Agricultural/operational prerequisites and stock-farming requirements
In order to fulfill the prerequisites for successful installation and operation of a biogas plant, the small farm in question must meet three basic requirements regarding its agricultural production system:
- availability of sufficient biomass near the biogas plant,
- use for digested slurry as fertilizer,
- practical use(s) for the biogas yield.
Farms marked by a good balance between animal husbandry and crop farming offer good prerequisites for a biogas tie-in. Unfortunately, however, such farms are rare in tropical countries. In numerous Third World countries, animal husbandry and stock farming are kept separate by tradition.
As the world population continues to grow, and arable land becomes increasingly scarce as a result, the available acreage must be used more intensively. In wet savannas, for example, the fallow periods are being shortened, even though they are important for maintaining soil fertility. In order to effectively counter extractive agriculture, animal husbandry must be integrated into the crop farming system, not least for its fertilizing effect. On the other hand, systematic manuring is only possible as long as collectible dung is allowed to accumulate via part-time or full-time stabling.
The installation of a biogas plant can be regarded as worthwhile, if at least 20-40 kg manure per day is available as substrate. This requires keeping at least 3 - 5 head of cattle, 8-12 pigs or 16-20 sheep/goats in a round-the-clock stabling arrangement. The achievable gas yield suffices as cooking fuel for a family of 4-6 persons. That, in turn, means that the farm must be at least about 3 hectares in size, unless either freely accessible pastures are available or extra fodder is procured. Crop residue like rice straw, sorghum straw, cornstalks, banana stalks, etc. should be chopped up, partially composted and mixed with animal excrements for use in the fermentation process (cf. chapter 3.2).
Fig. 3.2: Integration of a biogas plant into the agricultural production cycle (Source: OEKOTOP)
Adding a biogas plant to an integrated agricultural production system not only helps save firewood and preserve forests, but also contributes toward sustained soil fertility through organic fertilization and ensures the long-term crop-bearing capacity of the soil. Work involving the dissemination of biogas. technologies must account for and call attention to that complex relationship. If no organic fertilizing has been done before, a biogas plant will mean more work. Organic waste has to be collected and afterwards spread on the fields. Only if the owner is willing to invest the extra effort can the biogas plant be expected to serve well in the long term.
There are two central demands to be placed on the stock-farming system in relation to biogas utilization:
- permanent or part-time stabling or penning and
- proximity of the stables or pens to the place of gas utilization (usually the farmhouse).
If the distance between the stables/pens and the place of gas utilization is considerable, either the substrate must be hauled to the biogas plant (extra work) or the gas must be transferred to the place of use (cost of installing a supply pipe). Either of the two would probably doom the biogas plant to failure. The best set of circumstances is given, when
- the animal excrements can flow directly into the biogas plant by exploiting a natural gradient,
- the distance of flow is short, and
- the stables have a concrete floor to keep contamination like soil and sand from getting into the plant while allowing collection of urine.
Cattle pens
Dung from earth-floor pens has a very high total-solids content (TS up to 60%), and the urine is lost. Daily collection is tedious and there is no way to prevent sand from getting into the digester. Consequently, at the same time a biogas plant is being installed, concrete floors should be installed in such pens and provided with a collecting channel. This increases the total cost of the biogas plant, but is usually justified, since it lowers the subsequent work input, helps ensure regular feeding of the plant, reduces the chance of hoof disease and keeps sand and stones out of the digester. The overall effect is to enhance acceptance of the biogas plant.
The collecting channels can be designed as open gutters or covered ducts. Concrete split tiles serve well as construction material for the second (more expensive) version. The slots should be about 2 - 3 cm wide, i.e. wide enough to let the dung pass through, but not wide enough to cause injury to the animals.
Cattle dung dries rapidly in a hot climate, particularly if the pen has no roof. The cleaning water also serves to liquefy the dung and reduce its TS content to 5-10%. for the purposes of fermentation. The main advantage of this system is that the pens can be cleaned and the biogas plant filled in a single operation. The collecting channel should be designed to yield a floating-manure system with gates at the ends, so that a whole day's dung and cleaning water can collect at once. The advantages:
- easy visual control of the daily substrate input,
- prevention of collecting-channel blockage due to dung sticking to the walls and drying out,
- adding the substrate at the warmest time of day, which can be very important in areas with low nighttime temperatures.
Fig. 3.3: Pen with concrete floor and collecting channel for dung and urine.1. Water through, 2 Feeding through, 3 Collecting channel, 4 Sand and rocks, 5 Concrete (Source: OEKOTOP)
Intensive forms of animal husbandry often involve the problem of excessive water consumption for cleaning, which leads to large quantities of wastewater, dilute substrate and unnecessarily large biogas plants (cf. chapter 6). In areas where water is scarce, the digester drain-water can be used for scrubbing down the pens and diluting the fresh substrate, thus reducing the water requirement by 30-40%.
