For an average daily substrate feed rate of 50 kg per livestock unit (LSU = 500 kg live
weight) and a daily gas yield of 1 m3 biogas/LSU, the mass of the influent substrate will be
reduced by some 2% through the process of bio-methanation (volumetric weight of biogas:
1.2 kg/m3).
Viscosity
The viscosity of the slurry decreases significantly, because the amount of volatile solids is
reduced by about 50% in the course of a stable process of fermentation. In addition, the long
carbon chains (cellulose, alcohol and organic acids) are converted into short carbon chains.
Odor
The effluent sludge is much less odorous than the influent substrate (dung, urine). Given
sufficient retention time, nearly all odorous substances are completely digested.
Nutrients
The fertilizing properties of digested slurry are
determined by how much mineral substances and
trace elements it contains. In tropical soil, the
nitrogen content is not necessarily of prime
importance - lateritic soils, for example, are more
likely to suffer from a lack of phosphorus. All plant
nutrients such as nitrogen, phosphorous, potassium
and magnesium, as well as the trace elements
essential to plant growth, are preserved in the
substrate. The C/N ratio is reduced by the
simultaneous loss of carbon, thus generally
improving the fertilizing effect of the digested
sludge, since a lower C/N ratio (ca. 1:15) has a
favorable phytophysiological effect. Table 5 below
lists the approximate nutrient contents of various
substrates, whereby it should be remembered that
the actual values may vary considerably, depending
on fodder eaten by the animals.
Figure 43: Sludge disposal in
Thailand
Photo: Kossmann (gtz/GATE)
The phosphate content ("P2O5" is the form of phosphorous available for plants) is not
affected by fermentation. Some 50% of the total phosphorous content is available for plants
in the form of phosphate. Similarly, anaerobic fermentation does not alter the rate of plant-
available potassium (75 to 100% of the total potassium).
Nitrogen compounds
In contrast to the above nutrients, however, some nitrogen compounds undergo modification
during anaerobic digestion. About 75% of the nitrogen contained in fresh manure is built into
organic macromolecules, and 25% is available in mineral form as ammonium. The effluent
sludge contains roughly 50% organic nitrogen and 50% mineral nitrogen. The stated levels
can only be taken as approximate values, since they vary widely, depending on the type of
animal involved, the fodder composition, the retention time, etc. Mineral nitrogen can be
directly assimilated by plants, while organic nitrogen compounds must first be mineralized by
microorganisms in the soil.
Fertilizing effect of effluent sludge
Digested slurry is most effective when it is spread on the fields shortly before the beginning
of the vegetation period. Additional doses can be given periodically during the growth phase,
with the amounts and timing depending on the crop in question. For reasons of hygiene,
however, leafy vegetables should not be top-dressed.
Assuming that the soil should receive enough fertilizer to replace the nutrients that were
extracted at harvesting time, each hectare will require an average dose of about 33 kg N, 11
kg P2O5 and 48 kg K2O to compensate for an annual yield of 1-1.2 tons of, for example,
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