Determining the best digester for wastewater treatment plants

January 10, 2025

A closeup image of a digester in a wastewater treatment plant.

Determining the right digester is critical to increase wastewater treatment efficiency and mitigate operation challenges.

Many wastewater treatment plants (WWTPs) use digestion to meet federal regulations and reduce total sludge volume. The correct digester optimizes operations and maintenance, so it's crucial to make the best choice – aerobic or anaerobic – for your WWTP.

Aerobic digestion is more suitable for small and medium WWTPs because of its high-energy demand. On the other hand, anaerobic digesters are widely used in large-scale WWTPs to reduce the amount of waste-activated sludge (WAS).

This post explains the types of digesters to process wastewater solids and why municipalities should partner with an expert to determine the appropriate digester for their treatment facility.

Choosing the right digester for your plant

Aerobic and anaerobic digesters are used to reduce the volume of solids and are processes that significantly reduce pathogens in the solids before disposal. However, there are distinct differences between aerobic and anaerobic digesters.

We consider many factors when determining the type of digesters in a WWTP. They include:

Type of organic waste. Solids can be primary sludge (settled before secondary treatment), biological sludge (produced during secondary treatment) or a combination of both.

Operation mode. Aerobic digesters require oxygen. Anaerobic digesters operate without oxygen but require the sludge to be heated.

Design configuration/scale. Aerobic digesters are common for smaller WWTPs because of lower capital costs and operation ease. Anaerobic digesters are common for larger WWTPs because of their ability to produce large quantities of biogas, which can be used to heat the digester and/or generate power.

The following table highlights the differences between aerobic and anaerobic digestion systems.

Aerobic vs. anaerobic digesters

Aerobic digester

Anaerobic digester

The system requires an oxygen supply, such as blowers and diffusers or mechanical aeration devices, to the biomass.
Organic matter is degraded to carbon dioxide, water and nitrogen byproducts.
Secondary treatments, including activated sludge, fixed-bed bioreactors, moving bed bioreactors and membrane bioreactors, could be followed by aerobic digestion.

The system requires an oxygen supply, such as blowers and diffusers or mechanical aeration devices, to the biomass.
Organic matter is degraded to methane (biogas) and carbon dioxide.
The majority of WWTPs with aerobic digester systems use the biogas they produce as an energy resource to produce electricity or usable heat.

Advantages

Volatile solids reduction of 35%-50%
Production of odorless, biologically stable sludge.
Lower capital cost.Relatively easy operation.

Advantages

Volatile solids reduction of 35%-50%.
Energy recovery through the production of biogas.
Lower operational cost.Better mechanical dewatering characteristics.

Disadvantages

Higher operational cost.
No energy recovery through the production of biogas.
Poorer mechanical dewatering characteristics. The process is significantly affected by temperature.

Disadvantages

Higher capital cost.
Heating required.
The process and end product can have undesirable odors.
Supernatants can have high Biochemical Oxygen Demand (BOD) and ammonia loads.

Types

Conventional air aerobic digestion. This uses atmospheric air to provide oxygen. Liquid temperature significantly affects aerobic digestion, as most digesters are open tanks.
High-purity oxygen aerobic digestion. It is a modification of conventional aerobic digestion in which high-purity oxygen is used instead of air. It is best suited for cold-weather climates as the process is not as affected by changes in ambient air temperature.
Autothermal thermophilic aerobic digestion. Closed and insulated digester tanks are used to maintain increased operating temperatures. This decreases the required solid retention time.

Types

Single-stage high-rate anaerobic digestion. Heating, mixing, feeding and thickening occur in single-stage tanks.
Two-stage anaerobic digestion. A high-rate anaerobic digester is coupled in series with a second tank. The second tank is usually unheated and used for storage.
Separate sludge anaerobic digestion. Separate digester tanks are used for primary and biological sludge.

The right type of digester improves wastewater treatment efficiency and mitigates operation challenges. An expert can help you choose the best digester for your wastewater infrastructure.

Fehr Graham helps municipalities optimize wastewater treatment facilities

At Fehr Graham, we are a team of wastewater engineers committed to strengthening and improving wastewater infrastructure across communities in the Midwest and beyond. We have helped municipalities upgrade and retrofit their wastewater treatment plants.

One of our ongoing wastewater engineering projects involves planning, designing and engineering digester complex improvements for Fox Metro Water Reclamation District. We have helped Fox Metro operators procure digester covers and mechanical mixers. We have also designed a digester control building to house the electrical equipment and boilers. The NFPA 820-compliant improvements to the digester control building have ensured efficient wastewater treatment.

To learn how Fehr Graham can help you determine the right digesters for wastewater treatment plants, contact us or give us a call at 563.927.2060.

Lucas Elsbernd, a Professional Engineer and Senior Project Manager, finds innovative solutions to challenging water and wastewater projects. He manages municipal, commercial and industrial planning and design projects in the water resources environment. He fosters a positive client experience and collaborates with leaders throughout the firm on project pursuits and technical issues. Lucas is one of the firm’s water and wastewater experts. He has a strong technical knowledge of water and wastewater treatment design and construction engineering. He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it. or by phone at our Manchester office: 563.927.2060.

Determining the best digester for wastewater treatment plants

Choosing the right digester for your plant

Fehr Graham helps municipalities optimize wastewater treatment facilities

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