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CLOSE THIS BOOKBiogas Plants (GTZ, 1988, 85 p.)
5. Design of biogas plants
VIEW THE DOCUMENT5.1 Shape and static loading
VIEW THE DOCUMENT5.2 Bottom slab
VIEW THE DOCUMENT5.3 Spherical shell of masonry construction
VIEW THE DOCUMENT5.4 Masonry and mortar
VIEW THE DOCUMENT5.5 The parts of a biogas plant and their functions
VIEW THE DOCUMENT5.6 Floating gas drum
VIEW THE DOCUMENT5.7 Water-lacket plant
VIEW THE DOCUMENT5.8 Fixed-dome plants
VIEW THE DOCUMENT5.9 Large-scale plants
VIEW THE DOCUMENT5.10 Biogas plants in cold regions

Biogas Plants (GTZ, 1988, 85 p.)

5. Design of biogas plants

5.1 Shape and static loading

A biogas plant should be watertight. The gasholder must be gaslight. For this reason a biogas plant must have no cracks. But structures of masonry or concrete always crack. One can try to keep the cracks small. And one can determine the position where the cracks are to arise.

Cracks always arise where the tensile stresses are highest. Tensile stresses arise from tensile forces, flexure, displacements, settling and temperature fluctuations. When mortar or concrete sets, shrinkage cracks also form.

Stresses are high where the "external" forces are high. "External" forces are earth pressure, dead weight and applied load. Stresses are highest where the "internal" forces are highest. "Internal" forces are flexural, normal, gravitational and torsional forces.

The "external" forces can be reduced by favourable shaping of the structure. The liquid pressure and earth pressure are less in a low biogas plant. This is because both depend directly on the height (see Figure 57).

The "internal" forces can also be reduced by favourable shaping of the structure. If the "external" forces can act in one direction only, high "internal" forces arise. If, however, the "external" forces can be distributed in a number of directions, small "internal" forces arise. This is the case with all curved surfaces or "shells" (see Figure 16).

Fig. 16: Shape and load-bearing capacity Slabs will support a heavier load than beams for a given thickness of material. A curved shell supports more than a flat slab. A shell cuned in more than one dimension supports more than a shell of simple curvature. Curved structural components are more rigid; the stresses are smaller in them. Just imagine how thick the shell of a hen's egg would have to be if it were shaped like a cube!

Cracks arise where stresses are high. Particularly high stresses - "peak stresses" - arise at points where the stress pattern is disturbed.

Such disturbances occur at edges, angles, corners and under concentrated, applied or other loads. Disturbances arise along the line of intersection of surfaces. Cracks form at these points due to peak stresses.

Peak stresses always arise at the edges of angular structures. For this reason the gas space of a fixed-dome plant must never be angular.

Cracks arise owing to tensile stresses. If a component is under compression, it is free from cracks. The gas space of a fixed-dome plant should therefore always be under pressure at every point.

The liquid pressure of the fermentation slurry is directed outwards. The earth pressure is directed inwards. If the two forces balance reliably, the load on the structure is relieved. In a vaulted shape' the external loading is obtained even if the earth is stiff and cracked owing to drought (Figure 17-19).

Fig. 17: Same volume - different shape Different shapes have different stress patterns under the same load (a and b). The round shape has lower stresses. The angular shape has high stresses and many stress peaks. Different shapes are often loaded differently. In a vaulted shape, the loads acting in different directions are more reliably balanced than with a vertical wall (c and d).

Fig. 18: Pattern of stresses in a fixed-dome plant of masonry construction Top: empty; bottom: filled and with maximum gas pressure. The peak stresses shown are those resulting from the first approximation calculation. In practice they are reduced by deformation (with or without cracking). Positive (+) tensile stresses do not occur in the gas space.

