Aerobic vs. Anaerobic Septic Systems

[Bird]

I guess everyone knows by now that, after two years of drought, Texas has had lots of flooding problems this year. The Denton Record-Chronicle has a story this morning about the problems folks have been having with their septic systems. I don't know whether this link will work or not without you being registered, and I don't know how long the story will be available, but I'll try this to see.

If it doesn't work, I'll just quote a very small part of the story with information from an official with the county environmental health department:

Almost all the problems are from anaerobic septic systems which depend almost entirely on the soil absorbing sewage, he said.
About 90 percent of new septic tanks are aerobic . . . . because they're not as affected by outside influences, such as saturated soils.

For anyone moving into rural areas without sewer service, it's something to think about. Many of the old style (anaerobic) septic systems work well for many many years, but personally, having had both, I can sure understand why 90 percent of the new ones are aerobic; only way to go, in my opinion.

 

[Unclebuck257]

Bird,

Ours is about 9 years old and is an aerobic system. You're right in that this rain hasn't caused us any problems with our system, but some neighbors with the other system have had many problems due to the heavy rains. There are some negative points about aerobic systems but too much rain causing problems, isn't one of them.

Those mandatory 4Xs a year inspections, along with that $150.00-$250.00 per year fee for those inspections is one of the problems.

 

[Bird]

Yep, I paid $200 a year for those inspections and that was with a good reputable company. That is a drawback, since you could quite easily do them yourself, but the law doesn't allow that, probably because a lot of people would "forget" to do them.

 

[shaley]

Great topic Bird. Septic systems are one of the most misunderstood topics a home owner will ever incounter. The design choices really affect the up front cost of a lot/home. Up here Aerobic units are about 25K and a conventional is about 5K. What is the cost down your way? I'd be interested in what other are paying.

 

[Bird]

Steve, I'm afraid the prices I know about were so long ago that they wouldn't be valid anymore. I notice that newspaper article mentions $7,000 to $8,000 to replace marginal systems, so I would "assume?" that to be anaerobic systems. In 1995, I had a small anaerobic system put in for my parents' mobile home. It cost $3,800 back then; two concrete tanks and the leach fields were two 20' x 40' loops. In 1996, my brother had an aerobic system installed and in 1997, I had an aerobic system installed. Our total cost at that time was $5,000 each, plus about $20 for permit and notary fee, and I think $60 for a perk test (yep, government agencies can really be swift at times. We were required to have a perk test done by a licensed company or individual. If the soil didn't perk well, we had to go with bigger systems or an aerobic system which we were going to use anyway, but they still wanted the perk test). The next year, a brother-in-law had an aerobic system installed by a different company, but it only cost him a hundred dollars more than what my brother and I had paid. However, later the county went from $10 for the permit to $400 (how's that for inflation?). So I don't know what they've cost in the last 10 years or so, but I'd heard before that they were much more expensive in the east.

 

[Mickey_Fx]

I know what the words mean but from a practical standpoint, what's the difference in these 2 systems? How are they built different?

 

[Bird]

Mickey, if you'll do a google search, you can find some companies who manufacture the equipment, drawings, etc. I'll give you the greatly condensed version though.

Anaerobic septic systems have been around a long, long time. The sewage runs into an unvented underground tank where solids settle to the bottom and slowly dissolve or decompose with the assistance of certain bacteria. There are commonly two tanks, or a single tank divided vertically into two compartments. The liquid at the top of the first tank flows into the second tank, then out into "lateral lines", which are almost level, very slight slope away from the tank and with lots of perforations in those lines. The lines are commonly laid in trenches with gravel all around them so the liquid seeps out of the perforations and is slowly absorbed by the surrounding soil. In some cases, some of the moisture is "wicked" to the surface to evaporate. There are many different designs of tanks, metal was most common years ago, concrete in more recent times, different sized tanks, different lengths and layout of the lateral lines.

Aerobic systems may also have completely separate tanks, or simply divisions. The first tank is simply the same as the first septic tank. From there, the liquid flows into the second, or aeration, tank. A small air pump that runs continuously is pumping air to the bottom of that second pump, usually through about 3 small pvc lines and at the bottom are diffusers, so the liquid in the second tank is being continously aerated. I don't know how to describe it but the inside of that tank has a filter, and also has a funnel shape in the center. From that tank, the liquid flows through a line that has chlorine tablets so it's very mildly chlorinated. The third and final tank has a float valve, or switch, so that when the level reaches a certain height, the pump (a stainless steel water well pump in the ones I've seen) comes on and pumps the water out. I've heard of systems where the water is pumped, under pressure, through perforated lines into the soil, although the only ones I've seen had the water pumped out through 1" pvc pipe to one or more pop up sprinkler heads, so the water is used to water your lawn. My brother and brother-in-law only used one sprinkler head, while I used two. They were the type that would revolve 360 degrees.

Now maybe someone can elaborate on this condensed version.

 

[BTDT]

Now maybe someone can elaborate on this condensed version.


Bird, sounds like you have the scoop on the poop.

 

[shaley]

Here is a good read if you like details. Page 6 gets to the difference in the two technologies.

 

[RonMar]

The air pumped into the second tank enhances the bacteria and allows them to more completely digest the solids suspended in the water. The light chlorine treatment on the way out makes the water safe for above ground use. Basically a minature sewage treatment plant. Hence the need for regular inspections as a malfunction would result in untreated water being sprayed above ground.