A longtime Minnesota pumper shares the design he favors for impact probes and manhole covers

When pondering the “tools of the trade’’ editorial theme for this issue of Pumper, I’m reminded of my friend, Larry Fyle, of Nisswa, Minn., and his expertise at septic tank location and making the service routine more convenient for future visits to a residential onsite system.

Though Fyle is retired from pumping, and his service business continues with his son, Tony, he has some solid advice about making pump-outs go quickly and easily. While many of you utilize the same tools and techniques, a quick review might help technicians who are newer to the pumping business.

Fyle and many other sewage system evaluators in Minnesota carry an impact probe — sometimes custom-built — on their service trucks. Fyle showed me the probe he uses to find tanks, sewer pipes, distribution boxes, drop boxes, and underground trenches and beds.

Some probes use a T-handle and the probe has to be physically pushed into the ground. The impact probe is simply “hammered” into the ground until an object is detected. The impact probe is constructed so the probe rod can be “hammered” out of the ground as well.

Fyle demonstrated the probe for me. When the probe hits a concrete object like a septic tank, there is a distinct sound and feeling on the probe. A 4-inch plastic sewer pipe also has a distinct “feel.” It is a little “softer” than a concrete object.

The impact probe will also locate the rock layer in trenches and beds. Again, the top of the rock layer has a distinct feel, and not at all like a concrete tank or plastic pipe.

The impact probe also can be driven through the rock layer, letting the user know when it hits soil again. In this way, the depth to the rock layer can be determined and the thickness of the rock layer can be measured. The total length of a trench, or the size of a seepage bed, can easily be determined without any digging.

A variety of probes are available commercially, or a machine shop with the proper equipment should be able to construct Fyle’s impact probe design using the drawing included here.

Note the tip of the 5/16-inch rod is enlarged. This is to decrease the friction on the rod as it penetrates the soil. It is important the tip be larger than the diameter of the rod, according to Fyle.

Another critical feature is attaching the rod to the shaft piece. Unless the top of the rod is split before it is welded into the shaft piece, it will likely pull apart when the rod is “hammered” out of the ground.

An impact probe is a valuable tool for locating the various parts of an onsite sewage treatment system. This is particularly true of the older systems where there are no plans available, or no surface evidence where any part of the system is located.


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Another convenient product Fyle points out is the tapered and notched manhole cover. The top manhole cover is at the ground surface for easy location of the septic tank. Safety and preventing unauthorized entry are the reasons for having another cover at the top of the septic tank.

If a septic tank manhole cover has a single taper, it can be difficult to remove. The advantage of a notched opening with a combination taper and notch is that the cover rests on the top of the notch and is readily removed.

The method to remove the cover located on the top of the septic tank is shown in the drawing. A chain is attached to a handle in the cover and the cover can be lifted out of the manhole by several means.

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What about septic tank insulation?

In a cold, northern climate, what role would tank insulation play in septic system effectiveness?

In Minnesota and other states with cold winters and a potential for minimal snow cover, the major purpose of insulating a septic tank would be to prevent system freeze-up. In some cases additional heat is added to the tank, particularly if the system is used infrequently.

I am not a bacteriologist, but I have learned from colleagues that one of the optimum temperatures for anaerobic bacteria is the temperature of the human body. This is rarely achieved in a septic tank, but activity by the anaerobic bacteria in a septic tank would certainly increase as the temperature rises.

Typical septic tank temperatures in the summer in Minnesota are 50 to 60 degrees. Septic tank temperatures have become even cooler with the popularity of cold-water clothes washing.

Of course, heat is lost from the septic tank into the surrounding soil. An insulating layer of expanded polystyrene would reduce this heat loss and keep the tank temperature higher. Increased anaerobic bacterial activity would increase the rate of breakdown of sewage solids.

I believe it is a matter of record that septic tanks in the southern United States have a slower buildup of solids than tanks in northern regions. This is likely due to a difference in the temperature inside the septic tanks.

There is a trend for local sanitary codes to require pumping septic tanks every three years and to increase the required size of residential septic tanks. Given the trends, there is little need to worry about increased bacterial activity. The septic tank will be serviced and the solids removed long before any danger of the tank filling with solids. The tank needs to be kept from freezing under severe weather conditions. To prevent freezing is the reason for the insulation. n

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