Odours from a septic tank are not only offensive but may also be a sign that a health risk exists. Odours can come from five different places in the septic system: inside the home, near the septic tank, near the pretreatment area, near the soil treatment area or in the yard. Knowing the source of the odour is essential in treating the problem.
Consider plumbing problems which can cause septic odour in the house. Make sure that traps in basement drains are periodically filled with water. Check the plumbing vent pipes that vent gasses from the house up through the roof, these can sometimes get clogged with leaves and other debris. Finally examine the cover on the ejector sump pump basket for an improper seal.
Inspect manhole covers for strong septic odours coming from around the septic tank. If the manhole cover is not properly secured it may be easily put back in place to get a good seal. If the cover is a modern plastic one it may have a rubber seal that needs to be replaced. Weather stripping can provide a temporary seal but will need to be replaced anytime maintenance is done on the tank.
Understand the use of pretreatment areas and whether or not the system in question has one. Many modern septic systems will include a pretreatment area such as a aerobic treatment unit, constructed wetlands, recirculating sand and textile filters. If the system has one of these and there is a strong septic odour in that area, a licensed professional trained in the specific pretreatment unit should be called.
Inspect the soil treatment area if the septic odour is coming from there. Inspection pipes should be checked to make sure they are covered and not broken. If the ground around the area is soggy (and the rest of the ground is not) it could indicate that sewage is coming to the top and is a grave health risk. This problem requires calling a professional to correct it.
Check the height of plumbing vent pipes on the roof for septic odours in the yard. If the pipe is not high enough to allow the wind to carry the septic odours away they can fall back down causing a general septic odour in the yard. Carbon filters on the plumbing vent pipes can also help in mitigating the odour.