Sewage Specific Gravity

[weab]

Hello,

I am a structural engineer trying to design a sewage holding tank in a seismic zone. I'd like to know the specific gravity of typical of the combined fluid with solids, not just the solids. This fluid would be pumped into a tank just outside the restroom of an underground mine. This tank will eventually be pumped out but might be close to full for periods of time. You can see that with a full tall tank in a seismic zone, the specific gravity is important.

I have searched all of my old text books and scoured the internet and found nothing significant, nothing that I'd bet my job on. I found informal forums like this that said 1.04 (yes with a zero), or a range 1.4-1.6. Even in eng-tips, I found only one such question from 2001 with a single response which said 1.02. So I figure the answer is likely somewhere between 1.02-1.60. I don't mind using the high number of a range, but I also don't want to unnecessarily overdesign seismic overturning loads by 50%. Even using 1.6, I have no firm backing/proof.

I am hoping that I can get several of you to give me a quick numeric answer that most agree on and tell me the source you got this from, textbook or website or whatever. I'll try to track it down myself to use in my calculations. Whatever help you can provide is appreciated. Thanks in advance.

 

[LittleInch]

The issue is "Sewage" is a very vague term.

What sort of "solids"? Human excrement or something else?

So first look at what your sewage actually contains.

Can solid grit or dirt accumulate in your tank?

If not 1.05 sounds like a good number to me. Siimply based on logic

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

If it was all dried, compacted, solid, it would be 1.6.
I'd use that 1.02 as an average SG.
It floats, but not for long. SG<1
Then pretty much 1.00 until it settles and consolidates a bit.
Probably 1.05
Sludge at the bottom might get to 1.10, if not flushed for a while.
I wouldn't expect more than 1.05
Unless you are drying and compacting it, stick with a low SG.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[bimr]

The specific gravity of municipal sewage is generally within the range of 1.2 to 1.4.

 

[1503-44]

That must be after quite a bit of concentration. It cannot be a direct flow into a holding tank immediately outside a restroom facility.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[weab]

Thanks everyone. This helps as this is what I thought. My engineer has used a specific gravity of 1.8, which is obviously way too high.

1503-44 & bimr - Yes, there is little chance for any significant concentration, but maybe just a little. A worker flushes or washes hands, it goes to a grinder pump right outside the restroom and into a holding tank nearby. There is a vent pipe which could cause a little bit of evaporation, but the tank will be emptied no less than every 3 days. It's underground so temperatures are room temperature to slightly cool. The vent pipe will be piped to some safe place to handle it.

LittleInch - This is literally one toilet and one sink. It's not any huge system. Human excrement, just a little paper as many prefer to use a spray (this is in S.E. Asia), and hand wash water with a little dirt, etc. - that's all this should be.

I was hoping that someone could point to some sort of a source so I could point to something to talk my engineer out of S.G. of 1.8, even if not official info. Thanks for all of your assistance.

 

[1503-44]

Did a bucket test.
1.005
[flush]

Just use 1

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Point him towards a toilet and ask him where the extra 0.8SG comes from?

If you want a bit of margin go for 1.1

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Metcalf & Eddy lists medium strength municipal wastewater as having 220 mg/L of suspended solids. 10,000 mg/L is a 1% solution. That makes the suspended solids to be a .022% solution. The suspended solids have a 1.8 specific gravity. At best that would make the specific gravity to be 1.04.

 

[1503-44]

Wow! Nothings gonna float at 1.8
Needs splash guards.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[hacksaw]

The specific gravity of concentrated sulfuric acid is 1.8.

Waste water nominally 1.0+ depending on the water content.
Waste waster with SG=1.8 would be a concentrated slurry.

 

[CarlB]

Discussed in some other threads recently.
Recommended value varies,63-69 pcf.

https://www.eng-tips.com/viewthread.cfm?qid=506918

https://www.eng-tips.com/viewthread.cfm?qid=512832

 

[bimr]

Metcalf & Eddy lists medium strength municipal wastewater as having 220 mg/L of suspended solids. 10,000 mg/L is a 1% solution. That makes the suspended solids to be a .022% solution. The suspended solids have a 1.8 specific gravity.

If you have the suspended solids content, you can calculate the specific gravity yourself.

solids component of wastewater = 0.022 * 1.8 = 0.0396
pure water component = 0.978 * 1.0 = 0.9780
Adding this together gives the specific gravity of the solution.

Specific gravity of medium strength wastewater = 1.0176.

The weight of water is 1.0176 *62.48 = 63.6 lbs/cubic feet.

It is common to add a safety factor, particularly for plastic tanks, because plastic tanks are more likely to fail catastrophically.

 

[1503-44]

Yes. A safety factor would be good. Then they could use seawater, 64 pcf.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."