World Biogas Expo 2022 BANNER AD.
There's no Net Zero without Biogas: ending waste, delivering the circular economy, tackling the climate crisis.
AquaEnviro Biosolids 2021 Conference partner AD Banner
Image of egg-shaped anaerobic digesters at night.

Egg Shaped Anaerobic Digesters – Strange Looking Tanks Explained

Egg Shaped Anaerobic Digesters; Enormous eggs! Whatever next! Have you ever wondered what those huge “strangely other-world like” not quite “spherical” tanks are that you sometimes see?

Image of egg-shaped anaerobic digesters at night.
Scroll Down for more pictures of egg-shaped digesters!

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.

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

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

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

Image shows another egg-shaped digester.
CC BY-NC by leonelponce

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.

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

Egg-Shaped Digester Built in Pinellas County, Florida, USA

CC BY-NC by Massachusetts Clean Energy Center

Recent enhancements incorporated into the original design of the Egg-Shaped Digester (ESD™) anaerobic digester system have led to simplified mechanical systems, which have further improved the life cycle benefits of ESD systems.

Consequently, these improvements have allowed many owners to consider design-build construction of a digester facility. A few of the benefits of this construction approach include single source responsibility, accelerated construction schedule and lower capital cost.

This approach was recently implemented at a facility located in Pinellas County, Florida, USA. The County wanted to build a facility to treat solids that would produce a Class A product in a pelletized form, which could then be marketed and sold. via

Egg-Shaped Digesters for Sewage Sludge Treatment

The Egg-Shaped Digester is one of the most efficient and cost-effective anaerobic systems for sewage sludge treatment. Its double-curvature shape, small top liquid surface area and liquid mixing system help reduce scum, grit buildup and dead zones. Thus, the vessel does not have to be taken out of service for cleaning. The ESD system also provides a smaller footprint, improved reliability, enhanced process control and reduced maintenance, all leading to reduced energy demand. via

What Happens in an Egg-Shaped Digester?

Image shows several egg-shaped digesters.
CC BY-NC-ND by nsub1

The process of anaerobic digestion is what happens in any digester. 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.

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. via biofermenergy

What Fuels Anaerobic Digestion?

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

Tags: , , , , , ,
Previous Post
10 top silage clamp tips from Bock UK
Silage Clamps for AD Plants

Top 10 Silage Clamp Tips for the Care and Legal Operation of AD Feedstock Storage

Next Post
Sampling-during digester cleaning work

Digester Cleaning – Purposes and Services


    • Evelyn Lee
    • May 1, 2018

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

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Important: This site makes use of cookies which may contain tracking information about visitors. By continuing to browse this site you agree to our use of cookies.