Substrate types and management
• Cattle dung and manure
• Pig dung and manure
• Goat dung
• Chicken droppings
• Human excrements
• Manure yield of animal excrements
• The problem of scum
Cattle dung and manure
Cattle dung is the most suitable material for biogas plants because of the methane-
producing bacteria already contained in the stomach of ruminants. The specific gas
production, however, is lower and the proportion of methane is around 65% because of pre-
fermentation in the stomach. Its homogenous consistency is favourable for use in continuous
plants as long as it is mixed with equal quantities of water.
Fresh cattle dung is usually collected and carried to the system in buckets or baskets. Upon
arrival it is hand-mixed with about an equal amount of water before being fed into the
digester. Straw and leftover fodder or hay is removed by hand in order to prevent clogging
and reduce scum formation. Since most simple cow-sheds have dirt floors, the urine is
usually not collected. When it is, it usually runs along the manure gutter and into a pail
standing in a recess at the end of the gutter. The pail is emptied into the mixing pit - thereby
replacing some of the mixing water - in preparation for charging the digester. Urine can
considerably increase the gas production. A cemented stable floor, directly attached to the
mixing pit, is the best solution to make optimum use of dung and urine and to save time for
charging the digester.
Liquid cattle manure, a mixture of dung and urine, requires no extra water. However, the
simple animal housing found on most farms in developing countries normally does not allow
the collection of all animal excrement. Hence, most of the urine with its valuable plant
nutrients is lost.
Pig dung and manure
When pigs are kept in unpaved areas or pens, only the dung can be collected. It must be
diluted with water to the requisite consistency for charging the digester. This could result in
considerable amounts of sand being fed into the digester, unless it is allowed to settle in the
mixing vessel. Once inside the digester, sand and soil accumulates at the bottom and has to
be removed periodically. Some form of mechanical mixer should be used to dilute the dung
with water, since the odor nuisance makes manual mixing so repulsive that it is usually
neglected. Similar to cow stables, a cemented floor, sloping towards the mixing pit, is a
preferable solution.
Compared to cattle, pigs are more frequently kept on concrete floors. The water used for
washing out the pens yields liquid manure with a low solids content. Thus, whenever the
topography allows, the liquid manure should be allowed to flow by gravity into the digester.
Wash-water should be used as sparingly as possible in order to minimize the necessary
digester volume. Very frequently, the pig manure is collected in pails, which is advantageous,
even though a sand trap should be provided to prevent sand from entering the digester.
Goat dung
For goats kept on unpaved floors, the situation is comparable to that described for pigdung.
Since a goat farm is practically the only place where any substantial amount of goat dung
can accumulate, and then only if the animals are kept on straw bedding, the available feed-
stock for a biogas system will usually consist of a mixture of dung and straw bedding. Most
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