Floor Drain Odor

The floor drain in our basement emits a sewer odor mostly during the Fall (September – December). It doesn’t back up into the basement at all. The trap is full with water, but above the water line is an opening for the foundation french drain. This opening is cut through the cast iron pipe without any fittings to attach the foundation drain. There is another opening about 3 inches from the top that I think may be a cleanout for the drain. There is no fitting or plug for this opening either. A drain line for the laundry tub and the kitchen tees into the floor drain line somewhere past the floor drain trap. When the washer drains suds appear in the trap and I can see some small food scraps at the bottom of the trap.

I suspected that the unsealed cleanout (if that’s what it is) allows sewer gases to flow into the basement. Putting a rag into this opening has not stopped the odor though, so I don’t think it’s the source.

A plumber suggested that the drain lines may be cracked below the concrete. He thinks sewer gases are escaping from these cracks and the coming back through the foundation drain and out the floor drain. Replacement of cracked pipes would be expensive and he said we would not be able to reconnect the foundation train to the sanitary sewer (against current code). A sump pump installation would be needed to get ground water from a storm out of the house – more money. It seems foolish to spend a such a huge amount of money on a hunch.

A friend suggested installing a bell trap drain to eliminate the odors from coming back up through the drain. Is this the best thing to do or is there another solution more appropriate?