A septic tank should be pumped at least every 2 years. Those that have tanks that have not been pumped for 20 or 30 years are lucky to have a still functioning drain field. Those drain fields are likely in areas where the soil is remarkably capable of filtering effluent. Most septic systems fail when solids get past the baffles in the tank and enter the drain field, thereby clogging the drain lines. Rid X is not a miracle cure. Pumping your tank on a regular basis is the best thing you can do for a septic system. Now if there are only 2 people living in a home, you could go longer, but if there are 5 or 6 living there, you should pump it more often. Anyway, when I hear people say they have not pumped the tank in 20 years I get this feeling in my gut that what we have is likely a sluggish, nearing the end of it's life, drain field. There are exceptions but the exceptions are few. NOW, back to the odor thing. Do you or your neighbors have a grey water line? This would be a line where sink and laundry water are discharged to either a dry well, or to the surface. Where septic systems are failing people often do this to keep excess water from flowing into the septic drain field. These grey water lines can often emit a pretty foul odor to say the least. My other suspicions are that your tank has yet to fill with water which could be the cause of the odor coming up through the tank lid. Or, you have effluent rising to the surface due to a failed drain field. If your drain field is really green during times of drought, this is a sign of a slow field, if you have effluent at the surface this is a sign of a failed field. Did any water pour into your tank after or while it was being pumped???