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There are various membranes which may be used. This can range from simple stand-alone bladder tanks, to more sophisticated constructions, PVC ground sheets with dome on a concrete slab or part of a fixed silo.
FTS- Flexitank Systems manufactures to any engineering specified drawing. The membrane moves up or downward depending on the filling level. In the case of a dome structure the gas holder with base plate and bottom membrane seals the storage space to the concrete foundation. With the help of the anchoring ring, the membranes are attached to the foundation. Particular emphasis is on the dimensioning of the membranes. Each holder will be calculated individually in consideration of the loads (namely internal pressure, wind and snow loads). As a result, membranes with high tensile strength are selected. The PVC-coated membranes are flame retardant according to DIN 4102 B1, fungicide, and equipped with an increased UV protection.
An example of a more sophisticate construction is the concrete Silo with Bio Curtain (Johannesburg) , where the entire concrete floating slab (6 Tons) is held in place by a sealed centre pole and the gas constrained by a curtain as set out in the illustration:
The aforementioned project design, another FTS innovation, is just another example of FTS ability to optimise and combine design integrity and innovation, high performance functionality and effectiveness, to almost any client requirement.
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Gas storage tanks of varying volumes should be installed for changes in gas production (as well as gas consumption) rates while still maintaining stable pressure. The most viable method of reducing the costs associated with gas storage is to utilize the gas at the same rate that it is produced.
Floating Digester Cover: Using a floating pontoon cover in the digester, as mentioned above, can serve as a gas collection and gas storage mechanism.
Gas Storage Tank: A high-pressure storage vessel can also be used, but as this is quite costly for small-scale farm use, a medium-pressure storage vessel (<100 psi) may be a more viable option. A biogas storage tank is typically a spherical or cylindrical-shaped unit constructed from welded steel. Ensure that someone who is experienced with the design and construction of pressurized vessels is consulted due to the risk of leakage and/or explosion associated with pressurized gas.
REDUCING ODOR & CORROSION If the waste in the digester contains a high level of sulfates, hydrogen sulfide — which smells like rotten eggs — can be produced. However, if the digester is operating properly, odours should be minimal as both methane and CO2 are odourless gases, and are stored within an airtight storage vessel before being burnt, eliminating any problems associated with odour. Hydrogen sulfide is also highly corrosive, and thus corrosion can pose a real problem if measures are not taken to minimize it. To this end, hydrogen sulfide is best removed using specialized filters (filters filled with lead filings; a woodchip and iron oxide mix; or Sorb beads are typically used to remove hydrogen sulfide).
SAFETY CONSIDERATIONS
Methane is an extremely volatile gas, and when it mixes with air (at just 6-15% methane concentrations) there is a risk of explosion. Biogas is heavier than air and, as a result, sinks, displacing oxygen near the ground. When hydrogen sulfide is present, this gas not only presents an explosive threat but can also pose a poison hazard that could prove deadly in confined spaces. To ensure proper safety, if the biodigester is housed indoors, ensure adequate ventilation, and pressure gauges and/or pressure relief valves are vented to the outside. Having a Methane Gas Detector capable of measuring the values is important. Always take care when compressing or storing biogas, ensuring that suitable equipment and specialized storage tanks are used when biogas is stored under high pressure.
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