Tank Deterioration, Part 2

With readers’ help, maybe we’re getting closer to explaining why some concrete septic tanks are having issues

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I received another interesting letter concerning the deterioration of concrete septic tanks, particularly compartmented tanks. The letter from Lupe De la Garza of Texas may explain the reason for some of the concrete tank deterioration. He sent along a diagram that I’ve included here, with the following comments:

“This tank (of concrete construction, shown in the accompanying diagram) is what I use in Texas. I use the No. 1 tee fitting allowing air to flow from one chamber to another. We have no troubles with these tanks, but if you build a tank and use the No. 2 fitting and not No.1, then the tank is full of water and doesn’t allow gases from the second compartment to vent out. We have one company that builds two-piece tanks with seams in the middle of the tank; their tanks have no way of venting, so every time we open one up, the second chamber is thin on walls and top above the water level. So, it is how the tank is built; it doesn’t have anything to do with concrete itself.”

De la Garza goes on to conclude (I have modified his comments a bit for clarity):

“A concrete septic tank that is poured in two sections, and then put together in the middle, has no vent from the second to the first chamber. There is no way for the gases to escape from the second compartment. The concrete in the second chamber is thin above the liquid level after four years.

Not all concrete tanks have a problem. So it would be nice if you would not write these articles about concrete tanks in general.”

Lupe chastised me a bit as he felt I was unfairly criticizing concrete septic tanks. This of course was not my intent, but I do mean to challenge the concrete industry. Apparently action is needed to solve the problem of excessive deterioration in concrete septic tanks based on reports I have been receiving from readers.

Hopefully, we are together on a path to solve some or most of the problem.

 

CONCRETE MANUFACTURING VARIES

First of all, there apparently is a considerable variation in concrete septic tank manufacturing procedures. One reader reported that tanks made by one manufacturer did not deteriorate, while tanks from another manufacturer needed to be replaced in about 10 years.

In my opinion, there has to be considerable differences in the processes used to make concrete septic tanks. There may be a difference in the type of cement used, the wetness of the mix, and maybe in the curing of the concrete tank. Any one, or all of these procedures, will likely affect tank performance.

Now I will state my opinion of the situation from what I have gathered from readers.

One reader reported that older tanks were not deteriorating, but newer ones were deteriorating. Is it possible that the older tanks were single-compartment tanks, while the newer tanks were two-compartment tanks with no ventilation between the compartments? This is what De la Garza suggests: that inadequate ventilation of the second compartment is the problem.

I would suggest that other onsite professionals observe the ventilation, or lack of ventilation, between the compartments of compartmented septic tanks. I am of the opinion that this may be a major reason for the deterioration of the second compartment only.

 

OTHER POTENTIAL CAUSES

The absence of a scum layer was blamed by one reader for the deterioration of the concrete in the second compartment. I have difficulty accepting this as the reason for more deterioration in the second compartment. I am inclined to believe that the gases generated by anaerobic bacteria are more concentrated in the first compartment but are allowed to escape through the plumbing system vent pipe. I suspect that the gases in the second compartment do not have an opportunity to escape if the tank has improper ventilation.

One reader observed that pumping station concrete tanks were suffering severe deterioration. There should be very little hydrogen sulfide gas generated in the pumping tank.

Consider the operation of a septic system with a pumping tank. Effluent flows slowly into the pumping tank as sewage flows from the home. There will be considerable time before the liquid level controls in the tank start the pump. The air or gases in the pumping tank are slowly replaced by the effluent. When the pump operates, the liquid level drops quickly, likely in less than five minutes.

This causes a partial vacuum in the pump tank. That vacuum must be satisfied as dear old Mom Nature abhors a vacuum. We learned about that in high school physics. Does the gas flow in from the septic tank to reduce the vacuum? Is this gas mostly hydrogen sulfide? Should the pumping tank have a vent pipe to allow fresh air to satisfy the vacuum in the pump tank? Is the pump tank manufactured by different standards of concrete quality than the septic tank? Questions, questions, questions … What is the best solution? Please share your opinion with me and I will share them with other readers.

 

A WORD ON PLUMBING VENTS

One more observation by De la Garza: In a phone conversation, he mentioned an “automatic” valve is often installed in the vent pipe of the plumbing system when a homeowner is experiencing odor problems because of a low roof. The “automatic” valve does not allow gases to escape out from the plumbing system, or a septic tank, but when a toilet is flushed, the vent pipe allows air to flow down into the plumbing system so a vacuum is not created.

He always leaves one open vent, always the highest on the roof of the house. On flat roofs he extends one vent high to move gases away from the house. This seems to work very well.

And there you have it! Tank construction? Absence of a scum layer? Improper ventilation? What is the answer?



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