Fig. 19: Cracks in the gas space of a fixed-dome plant Angular gas spaces must on no account be used (a)! The transition from the roof arch to the wall must never be at a higher level than the lowest slurry line (b). Inlet and outlet penetrations must never be situated in the gas space (c). The gas space must remain undisturbed. Only the entry hatch at the top is allowed, because it can easily be checked.

A round shape is always a good shape, Because a round shape has no corners. Because its load pattern is more favourable. And because it uses less material. A round shape is often easier to build than an angular one (see Section 5.3). The rounder the better!

5.2 Bottom slab

The bottom slab is loaded at its edge by the weight of the digester wall. In the case of a spherical shell, the weight of the earth load also acts on it. The bottom slab distributes the weight over the ground of the site. The larger the foundation area, the less settlement will be experienced. The more even the loads, the more even the settlement. The more even the settlement, the less the risk of cracking.

A "rigid" shell distributes the weight better than a "soft" slab.

The weight of the fermentation slurry presses uniformly on the ground. Where the ground is of unequal consistency (e.g., boulders in loamy soil), loads must be distributed within the bottom slab. If the slab is too weak, it will break and cease to be watertight.

A "rigid" shell distributes the loads better than a "soft slab".

A vaulted shell is the best foundation shape. But a concial shell is easier to excavate. The only implement required is a straight piece of wood.

Building material available locally is used for the bottom slab. One of the following will be chosen on grounds of economy:

- quarrystone with a cement mortar filling and a cement floor,
- brick masonry with a cement floor,
- concrete.

Steel ring reinforcement at the outer edge increases the loadbearing capacity of the bottom. However, such reinforcement is not usually necessary. It is more important for the ground to be firm and clean. If the soil consists of muddy loam, it must first be covered with a thin layer of sand.

Fig. 20: The bottom slab A flat slab must be flexurally rigid if it is to distribute the edge loads over the entire surface (a). Shells ate flexurally rigid (b). Proceeding from a conical shell to a spherical shell (c). Possible forms of construction: Quarrystone with cement mortar (d). Masonry with cement floor (e) and concrete (f). Underneath the wall the bottom slab should be made out of massive concrete.

5.3 Spherical shell of masonry construction

The construction of a spherical shell from masonry (Figure 21) is completely problem-free. Every bricklayer can master this technique after once being shown how to do it. Concreting a vault, on the other hand, calls for much more skill and craftsmanship owing to the complicated formwork - the one exception being when the masoned shell is intended to serve as permanent formwork. A spherical shell of masonry is simple to construct because the radius always extends from the same centre. A trammel (A) is the only aid required. Bricks are stacked to get the right height for the centre. Lean mortar is used for the stack, which is subsequently demolished (M). No centring is necessary for laying the bricks.

Fig. 21: Construction of a spherical shell from masonry

When the bricks are laid, it is important for their tops to be parallel with the bottom edge of the trammel (B), from the very first course. The bricks are laid perpendicularly and centrally to the trammel (C). In the upper part - when the trammel is standing at a steeper angle than 45° - the first brick in each course must be held until the circle is complete. Each brick inbetween must be held only until the next brick is set. For this purpose, clamps (D) or counterweights of stones tied together (E) are used. The bricks can also be supported with sticks.

The mortar must be mixed from finely sieved sand (maximum particle size 3 mm). If the sand is too coarse, the mortar will be difficult to work. It has to "stick" to the sloping, narrow surface of the brick. Compo (cement/ lime) mortar is "stickier" than pure cement mortar. "Squeezed joints" (Q) should be used. The trowel should have straight sides, so that the squeezed-out mortar can be scraped off and reused (F). As in any masonry construction, the joints must be offset (G). The terminal ring is rendered. The last but one course of bricks is laid on end (J).

When backfilling, the footing point must be tamped particularly well: one man filling and two men tamping (H).

5.4 Masonry and mortar

The mortar and bricks should have about the same strenght. If the bricks are soft, the mortar must also not be too hard. If a good brick is thrown on to the ground three metres away, it must not break. If the bricks are of poor quality, the walls must be thicker. Mortar consists of sand, water and the binders. Cement gives a solid, watertight mortar. Cement mortar is brittle in masonry construction. Lime gives a soft, sticky mortar.

