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Building a biogas plant

Before building a biogas plant different circumstances necessary for its effective operation should be considered. Failure or unsatisfactory performance of a biogas plant occur mostly due to planning mistakes. The consequences of such mistakes may be immediately evident or may only become apparent after several years. Thorough and careful planning is, therefore, of utmost importance to eliminate mistakes before they reach irreversible stages.

Planning of a biogas plant should start from determining the potential biogas and biofertilizer production based on available substrate, and determining energy needs of the household.

In the case of biogas plant intended primarily as a source of energy, implementation should only be recommended, if the plant can be expected to cover the calculated energy demand.

The size of the biogas plant depends on the quantity, quality and kind of available biomass and on the digesting temperature and period. There are several ways to determine appropriate digester size.

Ratio of daily substrate input quantity and digester size.
Daily substrate input quantity is calculated from the retention time (digester turning time) and chosen digestion temperature. For mesophilic range digester turning time constitutes from 10 to 20 days and daily substrate input from 1/20 to 1/10 of overall substrate quantity in the reactor.

Digester size for a given substrate quantity
First based on the animal quantity experimentally calculated is the daily manure production (DM) that needs to be digested. After that the manure is diluted with water to reach 86% – 92% moisture content.

In big agricultural plants the ratio of manure and water normally stays within limits of 1:3 to 2:1. Therefore, the quantity of substrate loaded (D) is the sum of daily waste production (DM) and water that it is diluted with (DW).

For digestion of substrate in mesophilic temperature range it is recommended to use daily substrate input equal to 10% of overall substrate in digester (OS). Overall substrate quantity in the digester should not exceed 2/3 of digester volume (OD).

Therefore, digester volume (DV) is calculated according to following formula:
OS = 2/3 ОD, а ОD = 1,5 ОS
OS = 10×D
D = DM + DW.

Example 7. Rural household keeps 10 cattle heads, 20 pigs and 35 chickens. The quantity of daily excrements from 1cattle head is 55 kg, from 1 pig – around 4,5 kg and from a chicken – around 0,17 kg.

The quantity of daily waste in the household DM will equal 10×55 + 20×4,5 + 35×0,17 = 550 + 90 + 5,95 = 645,95 kg, approximately 646 kg. Moisture content of cattle and pig excrements is 86%, for chicken droppings – 75%.

To reach 85% moisture content it is necessary to add 3,9 liters of water to chicken droppings (around 4 kg).

Therefore daily substrate input quantity will constitute around 650 kg. Overall substrate content should be equal OS = 10×0,65 = 6,5 tons, and digester volume OD = 1,5×6,5 = 9,75 or approximately 10 m3.

Calculating daily biogas production
The volume of biogas generated each day depends on the type of used substrate and daily substrate input quantity.

Table 10. Biogas production for different substrate types. [8,18]

Substrate type Biogas yield
(m3 per 1 kg of
total volatile solids)
Biogas yield
(m3 per 1 ton of substrate
with 85 % moisture)
Cattle 0,250 – 0,340 38 – 51,5
Pigs 0,340 – 0,580 51,5 – 88
Chicken droppings 0,310 – 0,620 47 – 94
Horse 0,200 – 0,300 30,3 – 45,5
Sheep 0,300 – 0,620 45,5 – 94

Example 8. Rural household keeps 10 cattle heads, 20 pigs and 35 chickens. The amount of daily excrements from a cattle head is 55 kg, from 1 pig- 4,5 kg, from chicken – 0,17 kg.

Overall daily waste of he household will consist of 550 kg of cattle excrements (85% moisture), 90 kg of pig excrements (moisture 85%) and 5,95 kg of chicken droppings (moisture 75%). After diluting chicken droppings with water to reach 85% moisture content, the quantity of the substrate from chickens will be around 10 kg.

According to Table 10, biogas yield from 1 kg of:
• cattle excrements with 85% moisture is equal to 0,04 –0,05 m3;
• pig excrement with 85% moisture is equal to 0,05 – 0,09 m3;
• chicken droppings with 85% moisture is equal to 0,05 – 0,09 m3.

