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difference between greywater and septic?

Posted by ronbow (My Page) on
Mon, Nov 19, 12 at 12:54

Let me start by saying that I have never lived in a house with septic and a drainfield, but I had spent much time in a travel trailer. So I do understand the difference between fresh, grey and black water.

We are building a new house in an area without municiple sewer and without specific regulations regarding sewage treatment. So I am trying to do what's "right" rather than relying on local code.

In researching septic systems, I see that they all rely on a drainfield to handle liquid effluent. Drainfields can even be placed under the lawn where the kids play.

Home owners also spend energy and money to create grey water filtering systems in the hopes of safely using the grey water for irrigation. Isn't this exactly what the septic system does??

So my question is: If I am already installing a septic system, is there any point thinking about separate grey water plumbing and handling? Or should i just make sure that I have a properly designed drainfield?

Items that stand out from my research are:
1) extra filter on outflow from septic to make sure drainfield piping does not get clogged with solids
2) Closely spaced drainfiled irrigation to extend capacity and to avoid stripe effect of well irrigated and less irrigated grass
3) only plant grass over drainfield, nothing with roots that would seek out and invade the piping

Is that anything else that I need to consider before irrigating my entire back lawn with septic effluent? Or am I way off the mark?

Thanks,
Ronbow


Follow-Up Postings:

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RE: difference between greywater and septic?

If you choose a septic system and have the area install TWO leach fields (each sized for your whole house) and a valve at the tank output for alternating the fields. A filter on the tank output to block solids from the field(s) is also a good idea.


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RE: difference between greywater and septic?

" Isn't this exactly what the septic system does?? "

Not exactly.

A drain field is not for irrigation, just a place to allow the (still) contaminated water to soak into the earth and not be a heath hazard.

A septic tank only removes solids, it does nothing for biological contamination.


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RE: difference between greywater and septic?

Land grant universities in each state often have a lot of advice about installation and maintenance of septic systems. Here is an example of what is available:

http://waterquality.cce.cornell.edu/septic.htm

https://courses.cit.cornell.edu/crp384/2009reports/White&Chung_Gray Water Reuse.pdf


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RE: difference between greywater and septic?

Thank you for the replies and links.

I think my skepticism comes from the reported difficulties that people encounter creating and maintaining greywater systems. Much of my informaiton come from this very thorough site:
http://www.oasisdesign.net/greywater/misinfo/index.htm

I also read that 25% of American homes have septic. And the vast majority of the time they work without intervention. So it seems that modern septic system design is robust and reliable. It also seems to be inherently "green" since the off-water is (by desire or by necessity) going directly into a growing plant system rather than adding to the load on an energy consuming treatment station. The only application I can think of where you could use grey water but not black is the irrigation of vegetables. And even that seems frought with hazards (be super diligent about what goes into the grey water, don't store it more than 24 hours, etc.).

Presumably people try to save money on their drainfield and they install as little piping as required, and they wind up with green and brown striped grass (meaing some grass does benefit from the irrigation). If you were to combine your septic disposal needs with your irrigation desires, you could justify the expense of more densly placed irrigation pipes. We want to have a lawn, and we want the kids to be able to play on it, but we don't want to be so wasteful as to irrigate it with potable water.

So I guess my question boils down to: Is there any reason to invest in a grey water system to water the lawn, or can the same effect be achieved with septic tank effluent? The later even seems preferable because every single drop of water which enters my house winds up feeding my lawn.

Thanks.


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RE: difference between greywater and septic?

Sopmetimes grey water is reused within the house. Sometimes building codes interfere. Sometimes grey water reuse could interfere with the septic operation. There are plumbing fixtures, sink. toilet combos, that reuse grey water that seem to get around the pesky plumbing codes,

I don't know that much about septic systems except that everyone in my childhood village had them. We, and I assume others, had a separate dry well for grey water from the kitchen and laundry. I don't believe that the original septic tank had an extensive drain field if it had one at all. It might have been just a dry well. It was not working well enough whatever it was and a drain field was added. Codes change and it probably would have been required from the get-go at this point.


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RE: difference between greywater and septic?

"can the same effect be achieved with septic tank effluent?"