For masonry construction, cement mortar should always include a certain amount of lime. This makes it more workable, and the masonry becomes more watertight.

Mixing ratio:

Masonry mortar

2 (cement)

: 1 (lime)

: 10(sand)


1 (cement)

: 6 (sand)

Rendering mortar

1 (cement)

: 4 (sand)


1 (cement)

: 3 (sand)

The most important part of the mortar is the sand. It must be clean. It should not contain any loam, dust or organic matter. Mortar sand with a high proportion of dust or loam "eats up" much more cement than clean sand.

Fig. 22: Testing of mortar sand 1. Fines (loam, dust): Water glass 1/3 sand, 2/3 water. stir vigorously. Leave to stand for one hour. Measure fines. A maximum of 10 % of the amount of sand is permissible. 2. Organic matter: Bottle with stopper (not cork) to be filled with 1/3 sand and 2/3 soda lye (3 %). Shake repeatedly within an hour. Leave to stand for 24 hours. Water colour clear or light yellow: good; red or brown: bad.

The bricklayer or works foreman must check the sand before use (Figure 22). Sand may contain not more than 10% dust or loam, otherwise it must be washed. Soda Iye can be used to test whether the sand contains excessive organic matter. The following points are important when rendering:

- The rendering mortar must be compressed by vigorous, circular rubbing.
- All edges must be rounded.
- All internal angles must be rounded with a glass bottle.

5.5 The parts of a biogas plant and their functions

The feed material is mixed with water in the mixing tank (Figure 23). Impurities liable to clog the plant are removed here. The fermentation slurry flows through the inlet (Figure 24) into the digester. A stick is inserted through the inlet pipe' to poke and agitate the slurry. The bacteria from the fermentation slurry are intended to produce biogas in the digester (Figure 25). For this purpose they need time. Time to multiply and to spread through- ' out the slurry. The digester must be designed so that only fully digested slurry can leave it. Partitions (Figure 26) ensure that the slurry in the digester has long flow paths. The bacteria are distributed in the slurry by stirring (with a stick or stirring facilities, see Figure 27). If stirring is excessive, the bacteria have no time "to eat". The ideal is gentle but intensive stirring about every four hours. Optimum stirring substantially reduces the retention time.

Fig. 23: Mixing tank at inlet Grit and stones settle at the bottom of the mixing tank. For this reason the inlet pipe (p) should be 3-5 cm higher than the tank bottom. A round, cylindrical shape is cheapest and best for the mixing tank. If the tank is filled in the morning and then covered, the slurry heats up in the sun until the evening (c). Only then is the plug removed (s).

Fig. 24: The inlet The inlet must be straight. The axis of the inlet pipe should, as far as possible, be directed into the centre of the digester. This facilitates stirring and poking. The inlet should be as high as possible, so that gritty deposits do not block the inlet pipe. In fixed-dome plants, the inlet pipe must not pass through the gas space (a). For fibrous feed material, the diameter should be 200-400 mm.

Fig. 25: Path of the fermentation slurry in the digester Fresh fermentation material is lighter than fully digested sludge. For this reason the former quickly rises to the surface and then sinks only gradually. The digestion process has two phases. The better these phases are separated, the more intensive the gas production. The fermentation channel (A) satisfies these conditions best. Tandem plants are expensive and complicated (D). The deeper the digester, the lower and less uniform its temperature.

Fig. 26: Hemispherical plant with partition wall The principle of the fermentation channel is obtained by the fact that the inlet and outlet pipes are close together. The partition wall extends up above the surface level of the fermentation slurry. The gasholder must therefore float in a water jacket. The "horizontal KVIC gobar gas plant", which is similar in design, works perfectly with high gas production.

