What’s the Best Way to Remove Stubborn Solids?

Backflushing and mechanical agitation both play a role in effective removal of all sludges from a septic tank.
What’s the Best Way to Remove Stubborn Solids?
Jim Anderson, Ph.D., is an emeritus professor at the University of Minnesota Department of Soil, Water and Climate and recipient of the pumping industry’s Ralph Macchio Lifetime Achievement Award. Email Jim questions about septic system maintenance and operation at editor@pumper.com.

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Recently I received an email from a reader inquiring if backflushing is adequate to remove solids from the septic tank or if some type of mixing device should be used.

This is another case where the answer depends on a number of factors, including how long it has been since the last clean-out, the degree of sludge and scum accumulation, and the makeup of the scum and sludge layers.

When I started working in this industry, there was an interesting debate among pumpers about whether they could clean a septic tank through the 4-inch inspection pipes over the inlet and outlet baffles. In some cases there was another 4-inch access pipe to the middle of the tank. Rather than referring to inspection pipes, contractors who used this method to clean tanks referred to the openings as pumpout access.

When wastewater enters a septic tank, solids separate from the liquids and form layers of sludge and scum in the tank. The purpose of a septic tank is to hold the solids and break them down. It is the reason there are both inlet and outlet baffles. Effluent from the clear zone between the sludge and scum layers leaves the tank for the next treatment component. Solids that are hard to break down remain in the tank and build up over time, and eventually need to be removed.


Current recommendations are that if the depth of scum and sludge combined exceed 25 percent of the operating depth of the tank, the solids should be removed to make sure solids are not making their way downstream to the next treatment component. It is interesting to note that older publications I’ve reviewed recommended pumping the tank when scum and sludge reach 50 percent of operating depth. In the succeeding years it has been demonstrated that it is advantageous to clean the tank more often, leading to the current recommendation.

We now know that sucking liquid from the tank and then backflushing some of the liquid through these pipes does not adequately break up the solids in the tank for removal. When the tank is pumped, only a small percentage of the solids and usually virtually none of the scum solids are removed. As the tank refills, the scum will float to the top and has the potential to plug the outlet baffle. With the advent of effluent screens we now at least have a safety net to help prevent the scum from moving downstream and affecting other parts of the system, including the drainfield.

Another potential problem with pumping through the inspection pipes is that the baffles themselves can be damaged or removed, which defeats the purpose of the septic tank and the cleaning.

Most codes and manuals of practice require removal of solids through the maintenance hole or after removing the septic tank cover. This way the service provider can observe whether the process of taking a part of the liquid out and then returning it from the truck to the tank is breaking up the solids to the extent they can be removed from the tank. The service provider uses a spoon or shovel during this process to break up the scum. How compact the solids are will determine whether this is sufficient or if additional backflushes or other methods are needed.


When backflushing does not break up the solids so they can be pumped into the truck, other methods can be used. One is to inject air into the tank to try and mix the contents and break down the solids. The more common method is to use a mechanical mixer that acts somewhat like a baking mixer where the contents are mixed until they form a slurry that can be withdrawn by the vacuum pump.

Back to the question, the answer as I said is “it depends,” but in my opinion the service provider should have at the ready alternative tools that can be used to break up the solids for removal.

One last comment on cleaning tanks: I still hear from time to time that the pumper should leave a little in the tank to restart bacterial action after cleaning. This is unnecessary because incoming wastewater from the residence carries enough bacteria to start the anaerobic digestion process in the tank.


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