We smell sewer gas under our kitchen sink perodically. Sometimes it is strong other times weak. Season seems not to be a factor. We have a dish washer and a disposal. We have a double sink so there are two drains going into a single at the wall. The dishwasher drain goes into the disposal - we rarely use the disposal and it disperses a bio agent each time it is used - this does not seem to have a smell. All pipes appear to be secure. We have tried bleach, etc but seems only to be a temp. solution. What could possibly be the source? See attached photo of our piping config. under our sink.