It is still contaminated with all the bacteria of the human gut.

There are multi-tank aerobic treatment systems that digest the water, chlorinate, then de-chlorinate and produce water suitable for surface discharge.

They also require air pumps ad sometimes liquid pumps to move the contents around (though sometimes gravity can do the liquid movement if adequate grading is available).

'Stripes' usually only show up during droughts or with systems not buried deep enough.

Would you like your children playing on the nice lawn knowing that any dirt they touch is contaminated with human feces?


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RE: difference between greywater and septic?

Obviously most of those who have responded here do not know the true function of the leachfield.

Septic waste water treatment is a three part operation.

The septic tank is divided into two chambers by a baffle. The effluent enters the first chamber (sediment chamber) where and heavy non-digestable particulates quickly settle to the bottom of the chamber, while fecal matter may float or it may sink, but either way it quickly breaks down to a liquid slurry. The slurry then passes through the baffle into the second chamber, which is the "digester".

In the digester anaerobic bacteria (bacteria which thrives in the absense of air) consume the biodigestable materials in the slurry and the resultant gray water then goes into the leach field, where the treatment process is completed by aerobic bacteria (bacteria which thrives in the presence of air). The aerobic bacteria break the gray water down to water, carbon, nitrogen and a half a dozen other chemicals that are natural soil nutrients.

Quote:" I don't believe that the original septic tank had an extensive drain field if it had one at all"

That may be what you believe but it is totally false. Originally they required leach fields many times larger than what we are required today. Up until the mid 60's a leachfield for a 3BR house occupied nearly 3/4 of an acre.

As land prices went up and house sizes increased it became increasingly difficult to fit a septic system on the building lots, so they began systematically reducing the size of the leach fields. Today a leachfield is a mere fraction of what was required even as late as the mid to late 70's, but then perhaps that explains why we have to have our septic systems pumped every two years or so, when 30 years ago pumping a septic system was unheard of.


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RE: difference between greywater and septic?

"Home owners also spend energy and money to create grey water filtering systems in the hopes of safely using the grey water for irrigation. Isn't this exactly what the septic system does?? "
I've heard of 'graywater' collection systems that collect only laundry and other non-human-waste water in order to re-use it for irrigation. There is no interconnection with the septic system and no digestion of solids takes place in it. There's only a holding tank and a pump. It sounds like an expansion of the rain barrel or cistern. I've also heard that they are not allowed in most areas of the U.S.


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RE: difference between greywater and septic?

Lazypup, thanks for the explanation of how septic tanks work. That was an interesting read. Your comment about the historically infrequent, but now-frequent pumping of septic tanks makes sense of the fact that, in the old days, they did not seem to be installed to pump easily.

The home that I mentioned is a 3 br house in a rural area but within an (incorporated) village in New York. The yard was no where near a half acre, sloped, but sodden in the lower-lying areas during the spring. I am relatively certain that the original waste treatment system contained no drain field. I am relatively certain, on the other hand, that there was a simple dry well for the septic tank in addition to and separate from the one for the grey water. I was pretty young when the drain field was added.

I dredged up some fainter memories on my drive to work this morning. I remember a lot of talk among the neighbors, whose homes were mostly built within a decade, about "dry well trouble" and apparently ineffectual pumping of septic tanks. I remember seeing the dry wells uncovered for analysis associated with a lot of hand-wringing and tongue clicking. I think that most of those systems had troubles. Over the next few years I saw backhoes digging up several yards, trucks dumping crushed rock and piles of perforated pipes to be installed in the trenches. I think our yard was first. The exercise eliminated a wet, foul-smelling spot in the back yard in rainy weather.

There used to be a crude diagram in the basement with measurements of the distances to the elements of the waste water disposal system. I'll have to ask dad if it is still around. Some time ago, though much later than it should have, the village installed a modern sewer system and waste treatment facility so the original system is not in use.


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RE: difference between greywater and septic?

The discharge water is NOT 'clean' since it remains contaminated by all the normal bacteria of the human gut.

It is still not 'gray water' but black water with solids removed.

It is not safe for surface discharge or close contact.


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RE: difference between greywater and septic?

