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Regular operations

Daily operations
Substrate input
Daily substrate input portion and periodicity of feeding are very important for optimal operation of a biogas plant. Daily input portion is not constant and depends on the type of substrate, digestion temperature and solid matter concentration in substrate.
When daily substrate input does not exceed 1-5 % of digester volume biogas production is less optimal than when daily input reaches 10-20% of digester volume. However, with large input portions methane content of the biogas is decreasing and carbon dioxide content is increased, making gas less flammable.
Optimal daily substrate input for plants operating under mesophilic temperatures from the point of view of biogas quality is 6-10% of full digester volume and retention period of 10 – 20 days. Optimal daily substrate input for plants operating under thermophilic temperatures is 15-25% and retention period of under 8 days. When digester is operated under psychrophilic temperature it is recommended to load no more than 2% of overall digester volume daily. If digester is used for batch operation, substrate is loaded up to 2/3 of digester volume at once and is left to digest without additional feeds for 40 and more days.

Frequency of feeding and agitation
Daily substrate input should not be fed into digester at once, but by equal portions 4-6 times a day. After feeding the potion of new substrate it is recommended to agitate the substrate. Condition and operation of agitation system should be checked daily.

Using slurry color to control of digestion process
The progress of the digestion process can be judged by the intensity of biogas production and color of the fermented mass unloaded form the digester. Absence of biogas or low biogas generation is a sign of low activity of microorganisms and can be determined by the grey color of the fermented mass. This can happen due to lack of microorganisms that leads to slowing down of the process that can be revived by injecting nutrient solutions with high microorganisms concentration and with potential to generate biogas.
Slowing of the microorganism activity and acid formation can be caused by the excess of nutrients and is characterized by black color of the unloaded slurry and formation of a white film on its surface. Acids can be neutralized by introduction of vegetation ashes or lime water. If digester slurry has a dark-brown color with foam on it’s surface than digestion process can be considered as successful.

Substrate level control
A special problem of small scale fixed dome plants is the clogging up of the overflow point. This can lead to excessive pressure inside the digester and clogging of gas pipe. For preventing of clogging it is necessary to perform daily checks of the substrate level and condition of inlet and outlet openings.

• Controlling of the water traps
• Renewing the agents of the gas purification system
• Gentle cleaning of the drum of a floating drum plant
• Flexible pipes above ground should be checked for porosity

• deletion of scum from the substrate surface and cleaning the sediments from the bottom of he digester
• the whole plant and gas system should be checked for impermeability and pressure.