Solid and liquid agroindustrial waste materials, from slaughterhouses for example, and wastewater
from sugar/starch processing are not gone into here, since small-scale biogas plants of simple
design would not suffice in that connection (cf. chapter 6).
Waste from animal husbandry
Most simple biogas plants are "fueled" with manure (dung and urine), because such substrates
usually ferment well and produce good biogas yields. Quantity and composition of manure are
primarily dependent on:
- the amount of fodder eaten and its digestibility; on average, 40 - 80% of the organic content
reappears as manure (cattle, for example, excrete approximately 1/3 of their fibrous
fodder),
- quality of fodder utilization and the liveweight of the animals.
It is difficult to offer approximate excrement-yield values, because they are subject to wide variation.
In the case of cattle, for example, the yield can amount to anywhere from 8 to 40 kg per head and
day, depending on the strain in question and the housing intensity. Manure yields should therefore
be either measured or calculated on a liveweight basis, since there is relatively good correlation
between the two methods.
The quantities of manure listed in table 3.2 are only then fully available, if all of the anirnals are kept
in stables all of the time and if the stables are designed for catching urine as well as dung (cf.
chapter 3.3).
Thus, the stated values will be in need of correction in most cases. If cattle are only kept in night
stables, only about 1/3 to 1/2 as much manure can be collected. For cattle stalls with litter, the total
yields will include 2 - 3 kg litter per animal and day.
Table 3.2: Standard liveweight values of animal husbandry and average manure yields (dung
and urine) as percentages of liveweight (Source: Kaltwasser 1980, Williamson and Payne
1980)
Species
Cattle
Buffalo
Pigs
Sheep/goats
Chickens
Human
Daily manure
yield as % of
liveweight
dung urine
5 4-5
5 4-5
23
3 1 - 1.5
4.5
12
Fresh-manure
solids
TS (%)
16
14
16
30
25
20
VS (%)
13
12
12
20
17
15
Liveweight
(kg)
135 - 800
340-420
30- 75
30 - 100
1.5 - 2
50- 80
17