You seem to have the idea that the sewage treatment takes place in the septic tank and all the leachfield does is allow a place for the remaining liquid to be absorbed into the soil, but that is not true. Inside the septic tank the sewage is digested by anaerobic bacteria and converted from a thich slurry into a gray liquid, but the gray liquid discarging from the septic tank is still toxic effluent.

It must also be mentioned that the leachfield is not just the perforated pipes on the discharge side of the septic tank. The perforated pipes merely allow the gray liquid effluent to be absorbed into the soil below grade where Aerobic bacteria complete the breakdown process. Some ppl than ask the question, how can aerobic (air breathing) bacteria survive underground?The aerobic bacteria survives because normal atmospheric air can penetrate 3 to 4 feet into the soil. The amount of air penetration would not be enough for a person to breathe but it is more than sufficient to support the bacteria.

If the effluent were to go into a drywell as you suggest, the empty portion of that drywell above the effluent would quickly fill up with methane & sulphur-dioxide and there would be no oxygen to support the aerobic bacteria so the effluent would not finish the treatment process.

I would also like to correct a statement you made in your original post:

Quote; "We are building a new house in an area without municiple sewer and without specific regulations regarding sewage treatment."

That is not true, you may live in an area where code enforcement is marginal and many ppl elect to ignore the codes, but there is no area within the United States of America that does not have a plumbing code. If your local jurisdiction has no code, then the state code applies.

Taking this to the next level, the Plumbing Codes are not building codes, they are health codes, and they are enforced by your local board of health and the Federal EPA and while all the other trade inspectors can redtag your job, and demand certain changes be made before you can proceed with your project, a Plumbing Inspector is a sworn law enforcement officer with full arrest powers and they can not only redtag your job, a Plumbing inspector can arrest you under both civil and criminal charges for major violations of the plumbing code.


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RE: difference between greywater and septic?

"If the effluent were to go into a drywell as you suggest, the empty portion of that drywell above the effluent would quickly fill up with methane & sulphur-dioxide and there would be no oxygen to support the aerobic bacteria so the effluent would not finish the treatment process."

I can accept that. We have to assume that the systems were woefully inadequate as originally built.


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RE: difference between greywater and septic?

To the op:

Septic permitting and design is usually handled by the county health department, so forget about designing it yourself with minimal trench spacing to avoid striping and so forth.

To lazypups comments about drainfield size and older systems, he's not takeing into account variation in state rules and permitting. Here in NC, anything installed pre1977 will have about 125-250 linear feet of 3ft wide trench for a 3br home, hardly anywhere near 3/4 of an acre. By comparison, a modern 3br system will have between 400-540 linear feet of 3' wide trench on 9 foot centers.


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RE: difference between greywater and septic?

The discharge from a simple septic tank is still biologically contaminated.

It is NOT the same as gray water from bathing, washing clothes, etc.
This water is considered so minimally biolocigally contaminated it is NOT put into septic system, and used for irrigation in many places.

Of course come places (S. Korea) use black water directly in irrigation (like rice paddies).

The newer aerobic systems with chlorination and then de-chlorination (keeps the EPA happy) produce water safe for surface discharge.


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RE: difference between greywater and septic?

Thank you all for the insight and information. I was confused between what people ought to do and what they do do with their doodoo. Sites like the one below were misleading. But the historical perspective of shrinking lot sizes makes it all make sense. The pictures I've seen on the web of black water drainage fields under the family lawn are only for lack of a better option.

We live in the Israeli controled West Bank. The common approach in the smaller settlements like ours is to dig an "absorption hole" where all the sewage runs and eventually seeps into the ground. Any effort more than that seems disproportionate since our neighbors in the very large Arab villages (under control of Palestinian Authority) run a river of raw sewage above ground directly into the valley.

Since water is scarce and we don't feel right about using drinking water to maintain a lawn, I was looking for the most practical use of our effluent. The more appropriate approach for our situation seems to be to separate black and grey, to use an obsorption hole only for the black and to use the grey for the lawn. This keeps the hard-core sewage away from the people, reduces the load on the absorption hole, and makes the best use of the grey water.

Thanks again to everyone.

Here is a link that might be useful: back yard drainfield


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