Finding buried septic tanks can pose issues, especially if there are no records nor any obvious clean-outs to use for locating purposes. Here are some suggestions of ways to locate tanks that are buried.

1. Start at the county/local regulator. Some county records have as-builts that were drawn during system installation. I’ve seen some as-builts that triangulate the location using dimensions from two different locations. Triangulation gives very accurate locations for where to dig. Some county inspectors are now using GPS/GNSS to locate tanks and distribution cells with 100% accuracy. But there will always be those old tanks with no records at all.
2. Locate the clean-out in the basement if there is a basement. Open these carefully and be prepared for sewage to possibly pour out of clean-out in some instances. In the case of “hung plumbing” (through the basement wall as opposed to under the floor), we’ll place a plastic garbage can beneath the clean-out and plastic sheeting on the floor). Once the cleanout is open, measure out the tank with a tape measure. The side of the house the clean-out is facing (the wall it is going through) gives a good indication of what side of the house the tank is located (though not always).
3. Flushable transmitters. I’ve used Prototek flushable transmitters (and their locators) for decades with 99% accuracy. I remember the first time I ever used it. I got a call from a customer who could not find their septic tank cover. When I pulled up, the customer and his brother had about 30 holes dug in the yard. I flushed the transmitter, located the transmitter within minutes and stuck my flag right between two of their holes. When they stuck their shovel in the ground there they got the tink of hitting the manhole cover right below the surface. 
   1. Recommendations for using the flushable transmitter method:
      1. The transmitter is approximately the size of an egg. In bad cast iron or other pipe with buildup inside, be careful not to get this jammed in a pipe. Please note I’ve never had that problem.
      2. Make very certain to turn the transmitter on before you flush. I have a habit of turning the transmitter on and holding it near the locator to "test" that the transmitter is working (their batteries will die). The transmitters are expensive enough that you want to learn from my mistake that I made (only once!) and make certain it is turned on before you flush.
      3. The toilet has to have a good flush to get these to go down. If it’s an old toilet with no flush velocity, I place the transmitter in the toilet, then take a 5 gallon bucket and pour it in the toilet fast from about 3 feet above the toilet. I’ve only had to do that three or four times. 
      4. It’s possible to have some slight accuracy issues the deeper the pipe is and if the pipe is metal (cast iron). This issue seems to be rather minor as I’ve had great success with this method.
4. Sewer camera with transmitter. This seems to be the best and most used method of finding lost tanks. And if you don’t have a sewer camera, you really need to get one. The sewer camera is one of the most used and powerful tools I have and it is invaluable for giving your customers the best service possible. Some will say that sewer cameras are expensive. All important tools are expensive — charge for them. It’s another profit center. I do not look at it as an expense, I look at it as a way to provide an important service that I can charge for. Back to locating the tanks — find a clean-out. Send in the camera and watch the screen. The wonderful thing about today's sewer cameras is if the pipe is full of opaque water and you can’t see a thing, the transmitter will still locate what you are looking for. I cannot stress the use of a sewer camera in our business for many reasons and this is one of them. Sometimes when there is no clean-out available a toilet has to be temporarily removed for camera/locator access.
5. Thermal imaging (infrared). A reader asked if anyone had luck using thermal imaging or infrared cameras to find tanks. It just so happens my brother has a company that not only uses infrared/thermal imaging cameras, he and his staff are very highly certified in their use, so I asked about locating septic tanks.
   1. I was told that infrared cameras work by detecting and measuring heat. So let’s imagine we are outside on a hot August day. If you aim the thermal imaging camera at the ground all you will detect is the ground surface itself as that will be hotter than anything below grade. You need a 20 degree Fahrenheit difference between two objects to pick up the difference between them on an infrared camera. It would actually have to be 20 degrees colder outside than the temp in the tank itself.
   2. This winter they are going to test if it works because the ground will have to be cold for this idea to work. I’ll report back. 

In the meantime, keep using those flushable transmitters and sewer cameras for accurate tank locating.

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About the author
Todd Stair is vice president of Herr Construction, Inc., with 34 years’ experience designing, installing, repairing, replacing and evaluating septic and mound systems in southeast Wisconsin. He is the author of The Book on Septics and Mounds and a former president of the Wisconsin Onsite Water Recycling Association.