Let's pretend that my leach field suddenly malfunctions ... but we need to continue occupying our house ... so we route the septic tanks effluent through a progressively more aggressive series of 4 or 5 or 6 filters ...

The water input into our home is flow-limited by our whole house water filter ... so that's convenient - we can't produce water flow that would overpower similar filters on the other end ... (at least, not when they're new - see below ...)

I'm thinking an initial 100 micron 20gpm filter flowing into a 50 micron flowing into a 20 micron flowing into a 5micron ... and then you'd duplex the output of the 5 micron filter with a wye and flow, in parallel, to two 0.5 micron filters ... and the 10gpm filter would give you 20gpm total since you duplexed it.

At this point, we have water filtered to 0.5 microns coming out the end and still maintain 20gpm capacity.

I have two initial questions:

What is the status of effluent filtered to 0.5 microns - regardless of the practicality or expense ? How "dirty" is it and what is it dirty with ? How close, or far, am I to "clean" water that is no longer considered sewage ?

Is there some aspect of effluent that makes it particularly difficult or unwieldy to filter through normal "big blue" 20" housings and the various media one can fill them with ?

The difficulty that comes to my mind immediately is that filters like this decrease flow over time as they become filled and you might suddenly drop below, for instance, 12 gpm of total flow outbound, but be producing 13 or 14 gpm from the house ... and that would be bad. That's not that hard to solve, though ...


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    I don't think you can do it with filtration alone. You seem to be assuming all contaminants have the property of "largeness" - think about urine. Look into what NASA does to recycle waste on the ISS. Commented Oct 6, 2022 at 3:29
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    @Jasen yes, but the primary contaminant in seawater is salt, which is water soluble and can be washed away simply by bypassing a lot of the water. I.E. for every unit of salt-free water, you're intaking 8 units of 35,000 PPM salt water and ejecting back to the sea 7 units of 40,000 PPM salt water. That assumes you have lots of bypass water. Sewage processing is well understood, no need to reinvent the wheel. As for the "saturated leach field", real easy - have 2 leach fields and throw a valve, now you're on the other leach field. Commented Oct 6, 2022 at 3:35
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    To be clear, you're not asking when the water is potable, only when it is no longer sewage right? If so, where does your output then go? A creek? The forest?
    – P2000
    Commented Oct 6, 2022 at 4:01
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    A leach field contains trillions of organisms that process the effluent, so it's not just a filter.
    – Armand
    Commented Oct 6, 2022 at 4:43
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    VtC as hypothetical. From the help center: "You should only ask practical, answerable questions based on actual problems that you face. " (emphasis added)
    – FreeMan
    Commented Oct 6, 2022 at 12:10

4 Answers 4


Let's pretend that my leach field suddenly malfunctions ... but we need to continue occupying our house

So, you have the pump truck come on a regular basis to pump out your tank, while having a new leach field installed in the area set aside for the replacement leach field. And you minimize water use.

A septic tank and leach field is a filtration system. (For @Armand, I'll note that bio-filtration is involved, rather heavily.) There are alternative designs (Aerobic Treatment Units) for areas with poor leaching conditions that do the leach field's part in an additional tank with aeration. You could have one of those installed, but it's rarely cost effective if a leach field is an option.

That is the "filter" you need to get clean water out the other end.

Canister filters? Hah! NO.

You can also improve the odds for your leach field by having your septic tank sludge pumped out regularly BEFORE it's too much and clogs the field, and by retrofitting a filter on the output of the septic tank to prevent entry of large particles into the field. The former has always been true, the latter has become normal in the past 3 decades or so. Those filters are not terribly similar to your canister filters - they have to work on gravity head (3-4" water at the most) and are designed so they can easily be removed (plugging the output) for cleaning when needed (normally just at tank pump-outs. But far better a clogged filter than a clogged field.)


it can be done, but it's rarely economical as small scale.

For example it's done on the international space station, but shipping water there costs $10000 per glassful, so they have an incentive.

on a larger scale:

Your average municipal water treatment plant turns waste water into river water, and the freshwater treatment plant turns river water into potable water.

waste has lots of particles in it that need to be removed these would quickly clog an ordinary filter.

  • OK, but what clogs a 50micron filter doesn't clog a 100micron filter (and so on) so if 5 in a row is unwieldy (clogs too quickly) then ... just add a 6th in the front ? Each of these filter bodies is less than $100 and the high micron filters are not that expensive either ... even if I need to get to 0.2 micron @ 20 GPM that is still just a four-plexed point of use 0.2 micron filter ... it's not that interesting or expensive ... what's left after 0.2micron ?
    – user227963
    Commented Oct 6, 2022 at 4:22
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    there's still the problem with dissolved ions, .0005 micron needed to stop nitrates,
    – Jasen
    Commented Oct 6, 2022 at 4:27

The big problem you're going to have is viruses. When people originally figured out that microbes could be classified into bacteria and viruses (long before knowledge of DNA, etc.), the distinction was simply that a virus was something that you couldn't filter out, because it was too small.

There are lots of nasty viruses that can be present in human waste. Hepatitis is a very common example. New York now has polio transmission in the wild again, due to low vaccination rates in some communities.

At "toilet to tap" water treatment plants, there is a multi-stage process of treating waste water. There is a filtering stage, but it doesn't get rid of viruses. For that, you need a final stage where the viruses are killed with chlorine, ozone, or UV light.

Recycling gray water is a good environmental thing to do in places where water is scarce. But basically the definition of gray water is that it doesn't include anything from the toilet. (Stuff like shower water can contain small amounts of human feces, but in practice it isn't found to present much risk in the typical setup where people are using the gray water just to flush toilets or water plants.)


No F'ng way! You can filter out particulates but not bacteria! You'd have to have a HIGHLY DESIGNED system to turn effluent into a place to dispose of it that's legal. Probably involve UV irradiation / treatment. Probably nearly impossible to get a permit. Certainly not drinking water.

Since this seems like a hypothetical question, my hypothetical answer is if this happened, to get your septic tanks pumped on a regular basis until you can repair or replace your leach field (in my part of the country called a drain field). Unless soil conditions are quite unusual it would not be normal for a leach field to suddenly fail unless it was in a flood. Even if it got flooded by a leaking toilet, normally not using it for a few months allows it to "heal".

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    I think there are bacteria smaller than 0.5 micron - but not much smaller ... the smallest known bacteria is 0.2 micron so that's not too far off ... I don't think bacteria is the problem here.
    – user227963
    Commented Oct 6, 2022 at 3:51

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