Biodigester: how it works for biogas and for sewage

What a biodigester is

The Ocean Sewage Alliance describes biodigesters as anaerobic digesters: closed containers in which animal waste, human excreta and other organic waste are diluted with water and broken down by microorganisms without oxygen. The outputs are biogas, which is mostly methane and can be burnt, and a nutrient-rich slurry suitable as fertiliser.

The word covers two different jobs. On a farm or a homestead with cattle, a bio digester system makes cooking gas from manure. At a single house or commercial building, biodigester versions of the septic tank treat toilet waste on site, the same source notes.

Biogas biodigesters for farms and households

The development organisation SNV reported in February 2016 that the Tanzania Domestic Biogas Programme had built 12,000 bio-digesters benefiting more than 70,000 people, with 10,000 more planned for 2016–2017 with the government, the Norwegian Embassy and the Rural Energy Agency.

The UN’s Climate Technology Centre and Network sets out the numbers behind a typical farm unit in its biodigester technology brief:

Biogas biodigesters for farms and households

Figure	Value given by CTC-N
Manure needed	at least 100 kg a day
Size for that input	a 10 m³ system, for 5 head of cattle
Example set-up cost	USD 1,170: materials 960, labour 90, training 120
Fertiliser produced	3–4 litres per kg of excreta
Gas value	1 m³ of biogas replaces 0.5 kg of LP gas
Cooking time	a 5 m³ system gives 3–4 hours of cooking a day

The cost line is CTC-N’s worked example in US dollars, not a price in Tanzania; ask local builders for a current quote.

What a biogas biodigester needs to work

The CTC-N brief lists the limits that decide whether a unit keeps producing gas:

1. Warmth. An average temperature above 15°C; below that, gas output falls and insulation or a greenhouse cover is needed.
2. A steady feed. The size is set by the daily manure available, so the number of animals comes first.
3. No antibiotics in the feed. If animals have been treated, wait at least four days before using their manure, because antibiotics harm the bacteria in the digester.
4. A gas line. The methane has to be piped to where it is used, such as the kitchen.

Biodigester septic systems for toilets

For sewage, the Ocean Sewage Alliance lists septic tanks and latrines adapted for bio digestion among the types used at single residential and commercial sites. In a biodigester septic tank the solids are broken down by bacteria in closed chambers, and the liquid that leaves the tank still needs somewhere to go, such as a soak pit.

How a conventional septic tank is sized and emptied, and how it compares, is covered under septic tanks. The pit that takes the outflow from either system is described under soak pits.

Choosing the size of a biodigester

For biogas, start from the waste you have each day. CTC-N pairs 100 kg of manure a day with a 10 m³ digester; at the same ratio, 50 kg a day points to about 5 m³ (our calculation). For a toilet biodigester, ask the builder in writing how many users and how much daily wastewater the design assumes, and compare that with your building.

Kitovu does not sell biodigesters or biogas equipment.