• biogas yield from 550 kg of cattle excrements will constitute 22 – 27,5 m3;
• biogas yield from 90 kg of pig excrements will constitute 4,5 – 8,1 m3;
• biogas yield from 10 kg of chicken droppings will constitute 0,5 – 0,9 m3;
• overall biogas yield will be equal to 27 – 36,5 m3 per day.

Balance between energy demand and biogas yield
Energy demand for each individual household is calculated on the basis of the sum of all present and future consumption situations, such as cooking, lighting, energy production. It is also necessary to account for consumption of biogas for heating the substrate in digester which in condition of the Kyrgyz Republic amount to 10% to 25% depending on time of the year. Biogas volume, necessary for providing for energy demand of the household can be determined by the energy, consumed before building biogas plant. For example, burning of 1 kg of firewood analogous energy wise to burning 650 liters or 0,65 m3 of biogas, burning of 1 kg of pressed dung – to 0,7 m3 of biogas, and burning of 1 kg of coal – to 1,1 m3 of biogas.

Volume of biogas necessary for cooking can be determined from the daily time spent on cooking. Volume of biogas necessary to prepare 1 food portion for 1 person averages 0,15 – 0,3 m3 of biogas. For boiling 1 liter of water it is necessary to spend 0,03 – 0,05 m3 of biogas. For heating of 1 m2 of premises it is necessary to burn around 0,2 m3 of biogas per day. Household burners consume around 0,20 – 0,45 m3 per hour.

Example 9. A family of 4 people lives in a house of 100 m2 and keeps 20 cattle heads on farm area of 100 m2, and digests cattle manure on a biogas plant with digester volume of 15 m3.

Preparation of food for 4 people 3 times daily will require 1,8 to 3,6 m3 of biogas, and heating of the living premises with area of 100 m2 – around 20 m3 of biogas per day. For heating the substrate in the digester, for example, in September, 15% of produced biogas is necessary, which constitutes around 6 m3 of biogas per day.

For keeping 1 cattle head it is necessary to use around 3 liters of boiled water per day, and therefore for keeping 20 cows it is necessary to boil 60 liters of water, which will use up 1,8 – 3 m3 of biogas per day. For heating of the farm it is necessary to spend another 20 m3 per day.

Therefore, keeping of 20 cows will require burning of 21,8 – 23 m3 of biogas per day. For satisfying overall household energy needs it is necessary to use 49,6 – 52,6 m3 of biogas per day, which is covered by the production of biogas from 1100 kg of cattle manure – 44 – 55 m3.

Placement of the biogas plant
A golden rule of placing the biogas plant is: the plant belongs to the stable rather than to the kitchen. It is preferable to connect the mixing chamber and inlet directly to stable floor. A few meters of piping are more economic than daily transportation of manure from stable to biogas plant.

The level of stable floor has to be higher than the level of mixing chamber and then manure and urine will flow to the mixing chamber under gravitation. If the outlet of the biogas plant is elevated above the level of neighboring agricultural lands it will allow for easy spread of biofertilizer on these lands.

Design of the biogas plant
Throughout the world, a countless number of designs of biogas plants have been developed under specific climatic and socio-economic conditions. Choosing a design is essentially part of the planning process. It is, however, important to familiarize with basic design considerations before the actual planning process begins.

In countries with generally low temperatures, like Kyrgyzstan, insulation and heating devices are important for continuous operation of a biogas plant. The amount and type of substrate to be digested have a bearing on size and design of the digester and the inlet and outlet construction. The choice of design will also be based on the building materials which are available reliably and at reasonable cost.

Criteria for choosing design of a biogas plant:
Available space determines mainly underground or above-ground type of plant and in the case of above-ground construction – vertical or horizontal.
Existing structures may be used like a liquid manure tank, an empty hall or a steel container. To reduce costs, the planner may need to adjust the design to these existing structures.
Available substrate determines not only the size and shape of mixing pit but the digester volume (retention time!), the heating and agitation devices. Agitation through gas injection is only feasible with homogenous substrate and a dry matter content below 5%. Mechanical agitation becomes problematic above 10% dry matter.