Good question,

Bio digesters use organic waste to produce methane, a relatively clean fuel food waste by all entities that produce at least one ton of food waste per week commercialized. The process of anaerobic digestion is akin to the digestive processes that occur within mammals. Microorganisms break down biodegradable material in the absence of oxygen (composting, by comparison, requires aerating the pile to keep the process aerobic7). One of the end products is biogas, composed mostly of methane and carbon dioxide, with lesser amounts of water and gases such as nitrogen and hydrogen sulfide.6 Another end product is a residual digestate, a solid, nutrient-rich substance that can be used as fertilizer.

For those of you new to the topic, anaerobic refers to micro-organisms that thrive without oxygen, and a digester is basically a large fully enclosed tank. If you lock anaerobic organisms in a sealed environment without oxygen but with plenty of food scraps and other organic waste, they produce methane-rich gas through their digestive process. Essentially, it’s the natural process of decomposition revved up to optimal speed and efficiency. The leftover “sludge” is a relatively neutral substance, since decomposition has already occurred, and it can be dewatered and used as a soil enhancer. Anaerobic technology is nothing new, but it was not widely adopted in many organizations as a commercially and environmentally successful way to deal with livestock waste as well as human waste and food waste. CO-digesting food waste with human waste is also becoming a thing.

 Components of the Bio-gas Plant

The major components of the bio-gas plant are a digester tank, an inlet for feeding the kitchen waste, gas holder tank, an outlet for the digested slurry and the gas delivery system for taking out and utilizing the produced gas. This project is also useful for students to have a hands-on learning experience in constructing a Mini Bio-Gas Plant, using locally available material

Material Required:

1. Empty PVC can 50 ltrs capacity: 1 No. (to be used as Digester Tank)

2. Empty PVC can 40 ltrs capacity: 1 no. (to be used as Gas Holder Tank) (Make sure the smaller can fits inside larger one and moves freely)

3. 64 mm dia pvc pipe: about 40 cm long (to be used for feeding waste material)

4. 32 mm dia pvc pipe: about 50 cm long (fixed inside gas holder tank as a guide pipe)

5. 25 mm dia pvc pipe: about 75 cm long (fixed inside the digester tank as a guide pipe)

6. 32 mm dia pvc pipe: about 25 cm long (fixed on digester tank to act as outlet for digested slurry)

7. M-seal or any water-proof adhesive

8. Gas outlet system: Please see Step 4 below for required materials and construction

Tools required

Do not require many tools here. A hack saw blade for cutting the cans & pipes and a sharp knife for cutting holes on the cans are all the tools we need.

Additional accessories

A single burner bio-gas stove or a Bunsen Burner used in school laboratories

thanks a lot for the answer Prem Baboo, are there any technical considerations on the calculation of the fermenting time for example?

Dear Erick Javier

The reaction depends upon temperature maximum 50-600C when feed Food waste plus the fresh Cow Dung Slurry to be added to the  food waste to supplement the reaction process. It is used as a seeding material for the reaction process in the Up flow Anaerobic Sludge Blanket Reactor. Effective microorganism to be collected from source use to accelerate the reaction process. Also yeast is added for fermentation process to take place. The first reaction take 2 to 3 days or more it is depends upon temperature and bacteria generation. When reaction start once then the process is continuous and feed introduced every 24 hrs, and the continuous production of methane as per feed. Methane production is directly proportional to feed.

Please find attached herewith related article.

Regards,

Prem Baboo

While the initial answer of Prem Baboo is admirably complete, the Hestia digester used in the illustration (a photograph of a unit in Eugene, Oregon) would not be suitable for reliable use for two primary reasons. First, the EDPM membrane used on top as the gas storage facility has a very poor ability to retain heat (approximately equal to a thin sheet of single pane glass), and thus this digester cannot be kept warm enough (except with rather substantial heat input) to produce biogas reliably, day and night, year around. Second, at least as far as I know, the inlet on this unit is also the outlet, and the only means of agitation is to shove a stick into the inlet. The lack of agitation and the lack of any 'flow through" means that some fraction of the volume, perhaps the larger fraction, can be expected to turn into a dead zone, where digestion is not occurring. A cold digester, where only a portion of the volume is actively digesting even at the lower temperature will be disappointing for the builder or user.

Thus a good design for a digester must be heatable (which means it must be well-insulated), and it should have good agitation, except where other design features reduce the need for agitation.

I am working on a small design (4'x4'x4', 1.3 cu m) which is intended to meet these criteria. (Whether it will or not will require further experience to determine.) As an example of the design criteria of "insulated, therefore heatable", my calculations indicate that this unit could be kept at an operating temperature of 90 deg F on a 15 deg F day with an input of less than 700 BTU per hr. Attached is a picture:

Please also allow me to mention that for practical, small-scale digesters, my opinion is that the temperature of digestion is much better being confined to the mesophilic range (20-35 C, 70-90 F), rather than the thermophilic range (45-65 C, 115-145 F) because energy requirements are lower, and thermophilic digestion is far more sensitive to temperature fluctuations than is mesophilic digestion. Even small changes (2-3 deg C) can cause a thermophilic process to produce less biogas than might otherwise be expected, and research shows that it may take some time to recover. Because of its requirements, thermophilic digestion is really high tech digestion.

Meanwhile, if anyone wants to be kept up to date regarding progress on the digester, and when plans and workshops will be available, they should visit the website mentioned below and sign up for email notification.

David William House, author, The Complete Biogas Handbook, www.completebiogas.com