Fig. 27: Stirring facilities in the digester The impeller stirrer (a) has given good results especially in sewage treatment plants. The horizontal shaft (b) stirs the fermentation channel without mixing up the phases. Both schemes originate from large-scale plant practice. For simple household plants, poking with a stick is the simplest and safest stirring method (c). What matters is not how good the stirring arrangements are but how well the stirring is performed (see page 38).

The fully digested slurry leaves the digester through the outlet (Figure 28).

Fig 28: Outlet (overflow) of a floating-drum plant The outlet should be placed below the middle of the digester, otherwise too much fresh feed material will flow out of the plant too soon, thus reducing gas production by as much as 35 % (b). The height of the outlet determines the level of the surface of the fermentation slurry (c-f). This should be 8cm below the top edge of the wall. If this is not the case, difficulty will be experienced in painting. If the outlet is too low, digester volume is lost (d). If it is too high, the slurry will overflow the edge of the wall (e).

The biogas is collected and stored until the time of consumption in the gasholder. The prime requirement for the gasholder is that it must be gaslight. Floating gasholders are held by a guide.

In fixed-dome plants, the compensating tank acts as a storage facility for the slurry displaced by the biogas. In this case the gas is collected and stored in the upper part of the digester.

The gas pipe carries the biogas to the place where it is consumed. Condensation collecting in the gas pipe is removed by a tap or water trap. Flexible gas pipes laid in the open must be UV-resistant.

5.6 Floating gas drum

The gas drum normally consists of 2.5 mm steel sheet for the sides and 2 mm sheet for the cover. It has welded-in braces. These break up surface scum when the drum rotates.

The drum must be protected against corrosion. Suitable coating products are oil paints, synthetic paints and bitumen paints. Correct priming is important.

One coat is as good as no coat. Two coats are not enough. There must be at least two preliminary coats and one topcoat.

Coatings of used oil are cheap. They must be renewed monthly. Plastic sheeting stuck to bitumen sealant has not given good results. In coastal regions, repainting is necessary at least once a year, and in dry uplands at least every other year. Gas production will be higher if the drum is painted black or red than with blue or white, because the digester temperature is increased by solar radiation. Gas drums made of 2 cm wire-mesh-reinforced concrete or fibrocement must receive a gaslight internal coating.

The gas drum should have a slightly sloping roof (Figure 29), otherwise rainwater will be trapped on it, leading to rust damage. An excessively steep-pitched roof is unnecessarily expensive. The gas in the tip cannot be used because the drum is already resting on the bottom and the gas is no longer under pressure.

Fig. 29: The gas drum The gas drum should have a slightly sloping roof. When the cover plate is cut, a wedge (k) should be cut out. The cover plate must be rather larger than the diameter of the drum (see calculation at bottom left). In- accuracies can more easily be corrected if a lateral overhang of 2 cm is allowed.

Fig. 30: Forces on the gas drum The gas pressure and the weight of the metal itself give rise only to tensile forces in the jacket sheet. No reinforcements are necessary for these to be withstood (a). The loads from the guide tube must be reliably transmitted to the cover plate (b). A flange plate (b1 ) or angle iron (b2) is required for this purpose. The braces are stressed when the drum i,s rotated (c). They should not simply butt on to the metal but end in a corner (c, ) or at an angle (c2).

The side wall of the gas drum should be just as high as the wall above the support ledge. The floating-drum must not scrape on the outer walls. It must not tilt, otherwise the paintwork will be damaged or it will jam. For this reason a floating-drum always requires a guide (see Figures 31 and 32). The guide frame must be designed so that the gas drum can be removed for repair. The drum can only be removed if air can flow into it, either the gas pipe should be uncoupled and the valve opened, or the water jacket emptied.

