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Main criteria for choosing design of the digester is the possibility to implement it in practice and convenience from the point of view of maintenance and exploitation. Independently of design considerations the digester (fermentation tank) must meet the following requirements:

Water/gas tightness – water tightness in order to prevent seepage and the resultant threat to soil and groundwater quality; gas tightness in order to ensure proper containment of the entire biogas yield and to prevent air entering into the digester (which could result in the formation of an explosive mixture).
Insulation – insulation is necessary in climatic condition of the Kyrgyz Republic for effective operation of the biogas plant.
Minimum surface area – keeps cost of construction to a minimum and reduces heat losses through the vessel walls.
Structural stability – sufficient to withstand all static and dynamic loads (gas pressure, weight and pressure of substrate, weight of the covering), durable and resistant to corrosion to ensure long useful life of the plant.

Digester shapes
From the standpoint of fluid dynamics and structural strength, an egg-shaped vessel is about the best possible solution. This type of construction, however, is comparatively expensive, so that its use is usually restricted to large-scale sewage treatment plants.

Figure 22. Different digester and
loading and unloading systems

а – cylindrical digester loaded from the top;
б – cylindrical digester loaded from bottom;
в – cylindrical digester with 2 sections;
г – inclined digester;
д – channel digester with floating cover;
е – horizontal digester with sections.

The second-best shape is cylinder with conical or rounded bottom and top. Square digester shapes from concrete of bricks are not recommended for use because due to high pressure in the corners of such digesters will soon appear cracks. Another problem with square shapes is that solid particles will settle in the corners and disrupt the digestion process.

Digester can be divided with the help of inside partitions into several sections for preventing the scum and more complete substrate fermentation.

Digester building materials
Digesters can be constructed from the following materials:
Steel vessels are inherently gas-tight, have good tensile strength, and are relatively easy to construct. Susceptibility to corrosion both outside (atmospheric humidity) and inside (aggressive media) can be a severe problem. As a rule, some type of anticorrosive coating must be applied and checked at regular intervals.

Steel vessels are only cost-effective, if second-hand vessels can be used. When a vessel of appropriate size is available it is necessary to check inside and outside surface of the vessel for shells, corrosion, quality of welding, holes and other deficiencies that have to be repaired. Then the surfaces have to be cleaned and painted.

Plastics vessels for use as a digester can be made of flexible materials (sheeting) and rigid materials. Flexible materials can be easily damaged and are difficult to insulate for a year-round operation. Rigid plastic containers are quite stable and are not susceptible to corrosion and therefore are recommended for psychophilic temperature digestion of organic waste.

Concrete vessels have gained widespread acceptance in developing countries in recent years. The requisite gas tightness necessitates careful construction and the use of gas-tight coatings, linings and/or seal strips in order to prevent gas leakage. Most common are stress cracks at the joints of the top and the sides. The prime advantage of concrete vessels are their practically unlimited useful life and their relatively inexpensive construction.

Masonry is the most frequent construction method for small scale digesters in India and China. Only well-burnt clay bricks, high quality, pre-cast concrete blocks or stone blocks should be used in the construction of digesters.

Impermeability of digester[8]
When building a biogas plants with concrete, masonry of stone digester it is necessary to take care about water- and gas-tightness of digester by applying a cover of plaster. The plaster has to resist moisture and temperatures up to 60°C reliably. The plaster must be resistant to organic acid, ammonia and hydrogen sulfide.

Cement plaster with special additives
Good results in water- and gas-tightness have been achieved by adding ‘water-proofer’ to the cement plaster. For gas-tightness, double the amount of water-proofer is required as
compared to the amount necessary for water-tightness. The time between the applications of the layers of plaster should not exceed one day, as the plaster becomes water-tight after one day and the new plaster cannot adhere to the old plaster.

The following ‘recipe’ from Tanzania guarantees gas-tightness, provided the masonry structure has no cracks:
1. layer: cement-water brushing;
2. layer: 1 cm cement : sand plaster 1 : 2.5;
3. layer: cement-water brushing;
4. layer: cement : lime : sand plaster 1 : 0.25 : 2.5;
5. layer: cement-water brushing with water-proofer;
6. layer: cement : lime : sand plaster with water proofer and fine, sieved sand 1 : 0.25 : 2.5;
7. layer: cement screed (cement-water paste) with water-proofer.
The seven courses of plaster should be applied within 24 hours.

Bitumen coat with aluminum foil
Bitumen coats can be applied easily and remain elastic over long periods of time. Bitumen coats have to be applied only on dry surfaces. On the first still sticky bitumen coat, aluminum foil is mounted with generous overlaps. A second layer of bitumen is applied on the aluminum foil.

Disadvantage of the bitumen coats lies in their flammability and in the fact that it cannot be applied on wet surfaces. The drying of masonry structures requires several weeks, unless some heating device (e.g. a charcoal stove) is placed inside the digester for two to three days. Furthermore, the bituminous coat can be damaged by the up-and-down movement of the slurry.

Paraffin, diluted with new engine oil, is warmed up to 100 -150°C and applied on the plaster which has been heated up with a flame-thrower. The paraffin enters into the plaster and effects a ‘deep-sealing’. If paraffin is not available, simple candles can be melted and diluted with engine oil.

Digester sitting
Placement of the biogas plant depends on several factors, such as availability of free space, distance to living premises, placement of waste storage and stables and so on. Depending on the subterranean water level, convenience of loading and unloading of substrate the digester can have above-ground, partly covered or underground position. Digester can be placed on elevation base, or constructed indoors with the animal stables.

Digester should have a hatch necessary for periodical maintenance checks and repairs of the digester. Between the body of the digester and the hatch there should be a lining from rubber or special gas-tight composition. Underground placement is recommended whenever possible as it allows to cut on construction costs and costs of substrate loading equipment, improves thermoregulation and gives opportunity to use cheap insulation materials – clay and straw.

Insulation materials
Most of the biogas plants built in Kyrgyzstan are built without provision of insulation of the digester which allows plants to operate only during warm months.

Insulation materials should possess good insulation qualities, be cheap and readily available. Suitable insulation materials for biogas plants with partly underground placement are straw, clay, slag, pressed dung. Isolation should be made of layers, for example, for underground digester after preparing foundation pit, first a layer of polyethylene film for preventing contact of thermo isolation with soil should be placed. Then come the layers of straw and clay that are placed on the bottom of the pit. After that the digester is placed in the pit and the space left between the walls of the digester and pit walls are filled up with layers of insulation materials up to the upper part of digester, after which the last layer of clay mixed with slag no less than 300 mm thick.

Monitoring equipment
Monitoring equipment installed on digesters include: check of substrate level in digester, control of temperature and pressure in digester.