Why Don't Septic Tanks Just Explode?

[fessie]

I always wonder about this -- during a Cleaning Spasm (which is a rare event) I tend to do the whole house. Ammonia in the toilets, bleach in the sinks.

I know I'm not supposed to mix those. And I don't, within the house.

But they go into the septic tank together.

Shouldn't my front yard have blown up by now?

[silenus]

I don't think you approach the quantities or concentrations needed in a septic tank mix to have any problems.

[NinetyWt]

Three reasons off the top of my head:

Dilution: They get mixed up with a lot of water which is already in the septic tank.
Degradation: Chlorine bleach breaks down into other substances, in water.
Time: Water leaves the tank regularly. What you send down the drain now might not see what you sent down the drain an hour ago.

Jules may be able to expound a bit on water chemistry.

[fessie]

Hmmm -- don't they meet in the pipes, though?

How big IS a septic tank, anyway?

[NinetyWt]

Hmmm -- don't they meet in the pipes, though?

How big IS a septic tank, anyway?

The substances would be dissolved in several hundred gallons of water along with many other substances, in the tank. The time they spend in the piping going to the tank is minimal.

There is also a vent on the top of the tank which lets out any gases. If the gas were to form, it would dissipate out the vent.

It's bigger than a bread box.

COMPUTING EXAMPLE - An Example of Computing Septic Tank Size, Capacity or Volume in Gallons
One gallon of water has a volume of .1337 cubic feet. For a rectangular septic tank, multiply depth (or inside "height") in feet times width times length. Divide this figure by .1337 to establish the number of gallons in the septic tank.

Example 1: how many gallons is held in a a 4ft. deep x 5ft. wide x 8 ft. long septic tank? If the tank dimensions were 4ft. x 5ft. x 8ft. = 160 cubic feet. Using the conversion factor to convert cubic feet to gallons, 160 / .1337 = 1196 - or about a 1200-gallon tank.

One cubic foot of volume can contain 7.481 gallons of liquid. So a second approach to calculating septic tank actual size or capacity in gallons is to multiply the septic tank volume in cubic feet by this constant, which we round up to 7.5 gallons/cubic foot.

Example 2: how many cubic feet and how many gallons are held in a septic tank of typical dimensions of 4.5 ft. wide x 8.0 ft. long x 6 ft. high. (4.5 x 8 x 6) = 216 cubic feet. Since one cubic foot can contain 7.481 gallons, which we round up to 7.5 gallons per cubic foot: 216 x 7.5 = 1620 gallons of septic tank capacity - this is probably nominally a "1500-gallon septic tank".

From here.

[fessie]

Interesting! Thanks!

We do know not to send food scraps down the sink, as they just use up volume space.

[NinetyWt]

Interesting! Thanks!

We do know not to send food scraps down the sink, as they just use up volume space.

You're welcome.

No grease either. It will all coagulate together and make a big grease ball. A fellow engineer told me one time about seeing grease balls down at the treatment plant big as your car. Yuck!

[Jules]

What's in the septic tank is going to quickly use up the available chlorine, bonding it so it isn't available to bond with the ammonia and create chlorine gas. But if a tiny amount is produced in the tank, it's outside and not in an enclosed environment. If it so happens to seep to the surface without bonding to soil constituents (unlikely, unless the lid is askew,) it's not a big deal out there. I release small amounts of chlorine gas every time I change a cylinder, but since it's outside, it's OK as long as I keep my nose out of it. A snootful will clean out your sinuses awfully quick though.

I don't mind putting bleach in my septic tank in small quantities, but I don't want it to kill the good bacteria in there.

Re: grease balls. I keep that to a minimum too, but my septic-tank pumping buddy just cleans it out when he pumps tanks. He says every 3 to 5 years is good.

[fessie]

Do you recommend flushing down a packet of yeast once/month or so?


(interesting about the grease, I did NOT know. time to put a can by the sink)

[NinetyWt]

Yay Jules!

I told you she knew about water chemistry. BTW doesn't chlorine break down to some vaguely carcinogenic compounds in water with a lot of organics in it?

[Jules]

It does. Trihalomethanes come from hypochlorite if the water is loaded. I use well water, so it's not an issue (and chlorinate with gas anyway, so still less of an issue.) You just have to balance your chlorine levels to keep them from doing that. We sample for it in the summer, when it's likely to be at its worst.

I'm no septic tank expert. I just recently got one of my own, and don't have a wastewater license. Yeast probably isn't the bacteria in question though. I think you can buy packets designed to kick start your biological system. We bought a house that had been vacant for two years, and haven't had any problems. Yet.

[fessie]

What is this "cylinder changing" of which you speak?