Fig. 31: Floating drum guide frame An external guide frame (A) is cheapest. It is made of tubular steel, sectional steel or wood. The guide tube also acts as the gas outlet. With scheme (B), the open pipe is problematic. It cannot be reliably painted. The tidiest, but also the most expensive, solution is a guide with internal gas outlet (C). For the water trap (D) see also Figure 40. Guide frames for heavy gas drums must withstand large forces. All joints and anchor points must be just as strong as the pipes themselves.

Fig. 32: Unsuitable guidance systems for floating drums With these guides, the gas drum cannot be rotated. This means that floating scum cannot be broken up. The rollers and bearings must be lubricated. In arrangement (C), the paintwork of the drum is damaged. Plant (B) is jammed if only one of the guide rods is not vertical. A central guide tube is always better!

5.7 Water-lacket plant

The water-jacket plant (Figure 33) is a special case of the floating-drum plant. The drum floats in a water bath and not direct in the slurry. Water-jacket plants can handle substrates with a high solids content without danger of drum blockage due to crust formation.

Fig. 33: The water jacket

The floating-drum must be able to move freely up and down in the water jacket. It must be free to rotate. The inner braces must not rest on the inner edge of the wall (d). They must therefore begin offset at least 20 cm inwards (i). The water jacket must always be filled to the top, as the gas space will otherwise be reduced (c). A few drops of oil slow down the evaporation of water (g). The inner wall must either be gaslight at the base or rest on a ring of "gaslight" mortar (h). An overflow pipe can be installed to keep excessive rainwater from carrying off the oil film during the rainy season (k). The overflow pipe must not protrude into the water jacket.

The water-jacket is particularly suitable where human excrement is to be digested. Of all simple systems, the water-jacket plant is the cleanest. The gas drum rusts less in the water jacket than if it were floating directly in the slurry.

The water in the jacket evaporates quickly. For this reason the water level must be checked regularly. A few drops of used oil on the water surface prevent rapid evaporation and protect against corrosion (Figure 33,g). A rainwater overflow pipe can be quite helpful.

The inner wall of the water jacket is inside the gas space. Its upper part must receive a gaslight coating or rest on a gaslight ring, otherwise the gas will escape through the porous wall (Figure 33,h).

The water jacket must be kept absolutely free. If it is not, the floating drum cannot move up and down without impediment. The inlet or gas pipes must of course not be fed through the water jacket (Figure 33, f). The water jacket must be wide enough to allow objects inadvertently dropped into it to be retrieved (Figure 33, e).

The walls of the water jacket are as high as those of the gas drum.

If the drum is too high, the last gas cannot be used. The weight of the gas drum cannot then exert any more pressure on the gas (Figure 33, a).

If the walls of the ring are too high, unnecessary construction costs arise.

Fig. 34: Comparison of floating drums for water-jacket plants (A) and for plants with internal gas outlet (B): Bot types of plant are assumed to have the same gas-holding capacity. The distance between the top rim of the gas outlet pipe and the slurry level (A) depends on the shape of the drum. a: Overflow level or unpressurized slurry level; b: Pressurized slurry level; c: gasholder configuration as in A; d: Comparison of sheet metal cutouts for drum lids.

5.8 Fixed-dome plants

The top part of a fixed-dome plant (the gas space) must be gaslight. Concrete, masonry and cement rendering are not gaslight. The gas space must therefore be painted with a gaslight product.

Gastight paints must be elastic, This is the only way to bridge cracks in the structure.

Latex or synthetic paints (PVC or polyester) are suitable. Epoxy resin paints are particularly good. Polyethylene is not very gaslight. Hot paraffin coatings also serve well. The walls are first heated with a torch. Then hot paraffin (as hot as possible) is applied. Since the paraffin will only adhere to thoroughly dry masonry, it may have to be dried out first with the aid of a charcoal fire.

Fixed-dome plants produce just as much gas as floating-drum plants - but only if they are gaslight. However, utilization of the gas is less effective as the gas pressure fluctuates substantially. Burners cannot be set optimally.

Figures 35 and 36 show mayor details of the compensating tank.

