There are two basic types of tested biogas plants that have gained widespread acceptance in agricultural practice:
- floating-drum plants in which the metal gasholder floats on the digester, and
- fixed-dome plants in which gas storage is effected according to the displacement principle.
5.3.1 Floating-drum plants
A floating-drum biogas plant essentially consists of a cylindrical or dome-shaped digester and a movable, floating gasholder, or drum. The drum in which the biogas collects has an internal or external guide frame that provides stability and keeps the drum upright. Braces can be welded into the drum as a means of breaking up the scum layer when the drum is rotated. The digester is usually made of brick, concrete or quarrystone masonry with rendering, while the gasholder is normally made of metal.
Floating-drum plants are used chiefly for digesting animal and human excrements on a continuous-feed mode of operation, i.e. with daily input. They are used most frequently by:
- small-to-midsize family farms (digester size: 5 - 15 m³)
- institutions and large agroindustrial estates (digester size: 20-100 m³).
Advantages: Floating-drum plants are easy to understand and operate. They provide gas at a constant pressure, and the stored volume is immediately recognizable.
Drawbacks: The steel drum is relatively expensive and maintenance-intensive due to the necessity of periodic painting and rust removal. If fibrous substrates are used, the gasholder shows a tendency to get "stuck" in the resultant floating scum.
Floating-drum plants can be recommended as a mature, easy-to-operate, functionally capable means of producing biogas, particularly when reliability is deemed more important than inexpensiveness.
Floating-drum plants with gasholder in the digester (cf. fig. 5.6)
The dome shape is inherently sturdy, compact and material-sparing. The digester is easy to build, and the techniques can be learned by local craftsmen in a short time (cf. fig. 5.21).
Water-jacket plant (cf. fig. 5.7)
Water-jacket biogas plants are characterized by a long useful life and a more aesthetic appearance (no dirty gasholder). Due to their superior hygiene, they are recommended for use in the fermentation of night soil and for cases involving pronounced scumming, e.g. due to rapid evaporation, since the gasholder cannot get stuck in the scum. The extra cost of the masonry water jacket is relatively modest.
Cylindrical plant for quarrystone masonry and concrete (cf. fig. 5.8)
It is anything but easy to make a dome-shaped digester out of quarrystone masonry; it is much easier to build a concrete cylinder. In such cases, the classical (Indian) version with a cylindrical digester is quite practical Note: Quarrystone masonry consumes a lot of mortar.
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| Fig. 5.6: Floating-drum plant with internal guide frame. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 41 Discharge pipe, 5 Gas pipe, 51 Water trap (Source: Sasse 1984) |
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| Fig. 5.7: Water-jacket plant with external guide frame. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 5 Gas pipe (Source: Sasse 1984) |
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| Fig. 5.8: Cylindrical plant design for quarrystone masonry construction. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 5 Gas pipe (Source: KVIC) |
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| Fig. 5.9: Basic function of a fixed dome biogas plant. 1 Mixing pit, 2 Digester, 3 Gasholder, 4 Displaceinent pit, 5 Gas pipe -(Source: OEKOTOP) |
5.3.2 Fixed-dome plants
A fixed-dome plant comprises a closed, dome-shaped digester with an immovable, rigid gasholder and a displacement pit. The gas collects in the upper part of the digester. Gas production increases the pressure in the digester and pushes slurry into the displacement pit. When gas is extracted, a proportional amount of slurry flows back into the digester.
The gas pressure does not remain constant in a fixed-dome plant, but increases with the amount of stored gas. Consequently, a special-purpose pressure controller or a separate floating gasholder is needed to achieve a constant supply pressure. The digesters of such plants are usually made of masonry, with paraffin or bituminous paint applied to the gas-flled area in order to make it gastight.
Fixed-dome plants can handle fibrous substances in combination with animal excrements, since the motion of the substrate breaks up the scum each day. The plant is a continous-feed type, but can accept several days' worth of substrate at a time, if the displacement pit is large enough.
Fixed-dome plants must be covered with earth up to the top of the gas-filled space as a precautionary measure (internal pressure up to 0.1-0.15 bar). As a rule, the size of the digester does not go beyond 20 m³, corresponding to a gasholder volume of 3-4 m³. The earth cover makes them suitable for colder climates, and they can be heated as necessary.
Advantages: Fixed-dome plants are characterized by low initial cost and a long useful life, since no moving or rusting parts are involved. The basic design is compact and well-insulated.
Drawbacks: Masonry is not normally gaslight (porosity and cracks) and therefore requires the use of special sealants. Cracking often causes irreparable leaks. Fluctuating gas pressure complicates gas utilization, and plant operation is not readily understandable.
Fixed-dome plants are only recommended in cases where experienced biogas technicians are available for building them, and when the user is amply familiar with how the plant operates.
Fixed-dome plant with central entry hatch (cf. fig. 5.10)
The digester has the form of a hemispherical dome which is easy to build. Floating scum can be removed from the full digester through the central entry hatch.
Fixed-dome plant with suspended dome (cf. fig. 5.11)
Providing a separate foundation for the gas dome yields a statically advantageous, material-saving configuration that is very well suited for fixed-dome plants of ample size. The dome's foundation helps prevent cracking due to tensile stress, and the digesting space is made less expensive, since it can be built of thinner masonry, ferrocement rendering or - in the case of impervious soil - even left unlined.
