the sump pump (ejector pump) have a trap in its line before it enters your higher level sewer line. Is the pit sealed and vented? It worked before and now has stopped so likely we can find a fix without changing piping.