Are we supposed to be doing something out there?

[Jules]

Whoops, sorry. No, that was an aside to NinetyWt in regards to chlorinating water. You're not all that likely to have hundred pound cylinders of chlorine gas hanging around your house!

[NinetyWt]

What is this "cylinder changing" of which you speak?

Are we supposed to be doing something out there?

To expound on Jules' answer, municipal systems typically add chlorine to water for disinfection. The water department workers have to change out the chlorine bottles, which look sort of like cylinders. Then can get big, too - the well out here by my house uses 1-ton cylinders. Chlorine can kill you in small amounts, so she's also saying she is careful to keep from breathing the stuff when she's changing out bottles.

[plynck]

Septic tanks can build up methane, so yeah there is some possibility of building up to toxic levels. However, methane is lighter than air, and septic tanks are typically vented. In newer homes or buildings with plumbing vent stacks, the septic tank can vent back through the waste pipe and up through the roof. Our old office had a vent stack unfortunately situated rather close to our AC intake. They would have the tank pumped out on a Saturday because opening the tank cover would cause a quick release of gases up through the stack and back down through the air ducts. It wasn't awful, but it was noticeable.

A tank can vent back through something other than your house stack, but you probably wouldn't want to do that. You'd need to get it up high enough, and the odor can be fairly unpleasant.

Actually, even if your house discharges to a town/city sanitary sewer rather than a septic tank, it may still vent back through your house stack. (I live in a city that once had houses use S-traps at the discharge which will stop that, but that is the exception rather than the rule.) Actually, this is how a lot of cities find illegal (storm) connections to the sanitary sewer. They will put smoke bombs in the sewer, and watch the houses on the street for smoke coming out of basements (illegal sump pump discharge) or gutters (illegal downspout connections to the sewer).

After the septic tank, the discharge goes to a distribution box where it is divided into leaching trenches (the disposal field). If those trenches have grass over them, it should be fine. If there is pavement over the disposal field, then the pipes in the trenches are connected at the end with a header pipe, and that header is vented. The odors here are not typically as bad as what you'd get from your tank.

A lot of building inspectors like to use any major changes to your plumbing to get you to put in that vent stack if you don't have one. It's a good thing, in my opinion. It not only moves the noxious gases up, but also aids in proper drainage of sinks and tubs.

We did have a real problem with methane in my (engineering) company once, but it wasn't with a septic tank. We had designed and built a small unmanned pump station at the edge of an old landfill. The landfill was vented, but we had a huge low-pressure weather system move through the area one night. The pressure building up in the landfill forced the methane through the soil and in through the foundation of the building. Once a pump started up, the spark ignited the methane and blew the doors open. Most of the equipment inside was damaged. I was supposed to inspect the building that next morning...

[fessie]

and....???


(fascinating story btw 8-) )

[plynck]

and....???


(fascinating story btw 8-) )

And it's quite lovely here in the afterlife. The streets are paved with bacon.

No, the methane did its thing in the evening, and I arrived at the site the next morning to find a number of amused and bemused city employees.

[Carson O'Genic]

Actually, even if your house discharges to a town/city sanitary sewer rather than a septic tank, it may still vent back through your house stack. (I live in a city that once had houses use S-traps at the discharge which will stop that, but that is the exception rather than the rule.)

All codes to my knowledge have outlawed the S-trap due to their tendency to syphon dry. Whole house traps, or double venting, are also banned.
Current tank design limits back stink with a baffled atmosperic vented inlet.

Fessie, septic tank bacteria are anaerobic- baking/brewing yeasts require oxygen ( "rousing").

[plynck]

All codes to my knowledge have outlawed the S-trap due to their tendency to syphon dry. Whole house traps, or double venting, are also banned.
Current tank design limits back stink with a baffled atmosperic vented inlet.

[/quote]

Yes, I think they were banned in the '60s or '70s? My house was built in 1917. My plumber told me that there were (I think) three cities in Massachusetts that required the traps when houses were built. In looking at some diagrams, I think it might actually be a running trap:

http://chestofbooks.com/home-improvemen ... grams.html

I've had both the kitchen and the bathroom remodeled, and the building inspector never required the trap to be removed.

In our wastewater designs (say in pumping stations) we have sometimes used carbon canisters to take care of odor problems, but I hadn't seen it used with domestic systems.

[Inigo Montoya]

So how much Clorox would I need to pour down the drain to kill a 1200 gallon septic tank?

[Carson O'Genic]

So how much Clorox would I need to pour down the drain to kill a 1200 gallon septic tank?

25oz. But that is a conditional number, depending on contact time, temperature, agitation, amount lost in plumbing traps, etc.