Closed Loop Water System with Integrated Biogas Capture

I propose to build a simple proof of concept system which simultaneously solves many problems associated with water use sustainability.

Key Problems With Current Strategies

Essentially all municipal sewage systems around the world are still using the prehistoric systems pioneered in the indus valley civilization over 6,000 years ago.

We take fresh water, use and pollute it in our homes, then dump it into the sewer. Once in the sewer, municipalities dilute the sewage, sterilize it, and then dump it into the rivers and the ocean. This pollutes the rivers and ocean, wastes valuable organic solids, and drains our limited sources for fresh water.

There is a better way, and it’s far less complicated. If we are going to take water from aquifers, then it should go back to the aquifers rather than being dumped into the rivers or oceans.

Uses of Water

The first step in a more sustainable alternative system is a well which draws water up from the aquifer. That water is then filtered and used for three initial purposes:

    • Drinking
    • Bathing
    • Washing Hands

The water that does down the drains in sinks and showers is routed to a gray water holding tank where it is reused for flushing toilets. Overflow from the gray water tank simply flows into the digester.


Effluent flows into the biogas digester. Effluent includes all sewage from toilets and waterless urinals as well as household compost and any overflow from the gray water holding tank.

Quick Note On Bacteria: There are two kinds of bacteria, aerobic and anaerobic. All bacteria can survive only with or without oxygen. Aerobic bacteria can only survive with oxygen. Anaerobic bacteria can only survive without oxygen. The way cities sterilize sewage today is to simply deprive it of oxygen for a while, and then give it a lot of oxygen for a while. No bacteria can survive both stages of this process, so the sewage becomes completely sterilized.


Quick Note on Biogas: Biogas is methane, the same thing as natural gas. It can be used for anything natural gas can be used for: cooking, heating, generating electricity, etc.

A biogas digester is basically just an airtight anaerobic holding tank which uses bacteria to break down organic substances from compost or sewage and then it captures and store the biogas produced by this process. When sewage is deprived of oxygen, the anaerobic bacteria thrive, producing methane or biogas. Once they have eaten up all the organic material they can, they wait for more to enter the tank, and then begin producing biogas once again.

Some digesters will use something like a garbage disposal to help chew up the solid materials coming into the tank, accelerating the process of breaking it down for digestion.

Biogas is the most significant greenhouse gas contributing to climate change. It is 100x worse than carbon dioxide, so capturing it for use as fuel means reducing by 99% the impact of simply letting this gas escape into the atmosphere. Currently our municipal sewage processing systems simply pipe all that biogas out into the atmosphere, wasting an incredible amount of valuable biogas.

Effluent Filtration

Now that the system has digested the sewage, it can flow out through a multistage filtration process. The first step in filtration is to separate the liquids from the solids. The second step is to sterilize both the liquids and the solids the same way every municipal sewage system in the world does, through alternating aerobic/anaerobic processes which kill all the bacteria. Since we have already processed the sewage in a biogas digester, the anaerobic stage of sterilization is already done. We simply need to pump oxygen into the sludge coming out of the digester in order to completely sterilize it. For good measure, we can also pass it through an ultraviolet filter to add a layer of redundancy to the sterilization process.

Return To Sender

Now the sterilized liquid can be routed into a bioswale. This is similar to a septic system but far superior for several key reasons. Because a bioswale does not need to manage solid waste, it does not “get full” and therefore requires no regular maintenance. It captures any heavy metals using bioremediation techniques, and uses sunlight to denature any remaining complex organic molecules like drugs and hormones. The remaining pure water seeps down into the aquifer (a sufficient distance from the well), replenishing what we took from the aquifer.

Closed Loop

This system is a closed loop, meaning that it does not take water from the aquifer and dump it into the rivers and ocean. Instead it purifies that water  and returns it to the source. This means the system will keep working indefinitely in contrast to current municipal systems which simply destroy each source of water before moving on to find a new source to destroy.


The other huge benefit of this system is that the biogas being produced in the digester can be used to heat the water for hand washing and showers as well as powering cooking stoves and electric power generators.

One person’s sewage produces two hours of cooking biogas per day. A community using a system such as I have outlined here produces far more power than they need. In fact there are examples of farms that use biogas digesters to power all their buildings, equipment, and vehicles. The amount of biogas energy being wasted by current techniques is enormous.

Black Gold

We talked earlier about the solid wastes being extracted from the liquids flowing out of the digester. These solids are commonly composted with wood chips. This process takes a long time, a year or more. The solids are simply deposited in a holding pile along with wood chips, then they compost just like humanure or other composting toilet technologies.

After they are done composting, that literal sewage is converted to valuable mineral-rich soil ready to be used to grow food. This process can leverage the urban process to accelerate efforts like reforestation, or even the conversion of deserts into farmland and gardens. Everywhere there are people using toilets, the surrounding land could easily be made fertile using this method.

A Tighter Loop

Because this proposal relies on the use of aquifers as storage facilities where wells and bioswales draw and deposit fresh water, it relies on the assumption that you have an aquifer to leverage. This is not actually necessary in all cases. You could simply use additional stages of filtration such as distillation and/or bioreactors to purify the water sufficiently that it can immediately be reused rather than being dumped back into the aquifer. This more radical solution would be appropriate for extreme desert climates where the “yuck factor” is outweighed by basic survival needs.

For now, the aquifer replenishment method seems the easier sell!