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As a sustainability advocate I've always assumed that composting toilets were "better" than septic systems in almost any application.

However I've been reading up a bit on both system types (see Wikipedia links above) and now I'm wondering -- For a well-built and managed system, is a composting toilet always or usually more sustainable than a septic system?

Both systems require both primary and secondary treatment:

  • Composting toilet:
    1. Waste goes into a tank, where a carbon additive (such as sawdust or peat moss) must be added after each use.
    2. When the tank is nearly full, it's emptied into a secondary location for further decomposition, and to ensure all pathogens are neutralized. This could be a biogas digester to produce methane for energy production.
    3. The end result can be used as a soil additive.
  • Septic system
    1. Waste goes into a tank, where a microbial processes occurs, which generally requires a one-time process when the system is first put in use.
    2. When the tank is nearly full, it's emptied into a secondary location for further decomposition, and to ensure all pathogens are neutralized. This could be a biogas digester to produce methane for energy production.
    3. The end result can be used as a soil additive.

If I've understood correctly, the only (slight) difference is in the first step, in that the septic system requires less continual maintenance.

Obviously a composting toilet is a better choice in remote locations or where constructing a septic system doesn't make sense, but is there something else I'm missing?

  • It is possible to use a large covered pile of woodchips as the drainage field of a septic system. – Ian Ringrose Jan 7 at 16:07
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A. The composting toilet will use essentially no water. While the water in a septic system is recycled into ground water, this doesn't help. Water is plentiful, and not in short supply. Clean water suitable for drinking is not. A comment mentioned using rain water. A conventional residential toilet is used about 5 times a day. If low flow, call it 4 pee flushes (1 liter) and 1 turd flush (4 liters) So 8 liters a day. Per resident. A household of 4 uses 30 liters a day.

This Wikipedia article says current usage is 125 l/day usage.

30 l/day = 11,000 liters per year. In my climate (central Alberta) we get about 40 cm of precipitation per year. This would require perfect collection efficiency on a 27 m^2 roof. This ignores evaporation, leaky gutters, snow that slides off instead of melting (about 30% of my precip.) It also means you have to have non-freezing storage for essentially all of it. Roughly 1/3 of our precip is in June, 1/3 in winter as snow, and the remainder scattered. In a low precip year it can be substantially less than this.

Still, if I put gutters on all my roof, and collected it all, I could probably do this. So it's not impractical.

B. You do not have a large mass of concrete with it's embodied energy cost tied up in the septic tank.

There are alternative materials for tanks. Steel, and fiberglass are also used. You can ask a separate question as to which material has a smaller foot print over the life span of the tank.

C. You do not have the chemical external costs of making the PCV pipe for drainage field.

PVC as far as I've found is the only type of pipe available to construct septic drainage fields. In the bad old days clay tile was used. In theory you should be able to use weeping tile (corrugated polyethylene) but I've not seen it used.

Depending on the the design of the composting toilet, secondary processing may amount to storing a barrel in a warm place for a year.

The composting toilet will require a bale of coarse sawdust or fine wood chips. In our area, they give chips away. If you catch the power line pruners in your neighbourhood they will drop them off at your driveway. Buying them (here they are sold for horse bedding) costs $1200 for a 160 cubic yard walking trailer load delivered. Smaller bundles are sold as rodent bedding by pet stores. Based on the Sunny John design, this needs doing roughly on a yearly basis.

The largest expense for the Sunny John, is the room. If built as an outdoor unit, it amounts to a 5x5 foot insulated shed, with room underneath for 2 barrels. (Plastic, $20 each commonly, 2nd hand, $50 each wholesale new) Using salvaged glazing and thermal mass techniques it's self heating even in winter in Alberta, as long as it has a southerly exposure to sun. You would also need a dolly for barrel moving ($250) or hire it done periodically.

This particular design, the Sunny John, had this original link, but at this time you need to use the Wayback Machine to see the page.

It does not require addition of sawdust or peat moss after each use.

In a nutshell it uses a 45 gallon barrel as a repository, and was set up so that the barrel space was in a solar chamber. This kept the decomposition temperatures high enough to be continuously active, and dried excess water. When the drum was full, it was swapped out.

The easiest way to do the secondary process was to keep both barrels in the same chamber, and just swap positions. In the fall either mix it into your compost pile for an additional year of aging, or put it in your garden.

The downside of the Sunny John is that it doesn't retrofit easily. You basically are building a room with a 4-5 foot deep outside accessible 'basement'. Recommended for outside privy use or new construction. Would be very suitable for use in barns, shops, and other outbuildings.

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  • This gives few pointers, but doesn't provide a full answer I think? To consider: a septic tank uses water, but that can be rain and most if not all goes back into the soil. Septic tanks aren't necessarily all concrete, though the alternatives might not be better energy-wise. Similar for the piping. And what materials are composting toilets made of? Last but not least: the continuous addition (for the designs needing it that is) of sawdust etc is something which is hard to ignore from an ecological point of view unless it's sourced right next to your toilet. All in all: pretty hard to compare.. – stijn Dec 22 '19 at 17:15
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In remote and undeveloped locations, composting toilets are considered the first step toward civilization. On the other end of the spectrum, namely cities and large towns, a sewage system is the only option. The middle range is where septic systems become required by code.

When developing human habitats outside of sewer system range, I've had to consider the options. A flushing toilet is a creature comfort and it comes with an expense. I like it better than hooking up to a sewer for many reasons. The cost is lower because you pay a lot over the long term for sewer services, but the cost is higher compared to a composting toilet.

If you (and your guests) don't mind the inconvenience of a composting toilet, then that is the way to go. You get the best compost known to man at your disposal. You'll need to compost it thoroughly, at least a year in a microbial environment, but you cannot buy anything close to what you get in return. Because of our diet, well composted human excrement is superior in every way to anything available on the market.

Keep in mind that these same nutrients can be returned to you in a properly designed septic system, but there are some obvious concerns. Roots will invade your infrastructure as soon as it is convenient to them. They're looking for the water. There's another thing, perhaps even more important than the water, that they are all looking for. They pretty much all need good drainage. A well designed septic system provides this, and it's an ideal place for your fruit trees and other perennials to grow.

This is how I've dealt with that conundrum. I designed the septic system to be vastly above and beyond the needed capacity for the home. This provided a huge area of great drainage for trees and vines. The first few feet of the leach lines, where all the septic drainage goes, are not planted. Between the leach lines are huge compost pits that encourage the plant roots to go there instead. To further prevent too much water in the drain field, I plumbed and provided a greywater system tank that takes the bulk of the moisture for more precise watering on annuals. The key to my approach is to not use the septic system at all unless it's absolutely necessary.

I prefer to pee and poop in ways that are personally more sustainable to me, directly or through several composting toilets, but I do use the modern flushable toilet at times and I offer the option to my guests. It makes the home feel right to many, and it allows poopers to feel as if we are not forced into some primitive lifestyle with which we are not entirely comfortable.

It's clear to me that the right answer to your question is: "Both!" because it's not one over the other that matters. It's how you utilize that which is at your disposal (or for your disposal) that counts.

But to be honest, I'm pretty sure I wouldn't have installed a septic system if it hadn't been required by code. In view of sustainability, poop is a valuable thing and it's a shame to see our waste being wasted.

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