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4 - The liquid feedstock is then transferred to the digester tank – main biogas production After 2-3 days, the liquid feedstock is moved, via the pump room to the digester tank. Here the methanogenic bacteria continue to break down the organic matter and whilst doing so produce the majority of the biogas. The gas rises and collects at to the top of the digester, and then moved to the CHP engine, the upgrader unit or if into the final storage tanks. The digestate is moved to the separator, where it is separated into its liquid and solid forms. The main digester tank is maintained at 41-44 o C by using heating coils to circulate hot water around the tank. As with the Turbo Tank heat is taken from the CHP engine which burns biogas produced by the plant. The retention time in the main digester tank it about 15-20 days. This gives an average retention time of processing feedstock through both tanks of 18.3 days. The net volume of the digester tank is in the region of 7,200 m 3 . The digester tank has walls made of concrete, which is cast in-situ, insulated and then covered with cladding. The digester and storage tanks have a double membrane dome roof provided by Baur. The inner shell is the integrated gas storage and the outer membrane is a durable weather protection foil. The tanks are also interconnected with pipes, so gas pressures can be monitored and biogas moved around the plant. The biogas is also stored in the storage tanks. The pressure of the gas is monitored on all the domes and if this becomes too high there is a hydraulic pressure release valve to release biogas if needed.
The digester tank has four Mammut RF3 mixers from Paul Michl. The variable angle of the mixers is easily adjusted by a hydraulic unit, so the optimum positioning of the mixers can be controlled by the operators who set the mixing pattern, which will depend on the exact characteristics of the substrate within the tank. The Mammut mixers have a 4.5m Shaft and a 1600mm propeller. These are highly efficient mixers, even when the dry substance content if high. The mixing provided is strong and slow, but also gentle which is well suited to operating in the digester tank. As the gear mechanism is outside the tank this allows for easy maintenance, as no electrical components are situated within the tank. The mixer is mounted with anti-vibration bearings which are fixed to an instillation bracket. This robust gear unit has low noise during operation. The drive shaft has multiple bearings and an oil-bath lubrication. • The biogas produced is mainly methane, (50% and 65%), Carbon dioxide (48%-33%) some water, Hydrogen sulphide and some other impurities in small quantities. • One of the impurities in the biogas is Hydrogen Sulphide (H 2 S), biological desulphurisation which removes this can be facilitated by injection of small amounts of oxygen, or air, into the tanks. This occurs as the oxygen encourages the growth of a certain type of bacteria, called thiobacilli, which occur on the surface of the digestate and as they grow and use the H 2 S in the biogas and leave a yellow clusters of sulphur on surface of the digestate. This process can also be performed in the storage tanks.
Page 1 and 2: Qila Anaerobic Digestion plant Step
Page 4: 2 - The feedstock is moved to the V
Page 10: 5 - Biogas is moved to the upgrader
Page 13 and 14: 7 - Biomethane enters the National
Page 16: 9 - Digestate is removed from the m
Page 20 and 21: Qila Energy is a British biogas eng

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