Fig. 35: Correct height of compensating tank The bottom of the compensating tank is at the level of the zero line (filling line). The zero line is 25 cm below the head of the digester dome (c). Wrong: (a) the bottom of the compensating tank is too low. Part of the slurry is always in contact with air. Gas is lost. Unnecessary cost. (b) The bottom of the compensating tank is too high. The gas pressure rises very fast and to a very high level.

Fig. 36: Shape of compensating tank The shape of the compensating tank determines the height of the slurry surface and hence the gas pressure (cm WC). The lower the compensating tank, the lower and more even the gas pressure. However, the lower the tank, the larger the area exposed to atmospheric oxygen. Differences in building costs due to shape are slight.

Figure 37 shows details of the entry hatch.

Fig. 37: Detail of a fixed-dome entry hatch The gas pipe penetrates the shaft a few centimeters below the cover. The cover is sealed with screened and well-worked clay. The bottom of the cover is sealed with paraffin. Rocks are placed on the lid to weigh it down, and the shaft is filled with water to keep the clay gaslight. A few drops of oil keep the water from evaporating.

5.9 Large-scale plants

Large plants do not come under the heading of "simple" plants. For this reason they are not described in detail here. However, the designer must know that he cannot "simply" enlarge the plans for a "simple" plant to any degree.

The digester can be enlarged without major changes in the design. However, large floating drums quickly become awkward and heavy: to manufacture, to transport, to maintain.

A floating drum 5 m in diameter cannot be turned by one person. The surface scum in the plant is not broken. It will become more and more solid. Gas production will fall. In plants with digester volumes exceeding 50 m³, poking no longer provides sufficient agitation. Stirring or agitation facilities are required.

A floating drum with a diameter exceeding 5 m requires a more precise guide frame, otherwise the drum will tilt so badly that it jams. Water-jacket plants are particularly at risk in this respect.

In fixed-dome plants, the gas pressure also varies directly with size. If the shape of the structure is unaltered but the size is doubled, the gas pressure doubles. For this reason, large fixed-dome plants always require a separate gasholder and an agitator.

In large plants, large quantities of feed material and water must be obtained and mixed. Mechanical mixers become necessary. Large volumes of fermentation slurry require a larger drying area, as the thickness of the slurry layer cannot be increased indefinitely. Feed material or fermentation slurry often has to be stored for several weeks. This calls for large and expensive containers.

5.10 Biogas plants in cold regions

Simple biogas plants are usable only conditionally in tropical uplands or in temperate climatic zones. At latitudes as high as only 25 - 30°, gas production in winter generally falls to about half the summer level.

Whether it is worthwhile to heat a plant must be decided on an individual-case basis. In Europe, large-scale plants use up 20-30 % of their gas production for heating. Practicable heating systems for simple plants have not yet been developed.

Utilization of solar energy in the mixing tank (Figure 23) and insulation by covering with straw are insufficient where frost occurs. Floating drums have the highest heat losses. Underground fixed-dome plants maintain more even but generally lower temperatures. Fixed-dome plants with floating gasholders (Figures 3 and 52) may be a valid solution for cold regions although more expensive. Good results are obtained with roofed-over biogas plants. However, the cost of a "greenhouse" superstructure is relatively high. It is worthwhile only where low temperatures are combined with high insolation. Good results have been obtained by placing the plant under a compost heap. If the digester is surrounded externally by soft insulation, the wall cannot be "relieved of its load" by the earth pressure (see Figure 17).

Again, insulation must always remain dry. The only exception is special insulation with closed pores. Biogas plants are completely shut down in winter in the north of China; they are used for only six to eight months per year.

Where frost occurs, mixing and filling tanks must be roofed over. Transport of feed material is difficult in snow. It is essential to consider in detail how the plant is to be operated before commencing construction. Energy is particularly expensive in cold regions. This is why biogas plants have to be used in these regions. Unfortunately, appropriate types of simple plants have not yet been developed.