The big egg-shaped containers you see are "digesters," which are used for anaerobic digestion.
Anaerobic digestion is utilised in wastewater treatment facilities to stabilise both main and secondary sludges that settle out during aerobic wastewater treatment (Sewage Works).
The major output is biogas, or biomethane when cleaned up, which is a valuable renewable (mainly methane) gas.
The gas can be utilised to power the wastewater treatment plant's machinery and offices, and there may be enough left over to serve local houses.
The sludges contain a solids concentration ranging from 2 to 6% (that is, 20-60 grammes of Total Solids per litre).
Around 70% of the combined sludge is degradable, and in a normal single-stage CSTR reactor, up to 80% of it is digested, lowering the TS by roughly 50%.
Although mixed sewage sludge has a high concentration of carbs, lipids, and proteins, it is infamously difficult to digest.
The most recent designs of these facilities frequently incorporate a pre-treatment stage in which the entering sludge cells are broken apart to allow them to be digested more readily and quickly in the digester.
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