Composting Sewage For Biogas And Water

The normal way of processing sewage in America is to dilute it with incredible amounts of fresh water, sterilize the resulting sludge and then dump it into the rivers and the ocean. This is enormously wasteful and unsustainable for four reasons.

It wastes an incredible amount of fresh water, even in the desert where water is scarce. Even places like Arizona are doing this.

Secondly it wastes the valuable solid organic material that could become an enormous amount of fertilizer. I have talked to urban planners in Afghanistan who say they are baffled at the way so much organic waste is dumped into rivers as pollution rather than being used to create arable land and turn the desert into a garden.

Third, most of the contents of sewage are already water. We could be reclaiming that fresh water through distillation and purification. And in fact some arid regions are already learning how to do this.

Lastly, the way we process sewage today leads to biogas being released into the atmosphere. Methane is the main components of biogas, (as well as natural gas) and it is a one hundred-times-more potent greenhouse gas than carbon dioxide. Anything that we currently use natural gas for could be powered by biogas reclaimed from sewage, and it would mean drastically reducing the greenhouse effect of the biogas that is currently just being released into the atmosphere unburnt.

I’ll repeat that point because it’s not intuitive. One molecule of methane is 100x worse as a greenhouse gas than a molecule of carbon dioxide. When you burn a molecule of methane, it turns into a molecule of carbon dioxide, and you get two molecules of water which can then be condensed out as fresh water. This means burning methane gets you energy plus water and it removes that extremely dangerous methane from the atmosphere.

 

A Better Way

Capturing biogas during the reclamation of sewage is radically sustainable, as I explain in an upcoming VICE documentary, because it turns all these waste streams from outputs to inputs. It captures that valuable fresh water from the sewage and puts it to use as recycled fresh water. It captures that biogas to power stoves, heating, and even power generators. It doesn’t waste the enormous volumes of fresh water that cities are currently using to dilute sewage before they dump it into the rivers,. It protects the rivers from being polluted by sterilized sewage. And lastly it converts that solid waste into valuable fertilizer for growing food.

There is every reason to switch to sewage reclamation, and no reason not to.

 

Stage One: Biogas Reclamation

First let me say that a DIY solution would be far cheaper and more effective, but for the purpose of illustration here’s an example of a product that accepts your home sewage and compostables and converts them into biogas and compost…

Biogas Small

Now here’s a slightly larger example which is buried like a normal septic system but with gas capture integrated.

 

The basic idea in both cases is that you have a large sealed container like a cistern which has gas-traps (U bends) for input and output. You start by inoculating the container with composted manure from various animals in order to get the right bacteria into the system.

Now your organic waste goes in through the input gas-trap to the bottom of the tank where the mixture is anaerobic or without oxygen. This starts the biogas production process, and it displaces older material that has been in the tank for a while, pushing it up higher in the tank. This allows that exhausted older material to flow out to feed plant beds.

Meanwhile the gas is trapped at the top of the tank. This means the force of the water is applying pressure behind the gas to drive that gas out through your pipes to feed your natural gas appliances. For larger systems, you could also use a pump to compress that gas into cylinders for storage, sale, or transportation.

Examples

Arcosanti uses a very similar system. Their sewage flows through a tunnel down the mesa and it is dumped into the bottom of a retention pond far below the arcology. Here’s me exploring the tunnels…

Acrosanti tunnel

Once the sewage gets down to the bottom of the pond, it displaces older material upwards towards the surface where more oxygen is available. This means the sewage moves from an anaerobic process to an aerobic process.

 

Bacteria are either aerobic or anaerobic. They can survive in one setting but not the other. This is why cities use both processes, because it means anything from the original input is dead once it gets through both cycles. So at this point, all the original bacteria area dead, replaced with decomposers. The water on the surface of the retention pond evaporates over time, leaving solids below.

Unfortunately, paradoxically, Arcosanti is not using this extremely valuable material for any purpose. It just sits there in the pond.

 

Step 2: Fresh Water Extraction + Purification

This part is relatively easy. There are many best practices for purifying fresh water. Countless products exist for this purpose already, depending on the scale of your setup.

On a small scale (individual or family), I think the best option would be to allow the output of the biogas system to flow through a vortex filter (sometimes called swirl filter or siphon filter).

Vortex Filter

This is a technique for removing solid wastes which enjoys widespread use in aquaponics systems where fish water needs to be continuously cleaned to remove the solid wastes. One of the huge advantages of this kind of filter is that it does the hard work of separating the solids and liquids into separate pipes for you.

Next, you can use a multi-stage permanent filtration system to remove any particulates from the water. Here is an example from Taos Earthship Biotecture

Multi-stage permanent water filtration system

In this example, each stage of filtration is permanent. They are using spin-down filters which means the solids are collected at the bottom where the valves are, just like the vortex filter above. The only thing I would change would be to add a solids collection tube so that the valves can be easily purged into there. Then this process could simply be automated to purge the valves on a regular basis and keep the filters clean.

The last stage of this filter to the right is a ceramic drinking water filter. I would probably add a carbon filter before that to remove any lingering tastes or smells, and a UV filter to make absolutely sure that any complex organics like drugs, hormones, and protozoa are destroyed.

Now you have completely pure fresh water from sewage, and you didn’t even have to waste thirty times more water in the process.

All these filters have have purge lines which they direct the solids into. These can all run into a compost heap or outdoor planter where the solids can be used for anything from amending compost, feeding insects (to eat or feed to chickens), or even growing mushrooms.

 

Step 3: What To Do With The Fresh Water

Despite the fact that you now have perfectly purified fresh water that is completely safe to drink, many people will probably still feel some yuck factor. There is therefore some debate about what to do with this reclaimed water.

Israel is pumping its reclaimed water into the aquifers with the idea that it will filter through the land and into the many wells people have drilled over the years.

There is another option for removing the yuck factor which I think people will enjoy even more. The water can be used to water food crops and plants. These will produce an enormous amount of humidity in green houses which can then be condensed in any number of ways to provide fresh water which has been filtered and purified yet another time.

This is already being done in Taos and elsewhere. The main difference I propose to implement is simply that the sewage water should be reclaimed and reintroduced into the indoor water cycle, eliminating the need for new water inputs, and removing the sewage outputs; closing the open water loop. Taos is currently just dumping their sewage into outdoor septic beds which can’t grow any food because the sewage is not treated or sterilized so anything growing in it will be contaminated with dangerous bacteria.