How to build a plastic bag digester

A digester, of course, can be as simple as a tank with three holes in it:

• an inlet for the slurry, to feed the digester
• an outlet for the effluent, the digested slurry
• and a gas bung, to get the biogas out of the digester

Now, if you think about it— and Dr. T.R. Preston, who can be said to be the inventor of the plastic bag digester, obviously thought about it— you really don’t need a tank… You should be able to make a digester from anything that will hold water, like, say, a plastic bag.

So he did, in 1982.

This is no longer the day and age of the solitary inventor, however (even though they still exist), for at least three reasons. The first is that complex things like the bicycle and the airplane necessarily evolve over time. The second is that communications today are rapid and global, new circumstances require new adaptations, and as ideas spread, they plant themselves in just such changed circumstances. And third, lots of inventions these days are just very damn complicated, and to imagine and develop all the parts, they require the expertise of people with very different skill sets.

In any case, Dr. Preston has been involved in issues pertaining to technology, agriculture and poverty for many years. In 1980 or ‘81, he and two colleagues purchased and installed a 15 m³ (530 ft³/4,000 gallon) digester made of 1.5 mm/60 mil PVC. This digester came from Taiwan, and according to “The Characterisation of Production and Function of a 15m³ Red-Mud PVC Biogas Digester,” a paper by Pound, Bordas and Dr. Preston written in 1981, this digester was manufactured by the Union Industrial Research Laboratories.

Then according to Dr. Preston (private communication), the tubular plug-flow polyethylene biodigester was designed, installed and successfully tested for the first time in a house in Addis Ababa in Ethiopia, in 1982. This digester was described (in Spanish) in “Biodigestor de Baho Costo para la Produccion de Combustible y Fertilizante a partir de Excretas,” (“Low-cost biodigester for production of fuel and fertilizer from manure”) a paper written Dr. Preston and Dr. Raúl Botero when they were working in Colombia in 1985. Later, when he was in Vietnam and with Dr. Lylian Rodriguez, Dr. Preston wrote the justifiably famous “Biodigester Installation Manual” about plastic bag digesters, which is now widely available from many places on the web, and in several formats. (The link provided is to a web page version, hosted by FAO.)

Dr. Preston was initially talking about making these digesters from sheets, sealed together with a hot iron (set, he says, on “rayon”) to seal the sheets and cut semi-circles into a tube with two conical ends (as described in his 1983 publication “A Combined Digester and Gasholder PVC Plastic Tube Biogas Unit”), like this:

(I tried working with an iron to seal plastic to plastic, and I found that good results— for me at least— were impossible to achieve. The heat setting of the iron was only one critical parameter, which nevertheless could be set. The other two critical parameters were highly variable: the speed at which I moved the iron, and the pressure I applied. Maybe if I give up irrelevant stuff like sleeping, and just practice, practice, practice for a few weeks…?)

Continuing the history lesson, in 2003 Jaime Marti Herrero adapted the Preston/Botero design to the cold weather of the Bolivian highlands, as is discussed here. Jaime also has posted some of the best videos on biogas found on Youtube, for example, here.

Another useful reference— it has very good pictures of some elements of the digester— is “How to Install a Polyethylene Biogas Plant,” assembled from various sources by Francisco X. Aguilar.

Good pictures, yes: However, we suggest checking the “facts” and assertions made about biogas against other sources. For example, on page 7, this paper says (ALL CAPS as in original) that

YOU SHOULD NEVER CHARGE [fill] THE BIOGAS PLANT WITH CHICK­EN MANURE. This is not appropriate for biogas production.

With apologies to whomever, that’s just flat wrong and rather absurd. Chicken manure can have an abundance of ammonia because chickens and other poultry drop both feces and urine in the same load; and in some cases (depending entirely on what the chickens have been fed), chicken manure may have antibiotics (bad!) and even (worse!) heavy metals in it, but where the chickens are properly fed, too much ammonia can be addressed by dilution or mixing the manure with other substrates. There are plenty of biogas plants running quite happily and appropriately on a diet of chicken manure. It’s just flat-out silly to say “Don't use chicken manure”, particularly where one is Shouting At Us By Using All Caps.

A good many pertinent resources with regard to all things biogas are found at the excellent wiki once provided by Dr. Paul Harris (but no longer extant on the original server), and as concerns plastic bag digesters, see an archived page of that wiki found here. As well, although not a “how-to” reference, I recommend “Quantifying electricity generation and waste transformations in a low-cost, plug-flow anaerobic digestion system” which provides an excellent example of the full utility of plastic bag digesters.

Finally, both for its information on biogas and for the many, many other resources it provides, this mention:

Sometime in the late 70’s, an organization named VITA created a set of CDs with a vast collection of resources about all how-to aspects of village technology, from agriculture to water weirs, including a good deal of information about biogas. VITA went the way of all flesh (it died as an organization, in other words), and the CD set was picked up and might still be sold by Village Earth (I'm not sure; their website is found here). As well, you may want to take a look at Alex Weir’s site, CD3WD (here) where the whole VITA collection may be downloaded… Well, eventually. (There’s a lot of material, now available via BitTorrent.) A caveat, however: the new version of this collection, consisting of 6 DVD-sized chunks, is very difficult to use (at least as of March 2013) because it has no information about its contents except the filenames. Ouch. The older, smaller version is more focused toward alternative technology, and somewhat better described and organized, although— as welcome as these collections are, and with apologies to Alex Weir, the collector— “beautiful”, “elegant”, and “user-friendly” are not words that spring to mind when looking at what has been provided.

So, is there anything else you want to know about biogas? Well, not to bang on the drum, but we hear there is a (wait; checking that name… checking… ah: there) Complete Biogas Handbook… available. In spite of all the great information available for absolutely free on the Internet— and as we promised, a lot of the gems are linked from these pages— we feel very confident that the book can offer you important information you won’t find anywhere else.

Look around. You're sure to find someplace to buy it. (Hint: look toward the top of the page; glance to the right.)

And finally, if you’d rather learn All About Biogas in a fun, interactive workshop, and get yourself a nifty 200 gallon plastic digester kit while you’re at it, well there’s always the workshops. We hope to see you there.