Stables
Differentiation is generally made between:
- stabling systems with litter and
- stabling systems without litter, with the design details of the stalls appropriate to the type of animal kept.
For use in a biogas plant, any straw used as litter must be reduced in size to 2-6 cm. Sawdust has poor fermenting properties and should therefore not be used.
Cattle shelter
Variants suitable for connection to a biogas plant include:
- Stanchion barns with a slurry-flush or floating removal system (no litter) or dung collecting (with litter),
- Cow-cubicle barns with collecting channel (no litter).
Piggeries
The following options are well-suited for combination with a biogas plant:
- barns with fully or partially slotted floors (no litter),
- lying bays with manure gutter (no litter),
- group bays (with or without litter).
Fig. 3.4: Stanchion barn with floating gutter. 1 Collecting channel, 2 Stable, 3 Floating gutter leading to the biogas plant, 4 feeding aisle, 5 Feeding trough (Source: OEKOTOP)
Fig. 3.5: Cow-cubicle barn with floating gutter. 1 Collecting channel, 2 Cubicle, 3 Floating gutter leading to the biogas plant, 4 Feeding aisle, 5 Feeding trough (Source: OEKOTOP)
Fig. 3.6: Piggery with group bays (no litter). 1 Feeding aisle, 2 Feeding trough, 3 Floating gutter leading to the biogas plant, 4 Bay (pigpen) (Source: Manuel et Preas D levage No. 3, 1977)Liquid manure from swine normally has better flow properties than liquid manure from cattle, the main reason being that swine eat less fibrous material. Additionally, though, swine drop more urine than dung.
In tropical countries, few pigsties have fully or partially slotted floors. Most pigs are kept in group bays. Figure 3.6 shows a schematic representation of a piggery with bays of different size to accommodate animals of various weight categories. The animals are moved in groups from one bay to the next as they grow.
Chicken coops
Hens kept in battery-brooding cages never have litter. Despite the name, straw yards can be managed with or without litter.
In either system, the dry droppings are collected, transferred to the biogas plant and diluted to make them flowable. Feathers and sand are always problematic, since they successfully resist removal from the substrate. In many cases, the coop is only cleaned and disinfected once after the entire population is slaughtered. As a rule such systems are not suitable as a source of substrate for biogas plants.
- availability of sufficient biomass near the biogas plant,
- use for digested slurry as fertilizer,
- practical use(s) for the biogas yield.
Farms marked by a good balance between animal husbandry and crop farming offer good prerequisites for a biogas tie-in. Unfortunately, however, such farms are rare in tropical countries. In numerous Third World countries, animal husbandry and stock farming are kept separate by tradition.
As the world population continues to grow, and arable land becomes increasingly scarce as a result, the available acreage must be used more intensively. In wet savannas, for example, the fallow periods are being shortened, even though they are important for maintaining soil fertility. In order to effectively counter extractive agriculture, animal husbandry must be integrated into the crop farming system, not least for its fertilizing effect. On the other hand, systematic manuring is only possible as long as collectible dung is allowed to accumulate via part-time or full-time stabling.
The installation of a biogas plant can be regarded as worthwhile, if at least 20-40 kg manure per day is available as substrate. This requires keeping at least 3 - 5 head of cattle, 8-12 pigs or 16-20 sheep/goats in a round-the-clock stabling arrangement. The achievable gas yield suffices as cooking fuel for a family of 4-6 persons. That, in turn, means that the farm must be at least about 3 hectares in size, unless either freely accessible pastures are available or extra fodder is procured. Crop residue like rice straw, sorghum straw, cornstalks, banana stalks, etc. should be chopped up, partially composted and mixed with animal excrements for use in the fermentation process (cf. chapter 3.2).
Fig. 3.2: Integration of a biogas plant into the agricultural production cycle (Source: OEKOTOP)
Table 3.7: Biogas compatibility of farm types (Source: OEKOTOP)
| Type of farm | Characteristics of relevance to biogas generation | Rating as site for biogas plant |
| Stock farming only | Pasturing (nomadic, ranching, etc.) Intensive stationary fattening | unsuitable suitable |
| Crop farming only | Crop residue only; fermentation difficult | normally unsuitable |
| Mixed Agriculture | | |
| Stock farming for: | | |
| - animal power | Mostly nighttime stabling; only a few animals; 50% of dung collectible | possible |
| - meat production | | |
| extensive | Pasturing; no stabling; dung wasted | unsuitable |
| intensive | Fattening in stables; dung directly usable | suitable |
| - milk production | Frequently permanent stabling; all dung and urine usable | suitable |
| Crop farming: | | |
| - vegetables | Near house; crop residue and water available year-round | possible! |
| - field-tilling | | |
| unirrigated | 1 harvest per year, scarcity of fodder, long-distance hauling of water and manure | unsuitable |
| irrigated | 2-3 harvests per year; water available, small fields | possible |
There are two central demands to be placed on the stock-farming system in relation to biogas utilization:
- permanent or part-time stabling or penning and
- proximity of the stables or pens to the place of gas utilization (usually the farmhouse).
