The undeniable advantages of a flush toilet
In my last post on humanure composting I described how human feces can be composted, how this can be done in a simple, cheap, and efficient way, and why it is less wasteful of water and nutrients than using a conventional flush toilet.
Now, I want to return to the idea of flushing down our human wastes with water. This is not playing devil's advocate, I simply want to look at all possible advantages of that system, to see how they can be utilized in a sustainable way. Also, certain people may have a hard time to adjust to a system they are not familiar with, no matter how efficient it may be. There's nothing to be ashamed about this. We are toilet trained during at an early age, and what we learn during that formative period tends to stay with us all our lives. A flush toilet may in fact constitute a psychological necessity. This need is also important and should not be ignored, though there are real, tangible upsides to flush toilets.
The biggest advantage of water is the perfect seal it forms, blocking any smell from whatever has passed the curve in the pipe. This fact alone may be important enough to consider a flush system, preferably the kind that won't waste our resources like our conventional system does.
Can a flush toilet actually be sustainable?
The two most important questions to ask in this regard are: where does the water for our toilet come from, and where does it go? If its origin is a clean water source, such as wells or aquifers, or any elaborate, energy hungry, purifying systems, I would deem the use of such system for flushing toilets criminal, to say the least. Using potable water to get rid of excrement while others are dying of thirst used to be punishable by death in many cultures. On the other hand, the flush toilet becomes more acceptable if the water has been used, at a couple of times previously, as it happens in Earthships. There the water in the toilet has already been used twice: first to wash (hands, dishes, clothes, shower, etc.), then to water plants in the grey-water planter in the greenhouse.
No Earthship? No problem!
Earthships are amazing at catching water, filtering it, using and reusing it, and still re-reusing it. But what if we don't happen to be living in such a well designed structure? We can still use the water at least once, before flushing it down the toilet. For example, for one of the most common activities linked with the toilet: washing our hands. Japanese toilets have been famous for these kinds of features, but the same idea can be implemented with locally available materials. The picture on the right is my first attempt at this in our Mexican apartment. 
The other question regarding the water's final destination is equally important. Since there is no “out” or “away” where you could throw it, another option must be sought. Can humanure in water be composted? It sure can, once it has been removed from the water. California Red Worms do an amazing job at it. In one place in New Zealand I had the pleasure to help set up such as system. Inside of the house were a seemingly standard bathroom with a flush toilet and a kitchen with a garbage disposer in the sink. Both drained into the same bin behind the house, a concrete basin of 180x60cm, with a depth of 70cm (6 x 2 x 2.3 feet). In the bottom we laid a loop of drainage pipe, leading out through the drainage hole of the basin, and covered it all with gravel. On top of the layer of gravel we suspended a mesh basket. This is where the solids will remain, and where the worms live. The water would drain away immediately while the solids stayed feeding the worms. When the basket is full of compost it can be lifted out and emptied, something we estimated to happen every 1-2 years. Meanwhile, the occupant of the house could enjoy full western comfort in kitchen and bathroom, while knowing that their resources would not be wasted.
But what happens to the water?
Once it has left the worm-bin, the water flows through a series of basins. The first one is a filtration tank, with layers of bark, scoria, sand, and charcoal. The next step after this tank is the 97 liter (26 gallon) dosing chamber. This feature of the system ensures that when water enters the soakage trench, it spreads out evenly. Without the dosing chamber the water would dribble little by little, every time the facilities are being used, without much pressure, and accumulate in the lowest section of the trench. However, if everything is flushed at once, the entire pipe fills up evenly. This type of dosing chamber works with a pressure siphon. The lack of moving parts guarantees the low maintenance aspect. More details on it can be found here.
Eventually the water drains down the hill into the pipe inside the trench. The trench is 50 meters (164 feet) long, following the contour line of the hill. Around the pipe the trench is filled with more scoria as well as charcoal. This will soak up the water as soon as it comes out of the pipe, and more microorganisms can feed on it. A bit further down the hill a line of citrus trees make use of the water that passes onward through the trench. Still further down the hill, once it has passed through the filter, the trench, and the orchard, the water goes into a pond. A red aquatic weed grows on the surface, which can be skimmed off and used in the garden. It is very rich in nutrients and can be either dried, used as mulch, or mixed into the soil. In any case, it returns the excess nutrients back into the food circle, allowing us to eat our own wasted in form of healthy, tasty, nutritious food.
Other possibilities?
Yet another system I have seen doesn't even need a hill to flush and spread out the water. The concept is quite similar: Nutrient-rich blackwater from the toilet goes into an 'infiltrator', an inverted half-pipe buried underground, with about 60 cm (2 feet) of pumice or scoria under, around, and above it. This buried dome is empty inside, that's where all the water from the house drains into. The water seeps down, into the many tiny pockets in the volcanic rock, where microorganisms get to compost it. Fruit trees are planted all around the infiltrator, and sunflowers on top of it. Their roots reach around and under the half-pipe, taking up the water and the nutrients.
Design for the proper scale
With so many ways to utilize the nutrients from human wastes, whether with or without water, there really should not be an excuse for not composting humanure. No matter which option we go with, it's always important to design the system at the right scale: don't make it too small, which can be quickly overwhelmed by a sudden peak of visitors, but don't make it too large either, which would leave its dependents (worms, trees, etc.) without the water and nutrients they need. Most importantly, make sure all the water you use is taken up, to avoid ground-water contamination, as it is often the case for old-style outhouses.
If you liked this post, please check out my blog and my previous Steemit post on humanure composting:
Taking Care of Business 1 – Humanure Composting with a Bucket
Sources: 1, 2, 3, Pics: 1, 2, 3