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| Fig. 5.10: Fixed-dome plant with central entry hatch. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gas holder, 31 Entry hatch, 32 Gas cover, 33 Seal coating, 34 Rated break ring, 4 Displacement pit, 41 Outlet pipe, 42 Overflow, 43 Cover, 5 Gas pipe, 51 Water trap, 52 Cover (Source: Sasse 1984 / BEP Tanzania 1987 / OEKOTOP) |
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| Fig. 5.11: Fixed-dome plant with suspended dome. 1 Mixing pit, 11 Fill pipe, 2 Digester, 21 Digester rendering, 3 Gas holder, 31 Entry hatch, 32 Cas cover, 33 Seal coating, 34 Dome foundation, 35 Dome masonry, 4 Displacement pit, 41 Outlet pipe, 42 Overflow, 43 Cover, 5 Gas pipe (Source: BEP Tanzania 1987/ OEKOTOP) |
5.3.3 Other types of construction
In addition to the two most familiar types of biogas plant, as described above, a selection of special-purpose and otherwise promising designs are briefly presented below.
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| Fig. 5.12: Horizontal balloon-type biogas plant. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 4 Slurry store, 41 Outlet pipe, 5 Gas pipe, 51 Water trap, 6 Burden, 61 Guide frame (Source: OEKOTOP) |
Inflatable balloon plants (cf. fig. 5.12)
Inflatable biogas plants consist of a heatsealed plastic or rubber bag (balloon), the top and bottom parts of which serve as the gasholder and digester, respectively. The requisite gas pressure is achieved by weighting down the bag. Since the material has to be weather-resistant, specially stabilized, reinforced plastic or synthetic caoutchouc is given preference. The useful life amounts to 2 - 5 years.
Advantages: Standardized prefabrication at low cost; shallow installation suitable for use in areas with a high groundwater table.
Drawbacks: Low gas pressure requires extra weight burden, scum cannot be removed. The plastic balloon has a relatively short useful life, is susceptible to damage by mechanical means, and usually not available locally. In addition, local craftsmen are rarely in a position to repair a damaged balloon.
Inflatable biogas plants are recommended, if local repair is or can be made possible and the cost advantage is substantial.
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| Fig. 5.13: Earth-pit plant with plastic-sheet gasholder. 1 Mixing pit, ll Fill pipe, 2 Digester, 21 Rendering, 22 Peripheral masonry, 3 Plastic-sheet gasholder, 31 Cuide frame, 32 Wooden frame, 33 Weight, 34 Frame anchorage, 35 Plastic sheeting, 4 Slurry store, 41 Overflow, 5 Gas pipe (Source: OEKOTOP) |
Earth-pit plants (cf. fig. 5.13)
Masonry digesters are not necessary in stable soil (e.g. Iaterite). It is sufficient to line the pit with a thin layer of cement (netting wire fixed to the pit wall and rendered) in order to prevent seepage. The edge of the pit is reinforced with a ring of masonry that also serves as anchorage for the gasholder. The gasholder can be made of metal or plastic sheeting. If plastic sheeting is used, it must be attached to a quadratic wooden frame that extends down into the slurry and is anchored in place to counter its buoyancy. The requisite gas pressure is achieved by placing weights on the gasholder. An overflow point in the peripheral wall serves as the slurry outlet.
Advantages: Low cost of installation (as little as 1/5th as much as a floating-drum plant), including high potential for self help.
Drawbacks: Short useful life, serviceable only in suitable, impermeable types of soil.
Earth-pit plants can only be recommended for installation in impermeable soil located above the groundwater table. Their construction is particularly inexpensive in connection with plastic sheet gasholders.
Fig 5.14: Ferrocement biogas plant. 1 Mixing pit, 11 Fill pipe, 2 Digester, 21 Backfill soil, 22 Ferrocement, i.e. rendered lathing on surrounding soil, 3 Ferrocement gasholder, 31 Guide frame, 41 Outlet pipe, 5 Cas pipe, 51 Water trap (Source: OEKOTOP/BEP Caribbean 1986)
Ferrocement plants (cf. fig. 5.14)
The ferrocement type of construction can be executed as either a self-supporting shell or an earth-pit lining. The vessel is usually cylindrical. Very small plants (Vd <6 m³) can be prefabricated. As in the case of a fixed-dome plant, the ferrocement gasholder requires special sealing measures (provenly reliable: cemented-on aluminium foil).
Advantages: Low cost of construction, especially in comparison with potentially high cost of masonry for alternative plants.
Drawbacks: Substantial consumption of necessarily good-quality cement; participating craftsmen must meet high standards; uses substantial amounts of steel; construction technique not yet adequately timetested; special sealing measures for the gasholder.
Ferrocement biogas plants are only recommended in cases where special ferrocement know-how is available.
Fig. 5.15: Horizontal biogas plant (KVIC shallow design). 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 41 Outlet pipe, 5 Gas pipe, 51 Water trap (Source: OEKOTOP / KVIC 1978)
Horizontal plants (cf. fig. 5.15)
Horizontal biogas plants are usually chosen when shallow installation is called for (groundwater, rock). They are made of masonry or concrete.
Advantages: Shallow construction despite large slurry space.
Drawbacks: Problems with gas-space leakage, difficult elimination of scum.
Plants with separate gasholders
Masonry dome plants are sometimes equipped with separate gasholders. That approach always involves substantial extra cost and therefore is rarely recommended. Plants with separate gasholders are justifiable, when the points of gas consumption are a considerable distance away from the digester (at least 1 00 m).
Altematively, a separate gasholder could be useful for restoring the utility value of, say, a fixed-dome plant that has been found to leak at an elevated pressure level.
