Monday, December 14, 2009

Biogas - This shit might be worth something

In trying to figure out an effective closed system wastewater treatment solution for a single residence, I kept encountering issues with current septic tank designs and treatment of raw sludge. The current accepted practice is to settle the raw sewage and use leach fields to remove the excess water. Then perhaps every year the sludge is pumped out. Our level of technology with waste removal has really not changed much since Roman times where they sat on privies and the waste was taken away by running water underneath.

This seems to be a major waste of a potential renewable fuel source. In a closed residential ecosystem stored food and sustainable food production will be consumed and thus turned into human waste. This daily waste will continually input caloric energy into the sewage processing system. Somewhere in this sewage treatment process these joules should be reclaimed and turned back into usable energy to sustain the ecosystem.

A complete solution would be to design a specialized waste treatment process. In a house, all waste water collection points should be segregated in order to pre-sort various human waste in order to ease the processing.

Urine (high in urea nitrogen) should be collected using urinals and bidets and should go to a separate tank for processing. Urea can be concentrated and the high level of nitrogen is a valuable fertilizer for plants.

Grey water such as water from showers, sinks and clothes washing should be collected separately and can be used directly on plants for watering or (with a pre-UV treatment) be collected into tanks that can support floating aquaculture to remove impurities. Then this refined grey water can be further filtered and purified through reverse osmosis or distillation to provide pure drinking water.

Sludge is collected in low-flush toilets. Waste toilet paper should not be flushed but be collected and placed in a trash bin. Most 3rd world countries employ this method due to inadequate or constricted sewage pipes. When I lived in Pakistan, next to each toilet is a water pitcher which is used to wash your bum (with your left hand). This type of washing actually cleans better than using even a wet wipe. Anyway, pre-sorting paper waste would eliminate the need for maceration pumps that would be needed to break up the paper pulp for the bio reactor. Also with any design that uses any type of pump, reliability and maintenance will become an issue down the road. So with designs, passive systems should be the first on the list of requirements. Back to the bio reactor; for anaerobic reactions to occur properly, a higher temperature will increase the rate that the sludge will break down. Reasonable operating temperatures should be between 60-125 degrees. Excess water is also an issue with proper bio reaction since dilution of the sludge reduces the rate of decomposition. With a segregated waste collection system, grey water is not introduced which partially solves this problem. Low flush toilets should also help control water dilution. Surprisingly there are very few turn-key residential bio reactors on the market. One such system is produced by Sintex a plastics manufacturing company.

Here are the specs on their bio reactor which has been used in India: Sintex Bioreactor

This goal is to take the sewage and convert it to usable methane gas. Gas from the bioreactor can be collected into tanks. Methane gas is what is commonly known as natural gas. This gas can also be used with current 1kw methane fuel cell generators to produce electricity. This is a preferable method (than just burning the gas) since the C02 waste gas can be easily collected and pumped into a bioalgae or tanks filled with bacterial bioorganisms called methanogens which when exposed to sunlight can produce lipids or additional methane. This would enable a second cycle of methane production by the use of solar. Methane gas can be compressed or liquefied for long term storage into storage tanks. Since methane is stable, it results in a viable long term fuel better than bio-diesel which can become unstable over time.

In essence, we could provide ourselves with renewable source of energy with every flush. And I know we will be flushing down shit for many more years to come.


  1. Harley,

    I am extremely impressed with your post...I must however, make a small joke: why don't you do a test run at the office to see potential pitfalls firsthand. Also, heat + sludge makes me worry about smell...a small concern maybe, but a concern nonetheless.

    Maggie (Jason's girl) aka Future Resident. :)

  2. Hi Maggie (aka: FFB - fertile female breeder)
    Not sure if it could be done at the office yet. As for the smell...well that is the methane gas. So with this whole system it will be completely sealed and thus no smell. If there was a smell, then no one light a match. Somehow makes me think of lighting farts on fire and of youtube videos of kids making jenkum.