Summary: Biodigesters convert organic wastes into a nutrient rich liquid fertilizer and biogas, a renewable source of electrical and heat energy. Their use is widespread in developing countries, particularly India, Nepal, China and Vietnam. Biodigesters help families by providing a cheap source of fuel, preventing environmental pollution from runoff from animal pens, and reducing diseases caused by the use of untreated manure as fertilizer. 

Biodigesters produce biogas, an alternative fuel source

As organic wastes break down, whether in the ground, a compost heap or landfill, they release methane (a potent greenhouse gas that traps heat at 23 times the rate of carbon dioxide). A biodigester or biogas system is a waste-management solution that traps methane as it is produced, making it available for heating or cooking or even electricity generation. By preventing methane from venting freely into the atmosphere, these systems can help reduce emissions that contribute to climate change. 

Biogas is a sustainable substitute for the propane, kerosene, and firewood that many rural families in developing countries use for their domestic energy needs.  For those families that buy their fuel, a biodigester can save them hundreds of U.S. dollars every year. For those that gather their own firewood, it can reduce a family's workload (particularly women and girls) and help prevent the deforestation prevalent in many of these areas.

Biodigesters also create high quality fertilizer

In a biodigester, animal waste is coverted into biogas and fertilizer.

The Good:

Biodigesters:

• Provide clean and renewable energy. Families use less firewood, decreasing deforestation, save money and have accessible fuel.
• Reduce greenhouse gas emissions. The combustion of biogas produces lower greenhouse gas emissions than typical methane emissions from a waste lagoon or septic system.
• Reduce contamination of surface water, groundwater and other resources.
• Reduce odors and pathogens. Biodigesting sewage can reduce the parasitic and pathogenic bacterial counts by over 90%.
• Convert waste into high quality organic fertilizer. Families can obtain improved crop yields and save money.
• Can accommodate a wide variety of organic wastes including animal manure, night soil, crop stalks, straw, slaughterhouse wastes, biodegradable garbage and wastewater.

While waste needs to be added and effluent removed on a daily basis, biodigesters are typically very reliable systems that require little maintenance.

The Bad:

Biodigesters function poorly in colder climates unless an external heat source is applied. The methanogenic bacteria responsible for generating biogas require temperatures well above freezing (optimal temperature ranges - mesophilic: 30-40°C; thermophilic: 50-60°C), so biodigesters are not ideal in cooler areas.

In order to keep the anaerobic digestion process going continually, biodigesters require a daily amount of work and a consistent source of organic materials: 

• Each day, the waste to be added needs to be mixed with water and/or ground to a liquid state. Manure is naturally manure is water-soluble, but kitchen scraps, such as banana and orange peels need to be ground into smaller which can be time consuming.

• Each day, the biodigester effluent needs to be removed from the effluent tank.

Biogas provides 20% more energy than if dung/wastes were burned directly, but much less compared to fuels such as propane and natural gas. 

The Bottom Line:

Biodigesters are an excellent technological solution for families and farmers that want a combined sanitation and energy solution, who can manage the daily feeding required to produce adequate biogas, and who have use for lots of fertilizer.

Why it is good for XelaTeco:

Around the area of Quetzaltenango, biodigesters are currently not a widespread technology. However the socio-economic level and living conditions within many of the communities offer a good market for both small-scale (for single families) and large-scale (cow/pig farms) biodigesters.

The "Salchica" type biodigester 

A biodigester like this can create 3-4 hours worth of gas each day.

The salchicha biodigester consists of a sealed flexible plastic tube about one meter in diameter and between five and ten meters in length with PVC pipes in both ends. One pipe serves as the waste entrance; the other is the fertilizer exit.  At the highest point of the tube, in the center, a flexible hose is connected which pipes the gas to the kitchen or wherever it will be consumed.  The digester is empty of technology inside; everything that happens is a natural process which the biodigester simply takes advantage of.

Floating dome biodigesters

Floating dome biodigesters have been used in many parts of the world, such as India and Nepal.  They offer many advantages over the cheaper plug flow digesters, inlcuding greater product life expectancy and increased control over gas pressure. AIDG and XelaTeco are in the process of developing these larger-scale biodigesters for the Guatemalan market that can be used on small pig and cow farms.

Essentially there are four components to a floating dome digester: the digester tank, the floating dome, the influent chamber (feed pit) and the effluent chamber (outlet pit).  The digester tank consists of a concrete base with concrete block walls.  On one side of the tank, a pipe enters from the influent chamber; on the other, a pipe exits to the effluent basin.  Cast into the tank's concrete base are three protruding rebar posts, which align with three PVC tubes constructed into the floating dome.  The alignment of these two sets of parts ensures that the dome floats straight up and down within the tank.  The floating dome consists of a galvanized iron and sheet metal frame, covered in impervious fiberglass.  The dome is the storage vessel for the gas produced.  As gas generated during anaerobic digestion, the dome floats upward. As gas is used, the dome recesses downward into the tank.  The influent and effluent basins are also constructed of concrete block with a concrete base.  The water/waste mixture is added to the system via the influent basin; digested liquid exits via the effluent basin.