[Digestion] Dry fermentation in developing countries

Ken Calvert renertech at xtra.co.nz
Fri Oct 29 03:45:56 PDT 2010


Yvonne Hi again!   When you talk of security, I would think that the greatest danger would be from asphyxiation.  I don't see explosions as being a great danger.  When you crack open a digester, Methane is lighter than air and will quickly rise up out of the way.  But Carbon dioxide is much heavier than air and  will stay down in the box. if it is dug into the ground.   The cheapest system is to dig a trench in the ground  with one end sloping upwards to make a ramp that you can run a wheel barrow down into the trench..  The sides are plastered ferrocemento style, like throwing cow dung onto the sideof a wall to dry. The top cover is made from  PVC tarpaulin material and it is sealed into the ferrocemento sides by being pinned down into a small narrow trench made of cement and integral with the sides.
Then the sealing trench is filled with water or  better still a clay slurry.   However,  the real danger  is CO2 remaining down in the  digester and I would not recommend blowing it out with Jamies idea of a car exhaust either.  CO2 is so sneaky inthat you can't really smell anything an two good breathes of CO2 and you will be dizzy enough to fall down. Then you are really in trouble.  One to two minutes and you are a goner!          
  If security/safety is your main concern then  it takes two sheets of  nylon reinforced PVC tarp.  You lay one down on the ground as a base, with the edges turned up, and  you pour a concrete floor over the tarp, strong enough to support Men Wheelbarrows Tractors or whatever.  Then you heap up your digestable material on the floor  and then pull a top cover of PVC tarp over the top and seal it to the edges protruding from underneath the floor.  That seal is made from sliced up Polythylene pipe and does not require  water. Rather its a couple of tubes of  lubricating grease every time you seal  up a new batch of material.  
Being built on level ground the biogas goes upwards and the CO2 fraction spreads out over the ground and may kill a few low lying animals but not people or their dogs. 
Because the digester is so flexible it can be sucked virtually dry of biogas and the top will be sucked down on top of the substrate material. Then very very little gas will be wasted.  
If you were not so worried about explosions, I would be suggesting that one of those 12 volt blowers used to  pump up your air mattress for camping out,  or your swimming/paddling pool would do all the blowing and sucking that you would want.  In actual fact, with no air leaking in you can suck out  pure biogas with no danger at all.  It takes a 15% mixture of  gas in air to make an explosion.  However, even so, it would be wise to choose a blower that doesn't spark at the brushes.
Home me in a bit further on your vision and I will come back with more details of which ever way you want to go.  Ken C. 
 
  ----- Original Me

  ssage ----- 
  From: Voegeli, Yvonne 
  To: For Discussion of Anaerobic Digestion 
  Sent: Friday, October 29, 2010 10:06 PM
  Subject: Re: [Digestion] Dry fermentation in developing countries


  Dear all,

   

  Thank you very much for your feedback and ideas. We will have to think more about David's' suggestion using two flame traps.

   

  Ken is right; I should have given a bit more details on the project. I have to admit that we are at the very beginning... The project is located in Kumasi, Ghana, and our local research partner is the KNUST University of Science and Technology. The pilot plant shall be located at the university campus and the feeding material will consist of organic household waste or food waste from restaurants. In a first step, two Swiss students will work on the project for 6 months (start in December), spending 4 months on site. Their task is to identify the suitable material, that's why I can't give you the details on what exactly is available. At first, I was also thinking about a shipping container (what about corrosion?), but cement materials are also available as well as skilled workers. We will have to compare the different options and costs.

   

  On principle, the idea is that such a plant would be operated by the municipality or a private contractor (not by farmers), as our focus is on municipal solid waste. There are still many open questions, but my biggest concern was the security - if it is possible at all to construct a simple digester which is safe...

   

  I think the students might contact some of you directly for further questions later on.

  Thank you,

  Yvonne

    

   

   


------------------------------------------------------------------------------

  From: digestion-bounces at lists.bioenergylists.org [mailto:digestion-bounces at lists.bioenergylists.org] On Behalf Of Ken Calvert
  Sent: Freitag, 29. Oktober 2010 09:48
  To: For Discussion of Anaerobic Digestion
  Subject: Re: [Digestion] Dry fermentation in developing countries

   

    Yvonne Hi!  When you say developing countries, what sort of construction materials are you thinking of?   Would old 20 tonne shipping containers be available?

  Are cement materias and  tradesmen skilled in cement plaster work  readily available?   What about  PVC tarpaulin cloth?  Are there trucks on the roads that have waterproof covers  that can be strapped down  over a load?  What is the local soil and terrain?   Is it possible to dig say 2-3 metres deep in the soil and get a  smooth surface, or is the terrain too soft and sandy to  hold to the shape of a cistern or trench dug in the soil?   Is it too rocky to allow any sort of digging at all?  If you can home in a little more on the country side  and  local skills available  I can make several different suggestions.  Sincerely,  Ken Calvert.  www.coffee.20m.com

     

   

      

    ----- Original Message ----- 

    From: Voegeli, Yvonne 

    To: digestion at lists.bioenergylists.org 

    Sent: Thursday, October 28, 2010 9:53 PM

    Subject: [Digestion] Dry fermentation in developing countries

     

    Dear all,

     

    I like to start a new discussion on a different topic, namely the dry-fermentation process (discontinuous) similar to the digesters developed by the companies Bekon or Bioferm in Germany. The organic waste is filled batch-wise into simple garage-like digesters for about 30-40 days.

     

    Experts report that this process has a high potential for application in developing countries as it has the simplest design and the solid waste digesters are the least expensive ones. Also, substrates that lead to operating problems during wet fermentation due to their structure or the proportion of impurities they contain can still be processed using dry fermentation methods. However, up to date, there is no experience with this technology in developing countries. Therefore, our idea is to develop a dry-fermentation biogas plant adapted to the situation in developing countries using locally available material.  

     

    After visiting a Bekon biogas plant in Switzerland, I'm especially concerned regarding the opening of the digester for emptying and refilling. Before opening the door, the digester has to be vented in order to avoid a gas-air-mixture which is explosive. This installation seems to me rather complicated.

     

    - Is anybody of you familiar with discontinuous dry-fermentation systems and has an idea how the security can be assured in a developing country context? Is it possible to install a simpler venting system?

    - Likewise, when removing the drum from a floating-drum digester for emptying, how is ensured that at no time an explosive gas-air-mixture occurs?

     

    Thanks for your ideas,

    Yvonne

    °°° 

    Yvonne Vögeli
    Eawag / Sandec
    Überlandstrasse 133
    P.O. Box 611
    8600 Dübendorf
    Switzerland
    Phone: +41 (0)44 823 54 20
    Fax: +41 (0)44 823 53 99
    yvonne.voegeli at eawag.ch
    http://www.sandec.ch 

     


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