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Substrate types

Cattle manure
Cattle manure is the most appropriate substrate for digestion in biogas plants as methane-producing bacteria are already contained in cattle intestines. Homogeneity of cattle manure allows recommending it for use in continuous digestion biogas plants.

Normally fresh manure is mixed with water and freed from indigested straw to prevent scum and sediment formation. Cattle urine increases biogas yields significantly therefore it is recommended to build cattle farms with concrete floor and direct discharge of excrements into tank for mixing the substrate.

Pig manure

When keeping pigs in stalls with unpaved floors only manure can be collected. It has to be diluted with water for reaching adequate consistence for digestion. This can lead to considerable amount of sand and small rocks in digester if substrate is not especially left in mixing tank until it settles. In digester sand and earth concentrate on the bottom of digester and have to be periodically cleaned out. As in the case with cattle manure it is recommended to build stalls with concrete floor and direct discharge of excrements into tank for mixing the substrate.

Ship and goat manure
For sheep and goat kept without paved floors situation is quite similar to pigs manure. As goat farm is the only place for gathering necessary amount of manure and that only with straw bedding the substrate normally represents mixture of manure and straw. Most biogas systems digesting this type of substrate work in batch load regime when mixture of manure, straw and water is loaded without preliminary preparation into digester and is left there for a more prolonged period than pure manure.

Chicken droppings
For utilization of chicken droppings it is recommended to keep chickens in cages or on the perch located above a limited area. In case of floor keeping of chickens the share of sand, sawdust, and straw in manure will be too high. When using this substrate type it is necessary to count on possible problems beforehand and clean the digester more often than with other substrates.

Chicken droppings mix well with cattle manure and can be digested together. When using pure chicken droppings as a substrate there is a danger of high ammonia concentration.

If human faeces are digested in biogas plants the toilets have to be constructed in such a way that faeces are washed down by a small amount of water. The amount of water accompanying the night soil should be minimized by ensuring that no water taps or other external sources drain into the toilet bowls, and cleaning/flushing should be limited to rinsing out with about 0.5 – 1 liter water from a bowl [8].

Biogas yield is normally counted in liters or cubic meters on the kilogram of dry matter contained in substrate. Table 6 shows biogas yield on dry matter for different substrate types after 10-20 days of fermentation under mesophilic temperature.

For determining biogas yield from fresh manure with the help of table below it is first necessary to determine moisture content of the manure. A kilogram of fresh manure is dried and residue is weighted. The moisture content is then calculated with the help of the following formula:
(1 – weight of dry manure)×100%.

Table 6. Biogas yield and methane content for different substrate types [8, 18]

Type of substrate Biogas yield
(m3 per kg
of substrate)
Methane content
Cattle 0,250 – 0,340 65
Pig 0,340 – 0,580 65-70
Poultry droppings 0,310 – 0,620 60
Horses 0,200 – 0,300 656-60
Sheep 0,300 – 0,620 70

To calculate amount of fresh manure with a certain moisture content that corresponds to 1 kg of dry matter moisture content of manure in percent is subtracted from 100 and then divide 100 over the received value: 100: (100% – moisture content in %).

Example 1. If the moisture content of fresh cattle manure used as substrate is determined to be 85% than 1 kilogram of dry matter will correspond to 100:(100 – 85) = around 6,6 kilograms of fresh manure. It means that from 6,6 kilograms of fresh manure around 0,250 – 0,320 m3 biogas can be collected, and from 1 kilogram of fresh manure there will be 6,6 times smaller yield: 0,037 – 0,048 m3.

Example 2. If the moisture content of fresh pig manure is determined to be 80% than 1 kilogram of dry matter will be equal to 5 kilograms of fresh manure. With the help of the table we determine that 1 kilogram of dry matter (or 5 kilograms of fresh manure) yields 0,340 – 0,580 m3 of biogas. Therefore, 1 kilogram of fresh manure will yield 0,068 – 0,116 m3 of biogas.

