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Egg Shaped Anaerobic Digesters – Strange Looking Tanks Explained

Image of egg-shaped anaerobic digesters at night.Egg Shaped Anaerobic Digesters; Whatever next! Have you ever wondered what those huge “other worldy” spherical tanks are that you sometimes see?

They are anaerobic digestion process digesters, and very useful they are too!

They make sewage sludge much more wholesome and in particular they reduce sludge odour, but best of all they make large amounts of renewable energy to power the sewage works they form part of.

Egg Shaped Anaerobic Digesters – Very “Quatermass”!

Plus, many of them export their “green” electricity to the local power grid.

Origins of the Egg Shaped Digester

Egg-shaped digesters originated in Germany in 1950s and it is thought that all are used to treat sewage works sludge.

Egg Shaped Digester Design is Only Human

Design features are many, as follows:

– The steeply sloped bottom of the tank eliminates grit accumulation. Grit can easily be removed from the bottom, therefore, cleaning is not required.

  • Liquid surface area at the top is small, so scum can be kept fluid with a mixer, and removed through a scum door.
  • Egg-shaped digesters can be built with steel or concrete.
  • Steel construction is more common because concrete construction requires complex formwork and special
    construction techniques.

If you found our video interesting, now read more at our website.


This video was inspired by the presentation on egg-shaped digesters at:

More Information About Digesters Around the Web

A biogas digester (also known as a biogas plant) is a large tank where inside Biogas is produced through the decomposition/breakdown of organic matter through a process called anaerobic digestion. It’s called a digester because organic material is eaten and digested by bacteria to produce biogas.

Biogas is produced by the breaking down of organic, biodegradable waste or material (also known as biomass) such as vegetables, leaves, grass, weeds, leftover food scraps and such. When this organic breakdown happens it produces a gas, called biogas. via

A digester is a huge vessel [sometimes egg-shaped] where chemical or biological reactions are carried out. These are used in different types of process industries. via wikipedia

Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.

Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide and traces of other ‘contaminant’ gases. This biogas can be used directly as fuel, in combined heat and power gas engines, or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.

Small-Scale Digester: EUCOlino

Egg-Shaped Digesters are Used in the Wet AD Process CSTR or Plug Flow – as Explained Below

The wet anaerobic digestion process is applied to liquid waste streams that are conveyable by liquid pumping. Sometimes wet systems are called Low Solids AD (LSAD). The Wet AD process can be done in reactors of two main configurations, continuously stirred tank reactors (CSTR) and plug flow reactors. The theory of the CSTR is that, through rigorous mixing, the composition of the contents of the reactor in any given spot in the tank is the same as in any other spot in the tank. The theory of plug flow, on the other hand, is that the makeup of the contents at the head of the digester is different than that of the material leaving the digester just as the material flows through the digester in the pattern like a plug through a pipe. Wet systems commonly run at total solids levels between 2 and 8 percent. Wet systems will often start with a liquid manure or waste biosolids as the backbone of their feedstock load to provide a baseload buffering affect for enhanced process stability.

A key design parameter for any digester system is the overall loading rate. For any given project no two digester suppliers will provide a system of exactly the same size. Loading rates are commonly expressed as the number of days of retention time or the quantity of organic matter applied to a given tank volume. Common detention times for farm based manure digesters are roughly 20-30 days. Experience has shown that this time represents an optimum time where gas yield is maximized without over designing the residence time. Facilities that are co-digesting more complex wastes that include fats and proteins will commonly have retention times higher than 30 days. via

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One Response to Egg Shaped Anaerobic Digesters – Strange Looking Tanks Explained

  1. Evelyn Lee May 1, 2018 at 5:34 pm #

    Dear, I always wanted to know about this. Until today nobody told me. Thank you for posting this article.

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