If the distance between the stables/pens and the place of gas utilization is considerable, either the substrate must be hauled to the biogas plant (extra work) or the gas must be transferred to the place of use (cost of installing a supply pipe). Either of the two would probably doom the biogas plant to failure. The best set of circumstances is given, when
- the animal excrements can flow directly into the biogas plant by exploiting a natural gradient,
- the distance of flow is short, and
- the stables have a concrete floor to keep contamination like soil and sand from getting into the plant while allowing collection of urine.
Cattle pens
Dung from earth-floor pens has a very high total-solids content (TS up to 60%), and the urine is lost. Daily collection is tedious and there is no way to prevent sand from getting into the digester. Consequently, at the same time a biogas plant is being installed, concrete floors should be installed in such pens and provided with a collecting channel. This increases the total cost of the biogas plant, but is usually justified, since it lowers the subsequent work input, helps ensure regular feeding of the plant, reduces the chance of hoof disease and keeps sand and stones out of the digester. The overall effect is to enhance acceptance of the biogas plant.
The collecting channels can be designed as open gutters or covered ducts. Concrete split tiles serve well as construction material for the second (more expensive) version. The slots should be about 2 - 3 cm wide, i.e. wide enough to let the dung pass through, but not wide enough to cause injury to the animals.
Cattle dung dries rapidly in a hot climate, particularly if the pen has no roof. The cleaning water also serves to liquefy the dung and reduce its TS content to 5-10%. for the purposes of fermentation. The main advantage of this system is that the pens can be cleaned and the biogas plant filled in a single operation. The collecting channel should be designed to yield a floating-manure system with gates at the ends, so that a whole day's dung and cleaning water can collect at once. The advantages:
- easy visual control of the daily substrate input,
- prevention of collecting-channel blockage due to dung sticking to the walls and drying out,
- adding the substrate at the warmest time of day, which can be very important in areas with low nighttime temperatures.
Fig. 3.3: Pen with concrete floor and collecting channel for dung and urine.
Intensive forms of animal husbandry often involve the problem of excessive water consumption for cleaning, which leads to large quantities of wastewater, dilute substrate and unnecessarily large biogas plants (cf. chapter 6). In areas where water is scarce, the digester drain-water can be used for scrubbing down the pens and diluting the fresh substrate, thus reducing the water requirement by 30-40%.
Stables
Differentiation is generally made between:
- stabling systems with litter and
- stabling systems without litter, with the design details of the stalls appropriate to the type of animal kept.
For use in a biogas plant, any straw used as litter must be reduced in size to 2-6 cm. Sawdust has poor fermenting properties and should therefore not be used.
Cattle shelter
Variants suitable for connection to a biogas plant include:
- Stanchion barns with a slurry-flush or floating removal system (no litter) or dung collecting (with litter),
- Cow-cubicle barns with collecting channel (no litter).
Piggeries
The following options are well-suited for combination with a biogas plant:
- barns with fully or partially slotted floors (no litter),
- lying bays with manure gutter (no litter),
- group bays (with or without litter).
Fig. 3.4: Stanchion barn with floating gutter. 1 Collecting channel, 2 Stable, 3 Floating gutter leading to the biogas plant, 4 feeding aisle, 5 Feeding trough (Source: OEKOTOP)
Fig. 3.5: Cow-cubicle barn with floating gutter. 1 Collecting channel, 2 Cubicle, 3 Floating gutter leading to the biogas plant, 4 Feeding aisle, 5 Feeding trough (Source: OEKOTOP)
Fig. 3.6: Piggery with group bays (no litter). 1 Feeding aisle, 2 Feeding trough, 3 Floating gutter leading to the biogas plant, 4 Bay (pigpen) (Source: Manuel et Preas D levage No. 3, 1977)
In tropical countries, few pigsties have fully or partially slotted floors. Most pigs are kept in group bays. Figure 3.6 shows a schematic representation of a piggery with bays of different size to accommodate animals of various weight categories. The animals are moved in groups from one bay to the next as they grow.
Chicken coops
Hens kept in battery-brooding cages never have litter. Despite the name, straw yards can be managed with or without litter.
In either system, the dry droppings are collected, transferred to the biogas plant and diluted to make them flowable. Feathers and sand are always problematic, since they successfully resist removal from the substrate. In many cases, the coop is only cleaned and disinfected once after the entire population is slaughtered. As a rule such systems are not suitable as a source of substrate for biogas plants.
