The Beauty of Bio-Gas Digesters

In November, 2011, I took a trip to Nepal to visit an international development NGO called CHOICE Humanitarian.  I met the organization’s former CEO earlier in the year and was struck by the statistics he presented—contrary to many NGOs’ M.O.s, the rural villages that CHOICE works with have an abnormally high success rate after CHOICE’s involvement is over.  Their model of teaching leadership over the course of their development projects trains villagers to take control of their own further development, so when CHOICE leaves, they’re self-motivated and empowered to keep their own improvements going.

After five days volunteering in the office, getting to know CHOICE and how they operate, and exploring Kathmandu, I joined CHOICE’s Kiran Neupane on a trip to visit a few of the villages CHOICE works with.  We were accompanied by a few members of Kathmandu’s Rotary Club which was supporting some of their work and we stayed two nights in villagers’ homes. CHOICE and the villages we visited were preparing to launch a new home-stay-based trekking route the following month and we were a friendly group to give them a trial run.

The people were friendly, open and wonderful as Kiran checked on and showed us the progress in different villages.  CHOICE works on a variety of projects, letting the villagers decide their own priorities, and we saw a few schools and a health outpost tucked into the steeply sloping hills and cascading rice paddies.  But one recurring item really struck me as I saw them installed in many villagers’ households—the bio-gas digester.

What is a bio-gas digester and how does it work?

* Time to check your squeamishness at the door.  We’re about to talk about poop and pee.  We all make it, we all have to do something with it, and here’s a way to harness some good from it. *

how a bio-gas digester works - illustration

(click image to enlarge)

Bio-gas digesters collect human and animal waste (feces and urine) in an underground tank.  Microorganisms break down the waste within the tank by anaerobic digestion, which releases a mix of gases, mostly methane.  The gas rises and collects in a domed ceiling where it builds pressure.  A valve and a hose attach to the top of the tank, allowing it to be piped directly into the house where it is connected to a gas-burning stove.  The gas does not smell (like many might suspect) and does not make smoke.  When the tank reaches capacity, excess effluent that has already undergone digestion (“slurry”) is pushed out of the tank by natural pressure, through an overflow pipe and into a holding bin, where organic waste like leaves, grass or rice husks can be added to create high-quality fertilizer and mulch.

What’s most impressive is the range of benefits that bio-gas digesters provide beyond just the primary benefit of gas:

  1. Excrement from animals and humans is turned from waste to benefit
  2. The waste collected from one cow or buffalo, a couple goats and one family provides enough gas for regular cooking needs
  3. Families with more animals and a larger tank could also use gas for heating and lighting
  4. The gas is a renewable, clean-burning source of fuel
  5. Gas replaces wood or charcoal for cooking, eliminating the need to cut firewood and greatly reducing deforestation
  6. Families in unforested areas no longer have to pay for firewood or charcoal, saving them money
  7. The post-digestion slurry is (nearly) free of pathogens and creates excellent natural fertilizer and mulch for crops, eliminating the need for families to buy chemical fertilizers, saving them money and retaining more nutrients in the land
  8. Using gas instead of wood or charcoal eliminates respiratory health issues caused by inhaling smoke from open fires in kitchens and open spaces which affects women especially (who do most of the cooking)
  9. Toilets (or pit latrines) are needed to collect human waste, leading many families to have toilets for the first time. This is important for proper sanitation, which allows many families and children to avoid disease
  10. Gas provides fuel for boiling water, sterilizing it and saving families and children from waterborne diseases (many people already know to boil water, but where fuel is scarce or expensive, free gas may lead to families feeling freer to boil water more consistently)
  11. If used for lighting, families can avoid using kerosene and its dangers–smoke inhalation and risk of fires and burns
Nepalese woman with bio-gas digester

left to right: the lady of the household, toilet, animal waste input, purified water tank

The villagers we visited seemed very happy with their new bio-gas digesters. And as more see how they work for neighbors, they want them for their families too. One woman was not so sure about the slurry as fertilizer, though, believing that separating the gas must take some of the nutrients away from the fertilizer. So Kiran talked with her more, explaining how it works and how that’s not the case. He and CHOICE will continue to work with villagers to help them understand the benefits of bio-gas digesters and use them optimally.  That woman’s slight resistance was a good reminder that education and understanding is a big part of introducing new technologies, from the beginning through to adoption and follow-up.  And it’s important that people really understand better solutions in order to embrace and adopt them.

left to right: slurry runoff pit and pipe, the owners of the household, stacked drying corn, toilet (far right)

I’m very glad to have learned about bio-gas digesters and I’m impressed with all of the positive effects that ripple out from the central waste-to-energy benefit. As with all technologies, bio-gas digesters won’t necessarily be the best solution everywhere, in every geography, climate, economic situation or culture. But it’s a great one to know about and help implement where it is appropriate.

For more in-depth information about bio-gas and digesters, here’s a great page on Appropedia to start with >>

More on CHOICE Humanitarian Nepal >>

6 thoughts on “The Beauty of Bio-Gas Digesters

  1. Pingback: Prakti High Efficiency Stoves | BoP Designer

  2. Pingback: Assignment 5 | Systems, Sites and Buildings 2012

    • Thanks Ostin,

      I’m an enthusiast but not an expert. It looks like there are ways to do this, but the gas might have to be cleaned or filtered first. Looks like there is a lot of information available if you google it–I googled “storing gas cylinder from biogas digester” and got some good results. Here’s one that may be able to help:

      Hope that helps. Good luck!

Leave a Reply