Question:
Does anyone have ideas on why septic tank concrete can break down above the liquid level at homes with water softener discharges when the salt brine is not in direct contact with the affected area?
Answers:
- The final answer is: bacteria. Some species of bacteria oxidize sulfur into sulfuric acid, which attacks concrete in septic tanks and wastewater pipes. Fortunately, these bacteria are slow growers, but given the right environment, they can make a mess of concrete within a fairly short time span. Now, this may not be the only cause but it is a major one. And just because someone will ask: No, these bacteria are not the ones you would find in your typical septic or drain treatment.
Anaerobic bacteria in the septic tank produce sulfides. If the sulfides react with metals, mostly iron, from the water and waste, then they form the black sludge in the tank. If the pH is too low, then the sulfides can form hydrogen sulfide gas, the rotten egg smell. Since you can smell it, some of the hydrogen sulfide gas is in the vapor phase. In the vapor phase, this acid can attack the concrete and corrode it the same as any other acid. Look to the pH in the tank as the basic issue. I might also add that hydrogen sulfide gas is one of the most deadly gases known. We are lucky that we can smell it at concentrations far below the lethal level.
- I don’t know for sure, but there may be sodium ions in the vapor above the water level. Since sodium is much more reactive than calcium, it will replace the calcium in the concrete (the calcium silicates from the Portland cement form calcium hydroxide and a gel-like calcium silicate hydrate when water is added). This is why brine is used to recharge the media in the water softener: The sodium in the salt replaces the calcium from the hard water.
I have no data on what is present in the vadose portion of the tank, but I assume there could be sodium ions similar to sea spray in addition to the acid gasses (hydrogen sulfide) that are present in all functioning tanks.
- There are no sodium ions in the vapor above a solution of salt (sodium chloride). Ions only exist in solution. Ions in the vapor phase require special circumstances such as very high temperatures, several thousand degrees, and high vacuum. The corrosion caused by salt water on concrete is not so much on the concrete as on any rebar or iron reinforcing rods in the concrete. Several references to this can be found by searching under “salt water corrosion of concrete.”
Again, the main culprit is the hydrogen sulfide gas formed in the septic tank. The acid is the problem. As we know, concrete is washed with hydrochloric acid to clean it, and excess will actually dissolve the concrete.
