(If you use this information in a project for assessment please give proper reference to the source!)

Please be aware of the
aspects of running a digester project!

There are number of ways to approach the design, I have chosen to assume the digester is supplying gas to a small household and work backwards from the use to the input required and finally to the digester size. The calculations below are approximate, as I am an engineer and cannot see the sense in a high degree of accuracy when there are so many variables.

The VS content of various wastes is given below, taken from ASAE Standard D384.1DEC93 (ASAE, 1998)

Fresh Maure Production (kg per day per 1000 kg liveweight)

Animal | Dairy | Beef | Veal | Pig | Sheep | Goat | Horse | Layer | Broiler | Turkey | Duck |

Total | 86 | 58 | 62 | 84 | 40 | 41 | 51 | 64 | 85 | 47 | 110 |

VS | 10 | 7.2 | 2.3 | 8.5 | 9.2 | na | 10 | 12 | 17 | 9.1 | 19 |

For example a 60 kg pig will provide the 0.5 kg VS needed per day in 5 kg of manure. 15 litres of water must be added to this amount of solid to avoid scum formation (An et al, 1997), who also says 4-5 pigs are needed to provide the gas for a family of 4-5 (this may be because of dietry differences, decomposition of effluent outside the digester and difficulty in collecting all the manure).

Temperature (^{o}C) |
Retention Time (days) minimum recommended |

10 | 55 |

20 | 20 |

30 | 8 |

For 20 ^{o}C operating temperature and a retention time of 20
days 20 litres per day input gives a design capacity of 400 litres.

As the digester needs to be 5 to 10 times longer than its diameter it is possible to come up with a range of suitable dimensions for this capacity, allowing at least 10% extra volume for the gas head space.

For 1:5 proportions Diameter = cube root (4 x Volume / 5 / pi) and for
1:10 proportions Diameter = cube root (4 x Volume / 10 / pi) and pi = 3.14
or 22/7

eg. For a volume of 440 litres (or 0.44 cubic metres) Diameter = 0.48
m, with a length of 2.4 m, to 0.37 m, with a length of 3.7
m. A larger digester will extract more gas, be more robust and allow some
room for extra manure if necessary.

For any diameter of digester the required length can be found by Length = 4 x Volume / pi / Diameter squared. Knowing the Flat Width of a poly "tube" (which is half the circumference) Diameter = 2 x FlatWidth / pi

If you want to investigate this further I can e-mail you an EXCEL(5/95) model of a Continuous Flow Stirred Tank digester.

Images are also available.

You may now like to visit the pages on building
a "poly" digester as a start to your anaerobic digestion career.

Return to the Beginners Tour of Biogas

An, B. X., L. Rodriguez, et al. (1997). “Installation and performance of low-cost polyethylene tube digesters on small-scale farms.” World Animal Review. 88( (1)): 38-47.

Hill, D. T. (1983). “Simplified Monod Kinetics of Methane Fermentation of Animal Wastes.” Agricultural Wastes 5: 1-16.

Ram, M. S., L. Singh, et al. (1993). “Effect Of Sulfate and Nitrate
On Anaerobic Degradation Of Night Soil.” Bioresource Technology 45(3):
229-232.