Approximate values
If the weight of fresh daily manure collected by the household or a farm is known than it is possible to approximate daily biogas production as follows:
• 1 ton of cattle manure — 40-50 m3 of biogas;
• 1 ton of pig manure— 70 – 80 m3 of biogas;
• 1 ton of chicken droppings — 60 -70 m3 of biogas.
It has to be taken into account that these approximate values are provided for substrate with moisture content of 85% – 92%.

Biogas weight
Volume weight of biogas is 1,2 kilograms for a cubic meter, therefore when counting the quantity of biofertilizer it has to be subtracted from the weight of substrate [8].
For average daily load of 55 kilograms of fresh manure per cattle head and daily biogas yield of 2,2 – 2,7 m3 per cattle head the weight of the substrate will be reduced by 4 – 5% in the process of its digestion in biogas plant.

If a high volume of gas yield is observed but the gas is not flammable enough it often indicates that on the top of substrate in digester scum has formed. If gas pressure is very low it can also mean that there is scum that blocks gas outlet pipe. It is necessary to regularly remove scum from the surface of the substrate.

Removal of scum
Distinctive feature of scum that is formed on the top of substrate in digester is that it is not brittle, but quite viscous and can become very hard during a very short time period. It can be dissolved in the substrate by keeping it moist. This means that the scum can be watered or sunk in the substrate.

Substrate separation
Straw, grass, weed stems and even dry manure tend to float up on the surface of substrate and solid and mineral substances tend to settle on the bottom of digester and with time can close outlet pipe or lessen usable area of the digester. When substrate is well prepared, homogenous and does not contain too much water such problems do not arise.

Ready substrate
When fresh cattle manure is used the problem of scum does not arise. Scum forms when substrate contains solid and not well digested organic matter. Before constructing a biogas plant it is advisable to check animal fodder for the suitability of digestion in biogas plant. It might be necessary to fragment animal fodder and in that case these additional expenditures have to be taken into account. The problem of presence of solid particles in substrate is more serious when pig manure or bird droppings are used. Sand that chickens swallow and feathers that get into droppings make it a difficult substrate.

Research on chemical content of substrate before it is digested in biogas plant has been conducted by foreign scientist as well as by researchers in Kyrgyz Republic.

Table 7. Substrate content before its digestion in biogas plant [17]

Feed Moisture,
Humic acids per volatile solids,
Fulvic acids,
Manure 96-98 4 – 2 14.8 1.6 6.5
Manure and vegetation waste 96-98 4 – 2 28.3 3.7 7.5
Vegetation waste 96-98 4 – 2 33.5 4.0 7.3

Viscosity of substrate during digestion process becomes notably lower as the share of dry matter is reduced through fermentation by 50% in stable conditions.

Biofertilizer smell is less noxious than undigested substrate (manure, urine) and if the time of digestion is long enough almost all odorous substances are fully processed.

Nutrient content of biofertilizer is determined by the quantity of organic substances and chemical elements they contain. All plant nutrients such as nitrogen, phosphorus, potassium and magnesium as well as vitamins and microelements necessary for plant growth are kept intact in biofertilizer. C/N ratio (around 1:15) has favourable effect on the soil quality. Table 8 contains approximate nutrient content of biofertilizer.

Table 8. Biofertilizer nutrient content (grams per kg of dry matter) [17]

Feed Phosphate
Manure 3.05 5.64 3.25 0.98 1.75
Manure and vegetation waste 6.37 7.98 5.15 1.95 3.37
Vegetation waste 6.66 8.88 5.18 2.22 3.70

Phosphate and potassium
Phosphate (a form of phosphorus, directly assimilated by the plants) content does not change on the process of substrate digestion. In this form plants can assimilate around 50% of overall phosphorus content. Fermentation also does not influence the share of potassium from 75 to 100% of which can be assimilated by the plants.

Unlike phosphate and potassium, some of the contained in substrate nitrogen changes during fermentation process. Around 75% of nitrogen, contained in fresh manure becomes a part of organic macromoleculae and the left 25% are present in mineral form. After digestion in biogas plant around 50 % of nitrogen is in organic form and и 50% – in mineral. Mineral nitrogen can directly be assimilated by the plants and organic nitrogen first has to mineralize with the help of soil micro organisms.